Silylenes oligosilanes

Corriu et al. have reported that the coupling reaction of 2-(iV,iV-dimethylaminomethyl)phenyllithium with (McvSi)vSiCI 53 affords 2-(iV,iV-dimethylaminomethyl)-l-[tris(trimethylsilyl)silyl]benzene 894. No evidence has been found that the intramolecular iV-ligand coordinates to the silicon atom of 894. Upon UV irradiation, the trisilane forms a transient silyene 895, which has been trapped with 2,3-dimethyl-2,3-butadiene and triethylsilane to give the oligosilanes 896 and 897 as well as 898-900, (Scheme 126).859 Apparently, the bulk on the two ligands is insufficient to provide kinetic stabilization of the silylene intermediate 895. 

GENERATION OF SILYLENES BY THERMALLY INDUCED -ELIMINATION AND PHOTOEXTRUSION FROM OLIGOSILANES... 

By far more is known about the UV spectra of silylene-Lewis base complexes in matrices as well as in solution. To observe silylene-Lewis base complexes in matrices, an appropriate silylene precursor, in most cases a cyclic or acyclic oligosilane, is photolyzed at low temperature in an inert hydrocarbon or noble gas matrix, which is doped with small amounts (2-5%) of the respective Lewis base. In a few cases, 2-methyltetrahydrofuran (2-MeTHF), which serves as matrix material as well as a Lewis base, is used. When a rigid matrix such as a 3-methylpentane (3-MP) or 2-MeTHF matrix is used, the formation of the silylene complexes normally requires the matrix to be annealed in order to allow the molecules to diffuse through the softening matrix. In contrast, when the silylene is generated in an a priori soft matrix,... 

It is well established in organosilicon chemistry that oligosilanes extrude silylenes on irradiation.9 2440 The most frequently used precursor for this... 

As mentioned above, the photolysis of cyclopolysilanes was the earliest method for the generation of silylenes in matrices. Indeed, such extrusions from oligosilanes were used... 

Much attention has been focused on the detailed mechanism of silylene extrusion from oligosilanes. The following three possible pathways (A-C, equations 4-7) should be first taken into account. 

Recently, Tamao and coworkers have found that palladium catalyzed the skeletal rearrangement of alkoxy(oligosilanes) (equation 56), presumably via formation of a silyloxy base-stabilized silylene complex (106) (equation 57)256. 

The growth of interest in poly(silylene)s is indicated in Figure 5.1, which shows the number of literature publications on polysilanes (and oligosilanes) plotted versus time. Note that few publications appeared before 1980, but during the past 20 years scientific activity in this field has increased dramatically. The interest in polysilanes stems partly from their novel constitution and behavior and partly from their potential utility. The possibility that soluble polysilanes might be precursors to silicon carbide was evident when they were first synthesized, but other technological uses were not so obvious and arose only in the course of research. 

Photolysis of bulky permethylated Fp complexes such as FpSi[Si(CH3)3]3 does not cause deoligomerization, possibly because stable intermediate iron-silyl(silylene) complexes are not formed (27). Other less bulky transi-tion-metal-oligosilane complexes are also unreactive under the photolysis conditions. For example, the ruthenium analogues of the iron complexes, [( ri -C5H5)Ru(CO)2-Si ], are essentially photostable (23). Whether this behavior is due to the strength of the Ru-CO bond or to the enhanced stability of the Si-Si bond is not clear, and this problem is currently under investigation. 

Complicated reactions occur in the absence of trapping agents. Many signals can be observed besides those of aminosilanes in the Si spectrum, possibly oligosilanes by insertion of silylenes into Si-N bonds [8]. More investigations are necessary. 

The formal substitution of a saturated carbon atom in compounds 8, 13 and 16 by silicon results in precursors 21, 17 and 20, which could all be synthesized. In contrast to the results above, pulsed flash pyrolysis of these oligosilanes gave rise to the formation of only one C2H4Si2 isomer, namely 18. Actually, irradiation of matrix-isolated 2-silylsilacyclopropenylidene (18) led to silylene 19 in analogy to reaction 14—>15. 

We would like to suggest, that an attack of a chlorine takes place first at one of the silicon atoms of the tri- or tetrasilane under discussion to form a pentacoordinated silicon. In a second step, a silylene is generated which inserts into a silicon-chlorine bond of the disilane while the silicon skeleton of the oligosilane is shortened by one silicon atom. The source of the attacking chlorine is not yet known An intramolecular reaction with a chlorine of a chloromethylsilyl-sidechain and the central silicon might occur, or remaining traces of the aluminium trichloride, used for the chlorination, might induce the reaction. This is possible since only catalytic amounts would be necessary to cause the decomposition. 

In Sect. 2.3, generation of silylene complexes of transition metals was discussed on the basis of the reactivity of disilanyl-transition-metal complexes. The formation of silylene species in the presence of a catalytic amount of transition metals is also involved in the reactions of hydrodisilanes, which may readily form disilanyl complexes through oxidative addition of the Si-H bond prior to the activation of the Si-Si bond. Platinum-catalyzed disproportionation of hydrodisilanes affords a mixture of oligosilanes 89 up to hexasilane (Eq.45) [83]. The involvement of silylene-platinum intermediate was proven by the formation of a l,4-disila-2,5-cyclohexadiene derivative in the reaction of the hydrodisilane in the presence of diphenylacetylene. 

Similar generation of palladium-silylene complexes may be involved in reaction of oligosilanes with aryl isonitriles giving four-membered rearranged products 95, although the mechanism has not yet been clarified (Eq.49) [81,86]. 

Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review. Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc and the photoinduced electron-transfer reactions involving organosilicon compounds. ... 

The photochemical breakdown of polysilanes is of current interest in view of the importance of these compounds as photoresists. Three pathways have been identified in the photodecomposition of permethyl oligosilanes, namely, silicon-silicon bond cleavage, chain abridgement by elimination of a silylene (a silanediyl), and chain scission with formation of a silylene. The persistent radicals observed on irradiation of poly(di-n-alkyl-... 

Another interesting ring-contraction method is the photochemical extrusion of dimethyl-silylene from cyclic oligosilanes. Ando et al. used this method to cut out two Mc2Si units from dodecamethylhexasilacyclooctyne to yield octamethyltetrasilacyclohexyne (Scheme 8-9) [32, 33]. 

Summary 1,2-Dichlorodisilanes are prepared by reaction of silyl metal compounds with dichlorosilanes and subsequent chlorination. Alternatively, 1,2-dichlorodisilanes are obtained by insertion of a silylene in the Si-Cl bond of dichlorosilanes. Dehalogenation of 1,2-dichlorodisilanes with magnesium leads to cyclic oligosilanes or vicinally dimetallated species. Ab initio calculations show that coordinated disilenes are plausible intermediates in some of these reactions. 

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