Silols reactions

Treatment of the mixture with methylmagnesium bromide regenerates the starting silole. Reaction of two equivalents of bromine with l,l-dimethyl-2,5-diphenylsilole led to tetrabromosilacyclopentane from which a double /1-elimination afforded ( , )-l,4-dibromo-l,4-diphenylbutadiene (equation 48). 

Thermal cracking at 150°C of the dimer (mixed isomers) of the silole in presence of reactive dienophiles (maleic anhydride, tetracyanoethylene or dimethyl acetylenedicarboxylate) inevitably produced violent explosions arising from exothermic Diels-Alder reactions. 

Reaction of 1-boraadamantane with di-alkynylsilicon and -tin compounds <2001CEJ775> is accompanied by intramolecular 1,1 -vinylboration giving rise to siloles or stannoles 87 and permitting the enlargement of the 1-boraadamantane system by two carbon atoms (Scheme 39, pathway a). 

The reaction of the tetrayne pentasilane with triallylboranes furnishes a mixture of silole 121 and compound 122 in a 1 5 ratio (Scheme 51). An excess of AII3B did not induce further allylboration with the remaining C C bonds even upon prolonged heating <2002JOM(649)232>. 

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

Disubstituted silole derivatives are synthesized by the palladium-catalyzed reaction of (trialkylstannyl)di-methylsilane with terminal alkynes (Equation (107)).266 The mechanism is supposed to involve a palladium silylene complex, which is generated via /3-hydride elimination from LJ3d(SiMe2H)(SnBu3) (Scheme 62). Successive incorporation of two alkyne molecules into the complex followed by reductive elimination gives rise to the silole products. 

The l,l-bis(diethylamino) silols 304 and 305 are excellent precursors for the synthesis of other 1,1-difunctionalized silols (Scheme 43).374 The reaction of 304 and 305 with EtOH/AlCl3 and HC1 produces the silols 306 and 307 containing ethoxy groups or chlorine atoms attached to the silicon. The 1,1-dichloro silols 308 and 309 have been converted into the corresponding 1,1-difluoro silols 310 and 311 with ZnF2. Hydrolysis of 309 and 311 yields the fluorosilanol 312 and the silanediol 313, respectively (Scheme 43). 

Table 3 lists PAEs containing Si, Fe, and B. Silole containing Si in the five-mem-bered ring [157] has a low transition energy because of its low LUMO level [158], and its homopolymers have been prepared [159, 160]. 2,5-Dili-thiosiloles, which can be prepared by the ring-closing reaction of diethynyl-silane, serve as starting materials for various 2,5-disubstituted siloles such as 2,5-dibromosiloles and 2,5-di(trialkylstannyl)siloles [161,162]. PAEs with the silole ring have been reported as shown in Nos. 1-4 in Table 3. PAE-type polymers with Si atoms in the main chain have also been prepared (Nos. 12-16), and their optical properties including photoconductivity have been revealed [155,156].

Variously substituted siloles of type 129 could be synthesized by Tamao and coworkers by the reaction of l,4-diaryl-l,4-dilithio-l,3-butadienes of type 128 with chlorosilanes . The l,l -spirobisilole 130 was accessible by reaction of the diphenyl snbstitnted dilithium compound 128a with tetramethoxysilane (Scheme 47). All of the dilithium compounds 128a-e were obtained by reaction of 2,5-diaryltellurophenes 127a-e with i-butyllithium in diethyl ether. 

Compounds containing the silicon-carbon double bond have been prepared (Scheme 85) and derivatives of silabenzene and germabenzene have been characterized (Section 1.20.13). Methyllithium and n -butyllithium will alkylate the silole (126) showing the ring silicon to be the site for nucleophilic attack. The reaction is quantitative and no silole anion (127) was detected (Scheme 205) (8lJOM(2l8)C2l). 

The reaction of silole 44 with KH in THF or DME yields, after work-up with D2O, quantitatively the corresponding deuteriated silole (equation 52) metalated siloles have been suggested as intermediates80. In contrast, the analogous treatment of silole 45 with KH yields a mixture of three NMR spectroscopically characterized potassium compounds (equation 53). The main product of this reaction is the pentavalent silicate 46, which results from the nucleophilic attack of a hydride at the silicon center109. The other two products result from hydride addition to one of the ring carbons. 

The reductive dehalogenation of permethylated 1,1-dibromosilole 51 and the subsequent reactions were used by Tilley and coworkers to synthesize the metalated siloles... 

