Journal of the American Chemical Society
Communication
this observation is interesting because alkenylpalladium species are
known to be configurationally more stable. In spite of extensive efforts,
we were unable to activate CrCl2 by doping with Pd salts. We now
speculate that the Pd(OAc)2 used in the original work might have
been contaminated with some metal salt.
ACKNOWLEDGMENTS
■
We are grateful to the Eisai USA Foundation for generous
financial support.
(12) For examples, see: (a) Huggins, J. M.; Bergman, R. G. J. Am.
Chem. Soc. 1981, 103, 3002. (b) Hirata, Y.; Tanaka, M.; Yada, A.;
Nakao, Y.; Hiyama, T. Tetrahedron 2009, 65, 5037.
(13) In the presence of some Ni complexes, the coupling reaction
slowly progresses in MeCN with only free CrCl2, roughly
corresponding to the background coupling.
(14) The colors of the NiCl2 complexes are violet (1a, 2a, and 2b),
yellow (1b), green (1c), and blue (2c). The yellow and violet
complexes are known to be the α- and β-forms of 1b. See: Preston, H.
S.; Kennard, C. H. L. J. Chem. Soc. A 1969, 2682.
(15) With a lower Ni/Cr-ratio, the degree of Z → E isomerization
can be suppressed even for the trisubstituted (Z)-olefin E: in the
presence of 20 mol % Cr catalyst and (2 + 2) mol % Ni catalyst (the
second 2 mol % was added at 4 h), the coupling of trisubstituted (Z)-
vinyl olefin E was complete within 16 h, giving the product with Z/E =
3.2/1.
(16) An alkenylnickel(I) species was suggested to play a key role in
homodimerization. See: Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc.
1979, 101, 7547. Also see: Semmelhack, M. F.; Helquist, P. M.;
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(17) Aldehyde 5 readily eliminates benzyl alcohol even under weakly
basic conditions, yielding the corresponding α,β-unsaturated aldehyde.
This instability limited the choices and reaction conditions.
Experimentally, it was found that only simple organocuprates gave
the desired adduct in satisfactory yields.
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