Computational and Experimental Studies into Photoenamination/Diels–Alder Reactions

The scope of the photoenolization/Diels–Alder (oxa-PEDA) process is wide, whereas the corresponding aza-variant (photoenamination/Diels–Alder, aza-PEDA) is currently limited to N-silylimines. This work uses computational and experimental studies to better understand the challenges of the aza-PEDA reaction and hence expand its narrow scope. Prompted by a recent report that discussed a triplet mechanism, we reevaluate the mechanism of the aza-PEDA process on the singlet manifold. Several significant hurdles are presented upon switching from O to N: the 1,5-hydrogen atom transfer reaction that converts the triplet imine to the enamine is less energetically favored than the corresponding reaction in triplet ketones, the conformation required for this reaction is significantly less populated, and the photoexcited state can undergo radiationless decay. An otherwise promising N-tosylimine precursor was shown to favor an alternative pathway involving photochemical N–S bond cleavage. Aza-PEDA reactions were finally extended beyond N-silylimines by way of an N-methanesulfonylimine derivative.