Aldol Addition of Dimethylsilyl Enolate in the Presence of Polyisocyanate with Alkali Metal Salt Catalyst

Abstract

The polymers containing the aprotic polar solvent structures such as DMF, DMAc, and DMSO have been employed as the additives for several nucleophilic substitution reactions.1-6 These polymers, called ''solid co-solvent'', have been used with alkali metal bases and alkylhalide nucleophiles in two phases reaction system of organic solvent and water. In the works, the main role of the solid solvents is regarded as an effective phase-transfer agent. In addition, it has been pointed that the solid solvent provides a good reaction environment to enhance the substitution reactions by the effects of the polar solvent structure and hydrophobic main chain. Such previous findings suggest that the polymeric solvents possess the possibility to be utilized as the solvent itself and enable a solventfree reaction. The use of the solid solvents is preferable in the view of environmental problem and, further, the solid solvents are reusable. Consequently, the reaction system utilizing the solid solvents is thought to be an interesting ecological methodology.On the basis of the consideration mentioned above, we reported recently the effects of the solid poly(N,Ndimethylacrylamide), as the analog of DMF, on the alkali metal salt catalyzed aldol addition of dimethylsilyl (DMS) enolate to aldehyde. 7 In the work, the solvent structure in the polyacrylamide gel was demonstrated to be effective for holding an alkali metal salt and provide the reaction environment to give the corresponding aldol products in good yields. The results were comparable with those obtained in DMF solution by the use of analogous catalysts, which was reported previously by Hosomi et al. 8 In the examinations using the several kinds of the polyacrylamide gels, the derivative of Wang resin (PDMA), which was assumed to contain more flexible polyacrylamide chain, was found to be more suitable than the traditional cross-linked gel (PDMAG) for the aldol reaction (Scheme 1). In line with the obtained information, several polymeric solid solvents derived from Wang resin have been newly prepared and applied to the efficient aldol reaction catalyzed by alkali metal salts. In this work, we employ polybutylisocyanate (PBI) as the solid solvent, which has been prepared from Wang resin bounded pyrrolidine derivative according to the procedure reported by Okamoto et al. (Scheme 2) 17 The polyisocyanate PBI is also consisted of amide groups as the main chain and, further, assumed to be a kind of asymmetric polymer. Therefore, the use of PBI is expected to show several characteristics such as the effective holding of alkali metal salts and the possibility of stereoselective reaction. EXPERIMENTAL General1 H and 13 C NMR spectra were recorded on either a Bruker DMX-500 or a JEOL AL-300 spectrometer, using tetramethylsilane (TMS) as an internal standard in chloroform-d (CDCl 3 ). IR spectra were recorded on a Jasco FT/IR-230 spectrometer. The number-average molecular weight (M n ) and polydispersity (M w =M n ) were estimated on a Tosoh HPLC-8220 system with refractive index (...