Oxidative Alkenylation of Arenes Using Supported Rh Materials: Evidence that Active Catalysts are Formed by Rh Leaching

Abstract

This work focuses on the synthesis of supported Rh materials and study of their efficacy as pre‐catalysts for the oxidative alkenylation of arenes. Rhodium particles supported on silica (Rh/SiO2; ∼3.6 wt% Rh) and on nitrogen‐doped carbon (Rh/NC; ∼1.0 wt% Rh) are synthesized and tested. Heating mixtures of Rh/SiO2 or Rh/NC with benzene and ethylene or α‐olefins and CuX2 {X=OPiv (trimethylacetate) or OHex (2‐ethyl hexanoate)} to 150 °C results in the production of alkenyl arenes. When using Rh/SiO2 or Rh/NC as catalyst precursor, the conversion of benzene and propylene or toluene and 1‐pentene yields a ratio of anti‐Markovnikov to Markovnikov products that is nearly identical to the same ratios as the molecular catalyst precursor [Rh(μ‐OAc)(η2‐C2H4)2]2. These results and other observations are consistent with the formation of active catalysts by leaching of soluble Rh from the supported Rh materials.

Advances in Group 10 Transition-Metal-Catalyzed Arene Alkylation and Alkenylation

“…At 150 °C in the presence of 60 psig of ethylene and Cu­(OPiv) 2 , a TOF of 3.4 × 10 –2 s –1 (after 1 h) for styrene production using Pd­(OAc) 2 was determined. A TOF with similar magnitude (1.8 × 10 –2 s –1 ) was observed using [Rh­(η 2 -C 2 H 4 ) 2 (μ-OAc)] 2 as the catalyst precursor at 150 °C in the presence of 40 psig of ethylene and Cu­(OPiv) 2 as the oxidant . Thermolysis of Pd­(OAc) 2 in benzene in the absence of copper­(II) generates free acetic acid and Pd(0) .…”

“…In contrast, we have demonstrated that Pd­(OAc) 2 affords >2400 TOs of styrene after 24 h at 150 °C. Under some conditions, the formation of elemental Rh(0) or Pd(0) does not result in catalyst deactivation, since reoxidation to soluble molecular catalysts is feasible. ,,− In contrast, experiments with soluble Pt complexes are consistent with irreversible catalyst decomposition when Pt(0) is formed (Scheme ). (3) Pt­(II)-catalyzed ethylene hydrophenylation generates substantial diethylbenzenes.…”

“…A TOF with similar magnitude (1.8 × 10 −2 s −1 ) was observed using [Rh(η 2 -C 2 H 4 ) 2 (μ-OAc)] 2 as the catalyst precursor at 150 °C in the presence of 40 psig of ethylene and Cu(OPiv) 2 as the oxidant. 50 Thermolysis of Pd(OAc) 2 in benzene in the absence of copper(II) generates free acetic acid and Pd(0). 82 A similar reductive decomposition of [Rh(η 2 -C 2 H 4 ) 2 (μ-OAc)] 2 to form acetic acid and Rh(0) was also observed.…”

“… Also, our group has disclosed a series of studies using Rh catalysts (Scheme ). − Notable features of these reactions include (1) the use of Cu­(II) oxidants, for which we have demonstrated regeneration by dioxygen from air, ,, (2) the conversion of electron-deficient arenes, and (3) the selective formation of anti-Markovnikov addition products, 1-arylalkenes, with a >8/1 anti-Markovnikov/Markovnikov ratio in most reactions and higher selectivity under some conditions. ,, In an extension of Rh-catalyzed arene alkenylation, we recently reported oxidative conversion of unactivated arenes and alkenes to alkenylarenes using unpurified air or O 2 as the sole oxidant . This protocol uses a simple RhCl 3 salt ( 10 ) as a catalyst precursor, and >1000 TOs of styrene was demonstrated; however, under aerobic conditions benzaldehyde forms from the oxidation of styrene and is a major side product at high styrene TOs.…”

Mechanistic Studies of Styrene Production from Benzene and Ethylene Using [(η2-C2H4)2Rh(μ-OAc)]2 as Catalyst Precursor: Identification of a Bis-RhI Mono-CuII Complex As the Catalyst

“…Molecular complexes based on Ru, − Pd, − Ni, , Ir, − and Pt − have been reported to catalyze olefin hydroarylation (to produce alkyl arenes) and oxidative olefin hydroarylation (to produce alkenyl arenes) using unactivated arenes and olefins. Our group and others have reported Rh catalyzed aromatic C–H activation − and arene alkenylation using hydrocarbon substrates, ,− and we recently demonstrated oxidative olefin hydroarylation using [(η 2 -C 2 H 4 ) 2 Rh­(μ-OAc)] 2 ( 1 ) as the catalyst precursor. ,− These reactions incorporate the Rh catalyst precursor, arene, olefin and CuX 2 (X = carboxylate such as acetate, pivalate and 2-ethylhexanoate) and are generally heated between 150 and 165 °C. Potential advantages of the Rh-catalyzed alkenyl arene formation include: (a) selectivity for linear 1-aryl alkenes (>8:1 linear/branched ratio in most cases); (b) broad substrate scope including electron-deficient arenes; (c) under some reaction conditions, quantitative yield based on Cu­(II) oxidants as the limiting reagent; and (d) aerobic oxidation by recycling Cu­(II) with air, either during catalysis or in a step separate from catalysis.…”