Fluorosilanes, reaction

Lithium hydride is perhaps the most usehil of the other metal hydrides. The principal limitation is poor solubiUty, which essentially limits reaction media to such solvents as dioxane and dibutyl ether. Sodium hydride, which is too insoluble to function efficiently in solvents, is an effective reducing agent for the production of silane when dissolved in a LiCl—KCl eutectic at 348°C (63—65). Magnesium hydride has also been shown to be effective in the reduction of chloro- and fluorosilanes in solvent systems (66) and eutectic melts (67). 

This common way of synthesis has to cope with the problem that the reaction is nearly uncontrollable because of polycondensation [1], Intermediates, e.g., chlorosilanols, cannot be proved or isolated. However, starting with the alkaline hydrolysis of fluorosilanes, halosilanols, silandiols, and aminosilanols can be prepared and stabilized by bulky groups. 

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

Thiocarbonyl fluoride has also been prepared by reaction of bis(trifluoromethyl-thio)merkury and iodosilane (5i). The first product formed is trifluoromethyl-thiosilane, which spontaneously decomposes to thiocarbonyl fluoride and fluorosilane. 

Oxidations are observed in the reactions with benzothiophene20 and triphenylphos-phane 20 triphenylsilanc and chlorotriphenylsilane are converted into the fluorosilane 28,20-27 while iodoethane forms (difluoroiodo)cthanc (29).20 27... 

In order to illustrate this point, we report the opposite behavior of the ally and n -butyl anions in the substitution of an optically active fluorosilane, 1-NpPhMeSi—F. The first leads to inversion (91% IN) and the latter to retention (98% RN) (49), when the reactions with allyl- and n -butyllithium are carried out in the presence of a cryptand specific for lithium cation. The use of a cryptand makes it possible to study the difference in behavior of the anion species alone, since it avoids the possibility of electrophilic assistance by Li+ which could affect the apicophilicity of the fluorine atom. 

As a consequence, these two effects favor retention of configuration. Experimental trends show that inversion is only observed for soft nucleophiles, whose valence orbitals are usually diffuse 71). In this latter case, the unfavorable out-of-phase Nu—OR overlap prevails and an attack at the rear becomes possible (Scheme 9). However, the reaction is slow. For instance, the reaction of CH2=CH—CH2MgBr on a methoxysilane is 105-fold slower than the same reaction on a fluorosilane (8). 

Lithiated fluorosilanes such as 801 and 802 may be considered as fluoride donor adducts to silanimines and behave as the free silanimines (e.g. 803) in cycloaddition reactions with C=0 compounds. The stability of the four-membered cycloadducts obviously depends on the substituents (equation 274)355. 

Primary products of such condensations can be synthesized in the reaction of fluorosilanes with lithium amide (Scheme 3) so that the formation mechanisms of the ring compounds can be studied.15... 

Cyclodisilazane anions and cyclodisilazanes in cw-conformation are found in the reaction of dilithiated bis(silylamino)fluorosilanes with chlorotrimethylsilane. The dilithium salt of the corresponding bis(silylamino)chlorosilane is obtained. LiCl elimination in the presence of THF leads to the formation of silaamidides (which are isolated as dimers),19,26 four-membered cyclodisilazane anions and (thf)3Li-Cl-Li(thf)36 or Li(thf)/ cations (Scheme 10).19,27 Hydrolysis of dimeric silaamidides is a facile synthesis leading to cyclodisilazanes in the cw-conformation. Examples are shown in Figs. 1 and 2 (Scheme 11).19,27... 

Later, he found equilibrium between the six- and the four-membered-ring anion, which depends very strongly on the temperature at which the reaction is conducted.41 Higher temperatures lead to a better Si->N contact across the ring and therefore isomerization is possible. Substitution is preferred at low temperatures. We have studied the anionic rearrangement in reactions with fluorosilanes and boranes and found three more factors that influence the equilibrium between the six- and four-membered rings illustrated in Scheme 18.42 43... 

The reaction of equation 14 probably occurs stepwise, and it is a complex process involving substitution reactions and/or homo- and heterofunctional condensations. Numerous cyclodi-, tri- and tetrasilazanes (76) are obtained in the reactions of aminofluorosilanes (74) with lithium organyls via thermal LiF elimination of lithium aminofluorosilane derivatives (75) (equation 18)69-75. The primary products of such condensations in the reaction of fluorosilanes with lithium amide have been synthesized in order to study the mechanisms of their formation. An (R2SiFNLiH) compound was characterized by X-ray diffraction8,76 77. 

