Allyl ethers carbometallation

ZIRCONIUM-CATALYZED ENANTIOSELECTIVE CYCLIC CARBOMETALLATION OF ALLYLIC ETHERS, ALCOHOLS, AMINES, AND SULFIDES... 

Scheme 3.90 Catalytic asymmetric carbometallation-elimination of allyl ethers with ethyl-magnesium and chiral zirconocene derivatives [308].

Zirconocene-catalyzed carbomagnesation of internal allylic ethers proceeds by a different pathway. The carbometallation of allylic ethers with the internal double bond gives formally the S 2 substitution product 15 with the loss of the ether moiety (Scheme 7). A few examples are given in Table 4. Especially interesting, from the synthetic point of view, is ethylmagnesation of cyclic ethers (the last entry in Table 4), because it opens a pathway to the synthesis of functionalized homoallylic alcohols [14]. An asymmetric variant of this reaction gives products with ees>90% [15]. 

These organocopper intermediates were successfully trapped with electrophiles such as allyl bromide, acyl chlorides, and diphenylchlorophosphine, but not with benzaldehyde and methyl iodide. The resulting products were subjected to in situ oxidation with elemental sulfur to form stable alkenylphosphine sulfides 353 [103]. Owing to its synthetic potential, the regioselective carbometallation of substituted ynol ethers has recentiy witnessed a renaissance [91aj. Thus, the alkynyl ethers 354 were treated under different carbocupration conditions (Scheme 10.121). 

Normant has reviewed the carbometallation of alkynes to give stereospecific syntheses of alkenyl-metal derivatives, and their addition to aldehydes and ketones to afford allylic alcohols. Specific titanium-catalysed 5yn-hydromag-nesiation of 2-propynylic alcohols (Scheme 16), and the related zirconium-catalysed 5yn-carboalumination of propargylic alcohol itself (also Scheme 16) or its t-butyldimethylsilyl ether (c/. 3, 139 4,160) can both lead to allylic alcohols. 

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