Hydrogen abstraction Hammett reaction constants

The influence of substituents in Ar1 is characterized by negative p values (plot based on ordinary Hammett a constants)208. Reactions involving other types of phosphorus nucleophiles (e.g. PPh2Alk)208 and ArS02 ions as nucleophiles have also been studied209. The work has been extended to the study of a-azidosulfones but the reaction does not turn out to be analogous210. There is base-catalyzed abstraction of hydrogen ion from the a-carbon atom. 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

Japanese workers [26, 27] proposed Equation (24), where Ef is the resonance contribution of the substituent to the transition state and y its reaction constant. When the resonance contribution is very small the expression reduces to Hammett s equation. The hydrogen abstraction reaction of nuclear substituted cumenes toward the polystyryl radical was taken as the model reaction and given the arbitrary value of 7= 1.0 and r (cumene) = 0.0. Cammarata, Yau, Collett and Martin [28] related to Hammett s a by equation (25). 

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