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Research paper Nickel-catalyzed reduction of ketones with water and triethylsilane Nahury Castellanos-Blanco, Marcos Flores-Alamo, Juventino J. García Facultad de Química, Universidad Nacional Autónoma de México, Circuito Interior, Ciudad Universitaria, Mexico City 04510, Mexico article info Article history: Received 13 May 2017 Received in revised form 9 June 2017 Accepted 10 June 2017 Available online 19 June 2017 Keywords: Ketones Water Triethylsilane Nickel Phosphine Transfer hydrogenation abstract The acetophenone (1a) reduction using catalytically active nickel complexes and water is an efficient and sustainable method to access a new methodology of transfer hydrogenation of ketones. When triethylsi- lane (Et 3 SiH) was used as sacrificial agent to promote the transfer hydrogenation from water, 1-pheny- lethanol (2a) was obtained in excellent yield along with silanol (Et 3 SiOH) as the reaction’s driving force. Deuterium labeling studies were made using Et 3 SiD or D 2 O and these studies showed that both compounds participate as hydride sources for the ketone reduction. A scope of substrates was assessed, including a variety of mono/diketones, and a,b-unsaturated ketones, to yield the corresponding sec- ondary alcohols and saturated ketones. Additionally, asymmetric transfer hydrogenation of mono- ketones was studied for the mixture of nickel/(bisphosphine or phospholane) as catalyst precursor, using H 2 O/Et 3 SiO system and ethanol as hydrogen sources. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Transfer hydrogenation (TH) of ketones has attracted great attention in recent years due to its industrial application in the production of a wide range of alcohols and chiral compounds, which are useful in pharmaceuticals and fine chemicals prepara- tion [1–6]. Compared to the conventional hydrogenation, which uses highly flammable molecular hydrogen, TH is a safer and atom-efficient process, in which the reaction solvent (typically alcohols) is the source of hydrogen. TH of ketones is typically made using isopropanol [7–13] and formate [14–17] as hydrogen donors and the involved catalysts are regularly based on ruthenium [18– 22], rhodium [23–28] and iridium [29–33]. Particularly, TH reac- tions involving water as hydrogen source are uncommon, due to low solubility of substrates or instability of ancillary ligands in aqueous media [34–36]. Regarding this, only one methodology has been reported for the hydrogenation reaction of unsaturated ketones using magnesium/water system as hydrogen source, under catalytic palladium on activated carbon (Pd/C) [37]. As a result, the reaction of H 2 O and Mg forms Mg(OH) 2 to generate hydrogen, used in the hydrogenation of 4-phenyl-3-butene-2-one to obtain 4-phe- nyl-2-butanone in good yield (93%). However, to the best of our knowledge, there are no reports about the use of transition metals in homogeneous catalysts, that use water, from which the transfer of hydrogen into ketones can be made. Closely related examples of metal-mediated hydrogen transfer to unsaturated substrates such as alkenes or alkynes using water and transition metals (Ni [38], Pd [39–41] and Rh [42]) have been documented and require a sacrificial agent (S) favoring the hydro- gen release by formation of (S@O). In our search for efficient catalysts for TH processes, we recently reported the reactivity of the complex [(dippe)Ni(g 2 -C,C-alkyne)] (dippe: 1,2-bis(di-isopropyl-phosphinoethane), in the presence of water as hydrogen source [38]. The results showed the selective production of the appropriate cis- or trans-alkene from alkynes, along with Ni(OH) 2 and diphosphine monoxide (dippeO) that cor- respond to the final fate of oxygen in this reaction. Consequently, the use of a silane as a sacrificial agent (i.e. triethylsilane) allowed the selective preparation of cis-stilbene in 98% yield at 180 °C, within 48 h by formation of silanol or triethyldisiloxane. Nevertheless, in the case of ketones, the search for improved systems that may drive the selective reduction using simple mole- cules as hydrogen sources as well as the inexpensive metal-based catalysts still represents a major challenge. Therefore, we report herein the reactivity of [Ni(COD) 2 ] (COD = 1,5-cyclooctadiene) and the auxiliary ligand dippe using a variety of ketones in the presence of (a) water and (b) water/tri- ethylsilane as hydrogen donors. The results show the selective and efficient production of secondary alcohols from the corre- sponding ketones and saturated ketones from dienones; overall the process exhibits a strong dependence on the stoichiometric amount of water and silane. http://dx.doi.org/10.1016/j.ica.2017.06.035 0020-1693/Ó 2017 Elsevier B.V. All rights reserved. Corresponding author. E-mail address: [email protected] (J.J. García). Inorganica Chimica Acta 466 (2017) 324–332 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
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  • Inorganica Chimica Acta 466 (2017) 324–332

    Contents lists available at ScienceDirect

    Inorganica Chimica Acta

    journal homepage: www.elsevier .com/locate / ica

    Research paper

    Nickel-catalyzed reduction of ketones with water and triethylsilane

    http://dx.doi.org/10.1016/j.ica.2017.06.0350020-1693/� 2017 Elsevier B.V. All rights reserved.

