浅谈Hofmann重排反应在有机合成中的应用

讨论了Hofmann重排反应的机理以及烃基的结构、介质中的亲核试剂对反应的影响,并介绍了此重排在有机合成中的应用。     

聚丙烯酰胺分子量对其Hofmann反应的影响

协同效应;高分子效应;聚丙烯酰胺分子量对其Hofmann反应的影响     

Alkylation/elimination from azetidinium ylides: an entry to functionalized acrylonitriles

Azetidinium triflates were reacted in a one-pot two-steps sequence involving, generation of an azetidinium ylide, its alkylation with an halide, and final regioselective Hofmann elimination of the produced alkylated azetidinium ion to yield substituted α,β-unsaturated nitriles bearing an aminoethyl side-chain. The scope of this sequence was examined, and was found to depend both on the steric hindrance around the reactive center in the starting azetidinium salt, and on the nature of the reacting halide. Produced acrylonitriles were further used in DBU-catalyzed conjugate addition of amines, to yield 4-amino-2-aminomethyl-butyronitriles with fair diastereoselectivity, or, alternatively, to give C₂ symmetrical cyclopropanes.     

The synthesis of chiral functionalised morpholine N-oxides using a tandem Cope elimination/reverse-Cope elimination protocol

Hydroxylamine derivatives of (S)-prolinol have been generated using a Cope elimination. These undergo reverse-Cope elimination onto a pendant double bond to give morpholine N-oxides containing three contiguous chiral centres.     

Unusual properties of foams at low pressures

In addition to a high coarsening rate, foams at low pressures may show special properties. These include a bulk modulus going to zero, a Poisson's ratio approaching zero, and a zero binding energy, implying foam separation into individual bubbles at a critical pressure. We provide examples of calculations made for particular two-dimensional clusters with an ideal gas, which illustrate these features. We also discuss the difficulties in performing experiments at the low pressures at which the unusual properties show up.     

Preparation of poly(vinyl chloride) at low temperature by a photochemical method

A convenient method has been developed for the polymerization of vinyl chloride at low temperatures (to at least ?75°C) by using a tungsten–iodine rather than an ultraviolet lamp, and uranyl nitrate as sensitizer. The use of predominantly visible light minimizes degradation reactions sometimes encountered with ultraviolet light. Measurements of the fraction of racemic (or syndiotactic) diads and per cent crystallinity confirm that the products resemble polymers prepared by other techniques, such as polymerization initiated by a boron alkyl. It is concluded, therefore, that the method can provide a useful alternate to the more common, organometallic system. Measurements of torsional modulus as a function of temperature were also made. As the temperature of polymerization is lowered, the fraction of racemic diads and the per cent crystallinity are increased. The higher the per cent crystallinity, the higher the glass temperature, the broader the glass transition, and the higher the modulus in the rubbery state. Thus, the increased stereoregularity permits the development of a higher level of crystallinity which, in turn, restricts the mobility of the polymer chains.     

Stereoselective 1,3-dipolar cycloadditions of nitrones derived from amino acids. Asymmetric synthesis of N-(alkoxycarbonylmethyl)-3-hydroxypyrrolidin-2-ones

Diastereoselective asymmetric 1,3-dipolar cycloadditions of N-(alkoxycarbonylmethyl) nitrones derived from glycine, alanine and phenylalanine have been studied both experimentally and theoretically. Asymmetric induction is evaluated by either introducing a chiral group at the nitrone nitrogen atom or by using Oppolzer's sultam acrylamide. In both cases the sense of the asymmetric induction is the same, the (3R,5R)-isomer being preferentially obtained. The best results were observed with the chiral dipolarophile, which afforded an only isomer in all cases. The obtained isoxazolidines are easily transformed into the corresponding 5-substituted-3-hydroxypyrrolidin-2-ones. DFT studies are in a qualitative agreement with the observed experimental results.     

3-亚甲基六氢苯并呋喃-2(3H)-酮的简便合成

以氧化环己烯为原料,在正丁基锂作用下与乙腈经开环反应制得2-羟基环己基乙腈(2);2依次经水解、内酯化、二甲胺甲基化、甲基化和Hofmann降解反应合成了α-亚甲基-γ-丁内酯类化合物--3-亚甲基六氢苯并呋喃-2(3H)-酮,总收率30.4%,其结构经¹H NMR,¹³C NMR和HR-MS(EI)确证。     

An efficient modification of the Hofmann rearrangement: synthesis of methyl carbamates

A series of methyl carbamates was synthesized using NaOCl as an oxidant in the presence of KF/Al₂O₃/MeOH at reflux in excellent yields.     

