Isolation of an Acid/Base Complex in Solution Puts the Brakes on Nitrogen Inversion

Wash, Paul L.; Renslo, Adam R.; Rebek, Jr. Prof. Julius

doi:10.1002/1521-3773(20010401)40:7<1221::aid-anie1221>3.0.co;2-n

Cited by 35 publications

(8 citation statements)

References 25 publications

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“…We reported the inversion dynamics of prochiral N -ethyl- N -methyl- N -isopropylamine bound by H 2b and compared this process to that observed in the presence of simple carboxylic acids 42. In CD 2 Cl 2 , control acid H 3 has the same influence as pivalic acid on N -ethyl- N -methyl- N -isopropylamine inversion.…”

Section: Resultsmentioning

confidence: 98%

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Interaction Energies and Dynamics of Acid−Base Pairs Isolated in Cavitands

et al. 2008

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The use of capsules and cavitands in physical organic chemistry is briefly reviewed, and their application to the study of salt bridges is introduced. Carboxylate/ammonium ion pairs are generated within an environment that more or less surrounds the functional groups within a synthetic fixed The barriers to in-out exchange of several amine guests were determined to be in the range from 15 to 24 kcal mol −1 . Some parallels with enzymes are drawn: the receptor folds around the guest species; presents them with inwardly directed functionality; and provides a generally hydrophobic environment and a periphery of secondary amide bonds.

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Section: Resultsmentioning

confidence: 98%

“…When the cavitand is fitted with an introverted acid, as in H 2 , these amines are bound so tightly that the acid–base complex is stable to silica gel chromatography: the binding constants are K a > 10 5 M −1 . The structures of several complexes of H 2 and an amine guest have been studied previously using NMR 38,42…”

Section: Resultsmentioning

confidence: 99%

Interaction Energies and Dynamics of Acid−Base Pairs Isolated in Cavitands

et al. 2008

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“…2) (15)(16)(17). The introverted acid cavitand is a vase-shaped host, with cavity dimensions of Ϸ6 Å between the van der Waals surfaces of opposite walls and between the deepest part of the cavity and the amide carbonyls of the upper rim.…”

Section: Resultsmentioning

confidence: 99%

Self-fulfilling cavitands: Packing alkyl chains into small spaces

Purse

Rebek

2006

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

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The present research was undertaken to determine the conformations of alkyl groups that are forced into small spaces. Unbranched alkyl groups assume fully extended conformations in the free space of solutions, because any bends in the structure create steric clashes between hydrogens along the chain. We synthesized a series of alkyl esters of a carboxylic acid attached to the inner surface of a vase-shaped container structure. The shorter esters (ethyl and propyl) can be accommodated in extended conformations, and even small solvent molecules can share the container's space. Longer (butyl-octyl) esters adopt increasingly coiled conformations that writhe rapidly in the limited space of the cavity. Even longer esters (nonyl and decyl) can be synthesized, but their containers become distorted, and their spectra indicate slowed internal motions of the alkyl groups within the space. In general, alkyl groups are readily contorted when their internal strains are compensated by attraction with the inner surfaces and the proper filling of space.coiled conformations ͉ self-assembly T he lowest energy conformation of normal alkanes is a fully extended (anti) one in solution, the gas phase, and the solid state. Each bend creates gauche interactions involving repulsion between hydrogens on the C i and C iϩ3 carbons, and each gauche configuration is associated with an increase in energy of Ϸ0.55 kcal͞mol in the liquid state (1). The small energy involved means that, for a given alkane, a number of conformers are typically present in equilibrium. For example, n-heptane has 13 different accessible conformers in rapid interconversion under ambient conditions (2).The recent literature describes several systems in which the energetic preference for the anti conformation may be overcome by imposing geometric constraints on the size and shape of the space available (3-11). Naturally occurring protein receptors (12) and synthetic host molecules contain binding cavities suitable for hydrocarbons, and a variety of conformations have been observed by crystallography in the former and NMR in the latter. Normal alkanes are simply flexible and adept at filling spaces of diverse shape. We have examined normal alkanes in several hosts: hexameric pyrogallolarene assemblies encompassing a roughly cube-like space of Ϸ1,300 Å 3 (11); vase-shaped, water-soluble cavitands having an open-ended pocket of Ϸ200 Å 3 (7, 8); and a cylindrical molecular capsule having an inner space of 425 Å 3 (6, 9). The behavior in these systems fits neither Fischer's (13) lock-and-key model nor the Thoma and Koshland (14) induced-fit model: the alkane contorts to be accommodated; it assumes the size, shape, and chemical surface that are proper to the space on offer in the receptor. For example, n-alkanes shorter than the cavity of the cylinder (Fig. 1) are bound in an extended conformation; if the alkanes are slightly too long, they adopt a compact helical conformation with several gauche configurations. This conformation is shorter and thicker: it allows the alkane t...

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“…Other stereomutations implying a nondissociative mechanism are the configuration inversion of nitrogen atoms with aliphatic amines, oxygen atoms with ether ligands, or phosphorus atoms in phosphanes or phosphidos. , These inversion processes proceed via a pyramidal inversion (Figure ), which can be prevented by metal coordination or by steric hindrance.…”

Section: Introductionmentioning

confidence: 99%

Nondissociative Mechanism for the Inversion of the Configuration in Cyclopentadienyl Di(aryloxo)titanium Complexes: An Entropy Discussion

et al. 2017

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Titanium complexes with two dioxo bridges between the Cp and the metal can be synthesized by the reaction of [Ti{η 5 -C 5 H 3 -1,3-(SiMe 2 Cl) 2 }Cl 3 ] with different diols. Some of these complexes present a dynamic inversion process affording two diastereomers, A and B, which have been studied by X-ray diffraction and NMR techniques. The X-ray diffraction studies of [Ti{η 5 -C 5 H 3 -1,3-[SiMe 2 (κ 1 -1,2-O 2 C 6 H 4 )] 2 }X] X= Cl (2A), Me (4A), and Bn (5) complexes permit their absolute configuration to be assigned, and an epimerization mechanism has been supported by experimental evidence. Moreover, averaged activation energies (ΔH ⧧ , ΔG ⧧ ) and entropies (ΔS ⧧ ) for both the forward and reverse reactions were calculated from the Eyring−Polanyi equation from the NMR EXSY experiments; these are in agreement with the experimental evidence. Article pubs.acs.org/Organometallics

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Isolation of an Acid/Base Complex in Solution Puts the Brakes on Nitrogen Inversion

Cited by 35 publications

References 25 publications

Interaction Energies and Dynamics of Acid−Base Pairs Isolated in Cavitands

Interaction Energies and Dynamics of Acid−Base Pairs Isolated in Cavitands

Self-fulfilling cavitands: Packing alkyl chains into small spaces

Nondissociative Mechanism for the Inversion of the Configuration in Cyclopentadienyl Di(aryloxo)titanium Complexes: An Entropy Discussion

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