The solvent effect on the reaction constants of tert-butyl radical addition to 2-substituted allyl chlorides

The ρ values of free radical S_H2′ reactions have been determined in various solvents. The correlation of Hammett ρ with Taft's π* gives a W value of 0.70. The W value is a measure of susceptibility of the reaction constant to change in solvent polarity. However, the W value is 2.64 in the dissociation reactions of substituted benzoic acids. The free radical reactions are less susceptible to the solvent effect than ionic reactions and this could be rationalized in terms of the partial charge formed in the transition state of free radical reaction is less than that of heterolytic reaction. The ρ values in S_H2′ reactions might not reflect truly the partial charge separation at transition state, however, it might be a measure of the susceptibility of the reaction to the electronic effect of the substituents.     

Reactivity of free clusters

Recent studies of the reactions of free or isolated transition metal clusters with simple molecules will be reviewed. Cluster chemical reactions are carried out in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reactions can be categorized as chemisorptive (or surface) reactions, and bulk reactions. Chemisorptive reactions can be characterized asfacile, in which most cluster sizes of a given metal are equally reactive towards a particular reagent, oractivated, in which strong dependence of reactivity on cluster size is found. Under the normal operating conditions of the cluster reactor, a reaction may be kinetically controlled or at equilibrium. Following chemisorption, adsorbate decomposition and product desorption may occur. Specific reactions to be discussed include the reactions of iron clusters with ammonia and with water and the reactions of nickel clusters with ammonia.     

Highly enantioselective aldol reactions using a tropos dibenz[c,e]azepine organocatalyst

The four-step synthesis of a chiral primary tertiary diamine salt, possessing a tropos dibenz[c,e]azepine ring is described. It is shown that 3.5-5 mol % of this salt is capable of promoting highly enantioselective crossed-aldol reactions between cyclohexanone and a series of aromatic aldehydes. In all cases, the aldol reactions proceed with high diastereoselectivity for the anti-aldol product. The outcome of crossed-aldol reactions involving other cyclic ketones and acyclic ketones are also described. All examples involving cyclic ketones result in selectivity for the anti-aldol products, whereas acyclic ketones were found to favour the syn-aldol products. A discussion on the role of the chiral primary tertiary diamine salt in the catalysis of the aldol reactions is also presented.     

Cross-linked poly(4-vinylpyridine-N-oxide) as a polymer-supported oxygen atom transfer reagent

Oxygen atom transfer (OAT) reagents are common in biological and industrial oxidation reactions. While many heterogeneous catalysts have been utilized in OAT reactions, heterogeneous OAT reagents have not been explored. Here, cross-linked poly(4-vinylpyridine-N-oxide), called x-PVP-N-oxide, was tested as a heterogeneous OAT reagent and its oxidation chemistry compared to its molecular counterpart, pyridine-N-oxide. The insoluble oxidant x-PVP-N-oxide demonstrated comparable reactivity to pyridine-N-oxide in direct oxidation reactions of phosphines and phosphites in acetonitrile, but x-PVP-N-oxide did not react in other solvents. The polymer backbone of x-PVP-N-oxide, however, allowed for easy filtering and recycling in sequential oxidation reactions. In addition, x-PVP-N-oxide was tested as the stoichiometric oxidant in a copper-catalyzed OAT reaction to α-diazo-benzeneacetic acid methyl ester. The heterogeneous oxidant was much less reactive than pyridine-N-oxide, indicating that interaction with the metal catalyst was challenging. These results demonstrated a proof-of-concept that recyclable, polymer-supported OAT reagents could be a viable OAT reagents in direct oxidation reactions without metal catalysts.     

Understanding the mechanisms of [3+2] cycloaddition reactions. The pseudoradical versus the zwitterionic mechanism

Analysis of 12 three-atom-components (TACs) participating in [3+2] cycloaddition (32CA) reactions towards ethylene and acetylene allows establishing good correlations between the pseudodiradical character, the hardness η, and the nucleophilicity N index of the TAC with the feasibility of these non-polar reaction. These results allow the introduction of the pr index, which comprises the two aforementioned DFT reactivity indices. The increase of the pr index for an allylic-type TAC goes accompanied by a linear decrease of the activation enthalpy of the reaction. The present study makes it possible to establish a useful classification of 32CA reactions into zw-type reactions involving TACs with a high zwitterionic character, and pr-type reactions involving TACs with a high pseudodiradical character.     

