Substituent Effects on Singlet−Triplet Gaps and Mechanisms of 1,2-Rearrangements of 1,3-Oxazol-2-ylidenes to 1,3-Oxazoles

Freeman, Fillmore; Lau, Desirae J.; Patel, Atitkumar R.; Pavia, Paulo R.; Willey, Justin D.

doi:10.1021/jp8030286

Cited by 19 publications

(17 citation statements)

References 121 publications

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“…Singlet–triplet energy gap (Δ E s − t ) is often taken as an indication of stability for divalent species . Here, we discuss possible increase of stability for the unsubstituted carbene, cyclonona‐3,5,7‐trienylidene ( 1 CH2 ) through α and ά‐substitutions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* (Scheme ).…”

Section: Resultsmentioning

confidence: 99%

Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Koohi

Kassaee

Haerizade

et al. 2015

J of Physical Organic Chem

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Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (ΔEHOMO − LUMO), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization, and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1CH2) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ά‐SiMe2, ά‐NMe, ά‐PMe, ά‐O, ά‐S, ά‐CH2, ά‐cyclopropyl, and ά‐CMe2 (2SiMe2, 2NMe, 2PMe, 2O, 2S, 2CH2, 2cyclopropyl, and 2CMe2, respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant pπ orbital of the carbene center (Walshcyclopropyl → pπ carbene). Among them, the singlet closed shell 2NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (ΔEs − t = 27.1 kcal mol−1). In contrast, the least stable derivative is triplet 2SiMe2, which exhibits the lowest relative ΔEs − t of −5.5 kcal mol−1. The overall trend of ΔEs‐t is 2NMe > 2PMe > 2S > 2O > 2cyclopropyl > 2CMe2 > 2CH2 > 1CH2 > 2SiMe2. With one negative force constant, the unsubstituted 1CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.

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Section: Resultsmentioning

confidence: 99%

Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Koohi

Kassaee

Haerizade

et al. 2015

J of Physical Organic Chem

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“…ments of oxazol 2 ylidene (3, X = O) and five of its substituted derivatives into corresponding oxazoles. 7 In the work, 7 ab initio CBS QB3, CBS Q, and CBS 4M methods and the perturbation theory MP2, DFT with B3LYP and B3PW91 functionals (each with 6 31+G(d,p) and 6 311+G(d,p) basis sets), as well as the cluster methods CCSD, CCSD, and CCSD(T), and the configuration interaction methods QCISD and QCISD(T) were used. The influence of substituents on E ST values turned out to be relatively small and additionally difficult to interpret: for all 11 methods used and compared in the work 7 and six studied carbenes, the specified values are in the range of 82.0-68.5 kcal mol -1 .…”

Section: Quantum Chemical Methodsmentioning

confidence: 99%

Quantum chemical studies of azoles 2. Thermodynamic stability of neutral molecules and intermediates formed during electrophilic substitution of 1,2- and 1,3-azoles

