Organic Chemistry 2 · Claisen condensation and conjugate addition
Aza-Baylis–Hillman of an N-tosyl imine with MVK
Conditions: DABCO (cat.), THF
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The mechanism, step by step
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DABCO adds to methyl vinyl ketone
The DABCO nitrogen adds to the terminal CH2 of methyl vinyl ketone. The resulting zwitterion holds an ammonium ion next to a ketone enolate, drawn here with the charge on the α-carbon that reacts next.
The curved arrows
- The DABCO nitrogen lone pair bonds to the β-carbon of the enone.
- The C=C π pair moves onto the α-carbon, stabilized by the ketone.
Mannich-type addition to the imine
The enolate α-carbon attacks the imine carbon of the N-tosyl imine. The electron-withdrawing tosyl group makes the C=N carbon electrophilic and stabilizes the sulfonamide anion that forms.
The curved arrows
- The enolate carbon forms the C–C bond to the imine carbon.
- The C=N π pair moves onto nitrogen, giving a sulfonamide anion.
Protonate the sulfonamide nitrogen
Trace water (assumed proton shuttle) protonates the sulfonamide anion, giving the N–H sulfonamide and hydroxide. Moving the α-hydrogen straight to nitrogen inside the molecule is not drawn.
The curved arrows
- The nitrogen lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Remove the α-hydrogen
Hydroxide removes the hydrogen on the carbon between the ketone and the ammonium-bearing CH2, regenerating a ketone enolate. This sets up elimination of the amine.
The curved arrows
- The hydroxide lone pair takes the acidic α-hydrogen.
- The C–H pair stays on carbon, stabilized by the ketone.
E1cB release of DABCO
The carbanion re-forms the C=C bond and expels neutral DABCO. The catalyst is regenerated and the α-methylene ketone is formed.
The curved arrows
- The carbanion lone pair forms the C=C π bond.
- The C–N bond breaks and DABCO leaves with its lone pair.
N-(2-Methylene-3-oxo-1-phenylbutyl)-4-methylbenzenesulfonamide
The product is N-(2-methylene-3-oxo-1-phenylbutyl)-4-methylbenzenesulfonamide, racemic at the new benzylic stereocentre. The imine carbon has been joined to the α-carbon of methyl vinyl ketone with the C=C bond kept.
What you'll practice
Draw the DABCO-catalysed aza-Baylis–Hillman reaction of an N-tosyl benzaldimine with methyl vinyl ketone: zwitterionic enolate formation, Mannich-type addition to the imine, proton relay and E1cB release of DABCO.
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