Solvation Effect on Steric Bulk of Ionic Substituents: Imidazolium vs Imidazole

Perrin, Charles L.; Fabian, Miles A.; Armstrong, Kathleen B.

doi:10.1021/jo00097a028

Cited by 45 publications

(34 citation statements)

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“…By low-temperature NMR integration the equilibrium constants for ring inversion were measured. 88 Also, it follows from eq 10 that A Im -A R and A ImH+ -A R are given by -2RT A ImH+ -A Im = -RT ln(K e + /K e ) = -RT ln(K a cis /K a trans )…”

Section: Steric Hindrance To Ionic Solvationmentioning

confidence: 99%

The Logic behind a Physical–Organic Chemist’s Research Topics

Perrin

2016

J. Org. Chem.

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Abstract. Although my research has no common theme or defining area, a coherence connects the diverse topics, insofar as one project leads logically to another. Thus studies on mechanisms of hydrogen exchange in amides and amidines led to the influence of hydrogenbonding and to NMR methods for chemical kinetics, including 2D-EXSY spectroscopy.Another connection was the OH --catalyzed NH exchange in amines that had supported the hypothesis of stereoelectronic control. We therefore analyzed that hypothesis critically, tested it, found counterexamples, and proposed an alternative hypothesis. We next addressed one-bond NMR coupling constants in ethers and the reverse anomeric effect. The latter studies required a highly accurate NMR titration method that we developed to measure the additional steric bulk resulting from protonation of a substituent. This method is also applicable to measuring secondary isotope effects on acidity, and we could demonstrate that they arise from n-* delocalization, not from an inductive effect. Other studies included kinetic isotope effects for both dissociation and H exchange of aqueous NH 4 + , for C-N rotation in amides, and for a hydride transfer. The role of hydrogen-bonding led us to the rotation of NH 4 + within its solvent cage and then to the symmetry of hydrogen bonds. This review article is based on an address presented at the 249th National Meeting of the American Chemical Society in Denver CO, on the occasion of the James Flack Norris Award in Physical-Organic Chemistry. 1 It demonstrates the ways whereby one physicalorganic chemist, and physical-organic chemists in general, have investigated how molecular structure affects chemical reactivity. It also seeks to confirm my claim that "In my opinion, 2 physical organic chemistry represents the intellectual basis of organic chemistry. It asks (and answers!) fundamental questions about how chemical substances behave, and it rationalizes that behavior." 2 The reason that I won the James Flack Norris Award was for my "insight, rigorous analysis, and understanding of mechanisms and reactive intermediates", rather than any distinctive area of research with which I am associated. Instead of choosing one area to review here, I have tried to describe the path of my research for the past 40 years, as an expansion of an earlier article on this topic. 3 Each area seems to have engendered additional questions, leading to additional areas. Often those questions are marked by an intense skepticism, and the answers to those questions are supported by a logical analysis that I am proud of. I hope that this article will be as interesting, informative, and enjoyable to the reader as the research described here has been for me. A Beginner's ResearchMy Ph.D. thesis research was on mercuration of benzene, an electrophilic aromatic substitution that is accelerated by strong acid, and where we measured the H/D kinetic isotope effect. 4 To understand the origin of the acceleration it was necessary to consider acidity functions, including a new one, H...

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Section: Steric Hindrance To Ionic Solvationmentioning

confidence: 99%

The Logic behind a Physical–Organic Chemist’s Research Topics

Perrin

2016

J. Org. Chem.

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“…Copyright (1994) American Chemical Society. 30 This NMR titration method was used similarly to measure the effects of N protonation on the anomeric equilibria in N (D glucopyranosy1)imidazole (3) and its tetra O acetyl derivative 4 ( 31 ). In principle, the magnitude of any reverse anomeric effect could be measured as the increase in the proportion of the  anomer on protonation of an equilibrating mixture of anomers.…”

Section: Electrodeless Ph Titrationsmentioning

confidence: 99%

pH-Free Measurement of Relative Acidities, Including Isotope Effects

Perrin

2017

Measurement and Analysis of Kinetic Isotope Effects

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A powerful pH free multicomponent NMR titration method can measure relative acidities, even of closely related compounds, with excellent accuracy. The history of the method is presented, along with details of its implementation and a comparison with earlier NMR titrations using a pH electrode. Many of its areas of applicability are described, especially equilibrium isotope effects. The advantages of the method, some practical considerations, and possible pitfalls are considered.

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“…7,23 It was concluded that ∆A = 0.089, indicating only a marginal (albeit detectable) relative increase in the size of the imidazolium moiety. Although this (again) clearly ruled out the alternative steric explanation for the RAE observed in earlier studies, further confirmation was sought with alternative experimental methods.…”

Section: Recent Tests Of the Reverse Anomeric Effectmentioning

confidence: 99%

“…trialkylammonium): in these an ion-dipole pair may form in principle, but this would be expected readily to lead to oxocarbenium ion (not observed as discussed above), rather than recollapse to the substrate. It appears, however, that either the ion pair (23) or the exploded transition state (25) mechanism may operate, depending on the substrate and reaction conditions, as the following discussion shows. (In fact, the 'exploded mechanism' and the bimolecular nucleophilic reactions of glycosides, had been presaged in an elegant 1965 study by Lemieux and Hayami.…”

Section: Glycosidic Cases and The Exploded Mechanismmentioning

confidence: 99%

Some recent developments in stereoelectronic theory. Reevaluations of ALPH and the reverse anomeric effect

Chandrasekhar¹

2005

Arkivoc

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A critique of objections to the 'antiperiplanar lone pair hypothesis' (ALPH) and the 'reverse anomeric effect' (RAE) is presented. Whilst early fears that ALPH is incompatible with the Curtin-Hammett principle (CHP) were apparently unfounded, objections to ALPH based on the reactions of cyclic hemiorthoesters and amidinium salts are debatable. (The unreactivity of Kirby's bicyclic bridgehead acetal supports ALPH convincingly.) Objections to RAE were based on its apparent incompatibility with ALPH, studies on several models indicating (dubiously) the existence of a weak anomeric effect rather than RAE. However, this may well suggest that a normal anomeric effect is being offset by RAE. This apparently indicates different bases for the ground state and kinetic anomeric effects: the classical 'electrostatic gauche repulsive interaction' (EGRI) and the 'orbital interaction' (OI) respectively. The evidence against free oxocarbenium ions, but favouring the ion pair and the 'exploded transition state' (ETS) models apparently raises the question 'Whither ALPH?'. A rigorous application of the CHP to the stereochemistry of displacements at glycosyl anomeric centres, indicates that the α anomers react retentively via ion pairs, whereas the β anomers react with inversion via the ETS. (A substantial reassessment of the 'in situ anomerisation technique' is also indicated.)

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Solvation Effect on Steric Bulk of Ionic Substituents: Imidazolium vs Imidazole

Cited by 45 publications

References 0 publications

The Logic behind a Physical–Organic Chemist’s Research Topics

The Logic behind a Physical–Organic Chemist’s Research Topics

pH-Free Measurement of Relative Acidities, Including Isotope Effects

Some recent developments in stereoelectronic theory. Reevaluations of ALPH and the reverse anomeric effect

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