    ⇑ Corresponding author.E-mail address: [email protected] (J.J. García).

    Nahury Castellanos-Blanco, Marcos Flores-Alamo, Juventino J. García ⇑Facultad de Química, Universidad Nacional Autónoma de México, Circuito Interior, Ciudad Universitaria, Mexico City 04510, Mexico

    a r t i c l e i n f o a b s t r a c t

    Article history:Received 13 May 2017Received in revised form 9 June 2017Accepted 10 June 2017Available online 19 June 2017

    Keywords:KetonesWaterTriethylsilaneNickelPhosphineTransfer hydrogenation

    The acetophenone (1a) reduction using catalytically active nickel complexes and water is an efficient andsustainable method to access a new methodology of transfer hydrogenation of ketones. When triethylsi-lane (Et3SiH) was used as sacrificial agent to promote the transfer hydrogenation from water, 1-pheny-lethanol (2a) was obtained in excellent yield along with silanol (Et3SiOH) as the reaction’s drivingforce. Deuterium labeling studies were made using Et3SiD or D2O and these studies showed that bothcompounds participate as hydride sources for the ketone reduction. A scope of substrates was assessed,including a variety of mono/diketones, and a,b-unsaturated ketones, to yield the corresponding sec-ondary alcohols and saturated ketones. Additionally, asymmetric transfer hydrogenation of mono-ketones was studied for the mixture of nickel/(bisphosphine or phospholane) as catalyst precursor, usingH2O/Et3SiO system and ethanol as hydrogen sources.

    � 2017 Elsevier B.V. All rights reserved.

    1. Introduction

    Transfer hydrogenation (TH) of ketones has attracted greatattention in recent years due to its industrial application in theproduction of a wide range of alcohols and chiral compounds,which are useful in pharmaceuticals and fine chemicals prepara-tion [1–6]. Compared to the conventional hydrogenation, whichuses highly flammable molecular hydrogen, TH is a safer andatom-efficient process, in which the reaction solvent (typicallyalcohols) is the source of hydrogen. TH of ketones is typically madeusing isopropanol [7–13] and formate [14–17] as hydrogen donorsand the involved catalysts are regularly based on ruthenium [18–22], rhodium [23–28] and iridium [29–33]. Particularly, TH reac-tions involving water as hydrogen source are uncommon, due tolow solubility of substrates or instability of ancillary ligands inaqueous media [34–36]. Regarding this, only one methodologyhas been reported for the hydrogenation reaction of unsaturatedketones using magnesium/water system as hydrogen source, undercatalytic palladium on activated carbon (Pd/C) [37]. As a result, thereaction of H2O and Mg forms Mg(OH)2 to generate hydrogen, usedin the hydrogenation of 4-phenyl-3-butene-2-one to obtain 4-phe-nyl-2-butanone in good yield (93%). However, to the best of ourknowledge, there are no reports about the use of transition metalsin homogeneous catalysts, that use water, from which the transferof hydrogen into ketones can be made.

    Closely related examples of metal-mediated hydrogen transferto unsaturated substrates such as alkenes or alkynes using waterand transition metals (Ni [38], Pd [39–41] and Rh [42]) have beendocumented and require a sacrificial agent (S) favoring the hydro-gen release by formation of (S@O).

    In our search for efficient catalysts for TH processes, we recentlyreported the reactivity of the complex [(dippe)Ni(g2-C,C-alkyne)](dippe: 1,2-bis(di-isopropyl-phosphinoethane), in the presence ofwater as hydrogen source [38]. The results showed the selectiveproduction of the appropriate cis- or trans-alkene from alkynes,along with Ni(OH)2 and diphosphine monoxide (dippeO) that cor-respond to the final fate of oxygen in this reaction. Consequently,the use of a silane as a sacrificial agent (i.e. triethylsilane) allowedthe selective preparation of cis-stilbene in 98% yield at 180 �C,within 48 h by formation of silanol or triethyldisiloxane.

    Nevertheless, in the case of ketones, the search for improvedsystems that may drive the selective reduction using simple mole-cules as hydrogen sources as well as the inexpensive metal-basedcatalysts still represents a major challenge.

    Therefore, we report herein the reactivity of [Ni(COD)2](COD = 1,5-cyclooctadiene) and the auxiliary ligand dippe using avariety of ketones in the presence of (a) water and (b) water/tri-ethylsilane as hydrogen donors. The results show the selectiveand efficient production of secondary alcohols from the corre-sponding ketones and saturated ketones from dienones; overallthe process exhibits a strong dependence on the stoichiometricamount of water and silane.

    http://crossmark.crossref.org/dialog/?doi=10.1016/j.ica.2017.06.035&domain=pdfhttp://dx.doi.org/10.1016/j.ica.2017.06.035mailto:[email protected]://dx.doi.org/10.1016/j.ica.2017.06.035http://www.sciencedirect.com/science/journal/00201693http://www.elsevier.com/locate/ica

  • N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332 325

    2. Results and discussion

    2.1. Catalytic hydrogenation of acetophenone using water as hydrogendonor

    We first examinated the reduction of acetophenone (1a) undersimilar conditions as studied before for the reduction of ketones,but using water instead of ethanol as hydrogen source [43].

