Allylic carbanions 1,4-addition reaction with conjugated enones

The 1,4-addition reaction of stabilized allylic carbanions with conjugated enones is normally complicated by the ambident reactivity of the carbanion. Clean 7-selective conjugate addition to cyclic enones, however, has been observed with lithiated allylic sulfoxides, phosphine oxides and phosphonates... 

Selenium-stabilized carbanions can be also generated by 1,4-addition of nucleophilic reagents to a-selanyl a,[3-unsaturated carbonyl compounds. The conjugate addition of trialkylsilyllithium compounds to 133, followed by reaction with allyl iodide, afforded the addition products 134 with good m-stereoselectivity (R = Me dr 86 14 R = Ph dr 94 6) (Scheme 34).214 The addition of lithium dialkylcuprates to 2-phenylselanylcycloalk-2-enones has also been used for the synthesis of natural products.215,216... 

The butyllithium-generated anion of an allylic sulfone was reported to add to conjugate enones, but different regioselectivity was observed for 2-cyclohexenone (1,4-y) and 3-penten-2-one (1,4-a), as shown in equation (31). Clean 1,4-a additions to both acyclic and cyclic enones can be realized when the lithio carbanions are allowed to react in the presence of HMPA (2 mol equiv.) at -78 C. Under these conditions, allyl phenyl sulfone reacts with 2-cyclohexenone. 2-methyl-2-cyclopentenone and 3-methyl-2-cyclohexenone, giving the corresponding 1,4-a adducts as a 75 25 mixture of two dia-stereoisomers. In the reaction of allylic phenyl sulfone with an acyclic enone, 4-methyl-3-penten-2-one,... 

In order to prepare conjugated olefins, either an allylsulfone or an a,) -unsaturated carbonyl compound is needed. In the case of allylic sulfones, the delocalized carbanion can undergo either a- or y-addition, and the use of enals or enones can result in 1,2- or 1,4-addition products. In general, allylsulfonyl anions, derived from non-stabUized sulfones, undergo additions to carbonyls with excellent a-selectivities (reaction 3.17 [59] Scheme 3.16). The situation is more complex with a, -unsaturated compounds, but conditions have been defined that lead with high preference to adducts resulting from 1,2-addition (reaction 3.18) [60]. 

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