SYNTHESIS OF CHIRAL IMINES and AMINES ON SILICA SURFACES

We have devised simple, facile, and greener methods in the synthesis of several new chiral amines on silica surfaces. Several of these amines have the pyridyl, furyl, and other moieties in their molecular structures, thus affording additional ligands for coordination with other chiral molecules. We have also synthesized amines with ortho substituents which afford steric hindrance to the reactions. In all cases the reactions proceeded smoothly to give quantitative yields of the products. Specific rotations were obtained of pure samples of all the imines and amines. In the synthetic procedure, typically 2.0 g of activated silica were added to 10 mL of an ethereal solution of equimolar quantities (2.0x10-3 mol) of an aldehyde and a primary chiral amine. The flask was equipped with a stirring bar and a drying tube. The mixture was stirred briefly and allowed to stand for 15 minutes to 30 minutes at room temperature. Completion of the reaction to the imine stage was monitored by GC-MS. The reduction step was carried out in the same pot by adding 0.15 g of sodium borohydride, followed by the addition of a few drops of water, while cooling the flask in an ice bath. The flask was briefly stirred upon each addition of water. The progress of the reduction was monitored by GC-MS. Upon completion, the sample was filtered, and the silica was washed five times with 5 mL aliquots of ether. Traces of the starting aldehyde or amine if present, were removed by column chromatography prior to analysis. Products were analyzed by IR, GC, GC-MS, and proton and C-13 NMR, and by polarimetry