Photo-Fries Reactions of Aryl Esters

At the moment of their birth (i.e., when the excited singlet states of the aryl esters undergo lysis), the geminal radical pairs mustbe in positions that make their shape similar to that of their precursor ester the radical center of the acyl part is very near the oxygen atom of the aryloxy part, and all subsequent diffusion of the two species starts from this orientation. As a result, addition of an acyl radical to its aryloxy partner is favored spatially at the nearer adj acent ortho) position(s) than at the more distant meta and para positions. However, the ability of the acyl radical to add to each of the positions of an aryloxy radical is expected to depend on the energies of the adducts ° (which are keto intermediates that enolize thermally with time to the eventual products Equation 13.5 shows an example of the keto intermediate for acyl addition to the ortho position of phenoxy). The spin densities at the 2-, 4-, and oxy-positions of 1-naphthoxy (and of phenoxy) from ESR measurements and HF/6-31G level calculations (in parentheses) are shown in Fig. 13.1. Those for the 2-naphthoxy radical by ESR measurements and MNDO-UHF calculations are collected in Table 13.1. These considerations explain why the yields of products from addition at the meta positions are very low. 

FIGURE 13.1 Spin densities at different positions of phenoxy and 1-naphthoxy radicals. Values at the meta and meto-hke positions are not shown because they are very low. Reproduced with permission from Ref 43. Copyright (2001) Japanese Photochemistry Association. 

DYNAMICS OF RADICAL PAIR PROCESSES IN BUEK POEYMERS 

TABLE 13.1 Spin Densities (Atomic Units) of Carbon Atoms and Oxygen of the 2-naphthoxy Radical from MNDO-UHF Calculations and ESR Data 

Decarboxylation of excited singlets of aromatic esters (Eq. 13.6) is a concerted process, and it can account for a significant fraction of the reaction by appropriately substituted aryl esters, especially in bulk polymers and other media that are capable of imposing conformational constraints on guest molecules.Because photoinduced decarboxylation occurs before lysis of the aryl esters, it does not influence the rates at which the A B singlet pair react. For that reason, the relative yields of decarboxylation products need not be considered in analyses of the radical pairs unless their formation precludes sufficient radical pair production for their easy direct or indirect detection. 

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