Mechanistic Studies of Single-Step Styrene Production Catalyzed by Rh Complexes with Diimine Ligands: An Evaluation of the Role of Ligands and Induction Period

Abstract

Studies of catalytic benzene alkenylation using different diimine ligated Rh­(I) acetate complexes and Cu­(OAc)2 as the oxidant revealed statistically identical results in terms of activity and product selectivity. Under ethylene pressure, two representative diimine ligated rhodium­(I) acetate complexes were demonstrated to exchange the diimine ligand with ethylene rapidly to form [Rh­(μ-OAc)­(η2-C2H4)2]2 and free diimine. Thus, it was concluded that diimine ligands are not likely coordinated to the active Rh catalysts under catalytic conditions. At 150 °C under catalytic conditions using commercial Cu­(OAc)2 as the oxidant, [Rh­(μ-OAc)­(η2-C2H4)2]2 undergoes rapid decomposition to form catalytically inactive and insoluble Rh species, followed by gradual dissolution of the insoluble Rh to form the soluble Rh, which is active for styrene production. Thus, the observed induction period under some conditions is likely due to the formation of insoluble Rh (rapid), followed by redissolution of the Rh (slow). The Rh decomposition process can be suppressed and the catalytically active Rh species maintained by using soluble Cu­(II) oxidants or Cu­(OAc)2 that has been preheated. In such cases, an induction period is not observed.

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

“…Dihydrogen chemisorption of the Rh/SiO 2 material indicated a metal dispersion of ∼0.7 (H/Rh ratio), and the average metal particle size measured by TEM was found to be 2.2(5) nm (Figure 1, Table S1). The presence of metallic Rh(0) was confirmed with XPS, which showed a binding energy at 307.3 eV (Figure S1) [37,52,53] …”

“…Catalytic Arene Alkenylation using Rh/SiO 2 as Catalyst Precursor . We began our studies of catalysis using supported Rh materials with the conversion of benzene and ethylene to styrene using conditions that are similar to our previously reported catalysis using molecular Rh catalyst precursors [13,14,36–38] . All reactions were performed at least in triplicate with results reported with standard deviations.…”

“…A plot of peak area ratios versus molar ratios gave a regression line using hexamethylbenzene (HMB) as the internal standard. For the GC/FID instrument, the slope and correlation coefficient of the regression lines were 1.67 and 0.99 (styrene), 0.87 and 0.99 (trans‐stilbene), 1.40 and 0.99 (allylbenzene), 1.23 and 0.99 (α‐methyl styrene), 1.47 and 0.99 (cis‐β‐methyl styrene), 1.38 and 0.99 (trans‐β‐methyl styrene), respectively [37] . The turnover (TO) is defined as the product (i.e.…”

“…Recently, we reported Rh‐catalyzed arene alkenylation to directly synthesize styrene and 1‐aryl alkenes using molecular catalysts in homogeneous processes [13,14,36–41] . Based on the mechanistic studies, our initial proposed catalytic cycle involves (Scheme 3): a) Rh‐carboxylate assisted arene C−H activation, b) ethylene coordination and insertion into a Rh‐aryl bond, c) β‐hydride elimination, and d) alkenyl arene dissociation and oxidation of a Rh−H intermediate with CuX 2 (X=carboxylate) to regenerate the starting catalyst.…”

“… The proposed induction period pathway for Rh(I) mediated styrene synthesis using Cu(OAc) 2 as the oxidant, (X=carboxylate groups) [37] …”