Proton Affinities of N‑Heterocyclic Olefins and Their Implications for Organocatalyst Design

The
proton affinity (PA) of a range of structurally different N-heterocycles with an exocyclic double bond (= N-heterocyclic olefins, NHOs) has been determined using
DFT calculations on the BLYP/def2-TZVPP level. It was found that NHOs
belong to the upper end of the superbasicity scale, covering PA values
from 262 to 296 kcal/mol. Different types of NHOs are compared with
each other and with frequently employed organocatalysts. To boost
PA, (a) the ability to delocalize the positive charge and (b) steric
pressure/ring strain which can be relieved after protonation were
identified as key tuning parameters. Importantly, by analyzing PA
alongside partial charges and molecular electrostatic potentials,
it is shown that an increase of double bond polarization is not a
necessary prerequisite for high PA. In contrast, the more basic, more
sterically congested NHOs minimize unfavorable interactions by partly
pyramidalyzing the nitrogen atoms, rendering the olefinic bond less
electron rich and less polarized. These findings are in excellent
agreement with experimental evidence on NHO catalysis, not only providing
guidelines for a more rational design regarding PA/basicity but also
suggesting that NHOs could be specifically tailored toward either
nucleophilic or base-type reaction pathways.