Ethylmagnesiation

Hoveyda and coworkers have further demonstrated that allylic alcohols and ethers are good substrates exhibiting notable diastereocontrol (Scheme 79) . The ethylmagnesiation of allylic alcohol 129 affords the syn diol with 95 5 diastereoselectivity (70% yield). On the other hand, the reaction of the corresponding methyl ether 130 affords the monoprotected alcohol with opposite sense of diastereoselectivity (syrr.anti = 11 89). The corresponding f-butyldimethylsUyl ether 131 is recovered unchanged, and oxygen-free substrate 132 provides an equal mixffire of diastereomers. The chelation between the... 

Hoveyda and coworkers have also developed a number of useful asymmetric transformations by judicious modification of Zr-catalyzed ethylmagnesiation of alkenes into other reactions . 

Whereas ethylmagnesiation of allyl alcohols affords the syn diol with 95 5 diastereoselectivity, the same reaction on the allyl methyl ether affords predominantly the anti diastereomer with 90 10 selectivity [92] (Scheme 7-80). The reversal of stereoselectivity was attributed to the association of the magnesium alkoxide (and not of ethers) with the zirconium reagent (Scheme 7-80). 

In the case of an acyclic anti homoallyl ether or alcohol, the zirconium-catalyzed ethylmagnesiation reaction leads to the same sense of stereoinduction [93], whereas... 

Without internal coordination but with a stereogenic center a to the olefin, no stereoselectivity can be achieved ethylmagnesiation of 84 provides a 1 1 diastereomeric mixture of products as well as in the case of non-a-substituted homoallylic alcohols 85 [93], as described in Scheme 7-82. 

Scheme 7-103 Chiral zirconium catalyst for enantioselective ethylmagnesiation.

Scheme 7-104 Chiral zirconium catalyst for diastereoselective ethylmagnesiation of chiral allylic alcohols.

Recently, a novel C[-symmetric zirconocene dichloride CpCp ZrCla (Cp = C5H5, Cp = 1-neomenthyl-4,5,6,7-tetrahydroindenyl) was prepared, in which the alkene can only approach from one side. With this chiral ligand, the ethylmagnesiation of unsubstituted allylic amines or alcohols leads to good enantioselectivities [125]. 

Ethylmagnesiation of alkenes In the presence of CpjZrCh (5 mol. %), eihylmag-nesium chloride adds to alkenes to give an intermediate (a) that can react with various electrophiles (equation I). 

Ethylmagnesiation of homoallylic alcohols (ethers). Ethylmagnesium chloride can add to the double bond of homoallylic alcohols on catalysts with Cp2ZrCl2 with high stereoselectivity. In the case of the an/i-homoallylic alcohol 1 (R=H) the reaction proceeds with high an/i-selectivity in the case of both the alcohol and the methyl ether. In contrast to the reaction of the syn-homoallylic alcohol 3, the reaction proceeds with... 

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