Ethers, alkynic carbometallation

The carbometallation of allylboranes is known . Alkenes are less reactive than alkynes or allenes in reaction with allylboranes. Only highly reactive alkenes, such as vinyl ethers and cyclopropenes , are used. 

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. 

Alkynes can be treated with organometallic reagents to get c/s-carbometalated alkenes in the presence of suitable catalysts. Subsequently, the metal can be substituted by a proton during aqueous workup. Alternatively, the metalated alkene can be treated with an electrophile to introduce a second substituent to the alkene. Catalytic amounts of tris(acetylacetonato)iron allow the reaction of 3-pentynyl ethers with butyllithium at -20 °C in toluene to give 4-methyl-octen-3-yl ethers in high yields (Scheme 4—268). Addition of an electrophile other than before aqueous workup leads in a stereoselective reaction to the products of a syn carbometalation with subsequent displacement of the metal by the electrophile under retention of the configuration. ... 

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