NMR Spectroscopic Evidence for the Structure of Iminium Ion Pairs

Mayr, Herbert; Ofial, Armin R.; Würthwein, Ernst‐Ulrich; Aust, Nicola C.

doi:10.1021/ja972860u

Cited by 63 publications

(55 citation statements)

References 48 publications

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“…The cation and anion do not show strong interactions, although it is noteworthy that the oxygen atom O2 of the anion points to the empty p-orbital of the iminium carbon atom C1 (distance: 2.99 Å ). [19] The bond lengths and bond angles are close to standard values (see Figure 1). [20] …”

Section: Quantum Chemical Calculations Of Electrophilicitiessupporting

confidence: 68%

Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion

Böttger

Fröhlich

Würthwein³

2000

Eur. J. Org. Chem.

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The rate constants for the reactions of the 2‐azaallenium ion 1b+, the 2‐azaallylium ion 2a+ and the iminium ion 3+ with different nucleophiles were determined by 1H NMR spectroscopy. By correlation with the Linear Free Enthalpy Relationship (LFER) lg k20°C = s (E + N), developed by Mayr and Patz, the electrophilicity parameters E(1b+) = ‐3.7, E(2a+) ≈︁ ‐16 and E(3+) = ‐10.43 were obtained. They show that the relative reactivities of these ions are approximately 1012:1:106. Quantum chemical calculations (ab initio, DFT) of the methyl anion affinities for the ions 1b+,2a+ and3+ are in agreement with the experimental E values. The X‐ray structure of 3+·CF3SO 3− is reported for the first time; it shows no strong interaction between the cation and the anion.

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Section: Quantum Chemical Calculations Of Electrophilicitiessupporting

confidence: 68%

Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion

Böttger

Fröhlich

Würthwein³

2000

Eur. J. Org. Chem.

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“…The degree of interaction between model imine 7 111 and benzoic acid was assessed experimentally by 1 H NMR spectroscopy (Scheme 3). The chemical shift of an imine’s formyl hydrogen is highly sensitive to electrostatic perturbation, 112 and changes in imine chemical shift have previously been taken as evidence of imine protonation by stronger Brønsted acids. 113 No change in the signal was observed upon addition of a full equivalent of benzoic acid, thus providing further support for the prediction that imine protonation is highly endergonic in the reaction solvent.…”

Section: Resultsmentioning

confidence: 99%

Chiral Thioureas Promote Enantioselective Pictet–Spengler Cyclization by Stabilizing Every Intermediate and Transition State in the Carboxylic Acid-Catalyzed Reaction

et al. 2017

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An investigation of the mechanism of benzoic acid/thiourea co-catalysis in the asymmetric Pictet–Spengler reaction is reported. Kinetic, computational, and structure–activity relationship studies provide evidence that rearomatization via deprotonation of the pentahydro-β-carbolinium ion intermediate by a chiral thiourea•carboxylate complex is both rate- and enantioselectivity-determining. The thiourea catalyst induces rate acceleration over the background reaction mediated by benzoic acid alone by stabilizing every intermediate and transition state leading to up to and including the final selectivity-determining step. Distortion–interaction analyses of the transition structures for deprotonation predicted using density functional theory indicate that differential π–π and C–H···π interactions within a scaffold organized by multiple hydrogen-bonds dictate stereoselectivity. The principles underlying rate acceleration and enantiocontrol described herein are expected to have general implications for the design of selective transformations involving deprotonation of high-energy intermediates.

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“…However, these ion pairs also form strong noncovalent complexes with sulfinamide urea 21, wherein the sulfonate anion is hydrogen bonded in a bidentate fashion to the urea, and the iminium formyl and N-H protons are simultaneously engaged in H-bonding interactions with the sulfonate and sulfinamide oxygens ( Fig. 11) (53).…”

Section: Discussionmentioning

confidence: 99%

Attractive noncovalent interactions in asymmetric catalysis: Links between enzymes and small molecule catalysts

Knowles

Jacobsen

2010

Proc. Natl. Acad. Sci. U.S.A.

695

436

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Catalysis by neutral, organic, small molecules capable of binding and activating substrates solely via noncovalent interactions-particularly H-bonding-has emerged as an important approach in organocatalysis. The mechanisms by which such small molecule catalysts induce high enantioselectivity may be quite different from those used by catalysts that rely on covalent interactions with substrates. Attractive noncovalent interactions are weaker, less distance dependent, less directional, and more affected by entropy than covalent interactions. However, the conformational constraint required for high stereoinduction may be achieved, in principle, if multiple noncovalent attractive interactions are operating in concert. This perspective will outline some recent efforts to elucidate the cooperative mechanisms responsible for stereoinduction in highly enantioselective reactions promoted by noncovalent catalysts.enantioselectivity | H bonding | organocatalysis | transition state stabilization

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NMR Spectroscopic Evidence for the Structure of Iminium Ion Pairs

Cited by 63 publications

References 48 publications

Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion

Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion

Chiral Thioureas Promote Enantioselective Pictet–Spengler Cyclization by Stabilizing Every Intermediate and Transition State in the Carboxylic Acid-Catalyzed Reaction

Attractive noncovalent interactions in asymmetric catalysis: Links between enzymes and small molecule catalysts

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