Stepwise Diels–Alder: More than Just an Oddity? A Computational Mechanistic Study

We have employed hybrid DFT and SCS-MP2 calculations
at the SMD-PCM–6-311++G­(2d,2p)//6-31+G­(d)
level to investigate the relationship between three possible channels
for forming a Diels–Alder adduct from a highly nucleophilic
diene and moderately to highly electrophilic dienophiles. We discuss
geometries optimized using the B3LYP and M06-2X functionals with the
6-31+(d) basis set. The transition states and intermediates are characterized
on the basis of geometric and electronic properties, and we also address
the possibility of predicting detectability of a zwitterionic intermediate
based on its relative stability. Our results show that a conventional
Diels–Alder transition state conformation yields intermediates
in all four investigated cases, but that these are too short-lived
to be detected experimentally for the less activated reactants. The
stepwise trans pathway, beginning with a conjugate
addition-like transition state, becomes increasingly competitive with
more activated reactants and is indeed favored for the most electrophilic
dienophiles. Addition of a trans diene leads to a
dead-end as the trans intermediates have insurmountable
rotation barriers that prohibit formation of the second bond, unless
another, heterocyclic intermediate is formed. We also show that introduction
of a hydrogen bond donating catalyst favors a stepwise pathway even
for less activated dienophiles.