5-Hexenylmagnesium bromide

When the double bond is further removed, as in 5-hexenylmagnesium bromide, there is no evidence of a similar equilibrium.106... 

The simple insertion was favored by Walling and Buckler (25) since they could find no evidence for a chain process. However, Lamb (27) has recently reported that during autoxidation 5-hexenylmagnesium bromide undergoes cyclization to give eventually cyclopentylmethanol—a cyclization which is typical of the 5-hexenyl free radical. If this is true, either Reactions 21 and 22 or 21, 22, and 23 are indicated. In view of the close parallel between the autoxidations of all these electron-deficient molecules, some of which are essentially covalent in structure, the displacement scheme (Reactions 21 to 23) may have wider application than has been realized. 

Likewise, no isomerization (cyclization) was observed when 5-hexenylmagnesium bromide, a well-studied radical probe, was reacted with benzophenone. The authors concluded that this indicates that either the reaction is polar or, if SET, no free radical character is exhibited. ... 

Alkenylatioa of Crignard reagents.1 Facile vinylation of a Grignard reagent can be achieved with vinyl bromide with ferric chloride as catalyst and THF as solvent. For example, the reaction of 5-hexenylmagnesium bromide with vinyl bromide in THF at 25° catalyzed by ferric chloride gives 1,7-octadiene in 64% yield ... 

However, this mechanism was excluded on the basis of the reaction of the 2-pyridylsulfone (198) with 5-hexenylmagnesium bromide which did not give any 2-(cyclopentylmethyl)pyridine (199), which would be expected for a radical process. The only isolated compound was the ligand coupling type product (200) in 99% yield. 131 In spite of these suggestions, the mechanism is more likely to be a simple addition on the C-2 atom of the pyridine ring, followed by elimination of phenylsulfinic acid. 

The rates of reaction for the different Grignard reagents were largely linearly correlated to the anodic oxidation potentials of the reagents. 5-Hexenylmagnesium bromide produced l,2- iddi-tion product of which 50% was cyclized. 

Newer developments in the study of the reactions of benzophenone with Grignard reagents have been the use of radical probes. If the alkyl of the reagent has the ability to undergo isomerization as, for example, cyclization or cia-inms isomerization, while it exists as a free radical, it is possible to get information about the nitc of recombination of alkyl and kclyl in the formation of the various reaction products.. S-Hexenylmagnesium bromide has been used in the study of the free radical type mechanism by its reactions with oxygen and the rate of cyclization of the, S-hexenyi radical was known to he 10 s. ... 

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