New Umpolung Catalysts: Reactivity and Selectivity of Terpenol‐Based Lithium Phosphonates in Enantioselective Benzoin‐Type Couplings

Abstract

AbstractTwo new classes of terpenol‐based lithium phosphonates, i.e. phenylfenchyl phosphonates and 2,2′‐biphenyldiylbis(terpenyl) phosphonates, are employed as umpolung catalysts for the enantioselective cross benzoin coupling. The structural characteristics of the phosphonates were investigated by means of X‐ray, 31P‐NMR analyses as well as DFT computations. The chiral lithium phosphonates were found to catalyze the cross benzoin coupling with enantioselectivities up to 54 % ee. For the phenylfenchyl phosphonates, the catalyst reactivities highly depend on the substituents in benzylic position. Modification of benzylic >CH2 to >C(CF3)2 significantly increases the yield from 19 to 92 %. For phenylfenchyl phosphonate pre‐catalysts, substitution of the benzylic >CH2 group by >C(CF3)2 changes the sense of enantioselectivity from S‐ to the R‐configured benzoin product.