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Aza-Baylis–Hillman reaction

The aza-Baylis–Hillman reactionoraza-BH reactioninorganic chemistry is a variation of the Baylis–Hillman reaction and describes the reaction of an electron deficient alkene, usually an α,β-unsaturated carbonyl compound, with an imine in the presence of a nucleophile.^[1] The reaction product is an allylic amine. The reaction can be carried out in enantiomeric excess of up to 90% with the aid of bifunctional chiral BINOL and phosphinyl BINOL compounds,^[2] for example in the reaction of n-(4-chloro-benzylidene)-benzenesulfonamide with methyl vinyl ketone (MVK) in cyclopentyl methyl ether and toluene at -15°C.

In one study a reaction mechanism for a specific aza-BH reaction is proposed.^[3] Given a set of reaction conditions the reaction is found to be first-order in the triphenylphosphine nucleophile, MVK and the tosylimine concentration in the rate determining step in the presence of a Brønsted acid such as phenolorbenzoic acid. The presence of an acid facilitates the elimination reaction in the zwitterion by proton transfer which becomes much faster and no longer rate determining. A 6 membered cyclic transition state is proposed for this reaction step. Because this step is also reversible the presence of acid causes a racemisation process simply by mixing chiral aza-BH adduct, phosphine and acid.

Asymmetric aza-BH[edit]

Aza-BH reactions are known in asymmetric synthesis by making use of chiral ligands. In one study,^[4] for the first time, successful use was made of a chiral solvent based on an ionic liquid (IL).

This solvent is a condensation product of L-(−)-malic acid (available from the chiral pool), boric acid catalyzed by sodium hydroxide. When the sodium counter ion is replaced by a bulky ammonium salt the resulting ionic liquid has a melting point of −32°C.

This IL serves as the chiral solvent for the aza-BH reaction between N-(4-bromobenzylidene)-4-toluenesulfonamide and methyl vinyl ketone catalyzed by triphenylphosphine with chemical yield 34–39% and enantiomeric excess 71–84%.

References[edit]

External links[edit]

• https://www.organic-chemistry.org/Highlights/2006/30JanuaryA.shtm