aDépartement de chimie, Université Laval, 1045 avenue de la Médecine, Québec (Québec) G1V 0A6, Canada. Fax: +1 418 656 7916; Tel: +1 418 656 5034; E-mail: [email protected]
proReceived 00th January 20xx, Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Recent Progress in the Catalytic Carbene Insertion Reactions into the Silicon–Hydrogen Bond Hoda Keipour,a Virginie Carreras,a Thierry Ollevier a,*
The following review will explore the historical development of Si–H bond insertion reactions, giving an up-to-date account on the metal catalysts often employed, in addition to an assessment of their strengths and weaknesses. Diazo compounds have great synthetic potential as versatile reagents for the formation of metal carbenes, allowing the selective formation of C–C and C–heteroatom bonds and thus the introduction of functional groups into organic molecules. C–Si bond-forming methods, that introduce silicon motifs into organic molecules, rely on catalysts derived from metals such as rhodium, copper, iridium, silver, ruthenium, and iron to achieve the desired activities and selectivities.
Born in Sari, Iran, Hoda Keipour obtained her M.Sc. (2010) in organic chemistry in Professor Mohammad A. Khalilzadeh research group in Iran, where she focused on heterogeneous catalysis and organic chemistry applications. She is currently a Ph.D. student in synthetic organic chemistry at Université Laval, Québec, Canada, under the supervision of Professor Thierry Ollevier, where she is working on the development of new synthetic methods
for asymmetric insertion reactions of diazo compounds into Si–H and S–H bonds and asymmetric hydrosilylation of ketones, using copper and iron catalysis.
Born in Muret, France, Virginie Carreras received her M.Sc. (2016) in fundamental and applied organic chemistry from Université Paul Sabatier, Toulouse III, France. Previously, she has been an intern in organic chemistry at Université Laval, in Québec (2015), under the supervision of Professor Thierry Ollevier, then in D. R. Stuart Group at Portland State University, Oregon (2016), specialized in unsymmetrical diaryliodonium salts
synthesis. Recently, she started her Ph.D. in synthetic organic chemistry at Université Laval, Québec, under the supervision of Professor Thierry Ollevier. Her current research interest is the development of asymmetric iron catalysis using diazo compounds.
Born in Brussels, Thierry Ollevier obtained his B.Sc. (1991) and Ph.D. (1997) at the Université de Namur, Belgium, and was postdoctorate fellow at the Université catholique de Louvain, Belgium under István E. Markó (1997), NATO postdoctorate fellow at Stanford University under Barry M. Trost (1998–2000), then postdoctorate fellow at the Université de Montréal under André B. Charette (2000–2001). After an Assistant Professor appointment (2001) at Université Laval, he
became Associate (2006) and is currently Full Professor. Current research in his group aims at designing novel catalysts, developing catalytic reactions and applying these methods to chemical synthesis. He is active in the areas of Lewis acids, asymmetric catalysis, and synthetic green chemistry. He has served as an Associate editor of RSC Advances and was admitted as a Fellow of the Royal Society of Chemistry (2016).
1 Introduction
The catalytic insertion reaction of diazo compounds into X–H (X = heteroatom) bonds is a very powerful organic transformation due to the highly synthetic potential of the generated building blocks.1 In the past decades, chiral silanes have been particularly popular for stereoselective transformations in organic synthesis.2 Various methods using diverse catalysts have been discovered sharing the common aim to enhance the selectivity of these insertion reactions. Remarkable methodological advances have been made in the catalytic asymmetric and non-asymmetric diazo insertion reactions into the C–H bond. Crucial breakthroughs have been obtained for the diazo insertion reactions into the C–H bond and some reviews have summed up the progresses in this topic, 3-5 but almost no review has specifically focused on the advancement in catalytic carbene insertion reactions into the Si–H bond. Silicon constitutes
almost 30% of the mass of Earth’s crust. Whereas natural supplies of silicon are abundant, versatile methods for synthesizing organosilicon derivatives are not common. Metal-catalyzed carbene transfer reactions of diazo compounds have proved to be a useful methodology in organic synthesis.6, 7 Diazo compounds, which are commonly used as carbene precursors, have been extensively employed as versatile cross-coupling partners in various transition-metal-catalyzed reactions.8, 9 Diazo compounds can be converted into highly reactive free carbene intermediates under thermolytic or photolytic conditions. Because of their limited synthetic applications and their lack of selectivity in many chemical transformations, there has been a significant interest in the development of transition metal-catalyzed decomposition reaction of diazo compounds.10 Fischer carbenes11 and Schrock carbenes12 are various types of transition metal carbene complexes. Fischer carbenes have more electrophilic character because of the direct C→M donation which results in a positively charged carbon. They are usually found with low oxidation state metals, such as Mo, Cr, and W. On the other hand, Schrock carbenes are more nucleophilic because they form two polarized covalent bonds, giving a negative charge on the carbon atom, and are usually found with high oxidation state metals, such as Ti and Ta (Figure 1).13
Figure 1 Fischer carbenes and Schrock carbenes
The reactivity profile of transition metal carbenes is highly influenced by the nature of the substituents on the carbenes and the type of metal. Consequently, reviews on metal carbenes often classify them into three distinct categories: acceptor metal carbenes, acceptor/acceptor metal carbenes, and donor/acceptor metal carbenes.14 Generally, an acceptor substituent makes the metal carbene species more electrophilic as well as more reactive, whereas a donor group makes the metal carbene more stable and, thus, more selective in reactions (Figure 2).15 A wide variety of transformations can occur with these transition metal carbenes, such as X–H (X = C, N, O, Si, S, etc.) insertion reactions, cyclopropanation, ylide generation, and rearrangements. Their chemoselectivity was found to be dependent on the metal species, ligands, and substrates (Figure 3).16 C–Si bond-forming methods, introducing silicon motifs into organic molecules, rely on multistep synthetic routes to prepare and optimize the synthesis of reagents or catalysts. Insertion reactions of diazo compounds into the Si–H bond have been well-known since 1988. However, they have not received much attention compared to their N−H and O−H counterparts. Recently, there have been a few notable developments, particularly in the field of asymmetric reactions of diazo compounds into the Si–H bond, where chiral copper, rhodium, iridium, ruthenium, and iron catalysts have shown to be very promising. Various methods using diverse types of catalysts have been developed, with the objective of enhancing the selectivity of Si–H bond insertion reactions (Figure 4).
Figure 2 Classification of metal carbene species
Figure 3 Various transformations using metal carbenes
Figure 4 General scheme for diazo insertion reaction into the Si–H bond
The insertion reaction of carbene species into the Si–H bond was first reported by Kramer and Wright who studied the reaction of diazoalkanes with organosilanes.17 They mentioned that diazomethane reacts with organosilicon hydrides exclusively in the presence of ultraviolet light or copper powder. In this study, they suggested that the reaction involves the insertion of a carbene. The yields of methyl derivatives were shown to be dependent on the steric hindrance of the substituents on the silane.
