Binaphthyl backbone, 3,3 positions

Recently, Fan et al. reported a series of chiral BINOL-cored dendrimers via substitution at the 3,3 -positions of the binaphthyl backbone by different generations of Frechet-type dendrons (Figure 4.18) [73]. The proximity of the dendritic wedges to the catalytic center is expected to induce catalytic properties different from BINOL. In the absence of Ti(Oi Pr)4, the chiral dendrimer ligands showed much... 

Several miscellaneous polymer-supported chiral phosphine ligands and oxazoline ligands are depicted in Figure 6.8 and Figure 6.9, respectively. Binaphthyl monophosphine MOP [36] was anchored onto PS-PEG resin at the 2 -, 6- and O -positions of its binaphthyl backbone (compounds 84, 85 and 86) [37]. The polymeric phos-... 

Selective monosubstitution in the starting dibromide by PPh2 was realized at lowtemperatures [71]. Conversion of the second bromo substituent into the phosphorus diamide and final reaction with the diol produced the hybrid ligand. In the hydroformylation of 2,5-dihydrofuran, up to 91% ee with no isomerization was achieved, which is the best value reported to date. Noteworthy, the partially hydrogenated binaphthyl unit and the phenyl substituents in the 3,3 -positions are essential for the success of the asymmetric catalysis. Related ligands with a binaphthyl backbone, with or without Me-substituents in 3,3 -positions, resulted in poor conversions and stereoselectivities. 

To overcome the difficulty associated with the design of chiral guanidines as a catalyst, the binaphthyl-derived 22 was initially developed with the expectation that substituents at the 3,3 -positions of the binaphthyl backbone would break the planar symmetry of the guanidine skeleton to create an attractive chiral environment. This type of axially chiral organic base catalyst, particularly 22a, could indeed... 

In 2000, Rajanbabu et al. published the synthesis and coordination chemistry of the first chiral NHC containing a l.T-binaphthyl unit as the chiral element (Scheme 24) [69]. It contains two imidazolium rings linked to the l,T-binaphlhyl backbone in the 2 and 2 position through methylene bridges. This linkage was achieved by nucleophilic substitution and the imidazolium salts subsequently generated in an N-quaternization step with methyl iodide. 

There is also one example in which a chiral phosphoric acid has been employed as catalyst in the reaction. In particular, the addition of several cyclic p-ketoesters to methyl vinyl ketone was found to occur smoothly in the presence of several chiral phosphoric acids (Scheme 4.35). As mentioned earlier, a key feature of the chiral phosphoric acid catalyst is the backbone binaphthyl axial chirality together with the incorporation of bulky substituents at the 2 positions. In this case, 60b was identified as an appropriate promoter of the reaction leading to the corresponding Michael adducts in excellent yields, although with moderate enantioselectivity. In addition, the authors succeeded in applying this reaction to a procedure to carry out a subsequent Robinson-type annulation. 

Alternatively, atropisomery can be introduced in Kelliphite-type ligands by using an enantiopure binaphthol backbone (Figure 2.45) [44, 54]. Substituents in the 3,3 -positions of the binaphthyl unit enhanced the rate and enantiomeric excess values in the hydroformylation of vinyl acetate. After the best ligand was chosen and the reaction conditions were optimized, up to 80% ee could be realized. 

The synthetic route to the structurally related bis-thioureas of type 32 from (R)-5,5, 6,6, 7,7, 8,8 -octahydro-l,T-binaphthyl-2,2 -diamine was reported by Shi. The effectiveness of the acid-base synergistic catalysis of parent 32a and N,N-diisopropylethylamine was verified by application to the asymmetric Henry reaction of nitromethane with aromatic aldehydes (Scheme 7.57) [86]. Moreover, the Morita-Bayhs-Hillman reaction between 2-cyclohexen-l-one or 2-cyclopenten-1-one and aromatic aldehydes was found to proceed with moderate to good levels of enantioselectivity under the catalysis of 32b, which has additional 3,5-bis(trifluoromethyl)phenyl groups at the 3,3 -positions of the octahydrobinaph-thyl backbone, and l,4-diazabicyclo[2.2.2]octane (DABCO) (Scheme 7.57) [87]. 

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