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1.
Modern valence bond theory, in its spin coupled form, is used to elucidate the electronic rearrangements that take place during the course of the gas-phase hetero-Diels-Alder cycloaddition reaction of s-cis-acrolein (cis-1-oxabutadiene or cis-propeneal) and ethene. It is found that the most dramatic changes to the electronic structure occur in a relatively narrow interval of the reaction pathway soon after the transition state and that the system passes through a geometry at which it can be considered to be significantly aromatic. Although concerted, the reaction is markedly asynchronous, with the breaking of the carbon-oxygen pi bond, and the formation of the new carbon-oxygen sigma bond, "lagging behind" somewhat the other bond-making and bond-breaking processes. 相似文献
2.
The phenoxonium cation of a vitamin E model compound has been crystallized using the non-nucleophilic carborane and tetrakis(pentafluorophenyl)borate counteranions. The crystal structures confirm the assignment of the unusually stable phenoxonium cation and indicate that there is a substantial shortening of the carbon-oxygen bond lengths of the para-carbon atoms in the phenolic ring and a substantial increase of the carbon-oxygen bond length at the quaternary carbon. The crystallographic data are in excellent agreement with structural predictions from molecular orbital calculations. 相似文献
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[reaction: see text] Theoretical calculations of carbon-oxygen bond dissociation enthalpies in substituted methylperoxyl radicals (YCH(2)OO*) reveal that bond strengths are not governed solely by the stability of YCH2* radicals but are largely affected by hyperconjugation when Y is electron-donating or conjugating. In many cases, this hyperconjugative effect is greater than stabilization of the methyl radical by Y. All electron-withdrawing Y exert small destabilizing effects via inductive withdrawal of electrons from the polarized C-OO* bond. 相似文献
7.
Direct transition-metal-free intramolecular C-O bond formation: synthesis of benzoxazole derivatives
A direct base-mediated intramolecular carbon-oxygen bond formation has been developed without a transition-metal catalyst. In the presence of 2.0 equiv of K(2)CO(3) in DMSO at 140 °C, the intramolecular cyclization of o-haloanilides affords benzoxazoles in high yields. A mechanism via an initial formation of a benzyne intermediate followed by nucleophilic addition to form the C-O bond has been proposed. 相似文献
8.
As the first synthesis of sydnone derivatives having a carbon-oxygen bond at the 4-position of sydnone ring, 4-sydnonyl tosylate were successfully prepared by anodic substitution reaction. 相似文献
9.
Stereoelectronic effects in the ring cleavage of methyl glycopyranosides using dimethylboron bromide
《Tetrahedron letters》1987,28(22):2485-2488
Treatment of methyl D-glycopyranosides with dimethylboron bromide followed by a nucleophile gives acyclic products resulting from selective cleavage of the ring carbon-oxygen bond. The results are rationalized based on stereoelectronic considerations. 相似文献
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Naphthalene catalyzed lithiation of 1,3-dimethyl-2-phenylimidazolidine led to cleavage of the benzylic carbon-nitrogen bond, with formation of an intermediate dianion. Under similar conditions, 1,3-dimethyl-2-(4-chlorophenyl)imidazolidine underwent regioselective cleavage of the aromatic carbon-chlorine bond, leading to a 4-formylphenyllithium equivalent, whilst 1,3-dimethyl-2-(4-methoxymethylphenyl)imidazolidine underwent regioselective cleavage of the benzylic carbon-oxygen bond, leading to a 4-formylbenzyllithium equivalent. 相似文献
11.
[reaction: see text]. A convenient enantioselective synthesis of trans-hydroisoquinolones is described. This synthesis capitalizes on the ready availability of enantioenriched 2-substituted cyclohexenols by exploiting the asymmetry of an allylic carbon-oxygen sigma bond to control carbon-carbon bond formation in pinacol-terminated cyclizations of N-acyliminium cations. 相似文献
12.
Gernot M. Wallner Reinhold W. Lang Werner Platzer Christian Teichert 《Macromolecular Symposia》2002,181(1):399-410
Commercially available, highly transparent polymer films for transparent insulation applications were investigated systematically as to their relevant optical properties in the solar and infrared wavelength range. The photometric characterisation in the solar range and the calculation of non-spectral, solar optical film properties using models for scattering-absorbing media have shown, that the solar extinction is dominated by scattering occuring mainly at the surface. For various amorphous and semi-crystalline films the root-mean-square surface roughness correlated well with the solar optical thickness. Regarding high infrared absorptance in the wavelength range of about 10 μm the carbon-oxygen single bond is highly effective for commercial materials with maximum service temperatures of about 100°C. For 50 μm thick films of different polymer types with carbon-oxygen single bonds in the molecular structure a good correlation between the concentration of the functional corbon-oxygen group and the nonspectral, infrared optical thickness was found. 相似文献
13.
The reaction of cyclic ketals or acetals with 2,6-dibromophenylisocyanide in the presence of a catalytic amount of GaCl(3) results in the insertion of isocyanide into the carbon-oxygen bond of cyclic ketals and acetals. [reaction: see text] 相似文献
14.
