Halogenation and deuterium exchange in ethyl cyanoacetate. Enolisation mechanism and enol reactivity

Eberlin, Alex R.; Williams, D. Lyn H.

doi:10.1039/p29960001043

Cited by 10 publications

(6 citation statements)

References 16 publications

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“…They belong to three groups. (i) Derivatives of alkyl β,β-(di-bulky aryl) acetates 3a − c were observed as short-lived intermediates, 7a-c and (ii) malonic ester derivatives were detected by classical enol identification methods 7d. (iii) Two solid-state structures of compounds substituted with EWGs, i.e., a Meldrum's acid derivative 8 and pentakis(methoxycarbonyl)cyclopentadiene Cp-5 9 are the only enols of esters found in the Cambridge Structural Database (CSD).…”

Section: Introductionmentioning

confidence: 99%

Stable Enols of Carboxylic Esters. The Poly(methoxycarbonyl)cyclopentadiene Systems

Lei¹,

Cerioni²,

Rappoport³

2000

J. Org. Chem.

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The structures of the poly(methoxycarbonyl)cyclopentadienes C(5)H(6)(-)(n)()(CO(2)Me)(n)(), n = 5 (Cp-5), n = 4 (Cp-4), n = 3 (1, 2,4-; Cp-3) and n = 2 (1,2-; 1,2-Cp-2-I) were investigated. The X-ray diffractions of Cp-5 (known), Cp-4, and Cp-3 showed an enol of ester structure in the solid state. The enolic hydrogen forms a symmetrical hydrogen bond to a neighboring ester carbonyl, so that the vicinal "enolic" CO(2)Me groups in the 1, 2-C(=CO(2)Me)-C(CO(2)Me)(4) moiety are identical. The relevant X-ray parameters for the three enols are similar. The CP-MAS spectra of Cp-3-Cp-5 generally resemble their (13)C NMR spectra in CDCl(3) except for some differences of mostly <1 ppm. The (1)H, (13)C, and (17)O NMR spectra of Cp-3-Cp-5 in CDCl(3) are consistent with those of the hydrogen bonded enols. Most characteristic are the (1)H and (17)O signals of the OH groups at 19.7-20.1 and 221-225 ppm, respectively. Proton addition to sodium 1, 2-bis(methoxycarbonyl)cyclopentadienide gave a mixture of four 1, 2-bis(methoxycarbonyl)cyclopentadienes. The isomer (1,2-Cp-2-I) formed in 10-20% displays delta(O(1)H) at 19.3 ppm and is the enol analogue of Cp-5 whereas its main isomer (30-55%) (1,2-Cp-2-IV) has the ester structure. In CD(3)CN and DMSO-d(6) only one signal was observed at room temperature for each type of H, C, or O of Cp-5, suggesting a complete ionization to the symmetrical anion of Cp-5. In contrast, Cp-4 and Cp-3 in CD(3)CN at room temperature display OH signals in both (1)H and (17)O NMR spectra, and Cp-5 shows a broad OH signal in the (1)H spectrum at 240 K. The enol of ester structure is the main species, although exchange with the corresponding anion is possible. On standing in DMF-d(7) at room temperature, new signals are observed for Cp-3 and Cp-4. On raising the temperature in Cl(2)CDCDCl(2), Cp-3-Cp-5 show line broadening and appearance of new signals. These were ascribed to rearrangment and decomposition processes.

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Section: Introductionmentioning

confidence: 99%

Stable Enols of Carboxylic Esters. The Poly(methoxycarbonyl)cyclopentadiene Systems

Lei¹,

Cerioni²,

Rappoport³

2000

J. Org. Chem.

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“…We have also used the same procedure successfully earlier in the case of enolisation of ethyl cyanoacetate. 17 During exchange with D 2 SO 4 -D 2 O the singlet peak in the proton NMR spectrum from the methylene protons (at 3.6 ppm) was replaced by a triplet arising from the mono-deuterated species B (due to coupling with the deuterium atom). Over a longer time period there was evidence of conversion to the di-deuterated species C. Reaction was monitored by the decreasing singlet peak integral due to A.…”

Section: Results (A) Reactions Of 2-cyanoacetamide Camentioning

confidence: 99%

Halogenation of enol tautomers of 2-cyanoacetamide and malonamic acid

Eberlin

Williams

2002

J. Chem. Soc., Perkin Trans. 2

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Reactions of 2-cyanoacetamide (CA) with both bromine and iodine at low halogen concentration in aqueous acid solution were first order in halogen and CA, consistent with an interpretation involving rate-limiting halogenation of the enol tautomer. There was no acid dependence in the range 0.1-0.5 mol dm Ϫ3 . The observed rate constants for bromination and iodination were very similar, indicating that both probably occurred at the encounter limit. This enabled a value for K E , the equilibrium constant for enolisation, to be determined as 6 × 10 Ϫ10 . There was strong evidence that at low acidities (pH ∼ 4) enolisation is rate-limiting, but we were unable to achieve a fully zero-order kinetic dependence upon halogen. However we did obtain a value for the acid-catalysed enolisation rate constant k e of 2.7 × 10 Ϫ3 dm 3 mol Ϫ1 s Ϫ1 from measurements of rates of deuterium exchange with the solvent. The bromination of malonamic acid (ML) in dilute acid solution at low [Br 2 ] was zero-order in [Br 2 ] and we obtained a value of 1.3 × 10 Ϫ2 dm 3 mol Ϫ1 s Ϫ1 for the acid catalysed enolisation rate constant. There is some evidence which suggests that enolisation of ML takes place via the amide group rather than the carboxylic acid group. The results are compared with earlier studies.

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“…16 3-(Dimethoxymethyl)pentane-2,4-dione (10a). ν max (gas phase)/ cm Ϫ1 3005 (w), 2961 (m), 2944 (m), 2844 (w), 1718 (s), 1444 (w), 1363 (m), 1284 (w), 1199 (m), 1120 (s), 1084 (s); δ H (300 MHz) 2.17 (6H, s, COCH 3 ), 3 (12), 101 (51), 85 (100), 83 (14), 75 [(CH 3 O) 2 CH] ϩ (82), 69 (10), 55 (10), 47 (25), 43 (94).…”

Section: Thermolysis Of Dimethoxydimethyloxadiazoline (8a) In the Pre...mentioning

confidence: 99%

“…A similar thermolysis in which the oxadiazoline (0.15 g, 1.0 mmol) was in excess over 12 (0.11 g, 0.6 mmol) in 25.0 ml of benzene containing two drops of pyridine afforded 0.17 g of 13 (89%, based on anthrone) which was pure enough to characterize by NMR spectroscopy although it still contained a detectable amount of anthrone. 9-Anthracenyl dimethyl orthoformate (13 (18), 163 (31), 139 (12), 115 (5), 75 (100), 63 (5), 47 (30).…”

Section: Thermolysis Of Dimethoxydimethyloxadiazoline (8a) In Presenc...mentioning

confidence: 99%

Reactions of nucleophilic carbenes with enols

Couture¹,

Pole²,

Warkentin³

1997

J. Chem. Soc., Perkin Trans. 2

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Halogenation and deuterium exchange in ethyl cyanoacetate. Enolisation mechanism and enol reactivity

Cited by 10 publications

References 16 publications

Stable Enols of Carboxylic Esters. The Poly(methoxycarbonyl)cyclopentadiene Systems

Stable Enols of Carboxylic Esters. The Poly(methoxycarbonyl)cyclopentadiene Systems

Halogenation of enol tautomers of 2-cyanoacetamide and malonamic acid

Reactions of nucleophilic carbenes with enols

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