Homolytic vs. heterolytic

The Caradonna group studied the ability of 19 and 20 to catalyze the decomposition of peracids in MeOH by using TBPH as trapping reagent (44). Both raefa-chloroperbenzoic acid (raCPBA) and phenyl-peracetic acid (PPAA) were catalytically decomposed to yield 2,4,6-tri-tert-butylphenoxy radical, TBP, and HCHO no active oxygen was found at the end of the reaction. The mechanism of catalytic peracid decomposition (homolytic vs. heterolytic) was examined by using PPAA as substrate. Analysis of the PPAA decomposition products showed that 19 induced a heterolytic pathway no products derived from the benzyl radical were detected. Under similar conditions, the decomposition of PPAA by 22 was shown to follow a homolytic mechanism. 

Frankel, E.N., Neff, W.E. and Selke, E. Analysis of autoxidized fats by gas chromatography-mass spectrometry. IX. Homolytic vs heterolytic cleavage. Lipids 19, 790-800 (1984). 

Pincock, J. A. and Wedge, P. J., The photochemistry of methoxy-substituted benzyl acetates and benzyl pivalates homolytic vs heterolytic cleavage, /. Org. Chem., 59, 5587-5595, 1994. 

Homolytic vs. Heterolytic Fmgmentutiim Problem of dte Tran ent Radical... 

Okamoto et al. [47] examined the homolytic dehydrogenation of methanol in an aqueous media and found that only the C-H activation step was favorable. They had not, however, considered the heterolytic path. We calculate that the heterolytic activation of methanol is exothermic at —43 kJ/mol [24,25] (Eq. 19.8) and is expected to have a lower activation barrier as shown for heterolytic vs. homolytic water activation [40] (vide supra). 

In 1950, Derbyshire and Watos first considaed the possibility of electrophilic aromatic hydroxylation with electrophiUc hydroxyl (OH+) [41]. While in gas phase heterolytic cleavage of the 0—0 bond in HjOj molecule requires much more enagy than homolytic cleavage (ca. 300kcal/mol vs. 51 kcal/mol at 298 K), it becomes more favorable in polar solvents in the presence of Lewis or Brpnsted adds [42]. Similarly, peroxy acids (organic or inorganic) can provide electrophilic oxidizing species [42,43]. 

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