Subunit binaphthyl

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. 

Crown compounds containing a single 2,2 -binaphthyl subunit.116... 

Table 3.20. Crown compounds containing two 2,2 -binaphthyl subunits...

Table 7.4. Podands containing the biphenyl or binaphthyl subunit...

The common feature of compounds [5]-[15] is that the electrophoric units are linked by saturated spacers, thus establishing only weak electronic (through-bond or through-space) interaction of the Tt-systems. In contrast, the binaphthyl [16], the biperylenyl [17] and the bianthryl [18] as well as the structurally related homologues [19], [20] and [21] allow for a direct 7r,7r-interaction of the subunits it will be shown, however, that for both steric and electronic reasons the inter-ring conjugation can be weak and thus lead to electronically independent redox groups in a similar fashion as in [5]-[15]. 

The lowest excited triplet state in anthryl-oligothiophenes (634) were studied (96CPL(260)125). Also, binaphthyl oligothiophenes have been studied (09MM1710). The triplet state properties are studied with respect to coupling effects between the molecular subunits for a different chain length n and also for additional pyridine substitution (635). 

With the critical role of 3,3 -diaryl substituents of 1 in mind, Maruoka and coworkers examined the effect of 4,4 - and 6,6 -substituents of one binaphthyl subunit. As shown in (Scheme 5.4), the introduction of simple aromatic groups even at the 4,4 -position leads to a meaningful effect on the stereoselectivity of the phase-transfer-catalyzed alkylation of 2 [9]. 

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... 

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]. 

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). 

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... 

The incorporation of hydrophobic subunits such as aryl, alkyl, napthyl, binaphthyl, etc., into a macrocyclic structural framework leads to synthetic host molecules containing large endolipophilic cavities which have the potential of accommodating neutral organic guest species. 

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