Release of Terminal Alkynes via Tandem Photodeprotection and Decarboxylation of o-Nitrobenzyl Arylpropiolates in a Flow Microchannel Reactor

Photocleavable protecting groups (PPGs) offer a complementary protection paradigm compared to traditional protection groups. Herein, an o-nitrobenzyl (NB) PPG was employed to protect a variety of arylpropiolic acids. Upon a cascade of light-triggered photodeprotection in a microchannel reactor (residence times of 100-500 s), followed by Cu-catalyzed decarboxylation at 60 °C, the NB-protected arylpropiolic acid afforded a terminal alkyne. This terminal alkyne was further reacted in situ with an azide via click chemistry to yield a 1,2,3-triazole in a one-pot reaction. Furthermore, the effect of different substituents (methyl, vinyl, allyl, and phenyl) at the benzylic position on the rate of photodeprotection was studied. The quantum yields of photolysis for the benzylic-substituted esters were determined to be as high as 0.45 compared to the unsubstituted ester with a 0.08 quantum yield of photolysis.