Biochemie und Chemie
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A simple and sustainable one-step strategy for the preparation of electron-deficient aryl trifluoromethyl ethers (ArOCF3) from the corresponding phenols by electrochemical synthesis is presented. Anodic oxidation of trifluoromethane sulfinate (Langlois reagent) leads to direct O-trifluoromethylation of phenol-derivatives bearing fluorine, chlorine, bromine and nitrile substituents under mild conditions in yields up to 75% and in gram-scale. This electrochemical protocol provides an economic and green synthesis for an otherwise inaccessible class of molecules without the need for expensive or toxic reagents, oxidants or metal catalysts.
A sustainable strategy for O-trifluoromethylation of electron-deficient phenols by combining electrochemical synthesis with flow technology is presented. The reaction is optimized by screening experiments to establish a fast and efficient flow protocol. Simultaneous anodic oxidation of Langlois reagent and the phenols in a micro flow cell leads to direct preparation of trifluoromethyl ethers in yields up to 90%. This one-step protocol is tolerant of several functional groups, shows good regioselectivity and works without any chemical oxidants and catalysts by using electrical current as an inexpensive and sustainable reagent.
A B-factor for NOEs?
(2022)
Nuclear Overhauser effects (NOEs) are influenced by motion. Here, we derive exact, analytical results for a model of isotropic, harmonic fluctuations of atom positions that corresponds to the one underlying crystallographic B-factors. The model includes steric repulsion and yields closed-form expressions for the expected value of general invertible functions of the distance between two atoms, with the special case r-6 for NOEs. We discuss the implications for the definition of an NOE-based B-factor in solution NMR.
The photoelectron spectrum of H5C6-Te-CH3 displays in its low energy region overlapping bands of gas-phase conformers. Depending on the dihedral angle between the plane of the phenyl ring and the tellurium lone pair, the π conjugation amounts to only 0.1 eV and 0.3 eV, respectively. These values are compared to the considerably larger ones found for the analogous phenyl derivatives H5C6-X-CH3 with X = O, S and Se.
The He I photoelectron spectra of the series RnP(CN)3-n (R = Me, CF3; n = 0, 1, 2, 3) and of Me2ECN (E = N, P, As) are interpreted. The PE assignments are based on the comparison with the PE data of analogous halogeno and hydrogen derivatives as well as related cyano compounds and on simple MO considerations (composite molecule-approach). Hyperconjugative and inductive effects of the substituents CN, Me, F, Cl, and especially CF3 are assessed. The various effects of changing substituents or central atoms within the series are used to confirm the interpretation. The differing electronic structures of halogeno and cyano (pseudohalogeno) compounds are discussed on the basis of their PE spectra.
The well-resolved helium(I) photoelectron spectrum of H3C-Se—Se-CH3 exhibits distinct bands corresponding to 11 of the total 13 valence electron ionizations. The unequivocal assignment is supported by EHMO calculations including spin/orbit coupling. The two selenium lone pair ionizations differ by 0.23 eV; a split observed also for dimethyl disulfide and discussed within a general model for interactions between adjacent lone pairs.
CH3P(S)(NCO)2 reacts with [(CH3)3Si]2N-CH3, [(CH3)3SiNCH3]2CO and [(CH3)3Sn]3N to give the cyclic compounds 2a-2c. The structures are discussed on the basis of NMR and IR data. In 2 a and 2 b the (CH3)3Si-groups are easily and quantitatively replaced by protons with water under formation of (CH3)3Si-O-Si(CH3)3. By the reaction of CH3P(S)(NCO)2 with [(CH3)3Si]2S 4 is obtained, a cyclic compound with a sulfur atom of coordination number 2.
FP(S)(NCS)2 was used to investigate the scope of these reactions. With [(CH3)3Si]2NCH3 and FP(S)(NCS)2 5 is obtained, which reacts with S2Cl2 to yield 6, a bridged disulfur compound. This method may be useful for the systematic investigation of new cyclic compounds.
The rates of formation of three mono- and three dicarbenium ions of the triphenylmethyl type from the corresponding carbinols in trifluoro acetic acid - toluene mixtures were investigated using stopped flow techniques.
The reaction mechanisms are discussed in view of the obtained dependencies of the rate constants on acid concentration and temperature.