Mechanism of the Ru–Allenylidene to Ru–Indenylidene Rearrangement in Ruthenium Precatalysts for Olefin Metathesis

The intramolecular
allenylidene RuCl2(PR3)2(CCCPh2) to indenylidene
RuCl2(PR3)2(Ind) rearrangement that
occurs during the synthesis of Ru-based precatalysts for olefin metathesis
is presented. In the absence of acid, the ring closure via C–H
activation was shown to be unfavored for energy barriers up to 70
kcal/mol. Thus, it turned out to be HCl (or other acid) that plays
a crucial role during formation of the indenylidene, as the upper
energy barrier decreases to a reasonable 35 kcal/mol. Moreover, we
proved computationally that depending on the nature of the phosphine
the intramolecular rearrangement is either facilitated (PPh3) or slightly hampered (PCy3), which is in line with experimental
results.