Photoreduktion von 2‐phenylchinoxalin mit Alkoholen und Aminen

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Abstract
A mechanistic and kinetic analysis of the photoreduction of 2‐phenylquinoxaline (I) to 3‐phenyl‐1,2‐dihydroquinoxaline (II) in the presence of alcohols and amines is described. It is concluded that the reduction in alcohols proceeds exclusivly over an excited singlet state I (S1), probably of mixed n‐π*, π‐π* configuration. The triplet state which is also populated (ϕ;isc = 0.62) shows π‐π* configuration but remains inactive. A calculated pKa* 6.2 of I (S1) by means of a. Förster‐cycle, makes a mechanism which includes a first step photoprotonation of I (S1), followed by formation of semiquinone type radicals very probable.In the presence of amines the formation of II proceeds exclusively via the first excited triplet state of I. The calculated rate constants are strongly dependent on the solvent polarity and on the ionisation potentials of the amines. This demonstrates clearly the importance of a charge‐transfer mechanism. It is also shown that once the charge‐transfer from the amine to I (T1) has occurred, physical quenching by electron backtransfer is of no importance. At high amine concentration I (S1)‐quenching is observed.