Reactions of o-(alkoxymethyl)styrene derivatives with a stoichiometric amount of zirconocene-butene complex (zirconocene equivalent, "Cp(2)Zr") brought about an insertion of the zirconocene species into a benzylic carbon-oxygen bond. The oxidative insertion of Cp(2)Zr to the benzylic carbon-oxygen bond is a result of sequential reactions: (i) formation of zirconacyclopropane by the ligand exchange with o-(alkoxymethyl)styrene, (ii) elimination of the alkoxy group through an aromatic conjugate system giving metalated o-quinodimethane species, and (iii) transfer of zirconium metal to the benzylic position. Through use of a catalytic amount of "Cp(2)Zr", however, unprecedented homo-coupling reactions (dimerization) of o-(alkoxymethyl)styrene derivatives occurred to give a tetracyclic compound. On the other hand, reactions of o-(1-alkoxyisopropyl)styrene derivatives gave rise to the analogous tetracyclic compounds regardless of the amount of "Cp(2)Zr" (stoichiometric or catalytic). Heterocoupling product between o-(1-alkoxyisopropyl)styrene and styrene congeners was obtained in high cis stereo- and regioselectivity by treating o-(1-alkoxyisopropyl)styrene derivatives with "Cp(2)Zr" in the presence of an excess amount of styrene derivatives. 相似文献
15.
Cherif Behloul 《Tetrahedron》2005,61(39):9319-9324
The naphthalene-catalysed lithiation of Alloc- and Cbz-protected alcohols, amines and thiols in THF at 0 °C led, after quenching with methanol, to the recovery of the free alcohols, amines and thiols in short reaction times and with very good yields. The selectivity for the removal of the Alloc- or the Cbz- group in a polyfunctionalised substrate has been studied. The selective reductive cleavage of a benzylic carbon-oxygen bond was achieved in the presence of an allylic carbon-oxygen or carbon-nitrogen bond. This method represents a great improvement in comparison with the previously reported deprotection procedures by dissolving metals, since it avoids the use of the toxic liquid ammonia and, therefore, the need to perform the reaction at low temperatures. 相似文献
16.
An enantioselective synthesis of allylic esters has been achieved by a novel asymmetric alkylation of allylic gem-dicarboxylates. The catalyst derived from palladium(0) and R,R-1,2-di(2'-diphenylphosphinobenzamido)cyclohexene efficiently induced the alkylation process with a variety of nucleophiles to provide allylic esters as products in good yield. High regio- and enantioselectivities were observed in the alkylation with most nucleophiles derived from malonate, whereas a modest level of ee's was obtained in the reactions with less reactive nucleophiles such as bis(phenylsulfonyl)ethane. In the latter case, a slow addition procedure proved effective, leading to significantly improved ee's. The utility of the alkylation products was demonstrated by several synthetically useful transformations including allylic isomerizations, allylic alkylations, and Claisen rearrangements. Using these reactions, the chirality of the initial allylic carbon-oxygen bond could be transferred to new carbon-oxygen, carbon-carbon, or carbon-nitrogen bonds in a predictable fashion with high stereochemical fidelity. The conversion of gem-diesters to chiral esters by the substitution reaction is the equivalent of an asymmetric carbonyl addition by stabilized nucleophiles. In conjunction with the subsequent reactions that occur with high stereospecificity, allylic gem-dicarboxylates serve as synthons for a double allylic transformation. 相似文献
17.
(E)-O-Propargylic arylaldoximes were regioselectively converted, in the presence of copper catalysts, into their corresponding four-membered cyclic nitrones in good to excellent yields. The reactions proceeded via a tandem [2,3]-rearrangement and 4π-electrocyclization of the N-allenylnitrone intermediate and involved cleavage of the carbon-oxygen bond. 相似文献
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Migration of the ethynyl group, from sulfur to oxygen in acetylenic sulfoxides and sulfones leads to several documented examples of carbon-oxygen bond forming rearrangement reactions. 相似文献
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Silyl enol ethers derived from ketone functionalised rho-allyltricarbonyliron lactone complexes undergo highly diastereoselective carbon-fluorine and carbon-oxygen bond formation reactions with excellent control at the alpha-stereogenic centre. 相似文献
20.
In this paper an understanding of the physical relationships between the material structure and the temperature dependent
infrared optical properties of different transparent polymer films for solar applications is described. The infrared optical
properties are relevant for the heat transport of e.g. greenhouse and transparent insulation structures. The properties were determined based on infrared transmittance measurements
and the assumption of a constant index of refraction from the visible range. To establish structure-property-correlations
molecular structure parameters such as the concentration of carbon-oxygen single bonds and carbon-hydroxyl groups were determined.
For 50 μm thick films a good correlation between the concentration of the functional carbon-hydroxyl and the carbon-oxygen
group and the infrared optical thickness as well as the hemispherical emittance was found. This correlation fits well for
high and low infrared radiation absorbing polymeric materials consisting of carbon, hydrogen, and oxygen atoms. The carbon-hydroxyl
group appears to be slightly more effective than the carbon-oxygen single bond. Interestingly, the correlation works for polymers
with aromatic (PC, PET) and aliphatic (PMMA, CTA, ethylene copolymers) groups. 相似文献