Peterson alkenation titanium

Metallated allylsilanes have also been examined in terms of the effects of various gegenions on a-versus y-additions to aldehydes. Using allylaminosilane (49), Tamao and Ito have shown that transmetal-lation of the lithiated intermediate to copper using CuCN (1 equiv.) leads to a >95% preference for y-ad-duct (51 Scheme Other metals such as magnesium, zinc and titanium show a strong preference for a-attack (>95%), which ultimately leads to dienes (50) following Peterson alkenation. 

The methylenation of ketones and aldehydes by the Wittig reaction is a well-established and selective methodology. Unlike addition-elimination methods of alkene formation, the Wittig proceeds in a defined sense, producing an alkene at the original site of the carbonyl. The Wittig reaction is not considered here, but is used as the standard by which the methods discussed are measured. The topics covered in the methylenation sections include the Peterson alkenation, the Johnson sulfoximine approach, the Tebbe reaction and the Oshima-Takai titanium-dihalomethane method. 

The titanium species derived from sequential treatment of a-alkoxy-substituted allylsilanes with s-Butyllithium then Ti(0-i-Pr)4 engages in a Peterson alkenation reaction with aldehydes to give, via electrophilic attack at the a-terminus of the allyl anion, 2-oxygenated 1,3-butadienes which can be hydrolyzed to the corresponding vinyl ketone (eq 19). ... 

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