Pinacol Coupling Of Aromatic Aldehydes And Ketones Induced By Low Valent Vanadium Or Titanium

The pinacol coupling of carbonyl compounds affording pinacols is an important method for the formation of 1, 2-diols and one of the most powerful methods for constructing carbon-carbon bond. The coupling products have two newly formed stereo centers. And the products of the pinacol coupling reaction are a mixture of pinacol, alcohol and alkene. So, recent efforts have focused on the development of new methods and reaction systems to improve chemoselectivity and stereoselectivity of pinacol coupling.Based on the reported pinacol coupling of aldehyde and ketone mediated by low valent vanadium or titanium, some new reaction systems or methods were explored to improve the chemoselectivity and stereoselectivity of pinacol coupling.Aqueous vanadium (II) solution prepared by the reduction of vanadium pentoxide by aluminium and hydrochloric acid can induce pinacol coupling of aromatic aldehydes under ultrasound irradiation at room temperature. When. 4-CH₃C₆H₄CHO was substrate, only dl-pinacol was obtained. Using ethyl acetate as solvent, magnesium or aluminium and titanium tetrachloride can induce pinacol coupling of aromatic aldehydes or ketones. TiCl₄-Al-MeCOOEt system had better diastereoselectivity, when N,N,N',N'-tetramethyl -ethylene diamine (TMEDA) was added, the dl-selectivity was improved.Using ethyl acetoacetate as an ligand, the pinacol coupling of some aromatic aldehydes mediated by TiCl₄-Al in CH₂Cl₂ had good chemoselectivity and diastereoselectivity.