2 Metal carbene insertion reaction into the Si–H bond 2.1 Rhodium catalysis The RhII-catalyzed insertion reactions of a-diazoesters and a-diazoketones into the Si–H bond, resulting in the formation of a-silylesters and a-silylketones, was first reported by Doyle in 1988.18 In this study, the reactions were performed using 1 mol% of Rh2(OAc)4 at room temperature. The diazo compounds were slowly added to the mixture of silane sources and Rh2(OAc)4 by means of a syringe pump to minimize dimerization into the tri/tetra-substituted olefin. The desired insertion reaction products were obtained in high yields (up to 95%) (Scheme 1). A few years later, Landais demonstrated an efficient one-pot synthesis of a-(alkoxysilyl)acetic esters, proceeding by the Si–H
MLn
R1 R2δ+
δ– MLn
R1 R2
MLn
R1 R2
δ+δ–
Fischer carbeneselectrophilic in natureπ accepter metal ligandslow oxidation state
Metal carbene Schrock carbenesnucleophilic in naturenon π acceptor metal ligandshigh oxidation state
R1
R2M
R1
R2M
Acceptor metal carbeneAcceptor/Acceptor metal carbene Acceptor/Donor metal carbene
bond insertion reaction of carbenes, in the presence of RhII catalyst (Scheme 2, eq. a).19, 20 In this work, the synthesis of a-(alkoxysilyl)acetic esters was carried out using a slow addition of a-diazoesters into a mixture of chlorosilane and catalytic amounts of Rh2(OAc)4, followed by the nucleophilic substitution on the Si–Cl bond using ROH to form the desired a-alkoxysilanes in good yields (up to 82%), as shown in Scheme 2, eq. a. It was shown that a faster addition of a-diazoesters usually induced the dimerization of the metal carbene species, resulting in the formation of diethyl fumarate and maleate (Scheme 2, eq. b). He also reported that substitution of Rh2(OAc)4 with the less expensive Cu(acac)2 proved to be unsuccessful, owing to the fact that the amount of insertion product never exceeded 10% along with carbene dimerization. Landais also disclosed an interesting route to prepare optically active 1,2-diols.20 21, 22 The diol formation occurred after reduction of the esters using DIBAH, followed by an oxidation step to convert the C–Si bond into a C–OH bond.
Scheme 1 Synthesis of a-silylesters and a-silylketones using RhII-catalyzed decomposition of diazo compounds in the presence of organosilanes
Scheme 2 a) One-pot synthesis of a-(alkoxysilyl)acetic esters b) Diethyl fumarate and maleate as by-products of RhII-catalyzed insertion reaction
Menthyl a-methyl-a-diazoester was used with various silanes to produce chiral a-silylcarbonyl compounds in the presence of Rh2(OAc)4.21, 22 With trialkylsilanes, such as phenyldimethylsilane (PhMe2SiH) and triethylsilane (Et3SiH), 44% de was obtained, whereas (chlorodimethylsilane) ClMe2SiH furnished 34% de and bulky triphenylsilane (Ph3SiH) afforded low diastereoselectivity (6%) (Scheme 3). In addition, various a-diazoesters were also employed along with Rh2(OAc)4 catalyst in the same conditions to obtain various a-substituted-a-silylacetic esters (Scheme 4). The yields
were found to be relatively good and diastereoselectivities were in the same ranges as those formerly obtained.21, 22 The authors also carried out comparative studies between RhII-vinyl carbene insertion reactions into Si–H and O–H bonds (Scheme 5).23,
24 They reported up to 70% de for these transformations resulting in products of identical absolute configuration (S).
Scheme 3 Metal carbene insertion reaction into the Si–H bond using Rh2(OAc)4 as catalyst
Scheme 4 Carbene insertion reaction of (–)-menthyl a-diazoesters into the Si–H bond using Rh2(OAc)4 as catalyst
Scheme 5 Asymmetric insertion reaction of RhII-carbenes into Si–H or O–H bonds
In a further study, Landais investigated the RhII-catalyzed decomposition of a-vinyl-a-diazocarbonyl compounds in the presence of organosilanes and demonstrated an efficient and stereospecific synthesis of a-allylsilanes occurring in good yields.23 The a-vinyl-a-diazocarbonyl compounds were also prepared using corresponding b,g-(E) and (Z)-unsaturated esters and ketones. All underwent smooth insertion reactions regardless of the nature of the silanes used. This gave rise to good yields of a-allylsilanes (up to 80%) and other a-silylated carbonyl compounds (Scheme 6). Interestingly, in this study, the geometry of the double bond was totally retained during the insertion process allowing stereospecific access to (Z) or (E) a-allylsilanes.
α-silylesters α-silylketones
R1 (O)R2N2
O+ R3Si–H
Rh2(OAc)4 (1 mol%)CH2Cl2, rt
R1 (O)R2SiR3
O
HOEt
SiEt3
O
HOEt
SiPh3
O
HtBu
SiEt3
O
HPh
SiEt3
O
H
SiEt3
OPhH
(CH2)6CH3
SiEt3
O
O
SiEt3
O
SiEt3
94%
90%
89% 85%
95%
89%
88% 82%
1–2.1 equiv
HOEt
N2
O
1) Rh2(OAc)4 (0.3 mol%) R2ClSi–H (1.05 equiv)
HOEt
O2) ROH (1.2 equiv) NEt3 (1.2 equiv) CH2Cl2
SiR2OR1
HOEt
O
SiMe2OiPr
HOEt
O
SiMe2OBn
HOEt
O
SiMe2OEt
HOEt
O
SiMe2OtBu
HOEt
O
SiMe2O
HOEt
O
SiPh2OEt
HOEt
O
SiPh2OiPr
HOEt
O
SiMe2O
HOEt
O
SiMe2O
HOEt
O
SiMe2O Ph
74% 76% 73% 78% 74% 65%
71% 80% 82% 75%
a)
b)
HOEt
O
OEtHO
HOEt
O
HO
EtO
O(–)-menthylN2
O+ R3Si–H Rh2(OAc)4 (1 mol%)
CH2Cl2, rtO(–)-menthyl
SiR3
O2 equiv
O(–)-menthylSiMe2Ph
O
O(–)-menthylSiEt3
O
O(–)-menthylSiPh3
O
O(–)-menthylSiMe2Cl
O
70%, 44% de 72%, 44% de 66%, 6% de 74%, 34% de
Rh2(OAc)4 (1 mol%)CH2Cl2, rt
R = iBu: 75%, 40% deR = CH2Ph: 52%, 22% deR = CH2CO2tBu: 70%, 32% de
Scheme 6 Synthesis of a-allylsilanes using RhII-vinyl carbene insertion reaction into the Si–H bond
