The effect of HSAB on stereoselectivity: copper- and gold-catalyzed 1,3-phosphatyloxy and 1,3-halogen migration relay to 1,3-dienes.

The origin of stereodivergence between copper- and gold-catalyzed cascade 1,3-phosphatyloxy and 1,3-halogen migration from α-halo-propargylic phosphates to 1,3-dienes is rationalized with density functional theory (DFT) studies. Our studies reveal the significant role of the relative hardness/softness of the metal centers in determining the reaction mechanism and the stereoselectivity. The relative harder Cu(I/III) center prefers an associative pathway with the aid of a phosphate group, leading to the (Z)-1,3-dienes. In contrast, the relative softer Au(I/III) center tends to undergo a dissociative pathway without coordination to a phosphate group, resulting in the (E)-1,3-dienes, where the E type of transition state is favored due to the steric effect. Our findings indicate the intriguing role of hard-soft/acid-base (HSAB) theory in tuning the stereoselectivity of metal-catalyzed transformations with functionalized substrates.