Halides, alkyl from allylic halogenation

The 17-electron complex Ta(CO)4 (dppe), which is in equilibrium with its dimer, abstracts halogen from active halides such as allyl bromide, PhCHjBr, and t-BuI to give TaX(CO)4 (dppe) the 18-electron anion [Ta(CO)4(dppe)] reacts only slowly if at all with the same alkyl halides. 

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

The reaction of dibromocarbene with haloalkenes (vinyl halides, alkenes with a halogen more distant from the double bond) leads to the formation of 1,1-dibromocyclopropanes in poor to good yields. However, reactions with allyl halides, particularly substituted allyl bromides, require comment. These alkenes furnish, apart from 1,1-dibromocyclopropanes 1, the alkylation products of tribromomethyl anion 2 and, occasionally, the products of their further transformations (dibromocarbene adducts 3, products of elimination of hydrogen bromide 4 etc.) if they react with bromoform under phase-transfer catalysis conditions (Houben-Weyl, Vol.E19b, pi620). 

Differences in solubility of the reactants may for example be utilized as follows. Sodium iodide is much more soluble in acetone than are sodium chloride or sodium bromide. Upon treatment of an alkyl chloride or bromide with sodium iodide in acetone, the newly formed sodium chloride or bromide precipitates from the solution and is thus removed from equilibrium. Alkyl iodides can be conveniently prepared in good yields by this route. Alkyl bromides are more reactive as the corresponding chlorides. Of high reactivity are a-halogen ketones, a-halogen carboxylic acids and their derivatives, as well as allyl and benzyl halides. 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

Primary alkyl bromides react well in this sequence except for particularly reactive compounds (e.g., methyl bromide, allyl bromide) which give the vinyl halide by metal-halogen exchange. Secondary halides, as expected, suffer from elimination as a side reaction. Other electrophiles have been used successfully including D20, aldehydes and ketones, di-methylformamide,47 chlorotrimethylsilane,4 8 1,2-dibromoethane,4 and... 

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