Blanco then reported the synthesis of 3-silaglutarates by the reaction of a dihydrosilane with ethyl a-diazoacetate in the presence of Rh2(OAc)4 (Scheme 7).25 This method appeared to be generally easier to set up than the synthesis from a dichlorosilane and the lithium enolate of alkylacetates. He also found that the two methods are complementary because the metal carbene insertion reaction into the Si–H bond is sometimes competitive with the cyclopropanation in the case of alkenylsilanes. In addition, he described that it is possible to synthesize the triester analogue in 56% yield by treating hexyltrihydrosilane with ethyl a-diazoacetate in the presence of Rh2(OAc)4. The reaction of methyl a-phenyl-a-diazoacetate in the presence of PhMe2SiH and a chiral dirhodium(II) catalyst resulting in Si–H bond insertion of the intermediate metal carbene with varying degrees of enantioselectivity (up to 47% ee) was developed by Doyle and Moody.26 They reported that the enantioselective Si–H bond insertion reactions can be affected by the nature of the chiral dirhodium(II) catalysts used. The desired a-silylated product was obtained in good yield and ee's reaching 50%. They showed that, in general, among dirhodium(II) carboxylates and carboxamidate catalysts, carboxamidate ligands give better results. Among the tested catalysts, Rh2(5S-MEPY)4, appeared to be the best one, affording ee's in the range of 45–50% (Scheme 8). In 1997, Landais disclosed a kinetic study involving metal carbene intermediates during the insertion reaction into the Si–H bond.27 At first, he suggested that the insertion of Rh-carbenes into the Si–H bond occurs via a stepwise mechanism involving an oxidative addition of the Si–H bond to the metal carbene as an intermediate, followed by a migratory insertion of the hydrogen into the metal carbene center, the carbon-silicon bond will hence be formed after reductive elimination. He also demonstrated the kinetic isotope effect in the insertion reaction of an a-vinyl-a-diazoester in the presence of PhMe2SiH and PhMe2SiD, and Rh2(OAc)4 as catalyst. The corresponding a-silylesters were obtained in a 6:4 ratio (H/D) with a kinetic isotope effect of 1.5 (Scheme 9). Based on the obtained kinetic isotope effect, he indicated that the cleavage of the Si–H bond occurs in the rate-determining step and the magnitude of the kinetic isotope effect suggests that the transition state is either very early or very late. He also studied the Linear Free Energy Relationship (LFER) to accumulate information regarding the mechanism of the Si–H bond insertion reaction. It was thus concluded that the insertion proceeds through a concerted pathway with the development of partial positive character at the
silicon center and a relatively early transition state. In addition, he reported that the electrophilic attack of the Rh-carbene species into the Si–H bond would lead to a partial positive charge building up on silicon during the transition state. Here, by analogy with free carbenes, the putative metal carbene species can be regarded as the electrophilic species. This observation is in agreement with the concerted addition of the Si–H bond into an electrophilic Rh-carbene center via a transition state similar to that depicted in Figure 5. As a result, the retention of configuration at the silicon center along with the retention of the geometry of the double bond of vinyl Rh-carbene throughout the insertion reaction all seemed to be in keeping with such a mechanism.
Scheme 7 Synthesis of 3,3-disubstituted 3-silaglutarates
Scheme 8 The dirhodium(II)-catalyzed asymmetric Si–H bond insertion reaction of methyl a-phenyl-a-diazoacetate
Scheme 9 Competition experiment and kinetic isotope effect
Figure 5 Concerted addition of the Si–H bond into an electrophilic Rh-carbene center
Davies reported that a-allylsilanes or a-benzylsilanes can be obtained with high enantiomeric purity from the RhII-(S)-N-[p-(dodecylphenyl)sulfonyl]prolinate catalyzed decomposition of a-vinyl-a-diazomethanes or a-phenyl-a-diazomethanes in the presence of PhMe2SiH.28, 29 In this work, the RhII-prolinate catalyzed decomposition of methyl a-phenyl-a-diazoacetate in the presence of PhMe2SiH in dichloromethane resulted in the formation of a-benzylsilane in a very low ee (3%). Using pentane as the solvent at room temperature resulted in the formation of the product in 36% ee. He also reported the improvement in enantioselectivity by conducting the reaction at lower temperatures (e.g. –78 ºC) which led to the desired product in 85% ee. The same catalysts were also
(O)R2N2
O+ R3Si–H Rh2(OAc)4 (1 mol%)
CH2Cl2, rt(O)R2
SiR3
O
Ph
SiMe2PhOEt
OPh
SiMe2(5-Me-thienyl)OEt
OPh
Si(TMS)3OEt
O
OMe
O
SiMe2Ph
OMe
O
SiMe2Ph
Ph
SiMe2PhOEt
OPh
SiMe2(5-Me-thienyl)OEt
OC5H11
OEt
O
SiMe2Ph
OMe
O
SiMe2Ph
Ph Ph
SiMe2Ph
O
70% 72% 45% 73%
75% (Z:E 95:5) 80% 61% 76%
72% (Z:E > 98:2) 72% 65%
2 equivR1 R1
OMe
O
SiMe2PhC5H11
HOEt
N2
O
Rh2(OAc)4.xH2O (0.75 mol%)CH2Cl2, rt
2.5 equiv
SiR1
R2 CO2Et
CO2Et+ Si
R1
R2
H
H1 equiv
SiPh
CO2Et
CO2EtSi
Ph
tBuCO2Et
CO2Et
SiPh
AllylCO2Et
CO2EtSi
Ph
VinylCO2Et
CO2Et
SiCO2Et
CO2EtSi
CO2Et
CO2Et
64% 64%
24% 55%
62% 64%
Ph
N2
+(2 mol%)
CH2Cl2, 40 ºC
Rh2(5R-MEPY)4 : 70%, 45% eeRh2(5S-MEPY)4 : 69%, 47% ee
used for the decomposition of methyl a-vinyl-a-diazomethanes at –78 ºC, resulting in the formation of the a-allylsilanes in 77–95% ee (Scheme 10).
Scheme 10 RhII-prolinate catalyzed decomposition of methyl a-phenyl-a-diazoacetate and methyl a-vinyl-a-diazomethanes in the presence of PhMe2SiH
The stereospecific access to allylic systems using RhII-vinyl carbene insertion reaction into the Si–H bond was reported by Landais.24
These experiments were carried out by adding the a-styryl-a-diazoester to a suspension of the silane in dichloromethane and a catalytic amount of Rh2(OAc)4 (Scheme 11). In this study, the insertion process appeared to be stereospecific leading to (Z)- and (E)-allylsilanes in good yields accompanied with retention of the geometry of the double bond. He mentioned that the retention of the stereochemistry supports the hypothesis of a concerted mechanism. An extension of the process to a homochiral series was devised in this work as well using either a chiral auxiliary attached to the ester function or achiral a-vinyl-a-diazoesters and Doyle’s chiral catalyst Rh2(MEPY)4. In the former approach, using pantolactone as the chiral auxiliary gave diastereoselectivities of up to 70%, while the later approach produced the desired a-allylsilane with ee as high as 72%.
