- Directional properties of fluorenylidene moieties in unsymmetrically substituted N-heterocyclic carbenes. Unexpected CH activation of a methylfluorenyl group with palladium. Use in palladium catalysed Suzuki-Miyaura cross coupling of aryl chlorides
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Benzimidazolium salts having their two nitrogen atoms substituted by different 9-alkylfluorenyl groups (4a-e and 4g, alkyl1/alkyl 2 = Me/Et, Me/Pr, Me/n-Bu, Me/i-Pr, Me/Bn, Me/CH2SMe have been synthesised in high yields in two or three steps from N,N′-bis(9H- fluoren-9-ylidene)benzene-1,2-diamine (1). The imidazolium salts 4a-e were converted readily into the corresponding PEPPSI-type palladium complexes (PEPPSI = pyridine-enhanced precatalyst preparation stabilisation and initiation), while reaction of the methylthioether-substituted salt 4g with PdCl 2/K2CO3/pyridine afforded the palladacycle 5g resulting from metallation of the methyl group attached to the fluorenylidene moiety. NMR and X-ray diffraction studies revealed that the carbene ligands in 5a-5e behave as clamp-like ligands, the resulting metal confinement arising from a combination of the orientational properties of the fluorenylidene moieties that push the alkyl groups towards the metal centre and attractive anagostic interactions involving CH2(fluorenyl) groups. Complexes 5a-e were assessed in Suzuki-Miyaura cross-coupling reactions. Like their symmetrical analogues they displayed high activity in the coupling of phenyl boronic acid with p-tolylchloride but their performance remained slightly inferior to that of the related, symmetrical Et/Et complex 5h.
- Teci, Matthieu,Brenner, Eric,Matt, Dominique,Gourlaouen, Christophe,Toupet, Loic
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p. 12251 - 12262
(2014/08/05)
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- Room temperature dehydrogenation of ethane, propane, linear alkanes C4-C8, and some cyclic alkanes by titanium-carbon multiple bonds
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The transient titanium neopentylidyne, [(PNP)Ti≈CtBu] (A; PNP-≈N[2-PiPr2-4-methylphenyl] 2-), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η2-H2C-CH2)(CH 2tBu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C 4-C6 to form isolable α-olefin complexes of the type, [(PNP)Ti(η2-H2C-CHR)(CH2 tBu)] (R = CH3 (2), CH2CH3 (3), nPr (4), and nBu (5)). Complexes 1-5 can be independently prepared from [(PNP)Ti-CHtBu(OTf)] and the corresponding alkylating reagents, LiCH2CHR (R = H, CH3(unstable), CH 2CH3, nPr, and nBu). Olefin complexes 1 and 3-5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including 1H- 31P HOESY, and in the case of the α-olefin adducts 2-5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C-H coupled and decoupled 1H-13C multiplicity edited gHSQC, 1H- 31P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C 7 and C8) are also dehydrogenated by A to form [(PNP)Ti(η2-H2C-CHnPentyl)(CH 2tBu)] (6) and [(PNP)Ti(η2-H 2C-CHnHexyl)(CH2tBu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D 2C-CD2 with concomitant release of H2C-CH 2. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH2tBu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d12 can be dehydrogenated to 4 and 4-d12 with comparable rates (KIE = 1.1(0) at ~29 C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (O-CPh2; 2 equiv) and the diazoalkane, N2CHtolyl2. For complexes 3-7, oxidation with N2O also liberates the α-olefin.
- Crestani, Marco G.,Hickey, Anne K.,Gao, Xinfeng,Pinter, Balazs,Cavaliere, Vincent N.,Ito, Jun-Ichi,Chen, Chun-Hsing,Mindiola, Daniel J.
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supporting information
p. 14754 - 14767
(2013/10/22)
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- Method for producing alkyl-bridged ligand systems and transition metal compounds
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The invention relates to a method for producing highly substituted alkyl-bridged ligand systems on the basis of indene derivatives and transition metal compounds. Said alkyl-bridged ligand systems can be obtained in high yields using this method.
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- Enantioselective synthesis of 1,2-diamines via nucleophilic 1,2-addition to dibenzylamino-acetaldehyde-SAMP-hydrazone
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Nucleophilic 1,2-addition of RM/CeCl3 in THF or allyl Grignard reagent in toluene to dibenzylamino-acetaldehyde-SAMP-hydrazone 2 after reductive N-N bond cleavage leads to differently protected 1,2-diamines 4 in good yields and of high enantiomeric purity (ee = 92-99percent).The absolute configuration was determined by X-ray structure analysis on the 1,2-adduct 3e. - Keywords: SAMP-hydrazone; nucleophilic 1,2-addition; asymmetric synthesis; vicinal diamine; organocerium reagent
- Enders, Dieter,Chelain, Evelyne,Raabe, Gerhard
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p. 299 - 306
(2007/10/03)
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- On the Lewis Acidity Of Nickel(0), IX. - Alkyllithium Complexes Of Nickel(0)
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Tris(ethene)nickel(0) reacts with stoichiometric amounts of alkyllithium compounds LiR (R = C2H5, n-C3H7, n-C4H9) in ether/PMDTA as solvent at low temperature to afford the yellow crystalline complexes (PMDTA)2(LiR)2Ni(C2H4)2 (R = n-C3H7, n-C4H9, 5, 6) and (PMDTA)(LiR)Ni(C2H4)2 (R = C2H5, n-C3H7, n-C4H9, 3a, 7a, 8a).Ionic complexes of the type +-, which are known already for R = C2H5 (3b), are obtained in ether/TMEDA for R = n-C3H7 (7b) and n-C4H9 (8b) as suspensions but cannot be isolated without decomposition. (Et2O)2(C4H8O2)(Li2C4H8)(Ni(C2H4) (4) is obtained from Ni(C2H4)3 and 1,4-dilithiobutane in ether/dioxane. - With the exception of 4 all complexes undergo an alkyl/alkene exchange reaction with ethene in solution (ether, THF, toluene) at 0 deg C.NMR studies of the partially deuterated (PMDTA)(LiC2D5)Ni(C2H4)2 (-3a) show that the H/D distribution becomes statistical at this temperature.In the presence of ethene, 5-8 yield 3a or 3b with liberation of propene or 1-butene.The alkyllithium complexes of nickel(0) can be regarded as model compounds for the starting and product complexes of nickel(0) postulated to explain the "Nickel Effect" reaction of aluminium trialkyls and ethene.
- Poerschke, Klaus-Richard,Jonas, Klaus,Wilke, Guenther
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p. 1913 - 1920
(2007/10/02)
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