Bending Enamine Patterns of Stabilized Pentalenes into ?Polymethine Ylides?

We report the synthesis and structural study of 2-substituted 1,3-bis(dimethylamino)pentalenes. The two electrons donating substituents shift the formally anti-aromatic pattern toward more suitable polarized structures. A subtle steric trade-off can result either in planar cyclopentadienyl vinamidiniums or in distorted structures featuring a pyramidalized ylidic carbon adjacent to a stabilized pi-conjugated iminium (polymethine). This latter pattern mimics a postulated activated distorted geometry for key Breslow intermediates in the active site of thiamine-dependent enzymes. It highlights how the energetic drive to avoid anti-aromaticity can be used to access models for unconventional distorted conformations of organic molecules.

Automated summary

This paper reports the synthesis and structural study of 2-substituted 1,3-bis(dimethylamino)pentalenes. The substituents donate two electrons, shifting the aromatic pattern to more suitable polarized structures. Depending on the steric trade-off, either planar cyclopentadienyl vinamidiniums or distorted structures featuring a pyramidalized ylidic carbon adjancent to a stabilized pi-conjugated iminium can be obtained. This study provides insights into unconventional distorted conformations of organic molecules.