Scheme 11 Synthesis of a-allylsilanes using RhII-vinyl carbene insertion reaction into the Si–H bond
Afterwards, Hashimoto demonstrated that the enantioselective insertion reaction of methyl a-phenyl-a-diazoacetate into the Si–H bond was catalyzed by employing dirhodium(II) carboxylates incorporating N-phthaloyl-(S)-amino acids as chiral bridging ligands.30 In this work, the Si–H bond insertion reaction of methyl a-phenyl-a-diazoacetate was carried out with PhMe2SiH in the presence of 1 mol% of dirhodium(II) tetrakis[N-phthaloyl-(S)-
phenylalaninate], Rh2(S-PTPA)4 (Table 1). After studying the solvent effects, dichloromethane was found to be superior, allowing a smooth reaction to occur at −78 ºC, giving benzylsilane in 86% yield and 65% ee. It was also found that the enantioselectivity was highly dependent on the reaction temperature. In addition, further enhancements of up to 74% ee were shown to be possible by conducting the reaction at −90 ºC. On the other hand, toluene provided 80% yield and 52% ee at −78 ºC, but the reaction required much longer time to reach completion. Moreover, the effects of silicon substituents on enantiocontrol were also examined. Later on, Moody developed a parallel synthesis technique to identify several chiral dirhodium(II) carboxylates capable of affecting the enantioselective Si–H bond insertion reaction.31 He found that catalysts derived from a-hydroxyacids and N-arenesulfonyl a-amino acids are the most efficient families (Scheme 12). He also demonstrated that mandelic acid and N-benzenesulfonyl prolinate give relatively poor results in silane insertion reactions (8 and 12% ee, respectively, with PhMe2SiH).
Table 1 Enantioselective intermolecular Si–H bond insertion reaction of methyl a-phenyl-a-diazoesters catalyzed by chiral RhII complexes
Scheme 12 Enantioselective Si–H bond insertion reaction of methyl a-phenyl-a-diazoester with various silanes
Corey reported an effective process for the enantioselective synthesis of chiral a´-silylated-a,b-enones using a chiral RhII catalyst (Scheme 13).32 In this case, the best conditions were obtained using slow addition of the a,b-enone to the solution of Et3SiH and catalyst. N-Nonafluorobutanesulfonylproline (Nf-proline)–RhII complex gave (+)-(S)-6-triethylsilyl-2-methyl-2-cyclohexenone in 70% yield and 94% ee.
4
N
O
OSO2Ar
Rh
Rh
H
Ar =p-C12H25C6H4
pentane, –78 ºC
50%, 85% ee
R1
N2
+ PhMe2Si–HOMe
OR1
SiMe2PhOMe
O
SiMe2Ph
76%, 91% ee 64%, 95% ee
77%, 92% ee
Ph
SiMe2PhOMe
O
OMe
O
Catalyst (2 mol%)
SiMe2Ph
O
OMePh
SiMe2Ph
O
OMe
SiMe2Ph
O
OMePh
68%, 77% ee
SiMe2Ph
63%, 85% ee
OMe
O
1.1 equiv
N2
+ R3Si–HOR3
O
Rh2(OAc)4 (0.43 mol%)
CH2Cl2, rt
SiR3
R1 OR3
O
SiMe2Ph
PhOEt
O
SiMe2(Thien)
PhOEt
O
Si(SiMe3)3
PhOEt
O
SiMe2(iPrO)
PhOEt
O
SiF(iPr)2
PhOEt
O
SiMe2allyl
PhOEt
O
SiMe2(BrCH2)
PhOEt
O
R1
R2 R2
SiMe2Ph
PhOEt
O
SiMe2(Thien)
Ph OEt
O
SiMe2PhOMe
O
SiMe2Ph
tBu OEt
O
70% 72% 45% 85%
76% 98% 97% 80%
61% 76% 87%
2 equiv
Ph
N2
+ PhMe2Si–HOMe
OPh
SiMe2PhOMe
O2 equiv
Chiral rhodium catalyst(1 mol%)
*
OORhRhN2
Ph (2 mol%)
CH2Cl2, rtPh
R = iBu: 50% ee R = Me3Si: 55% eeR = Me(Me3SiO)2: 44% ee
TsHN CH2PhH
R3Si–H+O
OMeSiR3
O
OMe
1.1 equiv*
Entry Catalyst Solvent T (ºC ) Yield (%) ee (%) Conf.
Scheme 13 Enantioselective synthesis of 6-(S)-triethylsilyl-2-methyl-2-cyclohexenone by catalytic insertion reaction of a-diazoketone into the Si–H bond
In 2010, Panek showed that enantioenriched homoallylic ethers containing an a,b-unsaturated ester core could be directly obtained by Lewis acid catalyzed crotylation utilizing a chiral silane.33 In this study, the reagents were prepared by enantioselective insertion reactions of a-vinyl-a-diazoacetates into the Si–H bond using Davies’ Rh2(DOSP)4 catalyst or chiral CuI Schiff-base complexes. He also reported an efficient synthesis of chiral allylic silanes with C-centered chirality. These experiments allowed a comparison of chiral CuI vs RhII catalysis. Consistent with Davies’ studies, the Rh2(DOSP)4 catalyst provided both enantiomers of crotylsilane with excellent ee’s. Comparable selectivity was then achieved using tributylsilane (nBu3SiH) and PhMe2SiH (Table 2). In contrast, Rh2(S-DOSP)4 afforded the product with a moderate ee. The author also showed that the rhodium catalyst was ineffective for preparing the enantioenriched product owing to the poor solubility of Ph3SiH in pentane. In addition, his preliminary results demonstrated that using [(CH3CN)4Cu]PF6 and a diimine ligand affords the product with good selectivity (� 70% ee), which could further be improved to 97% ee by recrystallization from petroleum ether.
Table 2 Preparation of chiral a-allylsilane bearing a silyl group
Catalysta R3SiH T (ºC) Solvent Yield (%) ee (%)
1 Rh PhMe2SiH –78 hexane 65–70 88–97
2 Cu (R = H) PhMe2SiH 0 benzene 44–51 70–73
3 Cu (R = C10H21) PhMe2SiH 0 benzene 44–55 78
4 Rh nBu3SiH –78 pentane 48 86
5 Rh TMS3SiH –40 pentane 30 40
6 Rh Ph3SiH 0 pentane NR NR
7 Cu (R = H) Ph3SiH 0 Pentane 41/25b 70/97b
aCatalyst loading for Rh (1–5 mol%) and for Cu (5 mol%). bYield and % ee before and after recrystallization from petroleum ether.
