Mechanistic Studies of Palladium(II)-Catalyzed Hydrosilation and Dehydrogenative Silation Reactions

Abstract

The cationic Pd(II) complexes, [(phen)Pd(CH3)(L)]+[BAr‘4]- phen = 1,10−phenanthroline; L = Et2O, Me3SiC⋮CSiMe3; Ar‘ = 3,5-(CF3)2C6H3) catalyze the hydrosilation and dehydrogenative silation of olefins. Hydrosilation of ethylene, tert-butylethylene, 1-hexene, and cyclohexene by HSiR3 (R = CH2CH3, C6H5) occurs in the presence of 1 mol % [(phen)Pd(CH3)(L)]+[BAr‘4]-. The reaction of tert-butylethylene with HSi(i-Pr)3 in the presence of [(phen)Pd(CH3)(L)]+[BAr‘4]- yields neohexane and t-BuCHCHSi(i-Pr)3. Low-temperature NMR experiments revealed that the catalyst resting state for the silations of ethylene and alkyl-substituted olefins is [(phen)Pd(SiR3)(η2-H2CCHR‘)]+[BAr‘4]-. Evidence for rapid, reversible silyl migration at −70 °C was observed by 1H NMR spectroscopy. Deuterium labeling studies show that the intermediate Pd(II) alkyl complexes can isomerize via a series of β-hydride eliminations followed by reinsertions of olefin prior to reaction with DSiEt3. Styrene undergoes both hydrosilation and dehydrogenative silation in the presence of [(phen)Pd(CH3)(L)]+[BAr‘4]- or [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr‘4]- yielding ethylbenzene, R3SiCH2CH2C6H5 and trans-R3SiCHCHPh (R = CH2CH3, CH(CH3)2). 1H NMR spectroscopy revealed that the π-benzyl complexes [(phen)Pd(η3-CH(CH2SiR3)C6H5)]+[BAr‘4]- and [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr‘4]- are the catalyst resting states for the silation reactions of styrene.

“True” Chirality Transfer from Silicon to Carbon: Asymmetric Amplification in a Reagent‐Controlled Palladium‐Catalyzed Hydrosilylation

“…Based on reported mechanistic investigations, [8,9] our findings are consistent with the so-called "two-silicon cycle" depicted in Scheme 2. The stereogenic silicon center in (R)-5 functions as a stereochemical probe.…”

“…For our investigations, we selected the highly reactive catalyst 1 [7] (phen = 1,10-phenanthroline, Ar = 3,5-bis-(trifluoromethyl)phenyl), which had been introduced to hydrosilylation chemistry by Brookhart et al [8] Not only does 1 catalyze the hydrosilylation of internal alkenes with triorganosilanes, [8] its mode of reaction implies a feature of paramount importance to our purposes. Detailed elucidation of the catalytic cycle by Brookhart et al [8] and Widenhoefer and Perch [9] has revealed that carbon-silicon coupling occurs prior to carbon-hydrogen bond formation.…”

“…Detailed elucidation of the catalytic cycle by Brookhart et al [8] and Widenhoefer and Perch [9] has revealed that carbon-silicon coupling occurs prior to carbon-hydrogen bond formation. In contrast, a reversed order of bond formations is expected for hydrosilylations following the classic Chalk-Harrod [10] mechanism.…”

“…[11,13] We selected norbornene (2) as the substrate since it is not susceptible to b-hydride elimination in the course of the catalytic cycle. [8] The silanes (R)-3, (R)-4, and (R)-5 surveyed in this study were prepared in the desired enantiomeric purities according to known procedures. [14,15] Importantly, the oxidative addition of group VIII transition-metal centers into the silicon-hydrogen bond of silanes with silicon-centered chirality proceeds with stereoretention at silicon as verified for (R)-3.…”

Zirconocene-Catalyzed Silylation of Alkenes with Chlorosilanes

“…Intense interest has been focussed on the photophysical properties associated with the metal-to-ligand charge transfer chromophores in complexes of 2,2'-bipyridine and related ligands and upon binuclear and oligonuclear mixed-valence complexes of bridging ligands containing pyridine and pyrazine residues, [3] including hat itself.…”