Hydrogen exchange in amidinium ions. Chemically significant consequences of slow rotation about a carbon-nitrogen single bond

Perrin, Charles L.

doi:10.1021/ja00824a084

Cited by 9 publications

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“…Our NMR results revealed a discrepancy that led to the unexpected conclusion that ordinary primary amides RCONH 2 undergo acid‐catalyzed exchange via N ‐protonation, to form the intermediate RCONH 3 + , but with the novel feature that its deprotonation and C–N rotation are competitive on a picosecond timescale . In related studies on amidinium ions, we verified the equivalent “slow” rotation about the C–NH 3 + bond . We also demonstrated that amides with electron‐withdrawing substituents instead exchange via the imidic acid RC(OH) = NR′, which does not undergo E / Z isomerization during its lifetime.…”

Section: Researchmentioning

confidence: 61%

Half a century of research, teaching, service

Perrin

2014

J of Physical Organic Chem

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This is a summary of the research of Charles L. Perrin and his coworkers in the areas of aromatic mercuration, vibronic borrowing, ipso factors in electrophilic aromatic substitution, malonic anhydrides, mechanisms of proton exchange in amides, stereoelectronic control, the socalled reverse anomeric effect, an NMR titration method for highly accurate measurement of relative acidities, secondary deuterium isotope effects on acidity, symmetry of hydrogen bonds in solution, and nucleophilic addition to a para-benzyne diradical, along with Perrin's contributions to teaching and to service. Keywords:hydrogen bonds para-benzyne NMR titration isotope effects stereoelectronic control reverse anomeric effect vibronic borrowing electrophilic aromatic substitution malonic anhydrides proton exchange in amides Special thanks go to my dear wife Marilyn for her presence and support during these fifty years and for preparing a delicious dinner for the speakers and out-of-town guests the night before the symposium.After 50 years at UCSD I am still not retired but continue to enjoy teaching and research.

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Section: Researchmentioning

confidence: 61%

Half a century of research, teaching, service

Perrin

2014

J of Physical Organic Chem

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“…Figure 2 shows 1 H NMR spectra of benzamidinium ion in H 2 SO 4 solutions of increasing acidity. 12 Again, the NH signals show unequal broadening, and the one assigned as H E is broader. These spectra are convincing evidence that an N-protonated intermediate is such a strong acid that its lifetime is too short to permit rotational equilibration about the C-NH 3 + bond.…”

Section: Proton Exchange In Amidinium Ionsmentioning

confidence: 99%

“…In contrast, amidinium ions, which had been found to undergo NH exchange in H 2 SO 4 , can exchange only by the N -protonation mechanism via a dicationic intermediate (Scheme ). Figure shows 1 H NMR spectra of benzamidinium ion in H 2 SO 4 solutions of increasing acidity . Again, the NH signals show unequal broadening, and the one assigned as H E is broader.…”

Section: Proton Exchange In Amidinium Ionsmentioning

confidence: 99%

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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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“…N , N ′‐Amidinium ion pKa are high enough so that exchange rates at its different intramolecular =(R)N + H groups can be measured. These measurements have been performed in acid‐catalyzed and base‐catalyzed conditions. D 2 O‐catalyzed exchange in N , N ′‐dimethyl amididium ions have also been reported previously .…”

Section: Introductionmentioning

confidence: 99%

D₂O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule

Núñez

2017

J of Physical Organic Chem

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When N‐benzyl‐N′‐methylacetamidinium hydrochloride (pKa=11.8) is dissolved in D2O/DCl(1 M), an equilibrium of 2 54:46 stereoisomers in an ~2:1 =(R)Nδ+H(D) D/H ratio is formed. Therefore, 2 R =N‐benzyl (E and Z) and 2 R =N‐methyl (E and Z) groups attached to the corresponding H(D) (Z and E) for a total of 8 1H‐NMR signals are observed. Consequently, their rates of H and D transfer to D2O can be measured by means of the 1H‐NMR broadness (line shape) of the =(R)Nδ+H doublets and =(R)Nδ+D broad singlets. Acidity selectivity is observed for both processes. In fact, the relative proton and deuterium transfer rates follow the acidity order: =(PhCH2)Nδ+‐H(E) > =(PhCH2)Nδ+‐H(Z) > =(Me)Nδ+‐H(E) > =(Me)Nδ+‐H(Z). Proton transfer rates are in the range of 8 to 0.5 s‐1 with α = .92. This tendency is independently supported by the observed experimental chemical shift deuterium isotopic perturbation. The rate‐limiting step for proton exchange is the breaking of the hydrogen bond due to the fast amidine reprotonation (~1011 s). =(R)Nδ+D/=(R)Nδ+H equilibration is reached at ~80 s, and it can be measured by the relative =(R)Nδ+H versus =(R)Nδ+D signal integrations. The equilibrium of the 4 =(R)Nδ+H(D) centers is shifted toward deuterium, but they are further shifted in the more basic centers. Equilibrium is completely shifted toward D in the 4 centers when OD− contributes with the exchange process at pD > 3.

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Hydrogen exchange in amidinium ions. Chemically significant consequences of slow rotation about a carbon-nitrogen single bond

Cited by 9 publications

References 0 publications

Half a century of research, teaching, service

Half a century of research, teaching, service

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

D₂O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule

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Hydrogen exchange in amidinium ions. Chemically significant consequences of slow rotation about a carbon-nitrogen single bond

Cited by 9 publications

References 0 publications

Half a century of research, teaching, service

Half a century of research, teaching, service

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

D2O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule

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D₂O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule