Modification of regioselectivity in cycloadditions to C-70 under microwaveirradiation

The regioselectivity of the cycloaddition of N-methylazomethine ylide to C- 70 can be modified by using microwave irradiation as the source of energy. Under microwave irradiation and by choosing the appropriate solvent and irr adiation power, the 5-6 isomer is the major product, a situation that is in contrast to conventional heating where the 1-2 isomer predominates. Moreov er, isomer 7-21, which represents 13% of monoadducts under classical heatin g, is not formed under microwave irradiation and with ODCB as solvent. Theo retical calculations predict an asynchronous mechanism and suggest that the modification of the regiochemical outcome is related to the relative energ ies and hardnesses of the transition structures involved.