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 atom economic, implementing an inexpensive, non-precious 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 are effectively and selectively transformed to the respective internal E-alkenes. Preliminary results show chain walking isomerization at elevated temperature. Mechanistic studies were carried out including stochiometric reactions and in situ NMR analysis. These experiments are flanked by computational studies. Based on these, the catalytic process is initiated by liberation of “BH3” as a hydroborated alkene. 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.