Fragmentation radicals

Having gamed one electron the alkyl halide is now negatively chaiged and has an odd numbei of elections It is an anion radical The extra electron occupies an antibondmg oibital This anion radical fragments to an alkyl radical and a halide anion... 

Higher Hydrocarbons. The VPO of higher hydrocarbons is similar to that of the lower members of the series with two significant additional comphcations (/) the back-bitiag reactions of alkylperoxy radicals (eq. 32), particularly at positions 2 or 3 carbons removed from the peroxy position, and (2) above the NTC region, radical fragmentation (eq. 28). 

Some commercial photoinitiatois (Table 11) undergo a Noirish Type I photocleavage to form two initiating radical fragments direcdy for a benzoin... 

Single-electron transfer from a borate anion particle to the excited polymethine cation generates a dye radical and an aLkylphenylbotanyl radical. The aLkylphenylbotanyl radical fragments to form an active alkyl radical. It is the alkyl radical particles that initiate the polymerization reactions (101). 

Earlier sections have already provided several examples of radical fragmentation reactions, although this terminology was not explicitly used. The facile decarboxylation of acyloxy radicals is an example. 

The first step in cracking is the thermal decomposition of hydrocarbon molecules to two free radical fragments. This initiation step can occur by a homolytic carbon-carbon bond scission at any position along the hydrocarbon chain. The following represents the initiation reaction ... 

Bond dissociation energy, D (Section 5.8) The amount of energ r needed to break a bond homolytically and produce two radical fragments. 

The mechanism probably involves a Norrish type I cleavage (p. 318), loss of CO from the resulting radical, and recombination of the radical fragments. 

Another alternative for conversion of diols to alkenes is the use of the Barton radical fragmentation conditions (see Section 5.5) with a silane hydrogen atom donor.299... 

Among the most useful radical fragmentation reactions from a synthetic point of view are decarboxylations and fragmentations of alkoxyl radicals. The use of (V-hydroxy-2-thiopyridine esters for decarboxylation is quite general. Several procedures and reagents are available for preparation of the esters,353 and the reaction conditions are compatible with many functional groups.354 f-Butyl mercaptan and thiophenol can serve as hydrogen atom donors. 

Alkoxy radical fragmentation is also involved in ring expansion of 3- and 4-haloalkyl cyclohexanones. The radical formed by halogen atom abstraction adds to the carbonyl group, after which fragmentation to the carboethoxy-stabilized radical... 

Under UV irradiation, the photoinitiator cleaves into radical fragments that react with the vinyl double bond and thus initiate the polymerization of the monomer. If the latter molecule contains at least two reactive sites, the polymerization will develop in three dimensions to yield a highly crosslinked polymer network. 

Schuster and Brizzolara<61> have provided a very nice study of the photochemistry of 10-hydroxymethyl-A1 9-2-octalone (87). Schuster and Patel<13) previously used radical fragmentation reactions as probes for the nature of the intermediates in the cyclohexadienone rearrangement. This compound (87) was designed so that it could undergo a radical fragmentation reaction in competition with the usual type A rearrangement if the intermediate involved has radical character (n -> n triplet). Photolysis produced (88)—(92) ... 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

Radical fragmentation process using tin radical is applied to the conversion of nitro compounds into nitriles, as shown in Eq. 6.41.69... 

Figures 3.4 3.6 report the average mass spectra obtained, respectively, for frankincense resin, mastic resin and birch bark tar [16]. Moreover, in each spectrum the main ion and ion radical fragments are assigned, and for each material the main molecular markers are shown.

One problem we have had to overcome in developing metal-cluster oxidation-reduction photochemistry is the tendency of excited clusters to dissociate into radical fragments (for... 

Finally, an alternative approach employs a dissection into two radical fragments as shown below ... 

An alternative approach for determining the relative pi electronic stabilization of two torsional isomers utilizes a molecular dissection into two open shell radical fragments. This approach is illustrated by examining torsional isomerism in butadiene and 1,3,5-hexatriene. The ir MO s of the conformational isomers of 1,3,5-hexatriene can be constructed from the union of the n MO s of two formal allyl radicals. The two regiochemical modes of union of interest will be designated cis and tram ... 

Fig. 8. Pi orbital interactions obtaining in the union of two allyl radical fragments to form 1,3,5-hexatriene...

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