Disilanes hexamethyldisilane

The distance-dependent Pauling bond orders range from 1.00 in hexamethyldisilane with a SiSi bond of 235 pm in length (Fig. 4 standard d(l)) to 0.26 for hexakis(rert.butyl)disilane with an extremely elongated spacer distance of 270 pm between its bulky Si(C(CH3)3)3 half-shells [6b]. To rationalize the sometimes considerably weakened SiSi bonds - hexakis(rm.butyl)disilane does not dissociate into two radicals -, it has been proposed [6b,7] that additional attractive van der Waals interactions within the hydrocarbon wrapping contribute to the bonding within the respective organosilicon molecules. This assumption is further supported by the structure of hexakis(trimethylsilyl)disilane (Fig. 2), in which (presumably due to the considerable polarization Si -C5e-H5 calculated [5b]) extremely short non-bonded C(H3)-- (H3)C distances of only 352 pm are found. 

Thermolysis. Hexaphenyldisilene does not undergo thermolysis to Ph3Si. This might probably be due to a long Si-Si bond. However, hexamethyldisilane is believed to dissociate homolytically. If true, the end product is an isomer of the starting disilane and formed in quantitative yield. 

The catalytic effect of tetra-n-butylammonium fluoride in the homogeneous reduction of heterocyclic A-oxides and nitroarenes by hexamethyldisilane in tetra-hydrofuran can occur with EXPLOSIVE violence, but can be controlled by the slow addition of the disilane to the A-oxide (or nitroarene) and tetra-n-butylammonium fluoride to yield the parent heterocycle (>70%) (or azobenzene 84%). In a similar manner, azoxybenzene is converted into azobenzene (95%), and 4-nitropyridine-l-oxide, is reduced to azoxypyridine-l,l -dioxide (78%), with minor amounts of azopyridine-1, l -dioxide and azopyridine-1-oxide [5,6]. 

Disilanes 43-48 are allowed to react with BTSP to give the corresponding disiloxanes (equations 70 and 71), and the results are summarized in Table 15 . Compounds 44-47 and 48a react with BTSP exothermally at or below room temperature, while 44 reacted slowly at room temperature, and 48b and 48c and hexamethyldisilane required more drastic reaction conditions. The eight-membered ring 48d does not react with BTSP. 

The success of the reaction, as well as its stereoselectivity, is dependent on both the disilane and the bis-diene employed. For example, the ethoxycarbonyl-substituted bis-diene reacts with either hexamethyldisilane or 1,2-diphenyltetramethyldisilane to give high yields of isomeric mixtures of the carbocycle products. In contrast, hexamethyldisilane does not have sufficient activity in reactions with the other bis-dienes studied. In addition, stereoselective formation of only one isomer is observed in some cases. 

Prior to 1991, no high-yield double silylation had been reported using simple peralkyldisilanes such as hexamethyldisilane. In that year, Ito and Tanaka independently reported that palladium systems, with isocyanate and P(OCH2)3CEt ligands, respectively, promote insertions into the Si-Si bond of unactivated disilanes. 

Hexamethyldisilane appears to be stable up to about 500° C, but undergoes isomerization when passed in a quartz tube heated at 600° C to give trimethylsilane and trimethyl(dimethylsilylmethyl)silane in the molar ratio of approximately 1 4, along with the unchanged disilane (151,164). 

It has been confirmed that the availability of such reactions depends upon the existence of the metal-organosilicon compound. The cleavage of disilanes is only possible if there is at least one phenyl group in the compound e.g. it is not possible to cleave hexamethyldisilane. 

Photochemical chlorination of hexamethyldisilane yields chloromethyl-disilanes. The halogenation takes place3S6 on the methyl group ... 

The last reaction affords a newer and better method of forming mixed-substituent methyl/halogen disilanes by the reaction of trimethylchlorosilane with hexamethyldisilane in presence of A1C13 as a catalyst314. ... 

A different approach that even obviates the use of a preformed silyllithium reagent takes advantage of the cleavage of the Si-Si bond of a disilane by a copper salt. Hosomi and co-workers185 have reported on the reaction of various enones or enals 250 with hexamethyldisilane or l,l,2,2-tetramethyl-l,2-diphenyldisilane, catalyzed by copper(i) triflate-benzene complex (Scheme 61). The transformation requires heating to 80-100 °C in DMF or DMI and the presence of tri-/z-butylphosphine in order to stabilize the copper catalyst under these harsh conditions. The addition products 251 were obtained with high yield after acidic work-up. The application of the method to alkylidene malonates as the Michael acceptor was recently disclosed.1... 

