Vinylic C–H Activation of Styrenes by an Iron–Aluminum Complex

The oxidative addition
of sp2 C–H bonds of alkenes
to single-site transition-metal complexes is complicated by the competing
π-coordination of the CC double bond, limiting the examples
of this type of reactivity and onward applications. Here, we report
the C–H activation of styrenes by a well-defined bimetallic
Fe–Al complex. These reactions are highly selective, resulting
in the (E)-β-metalation of the alkene. For
this bimetallic system, alkene binding appears to be essential for
the reaction to occur. Experimental and computational insights suggest
an unusual reaction pathway in which a (2 + 2) cycloaddition intermediate
is directly converted into the hydrido vinyl product via an intramolecular sp2 C–H bond activation across
the two metals. The key C–H cleavage step proceeds through
a highly asynchronous transition state near the boundary between a
concerted and a stepwise mechanism influenced by the resonance stabilization
ability of the aryl substituent. The metalated alkenes can be further
functionalized, which has been demonstrated by the (E)-selective phosphination of the employed styrenes.