Asymmetric centers phosphorus atoms

Branching of the O-alkyl ester chain of organo-phosphorus compounds may introduce an asymmetric center, which together with an asymmetric substituted phosphorus atom creates a number of stereoisomers. Diastereoisomers may even be separated on a conventional capillary GC column. This is, for instance, the case with the nerve gas soman, which usually produces two peaks in a gas chromatogram. Although this is characteristic for identifying soman, it also increases the GC/MS detection limit by a factor of two. 

The spatial relationships that exist for the human body also exist at the molecular level because the molecules of nature exist as three-dimensional symmetrical and asymmetrical figures. One of the most common asymmetric molecules is a tetravalent carbon atom with four different ligands attached to it. The spatial arrangement of the atoms in this molecule is shown in Fig. 2, The carbon atom depicted in Fig. 2 is the asymmetric center of the molecule, and the molecule is a chiral stereoisomer. If the molecule and its mirror image are nonsuperimposable, the relationship between the two molecules is enantiomeric, and the two stereoisomers are enantiomers. Carbon is not the only atom that can act as an asymmetric center. Phosphorus, sulfur, and nitrogen are among some of the other atoms that can form chiral molecules. 

Atoms Other than carbon can be asymmetric centers. Any atom that has four different groups or atoms attached to it is an asymmetric center. For example, the following pairs of compounds, with nitrogen and phosphorus asymmetric centers, are enantiomers. 

Nitrogen, Phosphorus and Sulfur Atoms as Asymmetric Centers 232... 

Chiral Phosphorus Compounds Koizumi et al. 251 have prepared a series of chiral organophosphorus compounds (256) in which the phosphorus atom is the asymmetric center, whereby amino acid derivatives were used as chiral auxiliary reagents. 

Analytical Properties Shows chiral recognition for many racemates and is especially effective for substrates with a phosphorus atom at an asymmetric center however, the degree of chiral recognition is not so high in general Reference 6... 

Two kinds of chiral tertiary phosphine ligands have been used in asymmetric hydrogenation experiments involving rhodium complexes the Horner and Monsanto groups have concentrated on ligands whose chirality is centered at an asymmetric phosphorus atom, and the New Hampshire and Paris groups have focused their attention mainly on phosphides that carry chiral carbon moieties. 

The essence of asymmetric synthesis is the creation of asymmetric centers under the influence of other asymmetric centers in such a way that the resulting enantiomers or diastereoisomers are formed in unequal proportions. Most reactions in asymmetric synthesis that have been described involve the conversion of trigonal carbon atoms into asymmetric, quadrivalent carbon atoms, and this article will be principally concerned with such reactions, although, in many instances, the principles involved may also be applied to asymmetric reactions in which, for example, chiral phosphorus or sulfur atoms are formed. In all reactions in which are formed mixtures of enantiomers having one enantiomer in preponderance, it is possible to describe the stereoselectivity of the reaction in terms of optical yield (optical purity, or enantiomeric yield). The precise significance of these terms has been described in detail elsewhere,1 but, practically, where at a selected wavelength, [a] is the specific rotation of the reaction product and [A] is the specific rotation of a pure enantiomer, the optical yield = [a]/[A]. Thus, the value of the optical yield is a measure of the excess of one enantiomer over the other. 

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). ... 

Numerous biphosphines appear to be good chiral ligands for the near-quantitatively enantioselective hydrogenation of amino acid precursors The two phosphorus atoms are connected by a chiral backbone having one asymmetric center or two. The general structure is easily prepared by alkylation of a disubstituted phosphide ... 

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... 

A chiral molecule has a nonsuperimposable mirror image. An achiral molecule has a superimposable mirror image. The feature that is most often die cause of chirality is an asymmetric carbon. An asymmetric carbon is a carbon bonded to four different atoms or groups. An asymmetric carbon is also known as a chirality center. Nitrogen and phosphorus atoms can also be chirality centers. Nonsuperimposable mirror-image molecules are called enantiomers. Diastereomers are stereoisomers that are not enantiomers. Enantiomers have identical physical and chemical properties diastereomers have different physical and chemical properties. An achiral reagent reacts identically with both enantiomers a chiral reagent reacts differently with each enantiomer. A mixture of equal amounts of two enantiomers is called a racemic mixture. 

These compounds are important in asymmetric catalysis, in which a prochiral substrate is converted into a chiral product. There are basically three types of chiral phosphines. Firstly, there are phosphines of the type PRR R", where the chiral center is the phosphorus atom. Secondly, the substituent(s) or the molecule as a whole may be chiral. Finally, a coordinated phosphine complex may be chiral at the metal center. 

One of the routes leading to P-stereogenic phosphines is electrophilic substim-tion at the phosphorus atom of secondary phosphines, as a result of asymmetric catalysis in which a catalyst activates a phosphorus nucleophile or a carbon electrophile, creating an asymmetric environment, i.e., creating preference for one of Si or Re face sides at the reactive center [103-113]. Upon reaction with chiral metal complexes, racemic secondary phosphines are converted into diaste-reomeric metal-phosphide complexes A or B, which interconvert rapidly through the inversion at phosphorus. If the equilibrium A B is faster than the reaction of A or B with an electrophile E, then P-stereogenic phosphines 196, in which pyramidal inversion is slow, can be formed enantioselectively. The product ratio in this dynamic kinetic asymmetric transformation depends both on and on the rate constants ks and (Scheme 63). 

Most chiral organic compounds have at least one asymmetric carbon atom. Some compounds are chiral because they have another asymmetric atom, such as phosphorus, sulfur, or nitrogen, serving as a chirality center. Some compounds are chiral even though they have no asymmetric atoms at all. In these types of compounds, special characteristics of the molecules shapes lend chirality to the structure. 

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