Free silylene

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

Donor free silylene complexes are reactive intermediates in a variety of chemical reactions. In many cases, evidence for the coordinated silylenes involved has been obtained indirectly by means of trapping experiments [49-60]. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

As mentioned above, there have been no examples of stable diarylsilylene although dialkyl- or diamino-substituted silylenes are now available as stable crystalline compounds. However, Tokitoh et al. found that the overcrowded diarylsilylene-isonitrile complexes 139 act as a free silylene 138 in solution even at room temperature. Since there is a severe limitation in a study concerning the reactivities of silylenes under the conventional generating conditions, the reactivity of silylene-isonitrile complexes 139 as a masked silylene is of great interest. 

Trapping agent Rate constant ( free silylene) Rate constant (complexed silylene)... 

The first base-free silylene ruthenium complexes without a 7r-donor stabilization were prepared by Tilley and coworkers by displacement of a triflate ion by a noncoordinating lithium perfluorotetraphenyl borate (equation 34). The base-free silylene... 

Siliranes are also formed by the reaction of the cyclotrisilane [2-(Me2NCH2)C6H4]6Si3 with terminal and strained internal olefins under mild thermal conditions. The products obtained from the thermolysis of the siliranes thus prepared suggest a thermal equilibrium of the silirane with the cyclotrisilane and the corresponding alkene. This observation provides evidence for an equilibrium between the silylene and the cyclotrisilane and, moreover, proves that free silylenes are involved in the silylene transfer reaction48. 

There is a difficulty with the mechanism of Scheme 3 in that the fragmentation of a triplet diradical should conserve spin, yet neither triplet Me2Si nor triplet tetraphenyl-naphthalene have been detected. The diradical pathway for the photofragmentation of silanorbornadienes confirms an earlier proposal by Barton and coworkers52. There is always the possibility that diradical intermediates such as the singlet and triplet 13 S and 13 T could function as silylenoids. Thus, the assumption that products from pyrolysis of 7-silanorbornadienes are formed from free silylenes must be treated with caution. 

The inversion in selectivity between the insertion into an H—Si bond favored by the intermediate in the dehalogenation and the addition to a triple bond favored by the intermediate formed upon silirane pyrolysis suggests that different reactive intermediates are formed. Since free silylene is implicated in the silirane pyrolysis, the lithium-induced deiodination probably involves a silylenoid such as a complex of the silylene with Lil or with THF. For a further discussion of this possibility, see Section II.E. 

When the same dehalogenations were carried out in the absence of a trapping agent, however, the products depended on the nature of the halogen. With t-Bu2SiBr2 and / -15 u 2S i 12 the cyclotrisilane was obtained, but with /-BuiSiCli a disilane and a cyclote-trasilane were produced (equations 45 and 46). It follows that all three of these reactions cannot have proceeded through the free silylene and, in fact, none may have. 

Another conceivable, but probably less likely, explanation of these experimental results56 is that the triethylsilane somehow catalyzes the decomposition of silylenoid into free silylene, which can then insert into the Si—H bond. In any case, the intermediacy of silylenes in a-dehalogenations remains an open question, worthy of further study. 

Steady-state kinetic analysis of a competition experiment led to the conclusion that the siloxolane is formed by reaction of a vinylsilirane intermediate with acetone, and that the vinylsilirane arises from addition of the free silylene to butadiene. Since silylenes are known to react more rapidly with acetone than with butadiene, the kinetic analysis further suggested that the carbonyl sila-ylide dissociates more rapidly than it rearranges to the silyl enol ether shown in equation 64140. 

Carbonyl and thiocarbonyl complexes of Mes2Si have been observed in a remarkable series of experiments by Ando and coworkers144. Photolysis of Mes2Si(SiMe3)2 was carried out at 77 K in a soft (3-MP/isopentane) matrix in the presence of tetramethyl-2-indanone or its sulfur analog. The free silylene was formed initially but it slowly reacted with the carbonyl (or thiocarbonyl) compound to give the complex, which can be formulated as a silacarbonyl ylide (Scheme 8). Further photolysis converted these ylide complexes to the silaoxirane or silathiirane, reversibly. 

The nature of the bonding in silylene-metal complexes, as compared with the better known metal-carbene complexes, is a question of considerable interest. MO calculations on H2Si=Mo(CO)5 indicate that the Si—Mo bond consists of a cr-donor and --backbond component, like the carbon-metal complexes. The -component is, however, weaker than for metal carbenes251. Infrared C=0 frequencies for the base-free silylene metal complexes support this model. Theoretical considerations of the bonding in silylene-metal complexes are treated more fully in Section IV.E. 

The familiar Wurtz synthesis of polysilanes from dichlorosilanes and alkali metal evidently does not involve free silylenes. See R. G. Jones, R. E. Benfield, R. H. Cragg, A. C. Swain and S. J. Webb, Macromolecules, 26, 4878 (1993). 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

Removal of the base from a base-stabilized mononuclear silylene complex can induce the dimerization of the generated base-free silylene complex... 

Braunstein et al. recently reported an interesting reaction of a base-stabilized mononuclear silylene complex with platinum-ethylene complexes [Eq. (35)]. In this reaction, the Pt-bound ethylene ligand is displaced by the base-free silylene complex, but the products can be also regarded as a dinuclear complex where two metals are bridged by a silylene unit.67... 

All cationic phosphenium complexes reported to date are base-free species. In contrast, silylene complexes tend to be stabilized by forming base adducts. Most silylene complexes are base-stabilized species54 59 and base-free silylene complexes are still rare.59-63 Typical examples are shown in Scheme 7. A phosphite-phosphenium complex exists in a solid state and even in solution, and does not take an OMe bridging form between the two P atoms,64 whereas a silyl-silylene complex is not detected, but exists as an OMe bridging form between the two Si atoms.65... 

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. 

In addition, 5 undergoes many reactions which are not possible to carry out using transient silylenes. Some of these lead to novel products not obtainable by other syntheses. For example, 5 reacts with many metal carbonyls to give base-free silylene-transition metal complexes. Two examples are shown in Eq. 6 and 7 (LSi = 5). 

Silicon. The first base-stabihzed silylene complex [Cp Ru =Si(Ph)2NCMe (PMe3)2] was reported in 1987, and the first base-free silylene-containing complexes [Cp Ru=Si(SR)2(PMe3)2] (R = Me or Ph) were not synthesized until 1990, much later than for the heavier congeners Ge, Sn, and Pb. Recently, the silylene-containing... 

In the investigated set of heteronorbomanes the free silylenes have <5 Si in the range 428 42 ppm, while the molecules with one or two Si-O dative bonds show up in the region 143 29 ppm. The difference of approximately 300 ppm corresponds to the reported effect of a donor group on... 

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