Stereogenic center phosphorus

The development of the next major class of ligands occurred during the 1990s, with Burks DuPhos (42) family of phospholane ligands [222, 223]. (An individual member of the family is named after the substituent R in Me-DuPhos, R=Me.) This structure could be considered an improvement on the DIOP-derived ligands, where the stereogenic centers are now closer to phosphorus. In addition to the aromatic spacer of DuPhos, there is also the related BPE (43) family, where the spacer between the two phosphorus atoms is less rigid. In both series the phosphorus is... 

Chiral 2-alkyl-2-oxo-1,3,2-oxazaphospholanes 90 and 91 were chosen by Jommi and co-workers as a substrate for theoretical and experimental studies on the prediction of the diastereoselectivity of the amination reaction leading to a new stereogenic center a to the phosphorus atom. 

With the C2 symmetry of the chiral diamine, no new stereogenic center is created at the phosphorus atom, thus avoiding the need for a sometimes difficult separation of diasteroisomers, such as with amino alcohols. 

In 10, there are two remote stereogenic centers at phosphorus and it was isolated as an almost equimolecular mixture of the two sets of diastereoisomers, that is, (RR/SS) and (RS/SR). In each chiral diastereoisomer, the phosphorus atoms are nonequivalent and four distinct 31P resonances were obtained almost in a 1 1 1 1 ratio <1997J(P2)1445>. The kinetics of the thermal decomposition of 11 in dibutyl phthalate was studied. The high rate of decomposition was probably determined by mutual steric influence of the bulky dinitromethylene moieties <2006RJC499>. 

To achieve asymmetric C-P bond formation in the Abramov reaction, chiral phosphoryl components can also be used in which the phosphorus atom is a stereogenic center. In this case, the main problem is the synthesis of suitable enantio- or diastereomerically pure phosphites. 

Studies of structurally related phosphonoamidates possessing P- and C-stereogenic centers indicated that the alkylation of diastereoisomer (436) is mostly influenced by the chirality of the asymmetric phosphorus atom, while the alkylation of I diastereoisomer (437) depends on a combination of both the chirality of phosphorus and carbon atoms (Scheme 103). ... 

Considering that a metal DIOP complex is conformationally fiexible and the stereo genic centers may be too far from the substrate, Kagan synthesized a modified DIOP ligand 28 in which the stereogenic centers are closer to the phosphorus atom [31]. 

In early studies of the rhodium-catalyzed asymmetric hydroformylation, Horner-type monophosphanes with phosphorus as the stereogenic center were used73-1 18 143 172. Examples are ( + )-benzyl(methyl)phenylphosphane (BnCH3PhP )77- 78 115 and (-)-(/ )-methyl(phenyl)propylphosphane (CH3PrPhP )35. 

Recently, a highly effective new ligand, DuPHOS122, has been introduced. With two clcctron-rich phosphorus atoms embedded in C2-symmetric phospholane rings, its structure is fundamentally different from previously developed ligands. In contrast to other diphosphanes with a chiral backbone, such as Chiraphos or Diop, the stereogenic centers of DuPHOS are located in proximity to the coordination sphere. 

Although asymmetrically substituted carbon atoms are by far the most common type of stereogenic center in organic compounds, several other kinds of stereogenic centers are encountered. Tetravalent nitrogen (ammonium) and phosphorus (phosphonium) ions are obvious extensions. Phosphine oxides are also tetrahedral and are chiral if all three substituents (in addition to the oxygen) are different. Not quite... 

Due to the great importance of phosphines and phosphinites as chiral ligands for asymmetric catalysis, several hundred compounds of this type have been prepared. In this section, only those compounds which have been mentioned in this Houben-Weyl volume will be discussed. Because of their close relation to the specific topics, biaryl phosphines have already been mentioned in Section 6 and ferrocene derivatives in Section 7.1. All other phosphorus compounds are treated here. An excellent review on the synthesis of enantiomerically enriched compounds where phosphorus is a stereogenic center has recently been published73. 

For (-) castoramine (56) Rule 6 allows a priority determination that assigns the stereogenic nitrogen center as (R). Just as a nitrogen atom can be stereogenic, a phosphorus atom in a phosphine such as 53 can be considered chiral since the rate of inversion is negligible at temperatures up to = 130°C, as noted in Table 1.4. The lone pair electrons have the lowest priority by Rule 6 and the absolute configuration is (R). 

A wide range of alcohols, carbonyl compounds (particularly if the carbonyl group is close to the stereogenic center), lactones, phosphorus compounds, aromatic compounds, and sulfoxides have been successfully resolved on the different poly(saccharide) derivatives, Non-polar mobile phases are commonly used, although not exclusively, and the selectivity of the various cellulose polymers changes with the character of the ester or carbamate groups used to derivatize the polymer. 

As mentioned previously, the helicene-based chlorophosphite M,M,R) 25a, which is easily prepared from the reaction ofenantiopure (M,M,R)-[5 -HELOL 24a with phosphorus trichloride, has been successfirUy used to analyze the enantiomeric compositions of a variety of chiral materials such as alcohols, amines, or carboxylic acids. The reaction is conducted in an NMR tube where the analytes are added to the chiral helicene-based chlorophosphite in CDCI3. After 2 h reaction, the diastereomers ratios generated in situ are determined by P-NMR spectroscopy. Notably, this attractive method has proved to be highly efficient for analyzing the enantiomeric purities of a wide range of molecules whose stereogenic centers are far away from functional groups that react with phosphorus atom. The authors... 

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