Cationic organoaluminum compounds as intramolecular hydroamination catalysts Manish Khandelwal, Rudolf J. Wehmschulte * Department of Chemistry, Florida Institute of Technology,150 West University Boulevard, Melbourne, FL 32901, USA article info Article history: Received 1 August 2011 Received in revised form 13 September 2011 Accepted 15 September 2011 Keywords: Hydroamination Cationic Alkyl aluminum Aryl aluminum Catalysis Cyclization abstract Cationic dialkylaluminum and m-terphenylalkylaluminum compounds catalyze the intramolecular hydroamination of primary and secondary aminopentenes. The reaction rates are strongly dependent on the substrate and the catalyst substituents. The bulky species [Dipp*AlEt][CHB 11 H 5 I 6 ] (Dipp* ¼ 2,6- Dipp 2 C 6 H 3 e, Dipp ¼ 2,6-iPr 2 C 6 H 3 e), 4, was the most active catalyst. Although the neutral species DcpAlEt 2 (Dcp ¼ 2,6-(2,6-Cl 2 C 6 H 3 ) 2 C 6 H 3 e), 7 , and Dipp*AlEt 2 , 8, showed some catalytic activity, they were more than 25 times less reactive than their cationic counterparts [DcpAlEt][CHB 11 H 5 Cl 6 ], 3, and 4. The cyclization of secondary benzylaminopentenes with [Et 2 Al][CHB 11 H 5 I 6 ], 1 , was strongly dependent on the substitution of the C-2 olefinic carbon. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The intramolecular hydroamination of olefins allows the construction of nitrogen containing cyclic structures in an atom economical way leading to valuable precursors of biologically active molecules and natural products [1e3]. Base-catalyzed systems have been known for a long time [4], but the application of metal-based catalysts began with Marks’ report on lanthanide compounds [5]. Since then, numerous metals have been investigated including lanthanides [6e8], early [9e11] and late transition metals [12e14] and more recently alkaline earth metals [15]. Neutral complexes of the latter metals are isoelectronic with cationic group 13 species. As we have been working with cationic organoaluminum compounds [16,17], we were interested in their potential as hydroamination catalysts. Furthermore, there were only two reports regarding aluminum based hydroamination catalysts: one featured a four-coordinate aluminum amide featuring a bidentate phenylene-diamine ligand [18], the other a five-coordinate aluminum complex with a tridentate OCO-pincer ligand [19]. While these compounds showed some reactivity, we hypothesized that cationic low-coordinate organoaluminum compounds would be more reactive due to their electronic similarity to neutral magnesium and calcium based catalysts. Here, we report the activity of four cationic organoaluminum compounds as catalysts for intramolecular hydroamination of aminopentenes, a compar- ison with closely related neutral species and two examples of a secondary aminopentene substrate. 2. Experimental 2.1. General methods All experiments were conducted under a nitrogen atmosphere using standard Schlenk techniques or in a Vacuum Atmospheres dry box unless otherwise noted. Dry, oxygen-free solvents were used unless otherwise indicated. NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. 1 H NMR chemical shift values were determined relative to the residual protons in C 6 D 6 as internal reference (d ¼ 7.16 ppm), and 13 C NMR spectra were referenced to the solvent signal (d ¼ 128.39 ppm). Initial samples of the aminopentenes 2,2-diphenyl-4-methylpent-4-en-1-amine, A, 2,2-diphenylpent-4-en-1-amine, B, (1-allylcyclohexyl)-methan- amine, C, and 2,2-diphenyl-5,5-dimethylpent-4-en-1-amine, D, were provided by Dr. J. Koller and Prof. R. Bergman at the University of California, Berkeley. Larger amounts of A [20] and B [20] as well as the secondary amines N-benzyl-2,2-diphenylpent-4-en-1- amine, E [21], were prepared following the literature. The cata- lysts [Et 2 Al][CHB 11 H 5 I 6 ], 1 [22], [Et 2 Al][CHB 11 H 5 Cl 6 ], 2 [22], * Corresponding author. Tel.: þ1 321 674 7659; fax: þ1 321 674 8951. E-mail address: rwehmsch@fit.edu (R.J. Wehmschulte). Contents lists available at SciVerse ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.09.010 Journal of Organometallic Chemistry 696 (2012) 4179e4183
