Hammett parameters, oxidative addition

Oxidative addition. The oxidative addition of a phosphine to a low valent transition metal can be most easily understood by comparing the Ph2P-fiagment with a Cl- or Br- substituent at the phenyl ring electronically they are very akin, cf Hammett parameters and the like. The phosphido anion formed during this reaction will usually lead to bridged structures, which are extremely stable. 

Taking advantage of the reduc ohmic drop at ultramicroelectrodes, the oxidative addition of a series of / ara-substituted iodobenzenes to Pd (PPh3)4 could then be monitored in toluene, (e = 2.4) by following the decay of the zerov ent oxidation wave (compare Figure 8, previous page), and the results compared to those reported previously for THF (e = 7.6). The reaction was thus shown to have almost identical activation parameters (e.g., AH = 18 1 kcal.moW, AS = 1.7+1 cal.moH.K- in toluene) and lead to identical Hammett correlations (e.g., p = 2.3 0.2, in toluene) in both solvents, which supports a non-ionic mechanism. 

Sometimes the use of quantum chemically determined indices (1, 24, 57) or oxidation-reduction potentials (58) instead of the Hammett-Taft constants, the use of chromatographically obtained hydrophobicity parameter, ARm, (5, 59) instead of w or log P, or the addition of other variables such as those for hydrogen bonding (60), dipole moments (61),... 

Second-order rate constants for oxidation of phenol and chlorophenols by Ce(rV) in aqueous acidic solution do not correlate well with pA" values, C-NMR chemical shifts, or Hammett substituent constants nevertheless, a strict additivity of chlorine substituent effects was found in both characteristic and reactivity parameters. A proton-coupled electron-transfer mechanism is considered likely. ... 

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