Homolytic substitution intramolecular

It is estimated that thiophene reacts with phenyl radicals approximately three times as fast as benzene. Intramolecular radical attack on furan and thiophene rings occurs when oxime derivatives of type (112) are treated with persulfate (8UCS(Pt)984). It has been found that intramolecular homolytic alkylation occurs with equal facility at the 2- and 3-positions of the thiophene nucleus whereas intermolecular homolytic substitution occurs mainly at position 2. 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Formation of cyclic sulfoxide (R)-55 by t reatment of bromoarene (i )-54 with tributylstan-nane apparently proves that intramolecular homolytic substitution at the sulfur atom of the sulfoxide group proceeds with strict inversion of configuration (equation 50)103. 

Heterocyclic sulphoxides 65 mass spectra of 130-132 Hexahydronaphthalenols, synthesis of 310 Hofmann elimination 953 HOMO energies 1048, 1049 Homolytic substitution 1109 intramolecular 846 Horner-Wittig reaction 333 Hot electrons 892, 893 HSAB theory 282, 549 Hydrides, as reducing agents 934-941, 959 Hydrogen abstraction, photochemical 874, 876, 877, 879, 880... 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

The second, albeit scarcely applied, class of radical domino reactions indudes intramolecular homolytic substitutions, which can also result in the formation of rings. As a consequence, the radical is ejected with the displaced center and does not remain with the main ring hence, this type of reaction is normally featured as terminating step. The most typical complication occurring within substitutions is an undesired branching of a sequence caused by the released radical. 

Alkyl, vinyl and aryl radicals can give homolytic substitution at a heteroatom either inter- or intramolecularly thus forming C-heteroatom bonds. 

An example of C—Si bond formation concludes this overview of carbon heteroatom bond formation. Reflux of bromide 62 in benzene and in the presence of small amounts of (TMS)3SiH and AIBN afforded the silabicycle 63 in 88 % yield (Reaction 7.64) [76]. The key step for this transformation is the intramolecular homolytic substitution at the central silicon atom, which occurred with a rate constant of 2.4 x 10 s at 80 °C (see also Section 6.4). The reaction has also been extended to the analogous vinyl bromide (Reaction 7.65) [49]. 

It was proposed that an initially generated silyl radical 3, by reaction of i-BuO radical and polysilane 2, attacks another silicon atom in the same backbone to give a cyclic polysilane that contains an acyclic chain and another silyl radical (Scheme 8.1) [12]. The last silyl radical can either cyclize or abstract a hydrogen atom from another macromolecule, thus propagating the chain degradation. The reaction in Scheme 8.1 is an example of intramolecular homolytic substitution (ShO, a class of reactions discussed in Chapter 6. 

Aprotic diazotization of 86 (R = Me or Ph) produced 55% of thianthrene and lesser amounts of 1-phenylthio- (or methylthio) dibenzothiophen (Scheme 14) together with deaminated products. Comparable yields of 2-methyl-, 2-methoxy-, 2,7-dichloro- and 2,8-dichlorothianthrenes were obtained using this synthetic route. It is suggested that ring closure involves an intramolecular homolytic substitution at sulfur with loss of the 5-substituent as a radical [74JCS(P1)1272]. 

Photolysis of PTOC imidate esters generated amidyl radicals which can undergo intramolecular homolytic substitution reactions (Scheme 29).152 The ratio of exo to endo products observed for die reaction of each of die diiocarbonylimidazolide diastereomers (70) and (71) with B113S11H was found to be different indicating tiiat die intermediate radical (72) produced in each case was generated and reacted in a different conformation, respectively (Scheme 30).153... 

Selenopenams (V, Fig. 16) have been reported to be prepared in good to moderate yields in processes that presumably involved intramolecular homolytic substitution at selenium, starting from 2-azetidinone derivatives [283, 284]. 

Exposure of the diazonium salt 1107 to iodide ion generates an aryl radical, which undergoes intramolecular homolytic substitution at sulfur to liberate the acyl radical intermediate 1108. An exo-mode cyclization with concomitant incorporation of iodide then occurs to afford the 3-disubstituted-chroman-4-one 1109 in excellent yield (Scheme 275) <1997JOC5982>. 

A radical approach to cyclization is offered by the intramolecular homolytic substitution (ShO reaction at a silicon center. Reaction of phenyl bromoacetate with a stannylated silyl homoallyl ether under atom transfer conditions provides cyclic alkoxysilanes (Equation (119)).2... 

In a thorough study on the homolytic substitution reaction at silicon, Studer and Steen reported a mild method for the formation of cyclic five-membered alkoxysilanes based on tandem intermolecular add-ition/intramolecular homolytic substitution [137], whereby the reaction of homoallylic stannylated silylether 149 with ethyl iodoacetate in the presence of (Me3Sn)2 led to the desired Sni product 150 in high yield as the sole trans diastereoisomer (Scheme 49). While the 1,2 induction proved to be very high, leading in some cases to a unique diastereoisomer, 1,3 stereoselectivities were... 

A second strategy, analogous to nucleophihc substitution, involves intramolecular homolytic substitution at atom A with loss of a new radical L [Sni, Scheme 1, (2)]. hi this process the atom undergoing substitution (A) cannot be carbon, except for a few special situations, and hence the strategy is hmited in scope [8-10]. The central element in the third strategy is an... 

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