Cyclodisiloxanes

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

West and coworkers28 used /(SiSi) to discuss the structure of long-debated 1,3- cyclo-disiloxanes (12a) for which a few alternative structures were proposed. One such structure retains the a bond between two silicon atoms (12b). In a structure such 12a as 12b, the /(SiSi) should be close to the standard values of 80-90 Hz while for 12a 2/(Si—O—Si) will be observed with values of about 4 ppm. The values observed for a variety of 1,1,3,3-tetraaryl-l,3-cyclodisiloxanes are in the range of 3.85-4.02 Hz, which support structure 12a. There is another proposal for the structure of 1,3-cyclodisiloxanes (dibridged ir complex) and the authors suggested this structure is also consistent with the observed coupling constants. 

This result establishes that both oxidation of 52a and rearrangement of 53a to 1,3-cyclodisiloxane 55a take place with retention of configuration at silicon. Disiladioxethane 53a forms monoclinic crystals with space group C2/c. Extra electron density found above... 

Disilenes. Disilenes (3) can be prepared from l,2-dichloro-l,l,2,2-tetrasub-stituted disilanes (2) by reaction with lithium naphthalenide. Most disilenes are unstable to air, but 3 is fairly stable. Surprisingly, it is oxygenated to give 4, rather than the expected 1,3-cyclodisiloxane (5). 

In an earlier review article Schubert suggested that the shortest nonbonding Si-H contact can be estimated as 2.0 A,12 with the inexplicit assumption that any distance above this value is nonbonding, whereas shorter contacts correspond to the presence of some kind of Si-H bond. Schubert s structural criterion has been widely used in the 1990s to identify silane a-complexes both in experimental and theoretical studies.13,14 This criterion was obtained by summing up half of the nonbonding Si-Si distance in 1,3-cyclodisiloxanes (2.3-2.4 A)31,32 with half of the H-H distance of 1.85 A, at which the H-H interaction is supposed to be weak.12 Therefore, this... 

The reaction of stable disilenes with oxygen in solution is much faster. It eventually yields the same 1,3-cyclodisiloxane, but it clearly is not the initial product. Two intermediates have been isolated, a 1,2-disiladioxetane and a disiloxirane. At least one more is probably present. In spite of this progress, the mechanism of the oxidation reaction has not yet been elucidated918,92. 

At — 78°C, some of the disiloxirane 35 is still formed, but the major product is a thermally unstable isomer of the 1,3-disiloxane, apparently either a 1,2-disiladioxetane, 36, or a disilene perepoxide, 37. At 0°C, this isomer rearranges to the 1,3-cyclodisiloxane 34. The 1,2-disiladioxetane structure 36 was considered more likely, and equation 31 appears probable for the reaction. Perhaps the perepoxide 37 is the primary product, undergoing two competing secondary reactions reduction to 35 with a second molecule of the disilene and rearrangement to 3691a. 

---