Binaphthyl compounds

Stereoisomers that result from restricted rotation around single bonds are atropisomers. (5)-l,l -binaphthyl is an example of an atropisomer. The space-filling model is identical to the ball-and-stick model, but it shows the three-dimensional structure more clearly. The naphthyl groups cannot rotate around each other. Therefore, two nonsuperimpos-able, mirror-image isomers exist. 

Restricted rotation around 1-1 carbon-carbon bond 

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It is possible to translate the symbolism of the scissors and its basic structure into the field of molecules (cf. Fig. 11 b). Indeed, the scissor-shaped bulky binaphthyl compound equipped with two appending carboxy groups 1 (l,l -binaphthyl-2,2 -di-carboxylic acid, Fig. 11 c), as mentioned at the beginning (Sect. 1), strictly meets the general structure of an assumed coordinatoclathrate host (cf. Fig. 7). Also, the compound is in keeping with the considerations on an expectedly favorable lattice build-up (see Sect. 3.1). For checking, the crystal inclusion properties of 1 were studied in detail2). 

A number of optically active 1,1 -binaphthyl compounds, 8168 and 9,169 have been prepared with a view to use them as asymmetric catalysts. Compounds 8 (X = Br and OTf)170,171 and 9 [R3 = Me(OTf)2]169 have been used to resolve diols in their reaction with benzoyl chloride. Tin hydrides based on structure 8 (X2 = MeH,172 Bu H,173 and Me3CCH2H171) have been designed for carrying out enantioselective reductions. 

The salt precursor was prepared according to the following route as shown in Scheme 4. The desired enantiopure binaphthyl compound (5)-4 was made from 2-methyhiaphtalene (3) over five steps, which also included a resolution step [31-33]. The final precursor (5)-5 was obtained in 41% yield via a deprotonation of (5)-4 followed by the reaction with methyltrimethoxysilane and a subsequent reduction. 

CL Cooper, JB Davis, MJ Sepaniak. Mechanisms of enantiomeric resolution in cyclodextrin-modified capillary electrophoretic separations of binaphthyl compounds. Chirality 7 401-408, 1995. 

Substituted binaphthyl compounds can be synthesized in high optical yields using nucleophilic aromatic substitution reactions in which the chiral leaving groups are alkoxy moieties derived from naturally occurring alcohols28-29. For example, the condensation of 2-(l-alkoxynaphth-2-yl)-4.5-dihydro-4,4-dimethyl-l,3-oxazole with 1-naphthyllithium or 2-methoxy-l-naphthyl 2-magnesium bromide leads to (ft)- or (.S)-(l,T-binaphthyl-2-yl)-4,5-dihydro-4,4-dimethyl-l,3-oxazole derivatives. 

This type of catalytic strategy has recently been extended to enantio-selective addition of alkyllithiums to certain prochiral imines (Scheme 18) (35). Relevantly, in the presence of a small amount of a chiral ether ligand, 1-naphthyllithium reacts with a sterically hindered imine of l-fluoro-2-naphthaldehyde (conjugate addition/elimination) to afford a binaphthyl compound in greater than 80% ee. 

For the coupling of binaphthyl compounds, the authors note that 0.55 equivalents of PIFA and a temperature below 0 °C are necessary conditions to obtain the best yields (Table 5). Carbon-carbon bond formation occurs between the most highly oxygenated aryl rings of the naphthyl units. 

The first attempt at enantioselective ring opening of meso-epoxides by using a chiral selenolate was reported in 1988 [90]. Enantiomerically pure seleno-binaphthyl compounds 65-67 were synthesized and applied to the asymmetric ring opening of cyclohexene oxide (Scheme 47). 

The transoid-cisoid equilibrium in crowded binaphthyl compounds generates two enantiomeric cisoid forms which may interconvert. For these compounds, CTI requires heating to proceed (i.e. AG i = 23.5 kcal mol-1 for compound 2, h/2 = 14.5 min at 50 °C). Introduction of substituents increases the barrier to rotation and hence stabilizes the chiral configuration [6]. For example, (S)-1,T-binaphthyl-2,2 -dicarboxylic acid 3 could not be racemized at 175 °C in DMF (Fig. 13.IB). Depending on substituents, racemization may be favored by steric/electro-static repulsions and CTI of dicarboxylate 4 occurs at a lower temperature (AG = 24.4 kcal mol-1 for compound 2, = 51.5 min at 50 °C). 

Molecules with shapes analogous to screws are also chiral, since they can be right-handed or left-handed. There are several kinds of molecules in which steric factors impose a screwlike shape. A very important case is 1, T-binaphthyl compounds. Steric interactions between the 2 and 8 hydrogens prevent these molecules from being planar, and as a result, there are two nonsuperimposable mirror image forms. 

Scheme 10-130 Methyl 7-deoxycholate provides a chiral template for the enantioselective synthesis of the binaphthyl compound 401.

The complete transfer of axial chirality to central chirality during the Stevens rearrangement of binaphthyl compounds is demonstrated (eq 41). Treatment with an excess of BuLi results first into the expected Stevens [l,2]-benzyl shift, which... 

W. Hu, T. Takeuchi and H Haraguchi, Retention Behaviour of Binaphthyl Compounds in Enantiomeric Separation by Microcolumn LC with Micellar Bile-salt Mobile Phases, Chromatographia, 33 58 (1992). 

SCHEME 5 Examples of binaphthyl compounds in asymmetric synthesis. 

Wang Q, Chen X, Tao L et al (2007) Enantioselective fluorescent recognition of amino alcohols by a chiral tetrahydroxyl 1,1 -binaphthyl compound. J Org Chem 72 97-101... 

The nonbridged binaphthyl compounds / -Bl and R-B3 do not fit in the frame of the given description because of the very small HTP values and the... 

One example where Castro-Stephens reaction proved to be superior to other methods is the synthesis of the binaphthyl compound 43. In this case, Sonogashira reaction on 42 afforded only an undesired helicene product, where the Castro-Stephens coupling with 4 gave the desired bis-alkyne 43 in 40% yield. Later a Negishi coupling approach was even more successful, delivering the same product in 90% yield. ... 

To prepare salt 2 compound 5 was first prepared according to Scheme 16.2. Binaphthyl compound (S)-4 was prepared from 2-methylnaphthalene (3) over five steps. Owing to a resolution step the compound was obtained enantiopure [32-34]. In the last step precursor (S)-5 was synthesized in 41% yield as shown in Scheme 16.2. 

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