1. INTRODUCTION
As one of the important carbon synthons to prepare pharmaceutically
relevant and biologically active five- and six-membered carbo- and
heterocyclic compounds,[1-3] 1,6-diketones and
other acyclic long-chain diketones have attracted increasing attention
in recent years. Although a lot of synthetic methods have been reported
to yield 1,6-diketones, many of them were less efficiency and
selectivity, and used non-readily obtainable substrates and
synthetically harsh reaction conditions.[4,5]Thus, a more general and efficient methodology to construct
1,6-diketones is highly desirable.
Transition-metal-catalyzed ring opening and cross-coupling reactions
have been widely employed to synthesize 1,6-diketones from small
carbocyclic rings. The intrinsic strain of small carbocyclic rings has
been successfully exploited for the C−C bond
activation,[6] by releasing of strain compensates
to overcome the reachable thermodynamic barrier. Nevertheless, catalytic
self-coupling of metal homoenolates using rhodium-homoenolate derived
from β -carbon elimination (C−C activation) of cyclopropanol is
often restricted,[7-10] due to the possible facileβ -hydride elimination and isomerization
pathways.[11]
Recently, Ravikumar et al reported a rhodium-catalyzed C−C
activation of readily available cyclopropanols for one-step access to
diverse 1,6-diketones at room temperature.[12] The
catalyst [Cp*RhCl2]2 and additive
Ag2CO3 play an important role in
controlling the selectivity. As shown in Scheme 1,
by employing
[Cp*RhCl2]2 as catalyst and
Ag2CO3 as additive,
1-Benzylcyclopropan-1-ol (R ) would furnish
1,6-diketone P1 , whereas
the β -hydride elimination product monoketone P1’ could
not be obtained (reaction A). In contrast, the product would become
monoketone P2 in the absence of
[Cp*RhCl2]2 (reaction B). In
reaction C, the combination of
catalyst [Cp*RhCl2]2 and additive
AgOAc would also lead to monoketone P2 .
SCHEME 1 C−C activation of cyclopropanol reported by Ravikumar