Gold-catalyzed aerobic oxidation of dibenzylamine: Homogeneous or heterogeneous catalysis?

Au(OAc)₃ is applied as an effective catalyst of the selective oxidation of dibenzylamine to dibenzylimine using molecular oxygen as the only oxidant. When Au(OAc)₃ was preadsorbed onto CeO₂, the supported catalyst was more active than any homogeneous or heterogeneous catalyst known for this reaction. Although, some fascinating color changes in the early stage of the reaction indicated the formation of an amine complex, conventional filtration experiments proved the heterogeneity of the system. The fate of the active gold component was studied by in situ X-ray absorption spectroscopy (XANES) using a specially designed cell. These investigations revealed that in the early stage of the reaction Au(OAc)₃ is dissolved and subsequently reduced by the amine and the in situ formed gold nanoparticles are the real active species of the reaction. Formation of gold nanoparticles during dibenzylamine oxidation was proved independently by transmission electron microscopy. Our findings lead to a simple synthetic procedure using a commercially available gold salt, which upon interaction with the amine forms highly active and selective gold nanoparticles.     

Synthesis and thermal decomposition of Cr–urea complex

In this work, Cr–urea complex ([Cr(NH₂CONH₂)₆](NO₃)₃) was synthesized by direct solid-state reaction of chromium nitrate and urea, and its thermal decomposition reaction was studied for the first time to explore the possibilities of using the complex as precursor to nanosized chromium oxide. The formation of [Cr(NH₂CONH₂)₆](NO₃)₃ is confirmed from infrared spectroscopy and elemental analysis. Thermogravimetric and differential thermal analysis of the compound show a three-stage thermal decomposition in the temperature range from 190 to 430 °C. The result of X-ray diffraction (XRD) shows that the [Cr(NH₂CONH₂)₆](NO₃)₃ decompose at ~300 °C into α-Cr₂O₃ nanopowder with an average crystallite size of 33 nm.     

Biomimetical catalysis of α-Amino acid hydrolysis over chiral palladacycles

A 4.5-fold difference in catalytic hydrolysis rate constants between stereomer pairs was achieved as a result of the incorporation of bulky substituents into chiral cyclopalladated arylamines used as catalysts for hydrolysis of esters of optically active amino acids. An unexpected inversion of catalyst stereoselectivity depending on the bulkiness of substituents at the palladacycle a-carbon atom was discovered.     

Planar or nonplanar: what is the structure of urea in aqueous solution?

A combined quantum chemical statistical mechanical method has been used to study the solvation of urea in water, with emphasis on the structure of urea. The model system consists of three parts: a Hartree-Fock quantum chemical core, 99 water molecules described with a polarizable force-field, and a dielectric continuum. A free-energy profile along the transition of urea from planar to a nonplanar structure is calculated. This mode in aqueous solution is found to be floppy. That is, the structure of urea in water is not well-defined because the planar to nonplanar transition requires an energy of the order of the thermal energy at room temperature. We discuss the implications of this finding for simulation studies of urea in polar environments like water and proteins.     

Electrocatalytic oxidation of alcohols on gold in alkaline media: base or gold catalysis?

On the basis of a comparison of the oxidation activity of a series of similar alcohols with varying pK(a) on gold electrodes in alkaline solution, we find that the first deprotonation is base catalyzed, and the second deprotonation is fast but gold catalyzed. The base catalysis follows a Hammett-type correlation with pK(a), and dominates overall reactivity for a series of similar alcohols. The high oxidation activity on gold compared to platinum for some of the alcohols is related to the high resistance of gold toward the formation of poisoning surface oxides. These results indicate that base catalysis is the main driver behind the high oxidation activity of many organic fuels on fuel cell anodes in alkaline media, and not the catalyst interaction with hydroxide.     

Calcium-mediated hydroboration of alkenes: “Trojan horse” or “true” catalysis?

