Study On Asymmetric Allylation Of Aldehydes

The asymmetric allylation of aldehydes is one of the most important methods for preparation of chiral homoallylic alcohol, which is the building block in organic synthesis of fine chemistry and drugs. And chiral auxiliary, especially chiral allyboronate controlling asymmetric reaction is one of the common methods.Chiral homoallylic alcohol was prepared in this paper from asymmetric allylation of aldehyde with enantiomerically pure allylboronate tartramides such as chiral N,N'-diphenyl tartramide and N,N'-dibenzyl tartramide auxiliaries.Firstly, optical pure diethyl tartrates were obtained through esterification of D-(-)-or L-(+)-tartaric acid with ethanol under reflux with toluene as solvent, which were amidated with benzyl amine to give chiral D/L- N,N'-dibenzyl tartramide monomers. At the same time, D/L- N,N'-diphenyl tartramide monomers were prepared from direct amidation of tartaric acid with phenyl amine.Secondly, triallylborane was gained from allyl bromide-magnesium Grignard, prepared freshly from allyl bromide with metal magnesium powder of 60-100mu under nitrogen at 0-5℃, with boron trifluoride diethyl etherate under smooth reflux for about 3hrs. And the allyl boron difluoride was synthesized in the same way.Thirdly, chiral N,N'-diaryl 2-allyl-l,3,2-dioxaborolane-4,5-dicarbamides were prepared in two ways. One route was from D/L- N,N'-diaryl tartramides with triallylborane at room temperature; the other was from allyl boron difluoride with sodium D/L- N,N'-diaryl tartramide oxides which were synthesized from D/L-N,N'-diaryl tartramide with sodium hydride in diethyl ether under nitrogen. It wasfound that the first way is more effective and economic.We emphasized on the asymmetric allylation of benzaldehyde with N,N'-diaryl 2-aliyi-1,3,2-dioxaborolane-4,5-dicarbamides. High enantiomeric excess of 87.0% and yield of 93.6% could be obtained at -78℃ with toluene as solvent for 3 hours and further reacted for 15~20hrs at room temperature.Finally, the effect of subsititued groups was discussed. It was found that benzylamine and cyclohexanylamine favors asymmetric allylation and aliphatic aldehydes is more reactive than aromatic aldehydes.