Theoretical Studies on the Mechanism, Enantioselectivity, and Axial Ligand Effect of a Ru(salen)-Catalyzed Asymmetric Cyclopropanation Reaction

The mechanism of the Ru­(salen)-mediated
(salen = 1,2-cyclohexanediamino-N,N′-bis­(3,5-di-tert-butylsalicylidene)) cyclopropanation
reaction of styrene with ethyl diazoacetate (EDA) is explored with
density functional theory (DFT) methods. Five proposed reaction pathways,
including (1) a stepwise process containing the formation of carbene
species and cyclopropanation step (path a), (2) a one-step process
(path b), (3) a bis-carbene mechanism (path c), (4) a three-centered-intermediate
pathway (path d), and (5) the main side reaction of dimerization of
EDA (path e), are taken into consideration to determine the most favorable
mechanism. Computational results indicate that path a with a barrier
of 27.9 kcal/mol (trans) is superior to all other
pathways. The geometries of the critical transition states are picked
out for further analyses. It is found that the C–Ha group of the catalyst plays a key role in enantioselectivity. The
destruction of the active center (for example, a methyl group substituent)
can dramatically decrease the catalytic efficiency. In addition, the
axial ligands are found to mediate energy barriers of the formation
of carbene species and the cyclopropanation step in entirely opposite
directions. The natural bond orbital (NBO) analyses demonstrate that
carbene species show different characteristics of Fischer- or/and
Schrock-type complexes. This study may help to design and develop
more efficient catalysts for metal-mediated cyclopropanation reactions.