Abstract:The mechanism of the reaction between amines with dimethyl carbonate (DMC) has been investigated. Whereas in the absence of bases, they give methylation and carboxymethylation reactions without selectivity (B Al 2 and B Ac 2 mechanisms, respectively), in the presence of bases, the B Ac 2 mechanism prevails. The carbamate already formed reacts further with DMC via the B Al 2 mechanism to give the corresponding N-methyl derivative. Such pronounced double selectivity has been explained in terms of Pearson's Hard-Soft Acid-Base (HSAB) theory.Accordingly, N-methylcarbamates have been prepared from primary aliphatic and aromatic amines, either at reflux temperature of DMC (90°C) or at 230°C in autoclave. The reaction can be carried out in one step or through the isolation of the carbamate and the subsequent methylation reaction with DMC. This method is the direct synthesis, in high yield and selectivity, of secondary N-methylamines from the corresponding primary amines.
Solid acid catalysts based on the direct incorporation of the vanadium Keggin heteropolyacid (PMo11V) structure during the synthesis of silica by the sol-gel technique, in acidic media using tetraethyl orthosilicate (PMo11VSiO 2 1, PMo11VSiO 2 2, PMo11VSiO 2 3, and PMo11VSiO 2 4), were prepared and characterized by 31 P-NMR, FT-IR, XRD, and textural properties (S BET ). The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. A series of highly substituted hexahydropyrimidines were synthesized using these new materials, encapsulated in a silica framework, as catalyst in solvent-free conditions. This methodology requires short reaction time (1.5 h), a temperature of 80 °C in solvent free-conditions to obtain good to excellent yields of trifluoromethyl-hexahydropyrimidine derivatives. The Keggin catalyst embedded in the silica matrix is insoluble in polar media, which allows easy removal of the reaction products without affecting their catalytic activity.
Micellar Keggin heteropolyacid catalysts were prepared using hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide-CTAB), 1-hexadecylpyridinium chloride, and Keggin heteropolyacids H 3 PMo 12 O 40 and H 4 PMo 11 VO 40 as precursors. Four catalysts were prepared (PMo12C16, PMo11VC16, PMo12C 16Py, and PMo11VC16Py) and characterized by 31 P NMR, FT-IR, XRD, SEM analysis and textural properties (S BET ). The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. A series of bioactive 1,4-dihydropyridine derivatives such as nifedipine and nemadipine B were synthesized using these new materials, in a one-pot procedure in ethanol. This methodology requires a reaction time of 8 h, and a temperature of 78°C to obtain good to excellent yields of 1,4-dihydropyridine derivatives. The micellar Keggin catalysts are insoluble in polar media, which allows easy removal of the reaction products without affecting their catalytic activity. The leaching test showed that they have an excellent stability and can be used five times as heterogeneous catalysts without appreciable loss of the catalytic activity. Using the same material, unsymmetrical 1,4-dihydropyridines such as nitrendipine can be obtained through a sequence of steps in very good yield (78 %).
Graphical Abstract
Abstract:The synthesis of 5,5'(oxy-bis(methylene))bis-2-furfural (OBMF) from 5-hydroxymethylfurfural (5-HMF) was studied using bulk and alumina-supported Preyssler heteropolyacids. The formation of OBMF was related to the amount of Brönsted acid sites, the lowest yield to OBMF being obtained with supported heteropolyacids. However, the Lewis acidity of HPA supported on Al2O3 favored the formation of 2,5-dimethylfurane. The effects of solvent, catalyst loading, temperature and reaction time on the selectivity to OBMF from 5-HMF were studied in order to optimize OBMF production using bulk Preyssler heteropolyacids; a yield of 84 % to OBMF was obtained at 5 h and 343 K. These results demonstrate that bulk Preyssler heteropolyacid is a good candidate for OBMF synthesis under mild reaction conditions.
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