Enantioselective cyclization/hydrosilylation of 1,6-enynes catalyzed by a cationic rhodium bis(phosphine) complex

Article abstract of DOI:10.1021/ol027196n

(Matrix presented) Reaction of 4,4-dicarbomethoxy-1-octene-6-yne (1) with triethylsilane and a catalytic 1:1 mixture of [Rh(COD)₂]⁺ SbF₆^- and (R)-BIPHEMP (5 mol %) at 70°C for 90 min gave (Z)-1,1-dicarbomethoxy-

Full text of DOI:10.1021/ol027196n

ORGANIC
LETTERS
2003
Vol. 5, No. 2
157-159
Enantioselective Cyclization/
Hydrosilylation of 1,6-Enynes Catalyzed
by a Cationic Rhodium Bis(phosphine)
Complex
Harinath Chakrapani, Cong Liu, and Ross A. Widenhoefer*
P. M. Gross Chemical Laboratory, Duke UniVersity,
Durham, North Carolina 27708-0346
rwidenho@chem.duke.edu
Received October 29, 2002
ABSTRACT
Reaction of 4,4-dicarbomethoxy-1-octene-6-yne (1) with triethylsilane and a catalytic 1:1 mixture of [Rh(COD)2]+ SbF6- and (R)-BIPHEMP (5
mol %) at 70 °C for 90 min gave (Z)-1,1-dicarbomethoxy-3-(1-triethylsilyl)ethylidene-4-methylcyclopentane (2) in 81% isolated yield with 98% de
and 92% ee.
Substituted carbocycles represent one of the most prevalent
structural features of naturally occurring and biologically
active compounds.1 For this reason, considerable effort has
been directed toward the development of new and effective
methods for the synthesis of functionalized carbocycles. In
this area, transition metal-catalyzed approaches have shown
particular utility due to the high levels of selectivity and
efficiency often realized by transition metal catalysis.2 How-
ever, enantioselective transformations represent only a small
subset of the known transition metal-catalyzed cyclization
reactions,3 which is unfortunate given the propensity of the
naturally occurring carbocycles to display optical activity.
Cyclization/hydrosilylation of dienes,4,5 enynes,6 and
diynes7,8 has emerged as an effective route to the synthesis
of functionalized carbocycles. However, as is the case with
catalytic cyclization reactions in general, examples of asym-
metric cyclization/hydrosilylation remain quite limited. In
fact, the only examples of asymmetric cyclization/hydrosi-
lylation are the cyclization/hydrosilylation of 1,6-dienes
catalyzed by palladium pyridine-oxazoline complexes
(Scheme 1).4 Unfortunately, the cyclopentanes generated via
asymmetric diene cyclization/hydrosilylation are relatively
unfunctionalized. Because of this, we sought to develop an
effective procedure for the cyclization/hydrosilylation of
enynes to form the more functionalized alkylidenecyclopen-
tanes. Here we report the first examples of the asymmetric
Scheme 1
(1) (a) Hudlicky, T.; Price, J. D. Chem. ReV. 1989, 89, 1467. (b) Trost,
B. M. Chem. Soc. ReV. 1982, 11, 141.
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49. (c) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259.
(3) (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley:
New York, 1994. (b) Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I.,
Ed.; Wiley-VCH: New York, 2000.
(4) Widenhoefer, R. A. Acc. Chem. Res. 2002, 35, 905.
10.1021/ol027196n CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/24/2002