    Thus, treatment of (1a) with water (300 mmol), a variety of sol-vents (entries 1–5, Table 1) and a catalytic amount of [Ni(COD)2](2 mol%) and dippe (4 mol%) at 130 �C for 48 h, exhibited an impor-tant dependence on the polarity of solvent as can be seen in Table 1.When the transfer hydrogenation was initially carried out usingH2O (1 mL) as hydrogen source and solvent, in the presence of1.1 equiv of acetophenone 1a, the conversion was low (entry 1,Table 1), but this increased with the use of THF, resulting in forma-tion of 15% of 1-phenylethanol (entry 3). The reaction performed innonpolar solvents such as toluene or hexane resulted in poor ornull conversions (entries 4 and 5). The immiscibility of water inthese solvents was avoided by using THF as solvent and increasingthe temperature for the process, which resulted in an improvementof yield (15%). However, the system gave low yields since dippeproduced diphosphine monoxide, (iPr)2PCH2CH2P(O)(iPr)2 (dip-peO) as the final fate of oxygen; therefore, we decided to improvethe yield of 1-phenylethanol (2a) with the use of Et3SiH (seeTable 2) [38–41].

    2.2. Catalytic hydrogenation of acetophenone using Et3SiH/H2O

    The chemoselective reduction of 1a to produce 2a was possibleon using stoichiometric amounts of Et3SiH (1.2 equiv), in a goodyield (52%, Table 2, entry 2) compared with the reaction in theabsence of silane (15%, entry 1). The catalytic system performedvery modest using water as solvent and hydrogen donor (entry4), but in absence of water (entry 3) the yield of the hydrogenationproduct 2a was null. Since the transfer hydrogenation of 1a gavegood yields with the use of stoichiometric amounts of Et3SiH andwater; therefore, we optimized the amount of Et3SiH (equiv) andH2O (mmol) necessary to produce 2a (Fig. 1).

    According to the results in Fig. 1(a), the TH reaction appears toproceed most efficiently if an excess of Et3SiH is used to achievefull conversion of acetophenone to produce 2a (99%) and the useof less than 2.2 equiv gave poor to good yields of 2a (52–89%). Inaddition, the use of 2.2 equiv. of Et3SiH and increasing the amountof water (50–600 mmol), reached a maximum on using 300 mmol(Fig. 1(b)). Both time and temperature for this process were opti-mized (130 �C, 2 d) see SI (Figs. S1-S2). The improved yields of 2a

    Table 1Nickel-catalyzed transfer hydrogenation of acetophenone with water.a

    Entry Solvent 2a (%)b

    1 Water 62 Ethanol 93 THF 154 Toluene 25 Hexane –

    a Reaction conditions: 1.1 mmol of substrate, H2O (300 mmol), and 1 mL of sol-vent at 130 �C for 48 h.

    b Yields were determined by GC–MS, 2a used as internal standard.

    using water and silane demonstrated that Et3SiH participates assacrificial agent and hydride source [38,44–49]. Isotopic labelingexperiments regarding hydride insertion will be presented ahead(vide infra).

    The use of an alternative silane such as Ph3SiH decreased theyield (Table 3, entries 3 and 4), probably due to the increased reac-tivity and instability of phenylsilane in water [50–53]. Thus, thecombination of Et3SiH (2.2 equiv) and water (300 mmol), with[Ni(COD)2] (2 mol%) and dippe (4 mol%), gave the best catalyticsystem at 130 �C for 48 h (Table 3, entry 2) see SI, Figs. S4–S5.

    We investigated further on the mechanism of hydride insertionusing deuterium oxide (D2O) and Et3SiH, considering the mecha-nistic proposal depicted in Scheme 1. The reaction of [Ni(COD)2](a) with dippe (b) and substrate 1a at room temperature, THF-d8,for 24 h gives a mixture of compounds [Ni(dippe)2] as a singletat 53.8 ppm [43,54–56] (60%) in 31P{1H} NMR and two small dou-blets between 71 and 65 ppm (40%) with 2JP-P = 70.36 Hz, charac-teristic of similar nickel(0) complexes with diphosphine ligandsin agreement with g2-C@O coordination of ketones proposed forcomplex [(dippe)Ni(g2-C,O-acetophenone)] (d) (see SI, Fig. S10)[54–56].

    The reaction of complexes (c) and (d) with D2O and Et3SiH atroom temperature resulted in the formation of a small singlet at3.7 ppm in the 1H NMR spectra, assigned to OH in 1-phenylethanol(Fig. 2); this signal increased at high temperatures (70 �C, Fig. 2). Byusing this reaction mixture monodeuterated 1-phenylethanol 2b(C8H8DOH) was obtained at 100 �C for 15 h in THF-d8. On quench-ing the reaction to the open atmosphere the mass of monodeuter-ated product was confirmed (Fig. S7). The expected by-productswere also confirmed as the deutero-silanol (Fig. S6), diphosphinemonoxide (dippeO) and diphosphine dioxide (dippeO2) (SI,Fig. S10).

