Pyramidal Atomic Inversion

Lambert, Joseph B.

doi:10.1002/9780470147153.ch2

Cited by 145 publications

(7 citation statements)

References 194 publications

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“…Attempts made by Meisenheimer et al to isolate chiral tertiary amines of the type NRR 0 R@ suggested that rapid pyramidal inversion leads to optical inactivity. 5 In 1944, Prelog and Wieland 6 recognized the inherent chirality of Tröger's base 7 due to the two stereogenic nitrogen atoms related by C2 symmetry. They achieved enantiomer separation by liquid chromatography on a 0.9 m column using lactose hydrate as CSP (op 5 0.99), followed by fractional crystallization.…”

Section: Introductionmentioning

confidence: 99%

The stereodynamics of 1,2‐dipropyldiaziridines

et al. 2009

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N-alkylated trans-diaziridines are an intriguing class of compounds with two stereogenic nitrogen atoms which easily interconvert. In the course of our investigations of the nature of the interconversion process via nitrogen inversion or electrocyclic ring opening ring closure, we synthesized and characterized the three constitutionally isomeric diaziridines 1,2-di-n-propyldiaziridine 1, 1-isopropyl-2-n-propyldiaziridine 2, and 1,2-diisopropyldiaziridine 3 to study the influence of the substituents on the interconversion barriers. Enantiomer separation was achieved by enantioselective gas chromatography on the chiral stationary phase Chirasil-beta-Dex with high separation factors alpha (1-isopropyl-2-n-propyldiaziridine: 1.18; 1, 2-diisopropyldiaziridine: 1.24; 100 degrees C 50 kPa He) for the isopropyl substituted diaziridines. These compounds showed pronounced plateau formation between 100 and 150 degrees C, and peak coalescence at elevated temperatures. The enantiomerization barriers DeltaG(double dagger) and activation parameters DeltaH(double dagger) and DeltaS(double dagger) were determined by enantioselective dynamic gas chromatography (DGC) and direct evaluation of the elution profiles using the unified equation implemented in the software DCXplorer. Interestingly, 1-isopropyl-2-n-propyldiaziridine and 1,2-diisopropyldiaziridine exhibit similar high interconversion barriers DeltaG(double dagger) (100 degrees C) of 128.3 +/- 0.4 kJ mol(-1) and 129.8 +/- 0.4 kJ mol(-1), respectively, which indicates that two sterically demanding substituents do not substantially increase the barrier as expected for a distinct nitrogen inversion process.

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Section: Introductionmentioning

confidence: 99%

The stereodynamics of 1,2‐dipropyldiaziridines

et al. 2009

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“…15 The simplest and most common mechanism for thermal racemization of chiral sulfoxides is the pyramidal inversion mechanism, also called ''pyramidal atomic inversion.'' 16 In this mechanism, the racemization takes place without breaking bonds, via a transition state in which the four atoms of the sulfinyl group (oxygen, sulfur, and the two atoms connected to sulfur) lie in one plane. 14, 17 Mislow and colleagues reported in 1966-1968 kinetic evidence regarding several distinct pathways of racemization for simple sulfoxides.…”

Section: Introductionmentioning

confidence: 99%

Pyramidal inversion mechanism of simple chiral and achiral sulfoxides: A theoretical study

2007

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The pyramidal inversion mechanism of simple sulfoxides was studied, employing ab initio and DFT methods. The convergence of the geometrical and energetic parameters of H2SO and DMSO with respect to the Hamiltonian and basis set was analyzed in order to determine a computational level suitable for methyl phenyl sulfoxide (3), methyl 4-cyanophenyl sulfoxide (4), diphenyl sulfoxide (5), 4,4'-dicyanodiphenyl sulfoxide (6), benzyl methyl sulfoxide (7) and benzyl phenyl sulfoxide (8). The DFT B3LYP/6-311G(d,p) level was chosen for further calculations of larger sulfoxides. The barriers DeltaE calculated for the pyramidal inversion mechanism of sulfoxides 3-8 are in the range of 38.7-47.1 kcal/mol. These values are in good agreement with the experimental barriers for racemization via the pyramidal inversion mechanism. A resonance effect of a phenyl ring selectively stabilizes the transition state conformations, decreasing the energy barrier for pyramidal inversion by about 3 kcal/mol, compared to a similar molecule without a phenyl substituent. Introducing electron withdrawing groups (cyano) at the para positions of the phenyl ring(s) causes a further decrease of the energy barrier.

