Acidity of hydrocarbons.  XXV.  Secondary isotope effects in the lithium cyclohexylamide catalyzed .alpha.-deuterium exchange of ring-deuterated toluenes

Streitwieser, Andrew; Humphrey, Jamie

doi:10.1021/ja00991a015

Cited by 11 publications

(2 citation statements)

References 26 publications

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“…This is consistent with the calculated IEs per D in Table , which increase from ortho to meta to para. It is inconsistent with experimental ortho -, meta -, and para -deuterium kinetic IEs of −2.4, −0.4, and −1.8%, respectively, in the α deprotonation of toluene …”

Section: Resultscontrasting

confidence: 81%

Secondary Deuterium Isotope Effects on the Acidity of Carboxylic Acids and Phenols

Perrin

Dong

2007

J. Am. Chem. Soc.

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Secondary deuterium isotope effects (IEs) on acidities have been accurately measured by an NMR titration method applicable to a mixture of isotopologues. Deuteration definitely decreases the acidity of carboxylic acids and phenols, by up to 0.031 in the DeltapK per D. For aliphatic acids, the IEs decrease as the site of deuteration becomes more distant from the OH, as expected, but a surprising result is that IEs in both phenol and benzoic acid do not decrease as the site of deuteration moves from ortho to meta to para. The experimental data are supported by ab initio computations, which, however, substantially overestimate the IEs. The discrepancy does not seem to be due to solvation. The IEs originate in isotope-sensitive vibrations whose frequencies and zero-point energies are lowered upon deprotonation. In the simplest case, formate, the key vibration can be recognized as the C-H stretch, which is weakened by delocalization of the oxygen lone pairs. For the aromatic acids, delocalization cannot account for the near constancy of IEs from ortho, meta, and para deuteriums, but the observed IEs are consistent with calculated vibrational frequencies and electron densities. Moreover, the ability of the frequency analysis to account for the IEs is evidence against an inductive origin.

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Section: Resultscontrasting

confidence: 81%

Secondary Deuterium Isotope Effects on the Acidity of Carboxylic Acids and Phenols

Perrin

Dong

2007

J. Am. Chem. Soc.

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“…Previously it was indicated that deuterium behaves as an electron donor. The substituent parameters are equal, σ I = −0.001 and σ R = −0.0001, with respect to protium . Considering the sign and value of the Hammett constant, the expected pattern of pyrrole resonances may resemble the one established for [(2-CH 3 -TPP)Fe III (CN) 2 ] - with the largest upfield displacement for the 3-H resonance.…”

Section: Resultsmentioning

confidence: 82%

NMR Investigation of β-Substituted High-Spin and Low-Spin Iron(III) Tetraphenylporphyrins

et al. 1996

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The NMR spectra of a series of beta-substituted iron(III) tetraphenylporphyrin (2-X-TPP) complexes have been studied to elucidate the relationship between the electron donating/withdrawing properties of the 2-substituent and the (1)H NMR spectral pattern. The electronic nature of the substituent has been significantly varied and covered the -0.6 to 0.8 Hammett constant range. Both high-spin and low-spin complexes of the general formula (2-X-TPP)Fe(III)Cl and [(2-X-TPP)Fe(III)(CN)(2)](-) have been investigated. The (1)H NMR data for the following substituents (X) have been reported: py(+), NO(2), CN, CH(3), BzO (C(6)H(5)COO), H, D, Br, Cl, CH(3), NH(2), NH(3)(+), NHCH(3), OH, and O(-). The (1)H NMR resonances for low-spin dicyano complexes have been completely assigned by a combination of two-dimensional COSY and NOESY experiments. In the case of selected high-spin complexes, the 3-H resonance has been identified by the selective deuteration of all but the 3-H position. The pattern of unambiguously assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as an unique (1)H NMR spectroscopic probe to map the spin density distribution. The pyrrole isotropic shifts of [(2-X-TPP)Fe(III)(CN)(2)](-) are dominated by the contact term. In order to quantify the substituent effect, the dependence of isotropic shift of all low-spin pyrrole resonances and 3-H high-spin pyrrole resonance versus Hammett constants has been studied. The electronic effect is strongly localized at the beta-substituted pyrrole. The major change of the isotropic shift has also been noted for only one of two adjacent pyrrole rings, i.e., at 7-H and 8-H positions. These neighboring protons, located on a single pyrrole ring, experienced opposite shift changes when electron withdrawing/donating properties were modified. Two other pyrrole rings for all investigated derivatives revealed considerably smaller, substituent related, isotropic shift changes. A long-range secondary isotopic shift has been observed for [(2-D-TPP)Fe(III)(CN)(2)](-). The effect is consistent with a general spin density distribution mechanism due to beta-substitution. A fairly good correlation between the 3-H isotropic shift of (2-X-TPP)Fe(III)Cl and the Hammett constant has been found as well. The observed contact shift pattern of [(2-X-TPP)Fe(III)(CN)(2)](-) reflects spin pi delocalization into the highest filled MO equivalent to the unsubstituted porphyrin 3e(pi) orbital. To account for the substituent contribution, the semiquantitative Fenske-Hall LCAO method has been used to determine the molecular orbitals involved in the spin density delocalization. For low-spin complexes, (13)C pyrrole resonances of carbons bearing a proton have been identified by means of a (1)H-(13)C HMQC experiment. The reversed order of (13)C resonance patterns as compared to their (1)H NMR counterparts has been determined, e.g., the largest isotropic shift of 3-H has been accompanied by the smallest measured (13)C isotropic shift. Analysis of the isotropic shif...

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Secondary Deuterium Isotope Effect in an Electrophilic Aromatic Substitution ‐ Protodesilylation of Trimethylphenylsilane

Szele¹

1981

Helvetica Chimica Acta

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SummaryReaction rates for the protodesilylation of trimethylphenylsilane and of [2,4, 6-*H3]-trimethylphenylsilane by HC104, were measured in aqueous methanol ( 2 : 5 , v/v) and the secondary deuterium isotope effect for the reaction was found to be kH/kD3=0.79. The magnitude of the observed isotope effect supports a mechanism in which the rate-determining step is the proton transfer from the hydronium ion to the silane to form a a-intermediate.Introduction. -Recently we measured secondary deuterium isotope effects in reactions of benzenediazonium tetrafluoroborate, and these were used to differentiate between two possible reaction intermediates for the N,,-N,] rearrangement reaction that accompanies the dediazoniation [l]: the phenyl cation/N2 molecule pair and the benzenespirodiazirine cation (1). We concluded that the observed isotope effect on the rearrangement of [2,4, 6-2H3]-benzenediazonium tetrafluoroborate (kHlkD3 = 1.42) definitely excludes structure 1 as an intermediate, since for rate determining formation of 1 an isotope effect kHlkD3 smaller than unity is expected.

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Acidity of hydrocarbons. XXV. Secondary isotope effects in the lithium cyclohexylamide catalyzed .alpha.-deuterium exchange of ring-deuterated toluenes

Cited by 11 publications

References 26 publications

Secondary Deuterium Isotope Effects on the Acidity of Carboxylic Acids and Phenols

Secondary Deuterium Isotope Effects on the Acidity of Carboxylic Acids and Phenols

NMR Investigation of β-Substituted High-Spin and Low-Spin Iron(III) Tetraphenylporphyrins

Secondary Deuterium Isotope Effect in an Electrophilic Aromatic Substitution ‐ Protodesilylation of Trimethylphenylsilane

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