Ball demonstrated a strategy to utilize natural polypeptide ligands for the development of chiral dirhodium catalysts (Scheme 14).34, 35 Based on this strategy, a relatively poor initial “hit” of 45% ee was improved using parallel automated peptide synthesis. In addition, the combined effects of peptide libraries and facile dirhodium complexation proved to be efficient in the development of selective catalytic transformations. He also showed that the generation of a
small peptide library allowed the optimization of the peptide sequence to produce an effective catalyst for the enantioselective metal carbene insertion reaction into the Si–H bond (up to 99% ee). However, the selectivity was much lower for ortho-substituted aryl substrates (20−49% ee). A stereoselective synthesis of antibiotic (−)-virginiamycin M2 was disclosed by Panek in 2011.36 A convergent strategy was utilized which proceeded in 10 steps (longest linear sequence) from an enantioenriched silane (Scheme 15). This work started by establishing a reproducible asymmetric Si−H bond metal carbene insertion reaction into an a-vinyl-a-diazoester in order to prepare a chiral silane. Previously, a straightforward synthetic pathway was reported by Davies’ group to prepare enantioenriched a-allylsilanes bearing an ester functional group at the chiral center utilizing chiral Rh2(DOSP)4 catalyst.
aThe ee was determined after reduction into corresponding alcohol. Yield was based on 1H NMR relative calculation vs an internal standard.
Scheme 14 Asymmetric insertion reaction of methyl a-aryl-a-diazoesters into the Si–H bond
Scheme 15 Stereoselective synthesis of the antibiotic (−)-virginiamycin M2
Panek revealed that the use of excess PhMe2SiH is crucial for the success of the metal carbene insertion reaction process with high ee. Further experimentation revealed that increasing catalyst loading afforded the silane reagents with higher ee up to 97% (Table 3). Bonge and Hansen developed a method for metal carbene insertion reaction of a-halo-a-diazoacetates into the Si–H bond (Scheme 16).37 They found that Si–H bond insertion reactions between a-halo-a-diazoacetates and Ph3SiH or PhMe2SiH give the desired products in isolated yields of 51% and 25%, respectively. In addition, the yield of the desired product of the reaction with diphenylsilane (Ph2SiH2) was measured to be 80% using internal standard in 1H NMR. X-ray structural analysis of homoleptic RhII complexes made of enantiopure (R)-1,1ʹ-binaphthyl and (R)-(5,5ʹ,6,6ʹ,7,7ʹ,8,8ʹ-octahydro)binaphthyl phosphate ligands were disclosed by Lacour.
38 He also reported the possibility to introduce halogen atoms at the 3,3ʹ-positions. In this work, the isolated dirhodium complex (Rh2BNP4) was tested as catalyst (1 mol%) in enantioselective cyclopropanations and Si−H bond insertion reaction, affording chiral cyclopropanes and silanes in good yields but moderate enantioselectivities (up to 63% ee) (Scheme 17).
Table 3 Optimization of RhII-catalyzed carbene insertion reaction into the Si−H bond
Entry Catalyst (mol%) Silane (equiv) Yield (%) ee (%)
1 1 1.5 65 48
2 1 5 70 88
3 2 5 68 93
4 3 5 72 95–97
Scheme 16 Si–H bond insertion reactions with ethyl a-bromo-a-diazoacetate
Scheme 17 RhII-catalyzed Si−H bond insertion reaction of methyl a-aryl-a-diazoacetates
The RhI-catalyzed enantioselective Si−H bond insertion reaction of a-diazoesters and a-diazophosphonates has been developed by Xu.39 He reported that the use of a C1-symmetric chiral diene ligand enables the asymmetric reaction to proceed under exceptionally mild conditions and gives versatile chiral a-silylesters and a-silylphosphonates with excellent enantioselectivities (up to 99% ee). The mechanism and stereochemical pathway of this novel RhI-carbene-directed Si−H bond insertion reaction was also investigated by deuterium kinetic isotope effect experiments and DFT calculations. In addition, ethyl a-aryl-a-diazoacetates bearing either an electron-donating or electron-withdrawing substituent on each aromatic carbon were tested in the reaction with PhMe2SiH. For all of these substrates, the reaction was complete within 6 h at room
temperature and gave the corresponding insertion reaction products with good to excellent yields and enantioselectivities (91−97% ee) (Scheme 18). Other silanes, such as substituted aryl silanes, Et3SiH, and tripropylsilane (nPr3SiH) were also successfully applied in this transformation, affording the desired a-silylesters in moderate to good yields with excellent enantioselectivities (95−97% ee) (Scheme 19).
Scheme 18 Asymmetric Si−H bond insertion reaction of a-diazoesters using chiral rhodium catalysis
Scheme 19 Asymmetric Si−H bond insertion reaction of a-diazoesters using various silane sources
The asymmetric insertion reaction of a-diazophosphonates into the Si−H bond has been explored by the same group.39 He demonstrated that the expected Si−H bond insertion reaction was achieved under slightly modified conditions using the prepared RhI/diene complex as catalyst. He showed that the reactions proceed well to give the corresponding Si−H insertion reaction products in moderate yields with excellent enantioselectivities (92−97% ee). Various aryl-substituted silanes were also reported to be suitable for utilization in the reaction to afford highly enantioenriched a-silylphosphonates. In the case of the sterically bulky dimethyl(1-naphthalenyl)silane, a particularly high enantioselectivity (99% ee) was obtained (Scheme 20).
Scheme 20 Asymmetric Si−H insertion reaction of a-diazophosphonates using chiral rhodium catalysis
2.1.1 Reaction mechanism for rhodium(II) catalysis The Si–H bond insertion reaction mechanism using diazo substrates can involve two steps as suggested by Landais in 1997 and Wang in 2001.27, 40 First, the diazo decomposition catalyzed by the dirhodium complex occurs (Figure 6), which involves the complexation of the negatively polarized carbon of the diazo substrate to the axial site of the RhII catalyst. Afterwards, the RhII-carbene is generated via an irreversible N2 elimination. It was shown that the kinetics of the diazo decomposition obeys the first order with respect to the diazo substrate concentration. Moreover, the decomposition and the carbene formation are both dependent on the electronic effects bore by the diazo compound. The carbon bearing the diazo group is negatively polarized as shown in the mechanism in Figure 6, however, after the first step, the same carbon will form the carbene attached to the RhII (positively charged).
Figure 6 Proposed mechanism for carbene formation
As a result, an electron-withdrawing group bore by the phenyl group of the diazo stabilizes the diazo compound but destabilizes the RhII-carbene intermediate. Thus, the diazo with electron-withdrawing substituents decomposes more slowly than those bearing electron-donating substituents. On the contrary, an electron-donating group gives the highest chemoselectivity. In conclusion, the fastest reaction is more selective because the RhII-carbene intermediate is more stable. In other words, the step of
generating the RhII-carbene intermediate is the rate-limiting step in the overall transformation. The second step of this mechanism is the addition of the silane. Here, various parameters play crucial roles in controlling the enantioselectivity. As mentioned above, parameters such as the type of silane substituents, the chiral ligands used, the solvent, and the temperature either increase or decrease the enantiomeric ratios. 2.2 Copper catalysis In 1974, Watanabe reported the reaction of various silanes including hydropolysilanes with methyl a-diazoacetate in the presence of copper as catalyst, giving a-silyl- and polysilylesters in 44–90% yields (Scheme 21).41 He demonstrated that the relative reactivities of a series of m- and p-substituted phenyldimethylsilanes towards the active species, generated from the a-diazoacetate, correlate well with the Hammett σ constants for the ring substituents, with a r value of –0.26 and a correlation coefficient of 0.984. Based on the consideration of the observed r value, it was concluded that the reaction involves the insertion of the free carbomethoxycarbene into the Si–H bond via partially ionic early transition state.