2014

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The thermodynamic stability of 1,2 and 1,3 azole molecules, as well as of cationic and bipolar (carbenoid) intermediates in the gas phase and in aqueous solution formed by electro philic substitution (proton as a model electrophile) was compared based on the analysis of quantum chemical calculations performed at the DFT/B3LYP/6 31G(D) level of theory with zero point energy corrections. The differences in the chemical behavior of the isomeric 1,2 and 1,3 azole pairs, viz., pyrazole-imidazole, isoxazole-oxazole, and isothiazole-thiazole, were considered.Key words: 1,2 azoles, 1,3 azoles, electrophilic substitution, quantum chemical calcula tions, DFT/B3LYP/6 31G(D) method.In the previous communication, 1 we considered and discussed the results of quantum chemical calculations of 1,2 and 1,3 azoles, as well as of the intermediates that are formed via two alternative mechanisms of electrophilic substitution: the normal addition-elimination mechanism with the formation of intermediate cationic  complexes and the elimination-addition mechanism specific to azoles with the participation of ylide (carbene) intermedi ates. Such an ylide substitution is especially characteristic of 1,3 azoles; the corresponding experimental data is sum marized in the review. 2 The goal of the present work is to analyze the results of quantum chemical calculations reported previously 1 in or der to determine the relative thermodynamic stability of 1,2 and 1,3 azole molecules, as well as of cationic and bipolar (carbenoid) intermediates formed during electro philic substitution (proton as a model electrophile), and to consider the differences in the chemical behavior of isomeric 1,2 and 1,3 azole pairs: pyrazole-imidazole, isoxazole-oxazole, and isothiazole-thiazole. Structures 1-10, which characterize the transformation of the starting 1,3 and 1,2 azoles 1 and 6 into ylide substitution inter mediates 2-5 and 7-10, are shown in Scheme 1 and those related to the transformation of the same azoles 1 and 6 into the intermediates of the traditional addition-eli mination mechanism (cationic  complexes 11-16) are shown in Scheme 2. Quantum chemical methodIn the present work, we used the results of our quantum chemical calculations 1 done in terms of the density functional theory (DFT) using the B3LYP functional and the 6 31G(D) basis set with zero point energy corrections taking into account solvation effects using the overlapping sphere model (IEFPCM). 3 The appropriateness of this relatively simple DFT method with a modest basis set was justified in the work. 1 Thus, the results of calculations performed with this method for five membered heterocycles containing one heteroatom are in good agreement with the data obtained by ab initio Hartree-Fock and MP2 4,5 methods, which showed the relationship between the substrate selectivity and the ability of the heteroatom to delocalize a posi tive charge (N > O > S) and between the positional selectivity in the electrophilic substitution reaction of pyrrole, furan, and thio phene and the change in the st...

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“…In contrast, much less is known about such rearrangements in NHCs. Although the migrations of boron, nitrogen, carbon, and phosphorus have been observed experimentally, the previous quantum chemical investigations of the 1,2‐shift in NHCs have been mainly applied to studies of the migration of the hydrogen and methyl group . The hydrogen rearrangement in the carbene plane is a symmetry forbidden process, as it involves crossing molecular orbitals with the same symmetry.…”

Section: Introductionmentioning

confidence: 99%

“…The hydrogen rearrangement in the carbene plane is a symmetry forbidden process, as it involves crossing molecular orbitals with the same symmetry. An out‐of‐plane intramolecular migration is allowed, but the calculated barriers for the shift in model carbenes are rather large (ca. 40 kcal/mol for hydrogen and 50 kcal/mol for methyl migration).…”

Section: Introductionmentioning

confidence: 99%

“…An out‐of‐plane intramolecular migration is allowed, but the calculated barriers for the shift in model carbenes are rather large (ca. 40 kcal/mol for hydrogen and 50 kcal/mol for methyl migration). An intermolecular process is more favorable in energy: calculated activation energies ΔE for synchronous intermolecular hydrogen shift in imidazol‐ and thiazol‐ylidenes are small and lie in the range of 2–5 kcal/mol.…”

Section: Introductionmentioning

confidence: 99%

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1,2-migration inN-phosphano functionalizedN-heterocyclic carbenes

et al. 2014

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1,2-Migration of the phosphano-group to the carbene center in N-phosphano functionalized N-heterocyclic carbenes has been studied by density functional theory (DFT) calculations. An intramolecular mechanism with a three-center transition state structure seems to be most plausible for the isolated carbenes, while an intermolecular pathway catalyzed by azolium salts may be preferable for a migration proceeding in the course of generating the carbenes in situ. Our calculations show that amino-substitution at the phosphorus atom and an enhanced nucleophilicity of the heterocycle scaffold facilitate the phosphorus shift. Calculated singlet-triplet energy gaps do not correlate with thermodynamic stability of the studied carbenes and their disposition toward the 1,2-rearrangement.

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Substituent Effects on Singlet−Triplet Gaps and Mechanisms of 1,2-Rearrangements of 1,3-Oxazol-2-ylidenes to 1,3-Oxazoles

Cited by 19 publications

References 121 publications

Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Quantum chemical studies of azoles 2. Thermodynamic stability of neutral molecules and intermediates formed during electrophilic substitution of 1,2- and 1,3-azoles

1,2-migration inN-phosphano functionalizedN-heterocyclic carbenes

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