The photochemistry of diarylmethylammonium salts: Heterolysis vs homolysis
LE3 .A278 2010
Bachelor of Science
Photochemistry is the study of chemical reactions that may result from the absorption of electromagnetic radiation. A common consequence of electronic excitation for many molecules is bond cleavage. It can occur through two major competing pathways. Photo-heterolysis of an organic compound generates a carbocation intermediate, and photo-homolysis generates a radical intermediate. In general, homolytic reactions producing radical intermediates can give a large number of products due to the high reactivity of radicals and their tendency to react with oxygen. In contrast, carbocations produced by heterolytic reactions have more predictable chemistry, and in the presence of good nucleophiles, often a single product is formed. We sought to find a leaving group which gives primarily heterolytic photochemical bond cleavage for a wide variety of chromophores. Encouraged by the high yields of heterolytic products from the quinone methide case and the apparent heterolytic photochemistry observed in the limited available studies on trialkyl ammonium salts, we wanted to make derivatives bearing trialkyl amine leaving groups and investigate the homolytic to heterolytic cleavage ratio. If these proposed quaternary ammonium salts show primarily heterolysis and high quantum yields, trialkyl amine leaving groups might become the preferred leaving groups for conducting ionic photochemistry, and might greatly widen the application of such reactions in synthesis. Triethyl- 9- fluorenyltriethylammonium bromide ( 14) and triethylbenzyhydrylammonium bromide ( 15) were synthesized in a three step procedure from fluorenone ( 16) and benzophenone ( 17), respectively. They were characterized using 1H and 13C NMR xi spectroscopy. To study the photochemistry of 14 and 15, we irradiated them in a solution of 1: 1 water- methanol at 254 nm, the photoproducts were extracted and the ratio of heterolysis: homolysis was determined. For 14 the ratio of heterolysis: homolysis was 0.04: 1 and for 15 it was 3.3: 1. A mechanism is proposed which accounts for the unexpected heterolysis: homolysis ratios observed. Our results indicate that the trialkyl amine leaving group is better than most nucleophiles at promoting heterolytic bond cleavage in the excited state in the case of 14, however, this ability unfortunately does not translate to all chromophores, as is evident from the case of 15.
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