Photochemistry of Substituted Benzyl Acetates and Benzyl Pivalates: A Reinvestigation of Substituent Effects

Abstract

The photosolvolysis reactions, in methanol, of six substituted benzyl acetates (7a-9 and benzyl pivalates @a-9 were studied. Five major benzylic products were formed from two critical intermediates. The ethers (9) were formed from the ion pair, 15, and all of the other products (10-f4) were formed from the radical pair, 16. Quenching studies showed that only excited singlet state reactivity was important. The product yields were found to be highly substituent dependent. For instance, for the acetate esters, the yield of ether (9) varied from 2% for X = 4-OCH3 to 32% for X = 3-OCH3. Most of the differences in the yields could be attributed to ground state processes that occur after bond cleavage. The important competition is between electron transfer, converting the radical pair to the ion pair, and decarboxylation of RCOz'. The rates of electron transfer are shown to fit Marcus theory in both the normal and inverted regions. Direct heterolytic cleavage to form the ion pair is of minimal importance.

Oxidation of Benzylic Alcohols and Ethers to Carbonyl Derivatives by Nitric Acid in Dichloromethane

“…After 1 h, the observed degree of conversion was 26%, exclusively affording an isomeric mixture of (nitrophenyl)methyl acetates (31); no oxidation took place.…”

“…[31] 2-Chloro-2-methylpropanol (49) [32] and 2,2-dichloropropanol (53) [33] were prepared by NaBH 4 reduction of the corresponding aldehydes [34] in 82% (6.65 g) and 61% (7.85 g) yields, respectively, by a described procedure. [35] 1,2-Diphenylethanol (4ff, 5.23 g, 88%) [36] was obtained from deoxybenzoin (2ff) in a similar way.…”

Internal return in the photochemistry of ring-substituted 1-(1-naphthyl)ethyl esters of phenylacetic acid

“…Scheme 1 shows the possible pathways for the kinetic analysis. Equations [3] and [4] represent the rate of product formation from the intermediate ion pair and radical pair, respectively. The integrated ratio of these two equations gives the ratio of the two products from the two intermediates, [P1]IIPR].…”

“…Product yields as a function of the variation of Ar (substituent effects (l, 3), and phenyl(3) versus I-naphthyl(1)) and Scheme 1. General mechanism for the photoreactivity of arylmethyl esters in methanol.…”

“…These were chosen to allow a simultaneous study of both "0 equilibration and racemization. As with our previous studies (1,3), the substituents were chosen so that that the incipient 1-naphthylethyl radicals span a range of oxidation potentials that includes both the normal and inverted regions of the Marcus' correlation for the electron transfer rates that convert the radical pair to the ion pair. For comparison, ground state solvolysis results for 12b and 12c are also reported.…”