Silacyclopropane ring

Diazomethane ring expands the silacyclopropane ring to the silacyclobutane,... 

Silacyclopropane, 1,1 -difluorotetramethyl-decomposition, 1, 587 Silacyclopropanes, I, 575-581 ring expansion, I, 587 Silacyclopropenes, I, 583-587 minimum energy geometry, I, 587 ring expansion, 1, 587 7-Siladispiro[2.0.2.1 jheptane synthesis, I, 576... 

Silylene extrusion from siliranes in the presence of alkynes, notably bis(trimethyl-siiyl)acetylene, gives the silirene (35) in good yield (Scheme 41) (76JA6382). Compound (35) is more stable thermally than hexamethylsilirane and shows 2 Si NMR absorptions for the ring atom at 5 = 106.2 p.p.m., some 50 p.p.m. downfieid from those of silacyclopropanes, and about 100 p.p.m. downfieid from normal cyclic and acyclic tetraalkylsilanes. Notable reactions include alcoholysis and the insertion of aldehydes and ketones, dimethylsilylene... 

Though silacyclopropanes readily insert sulfur (Scheme 8), silacyclobutanes are not reported to react. Insertion does occur in the germacyclobutane ring on heating, however, to give the germatetrahydrothiophene. Selenium behaves similarly (Scheme 78) (68JOMI 12)143). 

Ando and coworkers found that the silylene Dip2Si 382 (Dip = 2,6-di-/-propylphenyl), formed on photolysis of the trisilane 382 in the presence of Cgo, gave an adduct assigned the structure of the silacyclopropane 384 which evidently arose from addition of the silylene across the C=C between two six-membered rings of the fullerene198. A segment of the structure of 384 is shown in equation 47. 

Silver is often used as a halophile. In the context of six-electron species, the role of silver atoms in carbene, nitrene, and silylene transfer reactions, including aziridination, CH insertion, ring expansion, and silacyclopropanation, has been reviewed.9... 

While silacyclopentanes and sitacyclohexanes do not appear to react in this way, it seems likely that the very strained three-membered ring in silacyclopropanes will. 

The relative reactivities of various silacyclopropanes were compared to gain insight into the reversibility of the reaction (Scheme 7.16).83 Woerpel and coworkers observed that the reactivity of the silacyclopropane toward the silver catalyst depended on the ring size, and that cyclohexene silacyclopropane was the most reactive. Notably, benzyl-substituted silacyclopropane 63d did not react when exposed to the silver complex. This lack of reactivity was interpreted as evidence in support of the irreversibility of silylene transfer to monosubstituted olefins. 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

Silacyclobutanes as well as silacyclopropanes undergo aldehyde insertion under catalysis by /-BuOK (Equation (77)).292 The reaction of silacyclobutanes with lithium carbenoids such as dihalomethyllithium and oxiranyllithium gives 2-substituted silacyclopentanes (Equation (78)). Treatment of l-(l-iodoalkyl)- and 1-oxiranyl-silacyclobutanes with a stoichiometric amount of an alkali alkoxide leads to silacyclopentanes by anionic 1,2-shift of the ring carbon adjacent to silicon. These ring-expansion reactions proceed probably via a pentacoordinate silane intermediate. 

Silver compounds are versatile catalysts for various cycloaddition reactions, including [2 + 1]-, [2 + 2]-, [3 + 2]-, and [4 + 2]-cycloadditions. An example for the silver-catalyzed formation of three-membered rings by [2+ l]-cycloaddi-tion is the silacyclopropanation reaction of mono- and disubstituted alkenes by silylene transfer from the cyclohexene silacyclopropane 432 that was reported recently by Woerpel et /.355,355a (Scheme 127). The reaction tolerates a number of functionalities in the substrate (OBn, OSiR3, BuTlC, etc.,) and is stereospecific with regard to the cisjtrans... 

Shimizu et al. claim that formation of the 1,3-disilacyclohexane ring 27 may be understood in terms of a silacyclopropane intermediate 26. The carbanion 25 formed by treatment of 24 with a base should generate silacyclopropane 26 by an intramolecular alkylation reaction. Since 26 should be highly reactive due to the strain energy, it should immediately be attacked at the silicon atom by another nucleophile 25 (Scheme 4) <1996CL1083, 1999ICA231>. 

A silacyclopropane 69 was proposed as an intermediate in one of two possible pathways involving the Grignard reaction of bis(chloromethyl)dimethylsilane with trimethylchlorosilane in THF. The intermediate was obtained from the intramolecular cyclization of GlCH2Me2SiCH2MgCl, which undergoes a ring-opening polymerization by nucleophilic attack (Scheme 17) <19998KC422>. 

Photolysis of cyclotrisilane 72, in the presence of five-membered ring compounds, produces l,l-di- /t-butyl-2,2-dimethyl-l-silacyclopropene 37 when reacted with cyclopentadiene (Equation 16), o o-3,3,6,6,7,7-hexa-/i t/-butyl-3,6,7-trisilacyclo-8-oxatricyclo[3.2.1.0 ]octane 38 and l,l-di-fem-butyl-2,2-dimethyl-l-silacyclopropane 39 when reacted with furan (Equation 17), and 2,2,6,6-tetra-/i m-butyl-2,6-disilabicyclo[3.1.01]hex-3-ene 40 when reacted with thiophene (Equation 18), among other products <19950M5695>. These products were characterized by H, and Si NMR, and for the furan derivative 38, also by X-ray diffraction studies. 

SYNTHESIS OF THREE-MEMBERED RINGS WITH ONE SILICON ATOM 1.09.6.1 Silacyclopropanes... 

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