Allyl dehalogenation

Alkyl, allyl, and aryl bromides are dehalogenated mainly with the formation of R R dimers in the presence of polypyridyl complexes of the metals of Group VIII. It has been demonstrated that the complexes [Co(bpy)3] + 203-204 [Ni(bpy)3]2+,205 and [Ni(phen)3]2+206 catalyze the reductive dimerization of allyl and alkyl bromides in organic 203 205 206 and aqueous micellar 204 solution. 

An instructive strategy to cyclize acyclic precursors to lactones was developed by Mulzer et al. (Scheme 7) [19]. The lactone 46 was formed via a halo-lactonization-dehalogenation [20] sequence from precursor 45, which in turn was build up by a Claisen rearrangement from the chiral allylic alcohol... 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

Iron carbonyls also mediate the cycloaddition reaction of allyl equivalents and dienes. In the presence of nonacarbonyldiiron a,a -dihaloketones and 1,3-dienes provide cycloheptenes (Scheme 1.5) [14,15]. Two initial dehalogenation steps afford a reactive oxoallyliron complex which undergoes a thermally allowed concerted [4 + 31-cycloaddition with 1,3-dienes. The 1,3-diene system can be incorporated in cyclic or heterocyclic systems (furans, cyclopentadienes and, less frequently, pyrroles). Noyori and coworkers applied this strategy to natural product synthesis, e.g. a-thujaplicin and P-thujaplicin [14, 16]. 

In a recently developed method, the single bond of a cyclopropene is obtained by 1,3-dehalogenation in an allylic system thus reaction of the iodochlorides (2) with an alkyl lithium at -78 °C leads to a range of cyclopropenes12) ... 

A well-defined complex, (IPr)Pd(allyl)Cl, provided an improved method for this transformation [ 168]. With very low catalyst loading (0.5-0.025 mol %), the dehalogenation of aryl chlorides could be carried out at 60 °C in less than 2 hours or in 2 minutes when micro-wave heating was used. 

Indium metal in water reduces a-halocarbonyl compounds and benzyl iodides to the corresponding dehalogenated products in excellent yields under sonication, though simple alkyl and aryl iodides remain inert under these conditions (Equation (97)).379 Similar dehalogenation in micellar systems in the presence of a catalytic amount of sodium dodecyl sulfate in water affords the corresponding parent carbonyl compounds in excellent yields (Equation (98)).380 The allylic iodide or acetate is reduced by indium into the corresponding 3-methylcephems and 3-methylenecephams in an aqueous system. The latter are converted into the former quantitatively under basic conditions (Scheme 110).381... 

The cleavage of a single bond (allylic or benzylic C-0 and C-N, C-X, C-S, N-N, N-O, 0-0 bonds, opening of cyclopropane, epoxides and aziridine by catalytic hydrogenation) is known as hydrogenolysis. Dehalogenation is favoured by basic conditions in the presence of a metal catalyst, Pd. The ease of dehalogenation decreases in the order I > Br > Cl > R... 

The ESR spectrum obtained for irradiated methanol solution of 1,3-butadiene or cis-1,3-pentadiene is similar to that of irradiated methanol solution of allyl chloride, in which case it is known that the electron leads to reductive dehalogenation (equation 24). 

Pd/C also catalyzes the PTC reduction of aryl and heteroaryl halides with sodium formate [52]. The reductive dehalogenation of allyl and benzyl halides with HCOOK can be carried out in a heptane/water system in the presence of water-soluble complexes of type PdCl2(sulfonated phosphine)2 and a PT agent... 

Cp)2Cr(NO)2]2 dehalogenates vicinal dibromoalkanes to form alkenes this complex does not abstract halogen from simple alkyl or allyl halides. 

Allyl sulfones. The dehalogenative cross-coupling of sulfonyl chlorides with allylic... 

In a related area, triarylphosphonium-supported tin reagents were very recently used for Stille crosscoupling reactions, dehalogenation reactions, cyclization reactions, and allylation reactions affording the desired products with very low tin pollution (5-15 ppm). ... 

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