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at SciVerse ScienceDirect
Journal of Organometallic Chemistry 696 (2012) 4179e4183
0022-328X/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.jorganchem.2011.09.010
a b s t r a c t
Cationic dialkylaluminum and m-terphenylalkylaluminum compounds catalyze the intramolecularhydroamination of primary and secondary aminopentenes. The reaction rates are strongly dependent onthe substrate and the catalyst substituents. The bulky species [Dipp*AlEt][CHB11H5I6] (Dipp* ¼ 2,6-Dipp2C6H3e, Dipp ¼ 2,6-iPr2C6H3e), 4, was the most active catalyst. Although the neutral speciesDcpAlEt2 (Dcp ¼ 2,6-(2,6-Cl2C6H3)2C6H3e), 7, and Dipp*AlEt2, 8, showed some catalytic activity, theywere more than 25 times less reactive than their cationic counterparts [DcpAlEt][CHB11H5Cl6], 3, and 4.The cyclization of secondary benzylaminopentenes with [Et2Al][CHB11H5I6], 1, was strongly dependenton the substitution of the C-2 olefinic carbon.
� 2011 Elsevier B.V. All rights reserved.
1. Introduction
The intramolecular hydroamination of olefins allows theconstruction of nitrogen containing cyclic structures in an atomeconomical way leading to valuable precursors of biologically activemolecules and natural products [1e3]. Base-catalyzed systems havebeen known for a long time [4], but the application of metal-basedcatalysts began with Marks’ report on lanthanide compounds [5].Since then, numerous metals have been investigated includinglanthanides [6e8], early [9e11] and late transition metals [12e14]and more recently alkaline earth metals [15]. Neutral complexesof the latter metals are isoelectronic with cationic group 13 species.As we have been working with cationic organoaluminumcompounds [16,17], we were interested in their potential ashydroamination catalysts. Furthermore, there were only tworeports regarding aluminum based hydroamination catalysts: onefeatured a four-coordinate aluminum amide featuring a bidentatephenylene-diamine ligand [18], the other a five-coordinatealuminum complex with a tridentate OCO-pincer ligand [19].While these compounds showed some reactivity, we hypothesizedthat cationic low-coordinate organoaluminum compounds wouldbe more reactive due to their electronic similarity to neutral
: þ1 321 674 8951.ulte).
All rights reserved.
magnesium and calcium based catalysts. Here, we report theactivity of four cationic organoaluminum compounds as catalystsfor intramolecular hydroamination of aminopentenes, a compar-ison with closely related neutral species and two examples ofa secondary aminopentene substrate.
2. Experimental
2.1. General methods
All experiments were conducted under a nitrogen atmosphereusing standard Schlenk techniques or in a Vacuum Atmospheresdry box unless otherwise noted. Dry, oxygen-free solvents wereused unless otherwise indicated. NMR spectra were recorded ona Bruker Avance 400 MHz spectrometer. 1H NMR chemical shiftvalues were determined relative to the residual protons in C6D6 asinternal reference (d ¼ 7.16 ppm), and 13C NMR spectra werereferenced to the solvent signal (d¼ 128.39 ppm). Initial samples ofthe aminopentenes 2,2-diphenyl-4-methylpent-4-en-1-amine, A,2,2-diphenylpent-4-en-1-amine, B, (1-allylcyclohexyl)-methan-amine, C, and 2,2-diphenyl-5,5-dimethylpent-4-en-1-amine, D,were provided by Dr. J. Koller and Prof. R. Bergman at the Universityof California, Berkeley. Larger amounts of A [20] and B [20] as wellas the secondary amines N-benzyl-2,2-diphenylpent-4-en-1-amine, E [21], were prepared following the literature. The cata-lysts [Et2Al][CHB11H5I6], 1 [22], [Et2Al][CHB11H5Cl6], 2 [22],
M. Khandelwal, R.J. Wehmschulte / Journal of Organometallic Chemistry 696 (2012) 4179e41834180
[DcpAlEt][CHB11H5Cl6] (Dcp ¼ 2,6-(2,6-Cl2C6H3)2C6H3e), 3 [17],[Dipp*AlEt][CHB11H5I6] (Dipp* ¼ 2,6-Dipp2C6H3e, Dipp ¼ 2,6-iPr2C6H3e), 4 [17], DcpAlEt2, 7 [17], and Dipp*AlEt2, 8 [17], weresynthesized according to the literature procedures.
