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Hydride Elimination (hydride + elimination)
Selected AbstractsSuppression of ,-Hydride Elimination in the Intramolecular Hydrocarboxylation of Alkynes Leading to the Formation of Lactones.CHEMINFORM, Issue 29 2007Zhibao Huo Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] ChemInform Abstract: Rhodium(II) Acetate Catalyzed Synthesis of Cyclic Enamides and Enamines via ,-Hydride Elimination.CHEMINFORM, Issue 30 2002Sengodagounder Muthusamy Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Role of Halide Ions in Divalent Palladium-Mediated Reactions: Competition Between ,-Heteroatom Elimination and ,-Hydride Elimination of a Carbon,Palladium Bond.CHEMINFORM, Issue 50 2001Zhaoguo Zhang Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] O -Acylated 2-Phosphanylphenol Derivatives , Useful Ligands in the Nickel-Catalyzed Polymerization of EthyleneEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2009Dmitry G. Yakhvarov Abstract The title ligands were prepared by O -acylation of 2-diphenylphosphanyl-4-methylphenol (1) or directly by double lithiation of 2-bromo-4-methylphenol and stepwise coupling with ClPPh2 and ClP(O)Ph2 or RC(O)Cl (R = Me, tBu, Ph, 4-MeOC6H4) to afford diphenylphosphinate 2 and carboxylic esters 3a,d. X-ray crystal structure analyses of 3b,d show conformations in which the P -phenyl substituents are rotated away from the ester group and the C(O)O , planes are nearly perpendicular to the phenol ring , plane. O -Acylated phosphanylphenols 2 and 3a,d form highly active catalysts with Ni(1,5-cod)2 (as does 1) for polymerization of ethylene, whereas phosphanylphenyl ethers do not give catalysts under the same conditions. The reason is the cleavage of the O -acyl bond upon heating with nickel(0) precursor compounds in the presence of ethylene. The precursors are P-coordinated Ni0 complexes, which are formed at room temperature, such as 4d obtained from 3d and Ni(cod)2 (in a 2:1 molar ratio), and characterized by multinuclear NMR spectroscopy. Upon heating in the presence of ethylene, the precatalysts are activated. Catalysts 2Ni and 3a,dNi convert ethylene nearly quantitatively, 2Ni slowly, and 3a,dNi rapidly, into linear polyethylene with vinyl and methyl end groups, and in the latter case, C(O)R end groups are also detectable. This proves insertion of Ni0 into the O,C(O)R bond of 3a,d ligands for formation of the primary catalyst. Termination of the first chain growing cycle by ,-hydride elimination changes the mechanism to the phosphanylphenolate,NiH initiated polymerization providing the main body of the polymer. A small retardation in the ethylene consumption rate with 3a,dNi catalysts relative to that observed for 1Ni and stabilization of the catalyst, which gives rise to reproducibly high ethylene conversion, is observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Ruthenium(II)-Catalyzed Cyclization of Oxabenzonorbornenes with Propargylic Alcohols: Formation of IsochromenesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2006Karine Villeneuve Abstract The ruthenium-catalyzed cyclization of a propargylic alcohol with an oxabenzonorbornene in methanol leads to an unanticipated isochromene framework. The catalytic cycle to form this product is believed to go through an oxidative cyclization of the two unsaturated partners with the ruthenium catalyst, followed by ,-hydride elimination, tautomerization andhydroruthenation. The ruthenacyclobutane thus obtained further undergoes [2+2] cycloreversion to form a ruthenium,carbene intermediate that atypically rearranges via a [1,3]-alkoxide shift and finally reductively eliminates to produce the desired compound. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Deposition of HfO2, Gd2O3 and PrOx by Liquid Injection ALD Techniques,CHEMICAL VAPOR DEPOSITION, Issue 3 2005J. Potter Abstract Thin films of hafnium oxide (HfO2), gadolinium oxide (Gd2O3), and praseodymium oxide (PrOx) have been deposited by liquid injection atomic layer deposition (ALD) and for comparison, have also been deposited by "thermal" metal-organic (MO) CVD using the same reactor. The ALD-grown films were deposited on Si(100) over a range of substrate temperatures (150,450,°C) using alternate pulses of [Hf(mmp)4], [Gd(mmp)3], or [Pr(mmp)3] (mmp = OCMe2CH2OMe) and water vapor. X-ray diffraction (XRD) analysis showed that as-grown films of HfO2 were amorphous, but these crystallized into the monoclinic phase after annealing in air at 800,°C. XRD analysis showed that as-grown Gd2O3 and PrOx films had some degree of crystallinity. Residual carbon (0.8,3.3 at.