Lewis Acid Activation of Carbodicarbene Catalysts for Rh-Catalyzed Hydroarylation of Dienes

The
activation of carbodicarbene (CDC)–Rh­(I) pincer complexes
by secondary binding of metal salts is reported for the catalytic
site-selective hydro-heteroarylation of dienes (up to 98% yield and
>98:2 γ:α). Reactions are promoted by 5 mol % of a
readily
available tridentate (CDC)–Rh complex in the presence of an
inexpensive lithium salt. The reaction is compatible with a variety
of terminal and internal dienes and tolerant of ester, alkyl halide,
and boronate ester functional groups. X-ray data and mechanistic experiments
provide support for the role of the metal salts on catalyst activation
and shed light on the reaction mechanism. The increased efficiency
(120 to 22 °C) made available by catalytic amounts of metal salts
to catalysts containing C(0) donors is a significant aspect of the
disclosed studies.