Disilenes disilanes

Evidence for a 7t-coordination was obtained through the reaction of various disilenes with Hg(OCOCF3)2, a reaction which leads regioselectively to bis(tri-fluoracetyl)disilanes. A disilene n-complex (79), which is stable up to — 50 °C, could be identified as an intermediate by spectroscopic methods. 

Thermochromism was already noted for the first stable disilene l.2a In dialkyl- or bis(trimethylsilyl)diaryldisilenes, the size of the aryl groups affects the thermochromism Tip-substituted ones (9,10) are thermochro-mic,10 while Mes-substituted ones (3,4) are not.9 Interestingly, tetrakisftri-alkylsilyl)disilanes 22 and 23 are highly thermochromic hexane solutions of 22 and 23 are light yellow below 0°C but dark red above 50°C.21 These observations were interpreted in terms of a thermal equilibrium between the bent and twisted conformations. Compound 24, having an absorption band at 480 nm, is dark red in solution even at room temperature. 

The coupling constant 7Si=Si is also informative concerning the nature of the Si=Si bond. The JSi=si observed for 13 is 155-158 Hz and that for 15 is 160 Hz, as compared with about 85 Hz for typical aryldisilanes.48 This indicates much greater s character in the crSi Si bond in disilenes than in disilanes, consistent with approximate sp1 hybridization of the Si atoms in the disilenes. Theoretical calculations on the parent disilene predict a JSi=Si value of 183.6 Hz.49... 

In the absence of electron-withdrawing ligands, disilanes possess only a little Lewis acidity consequently, they show a limited propensity to form hypervalent complexes. Starting from the symmetric methylchlorodisilanes ClMe2SiSiMe2Cl and Cl2MeSiSiMeCl2 as well as carboxylic acids and thiocarboxylic acids, Kawashima et al. have prepared rare examples of penta- and hexacoordinated disilene, namely compounds 886-889 (Scheme 125).855 856... 

Disilene and its isomer silylsilylene were neither available by standard vacuum flash pyrolysis of precursors 59-63, nor by the more elaborate method of pulsed flash pyrolysis of 60-63, a pulsed discharge in mixtures of argon and mono- and disilane74 or by the matrix photolysis of educts 59-66 using various light sources (Hg lamps, excimer laser)69,70,72, the microwave discharge in disilane 66 or the cocondensation of silicon atoms with SiFLt. 

Cathodic reduction potentials of disilenes were determined by cyclic voltammetry56. As shown in Table 18, tetraaryldisilenes are reduced at less negative potentials than dialkyl-diaryl derivatives. This is in sharp contrast to the fact that anodic oxidation potentials are similar for both types of these disilanes (see Table 13). 

In contrast to alkenes, disilene derivatives react very smoothly with various haloalkanes as shown in Eqs. (81) and (82). Tetramesityldisilene 1 reacts with tert-butyl chloride to give hydrogen chloride adduct 172 together with 2-methylpropene (173), while treatment of 1 with benzyl chloride affords l-benzyl-2-chlorodisilane 174.126 The reactions of tetrasilyldisilene 22 with haloalkanes proceed in a similar way to give l-halo-2-(haloalkyl)disilanes 175, whereas the reaction with carbon tetrahal-ides affords the corresponding 1,2-dihalodisilanes 176.127... 

Cycloaddition reactions of transient or isolable disilenes with heterocumulenes such as CX2 (X = S, Se) produce heterocyclic carbenes, for example, carbene 59, which has a disilane backbone. These carbenes are only transient species and were not isolated but were either trapped with C6o. or dimerization of the carbenes occurred to give the tetrathiafulvalene or tetraselenafulvalene analogues 28 <2002CEJ2730, 2005AGE7567>. 

One of the most fascinating reaction modes of disilenes is the ready formation of disiliranes, three-membered ring systems containing two silicon atoms and one heteroatom in the ring, which are hardly accessible by other routes. They are mostly formed by [2 + 1] cycloadditions of atoms or molecular fragments to the Si=Si double bond. One of the few examples of other accesses to this ring system is the reaction of AMithio-2,4,6-trimethylanilide 53 with the disilane 52, which affords the azadisilacyclopropane 54 in modest yield (equation 8)75. 

Indirect evidence for the validity of this assumption is provided by the structure [rf(Si-Si)=2.229(l) A, 359.9°], and especially the 1/si,Si coupling constant of the disilaoxirane 81 derived from the unsymmetrically substituted disilene 2996. With a Jsi,Si value of 123 Hz, this coupling constant is appreciably larger than those observed for other disilanes with a similar substitution pattern (85 Hz) and approaches the value of 160 Hz for the disilene 29. 

Disilenes. Disilenes (3) can be prepared from l,2-dichloro-l,l,2,2-tetrasub-stituted disilanes (2) by reaction with lithium naphthalenide. Most disilenes are unstable to air, but 3 is fairly stable. Surprisingly, it is oxygenated to give 4, rather than the expected 1,3-cyclodisiloxane (5). 

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). 

Similar results were obtained for the photolysis of poly(n-hexylmethyl-silane) using either methanol or n-propyl alcohol as the trapping reagent (Table II). The additional complexity of the disilane products was anticipated, because silyl radicals can abstract either a hydrogen atom or an alkoxy radical. Although the alkoxy disilanes could be produced by other routes (e.g., alcoholysis of Si-H bonds or addition to an intermediate disilene), these routes were considered unlikely on the basis of appropriate control experiments or the lack of a literature precedent. 

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. 

In the following text, results concerning the syntheses and reactions of disilenes of types 1-4 are presented. In addition, dehalogenations of the disilenes 2 and 4, and dehydrobromination of the disilane R HBrSi-SiBrHR, which possibly lead to disilynes of types 5-7, will be described (in some cases, these reactions obviously proceed via cyclotrisilenes as well as cyclotetrasilenes and even tetrasilabutadienes). The disilenes 1 (R = Ph), 3, and 4 and, (obviously) the disilyne 7, could be obtained under normal conditions due to an adequate steric shielding of the reactive >Si=Si< and -Si Si- entities with bulky silicon-bound groups. 

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