Si-0 bonds

In 1966, Atwell and Weyenberg studied the thermolysis of methoxy-substituted polysilanes to give silylenes such as SiMc2 and observed products as shown in equation (90) which could be attributed to the insertion of the silylene 

In 1977, Soysa, Okinoshima, and Weber demonstrated the insertion of SiMej into the Si-0 bonds of hexamethylcyclotrisiloxane. They also showed that SiMej was capable of deoxygenating (CHj)2S-0 to give MejSi=0 as a reaction intermediate. 

Very recently, Okinoshima and Weber discovered that (i) SiMePh does not insert into the Si-0 bonds of hexamethylcyclotrisiloxane, but into that of a 1,3-disilacyclopentane substrate, (ii) SiMePh is less reactive than SiMej, and (iii)the 1,3-disilacyclopentane substrate is a more reactive SiMej trapping reagent than is hexamethylcyclotrisiloxane. 

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This is expected as the Si-0 bond is a much stronger bond than that found on transition metals. 

Unsymmetrical hydroxydisiloxanes are formed by treatment of (ClMe2Si)20 with alkyllithium reagents followed by hydrolysis. Similar reactions may be carried out starting from ClMe2SiOSiMe2OSiMe2Cl, but the alkylations are less specific and cyclic siloxanes are formed in significant amounts in the hydrolysis step, Eq. (15) (66). The Si-0 bonds in the cyclic chlorosiloxane 7 are unaffected by KOH, so that the hydrolysis gives a 63% yield of 8 [Eq. (16)] (67). 

The majority of Si-0 bond lengths fall in the range 1.64 0.03 A (204), which is somewhat shorter than the expected 1.76 A estimated... 

In rare instances, one can also obtain displacement of oxygen by nitrogen as illustrated by reaction (3) (13). Reaction (3) proceeds despite the fact that the Si-0 bond is 25-30 kcal/mole stronger than the Si-N bond (14). 

The dinuclear isonitrile and alkoxycarbene complexes 6 and 7 were obtained from [Fe(CO)3( X-dppm) Tl2- j,2-Si(OMe)2(OMe))PtCl] by chloride substitution with isonitriles and 3-butyne-l-ol (or ( )-4-pentyne-2-ol), respectively [2], In these complexes, the organic ligand bound to Pt only occupies one coordination site, thus allowing the trimethoxysilyl ligand to display a T 2- i2-Si-0 bonding mode. 

By making use of the following bond energies, explain why C02 exists as discrete molecules whereas Si02 does not. Estimate the strength of the Si=0 bond. 

The potential sites of cleavage in the hydrolytic degradation of the trisiloxane surfactant, M2D-C3-0-(E0)n-CH3 (1) are illustrated in Fig. 5.5.3. The Si-0 bond (c) is a likely site of cleavage according to the chemistry of silicones and the relative instability of this bond to hydrolysis [23]. 

As it is well known, the (Si-0) bond in organosiloxanes may be considered to be polar or partially ( 50%) ionic.(12) Therefore, it can be cleaved by the attack of strong acids or bases. This is the main rationale behind the "equilibration" route to the synthesis of a wide variety of functionally terminated siloxane oligomers(12-14) from cyclic siloxanes and a.ordifunctional disiloxanes as shown in Scheme 3. 

These type of reactions are generally named as "equilibration" or "redistribution" reactions due to the nature of the processes. During the reactions the catalyst can only cleave the (Si-0) bonds in the cyclic or linear species including that of the "end blocker" and growing chains. However, the (Si-R) or (R-X) bonds are stable. Therefore at the end of the reactions the linear oligomers are functionally terminated and the minority (10-15%) cyclic side products are nonfunctional. After elimination of catalyst, the cyclic side products can usually be removed from the system by... 

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