- The value of 2JP–CO as a diagnostic parameter for the structure and thermal reactivity of carbonyl-stabilised phosphonium ylides
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A survey of 20 carbonyl-stabilised phosphonium ylides with recently reported X-ray structures shows a strong correlation between the C[dbnd]P to C[dbnd]O torsion angle and the value of 2JP–CO, with high values being associated with an anti configuration and low with syn. Seven new X-ray structural determinations are reported, several for types of ylide not crystallographically characterised before, and these also conform to this pattern. The value of 2JP–CO is then correlated with whether or not thermal extrusion of Ph3PO occurs to give alkynes for over 200 ylides and an empirical rule developed that the extrusion never occurs for ylides where this value is > 11 Hz. This is used to rationalise the anomalous behaviour of some trioxo ylides and cyclic ylides, two of which afford cycloalkynes, isolated after rearrangement as the isomeric 1,3-dienes. The rule also holds for a family of novel highly fluorinated ylides which afford fluorinated alkynes in good yield upon flash vacuum pyrolysis.
- Aitken, R. Alan,Boubalouta, Youcef,Chang, Da,Cleghorn, Lee P.,Gray, Ian P.,Karodia, Nazira,Reid, Euan J.,Slawin, Alexandra M.Z.
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p. 6275 - 6285
(2017/09/29)
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- Expanding the scope of biomass-derived chemicals through tandem reactions based on oxorhenium-catalyzed deoxydehydration
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New modes of DODH: Oxorhenium compounds act as deoxydehydration(DODH)/acid dual-purpose catalysts to transform biomass-derived diol substrates into a variety of commodity chemical precursors. The power of this approach is highlighted by a tandem [1,3]-OH shift/DODH of 2-ene-1,4-diols and 2,4-diene-1,6-diols, and by a DODH/esterification sequence of sugar acids to unsaturated esters for the production of polymers and plasticizers. Copyright
- Shiramizu, Mika,Toste, F. Dean
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p. 12905 - 12909
(2014/01/06)
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- Reducing characteristics of borohydride exchange resin-CuSO4 in methanol
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Reducing characteristics of borohydride exchange resin (BER)-CuSO 4 (cat.) were studied in methanol at room temperature. Carbon-carbon double bonds conjugated with benzene or carbonyl group were more rapidly reduced than was the case with isolated double bonds. Carbonyl groups were readily reduced, whereas esters and amides were inert, and nitriles were slowly reduced. High chemoselectivity was also observed in halide reductions: p-bromochlorobenzene and p-bromoiodobenzene were reduced quantitatively to chlorobenzene and bromobenzene, respectively. Aliphatic epoxides were inert to this reagent; however, styrene oxide derivatives were readily reduced to the corresponding deoxygenated products. Aliphatic azides were reduced slowly during 6 h, whereas phenyl azide was transformed to aniline in 1 h. Nitrocyclohexane was reduced at room temperature, but nitrobenzene, nitrosobenzene, azobenzene, and azoxybenzene required an elevated temperature (65 °C) for rapid reductions (1 h). Similarly, N,N-dimethylanihne N-oxide was reduced at room temperature, whereas pyridine N-oxide required refluxing. Finally, among the sulfur compounds tested, only diphenyl disulfide was reduced readily, and sulfide, aliphatic disulfide, sulfoxide, sulfone, and tosylate were inert to this reducing system.
- Sim, Tae Bo,Yoon, Nung Min
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p. 1101 - 1107
(2007/10/03)
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- Bimolecular formation of radicals by hydrogen transfer, 11: Transfer hydrogenation of conjugated cyclic dienes and trienes
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1,3-Cyclohexadiene, 1,3-cycloheptadiene and cycloheptatriene are smoothly hydrogenated to cyclohexene and cycloheptene, respectively, when heated to 260-340°C with an excess of 9,10-dihydroanthracene (DHA) in diphenyl ether or benzonitrile. On the basis of a mechanistic study a nonchain three-step mechanism is proposed which is initiated by a transfer of a hydrogen atom from DHA to the polyenes (retrodisproportionation). VCH Verlagsgesellschaft mbH, 1996.
