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Scheme 1

The features of the SPF photodecarboxylation (oxygen, temperature and concentration dependence) have a direct relation to the properties of the lowest triplet which clearly appears to be the precursor of the reaction. This state, of n,K* character, is characterised in neutral solution at room temperature by a T-T absorption with X,max = 600 nm and a lifetime in the microsecond domain in deaerated solution. Its decay rate depends on the presence of oxygen (kox ~ 109 1 mol1 s') and on the SPF concentration (kSPF ~ 2 x 109 1 mol"1 s-1) and was estimated to be ~ 2.5 x 104 s"1 at infinite dilution. Moreover, it increases by increasing temperature following an Arrhenius law with activation parameters AE* ~ 8-9 kcal/mol and preexponential factor ~ 1012 s"1. These findings indicate that photodecarboxylation actually proceeds from a higher triplet state T**, energetically close to T2(n,7i*). The electronic population of this latter, very similar to that of the n,7t* triplet of KPF, suggests a possible direct involvement of T2 itself in the reaction.

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