Allylic position

A white solid, m.p. 178 C. Primarily of interest as a brominaling agent which will replace activated hydrogen atoms in benzylic or allylic positions, and also those on a carbon atom a to a carbonyl group. Activating influences can produce nuclear substitution in a benzene ring and certain heterocyclic compounds also used in the oxidation of secondary alcohols to ketones. 

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 ... 

Allylic IS often used as a general term for molecules that have a functional group at an allylic position Thus the following compounds represent an allylic alcohol and an allylic chloride respectively... 

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. ... 

Chlorobutyl rubber is prepared by chlorination of butyl rubber (chlorine content is about 1 wt%). This is a substitution reaction produced at the allylic position, so little carbon-carbon double unsaturation is lost. Therefore, chlorobutyl rubber has enhanced reactivity of the carbon-carbon double bonds and supplies additional reactive sites for cross-linking. Furthermore, enhanced adhesion is obtained to polar substrates and it can be blended with other, more unsaturated elastomers. 

A large number of examples have been reported in the literature for the deuteration or tritiation of double bonds and for the hydrogenation of enones labeled at an allylic position. The following list of substrates, in addition to those already discussed, provides a representative cross section ... 

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... 

Similar oxidation with bromine of alkenes having branched perfluoroalkyls in the allylic position gives bromohydrm regioisomcrs and a dibromo derivative [J5] (equation 27)... 

Trifluoro- l-hydroxy-l-methylethyl)cyclohexene is monooxidized at both allylic positions in the nng with different agents. The major product is the rearranged product C when oxidation is accomphshed with chromiuin trioxide m methylene chlonde [42] (equation 34). 

The benzylic position in alkylbenzenes is analogous to the allylic position in alkenes. Thus a benzylic C—H bond, like an allylic one, is weaker than a C—H bond of an alkane, as the bond dissociation energies of toluene, propene, and 2-rnethylpropane attest ... 

Treatment of allylic substrates ISO, possessing suitable leaving groups X in tlieir allylic positions, witli organocopper reagents may result eitlier in an S 2-type process fa-attack) or alternatively in an S 2 one fy-attack), giving tlie substitution products 151 and 152, respectively fSclieme G.30) [Ij]. 

Treatment of an ethylidene malonic ester such as (a) with strong bases results in loss of a proton from the allylic position to produce the ambident ion fb). Alkylation of such carban-ions usually occurs at the carbon bearing the carbonyl groups, resulting in the establishment of a quaternary center and deconjugation of the double bond fc). 

The allylic position of olefins is subject to attack by free radicals with the consequent formation of stable allylic free radicals. This fact is utilized in many substitution reactions at the allylic position (cf. Chapter 6, Section III). The procedure given here employs f-butyl perbenzoate, which reacts with cuprous ion to liberate /-butoxy radical, the chain reaction initiator. The outcome of the reaction, which has general applicability, is the introduction of a benzoyloxy group in the allylic position. 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

Homoallylic systems may isomerize under hydrogenation conditions to allylic systems, causing hydrogenolysis to occur when it would not have been expected (39b,45a-45c). In these cases, if hydrogenolysis is unwanted, it is best to avoid those catalysts and conditions that favor isomerization. Double-bond migration to an allylic position may occur even if the double bond is required to leave a tetrasubstituted position (26a). 

Another method for preparing alkyl halides from alkenes is by reaction with jV-brotnosuccinimide (abbreviated NBS) in the presence of light to give products resulting from substitution of hydrogen by bromine at the allylic position—the position next to the double bond. Cyclohexene, for example, gives 3-bromo-cyclohexene. 

Why does bromination with NBS occur exclusively at an allylic position rather than elsewhere in the molecule The answer, once again, is found by looking at bond dissociation energies to see the relative stabilities of various kinds of radicals. 

Draw the alkene reactant, and identify the allylic positions. In this case, there are two different allylic positions we ll label them A and B. Now abstract an allylic hydrogen... 

For the construction of the I ring, the vinylic group introduced to activate the y-hydroxy epoxide moiety of 28 towards cyclization is an acrylic ester residue, which concomitantly allows cyclization on the allylic position, with formation of the tricyclic compound 29 containing the IJK fragment of the natural product, and fur-... 

At present it is believed that intermolecular chemical bonds are formed during the vulcanization of polychloroprene with ZnO not only due to the mobile chlorine in allyl position but also as a result of the reaction of the chlorine located directly at the double bond of the monomeric units chloroprene connected in the chain in 1,4-position as shown in the following scheme43. ... 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

Pathway A shows the most common reaction where the nucleophilic substitution reaction occurs at the electron-deficient carbon atom due to the strong electron-attracting character of the sulfonyl group. Nucleophilic displacements at the allylic position (SN2 reaction) are shown in pathway B. Pathway C is the formation of a-sulfonyl carbanion by nucleophilic attack on the carbon atom p to the sulfone moiety. There are relatively few reports on substitution reactions where nucleophiles attack the sulfone functionality and displace a carbanion as illustrated in pathway D3. 

A novel approach was developed very recently by Kita et al. [15]. DKR of allylic alcohols was performed by combining a lipase-catalyzed acylation with a racemization through the formation of allyl vanadate intermediates. Excellent yields and enantioselectivities were obtained. An example is shown in Figure 4.4. A limitation with this approach for the substrates shown in Figure 4.4 is that the allylic alcohol must be equally disubstituted in the allylic position (R = R ) since C—C single bond rotation is required in the tertiary alkoxy intermediate. Alternatively, R or R can be H if the two allylic alcohols formed by migration of the hydroxyl group are enantiomers (e.g. cyclic allylic acetates). 

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