Use of dissolving metals in the partial reduction of pyridines: formation of 2-alkyl-1,2-dihydropyridines

Donohoe, Timothy J.; McRiner, Andrew J.; Helliwell, Madeleine; Sheldrake, Peter

doi:10.1039/b101662h

Cited by 32 publications

(14 citation statements)

References 28 publications

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“…For instance, reductive methylation of the pyridine diester (75) gave a mixture of compounds (76) and (77) in a 5 : 1 ratio. However, the reaction of compound (75) with Na in NH 3 followed by quenching with allyl bromide gave a mixture of the allylated compounds (78) and (79) in a 1 : 2 ratio (Scheme 17.27 ) [88] .…”

Section: Partial Reduction Of Aromatic and Heteroaromatic Rings 433mentioning

confidence: 99%

Dissolving Metals

Yus

Foubelo

2008

Modern Reduction Methods

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17 IntroductionDissolving metals have been used extensively as reducing agents for more than a century but today they have been partially displaced from the central fi eld of organic synthesis by the use of other more selective methodologies, such as metal hydrides [1] and catalytic hydrogenations [2] . However, dissolving metals are still of interest for the selective reduction of specifi c polar functional groups (such as hindered cyclic ketones) and the reductive cleavage of some activated bonds [3] .Regarding the mode of action of electropositive metals in reduction processes, it is generally assumed that an initial single -electron transfer ( SET ) from the metal to the organic substrate takes place to give a radical anion [4] . Depending on the organic substrate and on the reaction conditions (mostly on the solvent), the highly reactive resulting radical anion can then decompose via a number of different routes. For instance, for compounds (I) with multiple C = X bonds (X = CR 2 , O, NR, S), the radical anion (II) could be protonated in the presence of a proton source (the solvent) to give a new radical (III) , which after a further SET from the metal, and further protonation of the resulting anion (IV) , would lead to the reduced product (V) . In the absence of a proton source, coupling of two radical anions (II) can occur to give fi rst the dianionic species (VI) and, after hydrolysis, compounds of type (VII) . Organic substrates (VIII) with activated bonds X − Y (X = CR 3 , OR, SR, NR 2 ; Y = CR 3 , OR, SR, NR 2 , Hal (F, Cl, Br, I)) also undergo reductive cleavage by means of electropositive metals through a SET mechanism. In this case, the initially formed radical anion (IX) suffers the activated X − Y bond scission to generate a radical (X) and an anion (XI) . Radical (X) would be converted to an anion after a new electron transfer from the metal, and fi nal protonation would yield the corresponding reduced products (XII) and (XIII) , respectively (Scheme 17.1 ).Most of the reductions by means of dissolving metals have been performed using highly electropositive metals, such as Li, Na and K, in liquid NH 3 as solvent. Less electropositive metals (Mg, Ca, Sr, Ba, Ni, Cu, Zn, In, Sn, Pb) have also been used, so by choosing the appropriate metal, solvent and cosolvent it is possible to

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Section: Partial Reduction Of Aromatic and Heteroaromatic Rings 433mentioning

confidence: 99%

Dissolving Metals

Yus

Foubelo

2008

Modern Reduction Methods

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“…[5] Wegen der Resonanzstabilisierung des N-aromatischen Kerns ist die Desaromatisierung von N-Heteroarenen oft eine kinetisch und/oder thermodynamisch ungünstige Reaktion. [7] In diesem Zusammenhang wurde basierend auf der Hydrierung als einfachster und atomeffizientester Reduktionsmethode eine Reihe komplizierter (Transfer)Hydrierungsmethoden zur selektiven Herstellung von (partiell) gesättigten azacyclischen Verbindungen aus N-Heteroarenen entwickelt. B. NaBH 4 oder LiAlH 4 )o der reaktiven Alkalimetall in stçchiometrischer Menge.D iese Methoden sind jedoch mit fehlender Chemo-und Regioselektivität, der geringen Toleranz funktioneller Gruppen sowie der Bildung großer Abfallmengen verbunden.…”

Section: Introductionunclassified

Katalytische Desaromatisierung von N‐Heteroarenen mit Silicium‐ und Borverbindungen

Park

Chang

2017

Angewandte Chemie

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Desaromatisierte N‐Heterocyclen sind als Strukturmotive für die organische Synthese und die chemische Biologie von Bedeutung. Die katalytische desaromatisierende Reduktion von nicht aktivierten N‐Heteroarenen mit Silicium‐ und/oder Borverbindungen als Reduktionsmittel ist eine der direktesten Alternativen zur Hydrierung. Bisher wurden jedoch nur vereinzelte Beispiele zur katalytischen Reduktion von N‐Heteroarenen mit Silan‐ oder Boranreduktionsmitteln beschrieben. In diesem Aufsatz wird über jüngste Fortschritte bei der katalytischen Reduktion von nicht aktivierten N‐Heteroarenen mit Hydrosilanen, Hydroboranen, Silaboranen und Diboranen berichtet. Dabei wird vor allem auf die chemische Reaktivität und Selektivität von Übergangsmetallkatalysatoren und metallfreien Organokatalysatorsystemen eingegangen. Des Weiteren werden die Funktionsweisen dieser Katalysatoren insbesondere auf der Basis experimenteller mechanistischer Erkenntnisse beschrieben.

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“…However, the synthesis of functionally substituted 1,2-dihydropyridines is often difficult; they are usually prepared by reduction [4][5][6] or functionalization [7] of pyridine derivatives.…”

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confidence: 99%