Kinetics and Mechanism of Allene Racemization Catalyzed by a Gold N‑Heterocyclic Carbene Complex

The kinetics of the racemization of 1,3-disubstituted allenes catalyzed by (IPr)­AuOTf (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidine) has been investigated. The rate of gold-catalyzed allene racemization obeyed the following second-order rate law: rate = k rac[allene]­[Au]. An analysis of the rate of the gold-catalyzed racemization of 1-aryl-1,2-butadienes as a function of allene electron donor ability established moderate depletion of electron density on the C1 allenyl carbon atom in the rate-limiting transition state for racemization. Analysis of the temperature dependence of the rate of racemization of 1-(p-tolyl)-1,2-butadiene established the activation parameters ΔH ⧧ = 8.3 ± 1 kcal mol–1 and ΔS ⧧ = −28 ± 4 eu. These observations were in accord with a mechanism for allene racemization involving turnover-limiting, intermolecular allene exchange followed by rapid allene stereomutation.