jo071004q_si_002.pdf (3.14 MB)
Insights on Co-Catalyst-Promoted Enamine Formation between Dimethylamine and Propanal through Ab Initio and Density Functional Theory Study
journal contribution
posted on 2007-10-26, 00:00 authored by Mahendra P. Patil, Raghavan B. SunojThe mechanistic details on enamine formation between dimethylamine and propanal are unraveled using
the ab initio and density functional theory methods. The addition of secondary amine to the electrophile
and simultaneous proton transfer results in a carbinolamine intermediate, which subsequently undergoes
dehydration to form enamine. The direct addition of amine as well as the dehydration of the resulting
carbinolamine intermediate is predicted to possess fairly high activation barrier implying that a unimolecular
process is unlikely to be responsible for enamine formation. Different models are therefore proposed
which could explain the relative ease of enamine formation under neat condition as well as under the
influence of methanol as the co-catalyst. The explicit inclusion of either the reagent or the co-catalyst is
considered in the transition states as stabilizing agents. The participation of the reagent or the co-catalyst
as a monofunctional ancillary species is found to stabilize the transition states relative to the unassisted
or the direct addition/dehydration pathways. The reduction in enthalpy of activation is found to be much
more dramatic when two co-catalysts participate in an active bifunctional mode in the rate-determining
dehydration step. The transition structures exhibited characteristic features of a relay proton transfer
mechanism. The free energy of activation associated with the two methanol-assisted pathway is found to
be 16.7 kcal/mol lower than that of the unassisted pathway. The results are found to be in concurrence
with the available reports on the rate acceleration by co-catalysts in the Michael reaction between enamine
and methyl vinyl ketone under neat conditions.