Scheme 21 Reaction of PhMe2SiH with methyl a-diazoacetate in the presence of copper
The use of Cu(acac)2 as an alternative to Rh2(OAc)4 was examined by Doyle.18 Cu(acac)2 promoted the reaction of a-diazoacetophenone with Et3SiH in 95% yield. However, the product of the reaction between a-diazoacetophenone and Et3SiH was a complex mixture relative to those obtained from the reactions performed using Rh2(OAc)4. In this case, the yield of the a-silylated ketone was low (< 60%) and the relative amount of the O-silylation product (12%) was comparable to that obtained with Rh2(OAc)4. Thus, Cu(acac)2, used in dichloromethane at reflux, appeared to be comparable to Rh2(OAc)4 regarding the effectiveness for the Si–H bond insertion reactions with the exception of the case with a-alkyl-substituted a-diazoketones. The insertion reaction of menthyl a-methyl-a-diazoesters into PhMe2SiH was next examined using three kinds of copper catalysts including Cu(acac)2, Cu(OTf)2, and Cu(OTf)2/sparteine (Scheme 22). Cu(acac)2 afforded a less stereoselective reaction than Rh2(OAc)4
and the desired a-silylester was obtained in only 50% yield and 32% diastereoselectivity. Reaction with Cu(OTf)2 alone resulted in even lower yields. However, it was found that a catalytic addition of optically active diamine sparteine with Cu(OTf)2 improved the yield up to 68% with the same range of diastereoselectivity. Surprisingly, the use of (+)-menthyl a-methyl-a-diazoester instead of the (–)-menthyl isomer increased the diastereoisomeric ratio. Landais described an easier route to synthesize a-(alkoxysilyl)acetic esters and their utilization.20, 22 As previously disclosed using Rh catalysis (Scheme 2), the synthetic pathway involved a two-step sequence carried out in one-pot: rhodium catalyzed Si–H bond insertion of a metal carbene, followed by a-alkylation of the esters. The key step of this work, however, was then carried out in the same manner as before, but in the presence of a copper catalyst prepared in situ from Cu(OTf)2 and an easily available Schiff-base (Scheme 23). Using this copper catalyst, he was able to obtain a-
silylesters in comparable yields to those reported using Rh2(OAc)4 (Scheme 2). The screening of various silanes finally demonstrated the higher catalytic activity of this copper catalyst compared to Cu(acac)2 and its similar activity to that of Rh2(OAc)4.
Scheme 22 Copper-catalyzed metal carbene insertion reaction of menthyl a-methyl-a-diazoesters into the Si–H bond
Scheme 23 Synthesis of a-silylaceticesters catalyzed by a copper Schiff-base complex
An asymmetric insertion reaction of methyl a-aryl-a-diazoacetates into the Si–H bond using a CuI catalyst associated with a chiral symmetric Schiff-base was developed by Jacobsen and Panek in 1998.42 In this study, the reaction of methyl a-phenyl-a-diazoacetate with PhMe2SiH in the presence of 10 mol% [Cu(OTf)]2·C6H6 and 12 mol% of a chiral Schiff-base diimine ligand afforded the insertion reaction product with moderate to good levels of enantioselectivities (Table 4). Also, a strong dependence of ee on temperature was observed, with up to 83% ee obtained in reactions carried out at –40 ºC. Both [Cu(OTf)]2·C6H6 and [(CH3CN)4Cu]PF6 proved to be equally effective as copper sources, whereas Cu(OTf)2 afforded significantly lower levels of enantioselectivity. In the next part of this study, a series of chelating ligands were screened in the reaction of methyl a-phenyl-a-diazoacetate with PhMe2SiH. As summarized in Table 4, several aryl substituted diimine ligands were also evaluated, with variations of the steric and/or electronic properties of the ligand. Among all of the used ligands, 2,6-dichlorobenzaldehyde ligand proved to be the most enantioselective for silane insertion. A survey done on other silane sources was conducted in the aim of optimizing the ee's of the insertion reaction products. Moreover, reaction of methyl a-phenyl-a-diazoacetate with the five silanes listed in Table 4 revealed a strong dependence of the enantioselectivity and the reaction rate on the silane structure itself.
Panek and Stavropoulos continued their previous studies on the copper-catalyzed Si–H bond insertion reactions with diimine chiral ligands.43 They used the same a-diazocarbonyl compound, hence permitting a direct comparison of the results obtained with the rhodium catalysis (Scheme 24). To completely understand the electronic properties of the transition state, a series of silane substrates were also evaluated in dichloromethane at 0 ºC and –40 ºC. No dependence on the enantioselectivity was found when using various silane sources. Based on the obtained results, they reported that levels of enantioselectivity are neither affected by the steric nor the electronic properties of the metal carbene intermediate, nor that of the silane substrates. Silane compounds were found to
only affect the stabilization of the positive charge developed on both the metal carbene carbon and the silicon atoms. This stabilization enhances the reactivity, however, does not influence the insertion selectivity. Here, once again the early transition state hypothesis seems to efficiently accommodate the insensitivity of enantiomeric ratio values. In addition, they proposed that the good levels of observed enantioselectivity are most likely due to the characteristics of the copper carbene cavity, as dictated by the specific interactions between the ligand moieties and the carbene.
Table 4 Asymmetric copper-catalyzed diazo insertion reaction into the Si–H bond
aRecrystallized yields and ee's are in parentheses. bReaction was run at –10 °C.
Scheme 24 Asymmetric copper-catalyzed diazo insertion reaction into the Si–H bond
Panek in his former study reported useful high values of selectivity for a CuI-promoted insertion reaction with methyl a-phenyl-a-diazoacetate utilizing C2-symmetric CuI bis-imine complexes.42, 43 A subsequent extension of the CuI catalyzed reaction with a-vinyl-a-diazoacetates was then investigated.33, 36 In fact, his evaluation of CuI C2-symmetric Schiff-base complexes demonstrated that they are viable options for promoting metal carbene insertion reactions into the Si−H bond thus capable of generating crotyl silanes. As a result, the treatment of a-vinyl-a-diazoacetates with chiral [(CH3CN)4Cu]PF6·(R,R)-diimine CuI complex, in the presence of
N2
+ PhMe2Si–HCopper catalyst (10 mol%)
CH2Cl2, rt
R = (–)-menthyl, Cu(acac)2: 50%, 32% deR = (–)-menthyl, Cu(OTf)2: 33%, 12% deR = (–)-menthyl, Cu(OTf)2/sparteine: 68%, 10% deR = (+)-menthyl, Cu(OTf)2/sparteine: 62%, 36% de
PhMe2SiH at 0 ºC, afforded the chiral allylic silane in a moderate yield (55%) and low enantioselectivity (12% ee). A significant selectivity dependence on diverse parameters, such as solvent and temperature, was observed. It was found that 5 mol% of [(CH3CN)4Cu]PF6·(R,R)-diimine with benzene as the solvent results in the insertion product with high selectivities up to 72% ee (Scheme 25). In addition, reactions conducted at 0 ºC exhibited higher selectivities than those carried out at room temperature.
