Photochemical Reactions of Alkenyl Phenylglyoxylates

Abstract

The photochemical reactions of alkyl phenylglyoxylate esters whose ester function contains double bonds of differing substitution, situated at varying distances from the carbonyl group, have been studied. The intramolecular Paterno `-Bu ¨chi reaction is the dominating photoreaction when an electron rich alkenyl group is situated at the proper distance as in 1′,5′-dimethylhex-4′-enyl phenylglyoxylate (6). When the distance between the excited carbonyl group and the same alkenyl function is increased, as it is in 3′,7′-dimethyl-6′-octenyl phenylglyoxylate (7), intramolecular γ-hydrogen abstraction (Norrish type II) products, as well as the products of intramolecular photocycloaddition, are observed. We suggest this to be the result of a competitive distant dependent electron transfer reaction between the excited carbonyl group and the alkene. A longer distance of separation between these two reactive functions decreases the efficiency of formation of the cycloaddition product. When the separation of the same alkene from the glyoxylate is shortened as in 4′-methylpent-3′-enyl phenylglyoxylate (5), electron transfer-induced remote proton abstraction leading to cyclol formation occurs, in addition to the Paterno `-Bu ¨chi reaction. Further reducing the distance as in 3′-methylbut-2′-enyl phenylglyoxylate (3) results in only the normal intramolecular (Norrish type II) and intermolecular hydrogen abstraction products. At a distance where both oxetane and cyclol formation are possible as in 5, decreasing the electron richness of the alkenyl group as in cis-3′-hexenyl phenylglyoxylate (cis-4) eliminates the oxetane product. Formation of cyclol is shown to be conformationally controlled in that a similar product is not observed to be formed from trans-3′-hexenyl phenylglyoxylate (trans-4). If both the electron richness of the alkenyl group and the distance of separation are decreased as in 2′-hexenyl phenylglyoxylate (2), only γ-hydrogen abstraction products result. Cis-trans isomerization of the olefin is not observed in either 2 or 4.

Controlled Release of Perfumery Aldehydes and Ketones by Norrish Type-II Photofragmentation of -Keto Esters in Undegassed Solution

“…83 ppm suggests that these two C-atoms should be directly connected to an O-atom, whereas the doublet recorded at 53 ppm indicates a C-atom that is further removed from the same O-atom. Our structural assignment is in agreement with the compound reported by Hu and Neckers [31].…”

Polystyrene‐Based 2‐Oxoacetates for the Light‐Induced Release of Fragrances Under Realistic Application Conditions

“…To assess the efficiency of our delivery systems, we compared the performance of fragrance conjugates 2a – 2c with that of the corresponding non‐polymeric profragrances 2‐phenylethyl 2‐oxo‐2‐phenylacetate ( 3 ) [ 18 ] and ( Z )‐3‐hexenyl 2‐oxo‐2‐phenylacetate ( 4 , [ 19 ] Figure ), as well as with the respective free aldehydes. The different compounds were deposited onto glass slides, and the light‐induced fragrance release was monitored by dynamic headspace analysis.…”

“…The amount of ( Z )‐3‐hexenal to be released from 2b was 22 wt% ( M = 98.15 × 0.36 = 35.3 g mol −1 ) and from 2c was 9 wt%; thus, to release 4.5 mg of ( Z )‐3‐hexenal, 20.5 mg of 2b and 50.0 mg of 2c had to be weighed in. The fragrance released from copolymers 2a – 2c was compared with that of the corresponding non‐polymeric profragrances 2‐phenylethyl 2‐oxo‐2‐phenylacetate ( 3 ) [ 18 ] and ( Z )‐3‐hexenyl 2‐oxo‐2‐phenylacetate ( 4 ), [ 19 ] the preparation of which has been reported in the literature. To release 4.5 mg of 2‐phenylacetaldehyde (47 wt%) in total, 9.6 mg of 3 had to be weighed in, and to release 4.5 mg of ( Z )‐3‐hexenal (42 wt%), 10.7 mg of 4 had to be weighed in.…”

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

“…[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. [63,64] A systematic study of the photoreaction in non-degassed solution revealed that both alkyl and ester chain fragmentations are in competition with each other.…”