Organometallics 1995, 14, 4209-4212
4209
Hydrogenation of Imines Catalyzed by a Zwitterionic Rhodium Complex Zhongxin Zhou,? Brian R. James,**$and Howard Alper*l+ Departments of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K I N 6N5, and University of British Columbia, Vancouver, British Columbia, Canada V6T 121 Received March 21, 1 9 9 P The hydrogenation of both aldimines and ketimines catalyzed by the zwitterionic rhodium complex, (y6-PhBPh&Rh+(l,5-COD), 1, was achieved in excellent yield with 9/1(v/v) THF and methanol as the solvent under 200-600 psi of H2 at 40 "C and the addition of 1equiv of 1,4-bis(diphenylphosphino)butane (DPPB). The extent of imine conversion is sensitive to the substrate structure and solvent employed.
Introduction Previous studies from one of our laboratories have demonstrated that the zwitterionic rhodium complex l1 kh' (COD)
1
is a useful catalyst for a variety of carbonylation reactions.2-8 This complex, either alone or in the presence of 1,4-bis(diphenylphosphino)butane(DPPB), has been found to be a highly regioselective catalyst for the hydroformylation of aryl and 1,l-disubstituted alk e n e ~ allyl , ~ ~acetates,6 ~ vinyl ethers,2 vinyl silanes,8 vinyl sulfones and sulfoxide^,^ as well as a,p-unsaturated ester^.^ Complex 1 is also an excellent catalyst for the inter- and intramolecular silylhydroformylation of a l k y n e ~ . ~Moderate J~ diastereoselectivity resulted from the hydroformylation of phenyl vinyl sulfoxide7and the hydrosilylation of prochiral ketonesll using 1 and a chiral ligand. Finally, an interesting example has been reported for the application of the zwitterionic complex 1to the stereoregular polymerization of phenylacetylene.I2 The catalytic hydrogenation of imines to amines (cf. eq 1)13-17 has attracted considerable interest in recent University of Ottawa. + University of British Columbia. +
@Abstractpublished in Advance ACS Abstracts, July 1, 1995. (1) Schrock, R. R.; Osborn, J. A. Znorg. Chem. 1970,9, 2339. (2)h e r , I.; Alper, H. J . A m . Chem. SOC. 1990,112, 3674. (3)Zhou, J. Q.; Alper, H. J . Chem. Soc., Chem. Commun. 1991,233. (4) Alper, H.; Zhou, J. Q. J . Org. Chem. 1992, 57, 3729. (5) Zhou, J. Q.; Alper, H. J . Org. Chem. 1992, 57, 3328. (6)Alper, H.; Zhou, J. Q. J . Chem. Soc., Chem. Commun. 1993,316. (7)Totland, K.; Alper, H. J . Org. Chem. 1993, 58, 3326. (8) Crudden, C. M.; Alper, H. J. Org. Chem. 1994,59, 3091. (9) Zhou, J. Q.; Alper, H. Organometallics 1994, 13, 1586. (10)Monteil, F.; Matsuda, I.; Alper, H. J . Am. Chem. Soc. 1995,117, 4419. (11) Goldberg, Y.; Alper, H. Tetrahedron: Asymmetry 1992,3,1055. (12) Goldberg, Y.; Alper, H. J . Chem. SOC.,Chem. Commun. 1994, 1209. (13) Fogg, D. E.; James, B. R.; Kilner, M. Inorg. Chim. Acta 1994, 222, 85. (14) Longley, C. J.; Goodwin, T. J.; Wilkinson, G. Polyhedron 1986, 5, 1625.
