Reactivity
for the Diels–Alder Reaction of
Cumulenes: A Distortion-Interaction Analysis along the Reaction Pathway
- Publication date
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Abstract
Cumulenes,
including allene, ketenimine, and ketene, can be employed
as dienophiles in Diels–Alder type reactions. The activation
energies of a Diels–Alder reaction between cyclopentadiene
and either the CC bond or the other CX (X = C, N,
or O) bond in cumulenes have been calculated by G3B3, CBS-QB3, M06-2X,
and B3LYP methods. The reactivity trend for the CC bond in
cumulenes is allene > ketenimine > ketene and that of the CX
bond in cumulenes is ketene > allene > ketenimine. Application
of
distortion-interaction analysis only at transition states does not
give a satisfactory explanation for these reactivities. By employing
distortion-interaction analysis along reaction pathways, we found
that the reactivity of the CC and CX bond in cumulenes
is controlled by both of its distortion and interaction energies.
The lowest distortion energy of allene leads to its highest reactivity;
the higher interaction energy results in higher activation energy
of ketene than that of ketenimine. Compared with the reactivity of
the CX bond in cumulenes, the CO bond in ketene has
the lowest activation energy to react with cyclopentadiene, due to
its lowest interaction energy, whereas the lower distortion energy
of ketenimine than that of allene leads to a higher reactivity. The
distortion energy of the reactants can be attributed to folding ability
and molecule strain. The corresponding interaction energy of the reactants
is controlled by orbital interaction, closed-shell repulsion, and
static repulsion