Syntheses, Structures, and Thermolyses of Pentacoordinate 1,2-Oxastibetanes: Potential Intermediates in the Reactions of Stibonium Ylides with Carbonyl Compounds

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 °C
in o-xylene-d10, the thermolyses of 3-phenyl-1,2-oxastibetane 14c were carried out at 220 °C in o-xylene-d10 and at 140 °C in acetonitrile-d3 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 α-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.