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