Silane-promoted cycloisomerization of functionalized 1,6-dienes catalyzed by a cationic (π-allyl)palladium complex

Article abstract of DOI:10.1021/ol9908981

(matrix presented) A 1:1 mixture of the (π-allyl)palladium complex (η³-C₃H₅)Pd(Cl)PCy₃ and NaB[3,5-C₆H₃(CF₃)₂]₄ in the presence of HSiEt₃ catalyz

Full text of DOI:10.1021/ol9908981

ORGANIC
LETTERS
1999
Vol. 1, No. 7
1103-1105
Silane-Promoted Cycloisomerization of
Functionalized 1,6-Dienes Catalyzed by
a Cationic (π-Allyl)palladium Complex
Ross A. Widenhoefer* and Nicholas S. Perch
Duke UniVersity, P. M. Gross Chemical Laboratory,
Durham, North Carolina 27708-0346
rwidenho@chem.duke.edu
Received August 2, 1999
ABSTRACT
A 1:1 mixture of the (π-allyl)palladium complex (η3-C H )Pd(Cl)PCy3 and NaB[3,5-C H (CF3)2]4 in the presence of HSiEt3 catalyzed the
3
5
6 3
cycloisomerization of functionalized 1,6-dienes to form 1,2-disubstituted cyclopentenes in good yield with high selectivity (typically >94%).
The protocol tolerated a range of functional groups and substitution at one of the allylic carbon atoms.
The cycloisomerization of enynes employing Mo,1 Ru,2 Ti,3
and especially Pd4 catalysts has proven an efficient route
toward the synthesis of functionalized carbocycles.5 In
contrast, diene cycloisomerization has remained less devel-
oped. For example, electrophilic d0 Sc6 and Zr7 metallocene
complexes readily catalyze the cycloisomerization of dienes
to form methylenecycloalkanes. However, the utility of these
procedures suffers from the limited functional group compat-
ibility and excessive air and moisture sensitivity of the
oxophilic catalyst. One approach toward ameliorating these
limitations is through employment of a late-transition-metal
catalyst. Unfortunately, early attempts to effect diene cy-
cloisomerization employing a late-transition-metal catalyst
required forcing conditions in an acidic medium.8 However,
the selective conversion of functionalized 1,6-dienes to
methylenecyclopentanes (A) has recently been demonstrated
(1) Trost, B. M.; Tour, J. M. J. Am. Chem. Soc. 1987, 109, 5268.
(2) (a) Chatani, N.; Morimoto, T.; Muto, T.; Murai, S. J. Am. Chem.
Soc. 1994, 116, 6049. (b) Nishida, M.; Adachi, N.; Onozuka, K.; Matsumura,
H.; Mori, M. J. Org. Chem. 1998, 63, 9158.
(3) Sturla, S. J.; Kablaoui, N. M.; Buchwald, S. L. J. Am. Chem. Soc.
1999, 121, 1976.
(4) (a) Trost, B. M.; Lautens, M. J. Am. Chem. Soc. 1985, 107, 1781.
(b) Trost, B. M. Acc. Chem. Res. 1990, 23, 34.
(5) (a) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259. (b)
Trost, B. M.; Krische, M. J. Synlett 1998, 1.
by employing either a Ru9 or a Ni10 catalyst.11 In contrast,
the selective conversion of 1,6-dienes to 1-methyl- or 1,2-
dimethylcyclopentenes (B or C) under mild conditions has
not been demonstrated.12 Here we report a palladium-
catalyzed, silane-promoted protocol for the selective conver-
(6) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990,
74.
(9) Yamamoto, Y.; Ohkoshi, N.; Kameda, M.; Itoh, K. J. Org. Chem.
1999, 64, 2178.
(7) Christoffers, J.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118, 4715.
(8) (a) Grigg, R.; Mitchell, T. R. B.; Ramasubbu, A. J. Chem. Soc., Chem.
Commun. 1980, 27. (b) Grigg, R.; Malone, J. F.; Mitchell, T. R. B.;
Ramasubbu, A.; Scott, R. M. J. Chem. Soc., Perkin Trans. 1 1984, 1745.
(c) Grigg, R.; Mitchell, T. R. B.; Ramasubbu, A. J. Chem. Soc., Chem.
Commun. 1979, 669.
(10) Radetich, B.; RajanBabu, T. V. J. Am. Chem. Soc. 1998, 120, 8007.
(11) See also: Heumann, A.; Moukhliss, M. Synlett 1998, 1211.
(12) Rajanbabu has reported 74% selectivity (91% yield) for the
conversion of dimethyl diallylmalonate to 4,4-dicarbomethoxy-1,2-dimeth-
ylcyclopentene catalyzed by a 1:2:2 mixture of [(η3-C3H5)PdCl]2, AgOTf,
and P(4-C6H4OMe)3.10
10.1021/ol9908981 CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/03/1999

