Chirality Centers Other Than Carbon

Atoms other than carbon may also be chirality centers. Silicon, like carbon, has a tetrahedral arrangement of bonds when it bears four substituents. A large number of organosili-con compounds in which silicon bears four different groups have been resolved into their enantiomers. 

Trigonal pyramidal molecules are chiral if the central atom bears three different groups. If one is to resolve substances of this type, however, the pyramidal inversion that interconverts enantiomers must be slow at room temperature. Pyramidal inversion at nitrogen is so fast that attempts to resolve chiral amines fail because of their rapid racemization. 

Phosphorus is in the same group of the periodic table as nitrogen, and tricoordinate phosphoms compounds (phosphines), like amines, are trigonal pyramidal. Phosphines, however, undergo pyramidal inversion much more slowly than amines, and a number of optically active phosphines have been prepared. 

Tricoordinate sulfur compounds are chiral when sulfur bears three different groups. The rate of pyramidal inversion at sulfur is rather slow. The most common compounds in which sulfur is a chirality center are sulfoxides such as  

The Cahn-Ingold-Prelog R,S convention is used to specify the configuration at sulfur, with the unshared electron pair considered to be the lowest ranking substituent on sulfur. Armodafinil, a drug used to treat sleep disorders, for example, has the R configuration. 

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Compounds with Chirality Centers Other than Carbon 232... 

COMPOUNDS WITH CHIRALITY CENTERS OTHER THAN CARBON... 

Section 7 16 Atoms other than carbon can be chirality centers Examples include those based on tetracoordmate silicon and Incoordinate sulfur as the chirality center In principle Incoordinate nitrogen can be a chirality center m compounds of the type N(x y z) where x y and z are different but inversion of the nitrogen pyramid is so fast that racemization occurs vrr tually instantly at room temperature... 

An effect observed with a number of compounds which have apparent chiral centers on elements other than carbon. Eor example, secondary and tertiary amines have a pyramidal structure in which the unshared pair of electrons is at the top of the pyramid. If the three substituents hnked to the nitrogen are all different, one might suspect that the tertiary amine would give rise to optical activity and be resolvable. However, rapid oscillation of the unshared pair of electrons on one side of the nitrogen to the other (hence, pyramidal inversion) in effect causes interconversion of the two enantiomers and prevents resolution. If the nitrogen is at a bridgehead, this umbrella effect is inhibited and optical isomers can be isolated. 

Since the most common cause of chirality is the presence of four different substituents bonded to a tetrahedral atom, tetrahedral atoms other than carbon can also be chirality centers. Silicon, nitrogen, phosphorus, and suit fur are all commonly encountered in organic molecules, and all can be chin rality centers under the proper circumstances. We know, for example, thal trivalent nitrogen is tetrahedral, with its lone pair of electrons acting a4 the fourth substituent (Section 1.11). Is trivalent nitrogen chiral Does compound such as ethylmethylamine exist as a pair of enantiomers j... 

Ephedrine and pseudoephedrine are structurally mirror images of each other. This is possible because they have a chiral center, the isopropyl carbon to which the nitrogen atom is attached. If the reduction is done in such a manner that the chiral nature of the substance is not jumbled (i.e. racemization), then ephedrine and pseudoephedrine give rise to "1" and "d" methamphetamine, respectively. The "1" form is several times more potent than the "d" form. Meth produced from phenylacetone is a racemic mixture, meaning that it is a 50-50 mix of the "1" and "d" forms of meth. Obviously, a batch of pure "1" form is most desirable, a racemic mixture is OK, and pure... 

An asymmetric carbon is also known as a chirality center. We will see that atoms other than carbon, such as nitrogen and phosphorus, can be chirality centers—when they are bonded to four different atoms or groups (Section 5.17). In other words, an asymmetric carbon is just one kind of chirality center. A chirality center also belongs to a broader group known as stereocenters. Stereocenters will be defined in Section 5.5. 

FIGURE 7.15 Chiral centers at atoms other than carbon. 

The atom with the higher atomic number has the higher priority. A lone pair of electrons (possible if the chiral center is an atom other than carbon) has the lowest possible priority. 

In most cases with more than two chiral centers, the number of isomers can be calculated from the formula 2", where n is the number of chiral centers, although in some cases the actual number is less than this, owing to meso forms.An interesting case is that of 2,3,4-pentanetriol (or any similar molecule). The middle carbon is not asymmetric when the 2- and 4-carbon atoms are both (/ ) [or both (5)] but is asymmetric when one of them is (/ ) and the other (5). Such a carbon is called a pseudoasymmetric carbon. In these cases, there are four isomers two meso forms and one dl pair. The student should satisfy himself or herself, remembering the rules... 

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