Effect of para-substituents and solvent polarity on the formation of triphenylboroxine.amine adducts

Density functional theory (B3LYP//6-311+G) calculations including Poisson-Boltzmann implicit solvent and NMR were used to study the formation of a series of para-substituted triphenylboroxine.amine adducts with respect to their phenylboronic acid monomers and free amine in solution. Our calculations suggest that the intermediate prior to forming trimer.amine is a dimer.amine adduct. Formation of dimer.amine can proceed via two pathways. Electron-donating substituents favor dimerization of two monomers before addition of the amine, and electron-withdrawing substituents favor formation of a monomer.amine adduct before addition of the second monomer. We also find that pi-electron acceptors destabilize formation of the dimer and trimer with respect to its monomers. Electron-withdrawing substituents favor adduct formation. Adduct formation is enthalpically stabilized by increasing the polarity of the solvent but differential solubility of the monomer compared to trimer.amine also has an effect on the equilibrium constant.     

Formation of para-substituted triphenylboroxines: a computational study

Density functional theory (B3LYP//6-311+G) calculations including Poisson-Boltzmann (PB) implicit solvent were applied to study the relative stability of triphenylboroxine (PhBO)(3) with respect to its phenylboronic acid monomers. In solution, formation of (PhBO)(3) is thermodynamically unfavorable; however, addition of an amine base results in the formation of stable 1:1 adducts of (PhBO)(3) and amine. Formation of 1:2 adducts is energetically unfavorable. We find that adduct formation is more exothermic than cleavage of the boroxine ring back to its monomers. pi-Electron-withdrawing groups in the para-position of the phenyl ring destabilize the boroxine ring with respect to its monomers. However, para-substituents that are net electron-withdrawing are found to stabilize formation of the 1:1 adduct.     

伯胺N1923从乙酸体系中萃取Hg(O)的研究

伯胺N₁₉₂₃在盐酸体系中对Hg(Ⅱ)具有很好的萃取性能^[1]。本文报导伯胺N₁₉₂₃从乙酸体系中萃取Hg(Ⅱ)的机理。     

Mechanistic evidence for an alpha-oxoketene pathway in the formation of beta-ketoamides/esters via Meldrum's acid adducts

A practical, one-pot process for the preparation of beta-keto amides via a three-component reaction, including Meldrum's acid, an amine, and a carboxylic acid, has been developed. Key to development of an efficient, high-yielding process was an in-depth understanding of the mechanism of the multistep process. Kinetic studies were carried out via online IR monitoring and subsequent principal component analysis which provided a means of profiling the concentration of both the anionic and free acid forms of the Meldrum's adduct 6 in real time. These studies, both in the presence and absence of nucleophiles, strongly suggest that formation of beta-keto amides from acyl Meldrum's acids occurs via alpha-oxoketene species 2 and rule out other possible reaction pathways proposed in the literature, such as via protonated alpha-oxoketene intermediates 3 or nucleophilic addition-elimination pathways.     

A highly selective rearrangement of a housane-derived cation radical: an electrochemically mediated transformation

To utilize housane-derived cation radicals as intermediates for the synthesis of the bicyclo (n.3.0) framework of natural products, a highly regioselective [1,2] shift of carbon to either a radical or an electron-deficient site is required. Herein we describe how this has been accomplished, provide a set of guidelines to assess housane oxidizability prior to its synthesis, and describe a synthesis of housane 18 that capitalizes upon the facility of [1,5] hydrogen shifts in substituted cyclopentadienes. The catalytic electrochemically mediated oxidation of 18 leads to a cation radical that engages in a rearrangement leading to the (4.3.0) adduct 23. The appearance of a catalytic current in the cyclic voltammogram of a solution containing the tris(aryl)amine and housane 18 is an excellent indicator that the amminium cation radical 14*+ is able to oxidize the housane and return the mediator to the original redox couple. DFT calculations show electron density on both the aryl and strained sigma framework in the HOMO of housane 18. From the spin density and electrostatic potential map for the cation radical, a picture where the spin resides on the side that is distal to the substituent emerges, while the hole is proximal to it. Both experiment and theory show that the rearrangement is best characterized as a [1,2] carbon shift toward an electron-deficient site and that migration toward the substituent-bearing carbon is much preferred over the alternative pathway.     

