Catalytic Synthesis of Superlinear Alkenyl Arenes Using a Rh(I) Catalyst Supported by a "Capping Arene" Ligand: Access to Aerobic Catalysis

Alkyl and alkenyl arenes are used in a wide range of products. However, the synthesis of 1-phenylalkanes or their alkenyl variants from arenes and alkenes is not accessible with current commercial acid-based catalytic processes. Here, it is reported that an air-stable Rh(I) complex, (5-FP)Rh(TFA)(eta(2)-C2H4) (5-FP = 1,2-bis(N-7-azaindolyl)benzene; TFA = trifluoroacetate), serves as a catalyst precursor for the oxidative conversion of arenes and alkenes to alkenyl arenes that are precursors to 1-phenylalkanes upon hydrogenation. It has been demonstrated that coordination of the 5-FP ligand enhances catalyst longevity compared to unligated Rh(I) catalyst precursors, and the 5-FP-ligated catalyst permits in situ recycling of the Cu(II) oxidant using air. The 5-FP ligand provides a Rh catalyst that can maintain activity for arene alkenylation over at least 2 weeks in reactions at 150 degrees C that involve multiple Cu(II) regeneration steps using air. Conditions to achieve >13 000 catalytic turnovers with an 8:1 linear:branched (L:B) ratio have been demonstrated. In addition, the catalyst is active under aerobic conditions using air as the sole oxidant. At 80 degrees C, an 18:1 L:B ratio of alkenyl arenes has been observed, but the reaction rate is substantially reduced compared to 150 degrees C. Quantum mechanics (QM) calculations compare two predicted reaction pathways with the experimental data, showing that an oxidative addition/reductive elimination pathway is energetically favored over a pathway that involves C-H activation by concerted metalation-deprotonation. In addition, our QM computations are consistent with the observed selectivity (11:1) for linear alkenyl arene products.