years, especially in the asymmetric hydrogenation of prochiral i m i n e ~ . l ~Both - ~ ~early (Ti) and late (Ir, Rh, Ru) transition metal complexes have been applied as the catalyst precursors, with neutral and cationic rho(15)Brune, H. A.; Unsin, J.; Hemmer, R.; Reichhardt, M. J . Organomet. Chem. 1989,369,335. (16)Bhaduri, S.; Sapre, N.; Sharma, K.; Jones, P. G.; Carpenter, G. J . Chem. Soc., Dalton Trans. 1990, 1305. (17)Basu, A.; Bhaduri, S.; Sharma, K.; Jones, P. G. J . Chem. SOC., Chem. Commun. 1987, 1126. (18) James, B. R. Chem. Znd. 1995,62, 167. (19) Bolm, C. Angew. Chem., Znt. Ed. Engl. 1993,32, 232. (20) Willoughby, C. A.; Buchwald, S. L. J . Am. Chem. Soc. 1992, 114, 7562. (21) Willoughby, C. A.; Buchwald, S. L. J . Org. Chem. 1993, 58, 7627. (22) Willoughby, C. A.; Buchwald, S. L. J . Am. Chem. SOC.1994, 116,8952. (23) Willoughby, C. A.; Buchwald, S. L. J . A m . Chem. SOC.1994, 116, 11703. (24) Burk, M. J.; Feaster, J. E. J . Am. Chem. SOC.1992, 114, 6266. (25)Burk, M. J.; Martinez, J. P.; Feaster, J. E.; Cosford, N. Tetrahedron 1994, 50, 4399. (26) Kang, Guo-J.; Cullen, W. R.; Fryzuk, M. D.; James, B. R.; Kutney, J. P. J . Chem. SOC.,Chem. Commun. 1988, 1466. (27) Cullen, W. R.; Fryzuk, M. D.; James, B. R.; Kutney, J. P.; Kang, Guo-J.; Herb, G.; Thorburn, I. S.; Spogliarich, R. J . Mol. Catal. 1990, 62, 243. (28) Becalski, A. G.; Cullen, W. R.; Fryzuk, M. D.; James, B. R.; Kang, Guo-J.; Rettig, S. J. Znorg. Chem. 1991, 30, 5002. (29) Ball, G. E.; Cullen, W. R.; Fryzuk, M. D.; Henderson, W. J.; James, B. R.; MacFarlane, K. S. Znorg. Chem. 1994,33, 1464. (30) Spindler, F.; Pugin, B.; Blaser, H.-U. Angew. Chem., Znt. Ed. Engl. 1990, 29, 558. (31)Chan, Y. N. C.; Osborn, J. A. J . A m . Chem. SOC.1990, 112, 9400. (32) Chan, Y. N. C.; Meyer, D.; Osborn, J. A. J . Chem. SOC.,Chem. Commun. 1990,869, (33)Levi. A.: Modena, G.: Scorrano. G. J . Chem. Soc.. Chem. Commun. 1975, 6. (34)Vastag, S.; Bakos, J.;Tdros,, S.;Takach, N. E.; King, R. B.; Heil, B.; Mark6, L. J . Mol. Catal. 1984,22, 283. (35)Kaean. H. B.:. Lanelois. Chem. . N.:. Dane. -. T. P. J . Orpanomet. 1975, 90, 553: (36)Bakos, J.; T6th, I.; Heil, B.; Mark6, L. J . Organomet. Chem. 1985, 279, 23. (37) Bakos, J.; T6th, I.; Heil, B.; Szalontai, G.; Pbrklnyi, L.; Fulop, V. J . Organomet. Chem. 1989,370, 263. (38) Lensink, C.; de Vries, J. Tetrahedron: Asymmetry 1992,3,235. (39) Brunner, H.; Huber, C. Chem. Ber. 1992, 125, 2085. (40) Lensink, C.; de Vries, J. G. Tetrahedron: Asymmetry 1993, 4, 215.
0276-733319512314-4209$09.00100 1995 American Chemical Society
4210 Organometallics, Vol. 14, No. 9, 1995
Zhou et al.
Table 1. Solvent Effects on the Hydrogenation of N-Benzylidenemethylamineby 1" entry no. 1
2 3
solvent MeOH CHzC12 CHZClmeOH (vlv 5/51)
4 5 6
DMEe benzene THI"
7 8 9
THFNeOH (v/v 9/1) THF/MeOH(v/v 5/5) THFNeOH (v/v 9/1Y
P(H2) (psi) 300 300 100 300 600 300 300 100 200 300 200 200 200
conversion i%)* e1
8 57c
79 > 99 (84d)
8 3 78 94 97 (god) '99 i94d) 8 55
a Reaction conditions: substrate, 3.4 mmol; (q6-PhBPh3)-Rh+(l,5COD), 1.2 mol %; mol of RWmol of DPPB, 111; temperature, 40 "C; solvent, 10 mL. Unless otherwise described reaction times are 24.5 i 0.5 h. Determined by 'H NMR with dibenzyl ether as the internal reference, N-benzylmethylamine was the only product. Time, 50 h. Isolated yield. e DME = 1,2-dimethoxyethane;THF = tetrahydrofuran. f Mol of RWmol of DPPB, 1/2.
dium(1) complexes being the most numerous. To our knowledge, there are no publications or patents on the use of zwitterionic rhodium complexes for the reduction of imines. We now wish to report that the zwitterionic complex 1is an excellent catalyst for converting imines to amines.
