New insight into the mechanism of the conjugate addition of benzenethiol to cyclic and acyclic enones and of the corresponding uranyl-salophen-catalysed version

A thorough kinetic investigation of the triethylamine-catalysed addition of benzenethiol to 2-cyclopenten-1-one in chloroform shows that the highest energy transition state is a complex of thiol, enone, and base in a 1:1:1 ratio, but whether formation or disruption of the enolate-triethylammonium ion-pair intermediate is rate-limiting is uncertain. Intervention of a second thiol molecule in the assembly of the transition-state complex is ruled out, at least at thiol concentrations not exceeding 0.1-0.2 M. Thiol addition is accelerated significantly by uranyl-salophen complex 1 and its diphenyl derivative 2. The complicated kinetics are described to high precision by means of ad hoc integrated rate equations in which associations to the metal catalyst of the enone reactant and addition product are taken into account. The kinetics are consistent with a four-body transition-state complex, whose formation results from the reaction of a (weak) thiol-base complex with a (strong) enone-uranyl-salophen complex. Open-chain and cyclic enones react at similar rates and respond to the presence of metal catalyst in much the same way. The relative catalytic efficiencies of ethyldimethylamine, triethylamine, and quinuclidine are determined essentially by differences in base strength, rather than steric bulk, in both the presence and absence of a metal complex. Only with the use of the relatively bulky Hünigs base is an adverse steric influence apparent, which is particularly severe in the reaction catalysed by the sterically demanding 2.