P-chiral bis(trialkylphosphine) ligands and their use in highly enantioselective hydrogenation reactions [9]

Full text of DOI:10.1021/ja973423i

J. Am. Chem. Soc. 1998, 120, 1635-1636
P-Chiral Bis(trialkylphosphine) Ligands and Their
1635
Use in Highly Enantioselective Hydrogenation
Reactions
Tsuneo Imamoto,*,† Junko Watanabe,† Yoshiyuki Wada,‡
Hideki Masuda,‡ Hironari Yamada,† Hideyuki Tsuruta,†
Satoru Matsukawa,† and Kentaro Yamaguchi§
Figure 1.
Department of Chemistry, Faculty of Science and
Chemical Analysis Center, Chiba UniVersity
Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Material R & D Laboratories, Ogawa and Co., Ltd.
Shoo-cho, Katsuta, Okayama 709-4321, Japan
Scheme 1
ReceiVed October 1, 1997
Optically active phosphines play a most important role as the
chiral ligands in various metal-catalyzed asymmetric reactions,
and numerous chiral phosphines have been designed and syn-
thesized over the past three decades.1 Among them, some P-chiral
phosphines such as (R,R)-1,2-bis[(o-methoxyphenyl)phenylphos-
phino]ethane (DIPAMP) were landmark discoveries at an early
stage in the history of asymmetric hydrogenation reactions.2,3
Thereafter, however, relatively less attention has been paid to
P-chiral phosphine ligands in the field of asymmetric catalysis.4
This is largely ascribed not only to the synthetic difficulty of
highly enantiomerically enriched P-chiral phosphines but also to
the fact that this class of phosphines, especially diaryl- and
triarylphosphines, are configurationally unstable and gradually
racemize at high temperatures.5
On the other hand, optically active trialkylphosphines are
known to hardly racemize even at considerably high temperature.6
On the basis of this fact, we designed a new class of P-chiral
phosphine ligands, 1,2-bis(alkylmethylphosphino)ethanes (alkyl
) tert-butyl, 1,1-diethylpropyl, 1-adamantyl, cyclopentyl, cyclo-
hexyl) (abbreviated as BisP*) (Figure 1).7 An important feature
of these ligands is that a bulky alkyl group and the smallest alkyl
group (methyl group) are bonded to each phosphorus atom. The
ligands would form five-membered C2-symmetric chelates, and
this imposed asymmetric environment might lead to high enan-
tioselectivity in asymmetric reactions. It is also anticipated that
these electron-rich trialkylphosphine ligands provide very high
catalytic efficiencies in transition-metal-catalyzed homogeneous
hydrogenations.8,9
The preparation of the designed P-chiral phosphine ligands and
their Rh-complexes has been accomplished using phosphine
boranes as the intermediates (Scheme 1).10 Evans et al. reported
that aryldimethylphosphine boranes were subjected to enantio-
selective deprotonation by a s-BuLi-(-)-sparteine complex,
followed by oxidative coupling using Cu(OPiv)2, to afford C2-
symmetric bisphosphine boranes in excellent enantiomeric ex-
cesses (ee), but they did not describe the reactions of alkyldi-
methylphosphine boranes.11 We applied their method to alkyl-
dimethylphosphine boranes 1a-e which were synthesized in one
pot from phosphorus trichloride. The reactions of 1a-c afforded
highly enantiomerically enriched C2-symmetric bisphosphines
2a-c along with minor amounts of the meso diastereomers.12
On the other hand, compounds 1d and 1e provided the corre-
sponding C2-symmetric products 2d and 2e with relatively low
† Department of Chemistry, Faculty of Science.
‡ Ogawa Co., Ltd.
§ Chemical Analysis Center.
(1) For representative reviews, see the following: (a) Noyori, R. Asymmetric
Catalysis in Organic Synthesis; Wiley & Sons: New York, 1994; Chapter 2.
(b) Ojima, I., Ed. Catalytic Asymmetric Synthesis; VCH Publishers: Weinheim,
1993; Chapter 1. (c) Koenig, K. E. Applicability of Asymmetric Homogeneous
Catalytic Hydrogenation. In Asymmetric Synthesis; Morrison, J. D., Ed.;
Academic Press: New York, 1985; Vol. 5, Chapter 3.
(2) Horner, L.; Siegel, H.; Buthe, H. Angew. Chem., Int. Ed. Engl. 1968,
7, 942.
(3) (a) Knowles, W. S.; Sabacky, M. J. J. Chem. Soc., Chem. Commun.
1968, 1445. (b) Knowles, W. S.; Sabacky, M. J.; Vineyard, B. D. J. Chem.
