Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the Carbene Electronic Structure and Cyclopropanation Reaction Mechanism

Present work addresses
the reactivity of several phenyl-substituted
metal–carbene complexes with 4-methylstyrene by means of density
functional theory OPBE simulations. Different paths that lead to cyclopropanation
were explored and compared to the olefin metathesis mechanism. For
this purpose, we chose four different catalysts: (i) the Grubbs second-generation
olefin metathesis catalyst, (ii) a Grubs second-generation-like complex,
in which ruthenium is replaced by iron, and (iii) two iron carbene
complexes (a piano stool and a porphyrin iron carbene) that experimentally
catalyze alkene cyclopropanation. Results suggest that the nature
of the applying mechanism is very sensitive to the coordination around
the metal center and the spin state of the metal–carbene complex.
Cyclopropanation by open-shell metal–carbene complexes seems
to preferentially proceed through a two-step radical mechanism, in
which the two C–C bonds are sequentially formed (path C). Singlet-state
carbenes proceed either through a direct attack of the olefin to the
carbene (path D) when the formation of the metallacycle is not feasible
or through a reductive elimination from the metallacyclobutane when
this intermediate is accessible both kinetically and thermodynamically
(path B).