Journal of the American Chemical Society
Communication
observed because of the higher reactivity. Therefore, the
conditions were reoptimized, and three key points were found:
(1) the use of VO(OPr-i)2Cl, (2) reaction with THF, and (3)
low temperature (−35 °C). Consequently, the cross-coupling
of boron enolate 1a and silyl enolate 2d gave the γ-keto ester
3ad in 91% yield with high selectivity (3ad:4a:5d molar ratio =
97:3:0; entry 16). The use of an aliphatic boron enolate, 1g,
was not a problem for the cross-selectivity (entry 17). Finally,
as an example of the most challenging combination, cross-
coupling between ketone enolates monosubstituted at the 2-
position (1f and 2e) was carried out. Notably, the cross-
coupling proceeded with high selectivity (75% yield, 3fe:4f:5e
molar ratio = 98:2:0; entry 18). Such selectivity is considered to
be controlled by the difference in the reactivities of boron and
silyl enolates. Oxidative cross-coupling of boron enolate 1a and
1-(trimethylsiloxy)cyclopentene (2f) at −35 °C gave 3af in
85% yield with 3af:4a:5f molar ratio = 94:6:0 (entry 19).
To gain insight into the mechanism, a radical clock reaction
was investigated using boron enolate 1j with a cyclopropyl
group at the 2-position (Scheme 3). Boron enolate 1j was
AUTHOR INFORMATION
Corresponding Authors
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from JST (ACT-C) is acknowledged. This
work was also partially supported by a Grant-in-Aid for
Scientific Research (C) (26410046) from the Japan Society for
the Promotion of Science.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
Figures S1−S5, Table S1, synthetic procedures, charac-
terization data, and NMR charts of the products (PDF)
D
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX