Reactive intermediates silenes

All silenes generated so far on the silylcarbene route are reactive intermediates themselves, which were characterized by typical subsequent reactions35 such as isomerization and dimerization or by trapping reactions (see below). However, photolysis of (silyl)diazo compounds in inert matrices at low temperature allowed the isolation and spectroscopic (IR, UV) characterization of several silenes (Scheme 2, Table 3). Irradiation of (dia-zomethyl)silanes 7 at X > 360 nm produced both diazirine 8 and silenes 10, but at shorter wavelength (X > 305 nm) the silenes were produced cleanly from both precursors the... 

Depending on the bulk of the group R1 the produced silenes are reactive intermediates (i.e. R1 = Me86-88, Et89, i-Pr89, CH2Ph89), or they are in a temperature-sensitive equilibrium with their head-to-head dimers (e.g. R1 = t-Bu)3,38,86. When R1 = 1-adamantyl3,87, no dimer was formed rather the pure silene was isolated and its crystal structure was obtained. 

Ishikawa and coworkers investigated the relative ease of migration of a Me3Si group to vinyl and phenyl groups in precursor compounds containing both groups133. In the case of the irradiation of compound 237 in the presence of methanol, silene 238 was found to be the major reactive intermediate, while with acetone and 2,3-dimethylbuta-l,3-diene the number and nature of different ene products can only be explained by the existence of silatriene 239 (equation 59). 

The addition reaction of alcohols to silenes is a strictly regiospecific process the OR group in the product is always attached to the silicon. The reaction is of special importance since silenes are easily trapped by reagents like methanol and t-butanol, and isolation of the adducts is normally taken as evidence for an intermediate silene. Furthermore, the alcohol addition to silenes is the prototypical reaction for the 1,2-addition of polar bonds across the Si=C double bond. A complete account on the alcohol addition to silenes, including the most recent mechanistic implications, is given in the chapter by Sakurai. We will therefore give only a brief survey of the reactivity of several families of silenes towards alcohols. 

The photochemistry of organosilicon compounds has been extensively studied, since many types of interesting reactive intermediates such as silylenes, silenes, disilenes and silyl... 

Experimentalists have been particularly well served by numerous reviews from some of the leading workers in the field. For example, Brook and Baines (76) have reviewed silenes, Wiberg (77) discussed M=C and M=N double bonds (M = Si and Ge), Cowley and Norman (78) have reviewed M=M (M, M = group 14 and 15 elements) double bonds, Raabe and Michl (79,80) have provided complementary reviews of multiple bonds to silicon, West (81) has reviewed disilene chemistry, Masamune (82) has reviewed the work of his group on Si=Si and Ge=Ge compounds, and most recently Barrau et al. (83) have summarized multiple bonds to germanium. Studies from the reactive intermediates era of this field are beautifully summarized by Gusel nikov and Nametkin (84). 

In another model study, Ishikawa and co-workers (42, 48) showed that for phenylpentamethyldisilane, the phenyl-substituted silyl radicals can undergo ortho radical addition to produce unstable silenes that can be subsequently trapped and identified (Scheme I). Recent spectroscopic studies have identified the silene as a reactive intermediate in this process (49). 

Direct irradiation of [(trimethylsilyl)ethynyl]pentamethyldisilane (94) afforded a mixture of reactive intermediates, which were detected and identified by flash-photolysis techniques and trapped as methanol adducts in continuous irradiation experiments. Amongst the intermediates identified was the 1-silaallene (95), which, although a minor photoproduct, was readily observable owing to its relatively long lifetime. The stereochemistry of the addition of carbonyl compounds to silenes generated by photolysis of meso- and rac-1,2-diethyl-1,2-dimethyldiphenyldisilane has been investigated. ... 

Given sufficient excitation energy, any molecule will fragment. Fundamental studies of the photochemistry of very simple organosilicon compounds, e.g. Me2SiH29 and Me4Si10, have been carried out at 147 nm. These indicated that a multiplicity of reactive intermediates including silylenes, silyl radicals, silenes and carbenes were formed, primarily as a result of homolysis of Si-H bonds if present, or in their absence Si-C and C-H bonds. Detailed mechanisms have been established in many cases. For example, mercury-sensitized... 

An interesting collection of reactive intermediates are involved in the decomposition of the diazo compound (35) or the diaziridine (36). The resulting carbene (37) in part rearranges to the silene (38) which is trapped as the silole (39), and in part rearranges to the silabenzene (40) which gives rise to the adducts (41) and (42) <83TL4245, 850M584>. 

The formation of the a-metalated species in the case of the Auner/Jones Type silenes (neopentylsilenes) 132 (which is deduced by formation of dimers 133 or by trapping) is achieved through addition of f-butyllithium to a vinylchlorosilane in an inert solvent79,80. The initially formed lithiated intermediate eliminates LiCl at ca 0°C (equation 31), the exact temperature depends on the substituents R. The multitude of transient silenes that are available through this method will be discussed in Section I.B.4.b on the reactivity of neopentylsilenes. The basic reactions of Cl3Si—CH=CH2 with t-BuLi have... 

The photochemical reactivity goes up with increasing number of methyl groups at the double bond and decreasing ionization potential28. Key intermediates in both the photochemical and the thermal oxidation of silenes 12, 7 and 13 are the siladioxetanes 14. These species are labile even in low-temperature matrices and could not be identified spectroscopically. Evidence for their formation comes from the observed oxidation products such as complexes 15 between silanones and formaldehyde and formylsilanols 16. 

Some of the reactions given in the sections above are important routes to reactive organosilicon intermediates such as silenes and silylenes, and because of their tendency to dimerize readily, to disilenes, the latter being formed when matrix-isolated silylenes are warmed up. It appears worthwhile to summarize some of the more useful reactions leading to silenes and silylenes, and their subsequent behaviors. The topics of silylenes97,142 and disilenes97,142,143 have recently been reviewed. 

Several examples were discussed earlier of the use of substituent effects for the elucidation of the mechanisms of silene reactions with nucleophilic reagents. For example, the trends in the rate constants for reaction of the series of 1,1 -diarylsilenes 19a-e with alcohols, acetic acid, amines, methoxytrimethylsilane and acetone all indicate that inductive electron-withdrawing substituents at silicon enhance the reactivity of the Si=C bond, and are consistent with a common reaction mechanism in which reaction is initiated by the formation of an intermediate complex between the silene and the nucleophile. 

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