Formation of epoxides from pentacoordinated organoarsenic compounds with a β-hydroxyethyl group

A series of pentacoordinated organoarsenic compounds (arsoranes) bearing a β-hydroxyethyl group (4a, 6a and 6b) was synthesized. The crystal structures were determined by single crystal X-ray analysis. Treatment of these arsoranes with KH almost quantitatively gave the corresponding epoxide. The reaction of 4a having an unsubstituted β-hydroxyethyl group with KH was monitored by 1H and 19F NMR in CD3CN, suggesting that a hexacoordinate arsenic anion was formed as the intermediate. However, a further stereochemical study of the epoxide formation suggested that the reaction proceeded in the SN2 manner and not in the ligand coupling reaction (LCR) of the intermediate hexacoordinate arsenic anion.