Carbon-carrying radicals

The radicals may further crack, yielding an olefin and a new free radical. Cracking usually occurs at a bond beta to the carbon carrying the unpaired electron. 

The secondary free radical can crack on either side of the carbon carrying the unpaired electron according to the beta scission rule, and a terminal olefin is produced. 

Numerous reports published in recent years have focused on carbon-centered radicals derived from compounds with selected substitution patterns such as alkanes [40,43,47], halogenated alkanes [43,48,49,51-57], alkenes [19], benzene derivatives [43,47], ethers [51,58], aldehydes [48], amines [10,59], amino acids [23,60-67] etc. Particularly significant advances have been made in the theoretical treatment of radicals occurring in polymer chemistry and biological chemistry. The stabilization of radicals in all of these compounds is due to the interaction of the molecular orbital carrying the unpaired electron with energetically and spatially adjacent molecular orbitals, and four typical scenarios appear to cover all known cases [20]. 

Allenylcobaloximes, e.g. 26, react with bromotrichloromethane, carbon tetrachloride, trichloroacetonitrile, methyl trichloroacetate and bromoform to afford functionalized terminal alkynes in synthetically useful yields (Scheme 11.10). The nature of the products formed in this transformation points to a y-specific attack of polyhaloethyl radicals to the allenyl group, with either a concerted or a stepwise formation of coba-loxime(II) 27 and the substituted alkyne [62, 63]. Cobalt(II) radical 27 abstracts a bromine atom (from BrCCl3) or a chlorine atom (e.g. from C13CCN), which leads to a regeneration of the chain-carrying radical. It is worth mentioning that the reverse reaction, i.e. the addition of alkyl radicals to stannylmethyl-substituted alkynes, has been applied in the synthesis of, e.g., allenyl-substituted thymidine derivatives [64],... 

ESR spectra of ion-radicals reveal the quantitative distribution of spin density. The ESR spectrum determines the HEC constant for the ith hydrogen, flT. The constant is directly proportional to the spin density at the ith carbon carrying the ith hydrogen. 

Generation of the carbon based radical in these processes involves the prior formation of a complex between manganese(lll) and the enol of the carbonyl reactant. Intramolecular electron transfer occurs within this complex. Addition to the olefin then takes place within the co-ordination sphere of manganese. When manganese is present in catalytic amount, the relative values of the equlibrium constants between manganese and both the carbonyl compound and the alkene arc important. If the olefm is more strongly complexed then no radical can form and reaction ceases. Reactions are usually carried out at constant current and the current used must correspond to less than the maximum possible rate for the overall chemical steps involved. Excess current caused the anode potential to rise into a region where Kolbe reaction of acetate can occur and this leads to side reactions [28]. 

The C—H bond is normally not very polar. As a result, the <7Ch and ch orbitals are widely separated and more or less symmetrically disposed relative to a. A sluggish reaction is expected with carbon free radicals, but a rapid reaction may be anticipated with both electrophilic and nucleophilic free radicals. Examples of both kinds of reactions are ubiquitous in organic chemistry. An ab initio investigation of the former, involving oxygen-centered free radicals, has been carried out [237], The reactivity spectrum may be modified by substitution on the carbon bearing the hydrogen atom. As we have seen in Chapter 7, all three kinds of substituents stabilize the carbon-centered free-radical intermediate. 

To disrupt the carbon—chloride bond at position 5 of the substrate anion radical, population of this bond with an unpaired electron should be increased. However, if a spin density at the carbon-carrying chlorine is too great, the initial chlorine-containing anion... 

Quantitative nitroxide-trapping experiments should be carried out under thoroughly de-oxygenated conditions since oxygen will act as a competitive radical trap for carbon-centred radicals. Consequently, several freeze-thaw degassing cycles using pressures <10 3 mm Hg are usually required. 

The radical chain mechanism involving allytin belongs to the fragmentation method family and can be schematically presented as in equation 38, which involves five steps (i) initiation step, (ii) reaction with the substrate, forming the carbon-centred radical, (iii) a possible evolution of this radical, (iv) addition of the newly formed radical to the allylic double bond and (v) /3-scission, regenerating the chain-carrying tin radical. 

Radical ring closure of an sp3 carbon-centered radical to a hydrazone group in compounds (174) can be carried out via 5-exo-trig manner. The silicon connection (Si-tethered) is removed by oxidative treatment with H202 to generate 2-amino-1,3-diols (175) (eq. 3.66) [189-191]. 

The same %-endo-trig cyclization can also be carried out with sp2 carbon-centered radicals, as shown in eq. 3.84. Thus, the preparation of mms -pyrano[3,2-c][2]benzox-ocines (216) was produced in good yields through %-endo-trig manner of aryl radicals derived from D-mannose pyranosides (215), using the Bu3SnH /AIBN system. 

Under the same photolytic conditions together with Bu3SnH, annulation of the cyclohexane ring can be carried out through the generation of an alkoxyl radical from sulfenate ester (9), 1,5-H shift, nucleophilic addition of the formed carbon-centered radical to methyl vinyl ketone, and finally, treatment with a polar sequence method as shown in eq. 6.6. 

In the case of a free-radical addition of hydrogen bromide, a type of addition limited to hydrogen bromide only, the attacking species is not proton, but a bromine atom that will join the carbon in such a way as to generate a more stable free-radical species. It will add to the carbon carrying two fluorines. In the intermediate free radical, the single electron is better accommodated at the carbon holding chlorine, because chlorine can disperse the electrons in its d-orbitals. 

In most cases, carbon-centered radicals carry the unpaired electron in p(7t)-type orbitals. Since C isotopes are present at a very low percentage (natural abundance, 1.11%), the spin distribution is monitored by adjacent H nuclei. The spin from the p-type orbital of the C atom is transferred to the adjacent H atom (H ) via n-spin polarization (Fig. 7.2a) however, spin transfer to more distant protons follows the model of a hyperconjugation mechanism illustrated in Fig. 7.2b. The closer the Cp-Hp is oriented toward the z-axis of the C p orbital, (0 = 0°) the bigger becomes the Hp hfc. When 0 is equal to 90°, the Hp hfc reaches its minimum value. This behavior is described by an empirical formula ... 

An early version was the Hofmann-Loffler-Freytag reaction (Scheme 1). Irradiation of a chloramine in acid leads to formation of the aminium radical, which can abstract a hydrogen to generate a caibon radical. Then the resulting carbon radical abstracts chlorine from another protonated chloramine, producing a chlorinated carbon and regenerating the chain-carrying radical. On treatment with base, the product... 

The mid-chain dehydrogenation of saturated fatty acyl derivatives is carried out by a large family of 02-dependent, nonheme diiron-containing enzymes known as desaturases. Both soluble and membrane-bound desaturases have been characterized. The mechanism of desaturation is thought to involve the stepwise syn removal of vicinal hydrogen atoms via a short-lived carbon-centered radical intermediate. The most common desaturase inserts a (Z)-double bond between the C-9,10 carbons of a stearoyl thioester however, many variations of this prototypical reaction have been discovered. Accounting for this diversity in terms of subtle alterations in active-site architecture constitutes a new frontier for research in this area. 

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