Palladium thiolato complexes [(L)Pd(R)(SR‘)], within which L is a chelating ligand such as DPPE,
DPPP, DPPBz, DPPF, or TRANSPHOS, R is a methyl, alkenyl, aryl, or alkynyl ligand, and R‘ is an aryl or
alkyl group, were synthesized by substitution or proton-transfer reactions. All of these thiolato complexes
were found to undergo carbon−sulfur bond-forming reductive elimination in high yields to form dialkyl sulfides,
diaryl sulfides, alkyl aryl sulfides, alkyl alkenyl sulfides, and alkyl alkynyl sulfides. Reductive eliminations
forming alkenyl alkyl sulfides and aryl alkyl sulfides were the fastest. Eliminations of alkynyl alkyl sulfides
were slower, and elimination of dialkyl sulfide was the slowest. Thus the relative rates for sulfide elimination
as a function of the hybridization of the palladium-bound carbon follow the trend sp2 > sp ≫ sp3. Rates of
reductive elimination were faster for cis-chelating phosphine ligands with larger bite angles. Kinetic studies,
along with results from radical trapping reactions, analysis of solvent effects, and analysis of complexes with
chelating phosphines of varying rigidity, were conducted with [Pd(L)(S-tert-butyl)(Ar)] and [Pd(L)(S-tert-butyl)(Me)]. Carbon−sulfur bond-forming reductive eliminations involving both saturated and unsaturated
hydrocarbyl groups proceed by an intramolecular, concerted mechanism. Systematic changes in the electronic
properties of the thiolate and aryl groups showed that reductive elimination is the fastest for electron deficient
aryl groups and electron rich arenethiolates, suggesting that the reaction follows a mechanism in which the
thiolate acts as a nucleophile and the aryl group an electrophile. Studies with thiolate ligands and hydrocarbyl
ligands of varying steric demands favor a migration mechanism involving coordination of the hydrocarbyl
ligand in the transition state.
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