Hitoshi Sakurai

Rh(II)-Catalyzed Skeletal Reorganization of 1,6- and 1,7-Enynes through Electrophilic Activation of Alkynes

The skeletal reorganization of 1,6- and 1,7-enynes leading to 1-vinylcycloalkenes using Rh(II) as a catalyst is reported. Two possible isomers of 1-vinylcycloalkenes, type I and type II, can be obtained, the ratio of which are highly dependent on the substitution pattern of the enynes used. Formation of type I compounds involves a single cleavage of a C-C double bond, the product of which is identical to that of enyne metathesis. In contrast, the formation of type II compounds involves the double cleavage of both the C-C double and triple bonds, which has an anomalous bond connection. The presence of both a phenyl group at the alkyne carbon and a methyl group at the internal alkene carbon facilitates the formation of type II compounds. The electronic and steric nature of the substituents on the aromatic ring also affects the ratio of type I and type II. The nature of a tether also has a significant effect on the course of the reaction. Experimental evidence for the intermediacy of a cyclopropyl rhodium carbenoid, a key intermediate in the skeletal reorganization of enynes, is also reported. In addition to the skeletal reorganization of enynes, Rh(II) complexes were found to have a high catalytic activity for some cycloisomerization reactions of alkyne derivatives, including the bicyclization of enynes to bicyclo[4.1.0]heptene derivatives and the cyclization of alkynylfurans to phenol derivatives.