Asymmetric organocatalytic reductions mediated by dihydropyridines

Connon, Stephen J.

doi:10.1039/b711499k

Cited by 176 publications

(47 citation statements)

References 164 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…[75] In this regard, it should not be forgotten that NADH (124), provides a metal-free hydrogenation in natural systems (Scheme 45) which bears some relation mechanistically to the metal-free reduction with the PH + /BH À systems. [76] In an early example, Sander et al showed that the strongly electrophilic carbene difluorovinylidene reacts directly with H 2 in an argon matrix at 20-30 K with practically no activation barrier to yield 1,1-difluoroethene (125; Scheme 46). [77] Formal insertion of examples of monoamino carbenes into the HÀH bond in solution were recently reported by Bertrand et al [35] (Scheme 18).…”

Section: Other Metal-free Catalytic Hydrogenation Reactionsmentioning

confidence: 98%

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

2009

View full text Add to dashboard Cite

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

show abstract

Section: Other Metal-free Catalytic Hydrogenation Reactionsmentioning

confidence: 98%

Frustrated Lewis Pairs: Metal‐free Hydrogen Activation and More

2009

View full text Add to dashboard Cite

show abstract

“…[40][41][42][43] It has been observed that the formation of related dihydropyridines (e.g. 1,6-dihydronicotinamide and dihydroacridines) proceeds via sequential proton transfer (PT) and electron transfer (ET) steps.…”

Section: Scheme 1 Homogeneous Reduction Of Co2 To Methanol By 12-dimentioning

confidence: 99%

“…120,121 In-deed, since the discovery of NADPH in the 1930's, related dihydropyridine compounds have been studied, especially in connection with their HT to various substrates containing C=C, C=N and C=O groups. [40][41][42][43] HT to carbonyls is obviously of particular interest here: the reactant CO2 and its reduced intermediates formic acid (HCOOH) and formaldehyde (OCH2) leading to CH3OH formation all contain C=O groups susceptible to HT.…”

Section: Scheme 2 Formation Of Pyridinium Radical (Pyh 0 )mentioning

confidence: 99%

Reduction of CO₂ to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

Lim¹,

Holder²,

et al. 2014

View full text Add to dashboard Cite

ABSTRACT:We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first component of the catalytic cycle, pyridine (Py) undergoes a H + transfer (PT) to form pyridinium (PyH + ) followed by an e -transfer (ET) to produce pyridinium radical (PyH 0 ). Examples of systems to effect this ET to populate PyH + 's LUMO (E 0 calc ~ -1.3V vs. SCE) to form the solution phase PyH 0 via highly reducing electrons include the photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and the photochemical [Ru(phen)3] 2+ /ascorbate system. We predict that PyH 0 undergoes further PT-ET steps to form the key closed-shell, dearomatized 1,2-dihydropyridine (PyH2) species. Our proposed sequential PT-ET-PT-ET mechanism transforming Py into PyH2 is consistent with the mechanism described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second component of the catalytic cycle, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first reduction step, i.e. reduction of CO2, are sequential in nature; by contrast, they are coupled in each of the two subsequent hydride and proton transfers to reduce formic acid and formaldehyde.

show abstract

“…[1b,c] We recently found that transitionmetal-catalyzed allylic substitution reactions were compatible with asymmetric dearomatization processes for various aromatics such as indoles, pyrroles, and phenols. [2, 3] However, suitable aromatic compounds are limited to electron-rich nucleophiles, and electron-deficient aromatic compounds, such as pyridines and pyrazines, have not been explored yet.To date, asymmetric dearomatization reactions of pyridine derivatives have focused on asymmetric hydrogenations, [4] transfer hydrogenations, [5] and Reissert-type reactions.[6] In most cases, preactivation by N-acylation or alkylation to generate highly electrophilic pyridinium intermediates is required. The removal of the protecting group from the nitrogen atom is always essential at a later stage.…”

mentioning

confidence: 99%