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

Abstract

We combine experimental and computational investigations to compare and understand catalytic arene alkenylation using the Pd(II) and Rh(I) precursors Pd(OAc) 2 and [(η 2 -C 2 H 4 ) 2 Rh(μ-OAc)] 2 with arene, olefin, and Cu(II) carboxylate at elevated temperatures (>120 °C). Under specific conditions, previous computational and experimental efforts have identified heterotrimetallic cyclic PdCu 2 (η 2 -C 2 H 4 ) 3 (μ-OPiv) 6 and [(η 2 -C 2 H 4 ) 2 Rh(μ-OPiv) 2 ] 2 (μ-Cu) (OPiv = pivalate) species as likely active catalysts for these processes. Further studies of catalyst speciation suggest a complicated equilibrium between Cu(II)containing complexes containing one Rh or Pd atom with complexes containing two Rh or Pd atoms. At 120 °C, Rh catalysis produces styrene >20-fold more rapidly than Pd. Also, at 120 °C, Rh is ∼98% selective for styrene formation, while Pd is ∼82% selective. Our studies indicate that Pd catalysis has a higher predilection toward olefin functionalization to form undesired vinyl ester, while Rh catalysis is more selective for arene/olefin coupling. However, at elevated temperatures, Pd converts vinyl ester and arene to vinyl arene, which is proposed to occur through low-valent Pd(0) clusters that are formed in situ. Regardless of arene functionality, the regioselectivity for alkenylation of monosubstituted arenes with the Rh catalyst gives an approximate 2:1 meta/para ratio with minimal ortho C−H activation. In contrast, Pd selectivity is significantly influenced by arene electronics, with electron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (α,α,α)-trifluorotoluene gives a 3:1 meta/para ratio with minimal ortho functionalization. Kinetic intermolecular arene ethenylation competition experiments find that Rh reacts most rapidly with benzene, and the rate of monosubstituted arene alkenylation does not correlate with arene electronics. In contrast, with Pd catalysis, electron-rich arenes react more rapidly than benzene, while electron-deficient arenes react less rapidly than benzene. These experimental findings, in combination with computational results, are consistent with the arene C−H activation step for Pd catalysis involving significant η 1 -arenium character due to Pd-mediated electrophilic aromatic substitution character. In contrast, the mechanism for Rh catalysis is not sensitive to arene-substituent electronics, which we propose indicates less electrophilic aromatic substitution character for the Rhmediated arene C−H activation.

Highly Anti-Markovnikov Selective Oxidative Arene Alkenylation Using Ir(I) Catalyst Precursors and Cu(II) Carboxylates

“…However, the lack of observed induction periods as well as the consistent change in linear/branched ratios seem to be most consistent with a homogeneous catalyst. Further, the proposed homogeneous mixed metallic Ir/Cu catalyst is consistent with our previous evidence for mixed metallic Rh/Cu and Pd/Cu catalysts for oxidative arene alkenylation. ,, …”

“…We propose that mixed metallic Rh/Cu and Pd/Cu species are likely active catalysts for styrene production (Scheme ). , Importantly, although our studies demonstrate that the structures shown in Scheme are likely the most active catalysts for the conversion of benzene, ethylene, and Cu­(II) carboxylate to styrene, we also propose that other mixed metallic species are viable catalysts . Thus, there is likely more than one active catalyst and the actual catalyst(s) can evolve with reaction conditions.…”

“…In addition to catalytic arene alkylation using arenes and olefins, Ru, , Rh, ,,− and Pd − catalysts for single-step oxidative arene alkenylation have been reported. The general mechanisms reported for arene alkenylation using Rh and Pd include arene C–H activation, olefin insertion into a M–aryl bond, and product-forming β-H elimination, which is followed by the dissociation of alkenyl arenes.…”

Palladium Catalysis: Dependence of the Efficiency of C−C Bond Formation on Carboxylate Ligand and Alkali Metal Carboxylate or Carboxylic Acid Additive. Part A: the C(sp2)−C(sp2), C(sp2)−C(sp3) and C(sp3)−C(sp3) Bonds

“…15H 2 O/air as the oxidant in propionic acid, no reaction happening in acetic acid (Scheme 16b) [98] or Pd(OAc) 2 and Cu(OCOt-Bu) 2 through a reaction involving heterotrimetallic cyclic PdCu 2 (η 2 -C 2 H 4 ) 3 (μ-OPiv) 6 (Scheme 16c). [99] The aerobic Pd(OAc) 2 /heteropoly acid/EtCO 2 H procedure (Scheme 16b) has been used for oxida-tive coupling of arenes with α,β-unsaturated aldehydes [100a] and acrylonitriles. [100b] Jacobs and co-workers observed that the solvent-free Pd II -catalyzed reaction of anisole with 4phenylbut-3-en-2-one occurred faster in the presence of benzoic acid and without induction period, affording high yield of the DHR product (Scheme 17).…”

Strain-Release-Driven Phosphine and Rhodium Catalysis: Facile Synthesis of Unsymmetrical Tetrasubstituted Alkenes

“…In addition, deuterium-labeling experiments were carried out at low conversion to investigate kinetic isotope effects. An average kinetic isotope effect (k H /k D = 2.03) of three independent experiments suggested that Rh-mediated C–H activation might be the rate-determining step in this reaction (Scheme d, See SI for more details).…”