Disilaoxiranes

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... 

The lone pairs on the oxygen atom in disiloxane, disilaoxirane, and 1,3 -cyclodis-iloxane have been shown [131] by the bond model analysis [132-134] to delocalize significantly to the silicon atoms throngh the interaction of the n-orbital... 

Reactions of 1 with epoxides involve some cycloaddition products, and thus will be treated here. Such reactions are quite complicated and have been studied in some depth.84,92 With cyclohexene oxide, 1 yields the disilaoxirane 48, cyclohexene, and the silyl enol ether 56 (Eq. 29). With ( )- and (Z)-stilbene oxides (Eq. 30) the products include 48, ( > and (Z)-stilbenes, the E- and Z-isomers of silyl enol ether 57, and only one (trans) stereoisomer of the five-membered ring compound 58. The products have been rationalized in terms of the mechanism detailed in Scheme 14, involving a ring-opened zwitterionic intermediate, allowing for carbon-carbon bond rotation and the observed stereochemistry. 

The early stages in the oxidation of disilene have been treated theoretically for the parent molecule H2Si=SiH2.95 The first intermediate along the reaction coordinate is the open-chain trans diradical 64 (Scheme 16), which is in equilibrium with a gauche form, 65. From the latter, closure to the 1,2-dioxetane 66 would probably be rapid. The open-chain form can react with a second molecule of disilene to give the diradical 67, which could collapse into two molecules of the disilaoxirane 68. If similar steps are followed in the oxidation of 3, they must be quite rapid, since the relative configuration at the silicon atoms is maintained in both products, 59a and 61a.93... 

Disilenes have much lower oxidation potentials than olefins , and consequently they are much more reactive toward 02. Typically, disilenes 93 react in solution with triplet oxygen to give 1,2-disiladioxetanes 94 as the major product, accompanied at room temperature by a smaller amount of disilaoxirane 95 (equation 93) °. ... 

The photochemistry of the above oxatrisilacyclobutane 200 (R = CH2CMe3) was also investigated. Photolysis resulted in the extrusion of a silylene and the formation of a diradical 202, which in the presence of cyclohexane afforded the dihydride 203 or in its absence cyclized to the disilaoxirane 204, as shown in Scheme 35. If the photolysis was done in ethanol, the silylene was trapped, and ring opening of the oxirane occurred at... 

Photolysis of cyclosilane rings containing heteroatoms also leads to silylenes, and sometimes this is the preferred route. An example is shown in equation 1444. Likewise, hexaneopentyl trisilaoxetane undergoes photolysis to give the silylene and a disilaoxirane... 

The general profile of the dioxygen oxidation of tetraaryldisilenes is shown in Eq. (63).7 Both in the solid state and in solution, the initial oxidation product of a tetraaryldisilene is the corresponding 1,2-disiladioxetane 145, whose intramolecular isomerization gives the thermodynamically more stable 1,3-disiladioxetane 146. All the steps of the oxidation occur intramolecularly and with the retention of stereochemistry around the Si-Si bond. While a small amount of disilaoxirane 147 is produced in the oxidation in low-temperature solution, 147 is converted to 146 smoothly in the presence of excess oxygen. 

The disilaoxiranes 6282,83 and 63s4 are readily accessible from the disilenes 9 and 13 respectively, by reaction with N2O or wctrt-chloroperbenzoic acid (m-CPBA). The absence of a reaction between N2O and the highly shielded disilene 13 suggests that the formation of 62 probably involves a [2 + 3] addition of N2O to the Si=Si bond with subsequent elimination of nitrogen (equation 10). 

Indirect evidence for the validity of this assumption is provided by the structure [rf(Si-Si)=2.229(l) A, 359.9°], and especially the 1/si,Si coupling constant of the disilaoxirane 81 derived from the unsymmetrically substituted disilene 2996. With a Jsi,Si value of 123 Hz, this coupling constant is appreciably larger than those observed for other disilanes with a similar substitution pattern (85 Hz) and approaches the value of 160 Hz for the disilene 29. 

At lower temperature in solution, the aerial oxidation of disilenes initially furnishes the 3,4-disiladioxetanes 119 together with small amounts of the disilaoxiranes 120, both of which react further in the solid state and in solution to give the cyclodisiloxanes 121 (Scheme 4). 

In a series of papers West and coworkers described the reactions of disilene 9 with several epoxides to give the disilyl enol ethers 125, the five-membered ring compounds 126 — the formal insertion products, as well as the products of epoxide deoxygenation, namely alkenes and the disilaoxirane 62 (equation 29)80,83,124 125. 

In conclusion, the anomeric effect at silicon, although smaller than for carbon, is significant especially for oxygen and it probably plays an important role in determining the preferred conformations of relevant silicon compounds such as disilaoxiranes. 

Since the isolation of the first molecular compound with an Si-Si double bond in 1981 [1] the chemistry of the disilenes has experienced an almost explosive development, as is reflected in numerous review articles [2]. Addition or cycloaddition reactions to this double bond provide an entry to a series of three- and four-membered ring compounds that are hardly accessible by other routes. Of particular interest among these systems are the disilaoxiranes and disiladioxetanes, both of which open up new questions about the bonding situation in small rings made up of main group elements. 

Scheme 4. Reaction of the disilene 4 with atmospheric oxygen or with m-chloroperbenzoic acid (m-CPBA) yields the 2,4-disiladioxetane 7 or the disilaoxirane 8, respectively.

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