Difluorosilylene

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. 

Earlier studies on the chemistry of difluorosilylene were focused on the reactions with nonmetal halides (8, 72, 77, 105, 106, 109) and unsaturated organic compounds (65-69, 77). [e.g., Eqs. (45)-(48)]. 

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)]. 

According to these earlier studies, a reaction mechanism involving diradical intermediates of the type (SiF2) (where n = 1, 2, 3, etc.) was established (70, 73, 80). This idea is particularly attractive if one considers the facts that in co-condensation reactions polymers are the major products obtained, and that volatile products of co-condensation reactions often contain (SiF2) units with n 1. The earlier ESR study also supported this idea (52). 

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.  

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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... 

Poly([7,8-bis(trifluoromethyl)tetracyclo [4.2.0.02 8.05 7]octane-3,4-diyl]-1,2-ethenediyl), 3457 Poly[borane(l)], 0134 crs-Poly (butadiene), 1480 Poly(l,3-butadiene peroxide), 1528 Poly(butadiyne), 1382 Poly(carbon monofluoride), 0336 Poly(chlorotrifluoroethylene), 0589 Poly(l,3-cyclohexadiene peroxide), 2380 Poly(cyclopentadienyltitanium dichloride), 1837 Poly(diazidophosphazene), 4781 Poly(dibromosilylene), 0282 Poly(difluorosilylene), 4324 Poly(dihydroxydioxodisilane), 4474 Poly(dimercuryimmonium acetylide), 0665 Poly(dimercuryimmonium azide), 4606 Poly(dimercuryimmonium bromate), 0253 Poly (dimercury immonium iodide hydrate), 4449 Poly (dimercury immonium perchlorate), 4006 Poly(dimercuryimmonium permanganate), 4603 Poly (dime thylketene peroxide), see Poly(peroxyisobutyrolactone), 1531 Poly(dimethylsiloxane), 0918 Poly(disilicon nitride), 4752 Poly(ethenyl nitrate), see Poly(vinyl nitrate), 0760 Poly(ethylene), 0778 Poly(ethylene terephthalate), 3256 Poly(ethylidene peroxide), 0831 Poly(furan-2,5-diyl), 1398 Poly(germanium dihydride), 4409 Poly(germanium monohydride), 4407 Poly(isobutene), 1578 Poly(methyl methacrylate peroxide), 1913... 

Dodecachloropentasilane, 4192 Fluorosilane, 4292 Hexachlorodisilane, 4185 Iododisilane, 4538 Octachlorotrisilane, 4188 Poly(difluorosilylene), 4324 Tetrachlorosilane, 4167 f Tribromosilane, 0286 f Trichlorosilane, 4130 f Trifluorosilane, 4327... 

Poly(difluorosilylene), 4324 Potassium antimonide, 4668 Potassium terf-butoxide, 1645 Potassium cyclopentadienide, 1840 Potassium diethylamide, 1679 Potassium octacyanodicobaltate(8—), 2875 Potassium—sodium alloy, 4641... 

Higher silicon halides Si X +2 with n > 2 but of limited molecular weight have been known for a long time, above all from chlorine, but a few from bromine, and none from iodine. In the case of fluorine, some higher silicon fluorides derived from difluorosilylene have been recently discovered (see p. 60). 

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. 

Inorganic Silylenes. Chemistry of Silylene, Dichlorosilylene, and Difluorosilylene... 

INORGANIC SILYLENES. CHEMISTRY OF SILYLENE, DICHLOROSILYLENE, AND DIFLUOROSILYLENE... 

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. 

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

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. 

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. 

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. 

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