Solvent effect on product distribution in photochemical pathways of alpha C-N versus beta C-C cleavage of n,pi* triplet-excited azoalkanes

The product distribution in;the photolysis of the triplet-excited azoalkanes 1a,b depends markedly on the type of solvent used; in contrast, the azoalkanes 1c,d, which undergo efficient deazatization from the singlet-excited state, display solvent-independent photobehavior. Thus, the aziranes 3 are produced essentially exclusively in polar protic solvents, while the housanes 2 predominate in nonpolar ones. The excellent correlation (r(2) = 0.963, seven solvents) of the azirane 3b yield with Gutmann's AN solvent parameter reveals that a combination of solvent properties such as the polarity and polarizability of the medium and the hydrogen-bonding ability is decisive for the photoproduct distribution. That the observed solvent dependence derives from bulk medium effects is borne out by the similar product distribution for the hydroxy-substituted derivative 1f to that for the azoalkane 1b in benzene, i.e., the intramolecular hydroxy functionality in the azoalkane If is ineffective in influencing the photochemistry of the triplet azo chromophore. Selective formation of the aziranes 3 from the triplet-excited azoalkanes 1 in the polar protic solvents is rationalized in terms of solvent stabilization of the more polar transition state for beta cleavage (azirane 3) over that for the alpha scission (housane 2). In marked contrast to the appreciable deuterium isotope effect for quenching of the singlet-excited azoalkanes by protic solvents, the lifetimes (laser-flash photolysis) and reactivity (product analysis) of the triplet azoalkanes are not affected by deuterium substitution.