Scheme 25 Copper-catalyzed asymmetric Si–H bond insertion reaction of methyl a-allyl-a-diazoacetate with PhMe2SiH
Zhou developed a highly efficient copper-catalyzed asymmetric carbene insertion reaction into the Si–H bond.44 Using chiral spiro-diimine ligands, he obtained a wide range of a-silylesters with excellent yields and enantioselectivities. He showed that the chiral spiro-diimine ligand exhibited much higher activity and enantioselectivity than the spiro-bis-oxazoline ligand. In this study, the insertion reaction of methyl a-phenyl-a-diazoacetate with PhMe2SiH was completed in 1 h at 0 ºC using the copper catalyst and the Si–H bond insertion reaction product was obtained in 95% yield with 93% ee (Scheme 26). Further improvement of the enantioselectivity (98% ee) for Si–H bond insertion reaction was also achieved by using Cu(OTf)2 instead of CuCl and lowering the reaction temperature to –60 ºC. Moreover, a variety of a-aryl-a-diazoacetates were utilized to conduct the Si–H bond insertion reaction under the optimal conditions resulting in high yields (85–95%) with excellent enantioselectivities (90–99% ee) (Scheme 27).
He also showed that the steric and electronic properties of substituents on the phenyl ring of the a-diazoesters slightly affected the enantioselectivity of the Si–H bond insertion reaction. He found that Et3SiH, nPr3SiH, and diphenylmethylsilane (Ph2MeSiH) are also suitable Si–H bond donors in addition to PhMe2SiH. Recently, Gouverneur developed a catalytic insertion reaction of 1-aryl 2,2,2-trifluoro-1-diazoalkanes into Si–H, B–H, P–H, S–H, and N–H bonds.45 These transformations enabled the synthesis of a large collection of novel and valuable chiral CF3-substituted molecules. She also found that enantioselective Si–H and B–H bonds insertion reactions catalyzed by CuI complexes derived from chiral bis-oxazoline ligands proceed in high yields with good to excellent enantioselectivities. In addition, she showed that the optimal conditions for the diazo insertion reaction into the Si–H bond consist of treating PhMe2SiH with an excess of the diazo reagent in dichloromethane at room temperature in the presence of [(CH3CN)4Cu]PF6 (Scheme 28). Under these conditions, the product of the Si–H bond insertion reaction was isolated in 88% yield. The Si–H bond insertion reaction of the more sterically demanding Ph3SiH was also done successfully, leading to the desired product in 77% yield (Scheme 28). In the same study, the reactivity of the model 1-aryl 2,2,2-trifluoro-1-diazoethanes was also probed in a range of heteroatom–hydrogen insertion reactions. For this class of diazoethanes, the reaction was performed with an excess of the nucleophilic component as well as an increased catalyst loading of 4 mol%. The reaction scope was broad, and a large collection of novel
chiral products were synthesized with a CF3 substituent located at the benzylic position. Similar to the reactions with 2,2,2-trifluoro-1-diazoethane, Si–H bond insertion reactions were feasible and the corresponding products were isolated in 61–98% yield (Scheme 29).
Scheme 26 Copper-catalyzed asymmetric Si–H bond insertion reaction of methyl a-phenyl-a-diazoacetate with PhMe2SiH
Scheme 27 Copper-catalyzed asymmetric Si–H bond insertion reaction of a-aryl-a-diazoacetate
Scheme 28 Copper-catalyzed Si–H bond insertion reaction
Moreover, the same group developed an enantioselective insertion reaction of diazo-CF3 into the Si–H bond using catalytic amounts of [(CH3CN)4Cu]PF6 and the large noncoordinating ion tetrakis[3,5 bis(trifluoromethyl)-phenyl]borate (NaBArF). She identified that Zhou’s chiral bis-oxazoline ligand, based on the spirobiindane scaffold, results in an optimal enantioselectivity. In the presence of this ligand, Si–H bond insertion reactions with PhMe2SiH and Et3SiH afforded enantiomerically enriched desired products, which were isolated in 99% and 80% yields, respectively, with 98% ee (Scheme 30).
Scheme 29 Copper-catalyzed 2,2,2-trifluoro-1-diazoethane insertion reaction into the Si–H bond
Scheme 30 Copper-catalyzed enantioselective Si–H bond insertion reaction
More recently, our group reported an efficient copper-catalyzed carbene insertion reaction of a-diazo carbonyl compounds into Si–H and S–H bonds.46 In this work, a wide range of a-silylesters was obtained in high yields (up to 98%) from a-diazoesters using 5 mol% of a simple CuI salt as catalyst (Scheme 31). We revealed that the reactivity of the substrate was influenced by the electronic properties of substituents on the phenyl ring of the methyl a-aryl-a-diazoacetates. Also, a-aryl-a-diazoacetates containing electron-donating groups such as methyl and methoxy need shorter reaction times to reach complete conversion and high yields. However, a-aryl-a-diazoacetates containing electron-withdrawing groups, such as Br, F, Cl, require an increased reaction time to reach completion.
Scheme 31 Copper-catalyzed Si–H bond insertion reaction of a-aryl-a-diazoacetate
We also studied the competition reaction of the intramolecular cyclopropanation reaction of methyl a-phenyl-a-diazoacetate using [(CH3CN)4Cu]PF6 as catalyst. It was shown that the cyclopropane was formed in 59% yield when the reaction was performed at 25 ºC without a silane (Scheme 32). In addition, the reaction of Et3SiH using CuI catalysis at –10 ºC and 25 ºC exclusively afforded the product of Si–H bond insertion reaction in almost quantitative yields. These competition experiments affirm the preference of Si–
H bond insertion reaction over intramolecular cyclopropanation. We have also found that the reaction of methyl a-phenyl-a-diazoacetate with Et3SiH, thiophenol, and styrene under CuI
catalysis afforded products of Si–H/S–H bond insertion reactions and cyclopropanation (Scheme 33). This competition experiment showed that the product of Si–H bond insertion reaction was formed in better yields vs S–H insertion reaction and cyclopropanation using a CuI catalyst at –10 ºC. The selectivity of the reaction was only moderate and running the reaction at a lower temperature (at –20 ºC) did not improve it. However, a-diazoketones led to a-silylketones in low to good yields (up to 70%) using 0.05 mol% of the same catalyst (Scheme 34). Moreover, it was acknowledged that the obtained a-silylesters can be readily used and transformed into other useful products as shown in Scheme 35, eq. a. Synthesis of a-allylsilane was also performed starting from a-hydroxysilane. Davies' procedure to synthesize various a-allylsilanes, originally taking advantage in the last step of a Rh2(OAc)4-catalyzed C–H insertion reaction through a b-lactone intermediate,29 was successfully improved by using the same copper catalyst employed for the Si–H bond insertion reaction (Scheme 35, eq. b).