    We studied next the use of both systems Et3SiH/D2O or Et3SiD/H2O with catalytic loads of [Ni(COD)2] and dippe (Table 4), theresults confirm that only one deuterium atom was incorporatedin each case (m/z = 123, entries 2–3), to produce 1-phenylethanolin good yields (82% and 89% respectively) compared to the com-plete protio-system Et3SiH/H2O (yield 100%), with a m/z = 122(Table 4, entry 1). The fragmentation pattern of each productallowed to establish the position where deuterium (D) was incor-porated (see SI, Figs. S7–S8); thus, D from D2O was incorporatedinto the quaternary carbon in the carbonyl group of 1a, and D fromEt3SiD was incorporated at the oxygen of the same group asdepicted in Scheme 1.

    To extend the scope of this methodology, TH was found to beapplicable to a variety of aromatic and aliphatic ketones (Table 5).As found, substitution on the benzene rings of the acetophenonewith only one fluorine did not inhibit the reaction (80%, entry 2),but the poly-fluorobenzenic ring (1-(perfluorophenyl)ethanone)severely decreased the yield (entry 3), perhaps due to an increasedp-acceptor capacity at the C@O moiety due to the presence of flu-orine substituents in the aromatic ring, making more difficult theoxidative addition of water, vide infra (step D to E Scheme 2). Agood yield was achieved for benzophenone (85%, entry 5) and inthe case of a bulky aliphatic ketone such as adamantan-2-one, anexcellent yield (94%) was obtained.

    A mechanistic proposal considering the experimental observa-tions above discussed is presented in Scheme 2 (vide supra); thus,seems to be likely that transfer hydrogenation proceeds via addi-tion of H2O to form E. Then a hydride addition into the carbonylmoiety produces F, followed by the addition of Et3SiH to releaseEt3SiOH along with the addition of a hydride to yield G (see SI,Fig. S8). The formation of a strong Si–O bond provides the drivingforce behind the reaction. The resulting nickel-hydride complexundergoes a reductive elimination of the secondary alcohol 2 andfurther coordination with ketone 1 closes the cycle.

  • Table 3Use of silanes.a

    Entry Silane (equiv) 2a (%)b

    1 Et3SiH 1.2 522 Et3SiH 2.2 993 PhSiH3 1.2 134 PhSiH3 2.2 22

    a Unless otherwise noted, the reactions were conducted using ketone 1a (1.1 mmol), water (300 mmol), [Ni(COD)2] (0.002 mmol), dippe (0.004 mmol) and THF (3 mL).b Yields were determined by GC–MS, 2a used as internal standard.

    Table 2Reduction of acetophenone using the Et3SiH-H2O system.a

    Entry Solvent Silane (equiv) H2O (mmol) 2a (%) b

    1 THF – 300 152 THF 1.2 300 523 THF 1.2 – –4 H2O 1.2 300 39

    a Unless otherwise noted, the reactions were made using ketone 1a (1.1 mmol), water (300 mmol), [Ni(COD)2] (0.002 mmol), dippe (0.004 mmol) and THF (3 mL).b Yields were determined by GC–MS, 2a used as internal standard.

    Fig. 1. (a) Effect of silane (equiv) on the TH reaction using 1a (1.1 mmol), H2O (300 mmol), THF (3 mL), 130 �C, and 2 d. (b) Effect of water (mmol) on the TH using 1a(1.1 mmol), Et3SiH (2.2 mmol), THF (3 mL), 130 �C and 2 d.

    326 N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332

    Next, we assessed the chemo-selectivity using closely relatedsubstrates (benzyl and a,b-unsaturated ketones, Tables 6 and 7).The results for benzyl (3a) (Table 6) gave low yield (20%) for thereduction product 2-hydroxy-1,2-diphenylethanone (3b) usingthe previously established catalytic conditions (entry 1), conse-quently we investigated the effect of auxiliary ligand, time andtemperature. A good yield and selectivity for 3b was obtained onincreasing the temperature to 180 �C, 48 h (77%, entry 4), a similaryield was achieved with dcype at 150 �C (66%, entry 3).

