362. Experiments on the mechanism of decarboxylation. Part I. Decomposition of quinaldinic and isoquinaldinic acids in the presence of compounds containing carbonyl groups

Dyson, Paul J.; Hammick, D. Ll.

doi:10.1039/jr9370001724

Cited by 55 publications

(19 citation statements)

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“…The reactivity of such a putative carbene is high, and complex formation according to a free carbene route is difficult. To date, free pyridylidenes, which were proposed by Hammick 70 years ago, 67 have been characterized only in the gas phase by mass spectrometry. 68,69 A recent computational study suggests that free 2-pyridylidenes may be stable if the heterocycle is extensively substituted by electron-donating amino groups.…”

Section: Pyridylidene Complexesmentioning

confidence: 99%

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

et al. 2009

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theme are found via the replacement or displacement of one N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared, 7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Section: Pyridylidene Complexesmentioning

confidence: 99%

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

et al. 2009

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“…1. First-order rate constants for the decarboxylation of substituted picolinic acids at 150" and ionic strength 1 .o.…”

Section: Pyridinemonocarboxylic Acidsmentioning

confidence: 99%

“…The mechanism of decarboxylation of pyridinecarboxylic acids was first examined by Hammick, who found that when the 2-, but not the 3-or 4-, pyridine and quinoline carboxylic acids are heated in high-boiling aldehyde or ketone solvents they give cr-pyridyl carbinols (1,2). He postulated that the reaction is a decarboxylation of the zwitterionic form of the amino acid to give a nitrogen ylid which acts as a nucleophile toward the carbonyl solvent as illustrated in eq.…”

Section: Introductionmentioning

confidence: 99%

Kinetics and Mechanism of Decarboxylation of some Pyridinecarboxylic Acids in Aqueous Solution

Dunn¹,

Lee²,

Thimm³

1972

Can. J. Chem.

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The first-order rate constants for decarboxylation of picolinic and five substituted picolinic acids in buffered aqueous solution.at 150" and ionic strength 1.0 increase as pH increases above 0, go through a maximum at pH near 1, then level off at about half the maximum. For quinolinic acid at 95" the rate maximum occurs at the isoelectric pH. It is therefore concluded that the anion decarboxylates about half as fast as the isoelectric species. Anion and isoelectric species show similar carboxyl-carbon kinetic isotope and substituent effects, so they probably decarboxylate by similar mechanisms. The methyl betaine of picolinic acid decarboxylates about 200 times faster than the anion. From these facts it is concluded that the isoelectric species which decarboxylates is probably zwitterion rather than neutral acid, and that anion and zwitterion decarboxylate by loss of carbon dioxide to form 2-pyridyl carbanion and ylid, respectively. The slower decarboxylation of isoelectric species compared to betaine suggests that only a small fraction of the isoelectric species is zwitterion. However, qualitative estimates from spectra of the fraction of zwitterion present in the isoelectric species at 150' suggest that this cannot be the whole explanation. The relative reactivities of the 2-and 4-positions are similar for decarboxylation and hydrogen exchange in pyridinium ions, but not in the unprotonated species.Les constantes de vitesse du premier ordre pour la dtcarboxylation de l'acide picolinique et cinq de ses derives substituks, en solution aqueuse tamponnee a 150" et de pouvoir ionique 1.0 augmentent avec un pH au-dessus de 0, passent par un maximum pour un pH proche de 1, et se stabilisent ensuite a environ la moitie de ce maximum. Pour l'acide quinolinique a 95", le maximum de vitesse se produit au pH isoelectrique. On a conclu par consequent que l'anion se decarboxyle a une vitesse qui est de moitie celle de l'espece isoelectrique. L'anion et l'espece isoelectrique montrent des effets similaires de substituants et de cinttique isotopique carboxyle-carbone, aussi les mecanismes de decarboxylation sont probablement voisins. La bttaine methylee de l'acide picolinique a une vitesse de decarboxylation de 200 fois superieure a celle de I'anion. Ces faits ont amenes a conclure que l'espece isoelectrique qui subit la decarboxylation est probablement le zwitterion plut6t que l'acide neutre et que la decarboxylation de I'anion et du zwitterion se fait par perte de gaz carbonique pour donner respectivement le pyridyl-2 carbanion et l'ylure. La dkcarboxylation plus lente de l'espece isoelectrique comparee a la betaine, laisse penser que seule une petite fraction de l'espece isoelectrique est a I'ttat de zwitterion. Toutefois, les estimations qualitatives a partir des spectres de la fraction de zwitterion prisente dans I'espece isoelectrique a 150°, suggere que cette explication ne puisse pas &tre la seule. Les reactivites relatives des positions -2 et -4 sont semblables pour la decarboxylation et l'echange d'hydrogene dans l...

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“…Decarboxylations, normally originate from carboxylate ion [124,125] and seem to occur much more frequently as monomolecular reactions. The carbanions which occur as intermediates in this case were successfully captured in some heterocyclics, using carbonyl compounds [126]. Hine et al [127] found that a close relationship exists between the rate of decarboxylation of various trihalogenoacetates and the rate of deprotonation by alkali, of the corresponding haloforms.…”

Section: (16)mentioning

confidence: 96%

Electrophilic Substitutions at Saturated Carbon Atoms

Köbrich

1962

Angew. Chem. Int. Ed. Engl.

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362. Experiments on the mechanism of decarboxylation. Part I. Decomposition of quinaldinic and isoquinaldinic acids in the presence of compounds containing carbonyl groups

Cited by 55 publications

References 0 publications

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

Kinetics and Mechanism of Decarboxylation of some Pyridinecarboxylic Acids in Aqueous Solution

Electrophilic Substitutions at Saturated Carbon Atoms

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