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p-quinone methide into the Cu–Si bond affords a p-allyl-copper
intermediate (C) that could isomerize to copper phenoxide E. At
this point two pathways are possible. Protonolysis in the presence
of MeOH would provide silane 2 with release of NHC–CuOMe to
restart the catalytic cycle. On the other hand, copper phenoxide E
could react directly with the silaborane to provide 2 and silyl-copper
complex B.
In conclusion, we have found that copper(I) salts can catalyze
the silylation–aromatization process of p-quinone methides. This
study represents the first silicon addition to a quinone methide and
provides new insight for the development of novel metal-catalyzed
transformations. Mono- and dibenzylic silanes can be prepared in
high yields under mild reaction conditions. We have also demon-
strated that dibenzylic silanes can be used as stable dibenzylic
carbanion equivalents. The development of asymmetric versions of
this and related transformations is underway.
Fig. 1 X-ray structure of compound 2g.
We thank the European Research Council (ERC-337776) and
MINECO (CTQ2012-35957) for financial support. M. T. and
A. P. thank MICINN for RyC and JdC contracts. We acknowledge
Dr Josefina Perles for X-ray structure analysis.
Notes and references
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Scheme 2 Functionalization of the C–Si bond and de-tert-butylation.
A possible mechanism for the silylation–aromatization reaction
of p-quinone methides is shown in Scheme 3. First, a silyl-Cu(I)–NHC
complex B is formed by reaction of a copper alkoxide A and the
silaborane reagent. Insertion of the exocyclic double bond of the
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Scheme 3 Plausible mechanism for the silylation–aromatization.
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