    Then, we explored the reactivity of a variety of a,b-unsaturatedketones (4a-4f, Table 7). Therefore, [Ni(COD)]2, diphosphine dippeand the mixture Et3SiH/H2O was tested in the reduction of chal-

    cone 4a. When the reaction was performed at 130 �C, 72 h, thereduced product 5a (91%) was obtained as amain product (Table 7).As expected, the use of a CH3 substituent instead of a Ph group inthe R2 position of the enone (4b, Table 7) gave a lower yield (5b,64%) due the increased electron density at the conjugated carbonylgroup. When the reaction was performed with other symmetricdienones, such as 1,5-diphenyl-1,4-pentadien-3-ones (4c-4e), thecorresponding saturated ketone was obtained preferably withboth, dba (dibenzylideneacetone, 4c) and the analogous structureusing CF3 in para position on the aromatic ring (67%-5c and 44%-5e). It was found that dienone 4d with electron-donating sub-stituent OCH3, was selective to yield the conjugate product 6d in

  • (2b, OH)

    (1a, CH3)

    (1a, Ph)

    (2b, Ph)

    0 h, 25 ºC

    15 h, 50 ºC

    8 h, 70 ºC

    15 h, 100 ºC

    (2b, CH3)

    THF-d8 THF-d8

    Fig. 2. 1H NMR spectra of reaction between 1b and D2O-Et3SiH (THF-d8, 300 MHz).

    Scheme 1. Preparation of monodeuterated 1-phenylethanol.

    Table 4Catalytic hydrogenation of 1a using deuterated systems.a

    Entry Deuterated System 1-phenylethanol (%)b m/z

    1 Et3SiH-H2O 100 1222 Et3SiH-D2O 89 1233 Et3SiD-H2O 82 123

    a Unless otherwise noted, the reactions were conducted using ketone 1a (1.1 mmol), water (300 mmol), silane (2.2 equiv), [Ni(COD)2] (0.002 mmol), dippe (0.004 mmol)and THF (3 mL).

    b Yields were determined by GC–MS, 2a used as internal standard.

    N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332 327

  • Table 5Scope of TH reaction.a

    Entry Reagent Product Conv. 1 (%) b Yield (%)b

    1 99 99

    2 80 80

    3 40 40

    4c 100 94

    5 85 85

    a Unless otherwise noted, the reactions were conducted using ketone 1a (1.1 mmol), water (300 mmol), silane (2.2 equiv), [Ni(COD)2] (0.002 mmol), dippe (0.004 mmol)and THF (3 mL).

    b Yields were determined by GC–MS, 2a used as internal standard.c Hydrosilylation product was obtained in 6% of yield.

    328 N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332

    high yield (95%, Table 7). An asymmetric 1-(4-(dimethylamino)phenyl)-5-(thiophen-2-yl)penta-1,4-dien-3-one, 4f, gives theenone 6f in good yield (87%) and a saturated ketone in lower yield(13%, 5f). The use of 4f allowed the isolation of complex [(dippe)Ni(g2-Ca,Cb-C17H17NOS)] (h) prior to be reduced since the distance C(8)-C(7) 1.325(5) Å is characteristic for a C@C bond (Fig. 4).

    The Ni(0) center has a slightly distorted trigonal-planar geome-try coordinated to the enone moiety in ang2-Ca,Cbway, where theCAC bond length of the olefin in the unsaturated ketone is enlargedby 0.99 Å upon coordination in compound (h) (1.429 Å) relative tothe free ligand (1.328 Å), similar to closely related complexes pre-viously reported [56]. This complex was also prepared indepen-dently in high yield reacting enone 4f with [(dippe)Ni(l-H)]2complex (see Section 4).

    2.3. Enantioselective ketone hydrogenation

    Considering the results of a Ni catalyst based on (P-P) biphos-phine ligand we were curious to use asymmetric (P-P) ancillaryligands for this transformation. Thus, we used commercially avail-able BPE (Bis-Phospholane Ethane) as ligand in the transfer hydro-genation reaction. The use of [Ni(COD)2] complex, (S,S)-BPE ligandand H2O/Et3SiH as hydrogen source was investigated in the hydro-genation of a 1-(4-fluorophenyl)-ethanone 1b to give the corre-sponding alcohol (S)-2b in good conversion (81%) with 40% ee at150 �C (Table 8, entry 1). The phospholane ligand (R,R)-BPE wascompletely inactive in the asymmetric hydrogenation of 1b(Table 8, entry 2).

    Our group recently reported on the use of nickel/phosphine cat-alyst for transfer hydrogenation of ketones using ethanol as a safeand cheap hydrogen source [43]. In the current case, using ethanolas reactant and solvent, yield a good stereo-induction ((S)-2b, 71%)

    (Table 8, entry 3). In a closely related report, Liu et al. informed thatthe complex [IrCp*Cl2]2 catalyzed the transfer hydrogenation ofacetophenones with ethanol as the hydrogen donor, but using sto-ichiometric quantities of tBuOK [57]. Herein, the reaction proceedin the absence of base. However, the performance of other chiralbisphosphines was rather modest.

    3. Conclusions

    In summary, we have developed a chemoselective transferhydrogenation of ketones and a,b-unsaturated ketones catalyzedby [Ni(0)] complexes, using water and triethylsilane as hydrogensource. This represents an attractive methodology to prepare sec-ondary alcohols from monoketones, diols from diketones and sat-urated ketones from a,b-unsaturated enones. Moreover, withnickel/(S,S)-phospholane catalyst and ethanol as the hydrogensource a monoketone 1b was hydrogenated to chiral alcohol (S)-2b in good yield and enantio-selectivity.

