Elimination reactions ketene-forming

On prolonged reaction with aryl isocyanates, the pyrido [ 1,2-a]pyrimidines (262) give pyrido[l,2-a]-s-triazines (266) in poor yield.328 The suggested pathway is the addition of the aryl isocyanate to form the 3,3-disubstituted product (263), which eliminates the ketene (265) the residual part of the molecule (264) reacts with an additional mol of aryl isocyanate to give 266. The ketene (265) forms the quinoline (267), by ring closure, which gives 268 by aryl isocyanate addition. The quinoline (267 R = CH2Ph, X = H) could be isolated. 

The effects of reactant structures on the ketene-forming elimination reactions of aryl esters of substituted phenylacetic acids (9) and (10) with secondary amines in acetonitrile (Scheme 2) have been studied in anticipation that the transition state might have E cB- kc E2 character.3 The reactions are second order for R2NH-MeCN and... 

Another more efficient catalytic version of the reaction consists of the interaction of ketones with chiral amines [6] to form enolate-like intermediates that are able to react with electrophilic imines. It has been postulated that this reaction takes place via the catalytic cycle depicted in Scheme 33. The chiral amine (130) attacks the sp-hybridized carbon atom of ketene (2) to yield intermediate (131). The Mannich-like reaction between (131) and the imine (2) yields the intermediate (132), whose intramolecular addition-elimination reaction yields the (5-lactam (1) and regenerates the catalyst (130). In spite of the practical interest in this reaction, little work on its mechanism has been reported [104, 105]. Thus, Lectka et al. have performed several MM and B3LYP/6-31G calculations on structures such as (131a-c) in order to ascertain the nature of the intermediates and the origins of the stereocontrol (Scheme 33). According to their results, conformations like those depicted in Scheme 33 for intermediates (131) account for the chiral induction observed in the final cycloadducts. 

Ynamines are obtained by thiol elimination from ketene 5, A -acetals (1-aIkylthio-l-dialkylaminoalkenes) in 40-50% yields on treatment with LiNEt, at 20 °C or with NaNHo in boiling piperidine, or by leading them over solid NaNH.j at 150-165 °C. In the first two procedures the formed ynamines are fractionally distilled from the reaction mixture (equation 83) . When elimination is effected with KNHj in HMPT, aqueous work-up leads to the hydration of the ynamine. Therefore, 1,2-dibromo-ethane is added to the reaction mixture. It functions as a proton donor for the... 

Enolates. The /f-H elimination from enolates involves formal elimination of ketene, and has been recognized in the decomposition of [Ru(Me) OC(CH2)H (PMe3)4] to [Ru(Me)(H)(PMe3)4], upon warming it in solution to 65°C. When this thermolysis was run in the presence of tert-butyl alcohol as a trap, tert-butyl acetate was formed in 10-15% yield, consistent with the formation of ketene during the course of the reaction [106]. 

Cp or Cp with elimination of N2 forming the corresponding carbenes Mn(CO)2(CR RML. The carbene precursor C H [C(N2)Ph]2 under similar conditions provides a bis-carbene complex. Reaction of the 7-membered carbocyclic carbene 69 with CO results in the n -ketene derivative 70. ... 

Ketenes are also formed by elimination reactions of esters, and the reaction of acetylated cellulose with tetra-n-butylammonium fluoride in tetra-hydrofuran has been suggested to result in ketene formation by an ehmination reaction by the ElcB mechanism in which fluoride ion acts a... 

In the cases where R is an ethoxyl group, thermolysis at 132 C induced a retro-ene reaction with the elimination of an ethylene to generate enyne-ketenes. Specifically, 72a was converted to the enyne-ketene 73, which then underwent a Moore cyclization reaction to form the biradical 74 (Scheme 17) (27). A 1,5-hydrogen shift afforded the biradical 75, which decayed via intramolecular routes to give the chromanol 76, the phenols 77 and 78, and the spiro ketone 79. 

