Pieter Oosterhoff

Photoinitiators and photoinitiation Part 11. The photodecomposition of some O-acyl 2-oximinoketones

The photodecomposition of several O-acyl 2-oximinoketones was studied using electron spin resonance and proton nuclear magnetic resonance chemically induced dynamic nuclear polarization techniques. Primary NO bond cleavage into α-ketoiminyl and acyloxy radicals from the triplet excited state was observed in all cases. α-Ketoiminyl radicals observable at T < −70°C underwent rapid fragmentation into a benzoyl radical and a nitrile at temperatures exceeding −60°C. Chemical evidence was provided by radical trapping using a thiol and a stable nitroxyl respectively. These experiments also showed that, for most O-acyl 2-oximinoketones 1, [E] ⇌ [Z] photisomerization was the major process, whereas for O-acyl 2-oximinoketones 2 photodecomposition was almost exclusive.

Photoinitiators and photoinitiation - Part 15. The photodecomposition of some [Z] O-sulphonyl 2-oximinoketones and some [Z] O-sulphamoyl 2-oximinoketones

Abstract The photodecomposition of some [Z]- O -sulphonyl 2-oximinoketones is studied by ESR spectroscopy. At low temperatures ( T O -sulphonyl (and O -sulphamoyl) fragments are split off as sulphonyloxy (and sulphamoyloxy) radicals in much the same way as acyloxy radicals in the case of O -acyl 2-oximinoketones studied before. At room temperature in the presence of a spin-trap, the trapped benzoyl radical is observed, which must have been formed by fragmentation of the α-ketoiminyl radical. The title compounds are shown to photoinitiate the polymerization of acrylates and to photoinduce the acid-catalyzed crosslinking of a melamine resin.

Photoinitiators and photoinitiation 10. The photodecomposition of some carboxylic esters of α-hydroxymethylbenzoin

Abstract The photodecomposition of some carboxylic esters of α-hydroxymethylbenzoin was studied using electron spin resonance (ESR) and proton nuclear magnetic resonance-chemically induced dynamic nuclear polarization ( 1 H NMR-CIDNP) techniques. Exclusive α-cleavage into benzoyl and α-hydroxy-α-acyloxymethylbenzyl radicals through the triplet excited state was observed. The α-hydroxy-α-acyloxymethylbenzyl radicals mainly undergo disproportionation with the other radicals present to give benzoylmethyl carboxylates. Elimination of carboxylic acids from the α,α-disubstituted benzyl radicals occurs to some extent, but at a much lower rate than the elimination of sulphonic acids from the corresponding α-hydroxy-α-sulphonyloxymethylbenzyl radicals generated from the sulphonic esters of α-hydroxymethylbenzoin.

The photoinduced α-cleavage of α-bromomethylbenzoin: the generation and behaviour of β-bromoketyl radicals

Abstract The photodecomposition of α-bromomethylbenzoin was studied by electron spin resonance and by radical trapping using I-dodecanethiol and a stable nitroxyl (TMPO) respectively. Photoinduced α-cleavage into benzoyl and α-hydroxy-α-bromomethylbenzyl (β-bromoketyl) radicals is observed. The β-bromoketyl radicals are found to undergo an extremely fast elimination of hydrogen bromide to give benzoylmethyl radicals. These β-bromoketyl radicals are also generated by hydrogen-abstraction from 2-bromo-1-phenyl-ethanol using triplet excited benzophenone. The only products observed are ‘unchanged’ benzophenone, acetophenone and hydrogen bromide. The same result is obtained on irradiation of 2-bromoacetophenone in the presence of benzhydrol. Possible mechanisms and implications are discussed.

Photoinitiators and photoinitiation. Part 13. The photodecomposition of sulphonic esters of α-hydroxymethylbenzoin ethers

Abstract The photodecomposition of some sulphonic esters of α -hydroxymethylbenzoin ethers is examined using ESR and radical trappingtechniques. Photoinduced α -cleavage into benzoyl and α -alkoxy- α -sulphonyloxymethylbenzyl radicals from a short-lived triplet excited state is observed. The α , α -disubstituted benzyl radicals undergo subsequent fragmentation, generating benzoylmethylradicals and sulphonic acids at a rate k el 7 s −1 . The fate of the alkyl ether part of the α,α-disubstituted benzyl radicals after elimination of sulphonic acid is unknown at present.

Photoinitiators and photoinitiation, 16 - The photodecomposition of some 1-phenyl-3-sulphonyloxy-1,2-propanediones

The photodecomposition of the title compounds is studied by ESR spectroscopy using a spin-trapping agent. The radicals observed indicate the breakdown of the title compounds and the simultaneous generation of sulphonic acid. Confirmative evidence for the photoinduced release of sulphonic acid is provided by the acid-catalysed crosslinking of a melamine resin.