    4. Experimental section

    Unless otherwise noted, all manipulations were performedusing standard Schlenk techniques in an inert-gas/vacuum doublemanifold or under an argon atmosphere (Praxair 99.998) in anMBraun UniLab glovebox (

  • Scheme 2. Proposed mechanism for TH of ketones with Et3SiH-H2O system.

    N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332 329

    purchased from Cambridge Isotope Laboratories and were storedunder 4 Å molecular sieves for 24 h before use. NMR spectra wererecorded at room temperature on a 300 MHz Varian Unity spec-trometer unless otherwise noted. 1H NMR spectra (d parts per mil-lion) are reported relative to the residual protio-solvent. 13C{1H}spectra give the characteristic carbon signal of each solvent. 31P{1H} NMR chemical shifts (d parts per million) are reported relativeto external 85% H3PO4. Coupling constants (J values) are given inHz. The following abbreviations were used for the NMR data:s = singlet; d = doublet; t = triplet, m = multiplet and br = broad.GC–MS determinations were performed using an Agilent Technolo-gies G3171A equipped with a column: 5% phenylmethylsilicone,30 m ⁄ 0.25 mm ⁄ 0.25 lm. Chiral HPLC analysis was performedon a Waters 1525, UV–visible dual (Waters 2487) instrument using

    Table 6Reduction of Benzyl using Et3SiH-H2O.a

    Entry Phosphine T (�C)

    1 dippe 1302 dippe 1503c dcype 1504c dippe 180

    a Unless otherwise noted, the reactions were conducted using ketone 1a (1.1 mmol),and THF (3 mL).

    b Yields were determined by GC–MS, 2a used as internal standard.c 1,2-diphenylethanone was obtained in 10% of yield.

    Chiracel OJ 4.6 ⁄ 250 mm 10 lm column at 25 �C and a mixture ofHPLC-grade hexanes and ethanol (90/10) as eluent, flowrate = 0.6 mL/min. 1H and 13C{1H} NMR spectra of the reductionproducts were obtained in CDCl3. Catalytic experiments were car-ried out in a 100 mL stainless steel Parr, T315SS reactor. Elementalanalyses (EAs) were performed by USAI-UNAM using a PerkinEl-mer microanalizer 2400.

    4.1. Reduction of acetophenone (1a) with [Ni(COD)2] and dippe, usingH2O/Et3SiH

    In a 150 mL Schlenk flask fitted with a Rotaflo valve, compound1a (0.065 g, 0.545 mmol, 50 equiv.) was reacted with [Ni(COD)2](0.003 g, 0.0109 mmol, 1 equiv.), dippe (0.0057 g, 0.0218 mmol, 2equiv.), H2O (327 mmol, 3 ⁄ 104 equiv, 5.8 mL) and Et3SiH(0.151 g, 1.30 mmol, 1.2 equiv.), using THF (3 mL) as solvent. Heat-ing was resumed until complete consumption of 1a was reached,according to GC–MS chromatography, which occurred after 2 daysat 130 �C. Exposing the system to the open atmosphere quenchedthe reaction and yellow colored or colorless solutions were formed.The mix was extracted 3 times with CH2Cl2 (15 mL), dried overNa2SO4, and concd. Identification of synthetic product (2b) wasbased on spectroscopic methods.

    4.2. Deuterium labeling using D2O

    An NMR tube with J. Young’s valve was charged in a run withsolution of 1a (4.36 mg, 0.036 mmol), [Ni(dippe)2] (c) (21.14 mg,0.036 mmol) using 0.7 mL of THF-d8, and to these was added 100equiv. of D2O (72.76 mg, 3.63 mmol) and 1 equiv. of Et3SiH(4.22 mg, 0.036 mmol). The mixture was then heated in a ther-mostatted silicon oil bath. For the reaction in THF-d8 at 100 �C,the NMR tube was heated and monitored over 15 h (Fig. 2, videsupra) The formation of [(dippe)Ni(g2-C,O-C8H8O)] (d) and mon-odeuterated product 2b was determined by 1H and 31P{1H} NMR(Figs. 2 and S10, respectively). After this time, the tube was openedto uncontrolled atmosphere; the remaining volatiles wereremoved in vacuum using an Schlenk line. Formation of a smallamount of a green solid was detected during the course of the reac-tion, identified as Ni(OH)2. The mix was extracted 3 times withCH2Cl2 (15 mL), dried over Na2SO4, and concentrated. Identificationof (2b), and phosphine oxides were made by GC–MSchromatography.