2.2. N-benzyl-4-methyl-2,2-diphenylpent-4-en-1-amine, F
This compound was prepared in analogy to E [21] using A asa precursor and obtained as a colorless, viscous oil. Yield: 0.58 g,85%. 1H NMR (400.13 MHz, C6D6): 0.78 (br s, 1H, NH), 1.14 (s, 3H,Me), 3.08 (s, 2H, CH2), 3.32 (s, 2H, CH2), 3.54 (s, 2H, CH2), 4.82 (s, 1H,]CH2), 4.90 (s, 1H, ]CH2), 6.98e7.19 (m, 15H, Ph). 13C{1H} NMR(100.62 MHz, C6D6): 25.1 (Me), 44.9(CH2), 50.8 (CPh2), 54.8(CH2Ph),55.5(CH2), 115.9(]CH2), 126.6,127.4,128.5,128.8,141.5, 143.3,147.9.
2.3. Hydroamination experiments
For a typical NMR scale experiment, 100 mmol of aminoalkene,10 mmol of catalyst, 2 mg of 2,6-di-tert-butylpyridine (10 mmol)and a known amount of hexamethylbenzene (2e4 mg,12e25 mmol) as internal standard were dissolved in C6D6(0.8 mL). The solution was transferred into a J. Young valve fittedNMR tube. The sealed NMR tube was then heated in a tempera-ture controlled silicone-oil bath at 135(�2) �C for the specifiedamount of time (see Tables 1 and 2), and the reaction progresswas monitored via 1H NMR spectroscopy. All product signalswere in good agreement with previously published data ofidentical products [18,20,21,23].
2.4. Hydroamination with in situ generated 1 (entry B9, Table 2)
A mixture of AlEt3 (6 mg, 52 mmol), [Ph3C][CHB11H5I6] (62 mg,54 mmol), was heated at 80 �C inside a J. Young type NMR tube until
Table 1Catalytic hydroamination of aminoalkenes (AeD) by cationic aluminum species (1e4).
Entrya Substrate Product Ca
A1 1
A2 2
A3 3
A4 4
B1 1
B2 2
B3 3
B4 4
C1 1
C3 3
C4 4
D1 1
1 ¼ [AlEt2][CHB11H5I6]; 2 ¼ [AlEt2][CHB11H5Cl6]; 3 ¼ [DcpAlEt][CHB11H5Cl6]; 4 ¼ [Dipp*a All reactions were carried out in C6D6 at 135 �C and 10 mol% catalyst loading.b Determined by integration of 1H NMR signals of reactant and product versus an intec Isomerized product.d 17.5 mol % catalyst loading in C6D6 at 135 �C.
the AlEt3 was consumed (2 d) (The reaction time for this reactionwas rather long due to the use of just an equimolar amount of AlEt3and crystalline instead of microcrystalline or amorphous tritylsalt.). A small portion of the solution (110 mg, ca. 8 mmol of catalyst)was taken into a separate J. Young type NMR tube, and hexame-thylbenzene (3 mg, 18 mmol) and 2,2-diphenyl-pent-4-en-1-amine(19 mg, 78 mmol) were added. The resulting mixture was heated at135 �C for 70 h while the progress of the reactionwas monitored by1H NMR spectroscopy. Conversion 98% Yield 82%with respect to theinternal reference. For conversion and yield after 30 h see Table 2,entry B9.