Mechanism of Oxidative Amidation of Nitroalkanes with Oxygen and Amine Nucleophiles by Using Electrophilic Iodine

Recently, we developed a direct method to oxidatively convert primary nitroalkanes into amides that entailed mixing an iodonium source with an amine, base, and oxygen. Herein, we systematically investigated the mechanism and likely intermediates of such methods. We conclude that an amine–iodonium complex first forms through N?halogen bonding. This complex reacts with aci‐nitronates to give both α‐iodo‐ and α,α‐diiodonitroalkanes, which can act as alternative sources of electrophilic iodine and also generate an extra equimolar amount of I⁺ under O₂. In particular, evidence supports α,α‐diiodonitroalkane intermediates reacting with molecular oxygen to form a peroxy adduct; alternatively, these tetrahedral intermediates rearrange anaerobically to form a cleavable nitrite ester. In either case, activated esters are proposed to form that eventually reacts with nucleophilic amines in a traditional fashion.     

Fries rearrangement of aryl formates: a mechanistic study by means of 1H, 2H, and 11B NMR spectroscopy and DFT calculations

1H, 2H, and 11B NMR spectroscopy has been used to study the mechanism of the Fries rearrangement of aryl formates promoted by boron trichloride by monitoring both the substrate and the Lewis acid. DFT calculations were employed to investigate the energetics of several reaction paths and to calculate NMR chemical shifts of key intermediates and products. After the formation of a 1:1 substrate-Lewis acid adduct, the rearrangement proceeds in two steps, beginning with the cleavage of the ester bond and the release of formyl chloride in situ, which, in turn, acts as a formylating agent, introducing an aldehydic functionality into the aromatic ring. The high regioselectivity (only the ortho product is obtained) is also accounted for by the proposed intermolecular, Lewis acid-assisted mechanism.     

Theoretical Analysis of Reactions between Phosphanylnitrenes and Boranes: The Fate of the Adducts

Ab initio molecular orbital calculations have been carried out to determine the minimum-energy pathways and thereby to probe the mechanism of reactions between phosphanylnitrenes (R(1)R(2)P&tbd1;N, R(1), R(2) = H, F) and boranes (H(2)XB, X = H, F, CH(3), and C(2)H(5)). Geometries have been determined using the MP2/6-31G(d,p) model, while relative energies have been estimated using, depending on the size of the system, the quadratic configuration interaction model (QCISD and QCISD(T)) with various basis sets including 6-31G(d,p), 6-311G(d,p), and 6-311++G(d,p). The stability of the primary complex adduct is strongly dependent on the substituents of the boranes. When the borane bears a H atom, the primary adduct is not at all stable and readily collapses to an amine isomer via a H-shift from B to N. This shift becomes more difficult if the substituent is F or CH(3). In the F case, a phosphorane isomer, owing to the strength of the P-F bond, turns out to be favored. When non-hydrogen boranes (BF(3) and B(CH(3) )(3) for example) could be used, the primary adducts could be stabilized and even exist as discrete intermediates. F substituents on the nitrene show no significant qualitative effect. In the H(2)PN + H(2)BC(2)H(5) reaction, a retro-ene reaction of the adduct directly gives rise to an amine product via a five-membered transition structure. In the reverse reaction of a HX molecule plus an iminoborane (RB&tbd1;N-PR(1)R(2) ), both 1,2-addition to B and N and 1,3-addition to B and P reactions are possible.     

Microcapsule enabled multicatalyst system

We present a new microencapsulated catalyst and report its use in a tandem multicatalyst reaction. Using an encapsulation technique, we developed an active, site-isolated amine catalyst that is capable of catalyzing the addition of nitromethane to an aldehyde. When a second Lewis acid catalyst is added, the nitroalkene intermediate is trapped and converted to the corresponding Michael adduct. We show that if the amine catalyst is not encapsulated, the two catalysts cannot function together. Moreover, if the two reactions are performed in sequence rather than in tandem, the first reaction results in an undesired dinitro product and the desired Michael adduct is not formed.     

A spectrophotometric study of the colour reaction between chloranil and aromatic amines

The colour reaction between aromatic amines and chloranil was studied spectrophotometrically. A 1 : 1 adduct formed in solution, with some dissociation. The effects of solvents, acid and alkali were examined. The adducts exhibited intense, and characteristic, broad absorption bands, which could be used for the detection and estimation of an aromatic amine, in the absence of substances of similar light absorption characteristics. A correlation between the basicities of an amine and the ight absorption of the adduct was established.     