Soc., Chem. Commun. 1972, 10. (c) Knowles, W. S.; Sabacky, M. J.; Vineyard,
B. D.; Weinkauff, D. J. J. Am. Chem. Soc. 1975, 97, 2567. (d) Vineyard, B.
D.; Knowles, W. S.; Sabacky, M. J.; Bachman, G. L.; Weinkauff, D. J. J.
Am. Chem. Soc. 1977, 99, 5946. (e) Knowles, W. S. Acc. Chem. Res. 1983,
16, 106.
(4) (a) Roberts, N. K.; Wild, S. B. J. Am. Chem. Soc. 1979, 101, 6254. (b)
Horner, L. Pure Appl. Chem. 1980, 52, 843. (c) Knowles, W. S.; Christopfel,
W. C.; Koenig, K. E.; Hobbs, C. F. AdV. Chem. Ser. 1982, 196, 325. (d)
Yoshikuni, T.; Bailar, J. C., Jr. Inorg. Chem. 1982, 21, 2129. (e) Horner, L.;
Simons, G. Z. Naturforsch. 1984, 396, 512. (f) Allen, D. G.; Wild, S. B.;
Wood, D. L. Organometallics 1986, 5, 1009. (g) Johnson, C. R.; Imamoto,
T. J. Org. Chem. 1987, 52, 2170. (h) Burgess, K.; Ohlmeyer, M. J.; Whitmire,
K. H. Organometallics 1992, 11, 3588. (i) Corey, E. J.; Chen, Z.; Tanoury,
G. J. J. Am. Chem. Soc. 1993, 115, 11000. (j) Nagel, U.; Krink, T. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1052. (k) Imamoto, T.; Tsuruta, H.; Wada,
Y.; Masuda, H.; Yamaguchi, K. Tetrahedron Lett. 1995, 36, 8271. (l)
Brenchley, G.; Fedouloff, M.; Mahon, M. F.; Molloy, K. C.; Wills, M.
Tetrahedron 1995, 51, 10581. (m) Brenchley, G.; Fedouloff, M.; Merifield,
E.; Wills, M. Tetrahedron: Asymmetry 1996, 7, 2809. (n) Vedejs, E.; Donde,
Y. J. Am. Chem. Soc. 1997, 119, 9293.
(7) A similar bisphosphine ligand, (S,S)-1,2-bis(methylphenylphosphino)-
ethane, was previously synthesized and employed as a chiral ligand in rhodium-
catalyzed asymmetric hydrogenation of N-benzoyl-R-aminocinnamic acid.4e
The enantioselectivity of the reduction was reported to be 22% ee.
(8) Burk and co-workers have demonstrated that Rh and Ru catalysts
bearing C2-symmetric bis(trialkylphosphines), 1,2-bis(trans-2,5-dialkylphos-
pholano)ethanes (BPE), exhibit not only very high enantioselectivities but also
exceedingly high catalytic efficiencies in asymmetric hydrogenations of various
olefinic and carbonyl substrates. (a) Burk, M. J.; Feaster, J. E.; Harlow, R. L.
Organometallics 1990, 9, 2653. (b) Burk, M. J. J. Am. Chem. Soc. 1991, 113,
8518. (c) Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L. J. Am.
Chem. Soc. 1993, 115, 10125. (d) Burk, M. J.; Harper, T. G. P.; Kalberg, C.
S. J. Am. Chem. Soc. 1995, 117, 4423. (e) Burk, M. J.; Gross, M. F.; Martinez,
J. P. J. Am. Chem. Soc. 1995, 117, 9375. (f) Burk, M. J.; Wang, Y. M.; Lee,
J. R. J. Am. Chem. Soc. 1996, 118, 5142.
(5) (a) Pietrusiewicz, K. M.; Zablocka, M. Chem. ReV. 1994, 94, 1375. (b)
Imamoto, T. In Handbook of Organophosphorus Chemistry; Engel, R., Ed.;
Marcel Dekker: New York, 1992; Chapter 1. (c) Kagan, H. B.; Sasaki, M. In
Chemistry of Organophosphorus Compounds; Hartley, F. R., Ed.; Wiley &
Sons: New York, 1990; Vol. 1, Chapter 3. (d) Valentine, D., Jr. In Asymmetric
Synthesis; Morrison, J. D., Scott, J. W., Eds.; Academic Press: New York,
1984; Vol. 4, Chapter 3.
(9) Inoguchi, K.; Sakuraba, S.; Achiwa, K. Synlett 1992, 169.
(10) (a) Imamoto, T.; Kusumoto, T.; Suzuki, N.; Sato, K. J. Am. Chem.
