Correlation between pKa and Reactivity of Quinuclidine-Based Catalysts in the Baylis−Hillman Reaction: Discovery of Quinuclidine as Optimum Catalyst Leading to Substantial Enhancement of Scope

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Abstract
The reactivity of a variety of quinuclidine-based catalysts in the Baylis−Hillman reaction has been examined, and a straightforward correlation between the basicity of the base and reactivity has been established, without exception. The following order of reactivity was established with pKa's of the conjugate acids (measured in water) given in parentheses: quinuclidine (11.3), 3-hydroxyquinuclidine (9.9), DABCO (8.7), 3-acetoxyquinuclidine (9.3), 3-chloroquinuclidine (8.9), and quinuclidinone (7.2). The higher than expected reactivity of DABCO, based on its pKa, was analyzed by comparing the relative basicity of DABCO and 3-acetoxyquinuclidine in DMSO. It was found that in aprotic solvent, DABCO was 0.6 pKa units more basic than 3-acetoxyquinuclidine, thus establishing a direct link between pKa of the amine and its reactivity. In contrast to previous literature work that reported the contrary, quinuclidine, which has the highest pKa, was found to be the most active catalyst. The reaction profile with quinuclidine showed significant autocatalysis, which suggested that the presence of proton donors might further enhance rates. Thus, a series of additives bearing polar X−H bonds were investigated and it was found that methanol, triethanolamine, formamide, and water all provided additional acceleration. Methanol was found to be optimum, and the powerful combination of quinuclidine with methanol was tested with a host of aldehydes and Michael acceptors. Not only were the reactions more efficient and faster than previously reported, but now new substrates that were previously unreactive could be employed. Notable examples include the use of acetylenic aldehydes and the employment of vinyl sulfones, acrylamides, δ-lactones, and even α,β-unsaturated esters bearing a β-substituent.