Light induced controlled release of fragrances by Norrish type II photofragmentation of alkyl phenyl ketones

Abstract

The use of alkyl phenyl ketones as delivery systems for the controlled release of fragrances was investigated by photoirradiation of undegassed solutions with a xenon lamp as well as natural sunlight. A large variety of precursor compounds was prepared efficiently in a few reaction steps from commercially available starting materials. The Norrish type II photofragmentation was found to be the predominant reaction pathway to yield the desired perfumery alkenes and acetophenones in polar and apolar solution. Systematic GC-MS analysis of the irradiated solutions allowed identification of a series of side products that are due to the presence of oxygen. A detailed analysis of the product distribution after irradiation was carried out for a series of 4-alkoxy-1-phenylbutanone derivatives. Besides the expected acetophenones, vinyl ethers and phenylcyclobutanols, the formation of alkyl formates, alcohols and 4-oxo-4-phenylbutanoates was observed. The product distribution as influenced by solvent polarity, precursor concentration and substituent effects was investigated. The utility of alkyl phenyl ketones as precursors for the light induced controlled release of fragrances under natural daylight conditions was also demonstrated.

Controlled Release of Volatiles under Mild Reaction Conditions: From Nature to Everyday Products

“…Although the "classical" Norrish type II photoreaction depicted in Scheme 6 was found to be the predominant reaction observed for the photoirradiation around 350 nm in non-degassed solution or in different practical applications, a series of side products have been identified which arise from the presence of oxygen. [56] Photoirradiation of 11 in acetonitrile for three hours afforded 43 % of acetophenone and 40 % of decyl vinyl ether as the expected Norrish type II reaction products, together with 9 % of decyl formate and 9 % of decanol, both of which are perfumery ingredients themselves, as well as unquantified amounts of benzoic acid. [56,60] Whereas the formation of decanol remained unexplained, the release of benzoic acid and decyl formate was attributed to the rearrangement of a cyclic species which can be formed by reaction of oxygen with the intermediate 1,4-biradical (Scheme 6).…”

“…[56] Photoirradiation of 11 in acetonitrile for three hours afforded 43 % of acetophenone and 40 % of decyl vinyl ether as the expected Norrish type II reaction products, together with 9 % of decyl formate and 9 % of decanol, both of which are perfumery ingredients themselves, as well as unquantified amounts of benzoic acid. [56,60] Whereas the formation of decanol remained unexplained, the release of benzoic acid and decyl formate was attributed to the rearrangement of a cyclic species which can be formed by reaction of oxygen with the intermediate 1,4-biradical (Scheme 6). [51,56] Alkyl or aryl a-keto esters (2-oxoesters) also undergo Norrish type II reactions in the presence of oxygen to form aldehydes (or ketones) together with a carboxylic acid (Scheme 7).…”

“…[56,60] Whereas the formation of decanol remained unexplained, the release of benzoic acid and decyl formate was attributed to the rearrangement of a cyclic species which can be formed by reaction of oxygen with the intermediate 1,4-biradical (Scheme 6). [51,56] Alkyl or aryl a-keto esters (2-oxoesters) also undergo Norrish type II reactions in the presence of oxygen to form aldehydes (or ketones) together with a carboxylic acid (Scheme 7). [51,61,62] Keto esters 16-20 release the corresponding aldehydes or ketones in good yields on photooxidation with a xenon lamp or outdoor sunlight.…”

“…[51] The most general group of precursors that undergo Norrish type II reactions are alkyl phenyl ketones. [55] As the photoreaction tolerates a broad variety of structural modifications in proximity to the carbonyl group as well as in the alkyl chain, it has been successfully used to generate acetophenones together with a multitude of different other fragrance molecules [51] such as alkenes and vinyl ethers from alkyl phenyl ketones (10 and 11), [56,57] aldehydes and ketones from phenacyl ether derivatives (12 and 13), [58] as well as esters and lactones from phenacyl acetals (14 and 15) [59] (Scheme 6). Although the "classical" Norrish type II photoreaction depicted in Scheme 6 was found to be the predominant reaction observed for the photoirradiation around 350 nm in non-degassed solution or in different practical applications, a series of side products have been identified which arise from the presence of oxygen.…”

Diastereomeric Discrimination in the Lifetimes of Norrish Type II Triplet 1,4‐Biradicals and Stereocontrolled Partitioning of Their Reactivity (Yang Cyclization versus Type II Fragmentation)

“…Keywords: diastereoselectivity · ketones · photolysis · radicals the Norrish Type II reaction is concerned, its potential as applied to photorelease processes, [18] photocyclization reactions, [19] asymmetric synthesis, [20] etc. has begun to unfold only recently.…”