SOLVENT EFFECT ON PRODUCT DISTRIBUTION IN PHOTOCHEMICAL PATHWAYS OF ALPHA-C-N-CLEAVAGE VERSUS BETA-C-C-CLEAVAGE OF N,PI-ASTERISK-TRIPLET-EXCITED AZOALKANES

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