Engineering reactions in crystals: gem-dialkoxy substitution enables the photodecarbonylation of crystalline 2-indanone

The photochemical reactivity of three 1,1,3,3-tetrasubstituted 2-indanones was investigated in solution and in crystals. While solution irradiation of tetramethyl-2-indanone 2 leads to 1-isopropenyl-2-isopropyl benzene 5 in high yield, crystals of 2 were completely inert. In contrast, samples of 1,3-bis(ethylenedioxy)-2-indanone 3 reacted efficiently in both media. While solution irradiation yielded benzocyclobutane 7 and o-xylylene 8, only the former was formed in the solid state. Finally, the 1,3-bis-thioketal analog 4 reacted sluggishly in solution but not at all in the solid state. Differences in reactivity between the three indanones were analyzed in terms of radical stabilization effects and possible intramolecular quenching mechanisms.