Synthesis and reactivity of 5-methylenehydantoins

5-Methylenehydantoin, as well as the N-mono- and N,N-di-protected derivatives, can be obtained by different synthetic routes. These compounds can undergo a large variety of reactions, such as Diels-Alder, epoxidation, methanol addition and conjugate addition reactions of different types of nucleophiles, including carbon (cyanide), nitrogen (piperidine) and sulfur (thiols, thioacetate) nucleophiles. The reactivity with electrophilic reagents, such as m-CPBA or methanol in acidic medium, and the need for Lewis acids to promote the conjugate addition reactions indicate that hydantoin is a poor electron-withdrawing group.     

Reactions of N-substituted 2,6(3,5)-dialkyl-1,4-benzoquinone imines with arenesulfinic acids

The regioselectivity in the reactions of N-arylsulfonyl-2,6-dialkyl-1,4-benzoquinone imines with arenesulfinic acids (1,6-, 6,1-, or 6,3-addition) is determined by steric factor, while in the reactions of N-aroyl-1,4-benzoquinone imines electronic effect of substituents in the quinoid ring is crucial. The reactions of N-arylsulfonyl-3,5-dimethyl-1,4-benzoquinone imines with arenesulfinic acids follow mainly the 1,4-addition pattern. N-(N-Arylsulfonylbenzimidoyl)-1,4-benzoquinone imines are capable of reacting in a way similar to both N-arylsulfonyl and N-aroyl derivatives.     

Stereoselective synthesis of N-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides

The stereoselective synthesis of N-acyl- and N-Boc-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides is described. These reactions effect formation of two bonds in a single operation and proceed with generally high levels of diastereoselectivity. In contrast to previously described reactions of γ-(N-arylamino) alkenes, these transformations proceed in high yield and high regioselectivity with both electron-rich and electron-deficient aryl bromides as well as vinyl bromide substrates.     

Reactivity difference between diphosgene and phosgene in reaction with (2,3-anti)-3-amino-1,2-diols

In reactions of (2,3-anti)-3-amino-1,2-diols with diphosgene and phosgene and their conversion into 1,3-oxazolidin-2-ones, some differences in the stereochemistry of the reactions have been found with these two reagents. The reactions with phosgene afforded the expected cis-oxazolidinones, and in the reaction with diphosgene under the same reaction conditions, the trans-oxazolidinones were also obtained.     

Copper-catalyzed Csp-chalcogen bond formation: Versatile approach to N-(3-(organochalcogenyl)prop-2-yn-1-yl)amides

This work describes the copper-catalyzed Csp-chalcogen bond formation through cross-coupling reactions of propynylbenzamides and diorganyl dichalcogenides at room temperature, under open atmosphere. Generally, the reactions have proceeded very smoothly, affording the new class of organochalcogen-containing propynylbenzamides in good yields. Notably, this protocol showed to be tolerant to neutral, electron-donating and electron-withdrawing substituents present in both reaction substrates. In addition, the applicability of the carbon-chalcogen bond as a reactive site has been proved by successfully employment of the N-methyl-N-(3-(butylselanyl)prop-2-yn-1-yl)benzamide as a precursor in palladium-catalyzed Suzuki and Sonogashira type reactions.     

Gold catalysed Suzuki-Miyaura coupling of arenediazonium o-benzenedisulfonimides

Arenediazonium o-benzenedisulfonimides have been used as efficient electrophilic partners in Au(I) catalysed Suzuki coupling reactions. The synthetic protocol is general, easy and produced either biaryls or heteroaryl arenes in good yields (51 positive examples, average yield 80%). o-Benzenedisulfonimide was recovered at the end of the reactions and was reused to prepare the starting salts for further reactions. Mechanistic insights suggest that the o-benzenedisulfonimide anion act as an electron transfer agent and promotes a catalytic cycle which does not require the presence of photocatalysts or external oxidants.     

