A Solvent Effect That Influences the Preparative Utility of N-(Silylalkyl)phthalimide and N-(Silylalkyl)maleimide Photochemistry

The photochemistry of selected N-silylalkyl-substituted phthalimides and maleimides has been investigated with the aim of exploring the generality and preparative consequences of an intriguing solvent effect on excited-state reaction chemoselectivities and quantum efficiencies. An example of this effect is found in the photochemistry of N-[(trimethylsilyl)butyl]phthalimide 10, where irradiation in MeCN leads to production of a mixture of four products that arise by excited-state intramolecular hydrogen-atom abstraction. In contrast, the benzoindolizidine 15 is the sole product produced by a single electron transfer (SET)−desilylation pathway upon irradiation of 10 in 35% H₂O−MeCN. Another example of this solvent effect is found in the photochemistry of the N-silylpropyl-maleimide 17. Irradiation in MeCN results in the production of the 2+2-dimer 19 whereas the pyrrolizidine 18 is generated exclusively by irradiation of 17 in 35% H₂O−MeCN. The results of fluorescence and triplet sensitization experiments suggest that the solvent effect has multiple sources including the control of the nature, reactivity, and intrinsic lifetimes of singlet and triplet excited states of the phthalimide and maleimide systems. The exploratory studies have clearly demonstrated the generality of this solvent effect and how it can be used to enhance the preparative utility of the photochemistry of N-(silylalkyl)phthalimides and N-(silylalkyl)maleimides.