Electron transmission study of the splitting of the .pi.* molecular orbitals of angle-strained cyclic acetylenes: implications for the electrophilicity of alkynes

Ng, Lily; Jordan, Kenneth D.; Krebs, Adolf; Rueger, Wolfgang

doi:10.1021/ja00390a005

Cited by 23 publications

(8 citation statements)

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“…The correlation of angle strain and electronic structure of cyclic alkynes is consistent with electron transmission spectroscopy data by Ng et al Comparison of vertical electron affinities (EAs), which according to the Koopmans’ theorem correlate with the unfilled orbitals energies, show a strong dependence of the LUMO energy on the degree of bending. For example, the EA of cyclooctyne, where the triple bond is bent by 21.5° increases by ∼1 eV in comparison with that in di- tert -butylacetylene.…”

Section: Baldwin Rulessupporting

confidence: 84%

Cyclizations of Alkynes: Revisiting Baldwin’s Rules for Ring Closure

2011

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Section: Baldwin Rulessupporting

confidence: 84%

Cyclizations of Alkynes: Revisiting Baldwin’s Rules for Ring Closure

2011

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“…These observations are interesting because the effect of alkyne bending on HOMO energies, a potential driving force for intermolecular interactions with electrophiles, is believed to be relatively small in comparison to the well-known enhancement of electrophilicity associated with LUMO energy lowering in bent alkynes, e.g., benzyne . Nevertheless, it is clear that the effect of the acceptor (∼2 kcal/mol) should be experimentally significant.…”

Section: Resultsmentioning

confidence: 92%

Selective Transition State Stabilization via Hyperconjugative and Conjugative Assistance: Stereoelectronic Concept for Copper-Free Click Chemistry

et al. 2011

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Dissection of stereoelectronic effects in the transition states (TSs) for noncatalyzed azide-alkyne cycloadditions suggests two approaches to selective transition state stabilization in this reaction. First, the formation of both 1,4- and 1,5-isomers is facilitated via hyperconjugative assistance to alkyne bending and C···N bond formation provided by antiperiplanar σ-acceptors at the propargylic carbons. In addition, the 1,5-TS can be stabilized via attractive C-H···F interactions. Although the two effects cannot stabilize the same transition state for the cycloaddition to α,α-difluorocyclooctyne (DIFO), they can act in a complementary, rather than competing, fashion in acyclic alkynes where B3LYP calculations predict up to ∼1 million-fold rate increase relative to 2-butyne. This analysis of stereoelectronic effects is complemented by the distortion analysis, which provides another clear evidence of selective TS stabilization. Changes in electrostatic potential along the reaction path revealed that azide polarization may create unfavorable electrostatic interactions (i.e., for the 1,5-regioisomer formation from 1-fluoro-2-butyne and methyl azide). This observation suggests that more reactive azides can be designed via manipulation of charge distribution in the azide moiety. Combination of these effects with the other activation strategies should lead to the rational design of robust acyclic and cyclic alkyne reagents for fast and tunable "click chemistry". Further computational and experimental studies confirmed the generality of the above accelerating effects and compared them with the conjugative TS stabilization by π-acceptors.

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“…The evidence for the resonant character of the electron scattering in the vicinity of 3.5 eV comes from the threshold electron energy-loss spectrum of Dance and Walker (1974) in which they observed the 3.6 eV feature associated with resonance-enhanced vibrational excitation of the parent molecule. Another support for the resonant origin of the 3.5 eV TCS structure results from the feature observed around 3.4 eV by Ng et al (1982) in the derivative of the transmitted electron current. Beyond the main maximum, over the energy range from 10 up to 370 eV, the cross-section energy function monotonically decreases.…”

Section: 3-butadiene H 2 C=c H -C H =C Hmentioning

confidence: 89%

Electron scattering from C₄H₆and C₄F₆molecules

Szmytkowski

Kwitnewski

2003

J. Phys. B: At. Mol. Opt. Phys.

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Absolute electron-impact total cross sections (TCSs) for C 4 H 6 isomers (1, 3butadiene and 2-butyne) and for hexafluoro-2-butyne (C 4 F 6) have been measured in a linear electron-beam transmission experiment as a function of impact energy, between 0.5 and 370 eV. For the C 4 H 6 molecule distinct differences in the shape and magnitude of TCS energy dependences of both isomers are apparent at low energies. In the 1, 3-butadiene TCS curve, the enhancement below 1.2 eV, the narrow maximum centred around 3.2 eV and the dominant broad peak at 9-9.5 eV are easily distinguishable; a very weak shoulder is located near 8 eV. The TCS energy function of 2-butyne demonstrates a remarkable resonant-like maximum located near 3.6 eV and much broader enhancement centred at 8 eV. Above 50 eV both C 4 H 6 curves merge together. The hexafluoro-2-butyne TCS has the maximum near 8 eV and very broad enhancement spanning between 20 and 80 eV; weak shoulders near 3 and 6 eV are also noticeable. The effect of perfluorination is indicated.

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Electron transmission study of the splitting of the .pi.* molecular orbitals of angle-strained cyclic acetylenes: implications for the electrophilicity of alkynes

Cited by 23 publications

References 1 publication

Cyclizations of Alkynes: Revisiting Baldwin’s Rules for Ring Closure

Cyclizations of Alkynes: Revisiting Baldwin’s Rules for Ring Closure

Selective Transition State Stabilization via Hyperconjugative and Conjugative Assistance: Stereoelectronic Concept for Copper-Free Click Chemistry

Electron scattering from C₄H₆and C₄F₆molecules

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Electron transmission study of the splitting of the .pi.* molecular orbitals of angle-strained cyclic acetylenes: implications for the electrophilicity of alkynes

Cited by 23 publications

References 1 publication

Cyclizations of Alkynes: Revisiting Baldwin’s Rules for Ring Closure

Cyclizations of Alkynes: Revisiting Baldwin’s Rules for Ring Closure

Selective Transition State Stabilization via Hyperconjugative and Conjugative Assistance: Stereoelectronic Concept for Copper-Free Click Chemistry

Electron scattering from C4H6and C4F6molecules

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Electron scattering from C₄H₆and C₄F₆molecules