Rearrangement phosphatoxy

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

In parallel with the development of the heterolysis of b-substituted alkyl radicals, a rearrangement reaction was observed and extensively studied in organic solvents. This rearrangement was first noted for b-(acyloxy)alkyl radicals (Scheme 5) by Surzur et al. [48] and, later, for b-(phosphatoxy)alkyl radicals by the Crich and Giese groups [49,50]. 

The rearrangements of b-(acyloxy), (i-(phosphatoxy)alkyl, and related systems have been reviewed [51,52] and representative kinetic data are given in Table 2 above. As revealed by isotopic labeling experiments, the acyloxy... 

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

Figure 7.6 Rearrangement of 2-acyloxy and related glycopyranosyl radicals (a) tetra-O-acetylglucopyranosyl, (b) tetra-O-acetylmannopyranosyl, (c) a phosphatoxy migration note how the a-face of the sugar is (d) reverse migration where the anomeric centre is stabilised, (e) failure of migration of a 5-thio radical.

The mechanism of the jff-(phosphatoxy)alkyl and y9-(acyloxy)alkyl rearrangements and their less well known but closely related cousins, the j5-(sulfatoxy)alkyl and yS-nitroxyalkyl rearrangements, has long presented a conundrum to workers in the field [Ij. Instances of reactions proceeding via pure 2,3-shifts [1] and pure 1,2-shifts... 

Extensive computational studies have been carried out on the )8-(acyloxy)alkyl and ff-( phosphatoxy)alkyl rearrangements by Radom and coworkers and by Zipse. These calculations in general support the possibility of concerted rearrangements taking place via 5-center-5-electron and 3-center-3-electron cyclic transition states [25, 26]. However, before such computations can be used as an aid in distinguishing between reaction pathways, it will be necessary for theoretical chemists to circumvent the present difficulties in calculating the radical ionic fragmentations. 

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