Silyl enol ethers dehydrogenation

In discussion of the dehydrogenation of silyl enol ethers using DDQ, Jung and Murai had proposed that DDQ abstracts a hydride producing a stabilized cation 70 which loses TMS+ to give the enone 23 (Scheme 3.31) [8]. 

Reinvestigating the reaction of cyclohexanone silyl enol ether 22 with DDQ, adducts were also observed [26]. At 25 °C, both C-C and C-0 adducts (75 and 76) were formed along with the dehydrogenation products 23 and 77. The ratio of... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

Elimination of the Pd enolate 253, formed by the transmetallation of 252, is a good synthetic method of the enones 254 [152], The silyl enol ether 255, prepared by trapping the conjugate addition product of cyclohexenone, is dehydrogenated with... 

Pd(OAc)2 to give the cyclohexenone 256. In clavulone synthesis, only the silyl enol ether in 257 reacts with Pd(OAc)2 to give the enone 258 [153]. The dehydrogenation can be carried out with a catalytic amount of Pd(OAc)2 using benzoquinone as the reoxidant. Cyclopentenone (260) is prepared from cyclopentanone (259) by using a supported Pd catalyst under 02 atmosphere [154], The enone 261 is converted to the dienone 263 via the dienol silyl ether 262 [155],... 

Side Note 12.4. Dehydrogenation of Ketones via Silyl Enol Ethers... 

A related reaction can be used to convert silyl enol ethers or enol acetates into enones. Phosphine-free Pd(OAc), is also the preferred catalyst for these dehydrogenations. 

The main side reaction of the hydrosilation reaction is the dehydrogenating silation reaction. Under certain conditions this reaction can be the main or even the exclnsive reaction. The reaction can occnr, not only with alkenes, bnt also with almost all known substrates. It achieves vinylsilanes from alkenes, silylalkynes from alkynes, and silyl enol ethers from ketones. ... 

In the synthesis of enone 18, ketone 17 is first converted into the corresponding silyl enol ether 31 using LDA and TMSCI under kinetic control. Then, the dehydrogenation with the IBX-4-methoxypyridine-A-oxide complex (11) takes place to provide the a, p-unsaturated ketone 18. 

Dehydrogenation of carbonyl compounds. Prior formation of silyl enol ethers is... 

The preparation of silyl enol ethers has been reviewed. A nontraditional approach to their preparation involves a dehydrogenative silylation using a silane, a metal... 

The dehydrogenative silylation becomes virtually exclusive when Co2(CO)8 and triethylamine are used for the reactions of ketones with HSiMe2Ph, HSiEt3 and HSiMe3187. The reaction provides a convenient route to silyl enol ethers (equation 58). 

However, such a marked ligand effect of the ester moiety was not observed in the case of methyl acetoacetate (23.5% e.e.)153 or ethyl benzoylacetate (62.3-68.4% e.e.)207. This may be due to the fact that the silyl ether could be produced not only by the hydrosilylation, but also by in situ transfer hydrogenation of the silyl enol ether (a mixture of E and Z isomers), which is formed in the first step of the reaction by dehydrogenative coupling of the enol with the hydrosilane. 

Some nucleophiles add to Af-fluoropyridinium salts to give dihydropyridines in which elimination of fluoride occurs in situ to give the 2-substituted pyridine, thus avoiding the need for a dehydrogenation step. The main disadvantages of this method are that the preparation of the pyridinium salts require the use of elemental fluorine and that some carbanions give only modest yields due to competitive reactions such as C-fluorination. However, silyl enol ethers do react efficiently stabilised heteronucleophiles (phenolate, azide) can also be used. Addition to N-fluoropyridinium salts shows a strong preference for attack at an a-position. ... 

Diketones are accessible via a number routes, for example by Michael addition of enolate to enone (or precursor Mannich base " ), by ozonolysis of a cyclopentene precursor, or by reaction of silyl enol ethers with 3-methoxyallylic alcohols. They react with ammonia, with loss of two mol equivalents of water to produce a cyclic bis-enamine, i.e. a 1,4-dihydropyridine, which is generally unstable but can be easily and efficiently dehydrogenated to the aromatic heterocycle. 

Normally, the A-arylimine is obtained by reaction of aldehyde and aniline in acidic condition. Either tetrahydroquinoline or its corresponding substituted quinoline can be generated in the Povarov reaction, depending on the reaction conditions. For instance, DDQ-promoted dehydrogenation, vacuum distillation under acidic condition, oxidation by air or Mn(OAc)3, and Pd/C-catalyzed aromatization of tetrahydroquinoline, provides the corresponding substituted quinolines in good to excellent yield. Since some tetrahydroquinolines are unstable under the reaction conditions, the corresponding substituted quinolines could be isolated as the sole products. Electron-rich olefin, such as vinyl enol ethers, vinyl sulfides, and silyl enol ethers, are widely used as dienophiles in the cycloaddition of A-aryl aldimines to obtain substituted tetrahydroquinolines. To access natural... 

Rh catalyses the hydrosilylation of alkenes and the dehydrogenative silylation of 1,5-dienes and alkynes, and the silylcarbonylation of alkynes to give p-silylacrylaldehydes with exceptional stereocontrol , ketones and enones give silyl enol ethers, 1,3-cyclobutane dione gives 3-hydroxycyclobutanone, and assists in the silylformylation of ethynyl pyirolidone derivatives to give pyrrolizidine alkaloids, and epoxides to give p-siloxy aldehydes, and... 

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