    Conv. 3a (%)b 3b (%)b 3c (%)b

    20 20 –52 – 5289 66 –87 77 –

    water (300 mmol), silane (2.2 equiv), [Ni(COD)2] (0.002 mmol), dippe (0.004 mmol)

  • Table 7Catalytic hydrogenation of a,b-unsaturated ketones with Et3SiH-H2O.a

    a, b-Unsaturated Ketone Yield (%)b

    4(a-f) 5(a-f) 6(c-f)

    4a R1: Ph R2: Ph

    4b R1: Ph R2: CH3

    5a 91 %

    5b 64 %

    4c R1: H

    4d R1: p-OCH3-Ph

    4e R1: p-CF3-Ph

    5c 67 %

    5d 5 %

    5e 44 %

    6c 32 %

    6d 95 %

    6e 34 %

    4f 5f 13 % 6f 87 %

    a Unless otherwise noted, the reactions were conducted using a,b-unsaturated ketones (1.1 mmol), water (300 mmol), silane (2.2 equiv), [Ni(COD)2] (0.002 mmol), dippe(0.004 mmol) and THF (3 mL).

    b Yields were determined by GC–MS, 4a used as internal standard.

    Fig. 4. ORTEP drawing of [(dippe)Ni(g2-Ca,Cb-C17H17NOS)] (h), showing 50% probability ellipsoids. Selected bond distances [Å]: C(10)-C(11) 1.429(5), P(2)–Ni(1) 2.1599(9), P(1)–Ni(1) 2.1621(9), Ni(1)–C(11) 2.002(3), Ni(1)–C(10) 1.975(3), C(8)–C(7) 1.325(5). Selected bond angles [�]: C(10)–Ni(1)–C(11) 42.10(2), C11-Ni(1)–P(1) 110.74(10), P(2)–Ni(1)–P(1) 91.15(3), P(2)–Ni(1)–C(10) 116.29(10).

    330 N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332

    4.3. Metal-catalyzed reduction of (1a) with [Ni(COD)2] and dippe usingd-labeled systems. (a) D2O

    In a 150 mL Schlenk flask fitted with a Rotaflo valve, chargedwith 1a (0.065 g, 0.545 mmol, 50 equiv.), [Ni(COD)2] (0.003 g,0.0109 mmol, 1 equiv.), dippe (0.0057 g, 0.0218 mmol, 2 equiv.),Et3SiH (0.277 g, 2.39 mmol, 2.2 equiv.), D2O (327 mmol, 3 ⁄ 104equiv, 5.95 mL) and THF (3 mL). The solution was heated with

    vigorous stirring at 130 �C during 48 h. After this, the Schlenk flaskwas opened in a well-vented hood prior to workup. A colorlesssolution was formed. The mix was extracted 3 times with CH2Cl2(15 mL), dried over Na2SO4, and concd. Identification of syntheticproduct monodeuterated 1-phenylethanol was based on spectro-scopic methods. During the reaction monitoring, yields and con-versions were determined by GC–MS chromatography (Table 4,entry 2). (b) Et3SiD. In a 150 mL Schlenk flask fitted with a Rotaflo

  • Table 8Enantioselective reduction of ketones.

    Entry Ligand Hydrogen Donor Conv. 1 (%) c ee (%) d

    1a (S,S)-iPr-BPE Et3SiH/H2O 22 22a (R,R)-iPr-BPE Et3SiH/H2O 0 03b (S,S)-iPr-BPE EtOH 71 714b (S,S)-iPr-Duphos EtOH 100 595b (S,S)-Et-Ferrocelane EtOH 72 396b (S)-BINAP EtOH 0 0

    a Unless otherwise noted, the reactions were conducted using ketone 1b (1.1 mmol), Hydrogen Donor (water (300 mmol), silane (2.2 equiv)), [Ni(COD)2] (0.002 mmol), (S,S)-BPE (0.003 mmol) and THF (3 mL).

    b The reactions were conducted using ketone 1b (1.1 mmol), hydrogen donor (ethanol (3 mL)), [Ni(COD)2] (0.002 mmol) and (S,S)-Ligand (0.003 mmol).c Conversions were determined by GC–MS analysis, 2a used as internal standard.d Determined by HPLC using a chiral column, and the configuration was determined as (S).

    N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332 331

    valve, charged with 1a (0.065 g, 0.545 mmol, 50 equiv.), [Ni(COD)2](0.003 g, 0.0109 mmol, 1 equiv.), dippe (0.0057 g, 0.0218 mmol, 2equiv.), H2O (327 mmol, 3 ⁄ 104 equiv, 5.8 mL), Et3SiD (0.29 g,2.4 mmol, 2.2 equiv.) and THF (3 mL). The solution was heatedwith vigorous stirring at 130 �C during 48 h. After this, the Schlenkflask was opened in a well-vented hood prior to workup. A color-less solution was formed. The mix was extracted 3 times with CH2-Cl2 (15 mL), dried over Na2SO4, and concd. Identification ofsynthetic product monodeuterated 1-phenylethanol was basedon spectroscopic methods. During the reaction monitoring, yieldsand conversions were determined by GC–MS chromatography(Table 4, entry 3).

