Alpha-substituent effect on the photodeprotection of 4,5-dimethoxy-2-nitrobenzyl acetate

Since the first report of caged ATP, o-nitrobenzyl groups have been widely used as photolabile protecting groups, and analogue compounds have been used to study hydrogen abstraction reactions. In this work, a caged compound containing acetic acid was used to directly analyse the multiple steps of the deprotection reaction, with the aim of elucidating how and during which steps the α-substituent (_αR) effect occurs. UV pump–visible probe spectroscopy was used to observe the ultrafast generation of biradicals and zwitterions as reaction intermediates. These experimental results and the results of DFT calculations suggested that the reaction proceeded in the four steps. During the _αH abstraction reaction, the small _αR volume results in a small energy gap between the S*(n-π*) and ground states and thus a fast rate of _αH abstraction. Conversely, during the cyclization reaction, the small _αR volume results in a high activation energy and thus a slow rate of cyclization.