Syntheses, structures, and thermolyses of pentacoordinate 1,2-oxastibetanes: potential intermediates in the reactions of stibonium ylides with carbonyl compounds.

[reaction: see text] Pentacoordinate 1,2-oxastibetanes 14a-d, which are formal [2 + 2]-cycloadducts of the reactions of stibonium ylides with carbonyl compounds, were successfully synthesized by the reactions of the corresponding bromo-2-hydroxyalkylstiboranes with NaH. The crystal structures of 14a and 14c were established by X-ray crystallographic analyses, showing their distorted trigonal bipyramidal structures and smaller C-Sb-O angles of the four-membered ring around antimony than the C-P-O angle of pentacoordinate 1,2-oxaphosphetane 3. The 1H, 13C, and 19F NMR spectra of 14a-d are consistent with the trigonal bipyramidal structure in the solution state. Although 14a did not decompose at all at 220 degrees C in o-xylene-d(10), the thermolyses of 3-phenyl-1,2-oxastibetane 14c were carried out at 220 degrees C in o-xylene-d(10) and at 140 degrees C in acetonitrile-d(3) to give the corresponding oxirane 28 with retention of configuration and cyclic stibinite 25. The formation of 28 is explained by apical-equatorial ligand coupling around antimony via a polar transition state, which is more favorable than olefin formation. In contrast, the thermolyses of 14c in the presence of LiBr and LiBPh4 gave oxirane 29 with inversion of configuration and the olefin 30, respectively. The formation of 29 and 30 is considered to proceed via an anti-betaine-type intermediate and hexacoordinate 1,2-oxastibetanide 36, respectively. Selective formation of 28, 29, and 30 in the thermolyses of 14c, which is regarded as an intermediate in the reaction of an alpha-phenyl-substituted stibonium ylide with a carbonyl compound, showed that the change of the reaction conditions controls the reactivity of a 1,2-oxastibetane compound.