Photogeneration of Amines from α-Keto Carbamates:  Photochemical Studies

Abstract

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 R-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. R-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.

365nm-Light-sensitive photobase generators derived from trans-o-coumaric acid

“…Although PBGs have less photosensitivity than PAGs in general, the former have the merit that erosion of metallic substrate is circumvented. Various kinds of PBGs have been developed, including acyloxyimines [3], oxime-urethanes [4], phthalimido carbamates [5], and α-keto carbamates [6]. o-Nitrobenzyloxycarbonyl-type PBGs are widely used to generate aliphatic amines by 254 nm-or 365 nm-light irradiation [7].…”

Photosensitivity Characteristics of UV Curable Organic-Inorganic Hybrids Sensitized with Benzoin Derivatives as Photobase Generators

“…Anionic UV curing is an excellent method because it is free from polymerization inhibition by oxygen, termination by water, corrosion of metal substrates, and volume shrinkage. Until now, several researchers have reported PBGs and demonstrated the advantages of PBGs in the field of photolithography [5][6][7][8][9][10][11]. The improvement of the quantum yield for photobase generation and of the basicity of photogenerated bases is strongly needed.…”

Synthesis and photoinitiation properties of a novel quaternary ammonium tetraphenylborate salt

“…PBGs can overcome this defect of PAGs and do not have such unfavorable properties. Cobalt-amine complexes, 2,3 acyl oximes, 1,[4][5][6][7][8] carbamates, 1,9,10 formamides, 11 and quaternary ammonium salts have been used as PBGs. [12][13][14][15][16] Among these PBGs, quaternary ammonium salts have been develop recently.…”