Fotoquímica de compostos Di- e Tricarbonilados Vicinais Cíclicos em presença de Olefinas

Abstract

Spectroscopic, kinetic and product studies were performed with several cyciic vicinal di- and tricarbonyl compounds, such as acenaphthenequinone (6), aceanthrenequinone (35), 1,2,3-indanetrione (18), 5-methoxy-1,2,3- indanetrione (22), 1H- benz[f]indene-1,2,3-trione (23), alioxan (14) anal dehydroascorbic acid (36), in the presence of olefins. The quenching of 6, 35, 18, 22 and 23 tripiets by olefins in, degassed benzene solution has been measured by laser flash photolysis. The alkenes studied inciuded cyclic, acyclic, isolated and conjugated dienes, and enol ethers. For 6 the quenching rate constant (kq) ranges from 2.1 x 106 M-1s-1 (for 1,5-hexadiene) to 6.0 x 108 M-1 S-1 (for L,3-dimethyl-2-butenel. A plot of log kq versus the ionization potential of the olefin is linear (r = 0.98) with a slope of -2,1/eV. On the other hand, the quenching rate constant for the tripiet of 35 by olefins is extremely low (<105 M-1s-1), which is in accord with the ππ* nature of its tripiet state. The tripiet state of the cyclic vicinal triketones 18, 22 and 23 is also quenched by olefins, however they are iess reactive than diketone 6. Steady state photolysis of 6 in the presence of several olefins led only to [2+2] and [4+2] photocycioaddition products. For 18 and 14, products derived from hydrogen abstraction and photocycioaddition processes were obtained, in all cases, electronic and steric effects are cleariy responsible for the observed product distribution