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.
Keywords: axis chirality; bond breaking; heterohelicene; inversion; racemization.
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