Reactivity of solids

A review of studies of reactivities of solids is presented, concentrating on localization and autolocalization phenomena. Some examples of feed-back and structure of the reaction zone are given. Plenary lecture     

Reactivity of Cyclanols Towards Quinaldinium Fluorochromate Oxidation

The kinetics of oxidation of cyclanols, viz., cyclohexanol, cyclopentanol, cycloheptanol and cyclooctanol by quinaldinium fluorochromate has been studied in aqueous acid medium at 313 K (±0.1 K). The cyclanols were converted to the corresponding cyclic ketones. The order of reaction was found to be one with respect to oxidant and fractional with respect to the substrate and hydrogen ion concentrations. Increase in the percentage of acetic acid increases the rate of reaction. The reaction mixture shows the absence of any free radicals in the reaction, which has ruled out the possibility of a one-electron transfer during the addition of acrylonitrile. The reaction has been studied at four different temperatures and the activation parameters were calculated. From the observed kinetic results a suitable mechanism was proposed. The relative reactivity order was found to be cyclohexanol < cyclopentanol < cycloheptanol < cyclooctanol. This was explained on the basis of I-strain theory.     

Rethinking Borole Cycloaddition Reactivity

Boroles are attracting broad interest for their myriad and diverse applications, including in synthesis, small molecule activation and functional materials. Their properties and reactivity are closely linked to the cyclic conjugated diene system, which has been shown to participate in cycloaddition reactions, such as the Diels-Alder reaction with alkynes. The reaction steps leading to boranorbornadienes, borepins and tricyclic boracyclohexenes from the thermal reaction of boroles with alkynes are seemingly well understood as judged from the literature. Herein, we question the long-established mechanistic picture of pericyclic rearrangements by demonstrating that seven-membered borepins (i. e., heptaphenylborepin and two derivatives substituted with a thienyl and chloride substituent on boron) exist in a dynamic equilibrium with the corresponding bicyclic boranorbornadienes, the direct Diels-Alder products, but are not isolable products from the reactions. Heating gradually converts the isomeric mixtures into fluorescent tricyclic boracyclohexenes, the most stable isomers in the series. Results from mechanistic DFT calculations reveal that the tricyclic compounds derive from the boranorbornadienes and not the borepins, which were previously believed to be intermediates in purely pericyclic processes.     

Reactivity of Arylhydrazones toward Molecular Oxygen

The kinetics and mechanism of reaction of arylhydrazones with molecular oxygen were studied by gas volumetry. The reaction rate was studied in relation to the structure of arylhydrazone and kind of the solvent. The inhibiting power of the compounds toward initiated oxidation of ethylbenzene was evaluated, and the most effective compounds were found, judging from the ratio of the rate constants of the reactions with molecular oxygen and with peroxy radicals arising in the course of ethylbenzene oxidation.     

Reactivity of Phosphaalkenes

Abstract

The reaction of triarylphosphaalkenes with oxidants (O₂, S₈, Se, Te, H₂O₂), with orthoquinones, and with Pt(O) - and Ni(O)-complexes is described.     

Reactivity of phthalocyanine precursors

Russian Chemical Bulletin - A higher reactivity of 4-nitrophthalonitrile as compared to phthalonitrile and 4-amino-phthalonitrile in the reaction with sodium methoxide in methanol was demonstrated...     

Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂N⁺. Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂NH^.+, some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂NH₂⁺. In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅C₆, leading to a novel iminium radical cation, Ar=N^.+Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.     

Umpolung Reactivity of Aldehydes toward Carbon Dioxide

Carbon dioxide is an intrinsically stable molecule. Therefore, its activation requires extra energy input in the form of reactive reagents and/or activated catalysts and, often, harsh reaction conditions. Reported here is a direct carboxylation reaction of aromatic aldehydes with carbon dioxide to afford α‐keto acids as added‐value products. In situ generation of a reactive cyanohydrin was the key to the successful carboxylation reaction under operationally mild reaction conditions (25–40 °C, 1 atm CO₂). The resulting α‐keto acids served as a platform for α‐amino acid synthesis by reductive amination reactions, illustrating the chemical synthesis of essential bioactive molecules from carbon dioxide.     

Reactivity and flammability of conjugated acetylenes

The thermal reactivities of model phenyleneacetylenes and diacetylenes and of poly(butadiynylene-1,3-phenylene) have been examined by differential scanning calorimetry and compared with the oxygen indices of the materials. No direct relationship was found between the heat of reaction and the oxygen index in these compounds. However, the heat of reaction, in general, does decrease with increasing molecular weight and is lower for rigid linear molecules than for related angular structures. These observations appear to be due to a lack of solid-state reactivity in the crystalline phase and retardation in the melt due to viscosity effects related to molecular size and rigidity. The synthesis of model compounds is described.     