    4.4. Catalytic reduction of ketones using [Ni(0)] in the presence of THF

    All reactions were made in a 150 mL Schlenk flask fitted with aRotaflo valve, typically charged with [Ni(COD)2] (0.022 mmol),dippe (0.044 mmol) and the corresponding ketone (1.1 mmol),silane (2.2 equiv), water (300 mmol) and THF (3 mL). The solutionwas heated with vigorous stirring at 130 �C for the correspondingreaction time. After this, the reactor was opened in a well-ventedhood prior to workup. Yellow colored or colorless solutions wereformed. The mix was extracted 3 times with CH2Cl2 (15 mL), driedover Na2SO4, and concd. Identification of synthetic product (2a)was based on spectroscopic methods. During the reaction monitor-ing, yields and conversions were determined by GC–MS chro-matography. Products and intermediates were characterized by1H NMR and 13C{1H} NMR after column chromatography purifica-tion using n-hexanes/THF mixtures using silica gel.

    4.5. Preparation of [(dippe)Ni(g2-Ca,Cb-C17H17NOS)] (h)

    The reaction between C17H17NOS (0.030 g, 9.65 ⁄ 105 mol) and[(dippe)Ni(l-H)]2 (0.034 g, 4.81 ⁄ 10�5 mol) in Methanol-d4.

    (1 mL) yield de mononuclear complex [(dippe)Ni(g2-Ca,Cb-C17-H17NOS)] (h). Immediate effervescence due to the reductive elim-ination of H2 was observed during mixing. The resulting solutionwas analyzed by NMR spectroscopy: 31P{1H} NMR (121.32 MHz,Methanol-d4, r.t.) d = 75,12 (d, JP-P = 57.02 Hz); d = 73,6 (d, JP-P =57.02 Hz). Slow evaporation of Methanol-d4 at �26 �C in a dryboxallowed the crystallization of the pure product. [MS-EI+, m/z (%)603 (3%)].

    4.6. X-ray structure determination

    The crystal for compound (h) was cryoprotected using Para-tone-N and mounted on glass fibers; then, the crystals were imme-diately cooled at 130 K using Cryojet cryostream (OxfordCryosystems device). Diffraction data were collected on an OxfordDiffraction Gemini diffractometer with a CCD-Atlas area detectorusing a radiation source graphite monochromator, kMoKa = 0.71073 Å. CrysAlisPro and CrysAlis RED software packageswere used for data collection and integration [59]. The double passmethod of scanning was used to exclude any noise. The collectedframes were integrated by using an orientation matrix determinedfrom the narrow frame scans. Final cell constants were determinedby a global refinement; collected data were corrected for absor-bance by using analytical numeric absorption correction [60] usinga multifaceted crystal model based on expressions upon the Lauesymmetry using equivalent reflections. Structure solution andrefinement were carried out with the SHELXS-2014 and SHELXL-2014 packages [61], WinGX v2014.1 software was used to preparematerial for publication [62,63]. Full-matrix least-squares refine-ment was carried out by minimizing (Fo2 � Fc2)2. All non-hydrogenatoms were refined anisotropically. H atoms attached to C atomswere placed in geometrically idealized positions and refined as rid-ing on their parent atoms, with CAH = 0.95–1.00 Å and with Uiso(H) = 1.2 Ueq(C) for aromatic methyne and methylene groups,

  • 332 N. Castellanos-Blanco et al. / Inorganica Chimica Acta 466 (2017) 324–332

    and Uiso(H) = 1.5 Ueq(C) for methyl groups. Crystal data andexperimental details of the structure determination of h are listedin the Supporting Information (see Tables S1–S4 at SI section)[62,63]. Crystallographic data have been deposited at the Cam-bridge Crystallographic Data Center number CCDC 1548929.Copies of the data can be obtained free of charge on applicationto CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. E-mail:[email protected].

    Acknowledgments

    We thank CONACYT (0178265) and PAPIIT-DGAPA-UNAM (IN-202516) for their financial support to this work. N.C.-B. thanksCONACYT for a graduate studies grant. Also, we thank Dr. AlmaArévalo for her technical support.

    Appendix A. Supplementary data

    Includes complete experimental procedures, selected NMRspectra, and GC-MS determinations of all products. This materialis available free of charge via the Internet. Supplementary dataassociated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.ica.2017.06.035.

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    Nickel-catalyzed reduction of ketones with water and triethylsilane1 Introduction2 Results and discussion2.1 Catalytic hydrogenation of acetophenone using water as hydrogen donor2.2 Catalytic hydrogenation of acetophenone using Et3SiH/H2O2.3 Enantioselective ketone hydrogenation

    3 Conclusions4 Experimental section4.1 Reduction of acetophenone (1a) with [Ni(COD)2] and dippe, using H2O/Et3SiH4.2 Deuterium labeling using D2O4.3 Metal-catalyzed reduction of (1a) with [Ni(COD)2] and dippe using d-labeled systems. (a) D2O4.4 Catalytic reduction of ketones using [Ni(0)] in the presence of THF4.5 Preparation of [(dippe)Ni(η2-Cα,Cβ-C17H17NOS)] (h)4.6 X-ray structure determination

    AcknowledgmentsAppendix A Supplementary dataReferences


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