Soc. 1985, 107, 5301. (b) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto,
T.; Sato, K. J. Am. Chem. Soc. 1990, 112, 5244. (c) Juge, S.; Stephan, M.;
Laffitte, J. A.; Genet, J. P. Tetrahedron Lett. 1990, 31, 6357. (d) Yang, H.;
Lugan, N.; Mathieu, R. Organometallics 1997, 16, 2089.
(11) Muci, A. R.; Campos, K. R.; Evans, D. A. J. Am. Chem. Soc. 1995,
117, 9075.
(6) Baechler, R. D.; Mislow, K. J. Am. Chem. Soc. 1970, 92, 3090.
S0002-7863(97)03423-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/06/1998

1636 J. Am. Chem. Soc., Vol. 120, No. 7, 1998
Communications to the Editor
Table 1. Rh-Catalyzed Enantioselective Hydrogenation of
R-(Acylamino)acrylic Derivatives (Eq 1)
cat.
cat.
en- pre- sub-
en- pre- sub-
sol-
% ee
sol-
% ee
trya cursor strate vent (config)b trya cursor strate vent
(config)b
1
2
3
4
5
6
7
8
9
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
5a MeOH 99.9c (R) 12
5b MeOH 98.4d (R) 13
5c MeOH 97.7c (R) 14
5d MeOH 95.9d (R) 15
5e MeOH 99.8e (R) 16
5f MeOH 97.3f (R) 17
5g MeOH 98.1g (R) 18
5h MeOH 83.6g (R) 19
5h C6H6 84.1g (R) 20
5i C6H6 55.3g (R) 21
5a MeOH 94.7c (R)
4b
4c
4c
4c
4c
4d
4d
4e
4e
4e
5h MeOH 20.1g (R)
5a MeOH 99.9c (R)
5b MeOH 98.6d (R)
5g MeOH >99.9g (R)
5h MeOH 82.4g (R)
5a MeOH 43.0c (R)
5h MeOH 93.0g (R)
5a MeOH 47.1c (R)
5h MeOH 89.3g (R)
5i MeOH 90.9g (R)
10
11
Figure 2. ORTEP drawing of rhodium complex [Rh((S,S)-3a)(nbd)]BF4
-
(4a): the NBD and the BF4 anion are omitted for clarity.
a Reactions were carried out at room temperature and an initial H2
pressure of 2 atm (for substrates 5a-g) or 6 atm (for substrates 5h
and 5i) using the catalyst precursors (4a-e) (0.2 mol %). b Absolute
configurations were confirmed by comparison of sign of optical rotation
and chiral HPLC or GC elution order, with configurationally defined
examples. c The ee (%) values were determined by HPLC using a Daicel
Chiral OJ column. d The ee (%) values were determined by HPLC using
a Daicel Chiral OJ column on the corresponding methyl ester. e The
ee (%) values were determined by HPLC using a Daicel Chiral OD-H
column. f The ee (%) values were determined by HPLC using a Daicel
Chiral OD-H column on the corresponding methyl ester. g The ee (%)
values were determined by chiral capillary GC using Chrompack’s
Chiral-L-Val column (25 m).
enantiomeric purity.13 Enantiomerically pure 2a-e were obtained
by direct recrystallization of the crude products or by recrystal-
lization after removal of the meso diastereomers by HPLC or
flash chromatography. Subsequent removal of the boranato group
was difficult by our conventional method,10a,b even with the use
of highly reactive cyclic amines such as 1,4-diazabicyclo[2.2.2]-
octane or pyrrolidine. An alternative method developed by
McKinstry and Livinghouse was found to be effective for this
transformation.14 Thus, the reactions with trifluoromethane-
sulfonic acid in toluene or tetrafluoroboric acid in dichloro-
methane, followed by treatment with aqueous KOH or K2CO3,
provided the desired bisphosphines 3a-e in high yields.15 These
bisphosphines were converted to the cationic Rh complexes 4a-e
by the reactions with [Rh(nbd)2]BF4.
The crystal structure of rhodium complex 4a was determined
by single-crystal X-ray analysis.16 The ORTEP drawing shown
in Figure 2 clearly confirms the expected C2-symmetric environ-
ment, where the bulky tert-butyl groups occupy the quasi-
equatorial positions and the methyl groups are located at the quasi-
axial positions to form a λ-chelate structure.
at room temperature and an initial H2 pressure of 2 atm in the
presence of the catalyst (0.2 mol %). The reactions proceeded
rapidly and were complete within 0.2-2 h. On the other hand,
the â-disubstituted ones (5h and 5i) required a higher H2 pressure
(6 atm) and a longer time (10-24 h) for completion of the
reactions. These results are summarized in Table 1.
