Alkyl halides outer-sphere electron-transfer

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

It has been shown that a complete shift in stereochemistry of the nucleophilic reactions of (29), with alkyl halides such as 2-bromobutane or cis-2-bromomethoxycyclohexane, from racemization to complete inversion, is induced by increase in the inner-sphere stabilization of the transition state from 0 to 3 kcal mol" This has been ascribed to competition between inner-sphere 5)vr2 and outer-sphere electron-transfer processes the former being extremely sensitive towards inner-sphere stabilization. 

Addition of excess CH3I to a solution of [Ni (tmc)]+ results in the rapid loss of the absorption (A = 360 nm, e = 4 x 103 M-1 cm-1) and appearance of a less intense band at A = 346 nm. A subsequent slower reaction gives rise to the weaker absorbance profile of [Ni"(tmc)]2+. The data are interpreted in terms of the formation of an organo-nickel(II) species followed by a slower hydrolysis with breaking of the Ni-C bond. Kinetic studies under conditions of excess alkyl halide show a dependence according to the equation — d[Ni1(tmc)+]/cft = 2 [Ni(I)][RX]. The data have been interpreted in terms of a ratedetermining one-electron transfer from the nickel(I) species to RX, either by outer-sphere electron transfer or by halogen atom transfer, to yield the alkyl radical R. This reactive intermediate reacts rapidly with a second nickel(I) species ... 

In contrast, the kinetics of the alkylation of trans-[Mo(N2)(L)(dppe)2r (L = CN or SCN) with nBuI show a first-order dependence upon both the complex and the alkyl halide (88). These kinetics and the retention of the ligand, L, in the diazenido product are consistent with the mechanism shown in Scheme 4. The electron-rich complexes (A) undergo rate-limiting, outer-sphere electron transfer to yield the alkyl radical and fra/is-[M(N2)L(dppe)2] (B). Subsequent, rapid attack of the radical on the coordinated dinitrogen of (B) yields the diazenido product (C). 

Modelling of outer sphere electron transfer 5 Application to organic redox systems 15 Modelling of dissociative electron transfer 21 Stepwise and concerted processes 23 Aryl halides 37 Alkyl halides 54 Other examples 63... 

The electron-transfer reactions of metal-carbonyl anions have been reviewed. Metal-carbonyl anions exhibit one-and two-electron reactions. The two-electron processes involving transfer of groups such as hydrogen, alkyl, and halogen between metal centers are related to the nucleophilicity of the anion involved. The one-electron processes are primarily outer-sphere electron transfers. However, in contrast to organic reactions, the metal-carbonyl anions can also undergo inner-sphere electron transfers. This is usually the case when an anion of low nucleophilicity transfers an electron to a metal-carbonyl cation or halide. 

For systems that are powerful excited-state reductants, photoreduction of alkyl halides is observed (6.16). This reaction was initially interpreted to be an outer-sphere electron transfer to form the radical anion, which rapidly decomposes to yield R- and X . Subsequent thermal reactions yield the observed products, an SrnI mechanism (Figure 3a). While such a mechanism, SrnI, appears plausible for a metal complex with E°(M2 /3M2 ) < -1.5 V (SSCE), it seems unlikely for complexes with E°(M2 /3M2 ) > -1.0 V (SSCE). Reduction potentials for alkyl halides of interest are generally more negative than -1.5 V (SSCE) (1/7). Alkyl halide photoreduction is observed for binudear d complexes whose excited-state reduction potentials are more positive than -1.0 V (SSCE) in CH3CN. 

An alternative pathway to outer-sphere electron transfer, which yields similar photoredox products with alkyl halides, is excited-state atom transfer (Figure 3b). Data obtained for Pt2(P20sH2)4 " indicate that alkyl... 

Miller, R. S., Sealy, J. M., Shabangi, M, Kuhiman, M. L., Fuchs, J. R., Flowers, R. A., II. Reactions of Sml2 with Alkyl Halides and Ketones Inner-Sphere vs Outer-Sphere Electron Transfer in Reactions of Sm(ll) Reductants. J. Am. Chem. Soc. 2000, 122, 7718-7722. 

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. ... 

Anionic dinitrogen complexes of MeP and W of the type trans - [M(N2)X(Ph2PCH2CH2PPh2)2] (X = N3, CN, SCN) are moderately strong reducing agents with oxidation potentials in the vicinity of -1 V versus SCE. They undergo an outer sphere electron - transfer with alkyl halides to produce M dinitrogen 17 - electron intermediates and alkyl radicals. Scheme E. The fates of both... 

Andrieux, C. P., I. Gallardo, and J.-M. Saveant, Outer-sphere electron-transfer reduction of alkyl halides. A source of alkyl radicals or of carbanions Reduction of alkyl radicals. J. Am. Chem. Soc. Ill, 1989 1620-1626. 

ScoOiOPorph/RCoooporph-1 The principle of the method is illustrated in Fig. 18 with the example of reaction (142). The rate constants obtained with the investigated nucleophiles (or with single electron donors—that is the question ) are compared to those of the reaction of a series of anion radicals with the same alkyl halide in the same medium. As discussed on p. 59, aromatic anion radicals behave in this reaction as outer sphere electron donors and the alkyl halide undergoes a dissociative electron transfer. For... 

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