Computational study of epoxy-amine reactions

Density functional theory calculations were carried out for the title reactions. Ethylene oxide and methylamine were adopted as reactants. Amine clusters (dimer, trimer, tetramer, and pentamer) were considered, because the combination of one oxide and one amine molecule gave a large activation energy. An amine tetramer was found to react favorably with the oxide via various zwitterionic intermediates. A back-side S(N)2 nucleophilic attack of one amine and the subsequent proton relay up to the front side provide a stabilized reaction field. The amine-alcohol mixed reactant may react readily with the oxide, because the alcoholic O-H group is in contact with the oxide oxygen with the strong hydrogen-bond stabilization.     

Organotitanium carbonyls. Reaction of tetrabenzyl-titanium with carbon monoxide

Tetrabenzyltitanium and its dicyclohexylamine adduct react with carbon monoxide to form acylbenzyltitanium compounds, which have been characterized. The presence of (PhCH₂)₄Ti(CO)₂ (amine) and (PhCH₂)₄Ti(CO) (amine) as intermediates was indicated by the IR spectra.     

A novel route for synthesizing esters and polyesters from the Diels-Alder adduct of levopimaric acid and acrylic acid

A novel route for the esterification of the Diels-Alder adduct between abietic acid, in its isomer form of levopimaric acid, and acrylic acid was established. The high purity Diels-Alder adduct was prepared starting from rosin acids. When the adduct was subjected to a condensation reaction in the presence of a cyclic carbonate ester and of an efficient amine catalyst, hydroxyalkyl esters were obtained. The corresponding linear polyesters were synthesized by the advanced polycondensation of the above intermediates at high temperature, under vacuum, and in the presence of some adequate polyesterification catalysts. In the work 1,3-dioxolan-2-one as cyclic carbonate ester, triethylamine as esterification catalyst, and toluene-4-sulfonic acid monohydrate or tetrabutyl titanate as polycondensation catalysts, were preferred for exemplifications. The polyesters were soluble in dimethylacetamide, trichloromethane, tetrahydrofuran, 1,1,2,2-tetrachloroethane, or 1,4-dioxane. The thermal and electric studies showed that the polymers were substances with good thermal stability and high dielectric properties.     

Rhodium-catalyzed cyclohydrocarbonylation approach to the syntheses of enantiopure homokainoids

Homologues of kainic acid, a naturally occurring potent glutamate receptor agonist, were designed based on a rigidified pipecolinoglutamic acid structure and can be regarded as homokainoids for their potential activities in the central nervous system. These novel homokainoids in an enantiomerically pure form were synthesized from enantiopure (R)- and (S)-Garner's aldehyde, featuring (i) the highly diastereoselective addition of alkenylcuprates to the acrylate intermediates and (ii) the Rh-catalyzed cyclohydrocarbonylation of homoallylic amine intermediates to construct the functionalized piperidine moiety in the key steps. For the introduction of a substituent at the 4- or 5-position of pipecolinoglutamic acid, a few different strategies were used, which successfully led to the formation of enantiopure homokainoids.     

Quantum mechanical studies of isomeric and conformeric structures of methyl‐chloro‐peroxide

Ab initio quantum mechanical studies are carried out for the isomeric structures and the torsional potential of methyl‐chloro‐peroxide. These species are important intermediates in the atmospheric reactions of methyl, methoxy, and methylperoxy radicals with chlorine dioxide, chlorine monoxide, and atomic chlorine, respectively. The calculations indicate that the peroxide form, CH₃OOCl, with a skew geometry for C, O, O, and Cl atoms, is the lowest minimum energy structure followed by CH₃OClO. The CH₃ClO₂ adduct is found to be much higher in energy. The calculated isomerization barriers are found to be relatively high to permit possible interconversion pathways. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Total synthesis of (-)-dendrobine

Cascading to alkaloids: An 18-step total synthesis of (-)-dendrobine is based on a reaction cascade with a key amine group (see scheme, Bn=benzyl). The amine is the initiator of the cascade and provides an efficient method for installing the stereocenters at C11 and C3. The overall transformation occurs stereoselectively only when the conversion is carried out without the isolation of intermediates.     

