A DFT study on the mechanism of Rh-catalyzed competitive 1,2- versus 1,3-acyloxy migration followed by [5+1] and [4+1] cycloadditions of 1,4-enynes with CO

The mechanisms of the [RhCl(CO)₂]₂-catalyzed intramolecular cycloadditions of 3-acyloxy-1,4-enynes (ACEs) by CO with concominant acyloxy (OAc) migrations have been extensively investigated using density functional theory (DFT) calculations. All geometries were optimized at the B3LYP/6-31 + G(d) level (LANL2DZ for Rh). The effect of the nature of alkynes, terminal vs internal, has been rationalized by modelling possible intermediates and transition states for competitive 1,2- vs 1,3-migrations of acyloxy group and for further possible transformations with terminal and methyl substituted starting enynes. Experimental and mechanistic studies indicate that the formation reaction of the functionalized cyclopentenones involved a Saucy-Marbet 1,3-OAc migration of ACEs with terminal alkynes and subsequent [4 + 1] cycloaddition with CO, while the reaction for cyclohexadienones proceeded by a [5 + 1] type cycloaddition ACEs of substituted alkynes involving a Rautenstrauch 1,2-OAc migration by carbonylation. The acyloxy migration step is found to be the rate determining step from the DFT study and the predicted regioselectivity is in harmony with the experimental result. The calculations presented herein will contribute to understanding and controlling this effective Rh (I) catalyzed transformation reactions of 1,2 and 1,3-acyloxy migration followed by cycloaddition for further bicyclic cyclohexadienone and cyclopentenone derivatives in obtaining custom made products.