Cyclic silanes

Combination of these various methods has led to a vast number of derivatives in which H is progressively replaced by one or more monofunctional group such as F, Cl, Br, I, CN, R, Ar, OR, SH, SR, NH2, NR2, etc. The cyclic silanes SisHm and Si6Hi2 were prepared in the late 1970s via (SiPh) which were themselves the first known homocyclic silane derivatives (F. S. Kipping, 1921) ... 

Kawakami et al. have prepared optically active bifunctional l,3-dimethyl-l,3-diphenyldisiloxanes.158,159 Strohmann et al. have prepared enantiomerically enriched Si-centered silyllithium compounds, which react stereo-specifically with triorganochlorosilanes.160-162 In solution, slow racemization of the silyllithium compounds takes place, which, however, can be circumvented by transmetallation with MgBr2. Oestreich et al. prepared new Si-centered cyclic silanes adopting the strategies developed by Corriu and Sommer.163 Bienz et al. have developed enantioselective routes for the preparation of C-centered chiral allenylsilanes.156,164-166... 

Disilanes and cyclic silanes can be polymerized via a number of routes, depending on their substituents, and are described in Section 3.11.4. 

On the other hand, product studies of volatile materials for the reaction of silane 4 (no allylic hydrogen available) with thermally generated t-BuO radicals at 46 °C revealed the formation of cyclic silanes in a 46 % overall yield... 

Anionic ROP mechanisms have been established for cyclic siloxanes such as (Mc2SiO)3 and (Mc2SiO)4 (Scheme 8.3) and the four-membered cyclic silane... 

Scheme 8.4 Proposed mechanism for the ROP of the cyclic silane (MePhSi)4.

The names of compounds containing silicon atoms as heteromembers (with or without other heteromembers) but not classifiable as (linear or cyclic) silanes, silazanes, silthianes or siloxanes are derived with the aid of the oxa-aza convention. Examples ... 

The reaction of cyclic silane 23 with lithium metal in THF opens an unexpected way to c-(MenSi6)Li (24). Whereas in tri-, tetra- and penta-cyclosilanes a Si—Si bond is cleaved under these conditions (Section VLB.3), 23 reacts with cleavage of a exocyclic Si—C bond (equation 35)85. The reasons for the different behavior of 23 toward lithium remain unclear. It was, however, suggested that the facile formation of 24 is due to its thermodynamic stabilization by electron delocalization into the silicon framework85. 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic generation from a trisilane 280153, thermolytic or photolytic decomposition of cyclic silanes 28114,154,155 and degradation of diazidosilanes 282153,156. The photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies157. The photolysis of w-diazo compounds 283 is the only frequently used reaction path to silenes 284 via a carbene-silene rearrangement8. 

Cyclic peralkylsilanes exhibit unique behavior which distinguishes these compounds from saturated catenates of carbon. In some ways, the properties of the cyclosilanes resemble these of poly-unsaturated or aromatic hydrocarbons. As shown in Table 12, five-membered permethyl cyclic silane shows a higher oxidation potential compared with... 

The mechanism given in equation 47 has been proposed for this reaction. The initially formed cation radical reacts with molecular oxygen to generate an intermediate, which may couple with a neutral cyclic silane to form species A. The intermediate A decomposes to the final product B and its cation radical B+", which could also be generated by direct anodic oxidation of the siloxane B. A further oxygen insertion step could take place via intermediate C+. ... 

Gilman s significant observation that triphenylchlorosilane is a catalyst in Wurtz-like reactions forming cyclic silanes appears to be due to intermediate formation of triphenylsilyl lithium (see Chapter 9 Cyclic Silanes). The triphenyl compounds are particularly good catalysts because the corresponding alkali-triphenylsilyl compounds are the most stable among the alkali-organo-silicon compounds. 

Some inorganic cyclic silanes with halogen substituents have been investigated, thoug their properties are almost unknown, and even their existence is controversial. 

The formation of various cyclic silanes results from an intramolecular tp3 SiLi elimination ... 

Pt-catalyzed copolymerization of ferrocenylsilane and SCB, disilacyclobutane, or cyclic silane monomers can be accomplished to provide random copolymers (Scheme 17) <1996MAR319, 1995MM401>. 

Inspection of Table 14.4.1 shows thatE(C-C) > E(Si-Si),. E(C-H) > (Si-H) and E(C-C) > E(C-O), but (Si-Si) < < (Si-0). Thus alkanes are much more stable than silanes, and silanes react readily with oxygen to convert the Si-Si bonds to stronger Si-O bonds. Although silanes do not exist in nature, some compounds with Si-Si and Si=Si bonds have been synthesized in the absence of air and in non-aqueous solvents. The silanes Si H2 +2 (n = 1 — 8), cyclic silanes Si H2n (n = 5,6), and some polyhedral silanes are known. The structures of tetrahedral Si Si ButU, trigonal-prismatic Si6(2,6- PrC6H3)6, and cubane-like Sis(2,6-Et2C6H3)8 are shown below ... 

Optically active binaphthylsilacycloheptanes. Jung and Hogan1 have prepared the optically active binaphthylic cyclic silanes 1 and 2, in which the chirality... 

There is also a well-documented mechanism of nucleophile-catalyzed racemization of silane that does not involve an ionic intermediate. This mechanism operates in at least some peculiar classes of cyclic silanes, having particular ability to form complexes with nucleophiles. Stevenson and Martin (287) studied carefully the kinetics of the inversion of a bicyclic spirosilane A [Eq. (70)]. The reaction was catalyzed by a weak nucleophile, a substituted benzaldehyde. The mechanism consistent with... 

The highly reactive potassium undecamethylcyclohexasilane is a suitable starting material for the synthesis of several cyclic silanes. Potassium undecamethylcyclohexasilane 1 has been synthesized by two methods. Both methods have some disadvantages, either the use of dangerous, highly toxic solvents (HMPA) [la] or reagents (Hg) [lb] and/or sequence of steps with a low yield. On the basis of these facts we tried to develop a simple new way to synthesize potassium undecamethylcyclohexasilane. 

Synthesis of Group IV- Undecamethylcyclohexasilanederivatives 1 reacts with zirconocenedichloride at -78 °C to give a dark red colored product (mp 188-190 °C), which is the first group IV derivative of a cyclic silane. A second substitution step does not occur even in the presence of two equivalents of potassium undecamethylcyclohexasilane. 

The chemical shift data available for cyclic silanes (32,62-71) show definite trends with the size of the ring into which the silicon atom is incorporated. Compounds [1 ] [6] in Table IV show the deshielding... 

Blinka and West have used MM to assess the conformational characteristics of cyclic silanes.They calculated the heats of formation from MM2 and then compared them to known relative stabilities. They assumed that the difference between their MM results and experimental numbers is the energy associated with O conjugation, which is not included in the MM2 force field. In this manner, they were able to discern quantitative measurements of o conjugation. 

Degradation of Larger Silicon Catenates. Although it seems clear that polymeric disilane derivatives photodecompose by bond homolysis to produce silyl radicals, model studies on larger silicon catenates indicate that their photochemistry may be more complex (Scheme IV). Cyclic silane derivatives seem to extrude monomeric silylenes upon irradiation to produce smaller cyclic silanes (52). The proposed silylene intermediates have been identified spectroscopically 49, 53), and trapping adducts have been isolated in solution. Exhaustive irradiation ultimately results in acyclic silanes, which... 

From our experience that cyclic silanes are formed more likely when using monosilanes with bulky substituents, we tried to electrolyze (undecamethylcyclohexasllanyl)dichlorosilane. However, in this... 

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