THE THERMOLYSIS OF EPSILON-HALODISILANES - A PREFERENCE FOR 1,2-SI SI-]O REARRANGEMENT OR SI-O CLEAVAGE OVER SI=O BOND FORMATION

Tris(trimethylsilyl)-2,2,2-trifluoroethoxysilane 6, tris(trimethylsily l)-2-fluoroethoxysilane 7, and tris(trimethylsilyl)2-chloroethoxysilan e 8 were synthesized and characterized by H-1, C-13 and Si-29 NMR, IR spectroscopy, and EI and CI mass spectrometry. Thermodynamic considera tions would suggest that, as a result of the driving force provided by the formation of a Si-F or Si-Cl bond, the thermolyses of these compo unds would lead to the formation of bis(trimethylsilyl)silanone 4. To examine this question, gas chromatography - mass spectrometry was as u sed a detection technique for products resulting from the high-pressur e thermolyses of 6-8. The elimination of (Me(3)Si)(3)SiCl appears to b e the major thermolytic pathway of decomposition for 8 at ambient or h igher pressures, although it is accompanied by the formation of other products, some of which could have arisen from the addition of various halosilanes to a silanone. Neither 6 nor 7 thermolyzed cleanly; the f ormer compound was essentially unreactive under the thermolysis temper atures used (850 degrees C). Of the products produced in the thermolys is of 7, no evidence for the formation of the silanone was obtained. I ndependently, mass spectrometry was used to study unimolecular reactio ns of molecular ions derived from 6-8. The major route to solitary ion s appears to involve a 1,2-trimethylsilyl migration from Si to O (9 -- > 10) prior to decomposition, for example, of the m/z 346 parent ion i n the decomposition of 6. The preparation of the ionized silanone may be a minor pathway. Some of the other fragmentation pathways for 6-8 a re discussed.