Reactivity of tetrahydrochromeno[2,3‐b]indoles: chromic indicators of cyanide

The synthesis and reactivity of a tetrahydrochromeno[2,3‐b]indoles are reported. Evidence for reversible ring‐opening is based on H/D exchange and trapping experiments. These compounds readily undergo reaction with tetra‐n‐butylammonium cyanide. The cyanide reaction is 10–100× faster when the solution is irradiated with 350 nm light. Reaction with trimethylsilyl cyanide occurs only with UV irradiation demonstrating photoreactivity. The rate of tetrahydrochromeno[2,3‐b]indole ring‐opening is greater for (i) Me substitution at the hemiaminal carbon (compared to Ph), and (ii) substitution of fluorine at the 9‐position of the indole. Under acidic conditions, the ring‐opened indolium ion is observed. Copyright © 2013 John Wiley & Sons, Ltd.     

Micellar effects on aromatic nucleophilic substitution by the ANRORC mechanism. Hydrolysis of 2‐chloro‐3,5‐dinitropyridine

The hydrolysis of 2‐chloro‐3,5‐dinitropyridine by sodium hydroxide in the presence of micelles of cetyltrimethylammonium bromide (CTABr), cetyltrimethylammonium chloride (CTACl) and sodium dodecyl sulfate (SDS) has been studied. The reaction follows a consecutive reaction path involving the formation of a long‐lived intermediate 3 and finally giving the product, 3,5‐dinitro 2‐pyridone 2 . The mechanism follows an addition of the nucleophile, ring opening and ring closure (ANRORC) reaction path. The rate constant was observed to be first‐order dependent on [OH^?]. The rate of reaction increased on increasing [CTABr] and, after reaching to the maxima, it started decreasing. The anionic SDS micelles inhibited the rate of hydrolysis. The results of the kinetic experiments were treated with the help of the pseudophase ion exchange model and the Menger–Portnoy model. The added salts, viz. NaBr, Na‐toluene‐4‐sulphonate, and (CH₃)₄NBr on varying [CTACl] and [SDS] inhibited the rate of reaction. The various kinetic parameters in the presence and absence of salts were determined and are reported herewith. Copyright © 2011 John Wiley & Sons, Ltd.     

Formation of pyrazol‐1,3,4‐thiadiazoles through 1,3‐dipolar cycloadditions of 3‐thioxo‐[1,2,4]‐triazepin‐5‐one with nitrilimines: an experimental and computational study

In this work the experimental results and the computational study of the title compounds and some ancillary compounds are reported. Two bicyclic pyrazol‐1,3,4‐thiadiazole derivatives were synthezised by reaction between 6‐dimethylaminomethylene‐3‐thioxo‐[1,2,4]‐triazepin‐5‐one 1 and several nitrilimines 2a–f to give corresponding spirocycloadducts 3a–f , which undergo a rapid rearrangement leading to the new bicyclic compounds, 4a–f and 5a–f . These obtained bicyclic products were characterized by ¹H and ¹³C NMR spectroscopy and finally by X‐ray crystallography. Theoretical calculations have been carried out using DFT methods to rationalize the formation of the two new bicyclic compounds. Two reaction types are involved in the formation of the compounds 4a–f and 5a–f . The first one is a 1,3‐dipolar cycloaddition (13DC) reaction between 1 acting as dipolarophile and 2a–f as dipoles. The results indicate that the cycloaddition between 1 and 2g , as model of 2a–c , takes place via a high asynchronous bond‐formation process. The regioselectivity obtained from the calculations is in complete agreement with the formation of the unique spirocycloadducts 3a–f . The second reaction leading to the formation of the final products is a domino process that is initiated by the quick and irreversible cleavage in a catalytic acid environment of triazepenic ring. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics and mechanisms of gas‐phase decarbonylation of α‐methyl‐trans‐cinamaldehyde and E‐2‐methyl‐2‐pentenal under homogeneous catalysis of hydrogen chloride

The kinetics of the gas‐phase elimination of α‐methyl‐trans‐cinamaldehyde catalyzed by HCl in the temperature range of 399.0–438.7 °C, and the pressure range of 38–165 Torr is a homogeneous, molecular, pseudo first‐order process and undergoing a parallel reaction to produce via (A) α‐methylstyrene and CO gas and via (B) β‐methylstyrene and CO gas. The decomposition of substrate E‐2‐methyl‐2‐pentenal was performed in the temperature range of 370.0–410.0 °C and the pressure range of 44–150 Torr also undergoing a molecular, pseudo first‐order reaction gives E‐2‐pentene and CO gas. These reactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor. The rate coefficients are given by the following Arrhenius expressions:

