- Identification and characterization of a new family of catalytically highly active imidazolin-2-ylidenes
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A new class of easily accessible and stable imidazolin-2-ylidenes has been synthesized where the side chains are comprised of substituted naphthyl units. Introduction of the naphthyl groups generates C2-symmetric (rac) and Cs-symmetric (meso) atropisomers, and interconversion between the isomers is studied in detail both experimentally and computationally. Complete characterization of the carbenes includes rare examples of crystallographically characterized saturated NHC structures. Steric properties of the ligands and an investigation of their stability are also presented. In catalysis, the new ligands show versatility comparable to the most widely used NHCs IMes/SIMes or IPr/SIPr. Excellent catalytic results are obtained when either the NHC salts (ring-opening alkylation of epoxides), NHC-modified palladium compounds (C-C and C-N cross-couplings), or NHC-ruthenium complexes (ring-closing metathesis, RCM) are employed. In several cases, this new ligand family provides catalytic systems of higher reactivity than that observed with previously reported NHC compounds.
- Luan, Xinjun,Mariz, Ronaldo,Gatti, Michele,Costabile, Chiara,Poater, Albert,Cavallo, Luigi,Linden, Anthony,Dorta, Reto
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p. 6848 - 6858
(2008/12/23)
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- Lewis base-catalyzed addition of trialkylaluminum compounds to epoxides
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A novel concept for catalytic epoxide alkylation has been developed. Lewis bases like phosphanes, arsanes, stibanes, and sulfides were found to catalyze the alkylation of symmetrical epoxides with trialkylaluminum compounds very effectively at a 5 mol % level. Cyclic as well as acyclic epoxides were readily alkylated in good yields. In reactions with terminal epoxides a significant enhancement of rate and/or regioselectivity was noted in the Lewis base-catalyzed process. Coordination of the Lewis base to the Lewis acidic aluminum reagent was proved by 27Al and 31p NMR spectroscopy and is proposed to form a more nucleophilic alkylating agent.
- Schneider, Christoph,Brauner, J?rg
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p. 4445 - 4450
(2007/10/03)
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- Lewis base-catalyzed addition of triethylaluminum to epoxides
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Lewis bases like phosphines, arsines, and antimonies catalyze the nucleophilic addition of triethylaluminum to epoxides very efficiently. They are proposed to coordinate to triethylaluminum with formation of monomeric and more reactive triethylaluminum Le
- Schneider, Christoph,Brauner, J?rg
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p. 3043 - 3046
(2007/10/03)
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- Microbiological Reduction of α,β-Unsaturated Ketones by Beauveria sulfurescens
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Microbiological reduction of α,β-unsaturated ketones was studied.Variously substituted cyclopentenones, cyclohexenones, and methylalkenones were reduced by Beauveria sulfurescens under low-aeration conditions.The reaction takes place only with a small substituent in the α-position and hydrogen in the β-position.The saturated ketone is always obtained, sometimes accompanied by saturated alcohol.Yields and optical purities of the products are excellent.
- Kergomard, A.,Renard, M. F.,Veschambre, H.
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p. 792 - 798
(2007/10/02)
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- Stereochemistry of Aliphatic Carbocations, 12. Alkyl Shifts between Secondary Carbon Atoms
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Several optically active, β-branched alkylamines have been synthesized from amino acids.The corresponding amines were obtained from isobutyraldehyde and 2-methylbutanal (37), respectively.The stereochemistry at the terminus of 1,2-methyl shifts has been elucidated in the nitrous acid deaminations of 4 and 21.Predominant, although incomplete inversion at the migration terminus is consistent with conformational control rather than neighboring group participation.The deamination of 31 involves a degenerate 1,2-ethyl shift and a nondegenerate 1,2-methyl shift, the reverse holds for 44.Complete inversion at the origin of the methyl migrations and the absence of subsequent retrogressive H shifts strongly support the intermediacy of methyl-bridged carbocations.Partial racemization at the origin of the ethyl migrations has been traced to proton shifts within ethyl-bridged intermediates.Rearranged open cations contribute significantly to the overall reaction if micelles are produced by self-aggregation of the alkylammonium ions.
