Homolytic cleavage-radical

Radical A fragment of a molecule possessing an unpaired electron as a result of homolytic cleavage. Radical chain mechanism A cycle in which propagation generates radical fragments, initiating further... 

Some of the evidence indicating that alkyl substituents stabilize free radicals comes from bond energies The strength of a bond is measured by the energy required to break It A covalent bond can be broken m two ways In a homolytic cleavage a bond between two atoms is broken so that each of them retains one of the electrons m the bond... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

Free-Radical Formation. Hydrogen peroxide can form free radicals by homolytic cleavage of either an O—H or the O—O bond. 

Diall l Peroxydicarbonates. Some commercially available diaLkyl peroxydicarbonates and their corresponding 10-h half-life temperatures (deterrnined in trichloroethylene solutions) are Hsted in Table 7 (45). These peroxides are active at low temperatures and initially undergo homolytic cleavage to produce alkoxycarbonyloxy radical pairs that may subsequendy decarboxylate to produce alkoxy radicals ... 

Homolytic cleavage of dlazonlum salts to produce aryl radicals is induced by titan1um(III) salt, which is also effective in reducing the a-carbonylalkyl radical adduct to olefins, telotnerization of methyl vinyl ketone, and dimerization of the adduct radicals. The reaction can be used with other electron-deficient olefins, but telomerization or dimerization are important side reactions. 

The broadest classification of reactions is into the categories of heterolytic and homolytic reactions. In homolytic (free radical) reactions, bond cleavage occurs with one electron remaining with each atom, as in... 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

Following the radical pathway" the next step is a homolytical cleavage of the N-R bond. The rearrangement to yield the tertiary amine 3 then proceeds via an intermediate radical-pair 4a. The order of migration is propargyl > allyl > benzyl > alkyl ... 

Alkyl halides can be reduced to alkanes by a radical reaction with tributyltin hydride, (C4H9)3SnH, in the presence of light (hv). Propose a radical chain mechanism by which the reaction might occur. The initiation step is the light-induced homolytic cleavage of the Sn— H bond to yield a tributyltin radical. 

It is important to emphasize that the hydroxy dithioketal cyclization can be conducted under mild reaction conditions and can be successfully applied to a variety of substrates.15 However, the utility of this method for the synthesis of didehydrooxocane-contain-ing natural products requires the diastereoselective, reductive removal of the ethylthio group. Gratifyingly, treatment of 13 with triphenyltin hydride and a catalytic amount of the radical initiator, azobisisobutyronitrile (AIBN), accomplishes a homolytic cleavage of the C-S bond and furnishes didehydrooxocane 14 in diastereo-merically pure form (95 % yield), after hydrogen atom transfer. 

The cyclization of imines 7, induced by the isopropyloxycarbonyloxy radical (i-PrOC02). obtained by homolytic cleavage of diisopropyl peroxydicarbonate, leads to dibenz[6,/][l,4]-oxazepines 8, accompanied by traces of biphenyls and benzoxazoles.424... 

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

A free-radical process consists of at least two steps. The first step involves the formation of free radicals, usually by homolytic cleavage of bond, that is, a cleavage in which each fragment retains one electron ... 

The second type of pyrolysis mechanism is completely different and involves free radicals. Initiation occurs by pyrolytic homolytic cleavage. The remaining steps may vary, and a few are shown... 

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

Hydrogen abstraction — The abstraction of a hydrogen atom H from a saturated carbon atom in a position allylic to the polyene chain can generate a resonance-stabilized neutral radical by homolytic cleavage of a C-H bond CAR = X - H. Then X - H -H R- X + RH. 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

An experimental probe for the presence of radical intermediates resulting from thermally induced homolytic cleavage of the N-0 bond was derived by incorporating an alkene into a model substrate to act as a potential intramolecular radical trap (Scheme 6.25) [11]. In a control experimental, thermal reaction of 73 gave the desired product 74 in 66% isolated yield. On the other hand, thermal rearrangement of the unsaturated compound 75 under our typical conditions gave the desired hydroxypyrimidinone 76 in only 38% isolated yield. When the vinyl ami-doxime mixture 75Z/E was heated in o-xylene at 125 °C in the presence of a... 

A crystal structure of the C02 derivative of (8), K[Co(salen)( 71-C02)], haso been reported in which the Co—C bond is 1.99 A, the C—O bonds are both equivalent at 1.22 A and the O-C-O angle is 132°.125 Carboxylation of benzylic and allylic chlorides with C02 in THF-HMPA was achieved with (8) electrogenerated by controlled-potential electrolysis,126 in addition to reductive coupling of methyl pyruvate, diethyl ketomalonate and / -tolylcarbodiimide via C—C bond formation. Methyl pyruvate is transformed into diastereomeric tartrates concomitant with oxidation to the divalent Co(salen) and a free-radical mechanism is proposed involving the homolytic cleavage of the Co—C bond. However, reaction with diphenylketene (DPK) suggests an alternative pathway for the reductive coupling of C02-like compounds. 

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