sion of 1,6-dienes to 1,2-disubstituted cyclopentenes within
minutes at room temperature under neutral conditions.
Table 1. Cycloisomerization of 1,6-Dienes Catalyzed by a 1:1
Mixture of (η3-C3H5)Pd(Cl)PCy3 (1c) and NaBAr4 (Ar )
3,5-C6H3(CF3)2; 1d) (5 mol %) in CH2Cl2 Containing HSiEt3 for
15 min at 25 °C
We recently reported several related procedures for the
cyclization/hydrosilylation of 1,6- and 1,7-dienes catalyzed
by cationic palladium phenanthroline and related com-
plexes.13 These complexes were initially targeted as diene
cyclization catalysts due to their high reactivity with respect
to olefin â-migratory insertion and/or σ-bond metathesis.14,15
Similarly, the cationic (π-allyl)palladium complex [(η3-
C3H5)Pd(OEt2)PCy3]+[BAr4]- (Ar ) 3,5-C6H3(CF3)2; 1) is
reactive toward olefin insertion and catalyzes the dimerization
of functionalized olefins.16 We therefore considered that 1
might also serve as a diene cyclization/hydrosilylation
catalyst. However, reaction of dimethyl diallylmalonate (2)
and triethylsilane (1.5 equiv) in the presence of 1 [generated
in situ from a 1:1 mixture of (η3-C3H5)Pd(Me)PCy3 (1a) and
HBAr4‚OEt2 (1b);16 5 mol %] at room temperature for 10
min gave none of the expected cyclization/hydrosilylation
product and instead yielded 4,4-dicarbomethoxy-1,2-di-
methylcyclopentene (3) as the exclusive product in 89% yield
and >97% isomeric purity (Scheme 1).
Scheme 1
Halide abstraction from the palladium chloride complex
(η3-C3H5)Pd(Cl)PCy3 (1c) with NaBAr4 (1d) also generated
an active cycloisomerization catalyst which converted 2 to
3 with a rate, yield, and selectivity comparable to the catalyst
generated from 1a and 1b (Table 1, entry 1).17,18 In addition,
the halide abstraction route to catalyst generation benefited
from the enhanced thermal stability and greater availability
a Yield refers to isolated material of >95% purity. b Determined by
capillary GC. c Minor isomer was 1,1-dicarbomethoxy-3-ethylidene-4-
methylcyclopentane.
(13) (a) Widenhoefer, R. A.; DeCarli, M. A. J. Am. Chem. Soc. 1998,
120, 3805. (b) Stengone, C. N.; Widenhoefer, R. A. Tetrahedron Lett. 1999,
40, 1451. (c) Perch, N. S.; Widenhoefer, R. A. J. Am. Chem. Soc. 1999,
121, 6960. (d) Widenhoefer, R. A.; Stengone, C. N. J. Org. Chem. Accepted.
(14) (a) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137.
(b) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118,
2436. (c) Brookhart, M.; Wagner, M. I. J. Am. Chem. Soc. 1994, 116, 3641.
(15) LaPoint, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997,
119, 906.
(16) DiRenzo, G. M.; White, P. S.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 6225.
(17) Halide abstraction employing NaBAr4 has been shown to form
cationic palladium complexes.14c Because no ether is present in the boronate
salt, the exact structure of the cationic species generated from 1c and 1d is
unknown.
of the precatalysts.16 However, a stoichiometric amount of
triethylsilane was crucial for efficient cycloisomerization. In
the absence of HSiEt3, palladium-catalyzed isomerization of
2 occurred slowly over 12 h at room temperature to form a
6:5:1:1 mixture of 3, 1,1-dicarbomethoxy-3-methyl-4-meth-
ylenecyclopentane,10 and two unidentified isomers in 65%
combined yield. Employment of <1 equiv of HSiEt3 or
employment of a bulkier silane led to a dramatic decrease
in the efficiency of palladium-catalyzed diene cycloisomer-
ization.19
(18) Typical experimental procedure: 2 (100 mg, 0.47 mmol) and HSiEt3
(80 mg, 0.70 mmol) were added sequentially via syringe to a solution of
1c (10 mg, 0.022 mmol) and 1d (24 mg, 0.024 mmol) in CH2Cl2 (8 mL)
at 0 °C. The resulting yellow solution was stirred at room temperature for
15 min to form a dark brown solution. Solvent was evaporated under
vacuum, and the oily residue was chromatographed (SiO2, 12:1 hexane-
EtOAc) to give 3 (89 mg, 89%) as a colorless oil.
The palladium-catalyzed protocol converted a range of
functionalized 1,6-dienes to 1,2-disubstituted cyclopentenes
(19) For example, dimethyl-tert-butylsilane failed to promote cyclo-
isomerization, while employment of dimethylphenylsilane led to 58% yield
for conversion of 2 to 3.
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Org. Lett., Vol. 1, No. 7, 1999