A J. Young valve fitted NMR tube was charged with [Ph3C][CHB11H5Cl6] (200 mg, 0.34 mmol) and AlEt3 (66 mg, 0.58 mmol) inbenzene (0.8 mL). The suspension was stirred at room temperatureuntil the red solid turned into a pale white solid. The solid waswashed with hexanes (3 � 1 mL) and dried (yield: 130 mg,0.29mmol, 85%). Benzene (0.8mL) andNH2CH2C(Ph)2CH2CHCH2,B,(138 mg, 0.58 mmol) were added to the dry solid. The mixture wasgently heated (80 �C) to obtain a clear solution. The productprecipitated out of the solution upon cooling to room temperature.Yield 126 mg (0.14 mmol, 50.5%). Mp 155e157 �C. 1H NMR(400.13 MHz, C6D6): �0.20 (q, J ¼ 8.2 Hz, 4H, AlCH2CH3), 0.70 (t,J ¼ 8.1 Hz, 6H, AlCH2CH3), 1.58 (s, 1H, CH-carborane), 2.92 (d,J ¼ 6.7 Hz, 4H, CH), 3.04 (s, 4H,NH2), 3.2 (s, 4H, CH2) 4.93 (d,J ¼ 10.2 Hz, 2H, CH’), 5.21 (m, 2H, CH), 5.39 (d, J ¼ 17.0 Hz, 2H, CH),6.95e6.99 (m, 12H, Ph), 7.07e7.11 (m, 8H, Ph). 13C{1H} NMR(100.62 MHz, C6D6): �2.7 (AlCH2CH3), 8.7 (AlCH2CH3), 40.7 (CH2),48.5 (CH2NH2), 50.9 (C(Ph2)), 120.6(]CH2), 128.9 (Ph), 129.7 (Ph),130.2 (Ph),133.1(CH),143.4 (Ph). 11BNMR (128.38MHz, C6D6):�23.1(d broad, 5B, BH), �5.3 (s, 5B, m-BCl), 1.1 (s, broad, 1B, p-BCl).
talyst Time (h) % Conversionb % Yieldb
5 >99 78
5 86 74
34 >99 85
1.5 >99 60
28 88 70 þ 15c
39 95 70 þ 10c
3 >99 83
0.25 >99 90
110 89 84
103 90 85
1d 98 88
70 0 0
AlEt][CHB11H5I6].
rnal standard.
Table 2Catalytic hydroamination of aminoalkenes (A and B) by neutral aluminum compounds.
Entrya Substrate Product Catalyst Time (h) % Conversionb % Yieldb
A6 No reaction 6 6 0 0
A7 No reaction 7 70 0 0
B7 7 70 73 50
B8 8 70 53 30 þ 10c
B9 AlEt3 þ [Ph3C][CHB11H5I6] 30 83 69 þ 18c
6 ¼ AlEt3, 7 ¼ DcpAlEt2, 8 ¼ Dipp*AlEt2.a All reactions were carried out in C6D6 at 135 �C and 10 mol% catalyst loading.b Determined by integration of 1H NMR signals of reactant and product versus an internal standard.c Isomerized product.
M. Khandelwal, R.J. Wehmschulte / Journal of Organometallic Chemistry 696 (2012) 4179e4183 4181
3. Results and discussion
3.1. Hydroamination of primary amines
The activity of four cationic organoaluminum compounds asintramolecular hydroamination catalysts was determined usingfour aminopentene substrates (Table 1). The complexes [Et2Al][CHB11H5I6], 1 [22], and [Et2Al][CHB11H5Cl6], 2 [22], differ only inthe anion, whereas [DcpAlEt][CHB11H5Cl6], 3 [17], and [Dipp*AlEt][CHB11H5I6], 4 [17], carry large m-terphenyl substituents. All fourcompounds are tight ion-pairs with four-coordinate aluminumcenters in the solid state and most likely also in aromatic solutions.
