Nitrogen-containing Chromophores

This chapter describes photoreactions of nitrogen containing chromophores which are different from the principal reactions of electronically excited carbonyl compounds. Five groups of substrates were included cyclic and acyclic imines, nitriles, amine oxides, azo and diazo compounds, and enamides. 

In contrast to these efficient intramolecular reactions most intermolecular photoreactions involving acyclic imines proceed with very low quantum yields. This is probably due to the rapid deactivation of the excited imine chromophore by C-N bond rotation. Cyclic imines, however, which lack this possibility, can undergo photoaddition to alkenes. 

2-(Methylthio)benzthiazole for example gives two products when irradiated in the presence of rraui-stilbene. This reaction was investigated by G. Kaupp and seems to proceed via two different biradical intermediates. Whereas one biradical leads to an azetane simply by radical recombination, the other rearranges to a seven-membered heterocycle. Thermolysis of the latter cycloadduct leads to an 1-azabutadiene which can be subsequently transformed into its geometrical isomer. This isomerization is reversible at another wavelength and thus constitute a simple photochromic system (see also Chapter 8). 

Direct irradiation of amino substituted azirines leads to neutral nitrile ylides which can be trapped with electron-deficient double bonds. H. Heimgartner described the synthesis of triazolines and oxazolines via trapping of a nitrile ylide with diethylazodicarboxylate and methyl trifluoroacetate, respectively. An elegant application of 1,3-dipolar cycloaddition chemistry is the photolysis of a tetrazole leading to a reactive nitrile imine which is trapped in an intramolecular fashion. 

Photochemical nitrogen extrusion also is the key reaction in a cross coupling reaction between reactive sides of a polymer chains as described by H. Meier. In this case a thia-diazole linked to a polyethylene chain (prepared from 3-pentanone in five steps) serves as the chromophore and double denitrogenation leads to chain dimerization with formation of a 1,4-dithiine. 

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