Scaffolds binaphthyl

In 2007, Maruoka et al. introduced chiral dicarboxylic acids consisting of two carboxylic acid functionalities and an axially chiral binaphthyl moiety. They applied this new class of chiral Brpnsted acid catalyst to the asymmetric alkylation of diazo compounds withA-Boc imines [91]. The preparation of the dicarboxylic acid catalysts bearing aryl groups at the 3,3 -positions of the binaphthyl scaffold follows a synthetic route, which has been developed earlier in the Maruoka laboratory [92]. 

The achiral analogue of the binaphthyl scaffold is the biphenyl backbone. An example comes from Chen et al. [362] who synthesised a bis-benzimidazolium salt starting from 2,2 -diaminobiphenyl (see Figure 3.114). In contrast to the bis-carbene in Figure 3.113, the... 

Figure 3,113 Synthesis of an axial chiral bis-carbene on a binaphthyl scaffold.

Note The axial chirality of the 1,T-binaphthyl scaffold is induced by the central chirality of the carbene unit [83,84]. This simplifies the issue of chirality from a synthetic point of view as the centrally chiral diamine is easier to purify than the axially chiral 1,1 -binaph-thyl scaffold that would otherwise be required. 

Figure 5.19 Axial chiral bis-carbene ligands based on the 1,1 -binaphthyl scaffold.

Note The IJ -biphenyl and the 1,1 -binaphthyl scaffolds have the same steric influence on the transition metal (ruthenium) centre of the second generation Grubbs catalyst. Therefore, the improved chiral resolution in the product is likely due to a favourite combination of axial and central chirality. 

The same authors then proceeded to synthesise the corresponding azolium salt based on the l,r-binaphthyl scaffold (see Figure 5.23) [79]. The same failure to synthesise either the free carbene or transition metal carbene complexes from it was estabUshed experimentally. Electronic destabilisation by a double annulation effect from the two phenyl (naphthyl) rings on the seven-membered azolium ring systan is indeed the most likely explanation. 

Although axial chirality may be more widespread owing to the 1,T-binaphthyl scaffold, planar chirality is plainly rising in general usage. The chief requirement is a planar scaffold and a discriminator between the obverse and the reverse of said plane. A convenient scaffold answering this simple definition is the well known ferrocene. With the obverse blank and the reverse a CpFe unit, only two different substituents on the same Cp ring... 

Axial chirality, especially when based on the l,l -binaphthyl scaffold seems to be favourable in asymmetric catalysis. However, no prediction can yet be made for specific ligand/substrate combinations. 

In the same area, these authors have also investigated the efficiency of various chiral sterically congested phosphane-amide bifunctional organocatalysts with a binaphthyl scaffold in asymmetric aza-Morita-Baylis-Hillman reactions of A -sulfonated imines with activated olefins such as methyl and ethyl vinyl ketones." The corresponding aza-Morita-Baylis-Hillman adducts could be obtained in moderate to excellent yields (37-98%) and moderate to good... 

A type of axial chiral sulfide based on the binaphthyl scaffold was reported by Uemura and coworkers in 2002. In the reaction of benzaldehyde with benzyl bromide in dichloromethane with the addition of n-Bu4NI, a 44% yield and 50% ee were obtained in 24h at a 5mol% catalyst loading. The 3,3 -phenyl derivative 9b was much less reactive and gave only a trace amount of the epoxide product [16]. 

A great improvement in this approach was obtained thanks to chiral bifunctional phase transfer ammonium salts having a binaphthyl scaffold with 3,3 -bis-(diarylhydroxy-methyl) substituents. These catalysts have demonstrated a superior level of enantiodiscrimination in the fluorination of cyclic 3-keto esters (Scheme 44.28). ... 

From these experiments it appeared that only the axial chirality of the binaphthyl scaffold of L5-a is responsible for the chiral induction. Therefore, the simplified ligand L5-b could be developed which displays an achiral gem-dimethyl substituted oxazoline ring (Fig. 10.7). This specifically designed ligand performs as good as, or even better than the analogous phosphino-oxazoUnes bearing two chirality elements. Authors also demonstrated that the double substitution of the 4-position of the oxazoline unit in L5-b is essential to the stereochemical control of the cycloisomerization reaction. 

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