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| - Rate Acceleration of the Baylis−Hillman Reaction in PolarSolvents (Water and Formamide). Dominant Role of HydrogenBonding, Not Hydrophobic Effects, Is Implicated
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| - A substantial acceleration of the Baylis−Hillman reaction between cyclohexenone and benzaldehydehas been observed when the reaction is conducted in water. Several different amine catalysts weretested, and as with reactions conducted in the absence of solvent, 3-hydroxyquinuclidine was foundto be the optimum catalyst in terms of rate. The reaction has been extended to other aldehydeelectrophiles including pivaldehyde. Attempts to extend this work to acrylates was only partiallysuccessful as rapid hydrolysis of methyl and ethyl acrylates occurred under the base-catalyzedand water-promoted conditions. However, tert-butyl acrylates were sufficiently stable to couplewith relatively reactive electrophiles. Further studies on the use of polar solvents revealed thatformamide also provided significant acceleration and the use of 5 equiv of formamide (optimumamount) gave faster rates than reactions conducted in water. Using formamide, further accelerationwas achieved in the presence of Yb(OTf)3 (5 mol %). The scope of the new conditions was testedwith a range of Michael acceptors and benzaldehyde and with a range of electrophiles and ethylacrylate. The origin of the rate acceleration is discussed.
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