Study Of Rare-earth Metals Amides Complexed With Phenoxy Modified Chiral Prolinols In Asymmetric Hydroboration

In this article,nine phenoxy functionalized chiral ligands L1-L9 were prepared bearing the structure of(S)-?(11)?-diphenylprolinol,while one chiral ligand L10 was synthesized using valinol.They were applied in the asymmetric hydroboration of aromatic ketones and methyl benzoylformate,respectively.Firstly,trisilylamino europium complex insitu catalyzed the asymmetric hydroboration of acetophenone ester using chiral L2,the corresponding ?-hydroxyester was observed in moderate enantioselectivity with poor chemoselectivity.However,it was found that the asymmetric hydroboration reduction of aromatic ketones could be smoothly transformed to chiral secondary alcohols under the catalytic reduction of trisilylamino ytterbium and chiral ligand L5 in-situ.The commonly encountered aromatic ketones were screened in the current asymmetric hydroboration reduction system well and the secondary alcohols with high yield and moderate to excellent ee values were achieved.1.Asymmetric hydroboration reduction of methyl benzoylforma te catalyzed by RE[N(TMS)2]3 with phenoxy-functionalized chiral pro linolsThe application of the easily available trisilylamino rare-earth metal complexes in the asymmetric hydroboration reaction of methylbenzoylformate was investigation using phenoxy-functionalized chiral prolinols.After routinely screening,it was found that the main reductive product 2a with 62% yield and 60% ee was obtained under the optimal conditions of 10 mol% Eu[N(TMS)2]3 and 20 mol% chiral ligand L2 at-10 o C in THF.2.Asymmetric hydroboration reduction of prochiral aromatic ketones catalyzed by chiral rare-earth metal complexes bearing phenoxy-functionalized chiral diphenylprolinolate ligandThe trisilylamino rare-earth metal complexes combined with phenoxy-functionalized chiral diphenylprolinolate ligands were used in the asymmetric hydroboration of acetophenone,and the products with moderate yield and ee value were obtained.During the investigation of the possible mechanism,it was found that trisilylamino ytterbium complex Yb[N(TMS)2]3 and two equivalents of phenoxy-functionalized prolinol L5(S)-2-(tert-butyl)-6-((2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)phenol)are the best combination system for the transformation.Subsequently,treatment of Yb[N(TMS)2]3 and L5 in 1:2 molar ratio yielded a new complex [L5Yb(L5H)],which is applied in the asymmetric hydroboration of acetophenones in the presence of additive pyridine.The outcomes are better than that of in-situ reduction.After the optimization of conditions,the ideal reaction conditions are as follows: 10 mol%[L5Yb(L5H)],2 equiv.HBpin,50 mol% pyridine in 1 m L DME,at-10°C for 8 h.The efficient asymmetric hydroboration reductions of various aromatic ketones were achieved in 90-99% yield and 68-95% ee.