Thermolysis disilenes

R R M were prepared by reduction of the dibromides R R MBr2 with lithium naphthalenide (method A), by thermolysis of a disilene (method B), and the ligand exchange of divalent group 14 element species (method C). In all cases except for the synthesis of Tip2PbS4, exclusively tetrasulfides R R MS4... 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

Thermolysis of compound 29 at 150-180 °G in the absence of trapping agents presumably proceeds via the competitive cleavage of the Si-Si and C-C bonds, resulting in simultaneous transient formation of the silene 30 and the disilene 31, which undergo fast cycloaddition to produce trisilacyclobutane 34 (Equation 5) <20020M2049>. 

The 1,2,3-azadisiletidine 47 and 1,2,3-azadisiletine 48 are, in a formal sense, the products of a [2+2] cycloaddition reaction between nitriles and disilene (Scheme 20). It can be assumed that the latter is the crucial intermediate formed during the thermolysis of hexasubstituted cyclotrisilane <1995TL8187>. 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Keywords Disilenes / Silylenes / Silaheterocycles / Thermolysis / Isonitriles... 

Silanones, the silicon analogs of ketones, are produced via rDA reactions. One attempt to prepare a suitable DA precursor for retrograde decomposition to a silanone met with unexpected results. The desired DA reaction between 2,2-dimethyl-l-oxa-2-silacyclohexa-3,S-diene (251) and perfluoro-2-but-yne was complete in one day at room temperature. The observed product was o-bis(trifluoromethyl)ben-zene, as the initial adduct apparently underwent retrodiene decomposition to yield the intermediate dimethylsilanone (252) (equation 111). The occurrence of this retro ene process at room temperature was not consistent with the analogous extrusions of silenes and disilenes that require elevated temperatures. However, the reaction sequence was substantiated by comparison with its carbon analog in which tetramethylpyran and dimethyl acetylenedicarboxylate react at room temperature to afford only acetone and the corresponding phthalate. Stable adducts that extrude silanones are also known. Reactions of 2-sili yrans and nudeic anhydride provide stable adducts, such as (253), that decompose upon thermolysis... 

While many examples of carbene oxidations have been reported, only four papers on the reaction of silylenes 1 with molecular oxygen have been published. The limited number of experimental studies on the oxygenation of silylenes is mainly due to the lack of suitable precursors.The photolysis of matrix-isolated trisilanes produces silylenes in close proximity to disilenes or other products of the precursor decomposition rather than matrix-isolated silylenes.Gas-phase thermolysis of disilanes and other thermal precursors requires very high temperatures, while the photolysis of diazidosilanes requires short-wavelength UV irradiation. In all of these cases, the yields of silylenes are rather poor. 

Silylenes, the carbene equivalent of organosilicon componnds, has been another intriguing target. Okazaki and coworkers carried out thamolysis of the hindCTed disilenes mentioned above with the expectation that a silylene, Tbt(Mes)Si , will be formed. The thermolysis was monitored by means of Si NMR spectra, bnt the genraation of silylene was not proved by NMR, although trapping experiments established the genaation of silylene. ... 

Several methods are available for generating disilenes but photolysis of masked disilenes is most convenient for mechanistic studies. 7,8-Disilabicyclo[2.2.2]octa-2,5-dienes, the formal adducts of the addition of disilenes to benzene, naphthalene, anthracene and biphenyl, are well established to generate the corresponding reactive disilenes by either thermolysis or photolysis. The parent 7,7,8,8-tetramethyl-7,8-disilabicyclo[2.2.2]octa-2,5-diene (10) generates tetramethyldisilene (11) in an argon matrix by photolysis at 10 Tetramethyldisilene (11) was also observed at 344 nm in UV spectra by 3-methylpentane (3-MP) and EPA (ether isopentane ethanol = 5 5 2) matrices (equations 2 and 3, respectively). On annealing the EPA matrix, a product of addition of ethanol to 11,... 

Another interesting method to obtain cyclotetrasilanes is the [2 + 2]cycloaddition of disilenes. Depending on the nature of R groups disilenes either are stable or undergo a dimerization reaction to a cyclotetrasUane. Tetrakis(trimethylsilyl)disilene, which forms either by thermolysis of methoxytris(trimethylsilyl)silane [104], by treatment of l,2-dipotassiotetrakis(trimethylsilyl)disilane with BrCH2CH2Br [96] or by reaction of the respective fluoride adduct with MgBr2 [105], dimerizes to octakis (trimethylsilyl)cyclotetrasilane (Scheme 6). 

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