Transition metal-catalyzed cycloadditions of cyclopropanes for the synthesis of carbocycles: C-C activation in cyclopropanes.

Transition metal-catalyzed cycloadditions of cyclopropanes have been well developed over the past several decades, leading to numerous new types of cycloadditions which are complementary to the traditional cycloadditions for the synthesis of carbocycles. Cycloadditions of vinylcyclopropanes (VCPs) and methylenecyclopropanes (MCPs) constitute two main aspects of this field. VCPs can act either as five-carbon synthons or three-carbon synthons, depending on whether the vinyl substituent is acting as an additional two-carbon synthon or not. As five-carbon synthons, VCPs are involved in [5+1], [5+2], [5+2+1], and [5+1+2+1] cycloadditions. As three-carbon synthons, VCPs are mainly involved in [3+2] and [3+2+1] cycloadditions. MCPs mostly act as three-carbon synthons and can have [3+2] cycloadditions with different π systems. Other types of cycloadditions involving MCPs are also reviewed, such as [3+1], [3+2+2], and [4+3+2] cycloadditions. Cycloadditions of some other unusual cyclopropane derivatives are also introduced briefly. The cycloadditions of VCPs and MCPs have found applications in total synthesis and some representative molecules are tabulated as selected examples.