The development of double axially chiral phosphoric acids and their catalytic transfer hydrogenation of quinolines.

The design of novel chiral catalysts for asymmetric synthesis is an active field of modern organic chemistry. Chiral phosphoric acid catalysts, pioneered by Akiyama et al. and Terada and co-workers, have been recently applied to a wide range of asymmetric organic transformations. Brønsted acids are used in these asymmetric reactions, mainly to protonate the electrophile, which is thereby activated and ready for attack by a corresponding nucleophile. To our knowledge, the previously reported chiral phosphoric acid catalysts are based on a limited number of backbone scaffolds, namely 3,3’substituted binol (1,1’-bi-2-naphthyl), 3,3’-substituted H8-binol, [5] taddol (a,a,a’,a’-tetraaryl-2,2dimethyl-1,3-dioxolan-4,5-dimethanol), vanol (3,3’-diphenyl-[2,2’binaphthalene]-1,1’-diol), and vapol (2,2’-diphenyl-[3,3’-biphenanthrene]4,4’-diol). The development of new chiral phosphoric acids with tunable backbones and the expansion of their application to other useful asymmetric organic transformations is still a great challenge for chemists. Herein, the design, synthesis, and application of novel double axially chiral phosphoric acid catalysts are reported. The rationale for our design of new chiral phosphoric acid catalysts originated from the observation that substitutents at the 3,3’-positions of binol are very important for achieving high selectivity. The use of 3,3’-nonsubstituted binol phosphate as a catalyst always gave low or even no enantioselectivity. c] We assumed that if the substitutents at the 3,3’positions of binol phosphate I possess a stable double axial chirality, then better performance in organocatalysis may be achieved compared to the mono axially chiral phosphoric acid catalysts based on the same scaffold. For the synthesis of the new catalyst, we removed the middle axial chirality of I to give the simplified compound II. Then for convenience, we further rationally modified II to obtain compound (R,R)-1 (Scheme 1) as the target catalysts. (R,R)-1 has double axial chirality and can be recognized as a “dimeric” form of (R)-