Atropisomeric Systems

Another example of chiral GC has been recently reported on the same chiral support for the analysis of enantiomers of 5,5 -dichloro-l,l -dimethyl-3,3, 4,4 -tetrabromo-2,2 -bip5UTole. Partial racemization of the optically pure sample occurred in the injector (10JC(A1217)2050). The high-temperature ranges, which are operated both in the column and in the injector, obviously require great care in the analysis of atropisomeric systems. 

The development of liquid chromatography on a chiral support gave a decisive impulse to the study of atropisomerism since resolution can be quantitatively performed under very mild conditions without derivatiza-tion. The analytical methods can be easily extrapolated to preparative scale. Several atropisomeric systems, which do not present suitable functional groups to perform classical resolution through diastereomer resolution, can be readily separated into optically pure enantiomers. Dynamic chiral HPLC fills the gap between barriers attainable by DNMR and by thermal racemization of pure enantiomers. Chiral HPLC opens the way to several unexplored domains in the field of atropisomerism. We have... 

Atropisomerism occurs in other systems as well. A sulfoxide (16), for example, forms atropisomers with an interconversion barrier with its atropisomer of 18-19kcal mol . The atropisomers of hindered naphthyl... 

MeOBIPHEP is the atropisomeric diphosphine 2,2,-bis(diphenylphosphino)-6,6 -dimethoxy-l,-l -biphenyl (100), has been synthesized. In the presence of SnCl2, this species is an efficient catalyst for the asymmetric hydroformylation of styrene. Asymmetric inductions are higher than those attainable using the system [PtCl2 (i )-(+)-BINAP ]/SnCl2, where BINAP is 2,2 -bis(di-phenylphosphino)-l,l,-binaphthyl. The influence of CO and H2 partial pressures on the catalytic activity of the (99)/SnCl2 system has also been studied.328 Complexes [PtMeCl(P-P)][(101), P-P = (5)-6,6,-(dimethoxybiphenyl)-2,2,-diylbis(diphenylphosphine) ((5)-MOBIPH) (102),... 

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). 

A. Atropisomerism about ferr-Alkyl-to-Triptycyl Bonds and Related Systems... 53... 

Ruthenium(II) complexes bearing atropisomeric diphosphine ligands have proved to be efficient systems for the hydrogenation of a wide range of prochiral substrates. A new catalytic system has been developed based on ruthenium complexes having SYNPHOS and DIFLUORPHOS as chiral diphosphanes (Figure 3.6). 

Next, other unsaturated carbocyclic systems are discussed, such as dibenzo-cyclo-heptadienes ( atropisomeric biphenyls), dihydropleiadenes, cycloheptatrienes and their benzo-derivatives. Most of these compounds exist in a half-boat or boat conformation. Tropones are considered in the light of recently published work. 

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