Pd(II) and Rh(I) Catalytic Precursors for Arene Alkenylation: Comparative Evaluation of Reactivity and Mechanism Based on Experimental and Computational Studies

We combine experimental and computational investigationsto compareand understand catalytic arene alkenylation using the Pd(II) and Rh(I)precursors Pd(OAc)(2) and [(& eta;(2)-C2H4)(2)Rh(& mu;-OAc)](2) with arene,olefin, and Cu(II) carboxylate at elevated temperatures (>120 & DEG;C).Under specific conditions, previous computational and experimentalefforts have identified heterotrimetallic cyclic PdCu2(& eta;(2)-C2H4)(3)(& mu;-OPiv)(6) and [(& eta;(2)-C2H4)(2)Rh(& mu;-OPiv)(2)](2)(& mu;-Cu) (OPiv= pivalate) species as likely active catalysts for these processes.Further studies of catalyst speciation suggest a complicated equilibriumbetween Cu(II)-containing complexes containing one Rh or Pd atom withcomplexes containing two Rh or Pd atoms. At 120 & DEG;C, Rh catalysisproduces styrene >20-fold more rapidly than Pd. Also, at 120 & DEG;C,Rh is & SIM;98% selective for styrene formation, while Pd is & SIM;82%selective. Our studies indicate that Pd catalysis has a higher predilectiontoward olefin functionalization to form undesired vinyl ester, whileRh catalysis is more selective for arene/olefin coupling. However,at elevated temperatures, Pd converts vinyl ester and arene to vinylarene, which is proposed to occur through low-valent Pd(0) clustersthat are formed in situ. Regardless of arene functionality, the regioselectivityfor alkenylation of mono-substituted arenes with the Rh catalyst givesan approximate 2:1 meta/para ratiowith minimal ortho C-H activation. In contrast,Pd selectivity is significantly influenced by arene electronics, withelectron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (& alpha;,& alpha;,& alpha;)-trifluorotoluenegives a 3:1 meta/para ratio withminimal ortho functionalization. Kinetic intermoleculararene ethenylation competition experiments find that Rh reacts mostrapidly with benzene, and the rate of mono-substituted arene alkenylationdoes not correlate with arene electronics. In contrast, with Pd catalysis,electron-rich arenes react more rapidly than benzene, while electron-deficientarenes react less rapidly than benzene. These experimental findings,in combination with computational results, are consistent with thearene C-H activation step for Pd catalysis involving significant & eta;(1)-arenium character due to Pd-mediated electrophilic aromaticsubstitution character. In contrast, the mechanism for Rh catalysisis not sensitive to arene-substituent electronics, which we proposeindicates less electrophilic aromatic substitution character for theRh-mediated arene C-H activation.