Crotyl Grignard reagents

For a review of allyl and crotyl Grignard reagents, see Benkeser, R.A. Synthesis, 1971, 347. 

The reaction of crotyl chloride with TMSCI under Barbier conditions yields the same ratio of products as the reaction of the preformed crotyl Grignard reagent with TMSCI in ether [Eq. (64) 130]. 

From the point of view of this reaction mechanism, the crotyl Grignard reagent must be considered the more stable covalent form since the methylvinylcarbinyl derivatives always predominate from such reactions. 

It was found that the main sideproduct in these reactions is the tertiary alcohol which contains two crotyl entities. This most probably originates from an addition reaction of the intermediate crotyl Grignard reagent to a ketone, formed in a preceding radical type reaction in which a hydrogen is abstracted from the aldehyde. 

On the basis of extensive NMR studies [60] it was concluded that the crotyl Grignard reagent had the primary structure but the possibility of a very rapid equilibration with a small proportion of the secondary reagent could not be excluded. 

With allylic Grignard reagents, the carbophilic addition on the thiocarbonyl of phenylisothiocyanate occurred as expected exclusively with inversion of the allylic chain. From crotyl and prenyl magnesium halides the compounds (4f) and (4g) were obtained in yields of 77 and 72%, respectively. 

The only olefinic acid isolated from the carbonation of the Grignard reagent prepared from the isomeric mixture of crotyl and methylvinylcar-binyl bromides is 2-methyl-3-butenoic acid (70%). Separation of the halides is unnecessary because of this fortunate allylic isomerization. 

In that same period, interest had grown in the structure of allylic Grignard reagents in solution and in the mechanism of their reactions. To account for the results of the reaction of ketones with the Grignard reagent prepared from crotyl bromide (1-bromo-... 

The second step in this scheme involves a four-center concerted mechanism to allow the crotyl structure to be retained in the bromomagnesio alkoxide. After a reaction time of 5 min the oc-methylallyl derivative was formed in almost 79% yield, together with 10% of the cis- and rmns-crotyl derivatives. After 193 hr only 0.8% of the a-methylallyl compound had remained among the reaction products, whereas the combined cis- and trans-products were formed in a yield of 83% 10% unreacted rm-butyl isopropyl ketone was found in the reaction mixture in the presence of presumed excess Grignard reagent. Some years later, such a reversibility was also demonstrated for the reaction of allylmagnesium bromide with fcrf-butyl isopropyl ketone and di-fcrt-butylketone [62, 63]. 

---