Mechanistic study of the light-initiated generation of free diazoalkanes: Towards photo-orthogonal synthesis

An in-depth mechanistic study for the photogeneration of free diazoalkanes from N-tosylhydrazone precursors by combining synthetic observations with spectroscopic and theoretical methods is presented. The N-tosylhydrazones have been previously established as donors for alkyl diazo species upon light irradiation, but exact mechanistic details of this photodissociation have remained elusive. Investigations of cyclohexane tosylhydrazone (CyTsH) by time-resolved FTIR spectroscopy proved the role of the deprotonated CyTs- as the light-harvesting species and revealed an intricate dependency of the thermal lifetime of the resulting diazoalkane on the deprotonating base. Computational studies including multiple approaches and levels of theory as well as rigorous benchmarking elucidated the dissociation mechanism via an allowed charge transfer state, a resulting destabilization of the dissociating bond, and a fast change of electronic character of the S1. These insights allow to suggest specific reaction conditions for photolabile or previously incompatible reaction partners thus paving way towards photo-orthogonal synthetic strategies.