Mechanism of acid-catalyzed proton exchange in amidinium ions

Perrin, Charles L.; Johnston, Eric R.; Ramírez, José

doi:10.1021/ja00540a021

Cited by 18 publications

(13 citation statements)

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“…The most logical place to expect the second protonation is on the side-chain C02H group, as shown in eq. [I], rather than in the guanidine system, in which further protonation would be very difficult; for instance the pK, of RC(=NH2+)-NH,+ has been estimated to be about -19 (22), and that of diprotonated tetramethylguanidine to be about -11 (23).…”

Section: Discussionmentioning

confidence: 99%

The cyclization of 2-substituted imidazolines in sulfuric acid

Cox¹,

Moore²,

McDonald³

1994

Can. J. Chem.

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A study of the rates of the cyclizations 1 + 3 and 2 + 4 as a function of medium acidity and temperature in aqueous sulfuric acid has been performed. The latter reaction is five times faster at all acidities. An excess acidity kinetic analysis reveals the probable involvement of a water molecule in the reaction in both cases. Mechanistic possibilities suggested by the observations are discussed; it is proposed that the water molecule acts as a base catalyst during the rate-determining ring closure. On a effectut une ttude des vitesses des cyclisations 1 + 3 et 2 + 4 en fonction de I'aciditC du milieu et de la temptrature, dans de I'acide sulfurique aqueux. A tous les niveaux d'aciditk, la deuxibme rtaction est cinq fois plus rapide. Une analyse de I'aciditC cinttique en excbs rCvble qu'une molCcule d'eau est probablement impliquCe dans la reaction, dans chacun des cas. On discute des diverses possibilitts de mtcanismes suggkrtes par ces observations; on suggbre que la moltcule d'eau agit comme catalyseur basique au cours de l'ttape dtterminant la vitesse de la fermeture du cycle.[Traduit par la rCdaction]

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Section: Discussionmentioning

confidence: 99%

The cyclization of 2-substituted imidazolines in sulfuric acid

Cox¹,

Moore²,

McDonald³

1994

Can. J. Chem.

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“…Saturation-transfer studies of acid-catalyzed exchange of amidinium ions ArC(NH 2 ) 2 + confirmed the observation, based on line-broadening (Figure ), that H E exchanges faster than H Z . There was a question regarding signal assignments, which seemed to undergo a solvent-induced shift on going from water to H 2 SO 4 , which could be confirmed with anionic lanthanide-shift reagents …”

Section: Site-to-site Rate Constantsmentioning

confidence: 99%

“…Saturation-transfer studies of acid-catalyzed exchange of amidinium ions ArC(NH 2 ) 2 + confirmed the observation, based on linebroadening (Figure 2), that H E exchanges faster than H Z . 20 There was a question regarding signal assignments, which seemed to undergo a solvent-induced shift on going from water to H 2 SO 4 , which could be confirmed with anionic lanthanide-shift reagents. 21 An alternative to saturation transfer is 2D-EXSY (exchange spectroscopy) NMR, which has become a very powerful method for exchange kinetics.…”

Section: ■ Site-to-site Rate Constantsmentioning

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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“…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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Mechanism of acid-catalyzed proton exchange in amidinium ions

Cited by 18 publications

References 4 publications

The cyclization of 2-substituted imidazolines in sulfuric acid

The cyclization of 2-substituted imidazolines in sulfuric acid

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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Mechanism of acid-catalyzed proton exchange in amidinium ions

Cited by 18 publications

References 4 publications

The cyclization of 2-substituted imidazolines in sulfuric acid

The cyclization of 2-substituted imidazolines in sulfuric acid

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