Decomposition of the ketenes resembles the three-centered HX elimination reaction (Section 2.4.5) and a carbene is formed. The energy available to CO is Ex plus any El associated with the CO elimination process. Rearrangement... 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

Tsuda and Oikawa (1989) investigated the photolysis of the 1,2-isomer of 10.89 (1,2-benzoquinone diazide) by means of MINDO/3 molecular orbital calculations with configurational interaction. These authors came to the conclusion that no ketocarbene of the type of 10.90 is formed, but that the rearrangement into the cyclopentadienyl ketene 10.94 is a concerted reaction in which the elimination of nitrogen and the rearrangement take place simultaneously. In the opinion of the present author the theoretical result for 1,2-quinone diazide is not necessarily in contradiction to the experimental investigations of Sander, Yankelevich et al., and Nakamura et al., as the reagents used were not exactly the same. 

In this cyclodecarbonylation reaction, a ketene species is unlikely to be the reaction intermediate as added alcohols produce no esters. As shown in Scheme 6.26, the ruthenium acyl species 72 is likely to be the intermediate [25], which is prone to decarbonylationto give ruthena-cyclohexadiene 73 this species undergoes subsequent reductive elimination to form 2H-indene. Addition of proton or Ru to species 74 generated the benzylic cation 75, which after a 1,2-aryl shift gave the observed products. 

The Rh-catalyzed ring-expansion reaction of thiazolidine 246 in the presence of a catalytic amount of KI (2 mol%) at 180°C and 65 atm of CO gave thiazolidinone 251 in 56-88% yields. Formation of thiomorpholinone 252 was not observed at all. Thus, a proposed mechanism of this reaction involves the elimination of a ketene molecule from a key intermediate 248 to form rhodathiazolidine complex 249, which undergoes migratory CO insertion, followed by reductive elimination to yield thiazolidinone 251 (Scheme 36). ... 

The reported gas-phase acylations with Nafion-H catalyst were generally carried out at the boiling point of the hydrocarbon to be acylated. The yield of aroylation reaction depends on the relative amount of the catalyst used. Optimum yields were obtained when 10-30% of Nafion-H was employed relative to the aroyl halide. Although this procedure allows very clean reactions with no complex formation and easy work-up procedures, it is presently limited to only aroylation. Attempted acetylation of aromatics with acetyl chloride under similar conditions led to thermal HC1 elimination from the latter to form ketene and products thereof. In the reaction of acetyl chloride by itself with Nafion-H, diketene was detected by IR and NMR... 

Although simple ketones and esters can not be allylated by Pd catalysts, they are allylated with allyl carbonates via their enol ethers of Si and Sn. In the allylation of the silyl enol ether 202 with allyl carbonate 200, transmetallation of 202 with the n-allylpalladium methoxide 201, generated from allyl methyl carbonate (200), takes place to generate the Pd enolates 203 and 204. Depending on the reaction conditions, allyl ketone 205 is formed by the reductive elimination of 203 [100]. When the ratio of Pd Ph3P is small, the a,/i-unsaturated ketone 206 is obtained by -elimination [101]. For example, the silyl enol ether 208 of aldehyde 207 is allylated with allyl carbonate (200) to give a-allylaldehyde 210 via 209. The a-allyl carboxylate 213 is obtained by allylation of ester 211 with allyl carbonate (200), after conversion of ester 211 to the ketene silyl acetal 212 [102], As the silyl group is trapped in these... 

In the reaction of allyl acetate 217 with ketene silyl acetal 218 of methyl acetate, using a Pd catalyst coordinated to DPPP, cyclopropane 220 is formed in addition to the expected allylacetate 219 [104], The cyclopropanation becomes main reaction when TMEDA, as a ligand, and thallium acetate are added [105]. The cyclopropanation can be understood by the attack of the enolate ion at the central carbon of 7r-allylpalladium to form the palladacyclobutane 221, followed by reductive elimination. 

Na2S204 works also as a SET reagent. Treatment of RfI (perfluoroalkyl iodide) with Na2S204 generates electrophilic radical, Rf, which reacts with electron-rich ketene dithioketal (32) through a chain pathway, to form Rf-substituted ketene dithioketal (34) via the elimination of hydrogen iodide. So, it looks like a substitution reaction of... 

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