- Kirmse, Wolfgang,Prolingheuer, Ernst-Christoph
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p. 104 - 128
(2007/10/02)
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- Short-Lived Intermediates. 8. Excited States, Regioselectivity, and Stereospecificity in the Photochemistry of (R,S:S,R)- and (R,R:S,S)-1,2-Dimethylbutyl Trifluoroacetate
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An investigation into the details of the type II photoelimination reaction in racemic (R,S:S,R)- (1a) and racemic (R,R:S,S)-1,2-dimethylbutyl trifluoroacetate (1b) in dodecane solution is presented and contrasted with similar studies of ketones.Stepwise statistical data extraction from quenching (1-dodecene) experiments gave the triplet excited state lifetimes (1a, 0.4; 1b, 2.1 ns), the fraction of reaction from each excited state giving alkene 2 (1a, 77 +/- 8percent singlet; 1b, 78 +/- 2percent singlet) and alkene 3 (1a, 76 +/- 5percent singlet; 1b, 76 +/- 6percent singlet), the regioselectivities of the singlet excited states (1a, 7.4 +/- 0.2; 1b, 4.3 +/- 0.1) and triplet excited states (1a, 6.6 +/- 0.9; 1b, 3.5 +/- 0.6), and stereospecificities of the singlet excited states (1a, 0.88 +/- 0.02; 1b, 0.74 +/- 0.01) and triplet excited states (1a, 0.59 +/- 0.10; 1b, 0.02 +/- 0.11).Dual quenching experiments gave singlet excited state lifetimes (1a, 2.6; 1b, 2.8 ns).The ester type II photoelimination reaction was shown to be markedly similar to analogous reactions in ketones.
- Gano, James E.,Chien, David Hou-Tar
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p. 3182 - 3188
(2007/10/02)
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- THE ATE COMPLEXES OF ALUMINIUM. REACTIVITY AND STEREOSELECTIVITY WITH RESPECT TO EPOXIDES AND CARBONYL COMPOUNDS. CATALYTIC ACTIVATION BY SALTS OF TRANSITION METALS
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When used in non-coordinating solvents (hydrocarbons) NaAlEt4 and LiAlnBu4 are good alkylation agents for epoxides.The presence of catalytic quantities of transition-metal salts, particularly NiCl2 or NiBr2, greatly accelerate the reactions, making them possible within a reasonable time in the case of disubstituted epoxides such as cyclohexene oxide, 2-3 epoxybutane, 1 phenyl-2,3-epoxybutane.In the case of aliphatic epoxides, dialkylmagnesium, NaAlEt4 and LiAlnBu4 lead mainly to alkylation of the least substituted carbon of the epoxide ring; while in the case of epoxides with C-O bond in the benzyl position, it is this carbon that is alkylated.The reaction always proceeds by total inversion of the configuration of the carbon in the epoxide ring, namely the site of the alkylation. NaAlEt4 is also good agent for alkylating carbonyl compounds when used in solvents of low basicity such as diethylether, or in totally non-coordinating solvents such as the hydrocarbons.The yields of the alcohol are greatly improved by using catalytic quantities of NiCl2.The behaviour of NaAlEt4 with 2-phenylpropanol is quite remarkable: in diethylether NaAl-Et4 gives predominantly the pair of enantiomers predicted by Cram's rule and with greater stereoselectivity than if EtMgBr was used, while in pentane the reaction is no longer stereoselective.Finally, with a cyclic ketone, 4-t-butylcyclohexanone, NaAlEt4 in diethylether and in hexane in the presence of NiCl2 gives predominantly the equatorial alcohol resulting from an axial attack, which is generally not favoured at all.
- Boireau, G.,Abenhaim, D.,Henry-Basch, E.
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p. 3061 - 3070
(2007/10/02)
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