Table 1. Asymmetric Cyclization/Hydrosilylation of 1,6-Enynes Catalyzed by a 1:1 Mixture of [Rh(COD)2]+ [SbF6]- and
(R)-BIPHEMP (5 mol %) in DCE at 70 °C
cyclization/hydrosilylation of functionalized enynes to form
silylated alkylidenecyclopentanes with up to 92% ee.
We have recently shown that cationic rhodium (()-BINAP
complexes catalyze the cyclization/hydrosilylation of 1,6-
diynes to form silylated 1,2-dialkylidienecyclopentanes.8
In addition, enantiomerically enriched cationic rhodium
(BINAP) complexes catalyze a number of highly enantiose-
lective cyclization reactions, including intramolecular olefin
hydrosilylation,9 intramolecular olefin hydroacylation,10 and
enyne cycloisomerization.11 For these reasons, we targeted
cationic rhodium BINAP complexes as catalysts for asym-
metric enyne cyclization/hydrosilylation. Although rhodium
BINAP complexes were not effective catalysts for enyne
cyclization/hydrosilylation, the closely related BIPHEMP
[BIPHEMP ) 6,6′-bis-(diphenylphosphino)-2,2′-dimethyl-
biphenyl] complexes proved effective. For example, treat-
ment of 4,4-dicarbomethoxy-1-octene-6-yne (1) with trieth-
-
ylsilane and a catalytic 1:1 mixture of [Rh(COD)2]+ SbF6
and (R)-BIPHEMP (5 mol %) at 70 °C for 90 min led to the
isolation of the silylated alkylidene cyclopentane 2 in 81%
yield with 98% de and 92% ee (Scheme 2).
(5) (a) Molander, G. A.; Nichols, P. J. J. Am. Chem. Soc. 1995, 117,
4415. (b) Molander, G. A.; Nichols, P. J.; Noll, B. C. J. Org. Chem. 1998,
63, 2292. (c) Molander, G. A.; Retsch, W. H. J. Org. Chem. 1998, 63,
5507. (d) Onozawa, S.; Sakakura, T.; Tanaka, M. Tetrahedron Lett. 1994,
35, 8177.
Scheme 2
(6) (a) Ojima, I.; Donovan, R. J.; Shay, W. R. J. Am. Chem. Soc. 1992,
114, 6580. (b) Muraoka, T.; Matsuda, I.; Itoh, K. Tetrahedron Lett. 1998,
39, 7325. (c) Ojima, I.; Vu, A. T.; Lee, S.-L.; McCullagh, J. V.; Moralee,
A. C.; Fujiwara, M.; Hoang, T. H. J. Am. Chem. Soc. 2002, 124, 9164. (d)
Molander, G. A.; Retsch, W. H. J. Am. Chem. Soc. 1997, 119, 8817.
(7) (a) Madine, J. W.; Wang, X.; Widenhoefer, R. A. Org. Lett. 2001,
3, 385. (b) Wang, X.; Chakrapani, H.; Madine, J. W.; Keyerleber, M. A.;
Widenhoefer, R. A. J. Org. Chem. 2002, 67, 2778. (c) Tamao, K.;
Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 6478. (d) Tamao, K.;
Kobayashi, K.; Ito, Y. Synlett 1992, 539.
(8) Liu, C.; Widenhoefer, R. A. Organometallics 2002, 21, 5666.
(9) Bergens, S. H.; Noheda, P.; Whelan, J.; Bosnich, B. J. Am. Chem.
Soc. 1992, 114, 2121.
(10) Barnhart, R. W.; Wang, X.; Noheda, P.; Bergens, S. H.; Whelan,
J.; Bosnich, B. J. Am. Chem. Soc. 1994, 116, 1821.
158
Org. Lett., Vol. 5, No. 2, 2003

3). Oxidative addition of the H-Si bond of the silane to a
Rh(I) species could form the Rh(III) silyl hydride species I.
Coordination and â-migratory insertion of the triple bond
of the enyne into the Rh-Si bond of I could form the
rhodium alkenyl complex II. Coordination of the pendant
olefin followed by â-migratory insertion into the Rh-C bond
of alkenyl olefin complex III could form the rhodium alkyl
complex IV. Formal C-H reductive elimination from IV,
coupled with H-Si oxidative addition, could release the
silylated alkylidenecyclopentane with regeneration of the
cationic Rh(I) complex I (Scheme 3).
Scheme 3
In summary, we have presented the first examples of
asymmetric enyne cyclization/hydrosilylation catalyzed by
a cationic, rhodium (R)-BIPHEMP complex. We are cur-
rently working toward the identification of more active and
more stereoselective enyne cyclization/hydrosilylation cata-
lysts.
Acknowledgment. Acknowledgment is made to the
National Institutes of Health (GM59830-01) for support of
this research. R.W. thanks Dupont, the Alfred P. Sloan
Foundation, the Camille and Henry Dreyfus Foundation, and
GlaxoSmithKline for additional financial support. We thank
Dr. Rudolf Schmid (Hoffmann-La Roche) for a generous gift
of (R)-BIPHEMP and Ms. Michele A. Keyerleber for
performing some preliminary experiments.
-
Mixtures of [Rh(COD)2]+ SbF6 and (R)-BIPHEMP
catalyzed the cyclization/hydrosilylation of enyne 1 with a
number of tertiary silanes to form silylated alkylidene
cyclopentanes 3-8 in 70-76% yield with 77-89% ee (Table
1, entries 1-6). In addition to enyne 1, several 1,6-enynes
underwent rhodium-catalyzed asymmetric cyclization/
hydrosilylation to generate the silylated alkylidenecyclo-
pentanes 13-16 in moderate yield with g80% ee (Table 1,
entries 7-10). Asymmetric enyne cyclization/hydrosilylation
was also applied to the synthesis of silylated pyrrolidine
derivative 18 (Table 1, entry 11).
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for new compounds and
determination of enantiomeric excess. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL027196N
On the basis of the proposed mechanism for rhodium-
catalyzed alkyne hydrosilylation12 and diyne cyclization/
hydrosilylation,6c,8 we propose a working mechanism for
rhodium-catalyzed enyne cyclization/hydrosilylation (Scheme
(12) (a) Takeuchi, R.; Nitta, S.; Watanabe, D. J. Org. Chem. 1995, 60,
3045. (b) Takeuchi, R.; Ebata, I. Organometallics 1997, 16, 3707. (c)
Takeuchi, R.; Nitta, S.; Watanabe, D. J. Chem. Soc., Chem. Commun. 1994,
1777. (d) Takeuchim R.; Tanouchi, N. J. Chem. Soc., Perkin Trans. 1 1994,
2909. (e) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics
1990, 9, 3127.
(11) Cao, P.; Zhang, X. Angew. Chem., Int. Ed. 2000, 39, 4104.
Org. Lett., Vol. 5, No. 2, 2003
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