Stereogenic Centers Other Than Carbon

Our discussion to this point has been limited to molecules in which the stereogenic center is carbon. Atoms other than carbon may also be stereogenic centers. Silicon, like carbon, has a tetrahedral arrangement of bonds when it bears four substituents. A large number of organosilicon 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. 

Verify that CH3NHCH2CH3 is chiral by trying to superpose models of both enantiomers. 

Phosphoms is in the same group of the periodic table as nitrogen, and tricoordinate phosphorus 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 substituents. The rate of pyramidal inversion at sulfur is rather slow. The most common compounds in which sulfur is a stereogenic center are sulfoxides such as  

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In general, we speak of enantiomerically enriched metal alkyls when we focus our interest on the stereogenic metalated carbon center. In the real case, these molecules are almost always diastereomerically enriched metal alkyls, due to the presence of stereogenic centers other than the metalated one alone. 

Section 7.16 Atoms other than carbon can be stereogenic centers. Examples include those based on tetracoordinate silicon and tricoordinate sulfur as the stereogenic atom. In principle, tricoordinate nitrogen can be a stereogenic center in 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 virtually instantly at room temperature. 

Isomerism is observed in the polymerization of alkenes when one of the carbon atoms of the double bond is monosubstituted. The polymerization of a monosubstituted ethylene, CH2=CHR (where R is any substituent other than H), leads to polymers in which every tertiary carbon atom in the polymer chain is a stereocenter (or stereogenic center). The... 

Stereogenic centers can also be prepared by carbon-carbon bond-forming reactions or reductions of functional groups other than alkenes. Some reactions are also summarized in Table 4 (144) for a comprehensive work on asymmetric catalysts, see Reference 145. In some cases, two stereogenic centers can be created. This result can be achieved either in a single step as... 

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