- Isomerization of 1,5-hexadiene catalyzed by bis-(cyclopentadienyl) lanthanide schiff base/NaH systems; Ln = Sm, Dy, Y, Er
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Catalytic isomerization of 1,5-hexadiene by Cp2Ln Schiff base/NaH (Schiff base = C14H14NO2, Ln = Sm, Dy, Y, and Er) systems was studied. The isomerization resulted in a mixture of 1,4-hexadiene, 2,4-hexadiene, 1,3-hexadiene, methylene-cyclopentane, and methylcyclopentene. 1,4-Hexadiene and methylenecyclopentane were the intermediate products, while 2,4-hexadiene and methylcyclopentene were the end-products. The effects of the nature of catalyst, temperature, amount of the catalyst, time and solvent, on the isomerization rate and product composition were also studied. The ratio of the linear to the cyclic product in the reaction depended on the amount of catalyst used.
- Yousaf, Muhammad,Qian, Yanlong,Saeed, Muhammad Khalid
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- Flash vacuum pyrolysis over magnesium. Part 1 - Pyrolysis of benzylic, other aryl/alkyl and aliphatic halides
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Flash vacuum pyrolysis over a bed of freshly sublimed magnesium on glass wool results in efficient coupling of benzyl halides to give the corresponding bibenzyls. Where an ortho halogen substituent is present further dehalogenation gives some dihydroanthracene and anthracene. Efficient coupling is also observed for halomethylnaphthalenes and halodiphenylmethanes while chlorotriphenylmethane gives 4,4′-bis(diphenylmethyl)biphenyl. By using α,α′-dihalo-o-xylenes, benzocyclobutenes are obtained in good yield, while the isomeric α,α′-dihalo-p-xylenes give a range of high thermal stability polymers by polymerisation of the initially formed p-xylylenes. Other haloalkylbenzenes undergo largely dehydrohalogenation where this is possible, in some cases resulting in cyclisation. Deoxygenation is also observed with haloalkyl phenyl ketones to give phenylalkynes as well as other products. With simple alkyl halides there is efficient elimination of HCl or HBr to give alkenes. For aliphatic dihalides this also occurs to give dienes but there is also cyclisation to give cycloalkanes and dehalogenation with hydrogen atom transfer to give alkenes in some cases. For 5-bromopent-1-ene the products are those expected from a radical pathway but for 6-bromohex-1-ene they are clearly not. For 2,2-dichloropropane and 1,1-dichloropropane elimination of HCl occurs but for 1,1-dichlorobutane, -pentane and -hexane partial hydrolysis followed by elimination of HCl gives E, E-, E,Z- and Z,Z- isomers of the dialk-1-enyl ethers and fully assigned 13C NMR data are presented for these. With 6-chlorohex-1-yne and 7-chlorohept-1-yne there is cyclisation to give methylenecycloalkanes and -cycloalkynes. The behaviour of 1,2-dibromocyclohexane and 1,2-dichlorocyclooctane under these conditions is also examined. Various pieces of evidence are presented that suggest that these processes do not involve generation of free gas-phase radicals but rather surface-adsorbed organometallic species.
- Aitken, R. Alan,Hodgson, Philip K.G.,Morrison, John J.,Oyewale, Adebayo O.
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p. 402 - 415
(2007/10/03)
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- Homogeneous Catalysis on Metal Clusters. The Isomerization and Selective Hydrogenation of Dienes, Alkenes, and Alkynes in the Presence of and . Spectroscopic Identification of the Reaction Intermediate...
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The complexes and catalyze under homogeneous conditions the hydrogenation of one double bond of dienes and show selectivity in the hydrogenation of triple and double C-C bonds; non-conjugated dienes are isomerized.Experimental evidence suggests that the cluster acts as a catalyst.Complexes and have been isolated in the reaction solutions.The former complexes have been identified by spectroscopy and the latter by X-ray diffraction methods.A reaction path is proposed.Crystals of are monoclinic, space group P21/n, with unit-cell dimensions a=10.960(7), b=17.064(8), c=10.162(5) Angstroem, β=106.07(2) deg, and Z=4.The structure was determined from diffractometer data by Patterson and Fourier methods and refined by full-matrix least squares to R=0.048.In the complex an allylic ligand (derived from a pentadiene) co-ordinated to a triangular osmium cluster has been found; although this type of structure has already been reported, this is the first osmium complex obtained by reaction with a diene.
