Substituent Effects,Reactant Preorganization,and Ligand Exchange Control the Reactivity in Rh~I-Catalyzed (5+2) Cycloadditions between Vinylcyclopropanes and Alkynes

摘要

Transition-metal-catalyzed cycloadditions have become powerful reactions for the construction of carbocycles.[{Rh(CO)2Cl}2]-catalyzed (5+2) reactions between vinylcyclopropanes (VCPs) and alkynes are an effective way to construct seven-membered rings.Experimental studies have shown that siloxy,alkoxy,or alkyl group substitution at the C1 position of the cyclopropane is required to confer adequate reactivity of the VCPs in these reactions.The cycloaddition of 1-siloxy-,1-alkoxy-,or 1-isopropyl-substi-tuted VCPs with methyl propiolate provides the (5+2) cycloadducts in high yields (84-93 %) in 10 min to 2 h at 40-80°C.Cycloaddition with the 1-methyl-substituted VCP is slower,giving 83 % yield in 8 h at 80°C.The unsubstituted VCP (R = H) is the slowest-reacting compound,giving only 23 % yield in 30 h at 80 °C.We have investigated the origins of these effects using density functional theory and show that substituent effects both on transition state energetics and on reactant preorganization influence the reaction rates.