Desilylation-elimination

OL-Methylenecyclohutanones. The reagent reacts regioselectively with activated alkenes (vinyl ethers, silyl enol ethers) to give cyclobutanones. These products undergo ring expansion with diazomethane to cyclopentanones. Both products undergo desilylative elimination in the presence of fluoride ion to form a-methylene ketones. 

Nitration of 2-methyl-l-(trimethylsiloxy)cyclohexene (31) with nitronium tetrafluoroborate leads to 2-methyl-2-nitrocyclohexanone (32 equation 12), while 6-methyl-l-(trimethylsiloxy)cyclohexene (33) gives under similar conditions a 30 70 mixture of cis- and fran5-6-methyl-2-nitrocyclohexanone (equation 13). The desilylative nitration of allylsilanes with nitronium tetrafluoroborate proceeds readily. The reaction is considered to pass through initial electrophilic attack of the nitronium ion on the allyl system followed by desilylative elimination. So, on treatment of l-(trimethylsilyl)but-2-ene (34) with nitronium tetrafluoroborate the product is 3-nitrobut-l-ene (35) and not l-nitrobut-2-ene (equation 14). ... 

It is possible for organosilyl compounds with a leaving group at the fi position to undergo E2-type reactions induced by cesium fluoride. In deprotonation-ehmina-tion, a strong base such as amide or alkoxide is required the conjugated acid produced by deprotonation (e.g. amine or alcohol) may react with the elimination product. In the case of cesium fluoride-induced desilylation-elimination, the formed conjugated acid is silyl fluoride which is unreactive. The production of unstable products such as cyclopropene (Scheme 2.13), cyclohexene-3-yne (Scheme 2.14), or orthoquinodimethane has been reported [24—29]. 

In fl-trimethylsilylcarboxylic acids the non-Kolbe electrolysis is favored as the carbocation is stabilized by the p-effect of the silyl group. Attack of methanol at the silyl group subsequently leads in a regioselective elimination to the double bond (Eq. 29) [307, 308]. This reaction has been used for the construction of 1,4-cyclohexa-dienes. At first Diels-Alder adducts are prepared from dienes and P-trimethylsilyl-acrylic acid as acetylene-equivalent, this is then followed by decarboxylation-desilyl-ation (Eq. 30) [308]. Some examples are summarized in Table 11, Nos. 12-13. 

The desilylation is also promoted by various Lewis acids, Yb(OTf)3 being among the most effective. This catalyst can be used in a one-pot sequence in which it promotes both the cycloaddition and subsequent elimination.48... 

Silyl enol ethers and silyl ketene acetals also offer both enhanced reactivity and a favorable termination step. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCl4. This reaction provides a method for introducing tertiary alkyl groups a to a carbonyl, a transformation that cannot be achieved by base-catalyzed alkylation because of the strong tendency for tertiary halides to undergo elimination. 

The reaction of diphenylmethane with dibenzylbarium in THF, or barium /-butoxide with trimethylsilyldiphe-nylmethane, Ph2CH(SiMe3), in the presence of BunLi forms barium diphenylmethanide by hydrocarbon elimination and desilylation mechanisms, respectively.304 Crystallized in the form of its 18-crown-6 ether derivative 116 (Figure 60), the complex displays direct Ba-C bonds of 3.065(3) and 3.097(3) A, and a longer contact at 3.39 A. 

The reaction starts with desilylation of nitronate. The a-nitro carbanion which is eliminated gives (through the transition state A) the coupling product, the anion B. The latter desilylates the next nitronate molecule to form the target product and continues the chain. 

Evidently, an increase in steric hindrance around the reduced fragment requires the presence of ammonium fluoride in the reaction mixture. It should be noted that potassium fluoride has no effect. It is highly probable that ammonium fluoride is required for slow elimination of HF, which gradually desilylates the nitroso acetal fragment thus facilitating its reduction. As can be seen from Scheme 3.284, many reduction products are derivatives of unnatural amino acids. Since the initial nitroso acetals can be prepared by silylation of simple acyclic AN, possibilities have been opened for the synthesis of unnatural amino acids from available AN. 

The isopropenyl side chain may derive by elimination of a tertiary alcohol or ether as in 202. Such a masking of the olefin avoids a possible competing vinylcyclopropane rearrangement. The correspondence of the cyclopentene of 202 with the vinylcyclopropane in 203 now becomes obvious. The presence of the dimethylcarbinol side chain now also offers the opportunity for its introduction by addition of a cyclopropyl anion to acetone. The feasibility of creating such an anion by fluoride initiated desilylation... 

On the other hand, the use of [Rh(CO)2Cl]2 as a catalyst results in ring opening of the siloxycyclopropanes 13 to the silyl enol ethers 14 with high stereoselectivity [10]. The 2-siloxyrhodacyclobutane 15a is proposed to undergo j8-elimination to give jr-allylrhodium 16a followed by reductive elimination to the silyl enol ether 14a. 1-Trimethylsiloxybicyclo[n.l.0]alkanes serve as / -metallo-carbonyl compounds via desilylation with a variety of transition metals [11]. The palladium-catalyzed reaction of the siloxycyclopropanes 17 under carbon monoxide in chloroform provides a route to the 4-keto pimelates 18. In the presence of aryl triflates, the 1,4-dicarbonyl compounds 19 are... 

Danishefsky dienes [98] cycloadd to Cjq in refluxing toluene or benzene [5, 38, 99-101]. The diene 103 adds in 60% yield to Cjq to give the desilylated ketone 104 [5,101]. Acid-catalyzed methanol elimination then furnishes the enone 105 in 82% yield (Scheme 4.17). As already described, this enone can be reduced by DIBAL-H to the corresponding alcohol for further functionalization. The same a,(3-un-saturated alcohol can also be obtained in better yield by Diels-Alder reaction of Cg0 with butadiene, followed by oxidation with singlet oxygen to the allylic hydroperoxide and PPhj reduction to the desired alcohol [101]. This sequence yields the allylic alcohol in 53%, starting from Cjq without the need of isolating intermediates. 

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