Unusual [1,2]- and [1,3]-M(CO)(5) shifts in Fischer carbene complexes: [4+3] and [3+3] annulation reactions of furan and pyrrole rings

The cycloaddition reactions of various C-heteroarylimines with alpha,beta-u nsaturated Fischer carbene complexes have been studied. Either [3 + 3] carb ocyclization or [3 + 3] and [4 + 3] heterocyclization reactions were accomp lished, depending on the structure of the imine and on the type of unsatura tion of the carbene complex. Imines derived from furan-, 4, benzofuran-, 5, and N-methylindole-2-carboxaldehyde, 7, gave C3 + C3 cycloadducts 10-12 on reaction with alkynyl carbene complexes 2a and 3. The reaction of N-unsubs tituted pyrrole imine 8a with alkenyl carbene complexes 1 involved the ring nitrogen atom of the pyrrole unit leading to C2N + C3 heterocycloadducts 1 4 and 15. Moreover, N-unsubstituted imines 8 and 9 underwent C2N2 + C3 hete rocyclization to alkynyl carbene complexes 2 and 3 to furnish zwitterionic metalpentacarbonyl containing heterocycles 17 and 18. We have Studied the t hermal behavior of heterocycles 17, finding that they led to bispyrrole 21 or indolizine 20 when heated at 120 degrees C in the presence and in the ab sence of carbon monoxide, respectively. The results obtained show that the [3 + 3] cyclizations are promoted by a [1,2]-M(CO)(5) shift, while in the c ase of the [4 + 3] cyclization, a [1,3]-M(CO)(5) shift follows the ring-clo sure step. The X-ray analysis of compound 17b, resulting from 1,3-pentacarb onyltungsten migration, is provided.