Diaroyl peroxides

Only a few diacvl peroxides see widespread use as initiators of polymerization. The reactions of the diaroyl peroxides (36, R=aryl) will be discussed in terms of the chemistry of BPO (Scheme 3.25). The rate of p-scission of thermally generated benzoyloxy radicals is slow relative to cage escape, consequently, both benzoyloxy and phenyl radicals are important as initiating species. In solution, the only significant cage process is reformation of BPO (ca 4% at 80 °C in isooctane) II"l only minute amounts of phenyl benzoate or biphenyl are formed within the cage. Therefore, in the presence of a reactive substrate (e.g. monomer), tire production of radicals can be almost quantitative (see 3.3.2.1.3). 

Diacyl peroxides may also undergo non-radical decomposition via the carboxy inversion process to form an acylcarbonate (Scheme 3.27).46 The reaction is of greatest importance for diaroyl peroxides with electron withdrawing substituents and for aliphatic diacyl peroxides (36) where R is secondary, tertiary or ben/,yl.157 The reaction is thought to involve ionic intermediates and is favored in polar solvents 57 and by Lewis acids.158 Other heterolytic pathways for peroxide decomposition have been described.150... 

Aroyloxy radicals are formed by thermal or photochemical decomposition of diaroyl peroxides (see 3.3.2.1) and aromatic peroxyesters (3.3.2.3) (Scheme 3.78) alkoxycarbonyloxy radicals are similarly produced from peroxydicarbonates (33.2.2). 

Dimethyl-I,l -biphenyl has been prepared by a wide variety of procedures, but few of these are of any practical synthetic utility Classical radical biarjl syntheses such as the Gomberg reaction or the thermal decomposition of diaroyl peroxides give complex mixtures of products m which 4,4 dimethyl-l.l -biphenyl is a minor constituent A radical process maj also be involved in the formation of 4,4 dimethyl-1, l -biphenyl (13%) by treatment of 4-bromotoluene with hydrazine hydrate 5 4,4 -Dimethyl-l,l -biphenyl has been obtained in moderate to good yield (68-89%) by treatment of either dichlorobis(4-methyl phenyl)tellurium or l,l -tellurobis(4-methylbenzene) with degassed Raney nickel in 2 methoxyethyl ether 6... 

Eberson introduced the concept of a dissociative ET to peroxides some time ago. There have been attempts to apply Marcus Theory to the ET reduction of diaroyl peroxides in connection with the mechanism of the chemically initiated electron exchange luminescence (CIEEL)." " The... 

Diaroyl peroxides and diacyl peroxides without a-branches are significantly more thermally stable than those with mono- or di-cr-substituents. Tlie primary use of most commercial diacyl peroxides (16. R1 = R2 = alkyl or aryl) is initiation of free-radical reactions. 

Additional information on the nature of the radical zwitterion and the oxyl radical comes from a kinetic study of the decarboxylation of aroyloxyl radicals (ArCOa ) generated by LFP of the parent diaroyl peroxide in CCI4 [227], showing that the presence of a 4-MeO substituent leads to a significant decrease in the rate of decarboxylation (Table 10). 

A review of some aspects of the kinetics of diacyl peroxides was presented in 1962. The rate law for decomposition of diacyl and diaroyl peroxides have the same form, eqn. (1). As an example, the overall rate coefficient k was given by 10 k = 7.00+3.21 [P]o for the decomposition of 5-phenyl valeryl peroxide in carbon tetrachloride at 77.1° The term [P]o refers to the initial peroxide concentration. Again the data are fitted to a f-order term in peroxide for the induced decomposition portion of the rate law. 

The values of k and ki at 119.4 °C were calculated to be 9.36 x 10 sec and 3.89 X 10 l. mole. sec, respectively. Induced decomposition of /-butyl perbenzoate is less important relative to unimolecular decomposition than it is in the benzoyl peroxide reaction. The values of the first-order (kj) and induced decomposition (kj) rate coefficients for the latter peroxide in toluene at 80 °C are 3.28 x 10 sec and 4.28x10 I. mole". sec , respectively . Some caution should be used in assuming that rate law (1) will be obeyed for all peresters in all solvents. It was stated previously in the diaroyl peroxide section (13.2.1), that the order in peroxide was dependent upon the mode of termination. It is then conceivable that the first-order rate coefficient could contain contributions from both unimolecular and induced decomposition reactions. The effect of concentration variation on the... 

Deoxygenation. The reagent abstracts oxygen from a diaroyl peroxide to give the diaroyl anhydride and triethyl phosphate. ... 

Finally, the formation of biaryls by C—C coupling can take place through two different mechanisms referred to as radical-radical (RRD) and radical-substrate (RSD) dimerization. A mechanism involving le oxidation of the phenol by the peroxide and biaryl coupling by preferential addition of the phenol radical cation to the ort/io-positions to the alkoxy group of the diaroyl peroxide has been suggested. 

SCHEME 104. Decomposition of diaroyl peroxides in neat phenols... 

Diacyl or diaroyl peroxides (36, R alkyl or aryl respectively) are given specific coverage in reviews by Fujimori, " Bouillion et ai., and Hiatt. They are sources of acyloxy radicals which in turn are sources of aryl or alkyl radicals. Commercially available peroxides of this type include dibenzoyl peroxide (BPO), didodecanoyl or dilauroyl peroxide (LPO), didecanoyl peroxide (42) and succinic acid peroxide (43). 

For diaroyl peroxides (36, R=aryl), m- and /(-electron withdrawing substituents retard the rate of decomposition while m- and /(-electron donating and all o-substituents enhance decomposition rates. The o-substiluent effect has been attributed to the sensitivity of homolysis to steric factors. 

---