Inversion of Diaza[5]Helicenes Through an N−N Bond Breaking Pathway

Abstract

1,1’,10,10’-Biphenothiazine and its S,S,S’,S’-tetroxide are diaza[5]helicenes with N−N linkages. Kinetic experiments on racemization together with DFT calculations revealed that they undergo inversion through the N−N bond breaking pathway rather than the general conformational pathway. In these diaza[5]helicenes with this inversion mechanism, the reduction of electronic repulsion in the N−N bond by modification of S to SO2 at the outer position of the helix led to a significantly higher inversion barrier, 35.3 kcal/mol, compared to [5]helicene. 1,1’,10,10’-Biphenothiazine S,S,S’,S’-tetroxide was highly resistant to acid-mediated N−N bond breaking and racemization under acidic conditions.