Chiral arsenic compounds

The most common structures of arsenic compounds are tetrahedral and pyramidal, which are similar when the sterically active lone pair is counted. Tetrahedral symmetry holds the potential for chirality and indeed many chiral organoarsenic compounds have been prepared. Arsenic may also use d orbitals for (d-d)n bonding and for hybridization with s2 and p3 orbitals, resulting in trigonal bipyramidal or octahedral structures. In the former the more electronegative substituents occupy the apical position. 

In molecules in which the nitrogen atom is at a bridgehead, pyramidal inversion is of course prevented. Such molecules, if chiral, can be resolved even without the presence of the two structural features noted above. For example, optically active 12 (Trdger s base) has been prepared. Phosphorus inverts more slowly and arsenic still more slowly." Nonbridgehead phosphorus," arsenic, and antimony compounds have also been resolved... 

Optical activity was first observed with organic compounds having one or more chiral carbon atoms (or centres) (i.e. a carbon substituted with four different groups). In the structures (1) to (17) the chiral carbons are specified with an asterisk. Subsequently compounds having chiral centres at suitably substituted heteroatoms (e.g. silicon, germanium, nitrogen, phosphorus, arsenic, sulphur, etc.) were also synthesised. Molecular dissymmetry, and hence chirality, also... 

The discoverj( of the stereoselective biomethylation of a prochiral arsinic acid by a microorganism opens up an exdting new route to optically active tertiary arsines. Numerous microorganisms reductively methylate arsenic(V) compounds . The biological synthesis of (+)-126 by Scopulariopsis brevicaulis was described in 1936. At that time, however, it was not recognized that simple tertiary arsines chiral at arsenic were configurationally stable and amenable to optical resolution. 

The stable pyramids associated with these heteroatoms should allow optically active compounds to be prepared. The dinaphtharsole (21), while potentially chiral at arsenic, undergoes rapid room temperature atropisomerism of the binaphthalene system which precludes isolation of individual enantiomers <93JOM(445)7l). Optically active dibenzoarsoles and -stiboles have been prepared... 

Chapter Z deals with chiral compounds containing, silicon, germanium, tin, lead, nitrogen, phosphorus, arsenic, and sulphur. 

Compounds containing a chiral nitrogen, phosphorus or arsenic atom—pages Z3-Z6. 

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