-methoxy substitution seems to facilitate the cleavage, which accounts for the better performance of the substituted vs the unsubstituted phenyl ketone catalyst. When applying the catalyst to the photochemical deracemization of other aza-1-isoindolinones, it was observed that the success of the reaction depends on the stability of the intermediate formed by back hydrogen atom transfer (bHAT) from the catalyst to the substrate. Racemic 6-aza-3-benzyl-1-isoindolinone and 7-aza-3-benzyl-1-isoindolinone could be successfully employed in a photochemical deracemization, because they offer either a nitrogen atom at N6 (6-aza) or the isoindolinone oxygen atom (7-aza) to form enamine and enol intermediates upon bHAT. 5-Aza-3-benzyl-1-isoindolinone leads, in a nonpolar solvent, only to high-energy intermediates and delivers a low enantiomeric excess under standard deracemization conditions.
Freund et al. (Sat,) studied this question.