The effect of Lewis acidity in carbonyl coupling reactions

摘要

Relative Lewis acidities of (+)-Pr(hfc)_3, (+)-Eu(hfc)_3 and (+)-Yb(hfc)_3 have been estimated by measuring the induced chemical shift of the aldehydic proton of benzaldehydein the presence of these lanthanide complexes. The order has been estimated as Pr(hfc)_3 > Yb(hfc)_3 Eu(hfc)_3. These lanthanide complexes and a few other non-lanthanide compounds such as BBr_3, BF_3OEt_2, TiCl_4 and SnCl_4 have been sued as Lewis acid catalysts in a coupling reaction between an aldehyde and (E)-1-methoxy-1-(trimethylsilyloxy)propene. Results show that the chemical yield of the reaction is in the same order as the Lewis acidity of the catalyst used. The observation is in consonant with the expectation that higher is the Lewis acidity more should be degree of activation of the carbonyl base by the Lewis acid.