-%) was detected in the HfO2 and PrOx films by Auger electron spectroscopy (AES), but not in the Gd2O3 films. The self-limiting behavior of the precursors was investigated at 225,°C by varying the volume of precursor injected during each ALD cycle and, in each case, oxide growth was not fully self-limiting. We propose a mechanism for this involving ,-hydride elimination of the mmp group, and also propose some general mechanistic principles which may influence the growth of oxides by ALD using other precursors. [source] Cobalt-Mediated Linear 2:1 Co-oligomerization of Alkynes with Enol Ethers to Give 1-Alkoxy-1,3,5-Trienes: A Missing Mode of ReactivityCHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2010David Leb, uf Dr. Abstract A variety of 1,6-heptadiynes and certain borylalkynes co-oligomerize with enol ethers in the presence of [CpCo(C2H4)2] (Cp=cyclopentadienyl) to furnish the hitherto elusive acyclic 2:1 products, 1,3,5-trien-1-ol ethers, in preference to or in competition with the alternative pathway that leads to the standard [2+2+2] cycloadducts, 5-alkoxy-1,3-cyclohexadienes. Minor variations, such as lengthening the diyne tether, cause reversion to the standard mechanism. The trienes, including synthetically potent borylated derivatives, are generated with excellent levels of chemo-, regio-, and diastereoselectivity, and are obtained directly by decomplexation of the crude mixtures during chromatography. The cyclohexadienes are isolated as the corresponding dehydroalkoxylated arenes. In one example, even ethene functions as a linear cotrimerization partner. The alkoxytrienes are thermally labile with respect to 6,-electrocyclization,elimination to give the same arenes that are the products of cycloaddition. The latter, regardless of the mechanism of their formation, can be viewed as the result of a formal [2+2+2] cyclization of the starting alkynes with acetylene. One-pot conditions for the exclusive formation of arenes are developed. DFT computations indicate that cyclohexadiene and triene formation share a common intermediate, a cobaltacycloheptadiene, from which reductive elimination and ,-hydride elimination compete. [source] Pincer-Type Heck Catalysts and Mechanisms Based on PdIV Intermediates: A Computational StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2010Olivier Blacque Dr. Abstract Pincer-type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain PdII centers, controversial PdII/PdIV cycles have been often proposed as potential catalytic mechanisms. However, pincer-type PdIV intermediates have never been experimentally observed, and computational studies to support the proposed PdII/PdIV mechanisms with pincer-type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving PdIV intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine-, phosphine-, and phosphite-based pincer-type Heck catalysts with styrene and phenyl bromide as substrates and (E)-stilbene as coupling product. The potential-energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that PdII/PdIV mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6-C6H3(XPR2)2}Pd(Cl)] [X=NH, R=piperidinyl (1,a); X=O, R=piperidinyl (1,b); X=O, R=iPr (1,c); X=CH2, R=iPr (1,d)] to yield cationic, three-coordinate, T-shaped 14e, palladium intermediates of type [{2,6-C6H3(XPR2)2}Pd]+ (2). An alternative reaction path to generate complexes of type 2 (relevant for electron-poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6-C6H3(XPR2)2}Pd(Cl)(CH2CHPh)] (4) and consecutive dissociation of the chloride ligand to yield cationic square-planar styrene complexes [{2,6-C6H3(XPR2)2}Pd(CH2CHPh)]+ (6) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate PdIV complexes of type [{2,6-C6H3(XPR2)2}Pd(Br)(C6H5)]+ (11), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes [{2,6-C6H3(XPR2)2}Pd(Br)(C6H5)(CH2CHPh)]+ (12). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes [{2,6-C6H3(XPR2)2}Pd(Br)(CHPhCH2Ph)]+ (13). Subsequent ,-hydride elimination induces direct HBr liberation to yield cationic, square-planar (E)-stilbene complexes with general formula [{2,6-C6H3(XPR2)2}Pd(CHPhCHPh)]+ (14). Subsequent liberation of (E)-stilbene closes the catalytic cycle. [source] | ||||||||||