LETTER
Rh(I)-Catalyzed Carbon–Carbon Double-Bond Formation
1647
ported in many previous cases.12 Migration and insertion
of the benzylidene group to the Rh–C bond produces in-
termediate E, and β-hydride elimination affords the prod-
uct 3a and generates the Rh–H compound. Catalyst A is
finally regenerated from the latter species in the presence
of Ag2O, which enters the next cycle.
According to some literature reports,13 the important in-
termediate D can also be generated through the following
four steps (Scheme 4, path b): First, a phosphine ligand of
[Rh(PPh3)3Cl] may be replaced by diazo benzyl carbene
to produce rhodium carbene intermediate A′. The chloride
anion then exchanges with the carbonate ion of Cs2CO3,
leading to the formation of complex B′ after releasing a
phosphine ligand. Further ligand exchange of B′ with 1
gives complex C′, in which the rhodium metal is directed
close to the C–H bond of the methyl group of 1. The C–H
activation then takes place, which is assisted by the oxy-
gen atom of the carbonate anion, producing rhodium car-
bene intermediate D after loss of CsHCO3.
References and Notes
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To further investigate the reaction mechanism, more ex-
periments were conducted in an attempt to trap some im-
portant intermediates generated during the reaction
process as direct evidence. Thus, the stoichiometric reac-
tion of [Rh(PPh3)3Cl] with 1 and 2a under similar reaction
conditions was conducted, but failed to reveal any mech-
anistic information. In this case, the desired product was
obtained in high yield, with no key intermediate being ob-
served either during or after the reaction by NMR or TLC
analysis, indicating that the key intermediates may be
formed in very low concentration that could not be ob-
served by using conventional tools. In many cases, these
rhodium carbene species are not stable and are difficult to
detect by traditional methods.14 Although the isolation of
the key intermediates was unsuccessful, the indirect evi-
dence mentioned above do provide some hints for the
C–H activation mechanism we proposed.
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In conclusion, an alternative method for the direct arylvi-
nylation of (quinolin-8-yl)methanone with substituted N′-
benzylidene-4-methylbenzenesulfono-hydrazide is de-
scribed.15 The desired 3-aryl-1-(quinolin-8-yl)prop-2-en-
1-one products are obtained in high yields through the cat-
alytic reaction with [Rh(PPh3)3Cl] in the presence of
Ag2O and Cs2CO3 at 130 °C for 48 hours. Two plausible
mechanisms involving the C–H activation and migratory
insertion of the carbene into the rhodium–carbon bond
were proposed to explain the formation of the products.
Further efforts to expand the scope of this reaction are
underway in our laboratories.
Acknowledgment
We thank the editor and one of the referees for helpful suggestions
on the reaction mechanism investigation. This work was supported
by grants from the NSFC (21072149, 20872108) and the Innovation
Foundation of Tianjin University.
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1643–1648