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)­(η2-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 °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 °C,
an 18:1 L:B ratio of alkenyl arenes has been observed, but the reaction
rate is substantially reduced compared to 150 °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.