• Products formation from α‐methyl‐trans‐cinamaldehyde
• α‐methylstyrene :
• β‐methylstyrene :
• Products formation from E‐2‐methyl‐2‐pentenal
• E‐2‐pentene :

The kinetic and thermodynamic parameters for the thermal decomposition of α‐methyl‐trans‐cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield α‐methylstyrene and carbon monoxide, whereas via (B) through a five‐membered cyclic transition state to give β‐methylstyrene and carbon monoxide. However, the elimination of E‐2‐methyl‐2‐pentenal occurs by way of a concerted cyclic five‐membered transition state mechanism producing E‐2‐pentene and carbon monoxide. The present results support that uncatalyzed α‐β‐unsaturated aldehydes decarbonylate through a three‐membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd.     

Substituent effects on the ring‐opening mechanism of gem‐dibromospiropentanes to related allenes: a theoretical study

Density functional theory computations at B3LYP and X3LYP levels were performed for ring openings of substituted gem‐dibromospiropentanes (R = ―H, ―Cl, ―Br, ―CH₃, ―SiH₃, ―OH, ―OCH₃, ―CF₃, ―BF₂, and ―SH) to related allenes. The conversion of spiropentanoids 5a , 5b , 5c , 5d , 5e , 5f , 5g , 5h , 5i , 5j to the corresponding allenes 7a , 7b , 7c , 7d , 7e , 7f , 7g , 7h , 7i , 7j can proceed in both concerted and stepwise mechanism except for R = ―H. Both ring‐opening mechanisms have similar activation energy barriers to open the spiropentanylidene ring and generate the structure of allene at all theoretical levels used herein. Generally the π electron‐donating group (―OH or ―SH) decreases the activation barrier for the follow‐up reaction of 1‐bromo‐1‐lithiospiropentanoid and free spiropentanylidene. Hence, both bearing electron‐donating substituents are more reactive than those with electron‐withdrawing group, and the first one to open the ring to the LiBr–allene complex does so more readily than the second. The sEDA index used to measure sigma‐electron excess/deficiency of the cyclopropylidene ring is mutually correlated for the studied systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Ring inversion process for a series of 3,5‐dialkyl‐1‐oxa‐3,5‐diazacyclohexanes: 1H DNMR study and semiempirical calculations

The ring inversion process for a series of 3,5‐dialkyl‐1‐oxa‐3,5‐diazacyclohexanes was studied using proton dynamic nuclear magnetic resonance (¹H DNMR) spectroscopy in conjunction with semiempirical calculations. At low temperature, the ring methylene protons decoalesced into two AB spin systems in a 2:1 ratio. Lineshape simulations of the DNMR spectra provided first‐order rate constants for magnetic exchange. The energy barrier for each inversion reaction was calculated from the respective rate constants. In general, as the size of the N‐alkyl group increased, the barrier to ring inversion decreased. A similar trend was seen in semiempirical calculations that modeled the ring inversion process. Copyright © 2010 John Wiley & Sons, Ltd.     

Surfactant‐assisted specific‐acid catalysis of Diels–Alder reactions in aqueous media

Surfactant‐assisted specific‐acid catalysis (SASAC) for Diels–Alder reactions of dienophiles 1 and 4 with cyclopentadiene 2 in aqueous media at 32 °C was studied. This study showed that acidified anionic surfactants (pH 2) such as sodium dodecyl sulfate (SDS) and linear alkylbenzene sulfonic acid (LAS) accelerate Diels—Alder reactions. Conversely, under similar reaction conditions (pH 2) these reactions are inhibited by (acidified) cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), dodecyldimethylammonium bromide (DDAB), and dodecylmethylammonium bromide (DMAB). A modest rate acceleration resulting from the surfactant hydrogen‐bonding capacity is also recorded for the Diels–Alder reaction of naphthoquinones ( 6 ) with cyclopentadiene ( 2 ) in aqueous media at 32 °C. Copyright © 2007 John Wiley & Sons, Ltd.     