The lithium compound has a polymeric lattice structure in the crystal, formed via Li F contacts. The LiNSiF part of the compound forms an eight-membered ring. The lithium is three-coordinated (sum of the angles = 359.3°)96,97. In the reactions of the lithium derivative with fluorosilanes, exocyclic substitution occurs, e.g., to give 119 (equation 33). The lithium compounds are stable up to about 100 °C. At higher temperatures Li—F elimination occurs and silyl coupled cyclodisilazanes (120) are obtained (equation 34)96,97. 

Starting from silyl triflates, fluorosilanes are obtained by reaction with LiF in ether84,85 (equation 51). 

This is the most common method for chloro-, bromo-, or iodosilanes.6 Often, auxilary bases such as triethylamine or pyridine are added. But when hydrazine is treated with fluorosilanes or silanes, no condensation is observed because of the reduced reactivity. Fluorosilanes only form adducts with hydrazines, so that the reaction is stopped at step (a). Because of the extremely strong Si—F bond energy, no cleavage of HF or N2H4-condensation is observed.3,6 In this case another preparation method must be chosen. 

Isomeric products are formed in the reaction of lithiated di-rm-butyl-methylsilylhydrazine with fluorosilanes. The formation of these isomers requires prior coordination of the Li+ ion with the two N atoms of the hydrazine unit. The crystal structure of the lithiated di-tert-butylmethyl-silylhydrazine 4 (9)14 exhibits two different silylhydrazide units I and II, which are bound by six Li+ ions to form a hexameric entity. 

Isomerism is observed also in the formation of tris(silyl)hydrazines (56a/b). The reaction of 46 with fluorosilanes leads to the formation of the isomeric products 67 and 68 or 69 and 70 [Eq. (16)]. 

The condition for the formation of isomers is a side-on coordination of the lithium in 47, the lithium derivative of 27. Depending on the bulkiness of the fluorosilane, this side-on coordination makes possible a substitution that is suitable for the system. In comparison, the reaction of lithiated 27 with iPr2SiF2 leads only to the formation of the tris(silyl)hydrazine 75,16 so that kinetic control of the reaction must be supposed ... 

The synthesis of array L7 is reported in Fig. 8.22. Compound 8.38 was reacted simultaneously with amines (Mi, two representatives), aldehydes (Mi, five representatives), and isonitriles (Ms, two representatives) to give 10 compounds (not all the combinations were reacted). The reaction was performed in trifluoroethanol (TFE), another hybrid fluorous-organic solvent (step a. Fig. 8.22), and after evaporation of the TFE, the crude product 8.39 was purified by two-phase extraction between fluorous solvents and benzene (step b). After evaporation of the solvent, the fluorous tag was cleaved with TBAF (step c) and a triphasic extraction (step d, Eig. 8.22) was performed to remove the fluorosilane tag and acid 8.38-related impurities extracted into the fluorous layer. Excess TBAE and TBAE-related impurities partitioned into the acidic aqueous layer. Yields and purities of the synthetic protocol are reported together with the structures of the library members L7a-j in Table 8.2. 

Thermolysis of both F3CSiH3 and F2HCSiH3 in the presence of efficient carbene trapping agents have shown that they decompose predominantly by a-fluorine shift to give fluorosilane and the fluorocarbenes CF2 and CHF, respectively [24]. Secondary reactions at elevated temperature in the absence of any... 

The explosion limit dropped to lower laser fluence upon increasing the pressure of the (fluoromethyl)silanes. Below the explosion limit, chemical changes of the (fluoromethyl)silanes could only be detected after irradiation with as many as 10 laser pulses. The reaction products revealed an almost quantitative reduction of the CF bond and afforded the gaseous fluorosilanes Sip4 and Sip3H, the hydrocarbons CH4 and C2H2, and different solid Si/C/F/H materials. 

The pioneering work of Hiyama has demonstrated that organosilanes (suitably functionalized) in the presence of a nucleophilic activator can undergo Pd-catalyzed cross-coupling reactions. The chlorosilanes, fluorosilanes and alkoxysilanes are used to couple with a variety of electrophiles. 

Silicon tetrafluoride reacts with pure silicon at low pressure in a flow system at 1200 °C to yield a transient species, SiF2, which can be condensed to yield a fluorosilane polymer, (SiF2) . Sihcon difluoride has a half-life of about 150 s in glass at low pressure in the gas phase. The chemistry of SiF2 shows many different reactions and has been reviewed. When condensed at —196 °C, a red-yellow deposit is formed,... 

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