History and Asymmetric Versions

A wide variety of silyl ethers can be employed, leading to functionalized homo-allylic alcohols or ethers. This three-component coupling reaction, which generates in a single operation a range of homoallylic ethers, does not require the initial and independent synthesis of the acetal (or ketal) derived from 6. 

The use of alcohol 46 is attractive as it avoids a preliminary silylation step. The second equivalent of allylsilane 1 is consumed whilst generating the catalyst (iodo-trimethylsilane) and is liberated in the form of propene 48. A year later, Seebach and Imwinkelried [21] employed dialkoxydichlorotitanium complex 49 instead of 

Mukaiyama rationalized this low selectivity by invoking the facile formation of benzylic oxonium cations which prefer to react via SN 1-type transition states with reduced chiral induction. His catalyst was prepared in situ by the addition of silver triflate to chlorodiphenylborane in a 1 1 ratio. 

The yields are good and the selectivity is excellent. The chiral auxiliary is prepared from the corresponding ketone by Noyori s hydrogenation (both enantiomers are accessible) and can be removed, in high yield after SMS condensation, by a two-step sequence involving a desilylation-debenzylation. 

Only one stereoisomer is generated in each case and further functionalization of this useful product can be performed easily. 

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