Alkyl halides Wagner-Meerwein rearrangment

Selective reduction of t-alkyl halides. The ate complex of B-butyl-9-BBN with n-BuLi selectively reduces tertiary alkyl halides to hydrocarbons in high yield without effect on primary or secondary halides. It does not reduce aryl or vinyl halfdes, but does reduce benzyl and allyl halides. The reduction involves a carbonium ion, and thus can proceed with Wagner-Meerwein rearrangements in certain systems. 

Fig. 14.4. Wagner-Meerwein rearrangement in the isomerization of an alkyl halide.

A Wagner-Meerwein rearrangement can be part of the isomerization of an alkyl halide (Figure 14.4). For example, 1 -bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [l,2]-shift A — B involved in this reaction again is an H atom shift. In contrast to the thermoneutral isomerization between carbenium ions A and B of Figure 14.3, in the present case an energy gain is associated with the formation of a secondary carbenium ion from a primary carbenium ion. Note, however, that the different stabilities of the carbenium ions are not responsible for the complete isomerization of 1-bromopropane into 2-bromopropane. The position of this isomerization equilibrium is determined by thermodynamic control at the level of the alkyl halides. 2-Bromopropane is more stable than 1-bromopropane and therefore formed exclusively. 

According to the data reported in the literature up to 1972, the gas-phase pyrolysis of alkyl halides was to be described in terms of a discrete polar transition state which yields products directly without the formation of an ion-pair intermediate. The only apparent exception to this consideration is the Wagner-Meerwein rearrangement of neopentyl chloride8,9. 

Simpler cases of Wagner-Meerwein processes are also known. For example, alkyl migration during addition of HX to alkenes (Chapter 6, Scheme 6.19) has already been noted. Similarly, to the extent that the same carbocations are generated during nucleophilic substitution reactions, the same processes occur Indeed, almost identical rearrangments will be encountered again in Chapter 8 in the discussion of derivatives of alcohols as they are here with alkyl halides. 

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