Modification of photochemical reactivity through the use of clathrates: the Norrish type I and type II reactions in Dianin's compound

Abstract

Dianin's compound (4-p-hydroxyphenyl-2,2,4-trimethylchroman) serves as host in a series of well-defined clathrate inclusion complexes with eleven linear, as well as branched chain, phenyl alkyl ketone guest molecules, chosen for their ability to undergo the Nomsh type I and type I1 photochemical reactions in solution. The photochemical reactivity of the guest ketones within the clathrate cavity was determined by irradiation of the inclusion complexes in the solid state. The results were compared to the photoreactivity of the ketones in polar as well as nonpolar liquid media. In general, the inclusion complex medium brings about an enhancement of type I over type I1 reactivity and causes an increase in type 11 fragmentation compared to type I1 cyclization. This change in reactivity is interpreted as resulting from the relatively restricted environment of the clathrate cavity coupled with the greater motion required for the type 11 process (y-hydrogen abstraction) compared to the type I reaction (a-cleavage), as well as from the greater steric requirements for type I1 cyclization (cyclobutanol formation) as compared to type I1 cleavage (1,4-hydroxybiradical scission). Chem. 63, 2719Chem. 63, (1985.I1 se forme des complexes d'inclusion bien dtfinis a I'intkrieur de clathrates du composk de Dianin, le p-hydroxyphknyl-4 trimkthyl-2,2,4 chromane, avec des phtnyl alkyl cttones, incluant tant des composts aliphatiques en chaines droites ou ramifikes, choisies pour leur facilitk de subir des rkactions photochimiques de Norrish des types I et 11, en solution. On a dttermink la rtactivitk photochimique des molkcules hBtes ? I I'intkrieur de la cavit6 en proctdant 21 I'irradiation des complexes d'inclusion Z1 I'ktat solide. On compare les rksultats obtenus avec les photorkactivitks des cttones tant dans des milieux polaires que dans des milieux liquides non polaires. D'une f a~o n gkntrale, les complexes d'inclusion augmentent la rtactivit6 du type I par rapport au type I1 et provoquent une augmentation de la fragmentation du type I1 par rapport ?I la cyclisation du type 11. On suggtre que ce changement dans la r6activitk provient d'un effet combint de I'environnement relativement restreint qui existe dans la cavitk du clathrate avec le fait que, par rapport a la rkaction de type I (coupure en a), le processus de type I1 (enltvement d'un hydrogkne en y) nkcessite de plus grands dtplacements; de plus, les besoins stkriques des cyclisations de type I1 (formation du cyclobutanol) sont plus importants que ceux requispour les scissions du type I1 (scission en hydroxybiradical-l,4).[Traduit par le journal]

Fotoquímica de alfa,alfa-dimetilvalerofenona adsorvida em celulose microcristalina

“…Nenhum outro produto foi detectado nestas condições além daqueles derivados das reações Tipo II de Norrish e a distribuição dos produtos obtida nestes solventes concorda razoavelmente com os dados da literatura 13,[24][25][26] . A Tabela 2 mostra claramente que a irradiação de amostras de butirofenona, valerofenona, hexanofenona, decanofenona e laurofenona, em celulose, preparadas tanto pelo Método A quanto pelo Método B, resulta em uma distribuição de produtos de ciclização/eliminação independente do tamanho da cadeia alquílica da cetona e do método de preparação da amostra.…”