Silirens

Clark [15] reported relaxation of the strain in the three-membered rings of fluorosnbstitnted phosphireninm 17 and silirene 19 by the Jt-ct interaction in comparison with the componnds 18 and 7. 

Barton and co-workers" performed flash vacuum pyrolysis (FVP) on trimethyl-silylvinylmethylchlorosilane (30), resulting in the production of trimethylchlorosi-lane (30%), trimethylvinylsilane (11.5%), and most interestingly, ethynylmethyl-silane (34, 11.9%). A proposed mechanism for the synthesis of 34 (Scheme 10) begins with the lo.ss of trimethylchlorosilane to form silylene 31, which can rearrange either to silaallene 32 or to silirene 33, both of which can lead to the isolated ethynylsilane. 

The first structure reports were of germirene and stannirene in the 1980s following closely the first silirene. In a pair of germirene499 and stannirene500 structures, the... 

In contrast to the somewhat complicated thermal behavior of siUranes, photolysis of silirene (4) readily leads to the loss of dimethylsilylene (Scheme 14.5... 

Evidence for the generation of a silylene was obtained in the presence of triethylsilane, which afforded 2,3-diphenyltetrasilane (82) in 79% yield. The reactions of some 1,3-diynes, R C=CC=CR (83), with silylenes afforded the silylene adduct. The course of this reaction strongly depends on the nature of the substituents R and R . When R is an alkyl group, the bis(silirene) (84) is formed initially but undergoes rearrangement to a bicyclohexadiene derivative upon longer photolysis (Eq. 9)... 

Subsequent evidence from the thermolysis of the silirene (29), which gave the two silins (30) and (31) as the major products even in the presence of another alkyne, tends to support a mechanism involving silirene dimerization (77JOM(l42)C45). Indeed, the presence, albeit in trace amounts, of pentaphenyltrimethylsilylbenzene (32) supports a silacyclopentadiene intermediate (Scheme 38), while the 1,2-disilacyclobutene (33) and diphenylacetylene give the disilin in only 1.2% yield (Scheme 39) (78JOM(162)C43). 

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... 

Irradiation of (38) with nitriles induces dimerization of the silirene with nitrile insertion to give the azadisilabicyclo[3.2.1]octa-3,6-diene (39), except with acrylonitrile, when the isoskeletal bicyclo[3.3.0]octa-3,6-diene (40) also results. It seems reasonable to suppose they form through [2+2] and [4 + 2] addition (Scheme 44) (78CC80). By way of contrast,... 

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). 

The photolysis of tris(trimethylsilyl)phenylsilane in the presence of a series of alkynes alforded the silacyclopropene through silylene addition to the triple bond. Those obtained from monosubstituted alkynes underwent photochemical isomerization to the disilanyl-alkyne through a 1,2-hydrogen shift (Scheme 48) (80JOM(190)117). Disubstituted alkynes form silirenes that can be isolated by preparative GLC. 

Silirenes (140, equation 32) could also be involved in the transition-metal catalyzed decomposition of bis(diazoketones) 139 which provides the electron-rich [4]radialenes 14266,67. While the formation of 142 directly from silirene 140 cannot be excluded a priori, it is more reasonable to assume that 140 undergoes twofold ring-expansion to form the cyclic cumulene 141, which then provides 142 by a cyclodimerization reaction. The intermediacy of 141 is corroborated by the isolation of the Diels-Alder product 14366. 

Compound 54 was also utilized as a photochemical source of dimethylsilylene to prepare the unusually substituted silirene 5634 as shown in equation 8. 

EtOH isopentane Et20) glasses at 77 K, four products 256, 257, 258 and 259 were obtained in overall >95% yield. The major product 256 was shown by isotopic labeling studies to have been derived from the silirene 260 formed by intramolecular coupling of the bis-carbene 261, and pathways for the formation of the other products were proposed. The photolysis of the bis-diazo compound 255 was studied in detail at 405 nm and it was found that the diazirine 262 was formed this on photolysis at 305 nm also gave 260 on the pathway to 256. The chemistry is shown in Scheme 46. 

A silirene may also generate a silylene. In the absence of catalyst, thermolysis of a silirene can give various products resulting from its decomposition into a silylene and an alkyne50. Thus siloles are formed together with 1,2-disilacyclobutanes, 1,4-disilacyclohexadienes and other products. With silirenes bearing bulky substituents on the ring carbon atoms, siloles become the major products (Scheme 9). 

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. 

Seyferth and coworkers observed an interesting insertion reaction of a silirene into the C=C bond of benzyne affording a 1,1-dimethyl-1-silaindene derivative (99) (equation 41)45c. 

Oxasiletene, the unsaturated analogue of oxasiletane, was first postulated by Seyferth and coworkers as a reactive intermediate in the reaction of l,l-dimethyl-2,3-bis(trimethylsilyl)-l-silirene with dimethyl sulfoxide25. The photochemical generation of siladienone intermediate 28 from (pentamethyldisilanyl)diazomethyl 1-adamantyl ketone... 

The structure of 91, a new bisaikylidenesilirane (sila[3]radialene) system, was confirmed by X-ray structure analysis (Figure 3c). Because of the high kinetic stability of 91 due to steric protection, aldehydes such as benzaldehyde as well as ketones did not react. However, [2 + 4] cycloaddition chemistry of 91 with 4-methyl-l,2,4-triazoline-3,5-dione resulted in the formation of a new ring-fused silirene (97) (equation 26). 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

Rearrangements of siliranes to alkylsilylenes by ring-opening w-elimination from silicon were discussed in Section II.D (see equation 34). Similar mechanisms have been written for rearrangements of silirenes to alkenylsilylenes5. 

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