Disilenes with ketones

The reaction of Brook s ketone with an excess of Et3GeLi followed by hydrolysis gave a 1 2 mixture of a trisilacyclobutane and (adamantoyl) adamantyl carbinol. It was suggested that this reaction involves the formation of a transient disilene. The structure of the trisilacyclobutane was established by X-ray analysis (equation 197)205. 

Disilene 1 reacts not only with aldehydes and ketones but also with some esters (Eq. 10)6 and acid chlorides (Eqs. 11 and 12).69 In the reaction... 

West and coworkers studied the photolysis of several adducts of disilenes with ketones, i.e. 1,2-disiloxetanes83. Based on the products obtained when the photolysis was carried out in ethanol as a trapping agent, it appears that the heterocyclic disiloxetane 179 decomposed to the silanone 180 and the silene 181, each trapped by ethanol to give the adducts 182 and 183, respectively (Scheme 29). In the absence of a trapping agent the silene photochemically rearranged to 184. A related 1,2-disilathietane 185 showed similar behavior (Scheme 29184. 

Polyphenylsilane, (PhSiH) , can be derivatized by free-radical hydrosilylation in the presence of a radical initiator. Alkenes, ketones and aldehydes react readily, to replace up to 93% of the Si-H bonds. This route can be employed to make polysilanes with hydrophilic groups, such as hydroxy, amino and carboxylic acid functions.43 Dialkylamino substituted polysilanes, made by the anionic polymerization of masked disilenes (see equation (17)), when treated with acetyl chloride give chloro-substituted poly silanes. The chlorine can then be displaced by other nucleophiles.27... 

The ene reaction of cyclic disilene 50 with enolizable ketones such as biacetyl and acetophenone gives initially a.v-adduct 199, which gradually isomerizes to the corresponding traKi-isomer 200 104,133... 

Stepwise radical mechanisms have been proposed for the apparent [2 + 2] cycloaddition of disilenes with ketones by Baines et a/.140,141 They have found that the reactions of tetramesityldisilene 1 with trara-2-phenylcyclopropane carbaldehyde (208a) and fra/M,fra/M-2-methoxy-3-phcnylcyclopropane carbaldehyde (208b), a mechanistic probe developed by Newcomb et al.,142 undergo characteristic cyclopropane ring-opening as shown in Eq. (99). 

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. 

Disilenes react with ketones, aldehydes, esters and acid chlorides by formal [2 + 21-cycloaddition to yield the corresponding disiloxetanes (equation 73)8,16. The reaction is non-concerted and proceeds through the initial formation of a 1,4-biradical intermediate, as has been shown by the products of reaction of tetramesityldisilene (110) with the cyclopropyl aldehyde 117 (equation 90)163. The absolute rate constants listed in Table 19 indicate there to be a significant difference in reactivity between the monophenyl-substituted disilene 103 and the 1,2-diphenyl-substituted derivatives 104, consistent with a steric effect on the rate of formation of the biradical intermediate. As would be expected, no kinetic deuterium isotope effect is discernible from the relative rates of addition of acetone and acetone- to these compounds. 

In spite of their bulky substituents, the presence of which is necessary to prevent oligomerization reactions, disilenes undergo numerous addition and cycloaddition reactions these have been summarized in several review articles [1]. For example, stable or marginally stable disilenes react with the C=0 and C=S groups of ketones [2-4] and thioketones [5] as well as with the triple bonds of acetylenes [2, 3] and nitriles [6]. Surprisingly, however, no cycloaddition reactions of disilenes with simple alkenes have yet been reported [1]. 

The bis(amino)silylene 1 (abbreviated as "SiN2") reacted readily at room temperature with benzophenone to give the 1,2,3-oxadisiletane 2 (Scheme 1 X-ray structure in Figure 1). These oxadisiletanes have only previously been observed by reaction of a disilene with a ketone [9, 10]. The formation of 2 is thought to proceed via an oxasilacyclopropane intermediate. The reaction of 1 with u(Me)C=0 yielded a similar disilaoxetane. 

Silanones, the silicon analogs of ketones, are produced via rDA reactions. One attempt to prepare a suitable DA precursor for retrograde decomposition to a silanone met with unexpected results. The desired DA reaction between 2,2-dimethyl-l-oxa-2-silacyclohexa-3,S-diene (251) and perfluoro-2-but-yne was complete in one day at room temperature. The observed product was o-bis(trifluoromethyl)ben-zene, as the initial adduct apparently underwent retrodiene decomposition to yield the intermediate dimethylsilanone (252) (equation 111). The occurrence of this retro ene process at room temperature was not consistent with the analogous extrusions of silenes and disilenes that require elevated temperatures. However, the reaction sequence was substantiated by comparison with its carbon analog in which tetramethylpyran and dimethyl acetylenedicarboxylate react at room temperature to afford only acetone and the corresponding phthalate. Stable adducts that extrude silanones are also known. Reactions of 2-sili yrans and nudeic anhydride provide stable adducts, such as (253), that decompose upon thermolysis... 

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