Allylic position halogenation

The direct introduction of a halogen atom (usually bromine) by means of V-haloamine (generally iV-bromosuccinimide) in the allyl position is known as the Wohl-Ziegler reaction ... 

Although alkenes typically react with chlorine and bromine by addition at room tern perature and below (Section 6 14) substitution becomes competitive at higher tempera tures especially when the concentration of the halogen is low When substitution does occur It IS highly selective for the allylic position This forms the basis of an industrial preparation of allyl chloride... 

Allenes react with other typical electrophiles such as the halogens and mercuric ion. In systems where bridged-ion intermediates would be expected, nucleophilic capture generally occurs at the allylic position. This pattern is revealed, for example, in the products of solvent capture in halogen additions and by the structures of mercuration products. ... 

Bromination with A-bromosuccinimide generally gives the same result as bromination with free bromine or hypobromous acid. The reaction is considered to proceed with a small concentration of free bromine and does not generate an appreciable concentration of acid. Conditions are therefore mild. In addition, A-bromosuccinimide has been used to brominate the allylic position of a, -unsaturated ketones in the presence of free-radical promoters or with irradiation, and thus gives access to dienones by dehydro-halogenation, for exaraple " ... 

Halogen attached directly to vinylic carbon or m an allylic position with... 

Oxygen functionalities, such as alcohols, ethers, and acetate groups, and halogens at the allylic position, deshield when the fluorine is at the terminal position and shield when it is at the internal 2-position (Scheme 3.43). 

This reaction is a special case of 4-1, but is important enough to be treated separately."4 Olefins can be halogenated in the allylic position by a number of reagents, of which N-bromosuccinimide (NBS)115 is by far the most common. When this reagent is used, the... 

Comparison of the stereochemical outcomes in entries 2 and 3 of Table 1 implies that the heteroatom functionality directs the oxidation reaction more effectively from pseu-doequatorial orientation (the O—C—C=C dihedral angle is 140°). Selective formation of yyw-epoxides is also known for olefins having carbamate18, acetal19, ether20 and halogen groups21 in allylic positions. There are many cases in the literature where the epoxidation... 

Halogens in the allyl position seem to be readily replaceable by hydrogen. Anglade (110) treated bis-2,4-(chloromethyl)anisole with Raney nickel and hydrogen in alcohol at ordinary temperatures. A 25% yield of 2,4-dimethylanisole was obtained, with ethyl ether formed as a by-product by interaction with the solvent. 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

In this way, pentadecanoic acid has been prepared in 58% over-all yield from palmitic acid. ° A modification of the process, whereby three carbon atoms may be removed, has been reviewed. In this procedure, the olefin is brofflinated in the "allylic position by N-bromosuccinimide. Dehydro-halogenation then gives a diene, R CH=CH—CH=C(C H5), which is Oxidized with the loss of three carbon atoms. 

Beside the Grignard and other C-C bond-forming reactions, a number of functional group transformations may also serve as an entry into allylic systems. Some of them, namely the reduction of a, -unsaturated carbonyl compounds (products of crotonic condensation), halogenation of alkenes at the allylic position with Af-bromosuccinimide (NBS) and epoxide isomerization, are shown in Scheme 2.56. 

Halogenation of saturated hydrocarbon polymers can hardly be controlled and is frequently assodated with chain degradation phenomena In contrast, the presence of randomly distributed olefinic unsaturations, allows selective halogenation reactions by adopting appropriate conditions. For instance, butyl rubber can be chiorinated or brominated in allylic positions and chloro-butyl or bromo-butyl rubber results The latter polymers are very interesting since they exhibit fast curing rates when sulfur and ZnO are introduced in the formulations. 

The reactivity of alkenes toward dichlorocarbene also decreases if there is a halogen in an allylic position. In this case side reactions, such as alkylation of trichloromethyl anion, halide exchange in the case of allylic bromides etc., take place. Although the yields of the products from these side reactions are low, they impede the isolation of the desired product, e.g. formation of 3, 4, and... 

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