Silenes cyclic—

Sakurai et al. have provided what is probably the most important mechanistic finding in the area of intermolecular additions of silenes in recent years, namely a detailed proposal for the mechanism of alcohol addition to the silicon-carbon double bond.68 A cyclic silene 116 was synthesized in the presence of various amounts of methanol and other alcohols, and varying proportions of methanol adducts 117 and 118 were obtained. It was concluded that the methanolysis involved two steps, the first being the association of the oxygen lone pairs with the sp 2-hybridized silicon atom of the silene. The second step, proton transfer, could occur in two ways. If the proton was transferred from the complexed methanol molecule (path a) its delivery would result in syn addition. However, if a second molecule of methanol participated (path b), it would deliver its proton... 

Sakurai and coworkers75 generated the five-membered silene 44 by a photochemical 1,3-silyl shift in the cyclic divinyldisilane 45 (Scheme 13). Since the silene 44 is constrained to be planar, no bond rotation is possible during the reaction. Contrary to the previous observations i.e. a simple two-step or a concerted four-centered mechanism, alcohols add to 44 nonstereospecifically, although in the cyclic silene bond rotation is prohibited. 

The photolysis of the rigid l-silabicyclo[2.2.1]heptene 250, which may be regarded as a cyclic vinylsilane, induces a 1,3-carbon shift to produce a cyclic silene intermediate 251, which is trapped by alcohols to give 252 (equation 62)141 142. 

The photolysis at 214 nm of a bridgehead silanorbornene 46 in alcohols has recently been reported by Steinmetz and Chen who proposed that a 1,3-Si to C migration occurred leading to the cyclic silene 47, which then reacted with the alcohol to give the final product 4830. The reactions are summarized in equation 6. 

Direct spectroscopic studies and calculations of cyclic silylene-to-silene and germylene-to-germene interconversions 99IZV2027. 

Subsequently, other members of the family of siienes (Me3Si)2Si= C(OSiMe3)R have been prepared, where R = Me, Et, i-Pr, CH2Ph, bicyclooctyl, CEt3, 1-methylcyclohexyl, and Mes. The first four siienes listed were not stable in inert solvents, and hence were not observable by NMR spectroscopy, since they rapidly reacted intermolecularly to give linear and/or cyclic head-to-head dimers.87 This is illustrated in Eq. (14) for the benzyl compound where the initially formed silene 5 yielded the cyclic head-to-head dimer 6 as well as the linear head-to-head dimer 7. The latter four siienes were all relatively stable and were characterized by NMR spectroscopy.105... 

The second form of head-to-head dimerization involved the formation of a linear (as distinct from a cyclic) species in which two molecules of silene form a silicon-silicon bond. If this follows the pathway suggested above in Eq. (25), the resulting 1,4-diradical must then disproportionate by hydrogen abstraction, forming a molecule saturated at one end and unsaturated at the other. Recent examples are given in Eq. (27).86... 

Wolff rearrangements were also observed when most of the same acylsi-lyldiazoalkanes were photolyzed in acetone instead of benzene.21 The ketenes 185 resulting from a 1,3-methyl migration of the silene were detected in addition to the expected ene product 186 derived from the reaction of the silene with acetone (or other enolizable ketones) (Eq. 58). When R = Ad, only the cyclic siloxatene 187 was formed under the same... 

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). 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

The potential surface for die gradient path addition of ethylene to silene and the possible existence and stability of intermediates in the thermal decomposition reaction of silacyclobutane has been explored.38 The energy maximum of die multi-step process corresponds to a cyclic transition state leading on one side to a planar silacyclobutane transition state which falls to ground-state puckered silacyclobutane and on the other side to a trans diradical which fragments to ethylene and silene. 

Sakurai, Kira and coworkers synthesized silene 253 from the cyclic divinyldisilane precursor 254 (equation 63)143. 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic generation from a trisilane 280153, thermolytic or photolytic decomposition of cyclic silanes 28114,154,155 and degradation of diazidosilanes 282153,156. The photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies157. The photolysis of w-diazo compounds 283 is the only frequently used reaction path to silenes 284 via a carbene-silene rearrangement8. 

Silenes, bearing an allylic hydrogen, frequently give linear non-cyclic dimers, in which two silene molecules form a Si—Si bond39,86,88,89 107 112. This linear head-to-head dimer can be formed by intramolecular disproportionation of the initially formed biradical (path B in equation 101). Of course, an ene -reaction (path A in equation 101)... 

When crotonate esters were employed the regioisomers 501 were mainly formed, accompanied by a significant amount of non-cyclic compounds 502 formally arising from insertion of the Si=C group in an ally lie C—H bond (equation 167). The reaction of silenes 149 and 150 with crotonate esters are the only examples known so far where the product distribution is different when the reaction is conducted under photolytic conditions or in the absence of light. Thus, in the dark reaction of 150 with methyl crotonate the relative ratio 501 (R1 = 1-Ad, R2 = Me) 502 (R1 = 1-Ad, R2 = Me) = 3.75 1 was obtained, while under photolytic conditions only minor amounts of 501 were detected245. The nominal insertion product 503 is also the major product in the reaction of methacrylate esters with 149 and 150 (equation 168). The minor product in this reaction is the l-sila-3-oxacyclohex-4-ene 504 (relative ratio 503 504 = 5.7 l)245. 

All known silenes react violently with molecular oxygen with the exception of the stable 1-silaallenes which are air-stable. Along with carbonyl compounds the products of the oxidation of silenes are cyclic siloxanes from the corresponding silanone, depending on the reaction conditions3,8,28. For example, when the stable adamantylsilene 150 is... 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

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