Aluminum alkoxide complexes from highly substituted 3,4‐epoxy alcohols: An NMR and computational study

The organoaluminum mediated epoxide ring opening of epoxy alcohols is a key step in the oxirane‐based approach for polypropionate synthesis. However, this reaction has shown unanticipated regioselectivities when applied to 2‐methyl‐3,4‐epoxy alcohols. In order to gain mechanistic insight into the factors controlling the epoxide ring opening process, diastereomeric 2‐methyl‐3,4‐epoxy alcohols were reacted with triethylaluminum in order to identify the aluminum complexes formed by these systems. Different epoxide–aluminum complexes were calculated using ab initio HF/[13s7p/11s5p] and B3PW91/6‐31G** gauge‐including atomic orbital (GIAO) methods and compared to the experimental NMR data. The calculated and experimental data correlates with the aluminum dimer complex (TIPSOCH₂CH(OAlEt₂)CH(CH)₃CHCH(O)(AlEt₃))₂ (VIII) for the systems favoring the nucleophilic attack at the external C4 epoxide carbon, while an unusual trialuminum species TIPSOCH₂CH(OAlEt₂)CH(CH)₃CHCH(O)(AlEt₃)₂ (X) is consistent with the systems favoring the internal C3 attack. The ²⁷Al NMR data established the tetracoordinated nature of the aluminum metal in all alkoxy aluminum intermediates, while the ¹³C NMR data provided insight into the aluminum‐oxygen coordination. The formation of the complexes was dictated by the stereochemical disposition of the substituents. These complexes are different from the generally accepted bidentate intermediates proposed for 2,3‐epoxy alcohols and simpler 3,4‐epoxy alcohols. Copyright © 2012 John Wiley & Sons, Ltd.     

Hydrolysis and retro‐aldol cleavage of ethyl threo‐2‐(1‐adamantyl)‐3‐hydroxybutyrate: competing reactions

The hydrolysis of ethyl threo‐2‐(1‐adamantyl)‐3‐hydroxybutyrate ( 1 ) and the parent ester ethyl 3‐hydroxybutyrate ( 4 ) has been studied experimentally and computationally. In the hydrolysis of threo‐ester 1 with 2 M NaOH, predominantly retro‐aldol product was observed, whereas the hydrolyzed product was present in a minor amount. When the reaction is carried out under the same conditions with the parent ester ethyl 3‐hydroxybutyrate ( 4 ), hydrolyzed product is exclusively observed. The competitive pathways, namely hydrolysis and the retro‐aldol reaction for 1 and 4 were investigated using DFT calculations in the both gas and solvent phase. The calculated results in the solvent phase at B3LYP/6–31 + G* level revealed that the formation of retro‐aldol products is kinetically preferred over the hydrolysis of threo‐ester 1 in the presence of a base. However, the parent ester 4 showed that the retro‐aldol process is less favored than the hydrolysis process under similar conditions. The steric effect imposed by the bulky adamantyl group to enhance the activation barriers for the hydrolysis of the ethyl threo‐2‐(1‐adamantyl)‐3‐hydroxybutyrate ( 1 ) was further supported by the calculations performed with tert‐butyl group at the α‐carbon atom of ethyl 3‐hydroxybutyrate ( 7 ). Copyright © 2010 John Wiley & Sons, Ltd.     

Conformational analysis of N‐phenyl‐ and N‐trifyl‐4,4‐dimethyl‐4‐silathiane 1‐sulfimides

N‐Substituted 4,4‐dimethyl‐4‐silathiane 1‐sulfimides [R = Ph ( 1 ), CF₃ ( 2 )] were studied experimentally by variable temperature dynamic NMR spectroscopy. Low temperature ¹³C NMR spectra of the two compounds revealed the frozen ring inversion process and approximately equal content of the axial and equatorial conformers. Calculations of the 4‐silathiane derivatives 1 , 2 and the model compound [R = Me ( 3 )] as well as their carbon analogs, the similarly N‐substituted thiane 1‐sulfimides [R = Ph ( 4 ), CF₃ ( 5 ), Me ( 6 )] at the DFT/B3LYP/6–311G(d,p) level in the gas phase and in chloroform solution using the PCM model at the same level of theory showed a strong dependence of the relative stability of the conformer on the solvent. The electronegative trifluoromethyl group increases the relative stability of the axial conformer. Copyright © 2010 John Wiley & Sons, Ltd.     