in good yield with high selectivity (Table 1). For example,
diesters (Table 1, entries 1-3), monoesters (Table 1, entries
4, 5), protected alcohols (Table 1, entries 6, 7), and protected
diols (Table 1, entries 8, 9) underwent palladium-catalyzed
cycloisomerization within 15 min at room temperature to
form the corresponding cyclopentenes in >70% yield with
g94% isomeric purity. In addition, dienes which possessed
substitution at an allylic carbon atom underwent facile
cycloisomerization in high yield and with good selectivity
(Table 1, entries 10-12). Dienes which possessed a substi-
tuted olefin underwent rapid cycloisomerization in high yield,
although considerable erosion of selectivity was observed
(Table 1, entry 13).20
mediate IV followed by reinsertion could form the palladium
tertiary alkyl intermediate V. Subsequent â-hydride elimina-
tion of the tertiary â-hydrogen atom would form the observed
cyclopentene and regenerate I.
Triethylsilane presumably serves two key roles in the
cycloisomerization process. Because HSiEt3 dramatically
increased the rate of cycloisomerization, the silane may
facilitate formation of the active palladium hydride species
I. The mechanism by which this may occur is unclear, as
direct reaction of a hydrosilane with an electrophilic late-
transition-metal alkyl complex typically leads to the forma-
tion of a metal silyl complex rather than a metal hydride
complex.15,21 In addition, because the selectivity of the
process increased considerably in the presence of HSiEt3,
the silane may also facilitate the conversion of palladium
alkenyl intermediate IV to palladium alkyl intermediate V.
In summary, a 1:1 mixture of (η3-C3H5)Pd(Cl)PCy3 (1c)
and NaBAr4 (1d) in the presence of HSiEt3 catalyzed the
cycloisomerization of functionalized 1,6-dienes to form 1,2-
disubstituted cyclopentenes in good yield with high selectiv-
ity. The protocol tolerated a range of functionality and allylic
substitution. We are currently working toward the develop-
ment of more efficient diene cycloisomerization catalysts and
toward elucidation of the role of silane in these isomerization
reactions.
We propose a plausible mechanism for palladium-
catalyzed diene cycloisomerization initiated by the cationic
palladium hydride intermediate I (Scheme 2). â-Migratory
Scheme 2
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the American Chemical
Society, for support of this research. Additional funding was
provided by DuPont. R.W. thanks the Camille and Henry
Dreyfus Foundation for a New Faculty Award.
Supporting Information Available: Experimental pro-
cedures and spectroscopic and analytical data for new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL9908981
insertion of an olefin into the Pd-H bond of I followed by
insertion of the pendant olefin into the Pd-C bond of
palladium alkyl olefin intermediate II could generate pal-
ladium cyclopentylmethyl intermediate III. â-Hydride elimi-
nation to form the palladium methylenecyclopentyl inter-
(20) 1,7-Dienes and terminally disubstitued dienes failed to undergo
cycloisomerization.
(21) (a) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462.
(b) Brookhart, M.; Grant, B. E. J. Am. Chem. Soc. 1993, 115, 2151. (c)
Marciniec, B.; Pietraszuk, C. Organometallics 1997, 16, 4320.
Org. Lett., Vol. 1, No. 7, 1999
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