Direct Detection of the Photorearrangement Reaction of Quinoline-Protected Dialkylanilines^†

The photolysis reactions of (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ)-caged amines have been investigated using time-resolved spectroscopy methods. Unexpectedly, an unconventional Hofmann-Martius rearrangement reaction with high yield and regioselectivity occurred during the photolysis of some CyHQ-protected dialkylanilines (such as compounds 1a and 2a). To have more insights into the mechanism of this unexpected photorearrangement reaction, we characterized the reaction intermediates directly using time-resolved spectroscopy. Our new results showed that the anionic form of compound 1a was photoexcited to the singlet excited state, then a heterolytic cleavage of the C-N bond took place to give CyHQ⁺ and the corresponding aniline. Thereafter, the recombined intermediate 6 was found to appear in about 19.7 and 44.3 ps for 1a (A) and 2a (A), respectively, before the generation of an ortho-substituted aniline (1b and 2b) via the excited-state deprotonation of 6. Thus, a logical photodynamic mechanism of this photoinduced Hofmann-Martius rearrangement reaction was deduced. This new insight into the reaction mechanisms may be helpful for the design of novel related photoactivatable aniline molecules and for understanding other similar photorearrangement reaction mechanisms.