Cationic palladium-catalyzed hydrosilylative cross-coupling of alkynes with alkenes

Article abstract of DOI:10.1021/ja0558342

A cationic palladium complex-catalyzed cross-coupling of alkynes with alkenes is presented, which occurs selectively under the hydrosilylation conditions using trichlorosilane. The unique reaction might be well understood in terms of an initial hydropalla

Full text of DOI:10.1021/ja0558342

Published on Web 11/08/2005
Cationic Palladium-Catalyzed Hydrosilylative Cross-Coupling of Alkynes with
Alkenes
Takamitsu Shimamoto, Motoharu Chimori, Hiroaki Sogawa, and Keiji Yamamoto*
Department of Materials Science and EnVironmental Engineering, Tokyo UniVersity of Science, Yamaguchi,
1-1-1 Daigaku-Dori, Sanyo-Onoda, Yamaguchi 756-0884, Japan
Received September 2, 2005; E-mail: yamamoto@ed.yama.tus.ac.jp
We wish to report herein a palladium complex-catalyzed cross-
coupling of alkynes with alkenes, which occurs selectively under
the hydrosilylation conditions. The unique findings originated from
the fact that, by using trichlorosilane as an addend, a palladium
complex, [(η3-C3H5)Pd(cod)]+[PF6]- (A) (cod ) cyclooctadiene),
at a 0.5 mol % level, was found to catalyze the cyclization-
hydrosilylation of a 1,6-heptenyne (B) evidently faster than
that of the corresponding 1,6-heptadiyne (C), as depicted in
Scheme 1.1
However, once triphenylphosphine (1 equiv) was added to the
Scheme 1
catalyst A, the same reaction as above in 3 h exhibited a quite
different reaction pattern as shown in eq 2.6 The initial hydro-
palladation directed the palladium center toward the â-position of
the alkyne regioselectively, resulting in giving almost a single
coupling product 5b in 58% isolated yield. The effect of a phosphine
ligand on a reversed regiocontrol in the hydropalladation of
1-alkynes would have a steric origin.
Although there are a variety of examples of the late transition
metal (Rh,2 Ni,3 Pd,4 and Pt5) complex-catalyzed cyclization-
hydrosilylation of R,ω-alkadiynes,2a,e,g,3a,4i,5b alkenynes,1,2c,g as well
as alkadienes,2f,4d it is worthy to note that trialkylsilanes are
necessarily of choice as addends in these cases, while HSiMenCl3-n
(n ) 0-2) and none of trialkylsilanes are applicable for our
reactions (Scheme 1).4i These facts may imply that the transfer of
a trialkylsilyl group to the π-coordinated alkyne site on these metal
centers, that is, the initial syn silylmetalation, must proceed much
easier than that of a trichlorosilyl group, whereas the syn hydro-
palladation must precede under our conditions.
With these intriguing circumstances, we have performed an
Although the exact nature of the catalysis is not clear yet, certain
scope and limitations as to the present hydrosilylatiVe cross-coupling
between alkynes and alkenes are examined, and results obtained
are listed in Table 1. As is seen in Table 1, similar reaction patterns
intermolecular version of the hydrosilylation of alkenyne as
mentioned above. Thus, in a 5 mL screw-capped test tube with a
stirring bar was placed a mixture of phenylacetylene (1.0 mmol),
1-hexene (3 equiv), trichlorosilane (1 M, CH2Cl2, 1 mL), and the
catalyst A (0.01 mmol, 1 mol %) under an argon atmosphere, and
the mixture was heated with stirring at 50 °C for 1-3 h. The
reaction mixture was either directly subjected to a bulb-to-bulb
distillation in vacuo or immediately treated with excess ethanol
and triethylamine in CH2Cl2 at 0 °C, and the two products were
obtained in 70% combined yield as triethoxysilyl derivatives after
a usual work up (eq 1).6 The result revealed that the initial
hydropalladation must take place at the alkyne site, directing
palladium at an R-position regioselectively, while the insertion step
of 1-hexene into the resulting vinylpalladium intermediate (carbo-
palladation) did undergo cleanly with rather poor regioselectivity,
a terminally silylated product being more or less predominant (vide
infra).7
to eqs 1 and 2 were substantially observed in most reactions of
1-heptyne (entries 1 and 2), methyl propiolate (entry 3), and again
phenylacetylene (entries 5 and 6) with 1-hexene or styrene,
respectively. However, the R-directing hydropalladation was found
to be not always exclusive, as shown in entry 1, even under
conditions of eq 1 (without PPh3). In addition, a reaction between
methyl propiolate and 1-hexene under added PPh3 (1 or 2 equiv)
to the palladium catalyst was exceptional (entry 4), where an R-1
coupling product predominated over a â-1 product. Reaction of
phenylacetylene with (E)-3-hexene (entry 7) again afforded a
â-isomer significantly without added PPh3 to the catalyst. In
addition, the major product is suggested to contain a single