Stereoelectronic effects in Menshutkin‐type SN2 reactions: theoretical study based on through‐space/bond orbital interaction analysis

Through‐space/bond orbital interaction analysis has been applied to investigate the stereoelectronic effects on stabilizing the transition state of Menshutkin‐type S_N2 reactions. The mechanism of how the substituent effects work on accelerating the reactions has been demonstrated from orbital interaction perspective. The geometrical structures and Mulliken charge distributions have been compared to elucidate the substituent effects for the S_N2 reaction center. It is found that the substituents lower the activation energies by strengthening the orbital interactions in the S_N2 reaction process. When electron‐donating and electron‐accepting substituents (–C₆H₅ and –CHO) are introduced to the same central carbon at the reaction center, the symmetry allows the π–π* interactions among the donor and acceptor in the transition state. It stabilizes the transition state much more than the reactant complex. And the π–π* interactions are estimated to decrease about 2.28 kcal/mol of the energy for transition state. The σ‐like orbitals of the partial bond around the central carbon are reactive, and the σ–π* orbital interactions stabilize the reactant complex much more than the π–σ* interaction. When the σ–π* and π–σ* interactions are deleted from the system, the activation energy increases and turns close to the values of the systems which are without such substituents. It can be concluded that the π–π*, σ–π*, and π–σ* interactions cooperatively accelerates the S_N2 reaction by stabilizing its transition state. Copyright © 2013 John Wiley & Sons, Ltd.     

Transient spectroscopic characterization of the ring‐opening reaction of tetrahydrochromeno[2,3‐dimethyl]indole

Tetrahydrochromeno is a structural variant of spiropyran that undergoes a reversible ring‐opening to generate a colored nitrophenolate intermediate. Earlier work confirmed this intermediate through trimethylsilyl cyanide trapping under continuous irradiation. We have performed transient absorption spectroscopy to further characterize the mechanism of the ring‐opening reaction. Excitation at 355 nm produced a transient species with an absorption maximum at 445 nm, which we assign to the nitrophenolate unit of the ring‐opened product. The transient absorption decays after ~970 ns with small optical density changes corresponding to a 0.15 quantum yield. Exposure to oxygen did not exhibit a significant deleterious effect on the photoisomerization of the chromeno dye. Time‐dependent density functional theory corroborated spectroscopic assignments of the starting chromeno and the putative ring‐opened intermediate. The excited state behavior of this system parallels the structurally similar oxazine system reported by Raymo and coworkers. The one significant difference is the longer lifetime of the photochemically generated intermediate from chromeno. Copyright © 2015 John Wiley & Sons, Ltd.     

The mechanism investigation of the imidazolidinone catalyzed five‐membered ring synthesis reaction

The intramolecular asymmetric Michael addition reaction catalyzed by imidazolidinone is investigated using the density functional theory calculations. The details of the reaction mechanism, potential energy surfaces, and the influence of the acid additive are investigated. The reaction process includes two stages. The first stage is Michael addition, in which the enamine complex is created and then the Michael addition is carried out. The second stage is a product separation stage which includes an enol‐keto tautomerization and a two‐step hydrolysis. The enantioselectivity is controlled by the Michael addition step which involves a new carbon–carbon bond formation. The calculation results provide a general model which may explain the mechanism and enantioselectivity of the title reaction. Copyright © 2012 John Wiley & Sons, Ltd.     

Investigation of N‐carbamoylamino acid nitrosation by {NO + O2} in the solid‐gas phase. Effects of NOx speciation and kinetic evidence for a multiple‐stage process

Nitrosation of N‐carbamoylamino acids (CAA) by gaseous NO + O₂, an interesting synthetic pathway to amino acid N‐carboxyanhydrides (NCA), alternative to the phosgene route, was investigated on N‐carbamoyl‐valine either in acetonitrile suspension or solventless conditions, and compared to the classical nitrosating system NaNO₂ + CF₃COOH (TFA), the latter being quite less efficient in terms of either rate, stoichiometric demand, or further tractability of the product. The rate and efficiency of the NO + O₂ reaction mainly depends on the O₂/NO ratio. Evaluation of the contribution of various nitrosating species (N₂O₃, N₂O₄, HNO₂) through stoichiometric balance showed the reaction to be effected mostly by N₂O₃ for O₂/NO ratios below 0.3, and by N₂O₄ for O₂/NO ratios above 0.4. The relative contribution of (subsequently formed) HNO₂ always remains minor. Differential scanning calorimetry (DSC) monitoring of the reaction in the solid phase by either HNO₂ (from NaNO₂ + TFA), gaseous N₂O₄ or gaseous N₂O₃, provides the associated rate constants (ca. 0.1, 2 and 10⁸ s^?1 at 25°C, respectively), showing that N₂O₃ is by far the most reactive of these nitrosating species. From the DSC measurement, the latent heat of fusion of N₂O_3, 2.74 kJ · mol^?1 at ?105 °C is also obtained for the first time. The kinetics was investigated under solventless conditions at 0°C, by either quenching experiments or less tedious, rough calorimetric techniques. Auto‐accelerated, parabolic‐shaped kinetics was observed in the first half of the reaction course, together with substantial heat release (temperature increase of ca. 20°C within 1–2 min in a 20‐mg sample), followed by pseudo‐zero‐order kinetics after a sudden, important decrease in apparent rate. This kinetic break is possibly due to the transition between the initial solid‐gas system and a solid‐liquid‐gas system resulting from water formation. Overall rate constants increased with parameters such as the specific surface of the solid, the O₂/NO ratio, or the presence of moisture (or equivalently the hydrophilicity of the involved CAA), however without precise relationship, while the last two parameters may directly correlate to the increasing acidity of the medium. Copyright © 2007 John Wiley & Sons, Ltd.     

