Thiophenol-mediated intramolecular radical cyclization: an efficient method for the synthesis of benzoxocine derivatives

A new, efficient, high yielding method for the synthesis of benzoxocine derivatives has been developed via a thiophenol-mediated intramolecular 8-endo radical cyclization. This method allowed the synthesis of the backbone of several sesquiterpenes.     

三氟乙烷与氟原子氢提取反应的动力学研究

采用双水平直接动力学方法研究了氢提取反应CH3CF3+F→CH2CF3+HF的反应机理.首先用MPW1K/6-311+G(d,p)方法优化了驻点的几何构型,并在相同水平上进行了频率分析,利用内禀反应坐标理论获得了最小的反应能量途径;随后,为了得到更准确的能量信息,采用MCG3-MPWPW91//MPW1K方法进行了单点能量校正;最后,根据变分过渡态理论计算了该反应在200² 000K温度范围内的速率常数.结果表明,理论计算值与已有的实验值吻合.     

Approaches for obtaining accurate rate constants for hydrogen abstraction by a chlorine atom

We have assessed computational methodologies for calculating the rate constants for hydrogen abstraction by Cl(?) for a selection of 12 reactions. For the conventional approach of calculating higher-level [B2K-PLYP/aug'-cc-pV[(T+d),(Q+d)]Z] single-point energies at lower-level [BH&H-LYP/6-31+G(d,p)] stationary points, large deviations from experimental rate constants are found in a number of cases in which the activation energy is very low. These discrepancies are due largely to deviations in the calculated activation energies and can be further traced to the inability of the low level to adequately locate the transition structures. We have examined several alternative approaches for calculating rate constants, namely, IRCmax, IRCmax at 0 K (ZK-IRCmax, with zero-point vibration energies (ZPVEs) incorporated), variational transition-state theory (VTST), and VTST with the inclusion of an Eckart tunneling correction (VTST+E). We find that the low level gives reasonable values for the ZPVEs and thermal enthalpy and entropy corrections that are required in such approaches. While the VTST+E approach yields the closest agreement with experimental rate constants for the systems considered, we find that the simpler IRCmax approach gives adequate values and is able to avoid the major shortcomings of the conventional approach in a cost-effective manner.     

Intermolecular hydrogen atom abstraction from the fluoranil triplet excited state: a Raman study

Intermolecular hydrogen abstraction reaction mechanisms in photoexcited ketones have traditionally been studied using time resolved absorption spectroscopy. Another approach is presented involving time resolved resonance Raman spectroscopy to study such reactions, using the fluoranil/isopropanol system as an example. It has been shown that vibrational spectra can be recorded starting from the triplet excited state to the product state (radical anion) via the intermediate state, which is the ketyl radical. Thus, it is demonstrated that following the reaction evolution in terms of structural (vibrational modes) details would prove to be useful not only for mechanistic investigation but also for structure-reactivity correlations in photoexcited systems.     

Intramolecular hydrogen atom abstraction with an eight-membered cyclic transition state in open-chain aliphatic aminium radicals

Hydrogen abstraction by the nitrogen in long-chain aliphatic gas-phase aminium radicals (amine molecular ions) takes place via six-, seven-, and eight-membered cyclic transition states. The subsequent fragmentation is particularly facile after eight-membered ring hydrogen transfer.     

Intramolecular hydrogen abstraction. Iodosobenzene diacetate,an efficient and convenient reagent for alkoxy radical generation

Photolysis of several hydroxy compounds in presence of iodosobenzene diacetate and iodine leads to alkoxy radical derivatives which undergo intramolecular hydrogen abstraction to produce cyclic ethers in good yield.     

New and efficient procedure for the preparation of unsymmetrical silaketals

[reaction: see text] A new and efficient procedure for the preparation of unsymmetrical silaketals via a three-step protocol without isolation of the intermediates is presented. The unsymmetrical silyl ethers and silanes can also be readily obtained via this sequence of reactions.     

Tunneling parameters for the hydrogen atom abstraction reactions of diphenylcarbene in a low temperature toluene matrix

Tunneling reaction rate constants of diphenylcarbene in a toluene matrix can be fit by an asymmetric Eckart barrier. The barrier heights in good agreement with theory.     

Thermal rearrangement of alkylaryldihydrosemibullvalene derivatives: hydrogen atom transfer processes

Alkylaryldihydrosemibullvalene derivatives, besides exhibiting vinylcyclopropane type rearrangements typical of tetraarylated analogues, also undergo H· atom transfer from methyl groups, most easily understood in terms of biradical intermediates resulting from alternative modes of cyclopropane ring scission.     

