Into the Blue: Ketene Multicomponent Reactions under Visible Light.

For the first time, a detailed study on the photophysical properties of variously substituted diazoketones and on their photoreactivity under blue LED irradiation was carried out. Despite very limited absorbance in the visible region, we have demonstrated that, independently from their structure, α-diazoketones all undergo a very efficient Wolff rearrangement. Contrarily to the same UV-mediated reaction, where photons can give rise to side processes, in this case, almost all absorbed photons are selective and effective, and the quantum yield is close to 100%. If the rearrangement is carried out in the presence of isocyanides and carboxylic acids/silanols, the photoreactivity is not affected, and the resulting ketenes can afford α-acyloxy- and α-silyloxyacrylamides through two distinct multicomponent reactions, performed both in batch and under continuous flow, with improved selectivity and broader scope. These photoinduced multicomponent reactions can be coupled with other visible-light-mediated transformations, thus increasing the diversity of the molecules obtainable by this approach.