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.

Typical decomposition by β-hydrogen elimination has limited the productive catalytic organometallic chemistry of late transition metal amido complexes. However, one reaction that has been shown to involve a late metal amido complex with β-hydrogens and elude extensive β-hydrogen elimination is the palladium-catalyzed amination of aryl bromides to give arylamines. The primary side products formed in these catalytic aminations are arenes, the products of aryl halide reduction. It would seem reasonable that both arylamine and arene products result from competitive reductive elimination of amine and β-hydrogen elimination from a common amido aryl intermediate. Our results do substantiate competitive β-hydrogen elimination and reductive elimination involving an amido group, but also reveal a second pathway to reduction that occurs when employing Pd(II) precursors. This second pathway for aryl halide reduction was shown principally by the observations that (1) stoichiometric reactions of aryl halide complexes or catalytic reactions employing [P(o-tolyl)3]2Pd(0) showed less arene side product than did catalytic reactions employing Pd(II) precursors, (2) increasing amounts of Pd(II) catalyst gave increasing amounts of arene product, and (3) reactions catalyzed by Pd(II) precursors showed amine:arene ratios at early reaction times that were lower than ratios after complete reaction. In addition to data concerning arene formation during Pd(II) reduction, we report data that demonstrate how electronic and steric factors control the relative rates for amine vs arene formation. The relative amounts of reduction product and amination product depend on the size of the phosphine and substitution pattern of the amide ligands. Systematic variation of phosphine size demonstrated that increasing the size of this ligand gave increasing amounts of arylamine product, increasing size of the amido group gave increasing amounts of arylamine product, while decreased nucleophilicity of the amide gave decreased amounts of arylamine product. Further, the presence of electron withdrawing groups on the palladium-bound aryl ring accelerated the reductive elimination reaction, relative to β-hydrogen elimination, and this result is consistent with previously observed acceleration of carbon−heteroatom bond-forming reductive eliminations with isolable palladium complexes.

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1999 mercaptans, thioethers mercaptans, thioethers (benzene compounds) Q 0580 -089Carbon-Sulfur Bond-Forming Reductive Elimination Involving sp-, sp 2 -, and sp 3 -Hybridized Carbon. Mechanism, Steric Effects, and Electronic Effects on Sulfide Formation.-Carbon-sulfur bond forming reductive elimination from saturated and unsaturated hydrocarbyl groups proceeds by an intramolecular, concerted mechanism. The reaction is facilitated by Pd-complexes with large P-Pd-P bite angles, electron-deficient carbon-bond ligands, and electron-rich thiolato ligands. The relative rates are fastest from complexes with alkenyl and aryl ligands, slower from complexes with alkynyl ligands, and significantly slower from complexes with alkyl ligands. -(MANN, G.; BARANANO, D.; HARTWIG, J. F.; RHEINGOLD, A. L.; GUZEI, I. A.; J. Am. Chem. Soc. 120 (1998) 36, 9205-9219; Dep. Chem., Yale Univ., New Haven, CT 06511, USA; EN)