diastereoisomer by an NMR analysis. Furthermore, reaction of
3-hexyne, an internal alkyne, with allylbenzene (entry 8) did
undergo the cross-coupling under hydrosilylation conditions, while
attempted reaction of 3-hexyne with (E)-3-hexene was found to be
very sluggish. Finally, in some of the above reactions, a small
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J. AM. CHEM. SOC. 2005, 127, 16410-16411
10.1021/ja0558342 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Hydrosilylative Cross-Coupling of Alkynes with Alkenes
Scheme 2. Proposed Catalytic Cycle
Acknowledgment. This work was partly supported by a Grant-
in-Aid from the Japan Society for Promotion of Sciences (No.
15550099) and was presented at the 85th Annual Meeting of the
Chem. Soc. Japan, March 2005 (Yokohama), Abstr. 1E2-08.
Supporting Information Available: Experimental procedures and
compound characterization data (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) Wakayanagi, S.; Shimamoto, T.; Chimori, M.; Yamamoto, K. Chem. Lett.
2005, 34, 160.
(2) (a) Ojima, I.; Donovan, R. J.; Shay, W. R. J. Am. Chem. Soc. 1992, 114,
6580. (b) Ojima, I.; Vu, A. T.; McCullagh, J. V.; Kinoshita, A. J. Am.
Chem. Soc. 1999, 121, 3230. (c) Ojima, I.; Vu, A. T.; Lee, S.-Y.;
McCullagh, J. V.; Moralee, A. C.; Fujiwara, M.; Hoang, T. H. J. Am.
Chem. Soc. 2002, 124, 9164. (d) Muraoka, T.; Matsuda, I.; Itoh, K.
Tetrahedron Lett. 1998, 39, 7325. (e) Muraoka, T.; Matsuda, I.; Itoh, K.
Organometallics 2002, 21, 3650. (f) Liu, C.; Widenhoefer, R. A.
Organometallics 2002, 21, 5666. (g) Chakrapani, H.; Liu, C.; Widenhoefer,
R. A. Org. Lett. 2003, 5, 157.
(3) (a) Tamao, K.; Kobayashi, K.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 6478.
(b) Tamao, K.; Kobayashi, K.; Ito, Y. Synlett 1992, 539. (c) For a related
Ni(0)-catalyzed dimerization of 1-alkynes, see: Lappert, M. F.; Nile, T.
A.; Takahashi, S. J. Organomet. Chem. 1974, 72, 425.
(4) (a) Widenhoefer, R. A.; DeCarli, M. A. J. Am. Chem. Soc. 1998, 120,
3805. (b) Widenhoefer, R. A.; Vadehra, A. Tetrahedron Lett. 1999, 40,
8499. (c) Widenhoefer, R. A.; Stengone, C. N. J. Org. Chem. 1999, 64,
8681. (d) Perch, N. S.; Widenhoefer, R. A. J. Am. Chem. Soc. 1999, 121,
6960. (e) Stengone, C. N.; Widenhoefer, R. A. Tetrahedron Lett. 1999,
40, 1451. (f) Perch, N. S.; Kisanga, P.; Widenhoefer, R. A. Organo-
metallics 2000, 19, 2541. (g) Wang, X.; Chakrapani, H.; Stengone, C.
N.; Widenhoefer, R. A. J. Org. Chem. 2001, 66, 1755. (h) Wang, X.;
Stankovich, S. Z.; Widenhoefer, R. A. Organometallics 2002, 21, 901.
(i) Uno, T.; Wakayanagi, S.; Sonoda, Y.; Yamamoto, K. Synlett 2003,
1997.
amount of simple hydrosilylation product from alkenes, but not
from alkynes, was usually observed. Especially, in the case of
hydrosilylation of allylbenzene, there was found an extensive
hydrosilylation product at the expense of diminished cross-coupling
with 3-hexyne (entry 8).
In conclusion, although further study must be undertaken before
extensive discussion, the unique reaction presented here might be
well understood in terms of (i) initial hydropalladation of a given
alkyne to form an alkenylpalladium species, (ii) the latter, in turn,
undergoes quickly and specifically an alkene insertion, and (iii)
the resulting homoallylic organopalladium species terminates one
catalytic cycle by undergoing substitution at the palladium center
with a trichlorosilyl group to give product(s), regenerating most
probably a hydrido-palladium species as an active catalyst (see
Scheme 2).8
(5) (a) Madine, J. W.; Wang, X.; Widenhoefer, R. A. Org. Lett. 2001, 3,
385. (b) Wang, X.; Chakrapani, H.; Madine, J. A.; Keyerleber, M. A.;
Widenhoefer, R. A. J. Org. Chem. 2002, 67, 2778.
(6) In the chemical formula, the designations R or â and 1 or 2 refer to the
carbopalladation at the R or â position of the alkyne employed, and C-Si
bond occurring at the terminal or internal carbon of the alkene counterpart,
respectively.
(7) Immediate reaction products consist of R’s (90%) and little â isomers
(∼8%), the latter being hardly isolated at this stage.
(8) Although the circumstantial evidence for intervention of HPdL+ species
in the proposed catalytic cycle appears to be quite probable, any premise
for the catalyst activation from a precatalyst A is problematic. We have
examined a stoichiometric reaction of complex A with HSiCl3 (2 equiv)
dissolved in CD2Cl2 in an NMR tube, which is sealed under freeze-
thawing, and observed rather slow evolution of both propene and free
cyclooctadiene by monitoring with 1H (270 MHz), the mixture forming
dark precipitates in an hour. However, by 29Si NMR (100 MHz, TMS
external standard), no significant data have been obtained.
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