- Castiglioni, Mario,Giordano, Roberto,Sappa, Enrico,Tiripicchio, Antonio,Camellini, Marisa Tiripicchio
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- Titanium Catalyzed Cyclization of 1,5-Hexadienes
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Cp2TiCl (1) and Cp2TiCl2 (2) combined with isopropylmagnesium bromide (molar ratio 1:1 and 1:2, resp.) catalyze the conversion of 1,5-hexadiene into a mixture of the five-membered ring compounds 3 and 4 as well as the linear isomeric hexadienes 5, 6, and 7.THF is most effective in both promoting cyclization as well as suppressing isomerization (3 -> 4 and 5 -> 6 or 7).The ratio of cyclic to linear products in reactions involving substituted 1,5-hexadienes is found to be dependent upon the position of the substituents.Substitution in the 2- or 2- and 5-position leads to the formation of the open-chain isomers 11 and 12 or 13 and 14, while 3,4-substituted 1,5-hexadienes react to give > 99percent of the five-membered ring systems 15 and 16 or 17 and 18.A reaction mechanism is discussed which involves Cp2TiH and Cp2(alkenyl)Ti intermediates.
- Lehmkuhl, Herbert,Tsien, Yen-Lung
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p. 2437 - 2446
(2007/10/02)
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- Solution-Phase Photodecarbonylation of the Bicyclohexan-3-one System
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Direct irradiation of solutions containing (-)-thujone (6) or (+)-isothujone (7) affords trans- and cis-5-methylene-6-methylhept-2-ene (8 and 9) in quantitative yield.The epimeric ketones give identical produc mixtures, which are somewhat temperature dependent.The product ratio remains constant throughout irradiation, and appropriate control experiments indicate that the observed diene mixture does not represent a rapidly established photostationary state.Limited epimerization of the ketones is observed although this process is much less efficient than extrusion of carbon monoxide.Quenching studies employing piperylene indicate that the majority of the decarbonylation products originate from a short-lived triplet state.Direct irradiation of solutions containing exo- or endo-2-methylbicyclohexan-3-one (10 or 11) yields trans- and cis-hexa-1,4-diene (12 and 13) in high yield.Again the epimeric ketones give identical product mixtures, which are slightly temperature dependent.Direct irradiation of solutions containing exo,exo-, exo,endo-, or endo,endo-2,4-dimethylbicyclohexan-3-one (16, 15, or 14) affords similar, but not identical, mixtures of trans,trans-, trans,cis-, and cis,cis-hepta-2,5-diene (17, 18, and 19) in high yield.Each of these product ratios is somewhat temparature dependent.These data are explained in terms of a mechanism involving biradical intermediates.Excitation followed by intersystem crossing leads to a short-lived triplet state which undergoes Norrish I cleavage toward the more substituted α position.The diene products are formed upon synchronous cleavage of the cyclopropane bond and loss of carbon monoxide.If internal rotation in the intermediate is rapid with respect to decay, the observed product ratios may be rationalized.Inefficient reclusure of the biradical species leads to limited epimerization of the bicyclohexan-3-ones.
- Cooke, Robert S.,Lyon, Gregory D.
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p. 7317 - 7322
(2007/10/02)
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- (Alkenyl-η3-allyl)bis(η5-cyclopentadienyl)titanium Complexes
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Bis(η5-cyclopentadienyl)titanium hydride (Cp2TiH), presumably formed in situ from bis(η5-cyclopentadienyl)titanium dichloride (1) and isopropylmagnesium bromide (2) adds to the conjugated C=C bonds of the alkatrienes 4, 5, 25, 36, and 45 to give the (alkenyl-η3-allyl)bis(η5-cyclopentadienyl)titanium complexes 7 and 10, 8, 26, and 30, 37, 46.The complexes 7, 8, and 26 with the alkenyl group in position 1 isomerize to give the compounds 27, 28, and 29 in which the C=C bond is conjugated with the allyl group.Compound 37 which contains an alkenyl group in a meso-position does not isomerize.In the case of isomycoren (40), TiH addition occurs primarily to the isolated C=C bond followed by intramolecular cyclization to give the bis(η5-cyclopentadienyl)(1-cyclopentyl-η3-allyl)titanium complex 41.
- Lehmkuhl, Herbert,Fustero, Santos
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p. 1353 - 1360
(2007/10/02)
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