Carbanions. 5. Preparation and proton and carbon-13 NMR spectroscopic structural study of the 4-hydridopyridyl anion and 4,4'-bis(hydridopyridyl) dianion. Absence of homoazacyclopentadienyl ion character

Oláh, George A.; Hunadi, Ronald J.

doi:10.1021/jo00317a013

Cited by 22 publications

(6 citation statements)

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“…Hence, the development of efficient methodologies for the construction of a DHP skeleton has attracted considerable interest in synthetic chemistry. While the reduction of preactivated pyridines with strong reductants is utilized as the conventional synthetic approach to 1,2- or 1,4-DHP derivatives, ,, several catalytic protocols for the direct reduction of pyridines have been described in recent years. Among them, the systems employing mild reducing reagents such as silanes and boranes instead of H 2 gas are considered to be efficacious because they may circumvent the use of highly flammable and highly pressurized hydrogen gas, as well as the obstacle of over-reduction to produce undesired piperidine derivatives (Figure a). , …”

Section: Introductionmentioning

confidence: 99%

Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate

2018

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N-Heterocyclic phosphenium triflates (NHP-OTf) 1 serve as efficient catalysts for the regio- and chemoselective hydroboration of pyridines under ambient condition with good functional group tolerance. Mechanistic studies indicate that a boronium salt, [(Py)·Bpin]OTf 4, is generated concomitant with NHP-H 5 via hydride abstraction from HBpin by 1 in the initial reaction step. Hydride reduction of the activated pyridine in [(Py)·Bpin]OTf 4 by NHP-H 5 affords the 1,4-hydroboration product selectively. Thus, the phosphenium species act as a hydrogen transfer reagent in the catalytic cycle.

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Section: Introductionmentioning

confidence: 99%

Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate

2018

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“…To test the viability of our approach to 1,2-DHPs, a judicious choice of protecting group had to be made to overcome the following challenges: (1) oxidation of dihydropyridines in the presence of air; (2) the nucleophilicity of the aziridine nitrogen had to be tempered to allow for an initial interaction of the activated Pt(II)-bound alkyne and the propargylic ester (see 1a → 2 ) without competing nitrogen addition to the alkyne; and (3) the reactivity of the aziridine nitrogen had to be tuned to allow for its addition to the metallocarbenoid functionality of 3 to afford 4 en route to 5a . Importantly, this addition should be rapid relative to a potential vinyl aziridine rearrangement of 3 …”

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confidence: 99%

Pt(II)-Catalyzed Synthesis of 1,2-Dihydropyridines from Aziridinyl Propargylic Esters

et al. 2007

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Pt(II)-catalyzed cycloisomerization of aziridinyl propargylic esters affords 1,2-dihydropyridines with regiodefined installation of substituents. A mild conversion of the 1,2-dihydropyridines to the corresponding substituted pyridines as well as chirality retention from the aziridinyl propargylic ester substrates have been demonstrated.Nitrogen-containing heterocycles comprise a large percentage of natural products and pharmaceutically important small molecules. 1 Methods directed toward the synthesis of azaheterocycles continue to be an important undertaking, and general strategies for the regio-and stereocontrolled introduction of substituents remain a challenge. Dihydropyridines (DHPs) have long been recognized as versatile synthetic intermediates 2 that provide ready access to a variety of substituted nitrogen-containing heterocycles such as piperidines (by reduction) 3 or to pyridines (by oxidation). 4 Historically, 1,4-dihydropyridines (1,4-DHPs) have been the most studied, whereas 1,2-DHPs have received relatively little attention. This has stemmed primarily from the ease with which these compounds can be accessed from pyridinium salts by nucleophilic addition at C(4) and the emergence of Hantzsch-type 1,4-DHPs as important calcium channel blockers. 5 The majority of synthetic approaches to 1,2-DHPs has also relied on nucleophilic addition into N-alkyl or N-acylpyridinium salts. 6,7 Unfortunately, regioisomeric mixtures of addition products (at the 2, 4, or 6 position) are often obtained if unsymmetrically substituted pyridinium salts, especially those that do not bear a substituent at the C(4) position, are employed. 8 More recently, other strategies for the synthesis of 1,2-DHPs have emerged that address some of these limitations. 9 Because of the lack of general methods for the regioselective synthesis of highly functionalized 1,2-DHP derivatives, their synthetic and biological potential remains largely unexplored. As such, there is a continued need for the development of more functional-group-tolerant methods that avoid the use of organolithium or Grignard reagents to access 1,2-DHPs as well as general strategies that afford access to bicyclic 1,2-DHPs. We envisioned that 1,2-DHPs (e.g., 5a, Scheme 1) could be realized from aziridinyl propargylic esters (see 1a) via aPt(II)-catalyzed cascade sequence. Thus, formation of zwitterion 2 from 1a via a 5-exo-dig cyclization of the ester carbonyl functionality onto the alkyne and subsequent rearrangement could afford metallocarbenoid 3. The strained [3.1.0] bicycle 4 could then result from nucleophilic attack of the aziridinyl nitrogen on the metallocarbenoid functionality. At this stage, irreversible valence bond isomerization coupled with the strain-release fragmentation of the aziridine was expected to produce 1,2-DHP 5a.The expectation that 5a could be formed from 1a is supported by our recent report of a Pt(II)-catalyzed method for the synthesis of acyloxycyclopentenones (e.g., 9, Scheme 2) using epoxide-bearing propargylic ester substrates (6)...

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“…(2 H, 2 CH, 6 q), 7. [3][4][5][6][7].S (5 H, Ph, H, m), 10.1 (1 H, COOH, s, exchanged with D20); IR (Nujol) p"" 3300Í-2500,1700,1160-1020 (five characteristic bands of dioxolane ring8) cm"1; MS, m/e 221 (M -1), 178,133, 115, 105 (base peak) 77, 43. 2-Phenyl-2,3-dihydroxybutanoic acid (7): threo and erythro mixture; mp 158-165 °C (from chloroform); NMR (acetone-d6) 0.9 and 1.2 (3 H, CH3, 2 d), 4.5 (1 H, CH, q), 6.0 (3 H, 3 OH, br, exchanged with D20), 7.3 (3 H, Ph H, m), 7.7 (2 H, Ph H, m); IR (Nujol) pmal 3300, 1690, 1250, 700 cm"1.…”

Section: Methodsmentioning

confidence: 99%

A novel reaction type promoted by aqueous titanium trichloride. Synthesis of unsymmetrical 1,2-diols

Clerici¹,

Porta²

1982

J. Org. Chem.

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Carbanions. 5. Preparation and proton and carbon-13 NMR spectroscopic structural study of the 4-hydridopyridyl anion and 4,4'-bis(hydridopyridyl) dianion. Absence of homoazacyclopentadienyl ion character

Cited by 22 publications

References 0 publications

Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate

Metal-Free Regio- and Chemoselective Hydroboration of Pyridines Catalyzed by 1,3,2-Diazaphosphenium Triflate

Pt(II)-Catalyzed Synthesis of 1,2-Dihydropyridines from Aziridinyl Propargylic Esters

A novel reaction type promoted by aqueous titanium trichloride. Synthesis of unsymmetrical 1,2-diols

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