Halide-displacing -migrations

Scheme 1-248. Preferential migration of the aryl ring in tram position with respect to the halide displaced.

In the reaction of aryl and alkenyl halides with 1,3-pentadiene (248), amine and alcohol capture the 7r-allylpalladium intermediate to form 249. In the reactions of o-iodoaniline (250) and o-iodobenzyl alcohol (253) with 1,3-dienes, the amine and benzyl alcohol capture the Tr-allylpalladium intermediates 251 and 254 to give 252 and 255[173-175]. The reaction of o-iodoaniline (250) with 1,4-pen tadiene (256) affords the cyclized product 260 via arylpalladiuni formation, addition to the diene 256 to form 257. palladium migration (elimination of Pd—H and readdition to give 258) to form the Tr-allylpalladium 259, and intramolecular displacement of Tr-allylpalladium with the amine to form 260[176], o-Iodophenol reacts similarly. 

Displacement of an allylic halide is complicated by side reactions involving migration of the double bond. A good example is the reaction of 7a-bromo-3 -acetoxy-A -steroids (201) which gives, besides the expected... 

The nitro group does not undergo migration of the naphthalene ring during the usual nitration procedures. Therefore, mono- and polynitration of naphthalene is similar to low temperature sulfonation, The nitronaphthalenes and some of their physical properties arc listed in fable 2. Many of these compounds are not accessible by direct nitration of naphthalene but are made by indirect methods, e.g nitrite displacement of diazonium halide groups in the presence of a copper catalysts, decarboxylation of nitronaphtbalcnccar-boxylic acids, or deamination of nitronaphthalene amines. They are nsed in the manufacture of chemicals, dye intermediates, and colorants for plastics. 

The pathway followed by the reaction is depicted in Figure B4.1. Methoxide anion adds to the a-haloalkenylborane generated by hydroboration of the haloalkyne, and induces migration of an alkyl group from the boron atom to the alkenyl carbon atom. The migration displaces halide anion from the alkenyl carbon atom and the centre is inverted. Finally protonolysis of the carbon-boron bond by acetic acid releases the (/f)-alkcne. 

In addition to the sulfonyloxy displacement reaction which may occur on halogenation with triphenyl phosphite complexes, the possibility of acetal migration (with appropriate structure) should not be overlooked. Thus, 1,2 5,6-di-O-isopropylidene-a-D-glucofuranose with triphenyl phosphite dibromide in benzene for 48 hours afforded " (presumably) 6-bromo-6-deoxy-l,2 3,5-di-O-isopropylidene-a-D-glucofiiranose, which was hydrolyzed to 6-bromo-6-deoxy-D-glucose in 10% overall yield. In some compounds, steric factors may prevent the introduction of halogen. With vicinal glycols and triphenyl phosphite methiodide, phosphonic ester formation occurs instead of halide production, but this reaction may be eliminated by use of triphenylphosphine in carbon tetrachloride. 

Among the many interconversions which occur readily in related cyclobutane, cyclopropane and open-chain frameworks, the stereospecific rearrangement of a-halo- or a-tosyloxycy-clobutanones into cyclopropanecarboxylic acid derivatives, has been shown to follow the semi-benzilic rearrangement mechanism, i. e. addition of nucleophiles to the carbonyl carbon atom and concerted displacement of halide (or tosyloxy) ions with 1,2-migration of the Ca — C carbonyl bond (see Section 4.1.2.2.5.). °... 

The degree of stereospecifidty decreases in the third cleavage step this decrease is the source of some racemization in the reactions of secondary alcohols. If ionization occurs more rapidly than displacement by halide, carbonium ions are formed, resulting in hydride or alkyl group migration. 

The Michael-induced RBR exploits the electrophilic nature of vinyl-substituted sulfones. Upon nucleophilic attack, the alkene double bond migrates to form an episulfone intermediate by displacement of an a-halide fScheme R.12T The product alkene, resulting from subsequent SO2 extrusion, is transposed by one position from the original alkene. The a-halide may... 

For substrates that lack an enolizable a-proton or for which a cyclopropanone intermediate would be highly strained, Favorskii products can be formed instead through a semibenzylic (also known as quasi-Favorskii) mechanistic pathway. This pathway involves addition of the base to the carbonyl followed by migration of an alkyl or aryl group with concomitant displacement of the halide. 

Intercepting the vinyllithium with a trialkylboron Lewis acid (eq 10) forms an ate complex that can then undergo iodine-promoted alkyl migration and elimination to give excellent yields of hindered trisubstituted alkenes. 4 Xhis transformation constitutes a formal Sn2 displacement of a secondary halide and appears generally applicable to cyclic and acyclic ketones. 

When enolization is not possible, a similar process may still occur, but the mechanism is different, and it is sometimes described as a semibenzylic acid rearrangement Figure 18.25 illustrates the process. The carbonyl is attacked by the base, as for the benzylic acid rearrangement, but this time, the migration displaces halide, as in the Favorskii. Some further examples are given in Figure 18.26 the last one is of the semibenzylic acid type. 

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