Chiral binaphthyl systems

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. 

More recently macrocycles with rigid and flexible aromatic and chiral binaphthyl systems with interesting fluorescence properties have been synthesized via hydroaminomethylation (Scheme 16) [62,63]. 

Cram and his coworkers have pioneered the use of bis-binaphthyl crowns as chiral com-plexing agents for ammonium salts and amino acid salts. In these systems, the chiral binaphthyl unit provides a steric barrier within the macrocycle which allows discrimina-... 

In Cram s first synthesis of a chiral bis-binaphthyl system, optically pure binaph-thol and diethylene glycol ditosylate were heated at reflux in tetrahydrofuran solution for 15 h with potassium f-butoxide, two products were obtained. The 1 + 1 product (mp 230—231°) was isolated in 5% and the 2 + 2 product (mp 123—126°) was obtained in 31% yield. The reaction is shown in Eq. (3.51). 

A new class of phosphines (30) containing only an axial element of chirality (atropisomerism) has been made (253, 254). An in situ 1 1 rhodium/2,2-bis(diphenylphosphinomethyl)-1,1 -binaphthyl system (30a) hydrogenated a-acetamidocinnamic acid to a 54% ee (S) using 50 atm H2, the solvent not being recorded (253). The corresponding diphenyl-phosphinite system (30b) in toluene-acetone was particularly effective (76% ee) for hydrogenation (95 atm) of a-acetamidocinnamic and a-acet-amidoacrylic esters (254). 

The Bolm protocol was recently used by Ellman et al. for the enantioselective oxidation of -butyl disulfide 22 [72], Excellent result was achieved in the formation of thiosulfinate 22 (91% ee, 93% yield) by using catalyst 20 (0.25 mol %) in a 0,5 mmol scale. In spite of extensive screening of chiral Schiff bases related to catalyst 20, better enantioselectivity was not realized. Chiral thiosulfinate 22 is a convenient starting material for the preparation of r-butyl sulfi-namides and t-butyl sulfoxides. Vetter and Berkessel modified the structure of the Schiff base moiety of catalyst 20 by replacing the aryl ring with a 1,l -binaphthyl system [73]. The corresponding vanadium catalyst realized 78% ee in the oxidation of thioanisol, which was better than that attained by the Bolm catalyst (59% ee). 

Oxidation of alcohols to carbonyl compounds using the stable nitroxyl radical TEMPO (41) as catalyst is a well-known preparative method [42, 43], Hypochlorite or peracetic acid is usually used as the final oxidizing agent and ca. 1 mol% of the catalyst 41 is used. In 1996 Rychnovsky et al. reported the synthesis of the chiral, binaphthyl-derived TEMPO analog 42 [44]. Table 12.1 lists the results obtained with 0.5-1 mol% of catalyst 42 [44], In these oxidation reactions 0.6-0.7 equivalents of sodium hypochlorite were used as the final oxidizing agent (plus 0.1 equiv. potassium bromide) in a two-phase system containing substrate and catalyst 42 in dichloromethane at 0 °C. As shown, the best selectivity factors (> 5) were observed for 1-phenylethanol and its derivatives as substrates. 

In practical terms, axial chirality often occurs when free rotation along an axis in the molecule is sufficiently hindered. A well known example is the binaphthyl system where free rotation around the common bond can be prevented by introducing bulky substituents in the 2,2 -positions. If these substituents are NHC, then an axial chiral bis-carbene results... 

This reagent has also been applied in the synthesis of a wide range of new diphosphines, many of which are chiral, including the l,r-binaphthyl system... 

The reactions proceed with retention of configuration at phosphorus. Various classical routes to alkylphosphine oxides have been applied in the synthesis of a range of potentially chelating and pincer-like ligands, e.g., (233), the binaphthyl system (234), the hybrid phosphine oxide-N-oxide (235), and the chiral pyridine bis(phosphine oxide) (236). A route to diarylmethylphosphine oxides is afforded by the palladium-catalysed reaction of aryl bromides with tet-rakis(hydroxymethyl)phosphonium chloride in the presence of a base. The diastereoisomeric system (237) has been isolated from the reaction of a cyclic... 

Iodorhodium(IIl) porphyrins also efficiently catalyze the reaction of ethyl diazoacetate with simple alkenes. generally providing the cw-isomers as the major product77 79110. The cis( tram ratio increases when bulkier porphyrins, such as tetramesitylporphyrin (TMP), are employed. The mechanism of this rhodium-catalyzed cyclopropanation with diazoacetate is interpreted as proceeding via carbene complexes79 80 111,112. Based on these results, asymmetric cyclopropanation of alkenes with ethyl diazoacetate is achieved if catalyzed by a chiral wall porphyrin81. An earlier described binaphthyl-system of this type82113114, introduced as an iodorhodium(lll) complex, 6, forms an extremely active catalyst and leads to m-cyclopropanes (preferred over the rran.v-products) with moderate to poor enantioselectivities if styrene, 1- and 3-phenylpropene are used as substrates (10-60% ee)81. 

The use of inorganic supramolecular compounds in catalysis has also been successful in recent years. Hupp etal. incorporated a Mn (III)-porphyrin see Porphyrin) epoxidation catalyst inside a molecular square, a system that shows enhanced catalyst stability and substrate selectivity as compared to the free catalyst. In another example, chiral metallocyclophanes were constructed from Pt(PEt3)2 units and enantiopure atropoisomeric l,E-binapthyT6,6 -bis-(acetylenes) and used in enantioselective diethyl zinc addition to aldehydes to afford chiral secondary alcohols. The first organometallic triangle based on Pt(II) and alkyne-di-substituted-binaphthyl system was reported and found to effect asymmetric catalysis reactions of aldehydes to alcohols with excellent conversion rates and enantiomeric excess/ ... 

Various oxidations with [bis(acyloxy)iodo]arenes are also effectively catalyzed by transition metal salts and complexes [726]. (Diacetoxyiodo)benzene is occasionally used instead of iodosylbenzene as the terminal oxidant in biomimetic oxygenations catalyzed by metalloporphyrins and other transition metal complexes [727-729]. Primary and secondary alcohols can be selectively oxidized to the corresponding carbonyl compounds by PhI(OAc)2 in the presence of transition metal catalysts, such as RuCls [730-732], Ru(Pybox)(Pydic) complex [733], polymer-micelle incarcerated ruthenium catalysts [734], chiral-Mn(salen)-complexes [735,736], Mn(TPP)CN/Im catalytic system [737] and (salen)Cr(III) complexes [738]. The epox-idation of alkenes, such as stilbenes, indene and 1-methylcyclohexene, using (diacetoxyiodo)benzene in the presence of chiral binaphthyl ruthenium(III) catalysts (5 mol%) has also been reported however, the enantioselectivity of this reaction was low (4% ee) [739]. 

In 34 and 35, there is one more element of chirality in the phosphite or phosphinite moiety, such as the binaphthyl system [44]. These modifications could potentially influence the origin of the stereochemistry in the asymmetric catalysis process, and sometimes excellent results have been obtained (quantitative conversions and enantiomeric excess values higher than 99%) in the case of 34 (R = Ph, = H, R = Ph) that is better than other known catalyst systems for the asymmetric hydrogenation of unfunctionalized 1,1-disubstituted terminal alkenes [44]. 

This host is chiral, possesses a C2 axis of symmetry, and the dihedral angle between the planes of the two naphthalene rings attached to one another can vary between 60° to 120°. Including a binaphthyl system in a crown ether... 

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