- 1,4-Bis-Dipp/Mes-1,2,4-Triazolylidenes: Carbene Catalysts That Efficiently Overcome Steric Hindrance in the Redox Esterification of α- And β-Substituted α,β-Enals
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As reported by Scheidt and Bode in 2005, sterically nonencumbered α,β-enals are readily converted to saturated esters in the presence of alcohols and N-heterocyclic carbene catalysts, e.g., benzimidazolylidenes or triazolylidenes. However, substituents at the α- or β-position of the α,β-enal substrate are typically not tolerated, thus severely limiting the substrate spectrum. On the basis of our earlier mechanistic studies, a set of N-Mes- or N-Dipp-substituted 1,2,4-triazolium salts were synthesized and evaluated as (pre)catalysts in the redox esterification of various α- or β-substituted enals. In particular the 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes overcome the above limitations and efficiently catalyze the redox esterification of a whole series of α/β-substituted enals hitherto not amenable to NHC-catalyzed transformations. The synthetic value of 1,4-bis-Mes/Dipp-1,2,4-triazolylidenes is further demonstrated by the one-step bicyclization of 10-oxocitral to (racemic) nepetalactone in diastereomerically pure form.
- Yatham, Veera Reddy,Harnying, Wacharee,Kootz, Darius,Neud?rfl, J?rg-M.,Schl?rer, Nils E.,Berkessel, Albrecht
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p. 2670 - 2677
(2016/03/12)
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- Preparation and application of 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene, a stable carbene
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The title compound 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene has been prepared in quantitative yield from the corresponding 5-methoxytriazoline precursor by simple thermal decomposition in vacuo via α-elimination of methanol. The carbene is a
- Enders, Dieter,Breuer, Klaus,Kallfass, Ulrike,Balensiefer, Tim
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p. 1292 - 1295
(2007/10/03)
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- Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands
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This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR3)(Cl)2Ru=CHR′, which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)2(Cl)2Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H2IMes)(IMes)(Cl)2Ru=CHPh, and the other of (PCy3)(Cl)(CO)Ru[η2-(CH2- C6H2Me2)(N2 C3H4)(C6H2Me3)], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydridocarbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBut. We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions.
- Trnka, Tina M.,Morgan, John P.,Sanford, Melanie S.,Wilhelm, Thomas E.,Scholl, Matthias,Choi, Tae-Lim,Ding, Sheng,Day, Michael W.,Grubbs, Robert H.
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p. 2546 - 2558
(2007/10/03)
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- Preparation of 1,2,4-triazolium salts and 1,2,4-triazolines
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A process for the preparation of 1,2,4-triazolium salts Ia STR1 where R1, and R2 and R3 are C-organic radicals, it being possible for R2 and R3 to be connected to give a 5- to 8-membered ring, R4 is hydrogen or an organic radical and A is an equivalent of an anion, by reacting an amidrazone II STR2 with a carboxylic acid III or a functional derivative (IIIa) of this acid, an anion A being formed from III or IIIa which, optionally, can be replaced by another anion. The triazolium salts have great industrial importance as catalysts for the preparation of acyloine from aldehydes.
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- 7. The chemistry of stable carbenes: Part 2 - Benzoin-type condensations of formaldehyde catalyzed by stable carbenes
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Stable carbenes derived from thiazole, 1 H-imidazole, and 4H -1,2,4-triazole are efficient catalysts for benzoin-type condensations of formaldehyde. Catalysts derived from N-substituted thiazolium salts trimerize formaldehyde to dihydroxyacetone (II). Catalysts based on 1,4-disubstituted 4H-1,2,4-triazol-1-ium salts give glycolaldehyde (I) as the main product and no II, whereas N,N′-disubstituted 1H-imidazol-3-ium salts yield mixtures of both products. The isolation of several intermediates in the catalytic cycle provide a better insight into the reaction mechanism.
- Teles, J. Henrique,Melder, Johann-Peter,Ebel, Klaus,Schneider, Regina,Gehrer, Eugen,Harder, Wolfgang,Brode, Stefan,Enders, Dieter,Breuer, Klaus,Raabe, Gerhard
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- Darstellung, Struktur und Reaktivitaet von 1,3,4-Triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-yliden, einem neuen stabilen Carben
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Stichworte: Carbene * Cycloadditionen * Heterocyclen * Insertionen
- Enders, Dieter,Breuer, Klaus,Raabe, Gerhard,Runsink, Jan,Teles, J. Henrique,et al.
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p. 1119 - 1122
(2007/10/02)
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