The photosensitised oxidation of amines.
thesisposted on 19.11.2015, 08:47 by R. F. Bartholomew
The photo-oxidation of amines sensitised by benzophenone and other ketones, fluorescein and thiazine dyes, and aromatic hydrocarbons has been examined. The major products obtained from the sensitised photo-oxidation of aliphatic secondary and tertiary amines were aldehydes and imines. Only aldehydes were obtained from aliphatic primary amines. The benzophenone sensitised photo-oxidation of N-alkylated anilines gave de-alkylated amines, amides and aldehydes. It is proposed that the primary process in all these reactions is the interaction of the amine with excited sensitiser, and that the products are formed by subsequent radical reactions. The reactions are not initiated by attack of singlet oxygen o the amine. The most important evidence presented in favour of a radical mechanism is as follows. 1. The types of products formed are compatible with a radical mechanism. The same types of products are obtained with all the sensitisers used, which implies that the sensitisers act by a common mechanism. 2. Only those compounds which are photoreduced by amines in deoxygenated solution act as sensitisers for the photo-oxidation of these amines. This implies that the primary process in photo-oxidation is the same as in photoreduction, namely the interaction of amine with excited sensitiser. 3. Plash photolysis of aerobic solutions of dyes or aromatic hydrocarbons with amines produced transient species which could only have "been formed "by the interaction of amine with the excited sensitiser. 4. Amines such as triethylamine and ethyldi-isopropylamine, which are efficient physical quenchers of singlet oxygen, are photo-oxidised efficiently with all the sensitisers used. This is evidence that singlet oxygen is not the active species in these photo-oxidations. Evidence from flash photolysis studies is also presented, which implies that the primary process with all the sensitisers involves electron transfer or exciplex formation. The fact that amines can compete effectively with oxygen for the excited sensitiser is due to the rapidity of such electron transfer processes.