Fischer carbene complexes in heterocyclic synthesis.: Selective cycloaddition reactions to 2-aza-1,3-butadienes

A structurally diverse set of Fischer carbene complexes are reacted with substituted 3-[(trimethylsilyl)oxy]-2-aza-1,3-butadienes 1, yielding 5- to 7-membered nitrogen-containing heterocycles in a selective manner. Aryl and heteroarylmetal carbenes 2 undergo [4 + 1] cycloaddition with 1 leading to pyrrolidone derivatives 3-5 or 6-9, depending on the C1-substituent of 1. The [(trimethylsilyl)ethynyl]carbene 10a gives rise to metal-containing and metal-free [4 + 2] cycloadducts 11 and 12, respectively, whereas the (phenylethynyl)carbene 10b furnishes azafluorenones 13 by a tandem [4 + 2] cycloaddition/pentaannulation process. In the case of alkenyl carbene complexes 14 the regioselective [4 + 3] cycloaddition is the only observed transformation. Thus, their reaction with the phenyl-substituted azadiene 1d-resulted in the formation of a approximate to 1:1 mixture of diastereoisomers 15 and 16, whereas in the case of the tert-butyl-substituted azadiene Ic the cis-diastereoisomers 15 are selectively formed. This heptaannulation is proposed to occur by a cyclopropanation/aza-Cope rearrangement.