Isomerization silene-silylene

For R = H the silole 51 had a at 278 nm while the silylene 48 absorbed at 250 and 480 nm. The isomeric silenes 49 and 50 absorbed at 2% and 270 nm, respectively. The UV absorptions for these species have been calculated and assigned, and their IR spectra have also been obtained. When R = Me, there was little change in the A.max, the four species absorbing at 280, 255 and 480, 312, and 274 nm, respectively. 

Two indirect routes to silenes, one derived from silylenes and the other from silylcarbenes, are of some generality and importance. Silylenes (e.g., Me3Si—Si—<]) (53) have been derived from the thermolysis of either methoxy or chloro polysilyl compounds. Thermolysis resulted in the elimination of trimethylmethoxy- or trimethylchlorosilane and yielded the silylene, which, based on products of trapping, clearly had rearranged in part to the isomeric silene [Eq. (5)]. Alternatively the silylene Me2Si has... 

Retro-ene reactions and 4 + 2 cycloreversions represent additional practical pyrolytic approaches to silenes. Silylene-to-silene isomerization is of mechanistic interest and has gone through a colorful history. Several other pyrolytic reaction types also lead to silenes (Chart 6). 

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

We theoretically studied the reactions of stable West silylenes 32 and 73 with phosphorus ylide H2C=PMe3.74 Similarly to the simplest analogs of carbenes, these compounds can form betaines in which the negative charge is localized on the silicon atom and the positive charge is localized on the phosphorus atom. These betaines can thermally decompose to form silenes (direction A, Scheme 39) or be isomerized to ylides via direction B. 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

Maier and coworkers have shown that it is possible to induce by irridation using the appropriate absorption band, a 1,2-hydrogen shift in silenes and in silylenes29,158. Thus, it is possible to switch photochemically between silylenes and silenes. Michl, West and coworkers have used this approach to isomerize dimethylsilylene 285 and 1-methylsilene 26 several times (equation 70). Due to the clean formation of 285 from the diazido precursor 286153 it was possible to measure the IR transition moment directions for both 26 and 285156. 

Data obtained from collision-induced dissociation experiments did not allow for a distinction of the isomeric metal-silene and-silylene species however, structure-specific ion-molecule reactions of the complexes with labeled ethene were used to clearly differentiate between the metal silene and the silylene. In this intriguing study, Jacobson and coworkers also bracketed the bond dissociation energies of the isomeric ions. 

A third photochemical access to silylenes, beside the isomerization of silenes and the photolysis of tri- or cyclopolysilanes, is the irradiation of geminal diazidosilanes, which under nitrogen loss often gives the corresponding divalent silicon species. However, silan-imines are frequent by-products164 or in some cases even the only products (Section IV). 

The mechanisms proposed by Caspar and coworkers in their approach to the problem have a common point, the formation of the intermediate 9. This silylene then undergoes two prototropic rearrangements initial isomerization to a silene, the 2//-silole (11), followed by a second to give the 1H-silole (2)19 (equation 3). 

Arrhenius parameters for the reversible interconversion of 1-methylsilene (MeHSi=CH2, 2a) and dimethylsilylene (3) via 1,2-H shift, an example of the well-known silene-to-silylene isomerization (equation 5), have been reported by two groups24,25. The earlier values of Davidson and Scampton [ a = 166 kJmol-1 and log(A/s 1) 13.5 in both... 

R = CH3) is small, and so also is the effect of silyl183. The calculated barriers are 42.2 kcal mol -1 in H2C=SiH2, 43.5 kcal mol"1 in H2C=SiH(CH3) and 42.8 kcal mol -1 in H2C=SiH(SiH3), all at the MP3/6-31G //6-31G level of theory183. The barriers may be smaller with substituents such as OH, which stabilize the silylene strongly, thus making equation 18 more exothermic. However, the isomerization barriers are still expected to be substantial so that at low temperatures substituted silenes are predicted to be kinetically stable towards 1,2-hydrogen shifts. Systematic studies on the effect of substituents on these isomerization barriers are needed for more quantitative estimations. 

A silene-to-silylene isomerization by a 1,2-shift of a trimethylsilyl group from silicon to carbon was originally proposed in order to account for the formation of 1,1,3-trimethyl-1,3-disilacyclobutane during the pyrolysis of allylpentamethyldisilane (equation 93)214. Since silenes are planar and since the rc-component of the C=Si double bond is quite strong ( 40 kcal mol ) 19, m, it is not easy for the molecule to align the migrating SiH or SiSi bond with the carbon p-orbital that forms the new bond. 

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