Trisilane, cyclic

The method employed for the first synthesis of stable disilene l,1 the photolysis of linear trisilanes, has been the most widely used synthetic method for disilenes. However, photolysis of cyclic trisilanes and dehalo-genation of dihalosilanes and 1,2-dihalodisilanes are also good routes in some cases (Table I). 

The first synthesis of a disilene by photolysis of the corresponding cyclic trisilane was reported by Masamune et al. in 1982.14 This method has been adapted for the synthesis of a variety of stable and marginally stable disilenes (Eq. 6).15-20 More recently, Kira et al. synthesized the first example of a stable tetrakis(trialkylsilyl)disilene 22 by this method.21... 

Correlation of the SiSi bond lengths in disilane derivatives X3Si-SiX3 with X = CH3, C(CH3)3 and Si(CH3)3 together with those of sterically overcrowded cyclic [7a] and linear [7b] trisilane derivatives versus their Pauling bond orders [5a,6b,7], expectedly, produces a linear regression (Fig. 4). 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic generation from a trisilane 280153, thermolytic or photolytic decomposition of cyclic silanes 28114,154,155 and degradation of diazidosilanes 282153,156. The photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies157. The photolysis of w-diazo compounds 283 is the only frequently used reaction path to silenes 284 via a carbene-silene rearrangement8. 

Laser flash photolysis of a cyclic trisilane led to three transient species observed by UV-visible spectroscopy.105 The shortest lived species was monitored at 530 nm and showed chemical behaviour consistent with diphenylsilylene. 

In review OW,7 only four types of synthetic methods are introduced for stable disilenes as shown in Scheme 1 (1) photolysis of linear trisilanes (method A), (2) photolysis of cyclic trisilanes (method B), (3) reductive dehalogenation of dihalosi-lanes (method C), and (4) reductive dehalogenation of 1,2-dihalodisilanes (method D). 

The proton NMR spectral data of organopolysilanes have often been published incidental to preparative studies (51, 54, 62, 74, 108, 119, 177, 187, 190). Only recently has a systematic investigation to determine the chemical shifts and coupling constants in linear and cyclic permethylated polysilanes, and to study the effects of substituents on the NMR properties of methyl derivatives of disilane and trisilane been reported (see Table V-VII) (206). 

An interesting reaction of disilane is the transition metal-catalyzed insertion of unsaturated hydrocarbons. The palladium-mediated reaction of cyclotrisilane 26 with phenylacetylene to afford the seven-membered carbosilane 51 (Equation 3) indicates that this general reaction scheme is also applicable to strained cyclic trisilanes <20040M490>. 

Photolysis of the acyclic trisilane 57 produces 1,2,4-trisilacyclopentanes 42 in moderate yields (67-79%) (Equation 4). Marker experiments suggest that the reaction proceeds via a bimetallic silylene bridged dimer, which collapses to give the cyclic carbosilane <20020M503>. 

Cyclic trisilane 323 upon steady-state photolysis (245 nm) was used for preparation of diphenylsilylene 324, the silicon analogue of singlet diphenylcarbene <2006JA14442>. Diphenylsilylene 324 was trapped by MeOH or triethylsilane to give diphenylmethoxysilane 325 and l,l,l-triethyl-2,2-diphenyldisilane 326 in 72 and 69% yield, respectively. 

Up-to-date, a variety of disilenes have been reported, owing to the improvement of several synthetic methods. For example, the photolysis of cyclic trisilanes (Scheme 5) and the reduction of dihalosilanes or 1,2-dihalodisilanes (Scheme 6) were found to be useful approaches to the corresponding stable silylenes when the precursors are available as stable compounds. 

It must be considered that silyl triflates are not stable in THF for an extended time, because they can initiate the cationic ring-opening polymerization (ROP) of cyclic ethers. Therefore, the silyl triflate (in diethyl ether) is added in drops to the (aminosilyl)lithium compound dissolved in THF. The reaction proceeds very quickly, and ROP of THF is not observed under these conditions. The trisilanes la - Ic can be converted by reaction with triflic acid into the silyl bis (triflates) 2a - 2c. These compounds are precursors for further chain elongation. The reaction with (diethylamino)-diphenylsilyllithium leads to the pentasilanes 3a - 3c. The reaction of bis(diethylamino)phenylsilyl-lithium [9] with silyl triflates proceeds analogously. The formation of the disilane 4 is shown in Scheme 2 as an example. Conversion with triflic acid and chain elongation with LiSiPh2(NEt2) leads to the branched tetrasilane 6. 

Photolysis of 2-phenylheptamethyltrisilane (97) generated methylphenylsilylene, which readily added to terminal, internal, and cyclic alkenes. Under the best conditions, irradiation of this trisilane with a low-pressure mercury lamp in the presence of alkenes that lacked methyl substituents yielded silacyclopropanes in 27-50% yield (Scheme 29). These silacyclopropane were detected by H NMR and were characterized as their methanolysis products <78JOM(i52)i55>. 

The photolysis of cyclic trisilanes constitutes an important source of disilenes, one of the newly accessible and exciting classes of organosilicon compounds (the other major source is from the dimerization of arylsilylenes, usually derived from the photolysis of linear trisilanes). The chemistry of cyclotrisilanes has recently been reviewed7. All cyclotrisilanes studied to date yield a disilene and a silylene when photolyzed at 254 nm64"72 (equation 42). 

Photolysis of cyclic trisilanes like 13 takes place with substantial retention of stereochemistry in the resulting disilane (equation 27), a result which has been explained... 

All known isolable two-coordinated silylenes are cyclic monomers stabilized by bulky substituted amino groups at silicon and in most cases supplemented by a Huckel-type delocalized electron system. They were obtained by reduction of the corresponding dihalogenated derivates [1]. Attempts to synthesize analogous pyrido- ornaphtho-l,3,2A, -diaminosiloles by the same route gave small amounts of the desired pure product or they failed [Id, 2]. This prompted us to investigate (i) the photolysis of bis(trimethylsilyl)diaminosilanes and (ii) the thermolysis of recently reported disilanes [3] and of the trisilanes as alternative methods to prepare stabilized low-coordinated silicon compounds. 

An interestng attempt was made by Ramsey [20] to determine the nature of the reactive excited state of a cyclic trisilane by means of an orbital symmetry analysis of its photofragmentation. The question addressed was whether the relevant excitation might be to a low-lying 3d orbital rather than to an antibonding valence orbital, as is usually assumed. 

Three unusual and interesting examples of silylene formation from photolyses are shown in Scheme 54. In the first, Fink and coworkers photolyzed the trisilane 294 at 254 nm and produced the relatively stable silylene 295. This on further photolysis gave rise to the sterically crowded silacyclobutadiene 296 which was trapped with several reagents. In the second example Michl and coworkers photolyzed the matrix-isolated bis-azide 297 to form the cyclic silylene 298, and this on further photolysis at selected wavelengths, using matrices and low temperatures, isomerized to the silacyclopentadienes 299 and 300 and finally to the l-sUa-2,4-cyclopentadiene 301. Finally, Sakurai and coworkers were able to convert the trisUane 302 to the cyclic divinylsilylene 303. 

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