The meta and para OH Substitution Effect on C‐Phenyl‐Nitrilimine Bond‐Shift Isomers

The geometric and electronic structure of 1,3-dipolar species, in particular of nitrile imines, can be surprisingly intricate. A particular example is the C-phenyl-nitrilimine, which exists as two energy minima that constitute bond-shift isomers. Questioning whether substituents in the phenyl ring could affect the existence of such bond-shift isomers, here we investigate the meta and para OH substituted derivatives. These two nitrile imines were generated in an argon matrix at 15 K by UV-irradiation of 2H-tetrazole precursors and found to photoisomerize to carbodiimides via 1H-diazirines. Using IR spectroscopy and quantum mechanical calculations, the C-(3-hydroxyphenyl)-nitrilimine was established to exist as two bond-shift isomers, with predominant propargylic and allenic type structures, whereas the C-(4-hydroxyphenyl)-nitrilimine was established to exist only as one species with a significant allenic type structure. The different effects of the OH substitution in meta and para positions on the bond-shift isomerism are rationalized with the support of Natural Resonance Theory and Hirshfeld atomic charges. Such fundamental knowledge, on the understanding of how substitution affects the structural characteristics of C-phenyl-nitrilimines, opens the door to modulate the chemistry of this important class of reactive compounds (e.g. in cycloaddition reactions) by appropriate tuning of their substitution (substituent type and position).