Photogeneration of Amines from α-Keto Carbamates:  Photochemical Studies

We investigate the design of new photoprecursors of organic bases and the steric and electronic factors that control their photocleavage to give free amines. The design strategy involves the protection of an amine with novel [(benzoinyl)oxy]carbonyl groups and substituted analogs. The resulting masked amines owe their photosensitivity to the rich photochemistry of the benzoinyl chromophore. The photochemistry of this chromophore allows for the clean photogeneration of free amine upon irradiation with UV light below 400 nm in both the solid state and in solution. The structure of the benzoinyl chromophore was varied to determine the optimal chromophore design. By varying the chromophore design, the influence of various steric and electronic effects on the photoliberation of free amines from α-keto carbamates could be gauged. Structural modification of the aryl rings was intended to probe the electronic factors of the photocyclization. Substitution at the 2 position was varied to investigate the steric factors involved in photocyclization. The practical potential of these photoactive carbamates as organic sources of photogenerated base was demonstrated spectroscopically (UV, IR, and NMR). GC-MS product studies also proved diagnostic in identifying photogenerated base. The thermal properties of this class of base photogenerators were also determined. α-Keto carbamates derived from 3',5'-dimethoxybenzoin and its substituted analogs appear particularly attractive. These carbamates undergo near quantitative photocleavage to give free amine along with the corresponding substituted benzo[b]furan photocyclization product. Preliminary evaluation of the solid state quantum yield for cyclohexylamine photogeneration from 3,3',5,5'-tetramethoxybenzoin cyclohexyl carbamate ranged from 0.03 to 0.08 depending on the exposure wavelength. The variation in photoefficiency correlates with the UV absorbance of the keto chromophore indicating direct excitation of the carbonyl group is the preferred pathway for photocleavage.