A Reaction Valley Investigation of the Cycloaddition of 1,3-Dipoles with the Dipolarophiles Ethene and Acetylene: Solution of a Mechanistic Puzzle

The
reaction mechanism of the cycloaddition of 10 1,3-dipoles with
the two dipolarphiles ethene and acetylene is investigated and compared
using the Unified Reaction Valley Approach in a new form, which is
based on a dual-level strategy, an accurate description of the reaction
valley far out into the van der Waals region, and a comparative analysis
of the electronic properties of the reaction complex. A detailed one-to-one
comparison of 20 different 1,3-dipolar cycloadditions is performed,
and unknown mechanistic features are revealed. There are significant
differences in the reaction mechanisms for the two dipolarophiles
that result from the van der Waals complex formation in the entrance
channel of the cycloadditions. Hydrogen bonding between the 1,3-dipoles
and acetylene is generally stronger, which leads to higher reaction
barriers in the acetylene case, but which also facilitates to overcome
the problem of a reduced charge transfer from 1,3-dipole to acetylene.
Mechanistic differences are found in the prechemical and chemical
reaction regions with regard to reactant orientation, preparation
for the reaction, charge transfer, charge polarization, rehybridization,
and bond formation. It is shown that similarities in the reaction
barriers as determined by CCSD­(T)-F12/aug-cc-pVTZ calculations result
from a fortuitous cancellation of different electronic effects. In
general, a caveat must be made with regard to oversimplified descriptions
of the reaction mechanism based on orbital theory or energy decomposition
schemes.