Mechanistic Insights into the Methylenation of Ketone by a Trinuclear Rare-Earth-Metal Methylidene Complex

Trinuclear
rare-earth-metal methylidene (CH22–)
complexes are an emerging class of compounds that serve as methylidene
transfer agents for methylenation of carbonyl compounds. Herein, the
reaction of a trinuclear scandium methylidene complex with acetophenone
was used as a model reaction of the multimetallic-cooperating methylidene
transfer case, and its detailed mechanism has been investigated by
the DFT approach. The analyses of Wiberg bond index, electron occupation,
the frontier molecular orbital, and natural charge provide us a clear
and comprehensive understanding of the CH22–/O2– group interchange process assisted by cooperating
multimetal sites. The mechanism presented here is markedly different
from conventional Wittig and transition-metal-mediated Wittig-type
reactions. In addition, the behavior of μ3-CH2 in a multinuclear complex system is also demonstrated. This
study not only enriches the chemistry of metal Wittig-type reactions
but also sheds light on the intermetallic cooperation for methylidene
transfer.