BBB
B
B
BBBB
B
C
B XX
X
XX
X
H
Al
X = I; 1X = Cl; 2
BBB
B
B
BBBB
B
C
B II
I
II
I
H
Ali-Pri-Pr
i-Pr
i-Pr4
Scheme 1. Structures o
They readily coordinate amine ligands to afford solvent separatedion-pairs such as [Et2Al(NC5H5)2][CHB11H5Br6] [22] and [DcpA-lEt(NH2-tBu)2][CHB11H5I6] [17]. The aminopentenes differ in thesubstitution pattern of the double bond as well as the centralquaternary carbon (Scheme 1).
The reactions were conducted with 10% catalyst loading in C6D6solution at 135 �C. The activity pattern for catalysts 1 and 2, i. e. the[Et2Al]þ species, closely follows that observed for C6H4-o-(3,5-tBu2C6H3NH)2AlNMe2(NHMe2) [18]. The nature of the anion,[CHB11H5I6]� or [CHB11H5Cl6]�, does not significantly effect theoutcome. The higher conversion rate for substrate B over C may beexplained by the ThorpeeIngold effect [24], and the failure of any
BBB
B
B
BBBB
B
C
B ClCl
Cl
ClCl
Cl
H
AlClCl
Cl
Cl3
Al
H2N
NH2
Ph Ph
Ph Ph
Et
Et
BBB
B
B
BBBB
B
C
B ClCl
Cl
ClCl
Cl
H+
-
5
f compounds 1e5.
HN Ph
R Ph Ph
NR
Ph Ph
Ph
[AlEt2][CHB11H5I6]
135 C, C6D6
R T(h) % conv.
H 0.5 >99
Me 11 64
R = H: E; R = Me: F
º
Scheme 3. Hydroamination of secondary aminopentenes.
M. Khandelwal, R.J. Wehmschulte / Journal of Organometallic Chemistry 696 (2012) 4179e41834182
conversion for D by unfavorable sterics in 1,2-disubstituted or 1,1,2-trisubstituted olefins [6]. Overall, the activity of the simple [Et2Al]þ
catalysts 1 and 2 is about twice as high as C6H4-o-(3,5-tBu2C6H3eNH)2AlNMe2(NHMe2) at a slightly lower temperature(135 versus 150 �C). A significant increase in activity was found forthe sterically more encumbered [Dipp*AlEt]þ cation, 4. The biggestincrease was observed for substrates B and C by factors ofapproximately 100 with respect to catalyst 1. The results for thecation [DcpAlEt]þ, 3, were mixed: there was also a significantincrease for substrate B, no difference for substrate C and a seven-fold decrease for substrate A.
Attempts to gain insight into the mechanismwere inconclusive.Addition of two equivalents of B to a solution of 2 led to theformation of the 1:2 adduct [AlEt2{NH2CH2C(Ph)2CH2CHCH2}2][CHB11H5Cl6], 5. Heating of this compound at 135 �C for 6 h led tothe evolution of some ethane, but extensive line broadening in the1H NMR spectrum prevented the analysis of the product. Similarly,heating of a 1:4 mixture of 2 and B at 135 �C for 6 h resulted in anNMR spectrum featuring mainly broad signals and some ethane.The 1H NMR spectra of the catalytic reactions (Table 1) also showedthe loss of the AlEt signals and the appearance of ethane, but thegaseous nature of ethane and its incomplete dissolution in C6D6prevented quantitative analysis. The generally acceptedmechanismfor lanthanide, calcium and lithium catalyzed hydroaminationinvolves the formation of ametal amide complex. Subsequently, theamide nitrogen attacks the olefin, which may have been activatedby coordination to the metal center. The observed ethane evolutionis in agreement with this mechanism (Scheme 2).
