C-N heterocoupling

The postulated C-N heterocoupling requires diffusion of the two radicals either in the solvent-solute surface layer or in the bulk solution. In both cases one expects that the reaction rate should decrease with increasing solvent viscosity. To achieve the latter, the CdS-Si02-catalyzed photoaddition of 2,5-DHF to azobenzene was conducted at pressures ranging from 0.1 to 120 MPa [164]. Both the formation rates of the addition and reduction products 13c and 16 (R = R = Ph) decrease with increasing pressure and from a plot of In(rate) vs. pressure activation volume AF are obtained as 17.4 + 3.4 and 15.8 + 2.3 cm mol , for 13c and 16, respectively (Figure 25). 

In no cases could a product arising from C-N heterocoupling be observed. Thus, in contrast to thermal routes, which usually involve the use of organometallic intermediates [211-213], the reaction is regioselective and much easier to perform. 

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