Mechanisms of the Au- and Pt-catalyzed intramolecular acetylenic Schmidt reactions: a DFT study

The reaction mechanisms of the PtCl4- and Au(I)-catalyzed intramolecular acetylenic Schmidt reactions were analyzed by means of hybrid density functional calculations at the B3LYP/6-31G*(LANL2DZ) level of theory for better understanding of the acceleration effect of ethanol solvent in PtCl4-catalyzed reaction and the different catalytic activities of Au and Pt catalysts. Calculations indicate the rate of the PtCl4-catalyzed reaction in noncoordinative solvent of 1,2-dichloroethane is limited by isomerization of the relatively stable chelate complex to the reactive π-complex of PtCl4 with the acetylenic moiety of homopropargyl azide substrate, which requires an activation energy of 29.6 kcal/mol. All nucleophilic cyclization, dinitrogen elimination, and 1,2-H shift of metal-carbene steps are quite facile. The generation of 2H-pyrrole intermediate in PtCl4-catalyzed reaction is completed by a ligand substitution reaction, and the final 2H-pyrrole to 1H-pyrrole isomerization is an intermolecular process with another 2H-pyrrole as a proton shuttle. When in ethanol solution, the favorable coordination of solvent molecules with PtCl4 could inhibit the chelation of PtCl4 with the homopropargyl azide. Besides, the alcohol coordination also facilitates the generation of 2H-pyrrole intermediate and the intermolecular isomerization of 2H-pyrrole to 1H-pyrrole. Consequently, the overall activation barrier of PtCl4-catalyzed reaction in ethanol solution is lowered to 21.5 kcal/mol, determined by the H-abstraction step of the intermolecular 2H-pyrrole to 1H-pyrrole isomerization. The basic steps in the Au(I)-catalyzed reaction are similar to those in the PtCl4-catalyzed one. However, no chelate complex could be formed from PR3AuSbF6 and homopropargyl azide, and the 2H-pyrrole generation step is much more favorable, indicating weaker interactions of Au(I) catalyst with the homopropargyl azide and the C-C double bond of 2H-pyrrole.