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.