Nitrosoalkenes: Synthesis and Reactivity

Some α- and β-halonitrosoalkenes 1 have been synthesized and characterized. The halogen atoms of the oxime precursors 2 can be substituted by alkoxy groups. Two kinds of cycloaddition reaction of 1 have been observed: (i) reaction of the NO group with dienes gives 3, 6-dihydrooxazine derivatives 6 which isomerise to epoxyepimines 7 in most cases of β-substituted nitrosoalkenes; (ii) if 4, 5-dihydrooxazines 22 are obtained, the cycloaddition of the nitrosoalkenes as 4π-component is presumed.     

How Oriented External Electric Fields Modulate Reactivity

A judiciously oriented external electric field (OEEF) can catalyze a wide range of reactions and can even induce endo/exo stereoselectivity of cycloaddition reactions. The Diels–Alder reaction between cyclopentadiene and maleic anhydride is studied by using quantitative activation strain and Kohn–Sham molecular orbital theory to pinpoint the origin of these catalytic and stereoselective effects. Our quantitative model reveals that an OEEF along the reaction axis induces an enhanced electrostatic and orbital interaction between the reactants, which in turn lowers the reaction barrier. The stronger electrostatic interaction originates from an increased electron density difference between the reactants at the reactive center, and the enhanced orbital interaction arises from the promoted normal electron demand donor–acceptor interaction driven by the OEEF. An OEEF perpendicular to the plane of the reaction axis solely stabilizes the exo pathway of this reaction, whereas the endo pathway remains unaltered and efficiently steers the endo/exo stereoselectivity. The influence of the OEEF on the inverse electron demand Diels–Alder reaction is also investigated; unexpectedly, it inhibits the reaction, as the electric field now suppresses the critical inverse electron demand donor–acceptor interaction.     

Intramolecular Reactivity of Triplet Carbonylcarbenes

Through a double cyclization triplet carbonylcarbenes add smoothly to double bonds in the δ and ε position, but do not react intramolecularly with C–H bonds (see scheme). The addition reaction fails if the keto group is replaced with an ester group.     

Reactivity of Polysaccharide Aldehydes toward N-Nucleophiles

The conversion of aldehyde groups in polysaccharide aldehydes to azomethine groups in reactions with amines and hydrazine derivatives was studied in relation to the structure of polysaccharide aldehyde and reaction conditions.     

Reactivity of dehydroacetic acid in organic synthesis

This review reports the reactivity of dehydroacetic acid, which considered to be a very reactive organic compound. The structure of the title compound was investigated. The condensation reactions with primary amines led to the preparation of a variety of heterocyclic ring systems, including bispyran, pyrazole, thiazole, pyridine, pyrimidine, thiazine, thiazepine, and diazepine ring systems. On the other hand, the condensation reaction with aldehydes led to the formation of 1,5-benzodiazepines, pyran, and pyranopyran heterocyclic ring systems. The reaction mechanisms were discussed. The bibliography includes 130 references.     

The Diverse Reactivity of Disilenes Toward Isocyanides

The addition of 2,6‐dimethylphenyl isocyanide and t‐butyl isocyanide to tetramesityldisilene was examined. In both cases, the initially formed product is an iminodisilirane; however, the iminodisiliranes are unstable under the reaction conditions and react with a second equivalent of the isocyanide to give either a 3‐silaazetidine or a novel bicyclic double enamine, respectively. Taken together with the previous examples in the literature, the results demonstrate that subtle differences in the steric bulk of the disilene or the electronic effects of the isocyanide can lead to dramatic differences in the reaction pathway.     

Reactivity of acid fluoride-terminated self-assembled monolayers on gold

This report describes the reactivity of acid fluoride (AF)-terminated self-assembled monolayers (SAMs) on gold toward amine and alcohol compounds and the potentiality of AF as a reactive intermediate for surface functionalizations. The AF group was generated in situ on a gold surface by reacting the terminal carboxylic acid group in the SAM of 16-mercaptohexadecanoic acid with cyanuric fluoride and pyridine under the optimized conditions. AF was found to be highly reactive toward various amine groups, such as primary and secondary amines, but it did not react effectively with alcohol. In addition, the amide coupling reaction by microcontact printing (microCP) was compared with the solution-based reaction: when amine-derivatized ferrocene compound was used for 1-min microCP on the AF-activated surface, the surface coverage of the reaction product was about 83% of 3.45 x 1014 cm-2, the coverage obtained in the solution-based reaction. On the basis of the high reaction efficiency of microCP, the AF-activated surface was also used as a platform for patterning a biological ligand, biotin.     

