Aryl esters singlet-state radical pairs

SINGLET-STATE RADICAL PAIRS FROM IRRADIATION OF ARYL ESTERS AND ALKYL ARYL ETHERS... 

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. 

Control over the course of the rearrangements of aryl esters can be achieved when the host matrix can interact with the ester and its intermediates through electrostatic or H-bonding interactions. Zeolites and Nafion membranes are two polymeric matrices in which such studies have been conducted. As mentioned above, the positions of the radical centers of the acyl radicals are known to be very near the oxygen atoms of their aryloxy partners at the moment of birth of the radical pairs from the excited singlet state of an aryl ester in all media all subsequent diffusion... 

In each of the aryl esters discussed above, the acyl radical formed upon lysis of the excited singlet state of the ester loses CO very slowly at the temperatures of the irradiations. At 296K, the rates of loss of CO by acetyl and propanoyl radicals in the gas phase are 4.0 and 2.1 x 10 s respectively." As aresult, no products from decarbonylation and rearrangement are expected" (or have been found) when either of the NA or NM isomers is irradiated in liquid solvents or bulk polymers, and kinetic information from photoproducts alone is limited to relative rates of radical pair processes (Scheme 13.3). For example, if no Fries products from 1-NA or 1-NM emanate from reencounter of radicals that have escaped from their initial cages, [2-AN]/[4-AN]/[l-NOL] = /c2a/ 4a/ nol-... 

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