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 Delta H-double dagger = 8.3 +/- 1 kcal mol(-1) and Delta S-double dagger = -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.