Alkene Isomerization Catalyzed by a Mn(I) Bisphosphine Borohydride Complex

An additive-free manganese-catalyzed isomerization of
terminal
alkenes to internal alkenes is described. This reaction is implementing
an inexpensive nonprecious metal catalyst. The most efficient catalyst
is the borohydride complex cis-[Mn(dippe)(CO)2(κ2-BH4)]. This catalyst operates
at room temperature, with a catalyst loading of 2.5 mol %. A variety
of terminal alkenes is effectively and selectively transformed into
the respective internal E-alkenes. Preliminary results
show chain-walking isomerization at an elevated temperature. Mechanistic
studies were carried out, including stoichiometric reactions and in
situ NMR analysis. These experiments are flanked by computational
studies. Based on these, the catalytic process is initiated by the
liberation of “BH3” as a THF adduct. The
catalytic process is initiated by double bond insertion into an M-H
species, leading to an alkyl metal intermediate, followed by β-hydride
elimination at the opposite position to afford the isomerization product.