The Importance of Hydrogen Bonding to Stereoselectivity and Catalyst Turnover in Gold-Catalyzed Cyclization of Monoallylic Diols

Density functional calculations and experiment were used to examine the mechanism, reactivity, and origin of chirality transfer in monophosphine Au-catalyzed monoallylic diol cyclization reactions. The lowest energy pathway for cyclization involves a two-step sequence that begins with intramolecular C–O bond formation by anti-addition of the non-allylic hydroxyl group to the Au-coordinated alkene followed by concerted hydrogen transfer/anti-elimination to liberate water. Concerted SN2′-type transition states were found to be significantly higher in energy. The two-step cyclization pathway is extremely facile due to hydrogen bonding between diol groups that induces nucleophilic attack on the alkene and then proton transfer between diol groups after C–O bond formation. Importantly, intramolecular proton transfer and elimination provides an extremely efficient avenue for catalyst regeneration from the Au–C σ-bond intermediate, in contrast to other Au-catalyzed cyclization reactions where this intermediate severely restricts catalyst turnover. The origin of chirality transfer and the ensuing alkene stereochemistry is also the result of strong hydrogen-bonding interactions between diol groups. In the C–O bond-forming step, requisite hydrogen bonding biases the tethered nucleophilic moiety to adopt a chair-like conformation with substituents in either axial or equatorial positions, dictating the stereochemical outcome of the reaction. Since this hydrogen bonding is maintained throughout the course of the reaction, establishment of the resultant olefin geometry is also attributed to this templating effect. These computational conclusions are supported by experimental evidence employing bicyclic systems to probe the facial selectivity.