Origins of differences in reactivities of alkenes, alkynes, and allenes in [Rh(CO)2CI]2-catalyzed (5+2) cycloaddition reactions with vinylcyclopropanes

Rhodium dimer [Rh(CO)(2)Cl](2) efficiently catalyzes the intra- and intermolecular (5 + 2) reactions of vinylcyclopropanes with alkynes and allenes, but not alkenes. This difference in reactivity is attributed to the difficulty of reductive elimination for the alkene. The computed reductive elimination transition structures show that the parficipation of the second Pi-orbital in acetylene and allene reduces the barrier by 9-15 kcal/mol, compared to ethylene, for which no such interactions are possible.