Crotylmagnesium chloride

An interesting selectivity in the transfer of alkyl groups (n-Alk>Me) is observed in these addition reactions. The regioselectivity of the reaction of crotylmagnesium chloride (213) with benzaldehyde strongly depends on the presence of various rare-earth metal chlorides. The a- to y ratio of products can be switched to the opposite by using only another metal salt. Yttrium trichloride gives exclusively y-product, while neodymium trichloride leads to 89% of the a-attack (with 92% of ( )-isomer) (equation 142) °. 

Yamamoto, Y. Maruyama, K. Regioreversed addition of crotylmagnesium chloride to carbonyl compounds in the presence of A1C13. 

The crotylmagnesium chloride prepared from the same starting reagents shows infrared spectra in the 1600 cm" region very similar to the bromide. Only small frequency... 

However, in a recently published, detailed study on the stereochemistry of the addition reaction of allylic Grignard reagents (mainly crotylmagnesium chloride) with a,/ -ethylenic ketones [15], addition to aryl-substituted enones occurred without any selectivity at all. The authors suggested that this was due to a single-electron transfer, since poor diastereoselectivity is observed when a carbon-carbon bond is formed by a radical- radical combination [64]. No further mechanistic details on this difference in reactivity were given. 

Benhallam, R., Zair, T., Jarid, A., Ibrahim-Ouali, M. Theoretical study of the stereochemistry of crotylmagnesium chloride s addition on a series of cyclic and acyclic enones. THEOCHEM 200Z, 626, 1-17. 

Terpenoids. In the presence of this Ti(IV) compound as catalyst, isoprene undergoes a regioselective insertion into aUyl—magnesium bonds as exemphfied for the reaction of crotylmagnesium chloride (1) in equation I. Evidently, (1) reacts in the isomeric form (lb). The products (2) can undergo usual Grignard reactions. 

Rearrangement of the adducts of crotylmagnesium chloride to styrene [Eq. (14)] was also reported (46). The cyclization appears to follow an unusual course in producing the larger of two possible rings. In previously reported organometallic (29) or radical (43) cyclizations, the smaller-ring cycloalkylmethyl skeleton is the kinetically determined product (i.e., cyclobutylmethyl rather than cyclopentyl, or cyclopentyl-methyl rather than cyclohexyl). 

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