Silenes with alcohols

Nucleophiles easily attack the Si=C double bond at the silicon. Complexes between diethyl ether and the parent silene H2Si=CH2 have been detected by low tonperature NMR and stable silenes of the Wibag -type form addncts with THF, triethylamine, pyridine and even with the fluoride anion . These complexes have been structurally characterized by X-ray structure analysis and details are presented in Section LC.l. Similarly, the initial step of the alcohol addition to silenes is the formation of a short-lived complex between the silene and the attacking alcohol , as is evident from the measured negative activation energy in the reaction of simple silenes with alcohols . ... 

Prior to 1985, much had been learned about the chemistry of the silicon-carbon double bond through the study of the reactions of silenes with a wide variety of reactants. Thus it was known that all silenes studied reacted readily with alcohols (particularly methanol) by regiospecific addition across the ends of the Si=C bond in which the MeO group became attached to silicon and the alcoholic H to carbon, as in Eq. (22). 

In 1975, we found that irradiation of pentamethylphenyldisilane with a low-pressure mercury lamp leads to the transient formation of a silene. In the presence of a trapping agent such as alcohol, the silene thus formed reacts with alcohol to give addition products, while in the absence of the trapping agent, it undergoes polymerization to give nonvolatile substances (5). 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

Phenyl(triphenylsilyl)carbene has also been trapped without the interference of a silylcarbene-to-silene rearrangement84. It undergoes 0,H insertion with alcohols and is oxidized to the ketone by DMSO the latter reaction is likely to include an S-oxide ylide (equation 56). 

Silenes also react efficiently with alcohols to give addition products. Indeed, addition is the most characteristic reaction of silenes and has been used for trapping silenes. Alcohols react regiospecifically to form alkoxysilanes. 

Rate constants for the reaction of silatrienes 52a-c with alcohols are listed in Table 6. According to the proposed mechanism, kq and k correspond to k and k2 of equation 15, respectively. It is interesting to compare the very low value of kq/kq (0.015-0.047) in the reaction of 52a-52c with methanol with the corresponding k /k2 value of 4.6 for the silene 44. The latter is about two orders of magnitude greater than the former. Apparently, silatrienes derived from aryldisilanes behave very differently from 44. 

The stereospecificity of methanol addition to neopentylsilenes has been investigated by Jones and Bates68. The mild thermal retro-Diels-Alder reaction (at ca 200 °C) of E and Z anthracene [4 + 2] cycloadducts 110 liberates stereospecifically the corresponding silenes 111, which are trapped by methanol. The ratio of the diastereomeric products 396a/396b coincides with the E/Z ratio of the precursors 110 (equation 117). In photochemical reactions of similar silene precursors, alcohols were used also to probe the decomposition mechanism69. 

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. 

By far the best source for 3a is (trimethylsilyl)diazomethane (19). It has already been mentioned that gas-phase pyrolysis of 19 alone yields products which are derived from intramolecular carbene reactions such as 1,3-C,H insertion and silylcarbene-to-silene rearrangement (see equation 20). Also, copyrolysis of 19 with alcohols or benzaldehyde allowed one to trap the silene but not the carbene 33 (see equation 5). Furthermore, solution photolysis of 19 in the presence of alcohols or amines did not give the X,H insertion products of the carbene but rather trapping products of the silene . On the other hand, photochemically generated carbene 3a did undergo some typical intermolec-ular carbene reactions such as cyclopropanation of alkenes (ethylene, frani-but-2-ene, but not 2,3-dimethylbut-2-ene, tetrafluoroethene and hexafluoropropene), and insertion into Si—H and methyl-C—H bonds (equation 39). The formal carbene dimer, trans-1,2-bis(trimethylsilyl)ethene, was a by-product in all photolyses in the presence of alkenes it is generally assumed that such carbene dimers result from reaction of the carbene with excess diazo compound. 

Some unusual behaviour was displayed by the benzodisilacyclobutane 84 as described by Ishikawa et al.95 When thermolyzed, it appeared to form the quinodimethane bis-silene species 85 shown in Scheme 13, as confirmed by trapping reactions with f-butyl alcohol, alkynes, or aldehydes, all of which added in a 1,4-manner (see Scheme 13). In the absence of a trapping reagent, 85 decomposed, but not to 86 as claimed earlier.95 ... 

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... 

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