Azoalkanes: photochemistry

This contribution is preceded by an earlier chapter, which covered the main topics of the photochemistry of cyclic azoalkanes for over three decades. Herein, we address the following mechanistic aspects of cyclic azoalkane photochemistry (1) the generation of azoalkane triplet states on direct irradiation and their photochemical transformations, and (2) the double inversion of the molecular skeleton in the formation of housane during photochemical nitrogen extrusion. 

Photochemistry of azoalkanes and enones is dominated by intersections of n-n and n-n states with the ground state as well as by the intervention of the triplet manifold. 

Adam, W. and Sahin, C. (1995) Photochemical decomposition of cyclic azoalkanes, in CRC Handbook of Organic Photochemistry and Photobiology (eds W.M. Horspool and P-.S. Song), CRC Press, Boca Raton, pp. 937-953. 

Turro, N.J., Cherry, W.R., Mirbach, M.F., and Mirbach, M.J. (1977) Energy acquisition, storage, and release. Photochemistry of cyclic azoalkanes as alternate entries to the energy surfaces interconnecting norbornadine and quadricyclene. Journal of the American Chemical Society, 99, 7388-7390. 

To study the photolysis of azo compounds, CIDNP was only recently introduced in the field of photochemistry. The CIDNP-effect consists of generating a geminate radical pair which still remembers the spin state of its precursor. So the multiplicity of the precursor can be determined from enhanced absorption or emission signals in azoalkane photolysis. The benzophenone sensitized photolysis of dia-zirine in deuteriochloroform leads to the triplet azo compound 24 which decomposes under elimination of a ground state nitrogen molecule and a triplet methylene 38>. This abstracts deuterium from deuteriochloroform to form the geminate radical pair 25. This can now recombine to give 26 or dissociate to afford the free radical products. 

Photopolymerization is traditionally initiated by direct photolysis of a precursor to provide free radicals via bond homolysis. Examples of such initiators include benzoin, and benzoin ethers, disulfides, and azoalkanes or dialkylperoxides. Hydrogen abstraction chemistry, typified by benzophenone photochemistry, is also recognized as extremely useful. However, a number of viable commercial photopolymer imaging systems are based upon ionic (especially cationic) polymerization. These systems will be discussed next. 

Adam W, Oppenlander T (1986) 185-nm Photochemistry of Olefins, Strained Hydrocarbons, and Azoalkanes in Solution, Angew. Chem. Int. Ed. Engl. 25 661-672. 

Early interest in the photochemistry of azocompounds was stimulated by its importance in free radical chemistry since the photolysis of azoalkanes has long been known to be a convenient source of alkyl free radicals. The emphasis, in the past decade, has shifted towards the mechanistic aspects and especially in the past few years many fine mechanistic studies have been published from different laboratories. It is quite evident, however, that a self-consistent picture of this field has just begun to unfold and at present many important details are still left unclarified. 

Fig. 4. Schematic illustration of the possible states involved in the photochemistry of azoalkanes.

The photochemistry of cyclic azoalkanes continues to attract attention. Diazirines are unique in their behaviour as elimination of nitrogen leads to the formation of carbenes, a process which is increasingly being employed for the generation of such species under mild conditions. 

A recent study on the rate of formation of products from excited hexafluoro-acetone in the presence of a vibrational relaxer and azoalkanes as triplet quenchers has produced data which the authors use to support the notion that vibrational relaxation in the singlet manifold is a multistep collisional process.75 76 The reaction between excited perfluoroacetone and ethane78 and the photochemistry of hexafluorobiacetyl vapour77 have also been discussed. 

A review of the phototochemistry of A-oxides has appeared and this includes a section on the photo-induced deoxygenation of heterocyclic A-oxides. A second review deals with the mechanistic aspects of the photochemistry of bicyclic azoalkanes and considers their photo-reduction by hydrogen donors, a process for which hydrogen-atom transfer and CT mechanisms have been suggested. 

Numerous reviews on the photochemistry and photophysics of ketones [18,23,25,78,79] and azoalkanes [38,43,80,81] are already available. This chapter focuses on intermolecular photoreactions of azoalkanes, which are compared with known data for ketones (see Structure 3.1). Unimolecular reactions such as the Norrish type-1 a-cleavage reaction of ketones [17,82-87] and their Norrish type-II reactions [20,24,73,83,88-91] as well as denitrogenation [8,9,43,68,92,93] and cis-trans isomerization of azoalkanes [43,92,93] are not discussed. The anphasis lies, besides data compilation, on mechanistic understanding, such that classical applications of azo compounds as dyes [94] or more recent apphcations of azo compounds in photochromic materials [10-12], or ketones as radical initiators in polymerization [95], are omitted as well. 

Subsequent studies exposed a pronounced medium effect on the product distribution. In the hquid-phase photolysis of the azoalkanes 3a,b, for which the triplet channel competes effectively with the singlet-state deazetation, the product distribution depends significantly on the type of solvent. In contrast, the azoalkanes 3c,d, with efficient denitrogenation in the singlet state, do not display solvent-dependent photobehavior. Thus, the aziranes 11 are produced almost exclusively in polar protic solvents, while housane formation is predominant in nonpolar solvents. It should be noted that the observed solvent dependence derives from bulk medium interactions, as similar product distributions are exhibited by the hydroxy-substituted derivative 3f and azoalkane 3b in benzene. Evidently, the intramolecular hydroxy functionahty in the azoalkane 3f does not influence the photochemistry of the triplet azo chromophore. 

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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