Reactions of the IO· radical resulting in hydrogen atom abstraction from halogen-containing molecules

A jet-stream kinetic technique and the resonance fluorescence method applied to detection of iodine atoms were used to measure the rate constants of the reactions of the IO^· radical with the halohydrocarbons CHFCl-CF₂Cl (k = (3.2 ± 0.9) × 10^?16 cm³ molecule s^?1) and CH₂ClF (k = (9.4 ± 1.3) × 10^?16 cm³ molecule s^?1), the hydrogen-containing haloethers CF₃-O-CH₃ (k = (6.4 ± 0.9) × 10^?16 cm³ molecule s^?1) and CF₃CH₂-O-CHF₂ (k = (1.2 ± 0.6) × 10^?15 cm³ molecule s^?1), and hydrogen iodide (k = (1.3 ± 0.9) × 10^?12 cm³ molecule s^?1) at 323 K.     

An efficient procedure for the preparation of 4-substituted 5-aminoimidazoles

The preparation of O-methylimidates from alpha-aminonitriles and their subsequent co-cyclization with primary amines to afford 4-substituted 5-aminoimidazoles was studied. It was found that the mildly acidic pyridinium p-toluenesulfonate efficiently catalyzed each stage of the reaction sequence: (a) the formation of the O-methylimidates, (b) their co-cyclization with a variety of primary amines, and (c) certain derivatizations of the resultant heterocycles. The developed reaction conditions tolerate a wide variety of alpha-aminonitriles and primary amine co-reactants. Thus, it is possible to easily prepare a diverse array of substituted heterocyclic compounds in good yield. The requisite alpha-aminonitriles were synthesized either from amino acids or by phase-transfer alkylation of a glycine anion equivalent. The unstable free 5-aminoimidazoles were normally protected in situ to provide derivatives (methyl imidates or N,N-dimethylamidines) that were amenable to characterization.     

The capacity of an α-nitroalkyl radical for hydrogen abstraction

Photochemical decomposition of 2-iodo-2-nitroadamantane in several hydrogen donating solvents, gives rise to formation of α-nitroalkyl radicals. Such ambident radicals can abstract hydrogen from the solvent via oxygen, resulting in a nitronic acid which decomposes exlusively into adamantanone. Alternately the abstraction can take place via carbon to give 2-nitroadamantane. The product distribution is strongly influenced by electron-withdrawing substituents in the hydrogen donor. The oxidation products derived from the solvent have been detected. All the experiments point towards a minor stabilisation of a carbon radical by a nitro group. INDO-calculations on the nitromethyl radical are in good agreement with this lack of stabilization.     

The selectivity of charged phenyl radicals in hydrogen atom abstraction reactions with isopropanol

The vertical electron affinity is demonstrated to be a key factor in controlling the selectivity of charged phenyl radicals in hydrogen atom abstraction from isopropanol in the gas phase. The measurement of the total reaction efficiencies (hydrogen and/or deuterium atom abstraction) for unlabeled and partially deuterium-labeled isopropanol, and the branching ratios of hydrogen and deuterium atom abstraction, by using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, allowed the determination of the selectivity for each site in the unlabeled isopropanol. Examination of hydrogen atom abstraction from isopropanol by eight structurally different radicals revealed that the preferred site is the CH group. The selectivity of the charged phenyl radicals correlates with the radical's vertical electron affinity and the reaction efficiency. The smaller the vertical electron affinity of a radical, the lower its reactivity, and the greater the preference for the thermodynamically favored CH group over the CH3 group or the OH group. As the vertical electron affinity increases from 4.87 to 6.28 eV, the primary kinetic isotope effects decrease from 2.9 to 1.3 for the CD group, and the mixture of primary and alpha-secondary kinetic isotopes decreases from 6.0 to 2.4 for the CD3 group.     

Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl radical in water

A combination of laser flash photolysis and competitive kinetic methods have been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a variety of organic substrates including alcohols, ethers, and carboxylic acids by the perfluoroalkyl radical, *CF(2)CF(2)OCF(2)CF(2)SO(3)(-) Na(+). Comparison, where possible, of these rate constants with those previously measured for analogous reactions in the non-polar organic solvent, 1,3-bis(trifluoromethyl)benzene (J. Am. Chem. Soc, 1999, 121, 7335) show that the alcohols react 2-5 times more rapidly in the water solvent and that the ethers react at the same rate in both solvents. A transition state for hydrogen abstraction that is more reminiscent of an "intimate ion pair" than a "solvent separated ion pair" is invoked to explain these modest solvent effects.