Chiral binaphthyl subunit

Although the conformationally rigid, N-spiro structure created by two chiral binaphthyl subunits represents a characteristic feature of 1 and related catalyst 9, Maruoka and coworkers have generally used their (S,S)- and (R,R)-isomers. Surprisingly, however, when the diastereomeric (R,S)-lc was used for asymmetric benzyla-tion of 2, the reaction was found to proceed very slowly, such that even after 60 h the... 

We designed and prepared the structurally rigid, chiral spiro ammonium salts 32 as a new C2-symmetric chiral phase-transfer catalyst (Fig. 4.1), and successfully applied this to the highly efficient, catalytic enantioselective alkylation of 28 under mild conditions [24]. A key finding was the significant effect of an aromatic substituent at the 3,3 -position of one binaphthyl subunit of 32 (Ar) on the enantiofacial discrimination, and (S,S)-32e was revealed as the catalyst of choice for preparing a variety of essentially enantiopure a-amino acids by this transformation (Table 4.3). 

The spiro-type phase-transfer catalyst (188, Ar = H) possessing a C2-symmetry axis provides a single type of asymmetric environment in contrast, a newly designed spiro-type phase-transfer catalyst (188, Ar H) has two different asymmetric environments. The substituents of the binaphthyl subunits affect enantioselectiv-ity, and the 3,5-bis[3,5-bis(trifluoromethyl)phenyl]phenyl group is the best substituent of those evaluated in the anti-selective aldol reactions of glycine SchifF base 186 with aldehydes (35) (Scheme 28.21) [94]. Similarly, simpMed chiral phase-transfer catalyst 189 bearing the 3,5-bis[3,5bis(trifluoromethyl)phenyl] phenyl substituent, which is prepared in a combinatorial approach from the readily available (S)-l,l -binaphthyl-2,2 -dicarboxylic acid, effectively catalyzes syn-selective aldol reactions [95]. 

A large number of chiral crowns have been prepared by numerous groups. The reader is directed to the tables at the end of this chapter to obtain an overview of these structures. It would not be useful to try to recount the synthetic approaches used in the preparation of all of these compounds we have chosen rather to subdivide this mass of compounds into three principal groups. The groups are (1) Cram s chiral binaphthyl systems (2) chiral crowns based on the tartaric acid unit and (3) crowns incorporating sugar subunits. These are discussed in turn, below. 

On the other hand, Maruoka and coworkers were intrigued with the preparation of symmetrical N-spiro-type catalysts to avoid the independent synthesis of two different binaphthyl-modified subunits required for 1. Along this line, 4,4, 6,6 -tetra-arylbinaphthyl-substituted ammonium bromide (S, S)-13 was assembled through the reaction of aqueous ammonia with bis-bromide (S)-14 on the basis of previous studies on the substituent effect of this type of salt. The evaluation of (S,S)-13 as a chiral phase-transfer catalyst in the alkylation of 2 uncovered its high catalytic and chiral efficiency (Scheme 5.9) [9]. 

Optical activity can also be induced on achiral azobenzene units when they are incorporated into the polymer main chains through covalent bonding to chiral subunits (34-37 in Fig. 14) [88-92]. Axially chiral amino- or carboxy-substituted biphenyl and binaphthyl and tartaric acid derivatives were used as the optically... 

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