Acceleration effect of five‐membered cyclic dithiocarbonate on an epoxy–amine curing system

A bifunctional cyclic five‐membered dithiocarbonate (DTC), having a bisphenol A structure, was found to be an effective accelerator for a epoxy–amine curing system comprised of bisphenol A diglycidyl ether and amine‐terminated polypropylene glycol. The acceleration effect was evaluated by monitoring the time‐dependence of the storage modulus of the reaction mixture with a dynamic mechanical analyzer. The reactions involved in the curing system were investigated in detail by performing a series of model reactions using the corresponding monofunctional monomers. This investigation revealed that (1) DTC reacted with amine rapidly, (2) the reaction afforded the corresponding adduct having a thiourethane and thiol moieties, and (3) the thiol reacted rapidly with epoxide. The thiourethane moiety incorporated into the resulting adduct effectively catalyzed the reaction of epoxide and amine, and this catalysis was the predominant mechanism for the acceleration effect arisen by the addition of DTC. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4606–4611, 2007     

One-pot synthesis of oxoisoindoline-1,2,3-triazole hybrid by a Ugi–click reaction

1,2,3-Triazole-3-oxoisoindoline-1-carboxamide system was successfully synthesized by using a combination of Ugi and click reactions. This two-step, one-pot synthesis was started by the reaction of 2-formyl benzoic acid, propargyl amine, and cyclohexyl isocyanide in ethanol. The resultant Ugi adduct underwent a copper-catalyzed click reaction, producing the desired products in good yields.     

Kinetics of the reaction between nitroxide and thiyl radicals: nitroxides as antioxidants in the presence of thiols

Cyclic nitroxides effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The present study focuses on the kinetics and mechanisms of the reactions of piperidine and pyrrolidine nitroxides with thiyl radicals, which are involved in free radical "repair" equilibria, but being strong oxidants can also produce cell damage. Thiyl radicals derived from glutathione, cysteine, and penicillamine were generated in water by pulse radiolysis, and the rate constants of their reactions with 2,2,6,6-tetramethylpiperidine-1-oxyl (TPO), 4-OH-TPO, and 3-carbamoyl-proxyl were determined to be (5-7) x 10 (8) M (-1) s (-1) at pH 5-7, independent of the structure of the nitroxide and the thiyl radical. It is suggested that the reaction of nitroxide (>NO (*)) with thiyl radical (RS (*)) yields an unstable adduct (>NOSR). The deprotonated form of this adduct decomposes via heterolysis of the N-O bond, yielding the respective amine (>NH) and sulfinic acid (RS(O)OH). The protonated form of the adduct decomposes via homolysis of the N-O bond, forming the aminium radical (>NH (*+)) and sulfinyl radical (RSO (*)), which by subsequent reactions involving thiol and nitroxide produce the respective amine and sulfonic acid (RS(O) 2OH). Nitroxides that are oxidized to the respective oxoammonium cations (>N (+)O) are recovered in the presence of NADH but not in the presence of thiols. This suggests that the reaction of >N (+)O with thiols yields the respective amine. Two alternative mechanisms are suggested, where >N (+)O reacts with thiolate (RS (-)) directly generating the adduct >NOSR or indirectly forming >NO (*) and RS (*), which subsequently together yield the adduct >NOSR. Under physiological conditions the adduct is mainly deprotonated, and therefore nitroxides can detoxify thiyl radicals. The proposed mechanism can account for the protective effect of nitroxides against reactive oxygen- and nitrogen-derived species in the presence of thiols.     

Mechanism and application of a microcapsule enabled multicatalyst reaction

In this paper, we describe the development and application of a multistep one-pot reaction that is made possible by the site isolation of two otherwise incompatible catalysts. We prepared a microencapsulated amine catalyst by interfacial polymerization and used it in conjunction with a nickel-based catalyst for the transformation of an aldehyde to a Michael adduct via a nitroalkene intermediate. The amine-catalyzed conversion of an aldehyde to a nitroalkene was found to proceed through an imine rather than a nitroalcohol. Kinetic studies indicated that the reaction is first order in both the nickel catalyst and the shell of the encapsulated amine catalyst. Furthermore, we provide evidence against interaction between amine and nickel catalysts and present kinetic data that demonstrates that there is a rate enhancement of the Michael addition due to the urea groups on the surface of the microencapsulated catalyst. We applied our one-pot reaction to the development of a new synthetic route for pregabalin that proceeds with an overall yield of 74%.