Bond cleavage, azoalkanes

Since czls-azoalkanes exhibit dipole moments of ca. (7... 10) 10 Cm (2... 3 D) [194], this solvent effect is best rationalized by assuming a decrease and final loss of the dipole moment during activation. Due to their dipole moments, czls-azoalkanes are more stabilized by polar solvents than the less dipolar activated complexes. The activation process corresponds to a synchronous, two-bond cleavage, probably accompanied by widening of the C—N=N bond angles [193]. A two-step, one-bond cleavage process via short-lived diazenyl radicals has been discussed [567], but this mechanism seems to be important only in the case of unsymmetrical azoalkanes, in particular arylazoalkanes [192]. 

Whereas C-H bond cleavage has been used only rarely, e.g. in adamantane [103], C-C bond rupture is more common, but only in strained carbocycles [104-106]. C-N bond cleavage is observed in azoalkane + decomposition [106d], although little synthetic use has been elaborated [107]. In contrast, C-Sn bond cleavage in alkyl-staimanes has been developed into a novel alkylation of a, -unsaturated ketones [108] and a versatile alkylation of electron-poor olefins [109]. Oxidative Si-Si [110] and Ge Ge bond cleavage [102] have also found use. 

C-N bond cleavage in cyclic azoalkane + with expulsion of N2 is apparently very rapid, because only follow-up products were detected [172]. In contrast, C-S bond cleavage in benzyl phenyl sulfide + is rather slow with k k 10 s [164]. 

From the temperature dependence of the direct and sensitized photolysis of azoalkanes 3, the activation energies were estimated to be >3.3 kcal mol and >10.5 kcal mol for a-CN-bond breakage in the singlet and triplet states, and >7.9 kcal mol for the P-CC-bond cleavage in the triplet state. ... 

Selective formation of the azirane 11 from the triplet-excited azoalkanes 3 in polar media is rationahzed in terms of solvent stabilization of the more polar transition state for the (i-CC-bond cleavage vs.that for the a-CN-bond scission. This solvent polarity effect, observed for the liquid-phase photolysis of azoalkanes 3, is similar to the photochemical behavior of the azoalkane 3a in the interior of the zeoHtes. The polar zeohte environment enhances the formation of azirane 1 la compared to the solution photochemistry in benzene. In contrast to zeolites, the formation of azirane 11 a, a triplet-state product, is completely suppressed in the crystaUine state and the housane 10a, a singlet-state product, is formed selectively. ... 

Dehnitive for triplet-state photochemistry, the a-CN-bond cleavage has been recently disclosed in the direct irradiation of azoalkanes 5. A prominent feature of the photolysis of azoalkanes 5 (Scheme 4) is the fact that the syn/anti product diastereoselectivity depends on temperature and bridgehead substitution. The temperature dependence of the direct photolysis of azoalkanes 5 stems from the competition between the singlet and triplet reaction channels (Scheme 4), the latter dominating at low temperature. Mechanistic details of this diastereoselective process is considered below, subsequent to a general discussion on the stereochemical inversion phenomenon in the photochemical nitrogen extrusion. 

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