Pyridines and their Benzo Derivatives: Reactivity at the Ring

Comins, Daniel L.; O’Connor, Sean P.; Al‐awar, Rima

doi:10.1016/b978-008044992-0.00602-7

Cited by 32 publications

(33 citation statements)

References 328 publications

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“…[5] We have shown that the addition of one or more equiv of pyridine to the iron(I) complex LFe(C 6 H 6 ) (L = β-diketiminate ligand, shown in Scheme 1) yields products in which the pyridine ligands are reduced by 1-2 electrons. [6] However, we have now discovered that addition of smaller amounts of pyridine leads to formation of a surprising new compound (1). Solutions of 1 are in equilibrium with the previously characterized pyridine-bound complexes and with LFe(C 6 H 6 ), as shown by 1 H NMR spectra of C 6 D 6 solutions of 1 with various amounts of pyridine ( Figure S5).…”

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confidence: 99%

“…[6] However, we have now discovered that addition of smaller amounts of pyridine leads to formation of a surprising new compound (1). Solutions of 1 are in equilibrium with the previously characterized pyridine-bound complexes and with LFe(C 6 H 6 ), as shown by 1 H NMR spectra of C 6 D 6 solutions of 1 with various amounts of pyridine ( Figure S5). Compound 1 can be isolated by adding 0.5 equiv of pyridine to LFe(C 6 H 6 ) in pentane, from which 1 precipitates in 51% yield.…”

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confidence: 99%

“…[1] However, pyridines are relatively unreactive in the absence of prior activation. [2] Direct cleavage of C-H bonds in unfunctionalized pyridines is uncommon, but has made substantial progress recently.…”

Section: Introductionmentioning

confidence: 99%

“…The presence of the hydride bridge was verified by adding 1 equiv of diphenylacetonitrile to 2, which affords a bridging iminyl complex (3) from formal [1,2]-addition of the Fe-H bond to the C-N triple bond. This reaction demonstrates that the hydride generated by the C-H activation is still reactive.…”

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confidence: 99%

“…More importantly, we were able to avoid the problematic pyridine equilibria by monitoring the kinetics of the reverse reaction, the conversion of 1 into LFe(C 6 H 6 ) in the presence of excess benzene, which should follow the same mechanism as the forward C-H activation. The kinetic experiments indicate the rate law v = k [1], with a zero-order dependence on [C 6 H 6 ] (see Supporting Information). We observed slower rates with the deuteride isotopologue of 1, showing a kinetic isotope effect (KIE) of k H /k D = 1.31 ± 0.01.…”

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C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

et al. 2016

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Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105-111 kcal/mol) C-H bonds of pyridine substrates are unknown. In this work, we report the rapid room-temperature cleavage of C-H bonds in pyridine, 4-tertbutylpyridine, and 2-phenylpyridine by an iron(I) species, to give well-characterized iron(II) products. In addition, 4-dimethylaminopyridine (DMAP) undergoes room-temperature C-N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl-bridged tetrairon(II) square. These facile bond-cleaving reactions are proposed to occur through intermediates having a twoelectron reduced pyridine that bridges two iron centers. Thus, the redox non-innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions. Graphical abstractAn iron(I) complex can activate C-H and C-N bonds in pyridines, using a mechanism that takes advantage of pyridine's redox activity. KeywordsPyridine; Iron; Redox-Active; Bond Cleavage Pyridines are widespread in pharmaceuticals, natural products, and polymers, and therefore their expedient synthesis is crucial. [1] However, pyridines are relatively unreactive in the absence of prior activation. [2] Direct cleavage of C-H bonds in unfunctionalized pyridines is uncommon, but has made substantial progress recently. [3] A recent exciting development has been the C-H arylation of pyridines using catalytic mixtures generated in situ from simple iron salts and an excess of Grignard reagent. [4] This recipe presumably generates reduced Correspondence to: Patrick L. Holland. a These authors contributed to this work equally, and should be considered co-first authors.Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.XXXX HHS Public Access Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript iron species that are the catalysts, but little is known about reduced iron-pyridine compounds and what stoichiometric steps to expect under these conditions. In this communication, we report well-characterized iron products that result from pyridine C-H bond cleavage. We also show an unprecedented C-N bond cleavage of 4-dimethylaminopyridine (DMAP) and construct a reasonable mechanistic scaffold that is based on the ability of pyridines to accept charge from low-valent iron.Pyridines are well-known redox-active ligands. [5] We have shown that the addition of one or more equiv of pyridine to the iron(I) complex LFe(C 6 H 6 ) (L = β-diketiminate ligand, shown in Scheme 1) yields products in which the pyridine ligands are reduced by 1-2 electrons. [6] However, we have now discovered that addition of smaller amounts of pyridine leads to formation of a surprising new compound (1). Solutions of 1 are in equilibrium with the previously characterized pyridine-bound complexes and with LFe(C 6 H 6 ), as shown by 1 H NMR spectra of C 6 D 6 solutions of 1 with various amounts of pyridine ( Figure S5). Compound 1 can be isolated by...

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C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

et al. 2016

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Tandem Condensation/Rearrangement Reaction of 2‐Aminohetarene N‐Oxides for the Synthesis of Hetaryl Carbamates

et al. 2018

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A new approach to hetaryl carbamates through a tandem condensation/rearrangement reaction of 2‐aminohetarene N‐oxides was developed. The developed reaction is suitable for both five‐ and six‐membered heterocycles and proceeds through the condensation of 2‐aminohetarene N‐oxide with trimethyl orthoformate followed by intramolecular N‐oxide oxygen transfer. For five‐membered hetarene N‐oxides (furoxans), the intramolecular rearrangement is catalyzed by a cyanide anion, while for six‐membered hetarene N‐oxides (azines), Lewis acid catalysis is sufficient. The developed protocol is characterized by operational simplicity and high efficiency, resulting in carbamoyl derivatives in good and high yields.magnified image

show abstract

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

et al. 2016

View full text Add to dashboard Cite

Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105–111 kcal/mol) C-H bonds of pyridine substrates are unknown. In this work, we report the rapid room-temperature cleavage of C-H bonds in pyridine, 4-tert-butylpyridine, and 2-phenylpyridine by an iron(I) species, to give well-characterized iron(II) products. In addition, 4-dimethylaminopyridine (DMAP) undergoes room-temperature C-N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl-bridged tetrairon(II) square. These facile bond-cleaving reactions are proposed to occur through intermediates having a two-electron reduced pyridine that bridges two iron centers. Thus, the redox non-innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions.

show abstract

Pyridines and their Benzo Derivatives: Reactivity at the Ring

Cited by 32 publications

References 328 publications

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

Tandem Condensation/Rearrangement Reaction of 2‐Aminohetarene N‐Oxides for the Synthesis of Hetaryl Carbamates

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

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