The first experimental evidence for the existence of a dimetalated silole 56 was obtained by Joo and coworkers from treatment of 1,1-dichlorosilole 55 with sodium in dioxane114. A red solid was isolated from this reaction, which yielded upon addition of electrophiles such as Me2HSiCl or Mc SiCl the corresponding 57. Upon treatment with t-BuCl or Me3SnCl, coupling products 58 were obtained (Scheme 14). 

Trapping of an intermediate silylene by reaction with diphenylacetylene to form a pseudo-pentacoordinate silole (209) was reported243. The silylene was obtained by a Ni(0)-catalyzed degradation of 92f (X = SiPh3, Y = Me, Z = H)243. 

The mechanistic study of this reaction has also shown that in the case of a substituted allyl group an exocyclic [l,3]-silatropic rearrangement is in competition (.Ea = 173-176 kJmol-1) with the endocyclic retroene reaction6. Thus the yield of silole reaches at most about 40%. However, this method can be used for the synthesis of C-methylated siloles having a Si—H bond (see Section II.B.2.e). 

In 1974, Caspar and coworkers suggested that 2 is formed in a gas-phase reaction of silicon atoms, resulting from the 31P (n,p) 31Si nuclear transformation, with 1,3-butadiene16. They proposed a mechanism in which the silylene 9, l-silacyclopent-3-ene-1,1-diyl, rearranges to silole 2. 

Attempts to isolate the silole monomer 2 or to trap it by reaction with maleic anhydride or perfluoro-2-butyne have failed. This is surprising given the successful trapping of 1-methylsilole5 and 1,1-dimethylsilole10. The authors conclude that silole 2 is not very reactive in Diels-Alder cycloadditions, except toward self-reaction19. 

Depending on the alkynes, the yields are variable, but they are very high in the case of disubstituted symmetrical alkynes (R = R = Me, Et, -Bu). The reaction is sometimes not specific, giving also a 1,4-disilacyclohexa-2,5-diene. It was recently used by Ishikawa and coworkers to obtain a 3,4-diethynylsilole (29) (Scheme 8) which is a precursor of polymers containing silole rings43 (see Section IV.C). 

The reaction of a silirene with an alkyne in the presence of a palladium catalyst allows cyclization of two molecules of the alkyne with the silylene, as in equation 9 above. For example, Seyferth and coworkers have prepared the silole 33 in 80% yield from 1,1-dimethyl-2,3-bis (trimethylsilyl) silirene and phenylacetylene (equation 10)45. Without catalyst, this reaction yielded the silole 34 and the ene-yne 35, resulting respectively from ring expansion and cleavage by PhC=CH of the silirene. Under UV irradiation, 35 alone was formed. 

This method did not afford C-unsubstituted siloles, in particular in the case of the reaction of a silirene with ethyne45d. 

These three metalloles are the lower C-alkyl substituted Si—H bond-containing siloles which were isolated. The possible substitution of hydrogen bonded to silicon by another atom or a functional group appeared very attractive. Scheme 13 shows the functionalization reactions of 1,3,4-trimethylsilole (63) described by Dubac and coworkers75. 1 -Fluoro-1,3,4-trirncthylsilolc (66) has been identified spectroscopically and chemically,... 

Siloles and 3//-siloles were suggested as reaction intermediates in the FVP of 1-allylsilacyclopentenes5 6 (see Section n.A.l, Scheme 1), on the basis of trapping experiments. They were spectrometrically detected during vacuum pyrolysis of other silacyclopentenes19 20 (see Section II.A. 1, Scheme 4). A 2//-benzosilole has also been postulated in the case of FVP of a 2-allylsilaindane106. 

The reaction between a disilene (cis- and fra s-MePhSi=SiPhMe) and a silole (1,1-dimethyl-2,5-diphenylsilole) is a stereospecific Diels-Alder addition, as in the case of an ethylenic dienophile128. A digermene reacts in a similar way129. 

The action of acids on siloles results in the cleavage of the two endocyclic Si—C bonds. Boiling concentrated hydrochloric acid, hydrogen bromide or glacial acetic acid promote the reaction, and the substituted butadienes, in which the geometry of the parent metallole is retained, are produced in high yield34,155 (equation 55). 

The mechanism of the cleavage is probably the same for both silole and stannole, consisting of a two-step protodemetallation reaction (equation 56). The product of the... 

The alkaline cleavage of siloles (by ammonia157 or sodium hydroxide155,157) leads to the same substituted butadiene as that obtained with the acid reaction (equation 57). 

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