1,2-DISILADIOXETANES - STRUCTURE, REARRANGEMENT, AND REACTIVITY

The low-temperature reaction of (E)-1,2-dimesityl-1,2-di-tert-butyldisilene (1a) with dioxygen gives (E)-1,2-dimesityl-1,2-di-tert-butyldisiladioxetane (2a), the structure of which has been established by X-ray crystallographic analysis. This result establishes that both oxidation of la and rearrangement of 2a to 1,3-cyclodisiloxane 4a take place with retention of configuration at silicon. Disiladioxetane 2a forms monoclinic crystals, space group C2/c. Extra electron density found above and below the Si-Si bond was shown by H-1 and solid-state Si-29 NMR to be due to 4a, arising from rearrangement of 2a in the solid. X-ray crystal structures were determined for crystals that were 5.6%, 15.2%, and 20.3% rearranged. Cell dimensions for the 5.6% rearranged crystal are a = 12.549 (7) angstrom, b = 12.174 (7) angstrom, c = 17.133 (7) angstrom, b = 103.94 (4)-degrees, V = 2529 (2) angstrom3 (Z = 4). The rearrangement of 2a in solution is first order with E(a) = +21.7 +/- 1.0 kcal mol-1, DELTA-H(double dagger) = 21.1 +/- 1.4 kcal mol-1, and DELTA-S(double dagger) = -9.0 +/- 4.5 cal K-1 mol-1. Isotope labeling studies with O-18 showed the rearrangement to be intramolecular in the solid and in solution. In the presence of phosphines or sulfides, 2a is partially deoxygenated to form disilaoxirane 3a.