Functionalised bis homocubyl systems as novel ligands and catalysts in asymmetric reactions
Asymmetric reaction is a very important class of reactions in organic chemistry where a new bond is created in an enantiocontrolled manner. The elegant and economically most attractive way to introduce chirality in a chemical reaction is by using a catalytic amount of chiral controller. The search for asymmetric catalysts that provide high yields and enantioselectivity is an ongoing quest for organic chemists. Current challenges focus on the development of enantioselective catalysts with high activity and broad substrate generality directed towards efficient and environment friendly methods for the synthesis of enantiopure compounds. There have been significant advances in the field of homogeneous asymmetric catalysis, which culminated in the award in 2001 of the Nobel Prize for Chemistry to Noyori, Sharpless and Knowles for their contributions to homogeneous asymmetric hydrogenation and oxidation.
The present inventors have found that chiral catalysts derived from bis-homocubane (BHC) would be superior to the existing class of catalysts for the following reasons:
(a) The proposed BHC-ligands are bidendate ligands with C1 symmetry; (b) Some of them are expected to chelate well with metal complexes; (c) The cage backbone will provide enough rigidity to bring selectivity in reactions; (d) The BHC moiety enhances the lipophilicity of the ligand, which could lead to more effective recycling of the ligand; (e) The source of chirality will be chiral cage backbone with 8 fixed chiral centers which can be obtained by photocyclization of dicyclopentadiene dicarboxylate or dicyclopentadienone followed by simple transformations; (f) since all the chiral centers are fixed/rigid, catalyst remains extremely stable, i.e. racemization doesn’t take place; (g) Since both the enantiomers of the catalyst are obtained after resolution, there is scope for synthesizing both the enantiomers of the product separately and stereoselectively.
The enantiomers of a polycyclic cage diol have been resolved via conversion to their diastereomers, separation of the diastereomers and hydrolysis. The enantiopure cage diol was employed as catalyst in the addition of phosphonates to activated imines (Mannich reaction) to afford the products in high yield and enantioselectivity. Several bis-homocubyl systems possessing functional groups such as hydroxyl, amino, azido, phosphonate/phosphate, imidazolyl, triazolyl, oxazolyl, nitrite, nitrile, tetrazole, nitro, carboxylate and keto have been synthesized for their application as ligands and organocatalysts in various asymmetric reactions