RESUMO
We here suggest and evaluate a methodology for the measurement of specific interatomic distances from a combination of theoretical calculations and experimentally measured (13)C kinetic isotope effects. This process takes advantage of a broad diversity of transition structures available for the epoxidation of 2-methyl-2-butene with oxaziridines. From the isotope effects calculated for these transition structures, a theory-independent relationship between the C-O bond distances of the newly forming bonds and the isotope effects is established. Within the precision of the measurement, this relationship in combination with the experimental isotope effects provides a highly accurate picture of the C-O bonds forming at the transition state. The diversity of transition structures also allows an evaluation of the Schramm process for defining transition-state geometries on the basis of calculations at nonstationary points, and the methodology is found to be reasonably accurate.
Assuntos
Alcenos/química , Aziridinas/química , Compostos de Epóxi/síntese química , Isótopos de Carbono , Compostos de Epóxi/química , Modelos Moleculares , TermodinâmicaRESUMO
We constructed a novel molecular conformational alteration system with an N-aryl-N-phenylacetamide structure, in which the N-aryl group consists of a hydroquinone-p-quinone system as a redox-dependent aromatic trigger. The amide conformation depended on the oxidation state of the aryl group, and the two states (compounds 2 and 3) were reversibly converted to each other by redox reactions. Such compounds would be applied as useful structural units for external stimulus-responsive control on the shape and function of large molecules or supramolecules.
Assuntos
Amidas/química , Modelos Moleculares , Conformação Molecular , OxirreduçãoRESUMO
The mechanism of the epoxidation of 2-cyclohexen-1-one with tert-butyl hydroperoxide mediated by DBU was studied by a combination of experimental kinetic isotope effects (KIEs) and theoretical calculations. A large 12C/13C (k(12C)/k(13C)) isotope effect of approximately equal to 1.032 was observed at the C3 (beta) position of cyclohexenone, while a much smaller 12C/13C isotope effect of 1.010 was observed at the C2 (alpha) position. Qualitatively, these results are indicative of nucleophilic addition to the enone being the rate-limiting step. Theoretical calculations support this interpretation. Transition structures for the addition step lead to predicted isotope effects that approximate the experimental values, while the predicted isotope effects for the ring-closure step are not consistent with the experimental values. The calculations correctly favor a rate-limiting addition step but suggest that the barriers for the addition and ring-closure steps are crudely similar in energy. The stereochemistry of these epoxidations is predicted to be governed by a preference for an initial axial addition, and the role of this preference in experimental diastereoselectivity observations is discussed.
Assuntos
Cicloexanonas/química , terc-Butil Hidroperóxido/química , Carbono/química , Química Orgânica/métodos , Cinética , Espectroscopia de Ressonância Magnética , Modelos Químicos , Modelos Moleculares , Conformação Molecular , Estereoisomerismo , TemperaturaRESUMO
A novel oxidative cyclization of quinone-arenols 5 leading to products 6 with a dibenzofuran-1,4-dione structure, which forms the core of several natural products, has been developed and applied to the synthesis of violet-quinone (4).
Assuntos
Benzofuranos/química , Quinonas/química , Ciclização , Estrutura Molecular , OxirreduçãoRESUMO
We investigated the reactions of various 1-naphthols (NPOHs; 1) with p-benzoquinones (Qs), such as 1,4-benzoquinone (BQ) and p-chloranil (CA), as pi-electron acceptors. With electron-rich NPOHs 1a-c, oxidative biaryl coupling and subsequent dehydrogenation reaction took place selectively to give the corresponding 2,2'-binapthyl-1,1'-quinones 3a-c in excellent yield. In the case of electron-deficient NPOHs 1e, f, two different types of reactions occurred in the presence of SnCl4 and ZrO2 under similar conditions: SnCl4 mediated oxidative dimerization and trimerization of NPOH, while ZrO2 promoted electrophilic arylation of Qs with NPOH. The resulting products 3 would be useful synthetic intermediates for naturally occurring diosindigo B, biramentaceone and violet-quinone.
Assuntos
Benzoquinonas/química , Naftóis/química , Compostos Policíclicos/síntese química , Eletroquímica , Transporte de Elétrons , Indicadores e Reagentes , Espectroscopia de Ressonância Magnética , Oxirredução , Espectrofotometria Infravermelho , Espectrofotometria Ultravioleta , Compostos de Estanho , ZircônioRESUMO
The combination of electrolysis and the Fe(III)(PA)3/O2/CH3CN system was investigated for allylic hydroxylation of octahydronaphthalene derivatives. Substrates with a bridgehead double bond gave the allylic alcohol with alpha-preference, while non-bridgehead olefin did not react smoothly. This system is a useful tool for alpha-selective allylic hydroxylation of octahydronaphthalene derivatives with a bridgehead double bond as model compounds for the AB fused ring of cholesterols.
Assuntos
Naftalenos/química , Ácidos Picolínicos/química , Colesterol/química , Eletroquímica , Hidroxilação , Espectroscopia de Ressonância Magnética , Conformação Molecular , Oxirredução , Espectrofotometria Infravermelho , Espectroscopia de Infravermelho com Transformada de Fourier , EstereoisomerismoRESUMO
The oxygenation reaction of cholesteryl acetate 1 was examined with the Fe(III)(PA)(3)/O(2)/MeCN system using an electrochemical method. The constant potential technique gave mainly the 7-hydroxylated product stereoselectively, along with the 7-oxo product. This oxygenation system is mechanistically unique, requiring iron catalyst, dioxygen, and both cathode and anode.