A new convenient synthesis of phenanthrene alkaloids from 1-arylmethyl-1,2,3,4-tetrahydroisoquinolines

Hofmann degradation of 1-arylmethyl-1,2,3,4-tetrahydro-2,2-dimethylisoquinolinium iodides with methanolic KOH gave stilbene derivatives, for example, (E)-N-2{2-[2-(phenyl)ethenyl]-4,5-dimethoxyphenyl}ethyl-N,N-dimethylamine. Photochemical electrocyclization of these stilbenes afforded the corresponding phenanthrene alkaloids/phenanthrene derivatives.     

Computational study of the structures of gaseous helium-3 at low temperature

A computational study of gaseous helium-3 at T=5.23 K, for number densities rho N<0.0021 A(-3), analyzing the different pair and triplet structures in both the r and the k spaces, is presented. Structures in r space (i.e., instantaneous, total continuous linear response, and centroids) are determined via path-integral Monte Carlo simulations in the canonical ensemble by utilizing the Aziz-Slaman and the SAPT2 interatomic potentials. Additional results obtained with the application of two closures for triplets in r space, the Kirkwood superposition approximation and the Jackson-Feenberg convolution, are also reported. Besides, an analysis of the nonsuitability of quantum hard spheres for describing this system is included. The pair structures in k space are fixed via Ornstein-Zernike schemes appropriate for dealing with quantum diffraction effects in fluids. The effect on the quality of the computed isothermal compressibilities brought about by increasing the sample size in the simulations and by the subsequent application of a grand ensemble correction to the asymptotic behavior of the canonical pair radial correlation functions is also investigated. Furthermore, it is demonstrated analytically that the methods of classical statistical mechanics for dealing with the higher-order direct correlation functions remain fully valid for studying the higher-order correlations of path-integral centroids. By taking advantage of this result, the triplet structure factors for the centroid (also for the instantaneous) correlations are computed by following a number of distinct closures and methods that involve triplet direct correlation functions. The latter computations are intended to explore an alternative scheme to the very expensive fixing of triplet structure factors through direct path integral simulations, an alternative which is expected to yield the main features of these triplet quantities for this gas. Comparison with experiment is made wherever possible, and the results presented allow one to explain the substantial structural features existing in gaseous helium-3.     

Competitive unimolecular reactions at low pressures. The pyrolysis of cyclobutyl chloride

The thermal decomposition of cyclobutyl chloride has been investigated over the temperature range of 892–1150 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via two competitive unimolecular channels, one to yield ethylene and vinyl chloride and the other to yield 1,3-butadiene and hydrogen chloride, with the latter being the major reaction under the experimental conditions. With the usual assumption that gas-wall collisions are «strong,» RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log k₁(sec^?1) = (14.8 ± 0.3) ? (61.1 ± 1.0)/Θ for vinyl chloride formation and log k₂(sec^?1) = (13.6 ± 0.3) ? (55.7 ± 1.0)/Θ for 1,3-butadiene formation, where Θ = 2.303 RT kcal/mol. The A factors were assigned from previous high-pressure low-temperature data of other workers assuming a four-center transition state for 1,2-HCl elimination and a chlorine-bridged biradical transition state for vinyl chloride formation. The activation energies are in good agreement with the high-pressure results which were obtained with a conventional static system. The difference in critical energies is 4.6 kcal/mol.     

Reaction of chromous chloride with 3-nitroflavenes. A novel synthesis of flavonols

Reaction of Chromium(II) chloride with 3-nitroflavene yields flavonol.     

Development of a new linker for the solid-phase synthesis of N-hydroxylated and N-methylated secondary amines

Merrifield resin was modified by the introduction of an ortho-nitrophenylethanal group that served as a linker moiety to attach amines to the resin by reductive amination. Resin-bound tertiary amines were shown to be readily transferred into the respective liberated N-hydroxylated or N-methylated derivatives by either an oxidation/Cope elimination or a permethylation/Hofmann elimination protocol. With these two divergent liberation/derivatization options, the new resin offers new flexibility in the solid phase synthesis of N-modified secondary amines, for instance in spider toxin synthesis.     

Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part II. Low temperature thermal oxidation

This paper deals with the kinetic modelling of unstabilised polyethylene thermal oxidation, particular attention being paid to the domain of low temperatures, typically below 80 °C. Experimental data show that the temperature dependence of the induction time t_i and the steady state rate of oxygen absorption r_S display a discontinuity at 80 °C. A model based on the hypothesis that this discontinuity concerns only the PO₂ bimolecular combination processes and is essentially explained by the competition between terminating and non-terminating PO₂ + PO₂ reactions, was proposed. With pertinent values of the Arrhenius parameters of the elementary reactions under consideration, the model fits well the experimental data (in the 40-200 °C temperature range) and is consistent with previously analysed results of radiochemical ageing. According to this model, 35-40% of the bimolecular PO₂ combinations would not be terminating at 45 °C and this proportion would increase with the temperature. Concerning terminations, the relative fraction of coupling processes, leading to peroxide bridges, would decrease relatively to the disproportionation processes when the temperature increases.