Results and Discussion It is well-known43that homogeneous hydrogenation is very sensitive to the solvent used in the reaction. Thus, solvent effects in the hydrogenation of N-benzylidenemethylamine (Table 1)were examined using H2 (300 psi), a catalytic amount of 1(1.2 mol %), and DPPB (Rh/DPPB 1/11 at 40 "C for 24.5 h. The conversion of N-benzylidenemethylamine to N-benzylmethylamine was very low when methanol, dichloromethane, dimethoxyethane (DME), o r benzene was used as the only solvent (Table 1, entries 1, 2, 4, and 5). The low conversions in methanol and DME were possibly due to the lack of solubility of the zwitterionic rhodium complex 1 in these solvents. The reaction conversion increased dramatically when methanol and dichloromethane were combined as the solvent for the hydrogenation reaction. Although tetrahydrofuran was not considered to be a good solvent for rhodium(1)complexTHF catalyzed hydrogenation of imines,14,24,26-28,34,35 was an excellent solvent for the zwitterionic rhodiumcatalyzed hydrogenation of N-benzylidenemethylamine (cf. Table 1, entry 6 ) , giving N-benzylmethylamine in 97% conversion (90%isolated yield). Addition of a small amount of methanol (9/1 THF/MeOH) to the reaction mixture resulted in a slight increase in imine conversion ('99% versus 94% a t P(H2) = 200 psi; Table 1, entries 6 and 71, which was confirmed by time dependent experiments. The conversion on N-benzylidenemethylamine, however, decreased to less than 10%when THFI MeOH (v/v 5/51 was used as the solvent, presumably due to decreased solubility of 1. Therefore, THF/MeOH (41)Oppolzer, W.; Wills, M.; Starkemann, C.; Bernardinelli, G. Tetrahedron Lett. 1990,31, 4117. (42) Bakos, J.; Orosz, A,; Heil, B.; Laghmari, M.; Lhoste, P.; Sinou, D.J . Chem. SOC.,Chem. Commun. 1991,1684. (43)James, B. R. In Homogeneous Hydrogenation; John Wiley &
Sons: New York, 1973.
(v/v 9/11 was chosen as the solvent system for this investigation. In the absence of an added phosphine ligand, rhodium black was formed during the reaction and the aromatic ring in the produced amine was further hydrogenated to give the saturated amine. The latter process could be inhibited by addition of 1equiv of DPPB. Note that addition of 2 equiv of DPPB significantly reduced the conversion of imine to amine (55% in 24.5 h under the same conditions shown in Table 1, entry 9). The hydrogenation of imines, catalyzed by 1 and added DPPB (100/1/1ratio of imine/l/DPPB) in 9/1 THFI MeOH, is sensitive to both electronic and steric effects, and the results of reactions of a variety of aldimines and ketimines are presented in Table 2. Reaction of Nbenzylidene-n-butylamine at 40 "C and 400 psi of H2 for 26 h afforded N-benzyl-n-butylaminein 98% conversion and 95% isolated yield (Table 2, entry 2). The reactivity of imines, and the yield of amines, decreased with an increase in the effective bulk of the N-substituent from n-butyl to sec-butyl to tert-butyl (Table 2, entries 2-4). Substitution of a hydrogen atom of the methyl group by phenyl (Ph-CH=N-CH2-Ph; Table 2, entry 5) resulted in a lower yield at 200 psi of H2, but dibenzylamine was isolated in 84% yield using 400 psi of H2. Under identical conditions, the methoxypropylimine (Table 2, entry 6 ) was also hydrogenated in high yield. N-Benzylideneaniline, containing