Kinetic study of the reactions of methoxy-substituted phenacyl radicals

Abstract

'The photochemistry of various mono-and dimethoxy-substituted a-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-a-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have .rr,.rre character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 x 10-5 M-I s-I, while for addition to the double bond in I,]-diphenylethylene the rate constant is 9.4 x lo7 M-' s-'. Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k 5 10' M-' s-I). In contrast, reaction with oxygen occurs with kc] = 2.5 x 10%~-' s-I. Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin.Key rvords: methoxyphenacyl radicals, a-bromoacetophenones, laser flash photolysis. Rksum6: Faisant appel ii la photolyse laser Cclair, on a ttudit la photochimie de diverses mono-ou dimCthoxy-a-bromoacCtophCnones dans des solvants oganiques. Les &tats singulets excitCs de courte durCe de vie se brisent pour donner des atomes de brome et les radicaux mCthoxyphCnacyles correspondants, avec des rendements quantiques allant de 0,13 ii 0,35. A I'exception de la 4-mCthoxy-a-bromoacCtophCnone (6), tous les autres substrats fournissent des ttats triplets facilement dCtectables; ils posskdent un caractkre .rr,.rr:"t une trks mauvaise habilitC ii enlever des hydrogknes. La dCgCnCrescence de 1'Ctat triplet ne contribue pas la formation du radical mCthoxyphCnacyle. Alors que les radicaux mCthoxyphCnacyles peuvent possCder une densite de spin allant jusqu'ii 0.3 au niveau de I'oxygkne du carbonyle, ce qui explique la bande semblable ii celle des alcoxy dans leur spectre d'absorption, leur rCactivitC chimique est dominee par le caractere radicalaire au site carbonC. L'habilitC des radicaux mCthoxyphCnacyles ii enlever des hydrogknes est modeste; par exemple, la rCaction du composC 6 avec le cyclohexa-1,4-dikne se produit avec une constante de vitesse de 2,6 x 10' M-' s-', alors que, pour l'addition ii la double liaison du 1,l -diphCnylCthylkne, la constante de vitesse est de 9,4 x lo7 M-' s-I. 11 est probable que les additions sur d'autres doubles liaisons seront plus lentes (par exemple, pour 12 + cyclohexa-l,3-dikne, k 5 10-5 M-' s-I). Par ailleurs, la rC...

Hydrogen vs. electron transfer mechanisms in the chain decomposition of phenacyl bromides. Use of isotopic labeling as a mechanistic probe

“…Typical half-lives for the p-methoxyphenacyl radical in solvents such as methanol, ethanol, 2-propanol, and acetonitrile are in the neighborhood of 10 IJ.S (1). Although these carbon-centered radicals seem to be rather unreactive, our work with a-bromoacetophenones shows that steady-state irradiation in alcohol solvents leads to remarkably rapid consumption of the starting material with concomitant formation of the corresponding acetophenone.…”

“…While 4-methoxyphenacyl radicals m a y have a spin density as a high a s 0.3 at the carbonyl oxygen, their reactivity x x is dominated by the radical character at the carbon site (1). Product studies o n the addition of phenacyl radicals to olefins…”

“…While the phenacyl radical has been written a s a carboncentered species, it can also b e written in its canonical form, as T h e photodecomposition of a-bromoacetophenone and related a n oxygen-centered radical (4), i.e., phenacyl bromides provides a convenient source of phenacyl radicals, reaction [ I ] (1)(2)(3). These radicals are readily detectohcH?…”

Initiator selectivity during free radical copolymerizations of styrene and methyl methacrylate initiated by anaerobic copper(II) oxidation of carbon‐13 labeled acetophenone

“…Photolysis, [33][34][35][36][37][38][39][40] electrochemical reduction, [41][42][43][44] and redox reactions [45][46][47][48][49][50] have been used to generate phenacyl radicals, and they have been used to some extent to initiate polymerization reactions. [51][52][53][54][55][56][57] In the study described herein, polymerization and copolymerization reactions involving styrene and methyl methacrylate were initiated by phenacyl radicals that were generated by the anaerobic oxidation of acetophenone.…”

Supramolecular Storage and Controlled Photorelease of an Oxidizing Agent using a Bambusuril Macrocycle

“…[35] We hypothesize that heterolytic cleavage occurs from the triplet state, which is analogoust ot he reaction of 2-hydroxyphenacyl moieties. [37,38] The photoreleaseo fi odide from pHP2 (4.76 10 À4 m)t or eplace S 2 O 8 2À (4.29 10 À4 m)i nside the BU3 (4.31 10 À4 m)c avity in situ in aD 2 O/CD 3 CN (1:1) mixture was monitored by 1 HNMR spectroscopy.The signals at d = 5.61 and 4.93 ppm, characteristic for the BU3·S 2 O 8 2À complex, shifted to those at 5.64 and 5.00 ppm, indicating the formation of ac omplex with I À (Figure S9). [37,38] The photoreleaseo fi odide from pHP2 (4.76 10 À4 m)t or eplace S 2 O 8 2À (4.29 10 À4 m)i nside the BU3 (4.31 10 À4 m)c avity in situ in aD 2 O/CD 3 CN (1:1) mixture was monitored by 1 HNMR spectroscopy.The signals at d = 5.61 and 4.93 ppm, characteristic for the BU3·S 2 O 8 2À complex, shifted to those at 5.64 and 5.00 ppm, indicating the formation of ac omplex with I À (Figure S9).…”