Silenes, reactions 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). 

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 ... 

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

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... 

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. 

The competitive migration to either a vinyl or phenyl group has been investigated by Ishikawa and coworkers133. While with traps other than methanol there is proof for the intermediacy of both silenes 238 and 239, formed from 237 (of equation 53), just one product, i.e., 400, is found for the reaction with the alcohol (equation 119). 

THF can also have an accelerating effect on reactivity, in cases where the weakly basic solvent can get directly involved in the reaction via a catalytic pathway and complexation with the free silene is weak. Such an effect has been observed for the reaction of alcohols with the aryldisilane-derived l,3,5-(l-sila)hexatriene derivatives 21a-c, as shown by the second-order rate constants for reaction of the three silenes by MeOH and TFE in isooctane, MeCN and THF solution (Table 11 note that the third-order rate constants for reaction of 21a-c with MeOH have been omitted)48. Table 11 also includes data for the reactions with acetone in the same three solvents, as an example of a reaction which has no catalytic component47. The rate constants for all three reactions decrease in the order isooctane > MeCN > THF for 21a, which complexes relatively strongly with the ether solvent, as demonstrated by the distinctive red shift in its UV absorption spectrum in THF (kmax = 460 nm) compared to isooctane and MeCN (kmax = 425 nm)48. Compound 21b exhibits a ca 10 nm shift of its absorption band in THF solution while none is detectable in the case of 21c, indicating that the equilibrium constant for THF complexation within this series of silenes decreases with increasing phenyl substitution at... 

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. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

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 . ... 

A modified Peterson reaction has been used to generate silenes which have then been converted to diols and lactones. The reaction involves the formation of the sila-Grignard reagents under standard conditions, followed by treatment with isobutyraldehyde to give the silene precursor 76. Silacyclohexene 76 was then produced by reaction with 1,3-pentadiene. High diastereoselectivity was observed and confirmed by 2D NMR experiments on the subsequent reduction and oxidation products. Thus, standard conditions converted silane 78 into diol 79 which was then oxidised to give lactone 80. The protocol can be expanded to achieve further functionalisation, for example in the synthesis of homoallylic alcohol 82. 

The relative rates of reaction of the silene Me2Si=C(SiMe3)2 with a series of amines, alcohols, phenols, thiophenols, dienes, and alkenes were obtained174 and are reported in Table VIII Section IV.C. 

Analogously, copyrolysis of 19 with an alcohol at 425 °C also leads to the silene-derived product, but not to the 0,H insertion product of the carbene38. The formation of 1-phenylpropene upon copyrolysis of 19 with benzaldehyde38 (equation 5) corresponds to another well-established silene-trapping reaction, namely [2 + 2] cycloaddition between... 

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