A scission

Alkoxy radicals can abstract hydrogen (eq. 12), but they may undergo a -scission reaction (eq. 16). 

Alkoxy radicals from hydroperoxides can undergo a -scission reaction (eq. 2) to yield an alkyl radical and a ketone. The higher stabiUty of the generated alkyl radical compared to that of the parent alkoxy radical provides the driving force for this reaction, and the R group involved is the one that forms the most stable alkyl radical. 

The second step is a -scission, the breaking of a carbon—carbon bond P to the charged carbon. The sum of the two reactions is the stoichiometry of the overall cracking reaction R H — RH + olefin. R+, a relatively stable carbenium ion such as the /-butyl cation, is a chain carrier. The role of the catalyst is to donate the proton to start the chain. This is a greatiy simplified representation. 

In a few cases, azo polymers were synthesized pho-tochemically. Azobenzoin compounds have photo cleavable benzoin groups. Being irradiated with UV light (A = 350 nm) 4,4 -azo-bis(4-cyanopentanoyl)-bis benzoin, ACPB, undergoes a-scission forming two free radicals per initiator molecule (Scheme 24). 

The benzoin ethers (75, R-alkyl R H) and the ot-alkyl benzoin derivatives (75, R=H, alkyl R =alkyl) undergo a-scission with sufficient facility that it is not quenched by oxygen or conventional triplet quenchers.276 This means that the initiators might be used for UV-curing in air. Unfortunately, it does not mitigate the usual effects of air as an inhibitor (Section 5.3.2). The products of a-scission (Scheme 3.53) are a benzoyl radical (13) and an ( -substituted benzyl radical (76) both of which may, in principle, initiate polymerization, 76 2"... 

It should be pointed out that not all benzoin derivatives (75) are suitable for use as photoinitialors. Benzoin esters (75, R=aeyl) undergo a side reaction leading to furan derivatives. Aryl ethers (75, R=aryl) undergo (3-seission to give a phenoxy radical (an inhibitor) in competition with a-scission (Scheme 3.54). Benzoin derivatives with a-hydrogens (75 R-Il) are readily autoxidized and consequently can have poor shelf lives. 

Depending on the nature of the substituent R, the radical 76 (Scheme 3.53) may be slow to add to double bonds and primary radical termination can be a severe complication (see 3.2.9).30 40The problems associated with formation of a relatively stable radical are mitigated with certain tx-alkoxy (77) and a-alkanesulfonyl derivatives (79).280 In both cases the substituted benzyl radicals formed by a-scission (78 and 80 respectively) can themselves undergo a facile fragmentation to form a more reactive radical which is less likely to he involved in primary radical termination (Scheme 3.55, Scheme 3.56). 

One of the peculiar features of sulfonyl radicals is the a-scission process, namely... 

Displacement (a-scission) and Free Radical Arbuzov (P-scission) Reactions. 58... 

The displacement of a ligand L from the phosphorus atom of a phosphoranyl radical can easily occur via a-scission of the L-P bond (Scheme 23). The fragmentation is a regiospecific process, i.e., the leaving group must be apical (Scheme 23) and it occurs via an intermediate o" structure (Fig. 5). 

The trapping of alkyl, alkoxyl and alkylthiyl radicals by trivalent phosphorus compounds, followed by either a-scission or p-scission of the ensuing phosphoranyl radical, is a powerful tool for preparation of new trivalent or pen-tavalent phosphorus compounds [59]. However, the products of these reactions strongly depend on the BDE of the bonds, which are either formed or cleaved. For example, the addition of phenyl radicals on a three-coordinate phosphorus molecule occurs irreversibly, while that of dimethylaminyl (Me2N ) or methyl radicals is reversible, the amount of subsequent P-scission (formation of compound C) depending on the nature of Z and R (Scheme 25). For tertiary alkyl radicals and stabilized alkyl radicals no addition is observed (Scheme 25) [63]. 

In the course of the 1990s, Yasui et al. [41b, 68] showed that, depending on the ligands attached to the phosphorus atom, phosphoranyl radicals may decay via three main processes a-scission, -scission and SET (Scheme 31). For example, in the presence of 10-methylacridinium iodide, phosphoranyl radicals generated from phenyl diphenylphosphinite decayed mainly via a-scission (Scheme 32) whereas phosphoranyl radicals generated from /so-propyl diphenylphosphinite decayed only via a SET process (Scheme 33). The reactivity of the phosphoni-umyl/phosphoranyl radical tandem has already been discussed in Sect. 3. 

Scheme 31 Decay of phosphoranyl radicals via a-scission, p-scission and SET. Reprinted with permission from [68]. Copyright 1994 Wiley Interscience...

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

Although both aliphatic and aromatic ketones have been studied, the aromatic ketones are more useful in commercial practice, since their absorptions occur at longer wavelength (lower energy) and their quantum yields are higher [Ledwith, 1977 Pappas, 1988]. Ketones and their derivatives undergo homolysis by one or both (often simultaneously) of two processes a-scission and electron transfer [Padon and Scranton, 1999]. a-Scission involves... 

For SR of higher hydrocarbons, Rostrup-Nielsen " and Tottrup " postulated a Langmuir-Hinshelwood-Houghen-Watson (LHHW) kinetic model. It was assumed that the hydrocarbon chemisorbs on a dual catalytic site, followed by successive a-scission of the C-C bond. The resulting Ci species react with adsorbed steam to form H2 and CO. The expressions were lit to data for SR of n-Cv on a Ni/MgO catalyst at 500°C the overall rate expression is " " ... 

The hydroxyquinoline (39-2) provides the starting material for a quinolone that incorporates a hydrazine function. Reaction of (39-2) with 2,4-dintrophenyl O-hydroxylamine ether (41-1) in the presence of potassium carbonate leads to a scission of the weak N-O hydroxylamine bond by the transient anion from the quinolone the excellent leaving character of 2,4-dinitrophenoxide adds the driving force for the overall reaction, resulting in alkylation on nitrogen to form the hydrazine (41-2). The primary amine is then converted to the formamide (41-3) by reaction with the mixed acetic-formic anhydride. Alkylation of that intermediate with methyl iodide followed by removal of the formamide affords the monomethylated derivative (41-4). Chlorine at the 7 position is then displaced by A-methylpiperazine and the product saponified. There is thus obtained amifloxacin (41-6) [48]. 

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