Theoretical calculations

Apeloig, Yitzhak

doi:10.1002/9780470772294.ch1

Cited by 10 publications

(8 citation statements)

References 115 publications

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“…The uncatalyzed rotational barrier around the double bond in these compounds should be lower than that in alkenes due to the contribution of the hybrid form 1a (eq 1) with the expected longer than normal C1C2 bond. However, the calculated CC double bond lengths of vinyl alcohol and of ethylene are almost identical, and even when the ethylene is substituted by bulky mesityl groups this is not appreciably reflected in bond lengths. The X-ray determined double bond lengths of systems 1 , X = OH, OPr- i , are only slightly (by ca.…”

Section: Introductionmentioning

confidence: 94%

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol¹

Rochlin

Rappoport

2003

J. Org. Chem.

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(E)-2-(m-methoxymesityl)-1,2-dimesitylethenol (3a) isomerizes in the absence of a catalyst in solution to a 1.0:0.9 E/Z (3a/3b) equilibrium mixture. In CDCl3 the isomerization is first order in 3a within a run, but the plot of the rate constant k(obs) vs the changing [3a]0 in different runs is a half-parabola, indicating self-catalysis by more than one enol molecule. At 0.09 M enol, the isotope effect k(3a)/k(3a)-OD = 2.1. In the presence of 0.025-0.25 M pyridine-d5, the k(obs) vs [pyridine-d5] plot displays a bell-shaped profile. The change in the shape of the OH signals of the 3a/3b mixture at 295-430 K in C6D5NO2 was followed by DNMR. The four signals of the diastereomeric 3a/3b mixture observed at 330 K coalesce at 350 K with barriers of 18.3 and 18.4 kcal x mol(-1) due to the diastereomerization of the vinyl propellers. The resulting two signals observed at >360 K further coalesce at 425 K with a barrier of 22.9 kcal x mol(-1) due either to oxygen-to-oxygen proton exchange or to E/Z isomerization. The estimated upper limit for the rate of proton exchange of k(ex) < or = (2-4) x 10(3) M(-1) x s(-1) at 425 K between the enol molecules is sufficiently slow to be a rate-controlling step in the isomerization. A process in which several enol molecules catalyze the isomerization is suggested, and several mechanistic routes are analyzed.

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Section: Introductionmentioning

confidence: 94%

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol¹

Rochlin

Rappoport

2003

J. Org. Chem.

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“…Although the extensive acidity (p K a ) listings of Streitwieser et al32a and Bordwell 32b do not include values for thioketones, we note that thioacetamide is seven p K a units more acidic than acetamide 32b. Another argument is that the enol/keto ratio is greater for thioketones, and the thioenoles are more acidic than enols; therefore, the thioketones are also more acidic.…”

Section: Resultsmentioning

confidence: 71%

Manifestation of Stereoelectronic Effects on the Calculated Carbon−Hydrogen Bond Lengths and One-Bond¹J_C_-_HNMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (CO), Thiocarbonyl (CS), and Methylidene (CCH₂) Groups toward C−H Donor Bonds

2004

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Theoretical examination [B3LYP/6-31G(d,p), PP/IGLO-III//B3LYP/6-31G(d,p), and NBO methods] of six-membered cyclohexane 1 and carbonyl-, thiocarbonyl-, or methylidene-containing derivatives 2-27 afforded precise structural (in particular, C-H bond distances) and spectroscopic (specifically, one-bond (1)J(C)(-)(H) NMR coupling constants) data that show the consequences of stereoelectronic hyperconjugative effects in these systems. Major observations include the following. (1) sigma(C)(-)(H)(ax)() -->(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() (Y = O, S, or CH(2)) hyperconjugation leads to a shortening (strengthening) of the equatorial C-H bonds adjacent to the pi group. This effect is reflected in smaller (1)J(C)(-)(H)(ax)() coupling constants relative to (1)J(C)(-)(H)(eq)(). (2) Comparison of the structural and spectroscopic consequences of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) hyperconjugation in cyclohexanone 2, thiocyclohexanone 3, and methylenecyclohexane 4 suggests a relative order of acceptor orbital ability C=S > C=O > C=CH(2), which is in line with available pK(a) data. (3) Analysis of the structural and spectroscopic data gathered for heterocyclic derivatives 5-12 reveals some additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y), pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)(), n(X) --> sigma(C)(-)(H)(ax)(), n(beta)(O) --> sigma(C)(-)(H)(eq)(), and sigma(S)(-)(C) --> sigma(C)(-)(H)(eq)() stereoelectronic effects that is, nevertheless, attenuated by saturation effects. (4) Modulation of the C=Y acceptor character of the exocyclic pigroup by conjugation with alpha-heteroatoms O, N, and S in lactones, lactams, and methylidenic analogues 13-24 results in decreased sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugation. (5) Additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugative effects is also apparent in 1,3-dicarbonyl derivative 25 (C=Y equal to C=O), 1,3-dithiocarbonyl derivative 26 (C=Y equal to C=S), and 1,3-dimethylidenic analogue 27 (C=Y equal to C=CH(2)).

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“…e The experimental Δ H ° f,298 (kJ mol -1 , ref ) are as follows: C 5 -2 : −365 ± 3; C 6 -2 : −378; 10 : −72.2; 11 ; −112.8; CH 3 COOH: −432; CH 3 COOCH 3 : −410. f Energies for the most stable enol conformers, see refs and . For other estimates of enol relative energies and heats of formation, see refs and .…”

Section: Resultsmentioning

confidence: 99%

Lactone Enols Are Stable in the Gas Phase but Highly Unstable in Solution

et al. 2002

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2-Hydroxyoxol-2-ene (C(5)-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C(4)-1), five- (C(5)-1), and six-membered rings (C(6)-1). The acid-base properties of C(4)-C(6) lactones and enols and reference carboxylic acid enols CH(2)=C(OH)(2) (3) and CH(2)=C(OH)OCH(3) (4) were also calculated in aqueous solution. The C(4)-C(6) lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-)(1) and acidities in the range of 1401-1458 kJ mol(-)(1). In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C(4)-1) to 14.5 (C(6)-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of -10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.

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Theoretical calculations

Cited by 10 publications

References 115 publications

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol¹

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol¹

Manifestation of Stereoelectronic Effects on the Calculated Carbon−Hydrogen Bond Lengths and One-Bond¹J_C_-_HNMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (CO), Thiocarbonyl (CS), and Methylidene (CCH₂) Groups toward C−H Donor Bonds

Lactone Enols Are Stable in the Gas Phase but Highly Unstable in Solution

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Theoretical calculations

Cited by 10 publications

References 115 publications

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol1

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol1

Manifestation of Stereoelectronic Effects on the Calculated Carbon−Hydrogen Bond Lengths and One-Bond1JC-HNMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (CO), Thiocarbonyl (CS), and Methylidene (CCH2) Groups toward C−H Donor Bonds

Lactone Enols Are Stable in the Gas Phase but Highly Unstable in Solution

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Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol¹

Stable Simple Enols. Self-CatalyzedE/Z-Isomerization of the Sterically Crowded 2-(m-Methoxymesityl)-1,2-dimesitylethenol¹

Manifestation of Stereoelectronic Effects on the Calculated Carbon−Hydrogen Bond Lengths and One-Bond¹J_C_-_HNMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (CO), Thiocarbonyl (CS), and Methylidene (CCH₂) Groups toward C−H Donor Bonds