- Synthesis of pentafluorobenzene-based NHC adducts and their catalytic activity in the microwave-assisted reactions of aldehydes
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N-Heterocyclic carbenes (NHCs) have been widely used in organometallic chemistry as ligands, as well as standalone organocatalysts in various reactions, mostly using aromatic aldehydes as substrates. We have previously demonstrated the efficiency of azolium-2-carboxylate zwitterions in the hydroxymethylation of aldehydes, especially aliphatic aldehydes, under microwave irradiation. In the present work, we report a series of pentafluorobenzene-based NHC adducts and their efficiency in the hydroxymethylation and self-condensation of aliphatic and aromatic aldehydes using microwave irradiation. The free carbenes are released under the reaction conditions and 1,3-dimesityl-2-(perfluorophenyl)imidazolidine and 1,3-bis(2,6-dimethylphenyl)-2-(perfluorophenyl)imidazolidine proved to be the most potent precatalysts.
- Papadaki, Evanthia,Magrioti, Victoria
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- New fluorinated catalysts for olefin metathesis
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New olefin metathesis catalysts, analogues of Grubbs II ones, bearing hexafluoroisopropylmethoxy groups in NHC ligand, provide high conversions in model ring closing metathesis of diallylmalonate and cross metathesis of allylbenzene with 1,3-diacetoxybut-
- Masoud, Salekh M.,Mailyan, Artur K.,Peregudov, Alexander S.,Bruneau, Christian,Osipov, Sergey N.
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p. 474 - 476
(2016/11/30)
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- Transfer Hydrogenation of Nitriles, Olefins, and N-Heterocycles Catalyzed by an N-Heterocyclic Carbene-Supported Half-Sandwich Complex of Ruthenium
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In the presence of KOBut, N-heterocyclic carbene-supported half-sandwich complex [Cp(IPr)Ru(pyr)2][PF6] (3) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) catalyzes transfer hydrogenation (TH) of nitriles, activated N-heterocycles, olefins, and conjugated olefins in isopropanol at the catalyst loading of 0.5%. The TH of nitriles leads to imines, produced as a result of coupling of the initially formed amines with acetone (produced from isopropanol), and showed good chemoselectivity. Reduction of N-heterocycles occurs for activated polycyclic substrates (e.g., quinoline) and takes place exclusively in the heterocycle. The TH also works well for linear and cyclic olefins but fails for trisubstituted substrates. However, the C = C bond of α,β-unsaturated esters, amides, and acids is easily reduced even for trisubstituted species, such as isovaleriates. Mechanistic studies suggest that the active species in these catalytic reactions is the trihydride Cp(IPr)RuH3 (5), which can catalyze these reactions in the absence of any base. Kinetic studies are consistent with a classical inner sphere hydride-based mechanism of TH.
- Mai, Van Hung,Nikonov, Georgii I.
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p. 943 - 949
(2016/05/09)
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- Synthesis of quaternary carbon stereocenters by copper-catalyzed asymmetric allylic substitution of allyl phosphates with arylboronates
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A copper-catalyzed asymmetric allylic substitution of γ,γ- disubstituted allyl phosphates with arylboronates has been developed for the construction of quaternary stereocenters. High regio- and enantioselectivities have been achieved by employing a hydroxy-bearing chiral N-heterocyclic carbene ligand, and both E and Z substrates provide the same enantiomer as the major product. The mechanistic aspect of this catalysis has also been investigated to find that a 1:1 copper/ligand complex is most likely responsible for the present asymmetric catalysis, and the reaction proceeds with almost perfect 1,3-anti stereochemistry with respect to the allylic electrophile.
- Takeda, Momotaro,Takatsu, Keishi,Shintani, Ryo,Hayashi, Tamio
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p. 2354 - 2367
(2014/04/17)
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- Effect of ligand substituents in olefin polymerisation by half-sandwich titanium complexes containing monoanionic iminoimidazolidide ligands-MAO catalyst systems
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Various half-sandwich titanium complexes containing iminoimidazolidide ligands, CpTiCl2[1,3-R2(CH2N)2CN] (1a-d) [R = Ph (a), 2,6-Me2C6H3 (b), cyclohexyl (c), tBu (d)], have been employed as the catalyst precursors for ethylene polymerisation, syndiospecific styrene polymerisation, and copolymerisation of ethylene with 1-hexene in the presence of MAO cocatalyst; 1d showed the highest catalytic activity for ethylene polymerisation whereas 1b showed the highest activity for syndiospecific styrene polymerisation.
