Density Functional Study on the Mechanism of Palladium(0)-Catalyzed Thioboration Reaction of Alkynes. Differences between Pd(0) and Pt(0) Catalysts and between Thioboration and Diboration

The hybrid density functional (B3LYP) calculations have been carried out to study the mechanism of acetylene thioboration reactions catalyzed by Pd(0)/Pt(0) complexes, and the results are compared to those for the diboration reactions. The main findings can be summarized as follows. (i) No oxidative-addition product of the S−B bond to the Pd(PH3)2 was found, therefore, the oxidative-addition mechanism proposed by Suzuki et al. is not the proper one for the thioboration reaction of alkynes. (ii) We propose a new mechanism involving the following steps:  (a) acetylene coordination to Pd(PH3)2, (b) dissociation of a phosphine ligand, (c) addition of the S−B bond to the metal center via a metathesis-like transition state, (d) isomerization of the resultant complex, accompanied by recoordination of the phosphine ligand, and (e) the reductive elimination of the alkenyl−thioboron product, R‘S−CHCH−B(OR)2. The rate-determining stage is found to be the addition of the S−B bond to the metal center via a metathesis-...