SUyl anions.1 A symmetrical disilane such as hexamethyldisilane is cleaved by TBAF (or CsF, but not by KF) to trimethylsilyl fluoride and the trimethylsilyl anion 1. An unsymmetrical silane is cleaved selectively at the less hindered Si atom. 

Organometals containing metal-metal bonds have become more important in recent years. Of especial interest are those compounds with both representative and transition metals. These have been known for over 30 years 130), but have become well known only comparatively recently 69) they provide a link (both literally and figuratively) between the two areas of organometallic chemistry. The growing commercial interest in the silicones has spurred research in organosilicon chemistry, especially polysilane chemistry. Work in this area has been reviewed by Kumada 159). The disilane fraction from the direct synthesis of methylchlorosilanes contains a mixture of compounds of type (CHa) -Clg. Sig, which are readily converted to hexamethyldisilane. This may then be converted to the chloride in a two-step synthesis 160) ... 

Two new classes of ligands, bicyclic phosphate 47 [45] and ferf-alkyl isonitriles [46] on palladium, enable bis-silylation with hexaalkyldisilanes, which have been regarded to be much less reactive than the activated disilanes (Eq. 20). Reactions of the terminal alkynes with hexamethyldisilane in the presence of these palladium catalysts afford (Z)-l,2-bis(trimethylsilyl)alkenes 48 in high yields. 

Fluorinated arylsilanes are conveniently prepared from fluorinated bromoarenes and disilanes in the presence of palladium catalysts (refs. 40- 42) and especially from hexamethyldisilane (refs. 40,41). 

Symmetrical tetramethylidisilane adds to dimethyl acetylenedicarboxylate in the presence of bis(triethylphosphine)palladium(II) chloride to yield the cw-adduct 286 . The palladium-catalysed disilylation of terminal acetylenes proceeds poorly when hexamethyldisilane is used in contrast, the disilane 287 affords good yields of the cf -adducts 288 . ... 

Besides alkali metals, certain very strong anionic compounds, such as alkyllithiums and alkali metal alkoxides can also be used to cleave disilanes (Eq. 2). Hexamethyldisilane can only be cleaved in strongly coordinating... 

Mizuno and his co-workers reported the photosilylation of electron-deficient alkenes by use of hexamethyldisilane and unsymmetrical disilanes. The photoreaction of l,l-dicyano-2-phenylethene and disilanes in acetonitrile in the presence of phenanthrene affords sily-lated dicyanoethanes in good yields. A key intermediate in this process is the silyl radical, generated by solvent (nucleophile)-assisted cleavage of the radical cation of disilanes. Silyl radical attack at the radical... 

A study of the molecular structure of disilane, hexachloiodisilane, and hexamethyldisilane has been carried out (2) with the aid of electron diffraction, and the Si—Si bond lengths obtained (2.32 0.03, 2.32 0.06, and 2.34 0.10 A, respectively) are roughly the same as that in elementary silicon. No information is available on the bond lengths of higher organo-polysilanes. 

The force constant of the Si—Si bond has been calculated as 1.3 x 10 dynes cm however, it is believed that this value is probably in error (f04). The dipole moments of several aromatic disilanes have been reported (2) which allowed one to calculate an aryl—Si—aryl valence angle of 115°. The dipole moment of 1,2-dichlorotetramethyldisilane was found (105) to be 1.75 debye in carbon tetrachloride and 1.35 debye in benzene. On the basis of the dipole moments, the infrared and the Raman spectra (in the gas, liquid, and solid state), information on the rotation about the Si—Si axis in 1,2-dichlorotetramethyldisilane was obtained. In the solid state, the chlorine atoms assume the tram position, whereas in the liquid and gas state the molecule exerts torsional oscillations about the Si—Si axis to a certain extent. The phase transformations of hexamethyldisilane were studied by NMR (80) and thermodynamically by means of differential thermal analysis (25). From such studies it appears that at higher temperatures rotations about both the Si—Si and Si—CH3 axes occur in combination with the overall molecular rotation about the molecular axis, whereas at lower temperatures all movements are hindered except for the Si—CH3 axial rotation. 

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