Scheme 32 Copper-catalyzed Si–H bond insertion reaction vs cyclopropanation
Scheme 33 Competition experiments: copper-catalyzed Si–H bond insertion reaction vs S–H insertion reaction and copper-catalyzed Si–H bond insertion reaction vs cyclopropanation
Scheme 34 Copper-catalyzed Si–H bond insertion reaction of a-diazoketones
Caselli and Vicente have demonstrated the ability of the catalytic system comprising [Cu(OTf)]2·C6H6 and pyridine-containing ligands for the insertion reaction of a-diazo carbonyl compounds into the Si–H bond (Scheme 36).47 In this work, a wide range of a-silylesters was obtained in moderate to very good yields (up to 96%) but with poor enantioselectivities (up to 46%). They have also mentioned that the catalytic system is quite robust and a remarkable TON of 30000 can be achieved. 2.3 Iridium catalysis Katsuki developed a highly enantioselective Si–H bond metal carbene insertion reaction into trisubstituted silanes in the presence of an appropriate iridium complex as catalyst using a-alkyl-a-diazoacetates (> 97% ee) or a-aryl-a-diazoacetates (> 99% ee) (Scheme 37).48 Also, Si–H bond insertion reactions into a prochiral disubstituted silane provided a new direct method for forming a stereogenic silicon center. This was for the first time achieved and was reported with high enantioselectivities (> 99% ee) (Scheme 38).
Scheme 36 Copper-catalyzed Si–H bond insertion reaction
a3 equiv of Et3SiH were used.
Scheme 37 Asymmetric Si–H bond insertion reaction with methyl a-aryl-a-diazoacetates
Scheme 38 Asymmetric iridium-catalyzed Si–H bond insertion reaction
In 2012, Che investigated the Si–H bond insertion reaction catalyzed by chiral iridium porphyrin.49 As shown in Scheme 39, D4 symmetrical chiral iridium porphyrin catalyzed carbene Si–H bond insertion reaction with various diazo compounds even at –80 ºC, to give products in high yields and good to high enantioselectivities. Among the a-diazo carbonyl compounds examined, methyl p-bromophenyl-a-diazoacetate appeared to be the most effective substrate giving products in 93% yield and 91% ee (Scheme 39).
Scheme 39 Si–H bond insertion reaction with methyl a-aryl-a-diazoacetate catalyzed by chiral iridium porphyrin
The catalytic functionalization of Si–H bond by means of the insertion reaction of carbene units was demonstrated by Caballero and Pérez in 2013.50 Decomposition of ethyl a-diazoacetate was achieved using a silver-based catalyst which was the first reported example of implementing this metal to promote this kind of transformation. In this study, competition experiments revealed that the relative reactivity of substituted silanes depends on the dissociation energy of the Si–H bond (tertiary > secondary > primary for ethyl substituted silanes). However, in the presence of bulky substituents, such an order reverts to secondary > primary ≈ tertiary (for phenyl substituted silanes). In addition, screening with other diazo compounds showed that N2C(Ph)CO2Et displays similar reactivity to that of ethyl a-diazoacetate, whereas other N2C(R)CO2Et (R = Me, CO2Et) results in lower conversions.
Recently, Bi developed metal carbene insertion reaction into Si–H, Sn–H, and Ge–H bonds using AgOTf as catalyst.51 In this study, AgI was shown to be an efficient catalyst for N-nosylhydrazone insertion reactions into the Si–H bond using various types of silane sources (Scheme 40).
Scheme 40 AgI- catalyzed carbene insertion reaction into the Si–H bond
2.5 Ruthenium catalysis The first example of an enantioselective insertion reaction of a-methyl-substituted a-diazoesters into the Si–H bond using Ru as catalyst was reported by Chanthamath and Iwasa.52 Using chiral RuII-pheox catalyst, they produced a wide range of a-silylesters with high yields and moderate to good enantioselectivities (Scheme 41). They also showed that other silane sources exhibit much higher activities and enantioselectivities than PhMe2SiH (Scheme 42). They established that the desired a-silylester was easily converted to the corresponding alcohol without any loss of enantioselectivity.
Scheme 42 RuII-pheox catalyzed enantioselective Si–H bond insertion reaction
2.6 Iron catalysis Our group developed an efficient iron-catalyzed metal carbene insertion reaction of a-diazo carbonyl compounds into Si–H and S–H bonds.46 A wide range of a-silylesters and a-thioesters was obtained in high yields (up to 99%) from a-diazoesters using a simple iron(II) salt as catalyst. Using the same catalytic system, a-diazoketones led to a-thioketones in moderate yields (up to 85%).53
2.7 Biocatalysis An interesting work has been reported by Arnold in 2016 for diazo insertion reaction into the Si–H bond using a biocatalytic system.54,
55 She has reported that iron haem proteins catalyze the formation of organosilicon compounds under physiological conditions via a carbene insertion reaction into the Si–H bond. In this study, the reaction proceeded in both vitro and vivo, accommodating a broad range of substrates with high chemo and enantioselectivities. She has also enhanced the catalytic function of cytochrome c from Rhodothermus marinus to achieve more than 15-fold higher total turnover numbers (TTN) than state-of-the-art synthetic catalysts. This biocatalyst was able to catalyse the reaction of a wide range of a-diazoesters into the a-silylesters in excellent yields and enantioselectivities (Scheme 43).
Scheme 43 Enantioselective Si–H bond insertion reaction using bio catalyst
3 Conclusions Various methods using diverse catalysts have been discovered, in a mutual objective to enhance the selectivity of insertion reactions into the Si–H bond. Diazo compounds, which are commonly used as carbene precursors, have been extensively employed as versatile cross-coupling partners in this transformation. Catalytic insertion reaction of diazo compounds into the Si–H bond is a powerful organic transformation, due to the highly synthetic potential of the generated building blocks. Carbon–silicon bond-forming methods that introduce silicon motifs into organic molecules rely on multistep synthetic routes to prepare and fine-tune the catalysts, using metals such as rhodium, copper, iridium, silver, ruthenium, and iron. Efficient catalytic Si–H bond insertion reactions are definitively of high potential and demonstrate future challenges which need to be addressed in this field.
Acknowledgements
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