Atropisomer BINAP

Asymmetric induction (See also Enantioselective) chiral ketones, 62, 106-107 chiral sulfoxides, 8-9 steroid synthesis, 27, 278-281 Asymmetric syntheses. See Enantioselective. .. Asymmetry of vesicle membranes, 351 dATP. See 2 -Deoxynucleoside 5 -triphosphates Atropisomers binap chelands, 102-103 Kemp s acid arylimides, 347 porphyrin oligomers, 348—349 5,10,15,20-tetraarylporphyrins, 253 Axial/equatorial stereoselectivity ... 

Atropisomerism occurs in other systems as well, including monopyrroles. Sulfoxide 16, for example, forms atropisomers with an interconversion barrier with its atropisomer of 18-19 kcalmoP. The atropisomers of hindered naphthyl alcohols, such as 17 exist as the ip-atropisomer (17a) and the ap-atropisomer (17b).Atropisomers can also be formed in organometalhc compounds, such as the bis(phosphinoplatinum) complex (see 18), generated by reaction with R-BINAP (see p. 1801). ... 

The usual solution is to use one chiral molecule containing two phosphorus atoms, and the most important of these is BINAP. BINAP is a chelating diphosphine the metal sits between the two phosphorus atoms, firmly anchored in a chiral environment. The chirality here is of an unusual sort, since BINAP, which you met in Chapter 14 (p. 319), has no chiral centres, it is one of the class of chiral molecules, known as atropisomers, whose chirality arises from the inability of the bond between the two naphthalene rings to rotate. 

Conformational isomers that are stable, isolable compounds are called atropisomets. The existence of chiral atropisomers has been exploited to great effect in the development of chiral catalysts for stereoselective reactions. An example is BINAP, shown below in its enantiomeric forms ... 

Anilides, bearing a sterically demanding orfho-substituent, are known to exist as atropisomers due to a high rotational barrier around aryl-nitrogen single bond. The cationic rhodium(I)/xyl-BINAP complex-catalyzed enantioselective [2+2+2] cycloaddition of 1,6-diynes with trimethylsilylynamides afforded axiaUy chiral anilides with good to excellent enantioselectivity (Scheme 21.21) [25]. 

Binaphthyl is one of many atropisomers possible for binaphthyl systems. Two important examples are the 2,2 -diol called BINOL and the 2,2 -diphenylphosphine derivative called BINAP. When chiral ligands are used, chiral synthesis becomes possible. That is, an achiral reactant can be stereospecifically converted to a chiral product. 

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