Density Functional Theory Study on the Mechanisms of Platinum‐ and Gold‐Catalyzed Cycloisomerizations of Biaryl Propargyl Alcohol and Indolyl Allenol to Phenanthrene and Carbazole

Abstract

AbstractThe mechanisms and selectivity of PtCl2‐ and AuCl‐catalyzed cyclizations of indolyl allenols as well as PtCl2‐ and InCl3‐catalyzed cyclization of biaryl propargyl alcohol were analyzed by means of DFT calculations to gain deeper insights into the reaction mechanisms and the different reactivity of Pt and In catalysts. The calculated results ruled out the mechanism proposed in the original study as a feasible pathway for PtCl2‐ and AuCl‐catalyzed cyclizations of indolyl allenols. Alternatively, by performing an exhaustive search on the potential energy surface, we found that PtCl2‐ and AuCl‐catalyzed cyclizations of indolyl allenols involve 6‐endo‐trig cyclization followed by water cluster‐assisted elimination of H2O, and the generation of product‐catalyst complex occurs by in situ‐generated H2O‐assisted deprotonation/proto‐demetalation sequence for Pt‐catalysis and by 1,2‐H shift for Au‐catalysis. In contrast, PtCl2‐catalyzed cyclization of biaryl propargyl alcohol proceeds through 6‐exo‐dig cyclization followed by direct hydrogen abstraction by hydroxyl group and 1,2‐H shift processes to afford vinylphenanthrene, while the same reaction in the presence of InCl3 catalyst offers fluorene via concerted 5‐exo‐tet cyclization/aromatization. The NBO analysis of the intermediate formed after the elimination of H2O in the catalytic cycles indicates that the nature of bonding is not quite carbene. The calculated results are very well in agreement with the experimental observations.