Site selection base ring design

Arai et al. also reported another BINOL-derived two-center phase-transfer catalyst 31 for an asymmetric Michael reaction (Scheme 6.11) [8b]. Based on the fact that BINOL and its derivatives are versatile chiral catalysts, and that bis-ammonium salts are expected to accelerate the reaction due to the two reaction sites - thus preventing an undesired reaction pathway - catalyst 31 was designed and synthesized from the di-MOM ether of (S)-BINOL in six steps. After optimization of the reaction conditions, the use of 1 mol% of catalyst 31a promoted the asymmetric Michael reaction of glycine Schiff base 8 to various Michael acceptors, with up to 75% ee. When catalyst 31b or 31c was used as a catalyst, a lower chemical yield and selectivity were obtained, indicating the importance of the interaction between tt-electrons of the aromatic rings in the catalyst and substrate. In addition, the amine moiety in catalyst 31 had an important role in enantioselectivity (34d and 34e lower yield and selectivity), while catalyst 31a gave the best results. 

Therefore an efficient substrate recognition site could be constructed around the activation site of the phosphoric acid catalyst, namely the acidic proton, as a result of the acid/base dual function and stereoelectronic influence of the substituents (STG). The BINOL derivatives were selected as chiral sources to construct the ring structure The C2 symmetry is crucial in the catalytic design because it means that the same catalyst molecule is generated when the acidic proton migrates to the phosphoryl oxygen. In addition, both enantiomers of the binaphthols are commercially available [52]. 

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