Theoretical studies of cobalt(I)‐catalyzed hydroacylation of vinylsilanes and alkyl aldehydes

Density functional theory (DFT) was used to investigate computationally cobalt(I)‐catalyzed hydroacylation of vinylsilanes and alkyl aldehydes to give ketones. Calculation indicated that cobalt(I)‐catalyzed hydroacylation had eight possible reaction pathways. In the cobalt‐hydride complexes IM2a and IM2b, the hydrogen migration occurred prior to the carbon–carbon bond‐forming reaction. In the complexes IM3a1 and IM3b1, the carbonyl elimination reaction occurred prior to the direct reductive elimination reaction. In the cobalt–carbonyl complexes IM4a and IM4b, the carbonyl insertion reaction was much easier to achieve than the decarbonylation reaction. The dominant reaction pathway was the reaction channel IM1a → TS1a → IM2a → TS2a1 → IM3a1 → TS4a → IM4a → TS5a → IM5a → TS6a → IM6a, and the reductive elimination reaction was the rate‐determining step for this channel, so the dominant product predicted theoretically was the linear ketone. Furthermore, the solvation effect was remarkable, and it decreased generally the free energies of the species. Copyright © 2015 John Wiley & Sons, Ltd.     

Catalysis by hydrogen chloride in the gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten‐3‐ol

A homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k₁ (s^?1) = (11.01 ± 0.31) ? (109.5 ± 2.8) kJ mol^?1 (2.303 RT)^?1 and for 3‐methyl‐1‐buten‐3‐ol log k₁ (s^?1) = (11.50 ± 0.18) ? (116.5 ± 1.4) kJ mol^?1 (2.303 RT)^?1. Electron delocalization of the CH₂?CH and C₆H₅ appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley & Sons, Ltd.     

The Baeyer‐Villiger oxidation versus aromatic ring hydroxylation: competing organic peracid oxidation mechanisms explored by multivariate modelling of designed multi‐response experiments

Peroxy acids can be used as the terminal oxidant for the Baeyer–Villiger oxidation of acetophenones and for direct ring hydroxylation of methoxy‐substituted benzenes. An oxidative system involving 3‐chloroperbenzoic acid (mCPBA) and 2,6‐dimethoxyacetophenone as model substrate was investigated by means of statistical experimental design, multivariate modelling and response surface methodology. The outcome of the organic peracid oxidation experiments was portrayed by a multi‐response matrix consisting of the yields of three different compounds; 2,6‐dimethoxyphenyl acetate, 1‐(4‐hydroxy‐2,6‐dimethoxy‐phenyl)ethanone and 3‐hydroxy‐2,6‐dimethoxy‐phenyl acetate. The optimized reaction protocol was utilized to investigate a series of various substituted acetophenones. The overall investigation revealed that both the molecular structure of the acetophenone substrate and the experimental conditions exhibited a substantial impact on whether the oxidation reaction follows the oxygen insertion or direct ring hydroxylation mechanism. An improved protocol for the direct ring hydroxylation was also obtained from the experimental and modelling described herein. Copyright © 2015 John Wiley & Sons, Ltd.     

