PHOTOGENERATION OF AMINES FROM ALPHA-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 am ine with novel [(benzoinyl)oxy]carbonyl groups and substituted analogs . The resulting masked amines owe their photosensitivity to the rich p hotochemistry of the benzoinyl chromophore. The photochemistry of this chromophore allows for the clean photogeneration of free amine upon i rradiation with UV light below 400 nm in both the solid state and in s olution. The structure of the benzoinyl chromophore was varied to dete rmine the optimal chromophore design. By varying the chromophore desig n, the influence of various steric and electronic effects on the photo liberation of free amines from alpha-keto carbamates could be gauged. Structural modification of the aryl rings was intended to probe the el ectronic factors of the photocyclization. Substitution at the 2 positi on was varied to investigate the steric factors involved in photocycli zation. The practical potential of these photoactive carbamates as org anic sources of photogenerated base was demonstrated spectroscopically (UV, IR, and NMR). GC-MS product studies also proved diagnostic in id entifying photogenerated base. The thermal properties of this class of base photogenerators were also determined. alpha-Keto carbamates deri ved from 3',5'-dimethoxybenzoin and its substituted analogs appear par ticularly attractive. These carbamates undergo near quantitative photo cleavage to give free amine along with the corresponding substituted b enzo[b]furan photocyclization product. Preliminary evaluation of the s olid state quantum yield for cyclohexylamine photogeneration from 3,3' ,5,5'-tetramethoxybenzoin cyclohexyl carbamate ranged from 0.03 to 0.0 8 depending on the exposure wavelength. The variation in photoefficien cy correlates with the UV absorbance of the keto chromophore indicatin g direct excitation of the carbonyl group is the preferred pathway for photocleavage.