- Nomura, Kotohiro,Fukuda, Hiroya,Katao, Shohei,Fujiki, Michiya,Kim, Hyun Joon,Kim, Dong-Hyun,Zhang, Shu
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p. 7842 - 7849
(2011/09/20)
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- Structure-Analytical Investigations of P-Substituted 1,3,2- Diazaphospholidines
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2-Diphenylphosphanyl-1,3,2-diazaphospholidines were prepared via metathesis from 2-chloro-1,3,2-diazaphospholidines and LiPPh2. For some of the products, symmetrisation to tetraphenyldiphosphane and 2,2′-bis-1,3,2- diazaphospholidinyls was observed. Most of the derivatives were characterised by single-crystal X-ray diffraction, which showed that all compounds studied feature elongated exocyclic P-Cl or P-P-bonds, respectively. The extent of this bond lengthening is in the P-phosphanyl-substituted species similar and in the P-chloro-derivatives less pronounced than in corresponding CC-unsaturated 1,3,2-diazaphospholenes. Structure correlation involving comparison of exocyclic P-X and endocyclic P-N distances suggests that n(N)/σ(P-X) hyperconjugation contributes strongly to the bond lengthening and induces a perceptible weakening of the P-P bonds in 2-diphenylphosphanyl-1,3,2- diazaphospholidines, which should render these compounds interesting substrates for P-P bond activation reactions. Copyright
- Puntigam, Oliver,Hajdok, Imre,Nieger, Martin,Niemeyer, Mark,Strobel, Sabine,Gudat, Dietrich
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scheme or table
p. 988 - 994
(2011/09/12)
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- 1,3-Diarylimidazolidin-2-ylidene (NHC) complexes of Pd(II): Electronic effects on cross-coupling reactions and thermal decompositions
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The synthesis of 1,3-diarylimidazolidin-2-ylidene (NHC) precursor, 1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride, (3b) has been extended to the electronically and sterically modified NHC precursors 3a (X = H), 3c (X = Br) and 3e (X = Cl) in order t
- Türkmen, Hayati,?etinkaya, Bekir
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p. 3749 - 3759
(2007/10/03)
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- New osmium-based reagent for the dihydroxylation of alkenes
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The cis dihydroxylation of alkenes is most efficiently accomplished by reaction with osmium tetroxide. Recently, the expense and toxicity of osmium tetroxide have led to a number of attempts to harness alternative osmium-based reagents, including microencapsulation and solid support techniques. We describe here the development of a new nonvolatile, stable, and recoverable osmium-based reagent devised for the stoichiometric cis dihydroxylation of alkenes. Although attempts to make this new dihydroxylation work with catalytic amounts of this reagent were unsuccessful, we did develop a sensitive test for free osmium tetroxide leached from the reagent in situ: this test may well have uses in probing future applications of derivatized osmium reagents.
- Donohoe, Timothy J.,Harris, Robert M.,Butterworth, Sam,Burrows, Jeremy N.,Cowley, Andrew,Parker, Jeremy S.
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p. 4481 - 4489
(2015/09/24)
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- Regarding the mechanism of olefin metathesis with sol-gel-supported Ru-based complexes bearing a bidentate carbene ligand. Spectroscopic evidence for return of the propagating Ru carbene
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Two isotopically and structurally labeled Ru-based carbenes (2-d 4 and 13) have been prepared and attached to the surface of monolithic sol-gel glass. The resulting glass-supported complexes (18-d n and 19) exhibit significant catalytic activity in promoting olefin metathesis reactions and provide products of high purity. Through analysis of the derivatized glass pellets used in a sequence of catalytic ring-closing metathesis reactions mediated by various supported Ru carbenes, it is demonstrated that free Ru carbene intermediates in solution can be scavenged by support-bound styrene ether ligands prior to the onset of competing transition metal decomposition. The observations detailed herein provide rigorous evidence that the initially proposed release/return mechanism is, at least partially, operative. The present investigations shed light on a critical aspect of the mechanism of an important class of Ru-based metathesis complexes (those bearing a bidentate styrene ether ligand).
- Kingsbury, Jason S.,Hoveyda, Amir H.
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p. 4510 - 4517
(2007/10/03)
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