Difluorosilylene reactions

Products from the Difluorosilylene Reactions under Various Experimental Conditions... 

The gas-phase reaction of cyclopentadiene gives the same products involving monomeric SiF2 units (Table IV). These are the products from genuine difluorosilylene reactions, quite parallel to the chemistry of other silylenes. 

Difluorophosphoryl(trifluoromethyl) peroxide, 16 135, 136 reactions of, 16 136 Difluoropolysulfanes, 16 302, 325-327 NMR of, 16 325, 326 separation of, 16 327 Difluorosilylene, 29 15-36 alternate layer reactions, 29 26-27 in nuclear recoil systems, 29 22 polymerization, 29 15 reaction mechanisms with alkenes and alkynes, 29 17-19 under co-condensation conditions, 29 28-32, 34-35... 

One of the most thoroughly investigated silylenes is difluorosilylene. The oldest observation was made by Troost and Hautefeuille639 in 1871, who postulated the existence of SiF2 in the transportation reaction of silicon with SiF4 ... 

Other silylenes besides difluorosilylene have been postulated. In many syntheses of polysilanes formed by dimethyldihalosilane with metal, dimethylsilylene is thought to be a shortlived intermediate. We see some aspects of these reactions in the section on cyclic and linear polysilanes. 

Difluorosilylene has always been considered the exceptional case of the silylene family. Unlike other silylenes, difluorosilylene had been believed to be quite inactive chemically in the gas phase (73). As a result, most studies of its chemistry were carried out under cocondensation conditions (73, 103, 104, 106, 108). At — 196°C, difluorosilylene forms a pale yellowish condensate, which polymerizes and forms small quantities of volatile perfluoropolysilanes (10% of the total yield) when the reaction trap is warmed to room temperature. 

The reactions of difluorosilylene with olefins and fluoroolefins [e.g., Eqs. (46) and (47)] appear to proceed by quite different pathways. Similarly, the reaction with benzene resulted in addition products (71, 107) [Eq. (49)], whereas reaction with fluorobenzene led to insertion products (107) [Eq. (50)]. 

When such an (SiF2) polymer (or oligomer) mechanism was cited in the literature, one could not but wonder whether difluorosilylene behaved as a carbene analogue at all. With considerable foresight, Gaspar and Herold stated more than a decade ago (39), in view of the poor Si—Si n interaction and the relative weakness of Si—Si single bond, it is very likely that characteristic reactions of difluorosilylene other than polymerization will be found in the near future. ... 

For both addition and insertion reactions, which one, the monomeric difluorosilylene or the diradicals -(SiF2) -, is the true reaction intermediate ... 

Since the difference in bond energies between Si—X and C—X decreases with increasing halogen atomic weight (Table III), somewhere along the line one should expect to observe both insertion and addition products from the same reaction, which would reflect the competitive preference of insertion vs. addition after the initial attack of the oligomeric difluorosilylene. 

Relative Yields of Volatile Products of the Reaction of Difluorosilylene with Vinyl Fluoride in Various Conditions... 

Having clarified the points mentioned above, it seems possible now to draw some conclusions about the reaction mechanism. The reactions of SiF2 in the gas phase seem rather simple. Difluorosilylene behaves quite similarly to the chemistry of other silylenes. The initial attack of SiF2 at the carbon-carbon double bond is followed by rearrangement whenever possible. Otherwise it may lead to polymerization. For example, CH2=CHF reacts according to the mechanisms in Scheme 6. 

A similar strategy faces difficulty in the case of addition products. The only products that could not be formed by dimerization of the corresponding silirane intermediates are the four-membered ring compounds (disilacyclobutanes in the reactions of alkenes and disilacyclo-butenes in the case of alkynes). But these four-membered ring compounds may well be formed by further insertion of difluorosilylene into intermediate three-membered silacycles, for example ... 

In the gas-phase reaction of difluorosilylene with cycloheptatriene (94), in addition to a small amount of unidentified products, compound (3) was obtained as the major product. Since only monomeric SiF2 is involved in the gas phase, the formation of (3) is best viewed as the result of further insertion of SiF2 into the initially formed silirane. This is the first experimental evidence for such insertion as far as difluorosilylene is concerned. 

As in the case of H2Si , difluorosilylene undergoes both insertion and addition reactions (Scheme 6), although in this case there is often more than one Sip2 unit in the product, which is perhaps not surprising in view of the formation of Si p2n+2 species on condensation. Insertion reactions occur readily with fluoroaUcenes, but addition occurs with other alkenes. This is presumably due to the high strength of the Si P bond formed in the insertion which drives the... 

The 2,3-disilabicyclo[2.2.2]octa-5-ene ring can be prepared by the co-condensation reaction between difluorosilylene and cyclohexa-1,3-diene (via initial formation of F2SiSiF2% which adds to the ring) <82JOM(226)2l> and 2,3-disilabicyclo[2.2.2]octa-5,7-dienes can be made from the Diels-Alder reaction of hexafluorobut-2-yne and stereoisomeric mixtures of 1,2-methyl-1,2-phenyl-1,2-disilacyclohexa-2,4-dienes, the products from which may both be used as disilene precursors (Scheme 6) <93JA11460>. 

OF3COF with difluorosilylene yields (F8Si0SiF2)20. The reaction of SiFa with CF3COCI at low temperature yields (95). Pyrolysis of hexa-... 

As mentioned in Section III.6, disproportionation between silicon and SiX4 molecules at high temperatures generates halogenated silylenes, SiX2. In practice, this type of reaction has been extensively employed in the formation of SiFj, as shown in equation (16). The chemistry of the difluorosilylene thus formed likely represents the most comprehensive study of all the known silylenes. 

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