Silylenes disilenes from

The reaction is initiated by extrusion of a silylene 2 from 3a or 3b, thus paralleling the well known equilibrium between hexamethylsilacyclopropane and dimethylsilylene [7]. In the absence of a silylene trapping reagent [8] dimerization of 2 to disilene 5 takes place. Addition of a third silylene to the Si=Si double bond eventually yields cyclotrisilane 1 [9]. The reversibility of the cyclotrisilane formation from 3a and 3b provides evidence, that the reverse reaction of 1 with olefins includes free silylenes 2 as reactive species as well. 

It is evident from the above material that the elimination of bridging silyl groups, as silylenes, disilenes or silenes, is a facile and useful photochemical reaction, but that occasionally interesting and unpredictable behavior may be observed. 

The chemistry of silylene-metal complexes has developed in quite another direction, however, from reactions of disilyl-metal complexes, leading to complexes of otherwise unstable disilenes such as Me2Si=SiMe2. Molybdenum and tungsten complexes have been particularly well investigated by Berry and co-workers,103 and platinum complexes have also been isolated.104 Readers interested in this field are directed to a 1992 review of silylene, silene, and disilene-metal complexes.105... 

Frequently, UV absorptions of the respective silylenes are not the only observed absorptions from 3-methylpentane matrix experiments. Since the corresponding disilenes are often formed upon annealing of the matrix, their spectra can be registered as well169. 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

The first 1,2,4-thiadisiletane results from the reaction of carbon disulphide and the hindered silylene [2,4,6- (Me3Si)2CH 3C6H2]MesSi (TbtMesSi ) formed from the Z-disilene precursor. The mechanism is thought to involve a skeletal rearrangement of the 3,3 -spirobi(l,2-thiasilirane) intermediate formed by silylene addition to each carbon-sulphur double bond (equation 31)64. 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

From the reactions of carbon monoxide in frozen matrices with Me2Si155 and several arylsilylenes Mes(R)Si (R = Mes, 2,6-diisopropylphenoxy, f-Bu)156, adducts were formed (see Section V.A.2). Theoretical calculations led to consideration of both a linear silaketene and a pyramidal n-donor base complex structure for the Me2Si(CO) adduct249. The observation that warming the carbon monoxide adducts of the arylsilylenes led to the formation of disilenes was interpreted as indicating the formation of a nonplanar complex that could dissociate as do other silylene-n-donor base complexes156. 

Apeloig and coworkers have trapped (Me3Si)2Si from the photoinduced [2 + 2] cycloreversion to (Me3Si)2Si=Si(SiMe3)2 and 2,2/-biadamantylidene of the corresponding 1,2-disilacyclobutane186. Since laser flash photolysis experiments indicated that the disilene was the sole primary photoproduct, the silylene was believed to be formed by photodissociation of the disilene. 

The first step of the retro-reaction involves loss of silylene 79, which could be trapped with 1-pentyne to give the known silirene 81 (equation 125). In the absence of a trapping agent, 79 recondenses to 77, probably by first dimerizing to the disilene Ar2Si=SiAr2 followed by 2 +1 cycloaddition to give 77 (equation 126). From the principle of microscopic reversibility, the fact that silylene is formed in the retro-reaction leads to the conclusion that 79 must also be an intermediate in the cycloaddition reaction. 

The above theoretical analysis for a variety of dimer structures of silylenes requires inevitably a definition of disilenes different from that of alkenes, molecules with carbon carbon double bonds. Geometry around a typical C=C double bond is planar and the double bond length (134 pm) is shorter than the corresponding single bond (154 pm). BDE of ethylene to two methylenes is ca. 170 kcal mol-1 which is 1.9 times larger than for the C C single bond (90 kcal mol-1 for H3C-CH3) the BDE of ethylene really almost doubles the BDE for ethane ... 

For the Z-to-E isomerization, a mechanism via silylsilylene complex 238 [Eq. (112)] has been proposed. Mechanisms including the following processes are excluded on the basis of the theoretical calculations (1) dissociation-association equilibrium between the corresponding disilene and Fe(CO)4, (2) removal of one CO from 236Z and then Si-Si bond cleavage forming the corresponding bis-silylene... 

The first structurally confirmed [2 + 4] adduct of a disilene and a 1,3-diene was compound 89, obtained from cyclopentadiene and 4174. The formation of the tricyclic compound 95 from furan and the cyclotrisilane 40 is probably initiated by a [2 + 4] cycloaddition of 41 to the five-membered ring to afford 94, which then undergoes a [2 + 1] addition at the newly formed double bond with the silylene 42 formed concomitantly in the photolysis of 40 (equation 16)74. 

Silylenes can be matrix-isolated not only in argon at very low temperature but also in hydrocarbon glasses at 77 K [16], Since they are singlets, silylenes do not abstract hydrogen from the hydrocarbon matrix. However, in general silylenes disappear immediately upon melting of the matrix. The usual first step is dimerization to the disilene if the substituents are sufficiently large, this product may be stable, as shown for an example in Eq. 3 [4]. 

We have shown in this paper that molecular orbital calculations at the ab initio level can be used to predict reliably the spectral transitions in silylenes, to evaluate the effects of substituents on the Si=Si multiple bond, to shed new light on existing experimental data and to direct future work towards the synthesis of novel isomers of disilenes. Although carbon and silicon are isoelectronic, multiple bonds to silicon differ dramatically from multiple bonds to carbon and analogies from carbon chemistry might therefore be entirely misleading when applied to silicon compounds. We believe that our studies have demonstrated the enormous power of modem computational methods and hope that this paper will prompt future theoretical studies and more importantly, theoretical-experimental collaborations in the field of organosilicon chemistry. 

The lower heats of formation of silylenes compared with carbenes translate into somewhat lower, albeit similar, reactivity. There is a general lesson here that slowing down the reactions of carbene-like species makes possible processes like dimerization that are more or less impossible for carbenes. We have already seen that slowing down their rearrangements allows the observation of intermolecular reactions of alkylsilylenes not observed for alkylcarbenes. The dimerization of silylenes to disilenes, discovered in the gas phase [36], was employed to brilliant effect by West and coworkers in the condensed phase synthesis of isolable disilenes [37]. While these were not the first disilenes, silylene dimerization opened the way for the preparation of many new molecules, including digermenes from the dimerization of germylenes [38]. 

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