an N-Ph group, was much less active than the N-Me analog, and reasonable yield of amine was only realized at higher H:! pressure (600 psi; Table 2, entries 7 and 8). Alkyl aldimines and ketimines were hydrogenated in good to quantitative yield (Table 2, entries 9-13). The aryl alkyl ketimine PhC(CH3)=NC4Ng gave the amine in 47% isolated yield using 600 psi of H2, and use of 1500 psi of HZincreased the yield by 19%(Table 2, entry 14). It was reported that RhCl(PPh& and [Rh(PPh& (NBDIIPF6could hydrogenate aldimines a t 25 "C and 1 atm of H2 in neat alcohol as the s01vent.l~No examples were presented, however, for the use of these catalysts in the hydrogenation of ketimines under the same conditions. Of note, the alcohol solvent was essential for activity in these Rh-based systems14 and for other effective imine hydrogenation catalysts;ls a possible chemical role for a coordinated alcohol, in providing H-bonded stabilization of a putative, coordinated r2imine moiety, has been suggested,14J8but clearly the data of Table 1 show that systems can be effective in the absence of alcohol. The zwitterionic rhodium complex 1-DPPB system shows comparable reactivity toward the hydrogenation of aromatic ketimines as rhodium(1)catalysts reported previously, such as [Rh(diene)Cll:!-P-P (P-P = chelating phosphines)26,28,34,36,38~42 and [Rh(NBD)(BDPP)I+(BDPP = 2,4-bis(diphenylphosp h i n ~ l p e n t a n e ) .However, ~~ the 1-DPPB catalyst system possesses higher reactivity for the hydrogenation of alkyl ketimines than the in situ produced cationic [Rh(diene)Cll~-P-P catalysts.26,28 In conclusion, the zwitterionic rhodium complex 1is an excellent catalyst for the hydrogenation of both aldimines and ketimines. The reaction is simple in execution and workup.
Hydrogenation of Imines
Organometallics, Vol. 14, No. 9, 1995 4211
Table 2. (~6-PhBPh~)-Rh+(1,5-COD)-Catalyzed Hydrogenation of Iminesa P(Hd (psi)
time (h)
conversion (yield, %y
200
25
>99 (94)
200 400
25 26
92 (71) 98 (95)
400
48
87 (79)
400
76
30 (27)
200 400
25 27
68 92 (84)
6
400
25
94 (86)
7
200 600
49 25
17 47 (40)
8
600
96
66 (53)
= C H ~ - N H - C H I ~
400
25
87 (78)
10
nCH2-NH-(CHz)3Me
400
18
'99 (93)
11
I nCH-NH-(CH2)3Me
600
68
96 (86)
12
600
20
>99 (94)
13
600
25
'99 (89)d
14
600 1500
96 48
53 (47) 70 (66)
entry no.
substrate
1
0
2
O
3
eCH=N-?H-CH2-Me
4
-
CH= N Me
C
H
=N-(CHz)3Me
Me Me e C H = N - ? - M eI
productb OCHZ-NH-Me
-
~ C H Z - N H - ( C H ~ ) ~ M ~
-
e C H z - N H - C H - C H 2I- M e Me Me eCH2-NH-C-Me
Me
5
I I
Me oCH2-NH-CHp
9
Me
lire
Reaction conditions: substrate, 4.0 i 0.2 mmol; catalyst, 1.0 mol %; DPPB, 1.0 mol %; THF/MeOH (v/v 911, 10 mL); temperature, 40 "C. Products were characterized by comparison of spectral data (IR, NMR ('H,I3C), MS) with literature results. c Conversions were determined by 'H NMR with dibenzyl ether or triphenylmethane as the internal reference; yields were isolated yields of purified products. Use of [Rh(COD)zI+OWinstead of the zwitterionic complex, under otherwise identical conditions, afforded the hydrogenation product in 3% yield. Also, use of [(DPPB)Rh(COD)lfBF4-or [(DPPB)Rh(COD)I+BPh4-as a substitute for the zwitterionic complex and DPPB, under otherwise identical conditions, gave the reduction products in 20% and 23% yield, respectively.
