Several chiral Lewis acid catalysts were prepared from lanthanoid trifluoromethanesulfonate (triflate) and a chiral sulfonamide ligand, and were applied to the Mukaiyama aldol reaction. The reaction proceeded in a catalytic manner with moderate enantiomeric excess.
Lanthanoid reagents are attracting much attention recently because of their potential ability in organic synthesis, l ) Especially of interest is their use in catalytic asymmetric reactions. Although many asymmetric catalysts of various metal species have been developed, there are only a few reports on successful catalytic asymmetric C-C bond forming reactions catalyzed by a lanthanoid complex. We have developed so far catalytic asymmetric nitroaldol 2) and Michael reactions 3) utilizing the basic character of lanthanoid complexes. From the course of our investigations on lanthanoid chemistry, herein we describe our preliminary results on the development of a Lewis acid chiral lanthanoid complex applicable to the Mukaiyama aldol reaction.
It is known that lanthanoid reagents can be used as a Lewis acid catalyst and in some cases chiral complexes such as NMR chiral shift reagent Eu(hfc)3, Eu(fod)34,5) and samarium(H) menthoxide 6) have been applied. But with the exception of Kobayashi's recent report on asymmetric Diels-Alder reaction catalyzed by a chiral ytterbium complex, 7) high enantiomeric excess have not been obtained and there is no report on successful asymmetric Mukaiyama aldol reaction 8) catalyzed by a lanthanoid complex. Moreover the structure of the complexes is generally uncertain. We feel that the development of more efficient Lewis acid lanthanoid catalysts and further studies to fully understand lanthanoid chemistry are necessary.
A chiral lanthanoid catalyst was prepared by reacting equimolar amounts of lanthanum triflateg) and a chiral bidentate ligand bistfiflamide of (IS, 2S)-1, 2-diphenylethylenediamine 10, 11) (deprotonated with excess of Nail) in THF for 12 h (Scheme 1). This catalyst was first applied to the reaction between benzaldehyde and ketene silyl acetal derived from ethyl isobutyrate. In TI-IF, this complex did not exhibit any catalytic activity for the reaction. After investigating some other solvents for the reaction, dichloromethane proved to be most satisfactory. Hence the catalyst-THF solution was evaporated in vacuo, and the residue was extracted with dichloromethane. In this dichloromethane solution, the aldol reaction proceeded in a catalytic manner using 20 mol % catalyst. 12) Eu and Yb catalyst were also prepared by this method from Eu(OTf)3 and Yb(OTf)3, respectively. Results of the reactions between some aldehydes and ketene silyl acetal catalyzed by these complexes are shown in Table 1.
Tf