Mechanism, radical with alkyl halides

The reactions are accelerated by radical initiators or by UV light, and inhibited by radical scavengers,128-129 and, under suitable conditions, the reaction of stannyl radicals with alkyl halides to give the corresponding alkyl radicals can be observed by ESR spectroscopy.130 Optically active alkyl halides react with racemisation,129 but vinyl halides may show some retention of configuration at low temperature. The reactions are therefore accepted to follow the radical chain mechanism which is depicted in Scheme 15-2. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Scheme 10.10. Radical mechanism in alkylation reactions with alkyl halides.

The potassium salt of the phthalodinitrile (ort/zo-dicyanobenzene) anion radical also reacts with an electrophile according to the electron transfer scheme. If the electrophile is tert-butyl halide, the reaction proceeds via the mechanism, including at the first-stage dissociative electron transfer from the anion radical to alkyl halide, followed by recombination of the generated tertiary butyl radical with another molecule of the phthalodinitrile anion radical. The product mixture resulting in the reaction includes 4-tert-butyl-1,2-di-cyanobenzene, 2-tert-bytylbenzonitrile, and 2,5-di(tert-butyl)benzonitrile (Panteleeva and co-authors 1998). 

These reagents are useful in the 1,4-addition of alkyl groups to a,(l-unsaturated carbonyl systems and can also be reacted with alkyl halides to produce larger alkanes (Following fig.). The mechanism is considered to be radical based. 

Kosower s radical (66) reacts with alkyl halides by a mechanism which transfers a halogen atom to the pyridinyl radical in the rate determining step (Py is 66) ... 

The Cr2+(aq) ion reacts readily with alkyl halides, apparently by a radical mechanism, to generate [R—Cr(H20)3]2+ ions the R—Cr bonds can then be cleaved by H30+. On the basis of these fundamental processes, Cr2+(aq) finds important use as a reductant for organic compounds, especially in aqueous DMF as solvent, with ethylenediamine present to complex the Crra produced. 

However, based on the mechanism for reduction of the non-cyclic R3M compounds (Section III.A.l) and the studies of the reaction of 4 (formed by chemical reduction) with alkyl halides discussed below, it seems to be more likely that the experimentally found two-electron reduction on a preparative time scale is due to a slow cleavage of the As—Ph bond in the anion radical. 

In 1962, Kuivila showed that the reaction of trialkyltin hydrides with alkyl halides (hydrostannolysis) (equation 1-6) was a radical chain reaction involving short-lived trialkyltin radicals, R3Sn, 12 and in 1964, Neumann showed that the reaction with non-polar alkenes and alkynes (hydrostannation) (equation 1-7) followed a similar mechanism,13, 14 and these reactions now provide the basis of a number of important organic synthetic methods. 

As stated above, CIDNP denotes the transient occurrence of anomalous line intensities in NMR spectra recorded during chemical reactions or shortly after their completion. The phenomenon was first observed in 1967 by Bargon, Fischer and Johnsen [35a] in thermal decompositions of peroxides and azo compounds, and, independently, by Ward and Lawler [35b] in the reactions of alkyl lithium with alkyl halides. It was immediately realized that the line anomalities are caused by populations of the nuclear spin states in the reaction products that deviate from the Boltzmann populations. After initial attempts of interpreting CIDNP by electron-nuclear cross-relaxation, the radical pair mechanism was developed in 1969 by Kaptein and Oosterhoff [36a], and independently by Closs [36b],... 

Tin alkynes rearrange to yield allene products in much the same way as do lithium alkynes, except that the reaction involves a radical mechanism. It is very similar to the reaction of allyl stannanes with alkyl halides, which substitutes the allyl group. Similar reactions are reported for allyl derivatives of cobalt, rhodium and iridium, but this work has not been extended to alkyne derivatives. 

Oxidative-addition reactions of alkyl halides also occur with metal ions such as thallium(I). In this case, the HOMO is an s orbital on the metal of a symmetry. The concerted mechanism is now impossible and only a two-step reaction can occur. The same is true for reactions of metals in the free state, such as zinc or magnesium, with alkyl halides. Note that free radicals may be formed, instead of ions, in all cases discussed. Free radical reactions are usually allowed by symmetry. 

Arsenic and Antimony. Three studies of reaction mechanisms for tetrahedral antimony(v) compounds are reported. These are of the reaction of trimethylantimony sulphide (MegSbS) with alkyl halides, where a four-centre transition state seems possible, the reaction of R4Sb+ cations with alkoxide ions, and the ageing of antimonic acid in aqueous solution. Both thermal and photochemical decomposition of pentaphenylanti-mony have been investigated. Whereas the products of the photochemical reaction are numerous, though all derived from phenyl radicals, the... 

The mechanism of the metathesis reaction of the new vanadium hydride [V(C5H5)H(C0)3] with alkyl halides is a free-radical chain process involving [V(CsH5XCO)3] based on normally applied criteria such as the similar rates for primary, secondary, and tertiary R and the formation of completely racemic... 

Oxidative addition of alkyl halides can also occur in certain cases by an outer-sphere electron transfer mechanism involving a coordinatively saturated metal center and an alkyl halide. This pathway is shown in Scheme 7.7. Oxidative addition by this initial outer-sphere electron transfer pathway tends to occur instead of an 5, 2 pathway when the electrophile is particularly susceptible to electron transfer, when the electrophile possesses some steric hindrance, when the electrophile possesses a weak C-X bond, and when the metal lacks an available coordination site. Because of the lack of a coordination site at the metal, the initial electron transfer occurs without prior coordination of the electrophile to the metal. This initial step parallels the electron transfer and subsequent radical chemistry that occurs when some carbanions are treated with alkyl halides. ... 

It has been proposed that a number of processes catalyzed or mediated by Ni of interest for organic synthesis proceed by radical intermediates. In particular, Ti -allyl Ni(II) complexes react with alkyl halides in polar solvents by a process for which a radical chain mechanism has... 

Such reactions, like the reaction of [IrCl(CO)(PMe3)2] with alkyl halides, occur according to a free radical mechanism, although in certain cases other mechanisms may also be possible. 

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