Carbon−Sulfur Bond-Forming Reductive Elimination Involving sp-, sp²-, and sp³-Hybridized Carbon. Mechanism, Steric Effects, and Electronic Effects on Sulfide Formation

Abstract

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.

Efficient, Nickel‐Catalysed Kumada–Tamao–Corriu Cross‐ Coupling with a Calix[4]arene‐Diphosphine Ligand

“…If one assumes that during catalysis a square planar [Ni(Ar)A C H T U N G T R E N N U N G (Ar')·1] complex is formed, itself displaying dynamics similar to that of 2 with temporarily enlarged PNiP angles, then it becomes clear that when the metal crosses the calixarene axis, the steric pressure exerted by the PPh 2 groups on the two Ni À aryl moieties becomes particularly strong and consequently facilitates the coupling step (Scheme 5). [26] These findings are in keeping with the observations made by Marcone and Moloy, [27] as well as by Hartwig et al [28] on diphosphines with large bite angles. The question whether the presence of the [a] i) 2, dioxane (1 mL), 100 8C, 1 h; ii) PhMgBr (1 mL in THF solution, 1 mmol), ArBr (0.5 mmol), decane (0.05 mL), dioxane (0.5 mL), 100 8C, 1 h. For the calculation of the TOF, we considered that the reaction began only once the substrates had been added to the reaction mixture.…”

The Mechanisms of the Stille Reaction

“…[226,227] This effect on the reductive elimination was also found by Hayashi et al [228] and van Leeuwen and co-workers [229] in the palladium-catalyzed cross-coupling reaction of Grignard reagents with aryl halides. In this process, the highest initial turnover frequencies were observed with ligands with large bite angles.…”

Highly Efficient and Functional‐Group‐Tolerant Catalysts for the Palladium‐Catalyzed Coupling of Aryl Chlorides with Thiols

“…[33,34] Palladium thiolates form easily and undergo relatively fast reductive eliminations with aryl groups. [35][36][37] Thus, the current limitations on the aromatic thiation could result from the notorious sensitivity of late metal catalysts to substrates containing reactive sulfur functionality. The lifetime and concentrations of the catalysts used for the coupling of haloarenes with thiols is likely to be limited by factors such as displacement of dative ligands by thiolates to form anionic thiolate complexes I or the formation of bridging thiolate complexes II that undergo slow reductive elimination ( Figure 1).…”