Isomerization of allylic halides

Isomerization reactions of allylic halides ((2), X = halogen) are well known (ref. 99, p, 720), and occur in both aprotic and protic solvents. Isomerizations which accompany solvolysis reactions of allylic chlorides are discussed in Section 2.2.2. 

Allylic chlorides isomerize considerably less readily than the corresponding bromides and iodides, and monosubstituted allylic chlorides such as a- and y-methyl-allyl chloride are reasonably stable. An a-aryl substituent or two alkyl substituents increases the lability of allylic chlorides considerably, and the kinetics of isomerization of a limited number of reactive allylic chlorides have been studied. 

The kinetics of the essentially irreversible isomerization of a-phenylallyl chloride to /raAz -cinnamyl chloride ((5), Ar = CeHs) 

The mechanism of the acid catalysis of a-phenylallyl chloride isomerization has not been investigated in detail. In aprotic solvents such as chlorobenzene, Bronsted acids probably catalyze the reaction by hydrogen bonding with chloride, which should stabilize the S i transition state. 

It has long been recognized that Lewis acids catalyze the isomerization of allylic chlorides (ref. 99, p. 721). The isomerization of 4-chloro-1,2-butadiene to 2-chloro-l, 3-butadiene in hexane solutions is first order in allylic chloride and first order in cuprous chloride.  

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There are several relevant aspects of rhodium enolate chemistry. Rh(I) catalyzes the isomerization of allylic aUtoxides to enolates. We welcome this reaction done in a direct thermochemical context analogous to the related isomerizations of allyl halides , ethers and allyl amines . From the enthalpies of formation of allyl alcohol and propanal (—185.6 and —124.5 kJmol ), and their respective gas phase deprotonation enthalpies (1564 and 1530 kJ mol 252,253 jj. jjg concluded that the rearrangement of the allyloxide to propen-1-olate is exothermic by 95 kJmoR. ... 

The substituent X moves two atoms along the chain, and the double bond moves in the opposite direction to the position where X is still an allylic group. This migration can occur so readily that it is reasonable to regard the isomerization of allyl halides as true tautomerism. For example, equilibrium between 2-butenyl bromide and 1-methylallyl bromide is set up in a few hours at 75° 14... 

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