It is noted that catalyst precursors 4a-c exhibited excellent
enantioselectivity in the hydrogenations of â-monosubstituted
derivatives and â-unsubstituted one (entries 1-7, 11, and 13-
15). However, use of these catalysts in the reactions of â-di-
substituted derivatives resulted in disappointingly low enantio-
selectivity (entries 8-10, 12, and 16). On the other hand, 4d
and 4e afforded low enantioselectivities in the reduction of the
â-monosubstituted derivatives (entries 17 and 19). Surprisingly,
these catalysts were effective for the â-disubstituted derivatives
to provide remarkably high enantioselectivity up to 93.0% ee
(entries 18, 20, and 21).17 The “steric matching” between the
ligands (3d and 3e) and the substrate olefins is considered
responsible for these results.
For the purpose of comparison, the rhodium complexes 4a-e
were employed as the catalyst precursors in asymmetric hydro-
genation of R-(acylamino)acrylic derivatives including â-disub-
stituted ones (eq 1). The reductions of â-monosubstituted
In summary, we have prepared a new class of P-chiral
phosphine ligands, 1,2-bis(alkylmethylphosphino)ethanes (3a-
e). The resulting Rh catalysts 4a-e are highly reactive, facilitat-
ing the reduction of R-(acylamino)acrylic derivatives in excellent
enantioselectivity. Further applications of these ligands to other
catalytic asymmetric reactions are in progress in our laboratory.
Acknowledgment. This paper is dedicated to Professor Carl R.
Johnson, Wayne State University, in commemoration of his 60th birthday.
We thank Professor Takao Ikariya, Tokyo Institute of Technology, for
valuable discussions.
Supporting Information Available: Experimental details for prepa-
ration of 3a-e and 4a-e and procedures for hydrogenation and
enantiomeric excess determinations (10 pages). An X-ray crystallographic
file, in CIF format, is available through the Web only. See any current
masthead page for ordering information and Web access instructions.
derivatives (5a-f) and â-unsubstituted one (5g) were carried out
(12) For example, the reaction of 1a afforded C2-symmetric product 2a
(67%, >99% ee) and the meso diastereomer (13%). Enantiomerically pure
2a was obtained by direct recrystallization of the crude product containing
the meso diastereomer from hot toluene one or two times.
(13) For example, the reaction of 1d afforded C2-symmetric product 2d
(46%, 75% ee) accompanied by the meso diastereomer (29%).
(14) (a) McKinstry, L.; Livinghouse, T. Tetrahedron Lett. 1994, 35, 9319.
(b) McKinstry, L.; Livinghouse, T. Tetrahedron 1994, 50, 6145.
(15) 3a-d were obtained as crystalline solids or oil: 3a, mp 23.5-25.5
°C; 3b, oil; 3c, mp 124-126 °C; 3d, oil; 3e, mp 21-23 °C. The absolute
configuration of 3a was determined by the single-crystal X-ray analyses of
its TfOH salt (3a‚2TfOH) and Rh complex 4a. The absolute configuration of
3e was determined by X-ray analysis of Ru complex 3e‚Ru(C4H7)2. The
configurations of other three phosphine ligands 3b-d were estimated by
analogy.
JA973423I
(17) The following are the highest ee values for the enantioselective
hydrogenations of â-disubstituted derivatives. Me-DuPHOS (99.4% ee);8e Me-
BPE (98.6% ee);8e BuTRAP (88% ee);18 DIPAMP (55% ee);19 [2.2]-
PHANEPHOS (51% ee).20
(18) Sawamura, M.; Kuwano, R.; Ito, Y. J. Am. Chem. Soc. 1995, 117,
9602.
(16) Crystal data for 4a: orthorhombic, P212121; a ) 20.500(8), b )
9.826(5), and c ) 11.517(3) Å; V ) 2319(1) Å3; Z ) 4; dcalcd ) 1.478 g
cm-3; F(000) ) 1064; µ(Mo KR) ) 9.06 cm-1; λ(Mo KR) ) 0.71069 Å;
3519 reflections measured, 3482 observed (I > 2.0σ(I)); 244 variables; R )
0.032, Rw ) 0.040, GOF ) 1.49. Flack parameter ) 0.010(1).
(19) Scott, J. W.; Keith, D. D.; Nix, G., Jr.; Parrish, D. R.; Remington, S.;
Roth, G. P.; Townsend, J. M.; Valentine, D., Jr.; Yang, R. J. Org. Chem.
1981, 46, 5086.
(20) Pye, P. J.; Rossen, K.; Reamer, R. A.; Tsou, N. N.; Volante, R. P.;
Reider, P. J. J. Am. Chem. Soc. 1997, 119, 6207.