Whereas trialkylalanes readily react with primary andsecondary amines under ethane elimination to give dia-lkylaluminum amides [25], the analogous reaction is more difficultfor cationic organoaluminum compounds [17]. We suggest thatunder the reaction conditions only one of the ethyl groups is dis-placed and that species such as [EtAlN(H)R{NH2R}n]þ may bepresent in the reaction mixture. This is further supported by theobservation, that the aminolysis of 1 with 2e3 equivalents of t-BuNH2 at 135 �C is incomplete even after 6 h and the observedstrong influence of the substituents on the aluminum catalysts. Thelatter strongly indicates that the AleC(Terphenyl) bond is notcleaved during the reaction.
For comparison, the effectiveness of the neutral organo-aluminum compounds Et3Al, 6, DcpAlEt2, 7, and Dipp*AlEt2, 8, wastested with substrates A and B under the same conditions as the
[RAlEt]+ + 3 NH2PhPh �
- EtH
R = Et, Dcp, Dipp*
R AlNH
H2N H2N
PhPh
PhPh
R'NH2 = NH2PhPh
Scheme 2. Possible
cationic species (Table 2). No reactionwas observed for substrate A,but a rather slow conversion took place for substrate B and thecatalysts 7 and 8 (70 h for 50 þ % conversion vs. 3 and 0.25 h for 3and 4, respectively). Apparently, the cationic nature of the catalystsis necessary for conversion in a reasonable time period. In situgeneration of the cationic catalyst (entry B9, Table 2) gives a resultsimilar to that of the preformed catalyst (entry B1, Table 1).
3.2. Hydroamination of secondary amines
Inspired by these results, we decided to test the reactivity ofsecondary aminopentenes E and F (Scheme 3) with catalyst 1.Hydroamination catalysts often display different activities towardprimary and secondary aminoolefins [26,27].
Interestingly, the conversion of both aminopentenes is reversedwith respect to their primary analogs A and B. Whereas substrate Abearing a substituted olefinic group was converted within 5 h, itsbenzylated derivative F could not be converted completely within11 h. On the other hand, aminopentene B, which is unsubstituted atthe olefinic functionality required 28 h for 88% conversion, whereasits benzylated counterpart E was converted within one half hour.This together with the fact that only a minor amount of ethane wasgenerated indicates that a different mechanism might have been atwork. Possibly, the alkene was activated by coordination to theLewis acidic aluminum center and subsequently attacked by thenucleophilic amine nitrogen. A similar but significantly faster
PhPh
- R'NH2
+ R'NH2R Al
H2N
NH
PhPh
PhPh
R Al
H2N
NH
PhPh
PhPh
NH2PhPh
NH
Ph Ph
+ +
+
catalytic cycle.
M. Khandelwal, R.J. Wehmschulte / Journal of Organometallic Chemistry 696 (2012) 4179e4183 4183
conversion preference for secondary aminopentenes was reportedfor the Et2Zn/[PhNHMe2][B(C6F5)4] catalytic system [28].
4. Conclusions
Cationic organometallic aluminum compounds catalyze theintramolecular hydroamination of aminopentenes. The reactionrates are strongly dependent on the substrate and the catalyst.Bulky substituents on the catalyst generally increase the reactivity.Control experiments with closely related neutral organoaluminumcatalysts showed that the positive charge on the aluminum centeris required for reasonable conversion rates.
Acknowledgments
Financial support for this work from the National ScienceFoundation (CHE 0718446) is gratefully acknowledged. We furtherthank the National Science Foundation (CHE 03422510) for thepurchase of the Bruker Avance 400 NMR spectrometer. We are alsograteful to Dr. Jürgen Koller and Professor Robert G. Bergman at theUniversity of California in Berkeley who provided us with initialsamples of the four primary aminopentenes.
Appendix A. Supplementary material
Supplementary data related to this article can be found online atdoi:10.1016/j.jorganchem.2011.09.010.
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