Selenium, chiral structures

Many of the molecules are chiral. Six of the above-mentioned compounds contain SeSe bonds and no isolated sulfur or selenium atoms (structural units of type —S—Se—S— or —Se—S—Se—), nine contain SeSe bonds and isolated sulfur or selenium atoms, and 13 contain no SeSe bonds. The number of SeS bonds in these molecules can be either 2, 4, 6, or 8. 

Se NMR spectroscopy is more and more recognized as a useful tool for the elucidation of molecular structures of selenium-containing compounds and intermediates. In other applications, Se nuclei were used as probes for detecting molecular chirality, intramolecular interactions, or bond situations. In addition, some methods have been invented where selenium-containing auxiliaries are applied to selenium-free compounds. In the following, applications in those fields are presented. 

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

The replacement of oxygen in phosphonic or a phosphinic acid by sulphur or selenium renders it much easier to obtain optically active forms of a simple product possessing a chiral phosphorus centre. As early as 1911, Ephraim failed to resolve anions of the type R2P(0)0, and so concluded that the anions were structurally symmetrical. However, the presence of an atom of, for example, sulphur, creates a non-symmetrical environment around the four-coordinate phosphorus atom, and permits the isolation of optical antipodes. Enantiomeric forms of a simple thiophosphonic acid derivative were first obtained by Aaron and Miller, who resolved the monoethyl ester of ethylphosphonoth-ioic acid, Et(EtO)P(S)OH, by fractional crystallization of its salts with various alkaloids. 

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