Of alkyl halides to alkanes

Tetra-p,3-carbonyldodecacarbonylhexa-rhodium, 288 of alkyl halides to alkanes Chromium(II) chloride, 84 Lithium aluminum hydride-Ceri-um(III) chloride, 159... 

Reduction reactions, of alkyl halides to alkanes (Continued)... 

CrCI2 reduction of alkyl halides to alkanes, of acetylenes to trans olefins, of epoxides to olefins, or of nitro compounds to oximes. 

Similar reactivity towards activated methyl compounds, especially methyl iodide, has been observed for Ni OEiBC [114, 115, 61, 116] and for Ni (tmtaa) (where tmtaa is the dianion of 6,8,15,17-tetramethyl-5,14-dihydro-dibenzo [b,i] [1, 4, 8, ll]tetraazacyclotetradecine) [112, 113]. Ni tmtaa), generated in situ by reduction of Nin(tmtaa) and observed by EPR, was proposed as the active catalyst in the reduction by NaBH4 of alkyl halides to alkanes. Recently, a different Ni(I) tetraazamacrocyclic complex, produced by irradiation of the Ni(II) parent compound with 5 MeV electrons, produced small amounts of methane from subsequently added CH3S-CoM [111]. The represented the first report of reactivity of any identifiably Ni(I) species with the natural substrate. 

Reactions.—New systems for the reduction of alkyl halides to alkanes include mixtures of lithium aluminium hydride with transition-metal chlorides (such as Ni or Co" chlorides) and some complex metal hydrides of copper, especially the THF-soluble Li4CuHs. Details of the application of sodium cyanoborohydride to the reduction of halides (and sulphonate esters) in dipolar aprotic solvents have been published. Virtually all other functional groups are inert to this convenient, mild, and efficient system, which has been used to develop a one-pot deoxygenation of primary alcohols (Scheme 39) via the iodides. The alternative cyanoborohydride reagents (55) and the polymeric (56) are also discussed. ... 

Methods reported this year for the reduction of alkyl halides to alkanes include the potassium-dicyclohexyl-18-crown-6 reduction of alkyl fluorides, sodium borohydride reduction of alkyl chlorides, bromides, and iodides (or sulphonate esters) under liquid-liquid phase-transfer conditions, and the selective reduction of tertiary alkyl, benzyl, and allyl halides with the borate (61). Continuing... 

Reduction of simple alkyl halides to alkanes by ethylenediamine complexes of Cr(II), denoted Cr"(en) occurs readily , e.g. for isopropyl chloride in aqueous dimethylformamide at 25 °C simple second-order behaviour is found with ki dependent on [en]/[Cr(II)] but reaching a limiting value of 1.6x 10 l.mole . sec . Competition studies between a mixture of two alkyl chlorides for Cr(II) was achieved by estimating alkane products by gas-liquid chromatography and... 

As noted earlier, [HCr(CO)5] also converts alkyl halides to alkanes, but the reactivity of the acyl chloride is much higher, such that it was possible to selectively convert the acyl chloride to an aldehyde in one step, without interference from the alkyl bromide functionality. A second equivalent of [HCr(CO)5] further reduced the alkyl bromide (Eq. (27)). 

Lithium aluminium hydride (LiAlH4), a strong reducing agent, reduces alkyl halides to alkanes. Essentially, a hydride ion (H ) acts as a nucleophile displacing the halide. A combination of metal and acid, usually Zn with acetic acid (AcOH), can also he used to reduce alkyl halides to alkanes. 

Alkyl halides are important starting materials in the synthesis and manufacture of a variety of organic compounds. Free-radical halogenation (Scheme 4.12) is an important methodology for the synthesis of alkyl halides from alkanes. Brominations (X = Br) tend to be very selective because of the... 

Primary and secondary alkyl bromides, iodides, and sulfonates can be reduced to the corresponding alkanes with LiBHEt3 (superhydride) or with lithium aluminum hydride (LiAlH4, other names lithium tetrahydridoaluminate or lithium alanate). If such a reaction occurs at a stereocenter, the reaction proceeds with substantial or often even complete stereoselectivity via backside attack by the hydride transfer reagent. The reduction of alkyl chlorides to alkanes is much easier with superhydride than with LiAlH4. The same is true for sterically hindered halides and sulfonates ... 

T. Imamoto, Reduction of Saturated Alkyl Halides to Alkanes, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 8, 793, Pergamon Press, Oxford, U. K., 1991. 

Reaction (414) is formally analogous to the oxidative addition of alkyl halides to noble metal complexes described earlier, and both homolytic and heterolytic processes can be envisaged. Heterolytic cleavage of C—H bonds represented in Eq. (415) is analogous to the interaction of the powerful oxidant Co3+ with alkanes in TFA in reaction (229). 

Inanaga found that the more powerful reductant obtained by the addition of HMPA to Sml2 allowed the reduction of alkyl halides to be carried out under milder conditions (Scheme 4.1).2 In the absence of HMPA, at room temperature, alkyl chlorides are not reduced and alkyl bromides react only slowly. On addition of the additive to Sml2, alkyl bromides are reduced in minutes at room temperature and alkyl chlorides are converted to the corresponding alkanes upon heating. A detailed discussion of the mechanism of alkyl halide reduction can be found in Chapter 3, Section 3.2. 

In the presence of a radical initiator (Z-), tributyltin hydride (RgSnH, R = CH3CH2CH2CH2) reduces alkyl halides to alkanes R X + RgSnH -> R H + RgSnX. The mechanism consists of a radical chain process with an intermediate tin radical ... 

In an important report, Janowicz and Bergman have described the photochemical incorporation of an alkyl group from an alkane into an iridium complex. The process does not appear to involve alkyl radicals, but is thought to occur via two successive concerted steps. This report may point the way to new catalytic processes for the functionalization of alkanes. The photoaddition of alkyl halides to alkenes catalysed by Cu complexes represents a novel process for formation of C-C bonds, and may have useful synthetic applications (Mitani et ai). The possibility of analogous intramolecular reactions comes readily to mind. 

The reduction of saturated alkyl halides to alkanes, as represented in equation (1), is the most fundamental reaction of reductive dehalogenations of organic halides. The importance of these reductions has stimulated considerable investigation, and a number of successful approaches have been reported hitherto. Numerous reducing agents or reagent systems are available and many of them have been applied to practical organic synthesis with notable success. 

This chapter surveys the reduction of saturated alkyl halides to alkanes. Reductive -eliminations of vicinal dihalides to alkenes are also described briefly. Reduction of vinyl and aryl halides is covered in this volume. Chapter 4.5 hydrogenolysis of allyl and benzyl halides is covered in this volume. Chapter 4.7, and reduction of a-halo-substituted carbonyl compounds CX—CO to carbonyl compounds CH—CO is covered in this volume. Chapter 4.8. 

A variety of metal hydrides reduce alkyl halides to alkanes. Sodium borohydride has been extensively used in aprotic solvents like DMSO, DMF, diglyme and sulpholane. Reduction with this hydride in the presence of phase transfer catalysts is even more advantageous. Sodium borohydride in the presence of nickel boride (Ni2B) has been... 

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