Silenes preparation

The facile photochemical sigmatropic 1,3-trimethylsilyl shift in polysilylacylsilanes from silicon to oxygen (equation 33) was utilized historically to prepare the first relatively stable silenes3 86 87. Silenes prepared by isomerization of acylpolysilanes bear, due to the synthetic approach, a trimethylsiloxy group at the sp2-hybridized carbon and relatively stable silenes of this type have in addition also at least one trimethylsilyl group at the silicon. These substituents strongly influence the physical properties and the chemical behaviour of these silenes. This is noticeable in many reactions in which these Brook -type silenes behave differently from simple silenes or silenes of the Wiberg type. 

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

Since that time, a few additional silenes or silaallenes have been prepared by this method references are given in Table I. 

References to the silenes so prepared since 1985 are listed in Table I. None of these silenes was stable, most undergoing head-to-tail dimerization to give 1,3-disilacyclobutanes in the absence of trapping reagents. Some interesting spontaneous silene-to-silene rearrangements were observed,52 which will be described in Section IV.E. 

Early studies by Sakurai101 demonstrated that vinyldisilanes readily rearrange via 1,3-silyl migration when photolyzed, thereby forming silenes, as shown in Eq. (11), which could be trapped by conventional methods. More complex silenes have been prepared more recently, particularly by Ishikawa et al.,66-67-69 and several references to these studies are given in Table I. 

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... 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

In view of the evident reactivity of the Brook-type silenes toward carbonyl compounds and the fact that these silenes were prepared by the photolysis of acylsilanes, it is natural to ask why the silenes apparently did not react with their acylsilane precursors. This question has been answered recently. On the one hand, as shown in Scheme 19, the silene Ph2Si=C(OSiMe3)Ad apparently did add in a [2 + 2] manner to its acylsi-... 

Irradiation of a solid film prepared from the polymer 11 in air afforded photodegradation products which are soluble in 2-ethoxy-ethanol. IR spectra of the products show strong absorption bands attributed to Si-OH and Si-O-Si stretching frequencies. In contrast to the products from 10, these products show absorption due to the Si-H bond. This result indicates that some of the silenes would be formed in this system. The intense absorption at 254 nm in the UV spectrum again disappeared after UV-irradiation. 

All results so far obtained prove the high synthetic potential of differently substituted neopentylsilenes. Especially silicon dichloro substituted silenes are useful for the preparation of a wide variety of new silacyclobutanes and -butenes. These SiC four membered ring compounds can be utilized as pre-... 

Dichloroneopentylsilene is formed in situ by reaction of trichlorovinylsilane with LirBu [1], The [2+2] cycloaddition to imines yields Si-dichloro functionalized 2-silaazetidines in a preparative scale [2], When aldimines are used as trapping agents for the silene, the resulting SiN-four membered ring compounds are isolated as syn/anti-isomers (syn/anti 2/1). 

It is an intriguing idea to stabilize low-valent silicon species, such as silyl cations, silylenes, silenes, and disilenes using intramolecularly coordinating ligands. Corriu et al. succeeded in the preparation of the first hypervalent silyl cation [(8-Me2NCioH6)2SiH]+l/2[l8]z 782 by the reaction of the hexacoordinated diorganosilane... 

Intramolecularly coordinated silenes have been prepared by Oehme et al.868-871 Adopting their general procedure for the synthesis of silenes (Scheme 33), 8-dimethylaminonaphth-l-yl lithium has been reacted with (Me3Si)3SiCHCl2 yielding the disilene (Me3Si)RSiC(SiMe3)2 929 via the intermediates 930-933 (R = 8-dimethyl-aminonaphth-l-yl) (Scheme 131). 

In a similar way, 2-(A, A -dimethylaminomethyl)phenyllithium and 2,6-bis(iV,iV-dimethylaminomethyl)-phenyllithium (Figure 4) have been used to prepare the intramolecularly coordinated silenes 934 and 935 (Figure 7). 

The alcohol 10 looks like it might be formed by the addition of a Grignard reagent to an aldehyde. In fact, Patrick Steel of the University of Durham prepared 10 (Tetrahedron Lett. 44 9135,2003) by Diels-Alder addition of the transient silene derived from 7 to the diene 8. More highly substituted dienes lead to more complex arrays of stereogenic centers. The intermediate silacyclohexenes, exemplified by 9, should also engage in the other reactions of allyl silanes. 

Silaethylenes (silenes) were extensively reviewed in 1979 and the first example stable enough to be bottled was prepared two years later. It results from the photolysis of the ketone (56) and is stable in the absence of air and other reagents. It melts at 92-95 °C and shows a ai NMR spectrum with the two Me3Si groups on the Si=C multiple bond nonequivalent, indicating restricted rotation. The IR absorption at 1135 cm-1 typifies a silaethylene (Scheme 85) (81CC191). 

Silathietanes can be readily prepared from the appropriate bis(chloromethyl)silane and KSH or by intramolecular hydrosilation in the presence of Wilkinson s catalyst (Scheme 97) (81JOM(204)13). Electron impact and photoionization mass spectrometry support the loss of silathione ions (R2Si=S) (R = Me, Et) indicating a transannular interaction, though decomposition by the loss of silenes and thioaldehyde also readily occurs (81JOM(214)145). 

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