Pentagon stability

Scheme 38 Pentagon stability by cycUc delocalization of a lone pair through vicinal O bonds favored by the orbital phase continuity...

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

A textbook error of the structure RAs = AsR of salvarsan, asphenamine, Ehrlich 606 [78] has been revised. The main component has a structure of a five-membered As ring (8) [79], which is favored by the pentagon stability. 

The pentagon stabilization has been found in a biochemical phenomenon [80], The hydrogen on the thiazolium ring 9 (Scheme 7) is easily ionized to afford the corresponding carbene 10, a key catalyst in enzymatic reactions for which thiamine (vitamin B-1,11) pyrophosphate is the cofactor. The pentagon stability is expected to contribute to this unusual deprotonation. A lone pair generated on the carbon atom in 10 can similarly delocalize through the vicinal C-N and C-S a bonds in a cyclic manner. 

A recent theory of pentagon stability [68, 77] suggests thermodynanic stability of 17 and 18 relative to hexazine. Lone pair electrons in the molecular plane are promoted by the orbital phase continuity to delocalize in a cyclic manner through o bonds of five-membered rings (Scheme 6). The n-rt conjugations also contribute to the relative stability of 17. 

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