2- silacyclopentanes

Diels-Alder reactions, I, 618 reactions, I, 617-620 reduction, I, 617 structure, I, 616 synthesis, I, 585, 586, 614-616 Silacyclopentane, 1-chloro-... 

Silacyclopentan-2-ol properties, I, 612 Silacyclopentan-3-ol properties, 1,612 Silacyclopent-3-ene, 1,1 -dichloro-NMR, I, 614 Silacyclopentenes... 

Silacyclopentanes (A) were the principal products in every case. The A/B ratio varied with R from a high of 22 when R2 = Ph2 to a low of 4 when R2 = Cl2 (see Table I). 

A cis/trans mixture of 1,2-dimethylgermacyclopentanes 56 and 57 was prepared as outlined in Scheme 1632. The isomeric germanes are stable and can be separated by spinning band distillation. Stereochemical assignments were made by comparison of the chemical shifts of the methyl carbons in the 13C NMR spectra with those of the analogous dimethylcyclopentanes and silacyclopentanes. 

Leighton has combined this concept of strained silacycles [62-66] with the asymmetric allylation chemistry in a series of pubhcations [60, 67-70], Leighton s aUyhc silacyclopentane 29 [67] (Scheme 20) works equally for allylation of aromatic and ahphatic aldehydes in the absence of additional Lewis bases (promoter activator) or Lewis acids with high yield and enantioselectivity. The mechanism of... 

Some chemical shifts for the known silacyclopentane-type structures are shown below as reference points concerning ring atoms and in part ( H) their immediate vicinity. The values are rounded to the first decimal place. 

Three or Four Heteroatoms including at least One Other Element Table 1 UV absorption maxima for silacyclopentanes. 

Silylene 132 showed absorption maxima at 260 and 440 nm in its UV-vis spectra, the latter of which is assignable to the (Si)-3p(Si) transition and close to those observed for dimethylsilylene (453 nm) and l-silacyclopentane-l,l-diyl (436 nm). Thus, the electronic structure of silylene 132 is not much per-... 

The intramolecular hydrosilation presents a convenient route to silacyclopentanes. It is catalyzed by chloroplatinic acid and gives only small quantities of the isomeric cyclohexane... 

The chemistry of silacyclopentanes resembles closely that of acyclic alkylsilanes. Strong bases are needed to cleave the silicon-carbon bond, while electrophilic attack occurs readily with halogens and strong acids, e.g. sulfuric. In addition, aluminum chloride will induce polymerization of silacyclopentanes unless one of the groups on silicon is carbofunctional (Scheme 154) (67MI12000). If this is chloromethyl, then ring expansion occurs (Scheme 155). 

Nucleophilic displacement on 1-chlorosilacyclopentanes is dominated by inversion, in contrast to silacyclobutanes (78JOM(154)203). For 2-oxa- and 2-thia-silacyclopentanes, however, both inversion and retention are observed. The stereochemistry is determined by the leaving group and the electronic nature of the nucleophile rather than ring strain... 

Silacyclopentanes can give inversion in some cases (Table XVII). 

Silacyclopentadiene, 1,1 -dimethyl-2,3,4,5-tetraphenyl-irradiation, 1, 618 structure, 1, 617 Silacyclopentadiene, 1-methyl-synthesis, 1, 615 Silacyclopentadienes Diels-Alder reactions, 1, 618 reactions, 1, 617-620 reduction, 1, 617 structure, 1, 616 synthesis, 1, 585, 586, 614-616 Silacyclopentane, 1-chloro-nucleophilic displacement, 1, 608 Silacyclopentanes, 1, 605-609 chemical properties, 1, 607-608 synthesis, 1, 605-607... 

Silacyclopentan-2-ol properties, 1, 612 Silacyclopentan-3-ol properties, 1, 612 Silacyclopent-3-ene, 1,1 -dichloro-NMR, 1, 614 Silacyclopentenes flash vacuum pyrolysis, 1, 615 synthesis, 1, 610-612 Silacyclopent-2-enes synthesis, 1, 610 Silacyclopent-3-enes properties, 1, 612 reactions... 

West (91), amplifying on these results, argued that since the solvolysis is bimolecular it must proceed either through a normal Sn2 bimolecular displacement or involve a rather stable pentacovalent intermediate. Both mechanisms. West believes, must involve a 5-coordinate transition state, and therefore may really be thought of as equivalent. West found that silacyclopentane was 13 times as reactive as diethylmethylsilane and 200 times as reactive as silacydohexane (which could be construed as evidence for I-strain in silacyclopentane). Since this order of reactivity is the same as that found in carbocyclic compounds, it was concluded that similar considerations of energy and entropy of reaction are encountered, a possibility that had also been advanced by Price. 

Carbenes 100, generated by UV irradiation of methyl (alkoxysilyl)diazoacetates 99, furnish l-oxa-2-silacyclopentanes 101 by 1,5-C,H insertion (equation 25)60. In order to suppress competing reaction pathways of the carbene intermediate (e.g. formation of a ketazine with excess diazo compound), the photolysis was carried out at high dilution, but, even then, yields were rather modest. Carbene insertion at CH2 seems to occur much more easily than at CH3 a preference of 3 0.6 1 for methylene insertion can be calculated from the isolated yields after correction for the number of C—H bonds. It should be noted that for both carbenes, 96 and 100, C,H insertion occurs only in the 1,5 mode whereas no 1,3-, 1,4- or 1,6-C,H insertion products could be detected. 

Hydrosilylation of divinyl ether has been applied for the synthesis of silacyclopentane 12 using Speier s catalyst (Scheme l)13. One of the two carbon-carbon double bonds was hydrosilylated first with a dialkyl(ethoxy)silane, giving silylethyl vinyl ether 10 in 53-59% yield, which was reduced with LiAlELr to hydrosilane 11. The intramolecular hydrosilylation of 11 affords silacyclopentane 12 in moderate yields (Scheme 1). The reaction with HSiEt2(OEt) gives 12a exclusively in 45% yield, while silacyclohexane 13b is formed as the minor product when HSiMe2(OEt) is used as the hydrosilane (12b/13b = 2.3/1 50% total yield)13. Other intramolecular hydrosilylation reactions useful in organic syntheses will be discussed in the section n.C. (vide infra). 

Remarkable dependence of the regioselectivity on the catalyst species employed is observed in the intramolecular hydrosilylation of allylic /V-dimethylsilylamines 63, i.e. the Rh-catalyzed reactions proceed via 5-endo cyclization to give l-aza-2-silacyclopentanes 64 whereas the Pt-catalyzed reactions yield l-aza-2-silacyclobutanes 65 via A-exo... 

The silanethiyl radical (7) generated from the silane thiol, which is prepared from the silane using first CuCl2 and then LiSH, abstracts hydrogen enantioselectively from silicon in ( )-fraws-2,5-dimethyl-l-phenyl-l-silacyclopentane, to bring about kinetic resolution of the latter. Such reactions are rare (equation 17)28. 

Similarly, ring expansion of fused ring siliranes also occurs stepwise with silabicyclo[3.1.0]hexane and silabicyclo[4.1.0]heptane (equation 34). With 1,1-dimethyl-1-silacyclobutane, sulphur insertion occurs to give the l-thia-2-silacyclopentane in 72% yield67. 

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