Reactivity of bromoalkanes in reactions of coordinated molecular decay

The results from experiments on reactions of the coordinated molecular decay of RBr bromoalkanes on olefin and HBr are analyzed using the model of intersecting parabolas (MIP). Kinetic parameters within the MIP are calculated from the experimental data, enabling calculation of the activation energies (E) and rate constants (k) of such reactions, based on the enthalphy of the reaction and the MIP algorithms. The factors affecting the E of the RBr decay reaction are established: the enthalphy of the reaction, triplet repulsion, the energy of radical R? stabilization, the presence of a π bond adjacent to the reaction center, and the dipole–dipole interaction of polar groups. The energy spectrum of the partial energies of activation is constructed for the reaction of coordinated molecular decay of RBr, and the E and k of inverse addition reactions are evaluated.     

Reactivity of indolizines in organic synthesis

This review reports the reactivity of indolizines. The reactions section covers, in general, electrophilic, oxidation, reduction, addition, cycloaddition, condensation, and Mannich and multicomponent reactions. The synthesis of bis-indolizines and cyclazines are reported. The reaction mechanisms are discussed.     

Reactivity of biarylazacyclooctynones in copper-free click chemistry

The 1,3-dipolar cycloaddition of cyclooctynes with azides, also called "copper-free click chemistry", is a bioorthogonal reaction with widespread applications in biological discovery. The kinetics of this reaction are of paramount importance for studies of dynamic processes, particularly in living subjects. Here we performed a systematic analysis of the effects of strain and electronics on the reactivity of cyclooctynes with azides through both experimental measurements and computational studies using a density functional theory (DFT) distortion/interaction transition state model. In particular, we focused on biarylazacyclooctynone (BARAC) because it reacts with azides faster than any other reported cyclooctyne and its modular synthesis facilitated rapid access to analogues. We found that substituents on BARAC's aryl rings can alter the calculated transition state interaction energy of the cycloaddition through electronic effects or the calculated distortion energy through steric effects. Experimental data confirmed that electronic perturbation of BARAC's aryl rings has a modest effect on reaction rate, whereas steric hindrance in the transition state can significantly retard the reaction. Drawing on these results, we analyzed the relationship between alkyne bond angles, which we determined using X-ray crystallography, and reactivity, quantified by experimental second-order rate constants, for a range of cyclooctynes. Our results suggest a correlation between decreased alkyne bond angle and increased cyclooctyne reactivity. Finally, we obtained structural and computational data that revealed the relationship between the conformation of BARAC's central lactam and compound reactivity. Collectively, these results indicate that the distortion/interaction model combined with bond angle analysis will enable predictions of cyclooctyne reactivity and the rational design of new reagents for copper-free click chemistry.     

Reactivity index based on orbital energies

This study shows that the chemical reactivities depend on the orbital energy gaps contributing to the reactions. In the process where a reaction only makes progress through charge transfer with the minimal structural transformation of the reactant, the orbital energy gap gradient (OEGG) between the electron‐donating and electron‐accepting orbitals is proven to be very low. Using this relation, a normalized reaction diagram is constructed by plotting the normalized orbital energy gap with respect to the normalized intrinsic reaction coordinate. Application of this reaction diagram to 43 fundamental reactions showed that the majority of the forward reactions provide small OEGGs in the initial stages, and therefore, the initial processes of the forward reactions are supposed to proceed only through charge transfer. Conversely, more than 60% of the backward reactions are found to give large OEGGs implying very slow reactions associated with considerable structural transformations. Focusing on the anti‐activation‐energy reactions, in which the forward reactions have higher barriers than those of the backward ones, most of these reactions are shown to give large OEGGs for the backward reactions. It is also found that the reactions providing large OEGGs in the forward directions inconsistent with the reaction rate constants are classified into S_N2, symmetric, and methyl radical reactions. Interestingly, several large‐OEGG reactions are experimentally established to get around the optimum pathways. This indicates that the reactions can take significantly different pathways from the optimum ones provided no charge transfer proceeds spontaneously without the structural transformations of the reactants. © 2014 Wiley Periodicals, Inc.