Why Does Gold(III) Porphyrin Act as a Selective Catalyst in the Cycloisomerization of Allenones?

The chemical basis (electronic and steric factors) through which [Au(TPP)]Cl acts as a highly efficient chemoselective catalyst (even more than other homogeneous Au(III) catalysts like AuCl 3 and [Au(salen)]Cl) in the conversion of allenone to furan is elucidated. The planar aromatic structure of porphyrin unit stabilizes the transition state for ring closure through symmetric orbital interactions between allenone and the catalyst to create a lower energy cyclization pathway. Even though AuCl 3 catalyzes the cyclization of allenone very efficiently, the inability for the reaction to terminate and the possibility for further reactions that lead to oligomerization of allenone through new C−C bond formation reduce its synthetic utility. A detailed mechanistic investigation on the dimerization of allenone in the presence of AuCl 3 provides a theoretical rationale for the previous experimental results. In the case of the [Au(salen)] + -catalyzed reaction, the higher activation energy required to cross the barrier for cyclization reduces its efficiency.