C O MMU N I C A T I O N S
Scheme 1
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3d, and 5 and NMR spectra.
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(15) Epoxidation using m-CPBA also affords the epoxide 3a with excellent
conversion, but the product crystallizes together with m-chlorobenzoic
acid. Attempts to perform a biphasic media reaction with a solution of
NaHCO3 afforded decanal, probably as a consequence of the aqueous basic
hydrolysis of the B-F bond in the trifluoroborates (see ref 9a) to a boronic
acid or borate that under the peracid conditions affords the enol leading
to the aldehyde. Other oxidizing agents such as Davis’s oxaziridine or
amine N-oxides leave the starting alkenyltrifluoroborate intact.
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(17) Oxidative B-C bond cleavage with Oxone/acetone is precedented: Webb,
K. S.; Levy, D. Tetrahedron Lett. 1995, 35, 5117-5120. We have
confirmed that both 1-alkenylboronic acids and the pinacolboronates
provide aldehydes quantitatively upon reaction with m-CPBA. Dioxirane
oxidation of 1-alkenylboronic acids also provides aldehydes quantitatively,
but the corresponding pinacolboronates affords an as yet unidentified
oxidation product (not an epoxide) that rapidly decomposes.
4-cyanophenyl bromide 7 as the electrophile. The previously
reported conditions found to be optimal for Suzuki coupling of
potassium alkyltrifluoroborates9a [PdCl2(dppf)‚CH2Cl2 (9 mol %),
Cs2CO3 (3 equiv) as base, heated at reflux in THF:H2O (10:1)]
afforded the 4-cyanophenyl-1,2-butanediol 8 with a 74% yield after
column chromatography. Although this type of cross-coupling/ring-
opening process is of interest, more importantly we could perform
the desired Suzuki reaction and retain the epoxide by simply
decreasing the amount of the water in the solvent mixture (THF:
H2O) to a 40:1 ratio, isolating the purified epoxyethylaryl compound
9 in 80% yield (Scheme 1).
In summary, we have found that potassium organotrifluoroborates
possess a reactivity that is complementary to other organoboron
compounds, permitting olefin epoxidation with dimethyldioxirane
with concomitant retention of the carbon-boron bond. This work
thus represents a significant expansion in the chemistry of organo-
boron compounds. We have described the first potassium R,â-
epoxytrifluoroborates, compounds that could have interesting
reactivity in analogy to the chemistry of oxiranyl anions. In addition,
we have successfully accomplished the first well-documented
B-alkyl Suzuki-Miyaura cross-coupling reaction with an epoxy-
alkyltrifluoroborate derivative. Studies directed toward the use of
R,â-epoxytrifluoroborates as oxiranyl anion equivalents as well as
the exploration of other oxidative processes of organotrifluoro-
borates are under investigation.
Acknowledgment. We thank Johnson & Johnson, Merck
Research Laboratories, Aldrich Chemical Co., Array BioPharma,
and Johnson Matthey for their generous support of our program
and Dr. Carmem Bernardi for synthesizing compounds 3a-c and
3e.
Supporting Information Available: Experimental procedures,
compound characterization data, and NMR spectra of 1, 2, 4(a-e)-9
(PDF). This material is available free charge via the Internet at http://
pubs.acs.org.
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its homocoupling product in a 1:1 ratio.
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