Conformational analysis of 4,4‐dimethyl‐4‐silathiane and its S‐oxides

4,4‐Dimethyl‐4‐silathiane and its S‐oxides [n = 0 ( 1 ), 1 ( 2 ), 2 ( 3 )] were studied experimentally by variable temperature dynamic NMR spectroscopy down to 103 K and the frozen ring inversion was revealed for all three compounds. The barriers for the degenerate ring inversion in 1 and 3 were measured to be 4.8 and 5.0 kcal/mol at the coalescence temperatures of 111 and 116 K, respectively, and practically coincide with the calculated barriers of 4.60 kcal/mol in 1 and 4.46 kcal/mol in 3 . The frozen equilibrium mixture 2‐ax/2‐eq contains 37% of the 2‐ax and 63% of the 2‐eq conformer. The ring inversion barrier proved to be ca. 4.8 kcal/mol. Calculations at the B3LYP/6‐311+G(d,p) level of theory showed the 2‐ax conformer to be 0.90 kcal/mol more stable than the 2‐eq conformer in the gas phase whereas in solution the relative stability of the conformers calculated using the PCM model at the same level of theory is inverted to become 0.19 (in CHCl₃) or 0.36 kcal/mol (in DMSO) in favor of the 2‐eq conformer. The chair–chair interconversion mechanism of sulfoxide 2 includes two intermediate energetically equivalent 1,4‐twist forms and the 2,5‐boat transition state: 2‐ax (chair) ? 2 (1,4‐twist) ? [ 2 (2,5‐boat)]^≠ ? 2 (1,4‐twist) ? 2‐eq (chair). The calculated ring inversion barriers are 5.1 ( 2‐ax → 2‐eq ) and 4.2 kcal/mol ( 2‐eq → 2‐ax ) in the gas phase, and 4.03 and 4.22 kcal/mol, respectively, in chloroform. Copyright © 2011 John Wiley & Sons, Ltd.     

Kinetics and mechanism for acid‐catalyzed disproportionation of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl

Disproportionation of cyclic nitroxyl radicals (NRs) in acid solutions is of key importance for the chemistry of these compounds. Meanwhile, the data reported on the mechanism of this reaction in dilute acids are inconsistent with those on the stability of NRs in concentrated acids. Here we have examined the kinetics and stoichiometry for the disproportionation of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl ( 1 ) in aqueous H₂SO₄ (1.0–99.3 wt%) and found that (1) the disproportionation of 1 proceeds by the same mechanism over the entire range of acid concentrations, (2) the effective rate constant of the process exhibits a bell‐shaped dependence on the excess acidity function X peaked at X = ?pK _1H+ = 5.8 ± 0.3, (3) a key step of the process involves the oxidation of 1 with its protonated counterpart 1H ⁺ yielding oxopiperidinium cation 2 and hydroxypiperidine 3 at a rate constant of (1.4 ± 0.8) × 10⁵ M^?1 · s^?1, and (4) the reaction is reversible and, upon neutralization of acid, disproportionation products 2 and 3H ⁺ comproportionate to starting 1 . In highly acidic media, the protonated form 1H ⁺ is relatively stable due to a low disproportionation rate. Based on the known and newly obtained values of equilibrium constants, both the standard redox potential for the 1H ⁺ / 3 pair (955 ± 15 mV) and the pH‐dependences have been calculated for the reduction potentials of 1 and 2 to hydroxylamine 3 that is in equilibrium with its protonated 3H ⁺ and deprotonated 3 ^? forms. The data obtained provide a deeper insight into the mechanism of nitroxyl‐involving reactions in chemical and biological systems. Copyright © 2008 John Wiley & Sons, Ltd.     

Mechanistic study on Mo(CO)6‐catalyzed intramolecular [2 + 2] or [2 + 2 + 1] cycloaddition reaction of 5‐allenyl‐1‐ynes

By means of density functional theory, the Mo(CO)₆‐catalyzed intramolecular [2 + 2] or [2 + 2 + 1] cycloaddition reaction of 5‐allenyl‐1‐ynes was investigated. All the intermediates and transition states were optimized completely at B3LYP/6‐311++G(d,p) level (LANL2DZ(f) for Mo). Calculations indicate that the complexation of 5‐allenyl‐1‐ynes with Mo(CO)₆ occurred preferentially at the triple bond to give the complex M1 and then the complexation with the distal double bond of the allenes generates the complex M5 . In this reaction, Mo(CO)₆‐catalyzed intramolecular [2 + 2] cycloaddition is more favorable than [2 + 2 + 1] cycloaddition. The reaction pathway Mo(CO)₆ + R → M5 → T7 → M12 → M13 → T11 → M18 → P4 is the most favorable one, and the most dominant product predicted theoretically is P4 . The solvation effect is remarkable, and it decreases the reaction energy barriers. Copyright © 2011 John Wiley & Sons, Ltd.