Mechanism of gold (I)-catalyzed double migration-benzannulation cascade toward naphthalenes: Deprotonation/protonation mediated by water

Abstract

The mechanisms of the gold (I)-catalyzed cycloisomerization of propargylic esters leading to unsymmetrically substituted naphthalenes have been investigated using density functional theory calculations done at the B3LYP/6-31G (d, p) (SDD for Au) level of theory. Solvent effects on these reactions have been explored by calculations that included a polarizable continuum model (PCM) for the solvent (CH2Cl2). Three possible pathways which lead to forming 1,3-diene, allene or carbenoid intermediate via an unprecedented tandem sequence of [1,3]- and [1,2]-migration of two different migrating groups were proposed. Calculations suggest that the [1,3]-rearrangement is a two-step process with activation free energies below 11.0kcal/mol for both steps. The following of [1,2]-migration reaction is also easy with an activation free energy of 20.4kcal/mol in CH2Cl2. The next step in the catalytic cycle is a [1,2]-hydride shift, and this step is the rate-limiting step (with a calculated activation free energy of 23.2kcal/mol) without water. In the presence of water, the direct [1,2]-hydride shift has been changed into a deprotonation/protonation process with an activation free energy of 9.7kcal/mol. The higher activation free energies for the intramolecular 6π electrocyclic ring closure process indicate that this step became the rate-determining one. Calculations show that a water-catalyzed [1,2]-hydrogen shift adopts a proton-transport catalysis strategy, in which the acetoxy group in the substrate is critical because it acts as either a proton acceptor when one water molecule or a water cluster is involved in catalysis. Our calculated results are consistent with the experimental observations of Gevorgyan et al. for the gold (I)-catalyzed cycloisomerization of propargylic esters leading to unsymmetrically substituted naphthalenes.