Experimental Section General Considerations. All 'H and 13C NMR spectra were recorded on a Varian 200 MHz Gemini spectrometer using CDC13 as the solvent. Mass spectra were obtained on a VG 7070 E mass spectrometer. Infrared spectra were run on a Bomem MB-100 FT-IR spectrometer. Solvents were dried and purified by standard methods. Column chromatography was performed with Merck Silica gel 60 (70-230 or 230-400 mesh) using solvent combinations determined via initial TLC analysis with Merck Silica gel 60 F254 plates (precoated). N-Benzylidenemethylamine was purchased from Aldrich and used as received. All other chemicals used for making imines and catalyst were purchased from Aldrich, Lancaster, or Strem chemical companies and used as received. All imines, prepared a t ambient temperature (aldimines) or at 70 "C (ketimines) using procedures described previousl3p4 and purified by Kugelrohr (bulb t o bulb) distillation under reduced pressure, were characterized by NMR PH, I3C), IR, and MS methods and compared with literature data, except for N-(lcyclohexylethy1idene)-n-butylamine(Cy(Me)C=N-C4H9), 'H NMR (CDC13, G(TMS) 0.00 ppm): major isomer, 3.22 (t, 2H, N-CHz), 2.15 (m, lH, CH), 1.74 (s, 3H, CHs-C=N), 1.851.50 (m, 7H), 1.45-1.10 (m, 7H), 0.93 (t, 3H, CHzCH3); minor (44)Taguchi, K.; Westheimer, F. H. J . Org. Chem. 1971,36, 1570.
isomer, 3.29 ( t ,2H, N-CHz), 1.91 (s, 3H, CHs-C=N), all other resonances are obscured by those of the major isomer. I3C NMR (CDC13, G(CDC13) 77.00 ppm, major isomer): 173.19 (C=N), 50.88 (N-CHz), 50.69 (CHI, 33.01 (CHz), 30.14 (2CHz), 26.11 (2CHz), 26.01 (CHz), 20.69 (CHz), 14.72 (CH31, 14.01 (CH3). IR v(C=N) 1656 cm-l (neat). MS: MC, 181 (4). Bp (Kugelrohr distillation): 60 "C/0.5 mmHg. Anal. Calcd for C ~ ~ H Z ~C,N79.49; : H, 12.78; N, 7.72. Found: C, 79.23; H, 12.65; N, 7.81. (q6-PhBPh3)-Rh+(l,5-COD) was synthesized according to the literature procedure.'
General Procedure for the Hydrogenation Reactions. To a 45-mL Parr autoclave fitted with a glass liner and stirring bar was added (q6-PhBPh3)-Rhf(l,5-COD) (0.04 mmol), substrate (4.0 mmol), 1,4-bis(diphenylphosphino)butane(DPPB) (0.04 mmol), dry THF (9 mL), and methanol (1 mL). The Hz line was flushed three times with Hz, the autoclave was fillvented three times with Hz t o displace the air, and subsequently the pressure was increased to the desired level with Hz. The mixture was stirred in the autoclave a t 40 "C (oil bath temperature, see Table 2 for reaction time). The excess Hz was released, the system was disassembled, and the solvent was removed from the reaction mixture by rotary evaporation. "he residues were analyzed by 'H NMR. In those cases where reactions were incomplete, a n internal reference (either dibenzyl ether or triphenylmethane) was added and the percent
4212 Organometallics, Vol. 14, No. 9,1995 conversion was determined by 'H NMR. The reaction mixture was separated by silica gel column chromatography using 41 (v/v) n-pentanelether a s the eluant followed by ether and then acetone. The product was purified further by Kugelrohr distillation under reduced pressure. All products were characterized spectroscopically (NMR (IH, I3C), IR, and MS) and compared with literature data, except for N-butyl-(1-cyclohexy1ethyl)amine (Cy(Me)CH-NH-CdHg). lH NMR (CDC13, S(TMS) 0.00 ppm): 2.72-2.42 (m, 3H, CH-N-CHz), 1.821.55 (m, 5H), 1.53-1.00 (m, 11H), 0.97 (d, 3H, CHs-CH), 0.95 (t, 3H, CH2CH3). 13C NMR (CDC13, G(CDC13) 77.00 ppm): 57.82 (CH-N),47.32 (N-CHz),42.82 (CH),32.56 (CH2),29.96 (CHz),27.86 ( C H z ) , 26.74 (CHz),26.63 ( C H 2 ) , 26.47 (CH2), 20.55
Zhou et al. ( C H z ) , 16.72 (CH3), 13.99 ( C H 3 ) . IR, v(NH) 3312 cm-' (neat). MS: (M HI+, 184 (100). Bp (Kugelrohr distillation): 65 "C/ 0.5 mmHg. Anal. Calcd for C12H25N: C, 78.62; H, 13.74; N, 7.64. Found: C, 78.90; H, 13.85; N, 7.97.
+
Acknowledgment. We are indebted to the Natural Sciences and Engineering Research Council of Canada for the award of a collaborative project grant in support of this research. We thank Johnson Matthey for a loan of some RhC13-3H20. OM9502087
