Bonding homolytic cleavage

N- lomoallylic enamides 239 derived from pymvate were subjected to free radical cyclization in presence of R3SnH and AIBN. The radical species generated from the C-halogen bond homolytic cleavage underwent a 5-endo-cyclization followed either by a 5-exo- or 6-fV/z/o-cyclization and reduction of the final radical species (Scheme 63)... 

The kinetic data on the relative rate of the C—R bond homolytic cleavage from radical 30 relative to that of the C—CH3 bond reveal also the relative stability of fluoro-substituted methyl radicals to the methyl radical (Scheme 1.72). Mono- and difluoromethyl radicals were formed from 30 preferentially in comparison with methyl radicals. Here again, mono- and difluoro-substitutions in a-substituted methyl radicals seem to stabilize the radicals [29]. 

However, H2 can bind in stable fashion to very electron-deficient metals, which are weak backbonders, nearly as well as to more electron-rich metals. Calculations show that for highly electrophilic metals the reduction in BD is almost completely offset by increased electron donation from H2 to the electron-poor metals. It is important to note there are two completely different pathways for cleavage of H-H and X-H bonds homolytic cleavage (OA) and heterolytic cleavage. Both pathways... 

Kita and coworkers use potassium bromide to depolymerize iodosobenzene 17 by generating an I—Br bond. Homolytic cleavage of the I—Br bond to generate the iodine-centered radical allows formation of the benzylic radical 18. The authors propose the benzylic radical is trapped by bromine radical to generate a benzylic bromide 19, a known byproduct of the reaction conditions. Conversion to the alcohol and subsequent oxidation yields the aryl ketone 20. While the ketone products are useful themselves, Ochiai utilizes this mechanism, though with a different hyperva-lent iodine reagent, to deprotect benzylic ethers such as 21 to alcohols 2. ... 

State correlation diagram for heteronuclear case in which both heterolytic bond cleavage products are energetically favored relative to homolytic cleavage. 

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... 

Homolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that one electron is re tamed by each of the initially bonded atoms... 

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

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 ... 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

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. 

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