On the kinetic analysis of thermoanalytical diagrams obtained with the “quasi-isothermal” heating technique

A comparative study of “n-order”, Avrami—Erofeev and diffusion-controlled reactions has been carried out using thermoanalytical data, calculated by assuming a “quasi-isothermal” heating technique. The results reported support the hypothesis that a single diagram obtained with this technique allows the discernment of Avrami-Erofeev, “n-order” and diffusion-controlled reactions. This does not occur when a linear heating program is used, as was reported in previous papers. In addition, it is shown that a single thermoanalytical curve obtained with a “quasi-isothermal” heating program does not permit the determination of the actual value of n of those reactions following “n-order” kinetics. On the other hand, it is proved that the kinetic analysis of two thermoanalytical curves obtained with a linear heating program and a “quasi-isothermal” heating technique, respectively, would provide an excellent procedure for discerning the proper mechanism of solid-state reactions.     

Stereoselectivity in metal carbene and Lewis acid-catalyzed reactions from diastereomeric dirhodium(II) carboxamidates: Menthyl N-acetyl-2-oxoimidazolidine-4(S)-carboxylates

The influence of a chiral menthyl group as the pendant ester substituent on the N-acetyl-2-oxoimidazolidine-4S-carboxylate ligands in chiral dirhodium(II) imidazolidinone catalysts has been examined. Significant match/mismatch influences are evident in the observed stereocontrol for carbon–hydrogen insertion reactions with diazoacetates, but these effects are minimal in cyclopropanation reactions. Steric restrictions prevent effective enantiocontrol in hetero-Diels–Alder reactions using these menthyl-substituted catalysts.     

Reactions of epoxides derived from internal perfluoroolefins with o-phenylenediamine and 2-aminophenol

The reactions of epoxy derivatives of internal perfluoroolefins with o-phenylenediamine and 2-aminophenol in dioxane gave 23–67% of the corresponding 2,3-bis(perfluoroalkyl)quinoxalines and 2,3-bis(perfluoroalkyl)-2H-1,4-benzoxazin-2-ols, respectively. When N,N-dimethylacetamide was used as a solvent, the main reaction pathway was anionic isomerization of epoxides into ketones which were then converted into 2-perfluoroalkylbenzimidazoles (in the reactions with o-phenylenediamine) or 2-hydroxy-N-perfluoroalkanoylanilines (in the reactions with 2-aminophenol). The reaction of 3,4-epoxydodecafluorohexane with 2-aminophenol in N,N-dimethylacetamide was accompanied by unusual cyclization to afford 2-pentafluoropropanoyl-2-pentafluoroethyl-1,3-benzoxazolidine.     

Solvent-free,catalyst-free aza-Michael addition of cyclohexylamine to diethyl maleate: Reaction mechanism and kinetics

The aza-Michael reaction is the addition of an amine to an electron deficient C=C double bond. This reaction is also used in the synthesis of precursors of polymeric networks. In this study, we paid attention to the kinetics and mechanism of the aza-Michael addition of cyclohexylamine (CHXA) to diethyl maleate (DEM) performed as a solvent-free, catalyst-free reaction and to concurrent reactions. In situ Raman spectroscopy, NMR spectroscopy and gas chromatography/mass spectrometry have shown the occurrence of three simultaneous reactions: (i) the aza-Michael addition of CHXA to DEM leading to diethyl 2-(cyclohexylamino)succinate, (ii) isomerization of DEM to diethyl fumarate (DEF), and (iii) the aza-Michael addition of CHXA to DEF formed by the reaction (ii). All of these reactions proceed with third order kinetics, first order in DEM or DEF and second order in CHXA. We propose a kinetic model that allows kinetic constants to be estimated. Furthermore, a numerical solution of the set of differential equations confirms the expected kinetic equations of reactions (i) and (ii) and gives values of rate constants comparable to the estimated ones. A DFT mechanistic study illustrates the structure of the reaction intermediates and transition states of all reactions and explains the contribution of the second amine molecule in the reaction mechanism.     

