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19.4: Reactions of Enolates with \(α\),\(β\)-Unsaturated Carbonyls

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    Reactions of enolates with \(α\),\(β\)-unsaturated carbonyls

    An \(α\),\(β\)-unsaturated carbonyl has a double bond in conjugation with a carbonyl. This gives it special reactivity since it is electrophilic at two different positions – the carbonyl carbon and the \(β\)-position. Attack at the carbonyl carbon is called 1,2-addition, while attack at the \(β\)-position is called 1,4-addition. Why does 1,4-addition even occur? Because in the LUMO, the largest coefficient lies on the \(β\)-carbon. We can see this from the resonance forms below:

    Screen Shot 2023-01-04 at 10.36.49 AM.png

    Under thermodynamic control, 1,4-addition (also known as conjugate addition) is favored (maximal orbital overlap). Under kinetic conditions, 1,2-addition is favored (electrostatic interactions dominate, and the carbonyl carbon has more positive charge).

    This all matters because enolates are softer nucleophiles and will attack in a 1,4-manner. Other reagents (RMgBr, LAH, RLi) are hard nucleophiles and will attack in a 1,2-fashion. Other soft nucleophiles include CN, RS, N3, and RNH2, and will undergo conjugate addition. In addition, cuprates will add in a 1,4-manner, yielding conjugate addition products.

    Screen Shot 2023-01-04 at 10.36.57 AM.png

    Michael addition – addition of an enolate to an \(α\),\(β\)-unsaturated carbonyl with 1,4-regioselectivity. Retrosynthetically, this is an important transformation.

    Screen Shot 2023-01-04 at 10.37.05 AM.png

    Robinson annulation – a three-step sequence involving Michael addition, followed by an aldol reaction, following by E1cb elimination. Retrosynthetically, this fused ring synthesis is important to recognize.

    Screen Shot 2023-01-04 at 10.37.14 AM.png


    19.4: Reactions of Enolates with \(α\),\(β\)-Unsaturated Carbonyls is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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