Three-component carbon–carbon bond-forming reactions catalyzed by a Brønsted acid–surfactant-combined catalyst in water
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Introduction
Mannich and related reactions provide basic and useful methods for the synthesis of β-amino carbonyl compounds, which constitute various pharmaceuticals, natural products, and versatile synthetic intermediates.1 Conventional protocols for proton-catalyzed three-component Mannich-type reactions of aldehydes, amines, and ketones in organic solvents (Eq. (1)) include some severe side reactions and have some substrate limitations, especially for enolizable aliphatic aldehydes. Therefore, some modern variants of Mannich reactions have been developed so far.2 Among them, Lewis acid-mediated reactions of pre-formed imines and silyl enolates (Eq. (2)) as substrates provide one of the most efficient methods.3 Furthermore, three-component Mannich-type reactions of aldehydes, amines, and silyl enolates have also been reported and greatly extended their synthetic utility.4Recently, organic reactions in water without use of harmful organic solvents have attracted much attention, because water is a cheap, safe, and environmentally benign solvent.5 In the course of our investigations to develop new synthetic methods in water, we have recently found that a combination of a water-stable Lewis acid (e.g. scandium triflate) and an anionic surfactant (e.g. sodium dodecyl sulfate (SDS)) provides an efficient system for some Lewis acid-catalyzed reactions in water.6 Moreover, we have also synthesized more simplified catalysts, ‘Lewis acid–surfactant-combined catalysts (LASCs)’, such as scandium tris(dodecyl sulfate) (Sc(DS)3).7 These LASCs form stable colloidal dispersion systems with organic substrates and function as effective Lewis acids in water. These catalytic systems have been successfully applied to three-component Mannich-type reactions of aldehydes, amines, and silyl enolates in water.6., 7.
As an extension of these studies, we planned to develop a ‘Brønsted acid–surfactant-combined catalyst (BASC)’, composed of a Brønsted acidic group and a hydrophobic moiety. Although conventional procedures of Mannich-type reactions use a Brønsted acid catalyst, they need organic solvents for water-insoluble substrates. We expected that a BASC would efficiently catalyze Mannich-type reactions in water by activating the intermediate imines and by creating effective colloidal dispersions.8 Here we report BASC-catalyzed reactions of aldehydes, amines, and silyl enolates, parent ketones, Danishefsky's diene, or allyltributyltin in water.9 In addition, microscopic observation of colloidal particles created by the BASC and organic substrates is presented.
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Results and discussion
First, we searched an efficient catalyst for Mannich-type reactions of silyl enolates with imines generated in situ from aldehydes and amines in water. Various catalysts were tested for the reaction of benzaldehyde, o-anisidine, and 1-phenyl-1-(trimethylsiloxy)ethene in water, and selected examples are summarized in Table 1. Although the reaction in the presence of SDS alone afforded the desired β-amino ketone in a very low yield (entry 1), addition of a catalytic amount of HCl slightly
Conclusions
Three-component reactions of aldehydes, amines, and various nucleophiles are efficiently catalyzed by DBSA, a BASC, in water. Aromatic, heteroaromatic, and aliphatic aldehydes can be successfully used as the aldehyde component. Moreover, these reactions, which proceed sluggishly in organic solvents, attest to the unique property of water as a reaction medium. DBSA forms stable colloidal particles in the presence of the substrates in water, and this colloid formation plays an important role in
General procedure for Mannich-type reactions of aldehydes, amines, and silyl enolates
An amine (0.25 mmol), a silyl enolate (0.38–0.75 mmol), and an aldehyde (0.25 mmol) were successively added to a solution of DBSA (0.025 mmol) in water (1.5 mL). The resulting mixture was stirred at 23°C for 2 h, and then quenched with saturated aq. NaHCO3 (5 mL) and brine (5 mL). The mixture was extracted with ethyl acetate, washed with brine, dried over Na2SO4, concentrated, and purified by silica gel chromatography to give the desired product.
General procedure for Mannich-type reactions of aldehydes, amines, and ketones
To a solution of DBSA (0.0025–0.075 mmol, 1–30 mol%) in H2O
Acknowledgements
The authors are grateful to Professor Takeshi Iwatsubo (The University of Tokyo) for light microscopy observation. This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan. Y. M. thanks the JSPS fellowship for Japanese Junior Scientists.
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