Reactions of thebaine with cis- and trans-disubstituted dienophiles

Thebaine has been shown to undergo Diels-Alder reactions with trans-disubstituted ethylenes containing relatively bulky groups. These were correlated with reactions in which the corresponding cis-disubstituted dienophiles were used. Reductions, Grignard reactions and base-catalyzed rearrangements of the adducts are described.     

Reactivity of quinones as alkyl radical acceptors

The enthalpies of the addition of 11 alkyl radicals to ortho-and para-benzoquinones and substituted para-benzoquinones and the enthalpies of formation of various alkoxyphenoxyl radicals have been calculated. Experimental data for the addition of alkyl radicals to quinones are analyzed in terms of the intersection of two parabolic potential curves, and parameters characterizing this class of reactions are calculated. The classical potential barrier of the thermally neutral reaction of alkyl radical addition to benzoquinone is E _e,0 = 82.1 kJ/mol. This class of reactions is compared to other classes of free-radical addition reactions. The interaction between the electrons of the reaction center and the π electrons of the aromatic ring is a significant factor in the activation energy. Activation energies, rate constants, and the geometric parameters of the transition state have been calculated for 40 reactions of alkyl radical addition to quinones. Strong polar interaction has been revealed in the addition of polar macroradicals to quinones, and its contribution to the activation energy has been estimated. Kinetic parameters, activation energies, and rate constants have been calculated for the reverse reactions of alkoxyphenoxyl radical decomposition to quinone and alkyl. The competition between chain termination and propagation reactions in alkoxyphenol-inhibited hydrocarbon oxidation is discussed.     

Tandem enantioselective organo- and biocatalysis: a direct entry for the synthesis of enantiomerically pure aldols

Direct proline-catalyzed aldol reactions were linked in sequence with lipase-catalyzed kinetic resolutions to afford enantiomerically pure (>99% ee) aldol adducts in higher conversion than a standard resolution of racemic materials. The combination of organocatalytic aldol reactions and enzymatic kinetic resolutions provided exclusively either the (R)- or (S)-enantiomer of the aldol adducts even though an (R)-selective lipase catalyzed the kinetic resolutions. Furthermore, the one-pot tandem reactions are inexpensive, are operationally simple, circumvents the use of organic solvent and are environmentally benign.     

Organocatalytic SOMO reactions of copper(I)-acetylide and alkylindium compounds with aldehydes

The derivatisation of aldehydes in their α-position is an important facet of organic synthesis. Organocatalytic radical reactions afford α-functionalised aldehydes via a SOMO activation pathway. New organo-SOMO reactions of aldehydes with copper(I)-acetylide and alkylindium reagents are detailed. These reactions proceed well under the catalysis of chiral imidazolidinones. The corresponding functionalised aldehydes were obtained with acceptable yields, but with only low enantiomeric ratios.     

Non-isothermal two-membrane reactors for reversible gas phase reactions

The performance of a non-isothermal two-membrane reactor for reversible chemical reactions in gas phase has been analyzed by numerical simulation. The analyzed reactions were of the form: aA = bB + cC. Two membranes, that are permeable to all the components of the reaction mixture, are supposed to be the most permeable to one of the two reaction products, satisfying the condition of reverse products permselectivities. The reactant is taken to be the slowest permeating component. A negative temperature influence on the permeabilities of components has been assumed. Co-current plug flow pattern has been accepted. It has been shown that it is possible to enhance reactant conversion above that of a conventional reactor for both endothermic and exothermic reversible reactions, including adiabatic and non-adiabatic case. By using a two-membrane reactor, considerable lowering of feed temperatures is enabled for an endothermic reaction. For endothermic reactions, there is the optimum feed temperature, whereas for exothermic reactions, the higher the temperature, the lower is the attained conversion. In reactor design, the optimal external heat exchange for both endothermic and exothermic reactions can be determinated.