Silacyclopropane

Gordon, M.S. The isomers of silacyclopropane Chem. Phys. Lett. 76 163-168, 1980. 

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... 

The coordination compound 76 was stable enough for isolation and recording of its NMR spectra, from which a rigid silacyclopropane structure could be deduced. The mechanism of complex formation has also been investigated in detail by matrix techniques. 

Hydrogen bonding between an SiOH group and fluorine occurs in the platinum complex 44 [prepared by hydrolysis of a bis(alkylidene)-silacyclopropane] in which the SbF6 anion hydrogen bonds to the platinum-containing cation (O - F distance 2.77(2) A) (249). The platinum complex 45 is not reported (250) to form hydrogen bonds, and a more recent study (210) has confirmed that there do not appear to be any OH OH or OH tt interactions. 

Novel thermal and metal-catalyzed di-tert-butylsilylene 161 transfer reactions have been reported by Woerpel < / /.308-312 The transfer reactions required the inital preparation of cyclohexene-derived silacyclopropanes 169-171, which has been achieved by trapping of di-fert-butylsilylenoid, generated from /-Bu2SiCl2 and lithium, with cyclohexenes (Scheme 26).305 It is noteworthy that these reactions occur with remarkably high diastereoselectivities when 2-substituted cyclohexenes are used. The silacyclopropanation of 169 with functionalized cyclopentenes under thermal conditions (115°C) has provided /razy-silacyclopropanes, such as 172, with diastereoselectivities up to 96 4, whereas no silacyclopropanes were obtained from the direct reaction of the same cyclopentenes with /-Bu2SiCl2 in the presence of lithium (Scheme 26).308... 

The silacyclopropanation of acyclic and cyclic alkenes with 169, catalyzed by AgOTf, occur at room temperature or even below to yield new cyclosilapropanes 173-177. In the case of chiral /3-pinene, the silacyclopropanation occurs enantioselectively (dr > 95 5) (Scheme 26).312 Mechanistic studies have been undertaken, which suggest that silyl silver complexes play an important role in the catalytic cycle of the silylene transfer.310... 

In 1964, silacyclopropanes were suggested as intermediates in the dehalogenation of chloromethylsilanes using Na/K vapor. The ultimate product, dimethylvinylsilane, also results from dimethylsilylene and ethylene, providing further support to the silirane as intermediate (Scheme 1) (64JA1442). 

However, the only evidence that the intermediate is a silacyclopropane comes from the photolytic generation of phenylmethylsilylene in the presence of 2,3-dimethylbuta-1,3-diene. Methanol gives (8), indicating a 1,4-addition to a vinylsilacyclopropane (Scheme 19) (75JOM(86)C23). Also the addition of dimethylsilylene to 1,3-cyclohexadiene gives 7-silanor-bornene (9) and 3,3-dimethyl-3-sila-l,4,6-heptatriene (10), both of which can be formed... 

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... 

The suggestion that a silacyclopropane intermediate is formed on photolysis of styryl-disilanes (Scheme 46) (76JA7424) prompts the use of disilanylalkynes as precursors to silirenes. The silirene (36) is formed by a 1,2-silyl migration as the major product using (42), but the silapropa-1,2-diene (43) also results. Both products can be trapped as methanol or acetone derivatives (Scheme 47) (77JA3879, 79JOM( 179)377). 

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). 

The extrusion of silylenes by pyrolyzing silacyclopropanes provides one of their more interesting reactions. Silacyclobutanes generate equally novel pyrolysis products. 

The germacyclobutane will behave like the silacyclopropane in eliminating a carbene equivalent. The germylene so formed then reinserts into the germacyclobutane, giving the 1,2-digermacyclopentane (60 Scheme 88) (70DOK(194)1096). 

Silylenes were shown for the first time by Seyferth and Annarelli67 to be generated in the course of a thermally induced [l+2]-cycloreversion of silacyclopropanes. This methodology was used to generate transient dimeth-... 

The classical open /l-silylethyl cation 9 and /f-silylvinyl cation 11 are no minima at higher level of theory3,4. They collapse to the bridged protonated silacyclopropane 21 and silacyclopropene 22, respectively. On the basis of their calculated structures (Figure 3) both cyclic molecules are best described as -complexes between a silylium ion and ethene or acetylene, respectively. 

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