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“…Literature data on the barriers to pyramidal nitrogen inversion in macrobicycles containing bridgehead nitrogen atoms are scarce . In the case of simple acyclic alkylamines, they lie in the 21−40 kJ mol -1 range . For example, dibenzylmethylamine has Δ G c ⧧ = 25.1 kJ mol -1 at 127 K 12a.…”

Section: Discussionmentioning

confidence: 99%

“…When not hindered, interconversions between the ii , io ( oi ), and oo forms proceed via nitrogen inversion rather than homeomorphic isomerization . The nitrogen inversion free energy barrier of [7.3.1]- 1 has been determined by variable temperature 13 C NMR spectroscopy: Δ G 218 ⧧ = 39.3 kJ mol -1 , which is at the upper limit of the range observed for simple amines but much lower than the values obtained for systems with CNC angle strain, such as aziridines or 7-azanorbornanes and related systems .…”

Section: Introductionmentioning

confidence: 91%

The in, out Asymmetric Pseudo-Triple Helical Form of a D₃_h Diaza-Macropentacycle

et al. 2007

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A sterically encumbered [N(2)S(6)] macropentacycle (5) related to diazamacrobicycles and cryptands has been synthesized in 53% yield by the [1+1] condensation reaction between functionalized macrocyclic and macrotricyclic precursors. A macrononacycle (18) resulting from the corresponding [2+2] condensation was isolated in 7% yield from the reaction mixture. Both compounds showed broad features in their room-temperature (1)H NMR spectra, but their maximal average symmetry (D(3h) and D(2h), respectively) was achieved at high temperature (380 K). At low temperature (200 K, CD(2)Cl(2) solution), the macropentacycle is "frozen" to a single asymmetric (C1) conformation on the (1)H NMR time scale, which has also the molecular structure observed in the solid state by X-ray crystallography: pseudo-triple helical ( not equalC(3)) shape, io (in, out) form resulting from the endo/exo configuration at the nitrogen bridgehead atoms, and similar orientations of the tosyl substituents. The solution dynamics of the molecule can be described by coupled bridgehead nitrogen inversion, triple helix symmetrization, and reversal of triple helix handedness, with DeltaGc = 54.2 kJ mol(-1) in CD(2)Cl(2) at 300 K. Adoption of the io form by macropentacycle 5 in the crystal and in solution at low-temperature most probably results from the steric crowding and strain introduced by the [15]ane-N(2)S(2) macrocyclic bridging subunits.

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Pyramidal Atomic Inversion

Cited by 145 publications

References 194 publications

The stereodynamics of 1,2‐dipropyldiaziridines

The stereodynamics of 1,2‐dipropyldiaziridines

Pyramidal inversion mechanism of simple chiral and achiral sulfoxides: A theoretical study

The in, out Asymmetric Pseudo-Triple Helical Form of a D₃_h Diaza-Macropentacycle

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Pyramidal Atomic Inversion

Cited by 145 publications

References 194 publications

The stereodynamics of 1,2‐dipropyldiaziridines

The stereodynamics of 1,2‐dipropyldiaziridines

Pyramidal inversion mechanism of simple chiral and achiral sulfoxides: A theoretical study

The in, out Asymmetric Pseudo-Triple Helical Form of a D3h Diaza-Macropentacycle

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The in, out Asymmetric Pseudo-Triple Helical Form of a D₃_h Diaza-Macropentacycle