Allylic anions oxygen-substituted

From this point on, the regioselectivity of substituted allyl anions is much less regular, and somewhat less explicable. For a start, X-substituted allyl anions react with carbonyl electrophiles with a selectivity. This is explicable, but it is determined by the site of coordination by the metal, not by the frontier orbitals. We can contrast the reaction of the oxygen-substituted lithium anion 4.57 with an alkyl halide, which is y selective, as usual, and the reaction of the zinc anion 4.58 with a ketone, which is a selective.304 The oxygen substituent coordinates to the zinc cr-bound at the y position, and the aldehyde is then delivered to the a position in a six-membered cyclic transition structure 4.59. The same reaction with the lithium reagent 4.57 gives a 50 50 mixture of a and y products, and so lithium is not so obviously coordinated in the way that the zinc is. This type of reaction is often brought under control in the sense 4.59 for synthetic purposes by... 

Addition of triethylaluminum or triethylborane to (61f) produces the ate complexes, which react with aldehydes at the a-position. In contrast to the ate complex of the oxygen-substituted anion, the ate complex of (61f) produces low diastereoselectivity. Reaction of the boron ate complex with y,y-dimethylal-lyl chloride and bromide occurs at the a-position with inversion of the allyl unit. 

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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