- Morgenthaler, Jens,Ruechardt, Christoph
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p. 1529 - 1532
(2007/10/03)
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- Photoinduced Reactions of Cr(CO)3-Coordinated 1,3,5-Cycloheptatriene Cycloaddition with an Alkyne and Catalytic 1,6-Hydrogenation
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Irradiation of Cr(CO)3(η6-1,3,5-cycloheptatriene) (1) in the presence of bis(trimethylsilyl)ethyne results in cycloaddition of the alkyne to the cycloheptatriene ligand, yielding Cr(CO)3(η4:2-7,8-bis(trimethylsilyl)bicyclonona-2,4,7-triene) (2, 60percent yield, Π = 0.08), which upon oxidative decomposition with Ce(IV) releases the organic cycloadduct, 7,8-bis(trimethylsilyl)bicyclo-nona-2,4,7-triene (3).Photocatalytic hydrogenation of 1,3,5-cycloheptatriene in the presence of complex 1 affords 1,3-cycloheptatriene.This process involves 1,6-addition of hydrogen, as demonstrated by using D2.Key Words: Chromium complexes/ Cycloheptatriene, hydrogenation of/ Photoreactions/ Photocatalysis/ Alkynes, cycloadditon with
- Fischler, Ingrid,Grevels, Friedrich-Wilhelm,Leitich, Johannes,Oezkar, Saim
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p. 2857 - 2862
(2007/10/02)
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- Rearrangements of 1-Cyclohexenylmethylenes and Their Relevance to the Mechanism of the Phenylcarbene Rearrangement
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Gas-phase pyrolysis of 1-cyclohexenyldiazomethane (17) and (2-methyl-1-cyclohexenyl)diazomethane (36) leads to the generation of cyclohexenylmethylenes, 13 and 27, respectively, whose intramolecular rearrangement mechanism can be inferred from the stable end products.These substituted vinylmethylenes undergo intramolecular ?-addition, but apparently do not participate in all-carbon Wolff rearrangement.The relevance of these results to the mechanism of the phenylcarbene rearrangement is discussed, and it is suggested that a ?-route with a bicycloheptatriene-like transition state may operate.An attempt to generate 1,3-cyclohexadienylmethylene is described.
- Miller, Paula C.,Gaspar, Peter P.
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p. 5101 - 5107
(2007/10/02)
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- Regioselective Hydrogenation of Conjugated Dienes Catalyzed by Hydridopentacyanocobaltate Anion using β-Cyclodextrin as the Phase-Transfer Agent and Lanthanide Halides as Promotors
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β-Cyclodextrin is a useful phase-transfer agent for the hydrogenation of conjugated dienes to monoolefins catalyzed by the in situ generated hydridopentacyanocobaltate anion.This reaction, which usually proceeds by 1,2-addition to the diene, is promoted by cerium or lanthanum chloride.Polyethyleneglycol (PEG-400), with or without added lanthanide, can also used as the phase-transfer agent for the reduction process.
- Lee, Jong-Tae,Alper, Howard
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p. 1854 - 1856
(2007/10/02)
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- LES (TRIMETHYLSILYL)BICYCLO-ALCENES, NOUVEAUX SILYLCYCLOPROPANES BICYCLIQUES FONCTIONNELS
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A convenient synthesis of new silylbicyclo-alkenes is reported involving a facile and rapid process and giving both endo and exo monosilyl isomers as well as gem-disilyl derivatives.The behaviour of these species towards acids has been investigated.Both the size of the large ring and the double-bond position play a role in the regiochemistry of the electrophilic attack.These results bring to ligth new perspectives for functionalization of cyclololefins via cyclopropanation.
- Grignon-Dubois, M.,Dunogues, J.,Ahra, M.
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p. 216 - 225
(2007/10/02)
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- Cyclobutene Photochemistry. Nonstereospecific Photochemical Ring Opening of Simple Cyclobutenes
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The photochemistry of bicyclohept-6-ene, bicyclooct-7-ene, and cis- and trans-3,4-dimetylcyclobutene has been investigated in hydrocarbon solution with monochromatic far-ultraviolet (185 and 193 nm) light sources.All of these simple cyclobutene derivatives undergo ring opening to yield the isomeric 1,3-dienes, and the latter three open nonstereospecifically to yield mixtures of the possible geometric isomers.The isomeric 3,4-dimethylcyclobutenes yield different mixtures of the three 2,4-hexadiene isomers, and in each case the mixtures are weighed in favor of the orbital symmetry forbidden isomer(s).Attempts have been made to analyze the relative isomeric diene yields from ring opening of bicyclooctene and the isomeric 3,4-dimethylcyclobutenes within the context of the purely disrotatory, adiabatic ring-opening mechanism that recent ab initio calculations suggest should be possible.While the results for the former compound are consistant with this mechanism, analysis of the relative yields of the isomeric 2,4-hexadienes from photolysis of the latter two compounds indicates that photochemical ring opening by the formally forbidden, conrotatory pathway may compete to some extent with disrotatory ring opening.
- Clark, K. Brady,Leigh, William J.
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p. 6086 - 6092
(2007/10/02)
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- DIRECT PHOTOLYSIS AT 185 nm OF SIMPLE CYCLOBUTENES. MOLECULAR ELIMINATION OF ACETYLENE
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Direct photolyses at 185 nm of bicyclohept-6-ene and bicyclooct-7-ene in pentane afforded acetylene and the fragment cycloalkenes as the major photoproducts, whereas the Woodward-Hoffmann allowed ring-opening giving rise to 1,3-diene was a minor process.
- Inoue, Yoshihisa,Sakae, Mitsuhiro,Hakushi, Tadao
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p. 1495 - 1498
(2007/10/02)
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