The hydroboration of 1,1-diphenylethylene (DPE) with catecholborane (HBcat) proceeds at 100 °C. For conversion at room temperature three different organocalcium catalysts have been investigated: the calcium hydride complex [DIPPnacnacCaH·(THF)]₂ (1, DIPPnacnac = CH{(CMe)(2,6-iPr₂C₆H₃N)}₂), Ca[2-Me₂N-α-Me₃Si-benzyl)₂·(THF)₂ (2) and DIPPnacnacCa(H-BBN)·(THF) (3, BBN = 9-borabicyclo[3.3.1.]nonane). Although up to 96% conversion of DPE is found, the product of the reaction is not the expected Ph₂CHCH₂Bcat but (Ph₂CHCH₂)₃B is formed instead. Organocalcium compounds catalyze the decomposition of HBcat to B₂(cat)₃ and BH₃ (or B₂H₆) and the latter is involved in hydroboration of DPE. The calcium-catalyzed decomposition of HBcat was investigated with ¹¹B NMR and the signals were assigned to the following species: B₂(cat)₃, B(cat)₂^?, HBcat, BH₃(THF), BH₄^? and B₂H₇^?. A tentative mechanism for the formation of these species was proposed. The intermediate DIPPnacnacCa(BH₄)·(THF)₂ (5) was independently prepared by reaction of 1 and BH₃(Me₂S) and was structurally characterized by X-ray diffraction. Stoichiometric reaction of 1 with pinacolborane (HBpin) gave a trimeric complex [DIPPnacnacCa(H₂Bpin)]₃ (6) which was structurally characterized by X-ray diffraction. This complex does not react with DPE, also not at elevated temperatures. The possible equilibrium between 6 and 1/HBpin is therefore fully at the side of 6. As 6 is unstable in the presence of HBpin, no further catalytic conversions have been investigated.     

Autoxidation catalysis with N-hydroxyimides: more-reactive radicals or just more radicals?

A first-principles analysis reveals that the catalytic efficiency of nitroxyl radicals in alkane oxidations is not simply correlated with their reactivity toward the substrate.     

Density functional theory study on the mechanism of Rh-catalyzed decarboxylative conjugate addition: diffusion- and ligand-controlled selectivity toward hydrolysis or β-hydride elimination

The mechanism of Rh-catalyzed decarboxylative conjugate addition has been investigated with Density Functional Theory (DFT). Calculations indicate that the selectivity toward hydrolysis or β-hydride elimination of the investigated reaction is a compromise between diffusion control and kinetic control. Ligand control can be adjusted by modifying the intermolecular interaction between the Rh(I) enolate intermediate and water.     

XRD and VCD: a marriage of love or convenience? Honeymoon around a cyclic urea derivative

The structures and absolute configurations of the enantiomers (3aR,8aR)‐2,2‐dimethyl‐4,4,8,8‐tetraphenyl‐4,5,6,7,8,8a‐hexahydro‐3aH‐1,3‐dioxolo[4,5‐e][1,3]diazepin‐6‐one 0.33‐hydrate, C₃₂H₃₀N₂O₃·0.33H₂O, (Ia), and (3aS,8aS)‐2,2‐dimethyl‐4,4,8,8‐tetraphenyl‐4,5,6,7,8,8a‐hexahydro‐3aH‐1,3‐dioxolo[4,5‐e][1,3]diazepin‐6‐one 0.39‐hydrate, C₃₂H₃₀N₂O₃·0.39H₂O, (Ib), have been elucidated unambiguously using the complementary power of single‐crystal X‐ray diffraction (XRD) and vibrational circular dichroism (VCD). The enantiomers crystallize in the Sohncke space group P2₁2₁2 and pack as dimers stabilized by two symmetric hydrogen bonds involving one amide group each of the cyclic urea moiety. This double interaction is capped by a water molecule that partially occupies a site lying on the twofold axis and forms an uncommon hydrogen bond between the two monomers. A comparison between the solid‐state VCD characterizations and the Bayesian statistics on Bijvoet differences determined from the XRD measurements reveals a tendency towards the correct determination of the absolute configuration by this latter method.     

Heterogeneous or homogeneous catalysis? Mechanistic studies of the rhodium-catalyzed dehydrocoupling of amine-borane and phosphine-borane adducts

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.     

The mechanism of ring-opening polymerization of L-lactide by ICl3 catalysts: Halogen bond catalysis or participating in reactions?

The mechanism of ring-opening polymerization of L-lactide by iodine trichloride (ICl₃) catalyst has been explored by using density functional theory (DFT) calculations and three catalytic pathways were proposed. The first and second pathways belong to the halogen bond catalysis, and the third pathway involves the ICl₃ catalysts participating in reactions. When the carbonyl group was maintained involved in the reaction and activated catalytically by the halogen bond, there are two possible pathways. The first pathway involves only one transition state, and the second pathway requires two transition states. There is another pathway in which ICl₃ directly participates in the reaction, it is named the third pathway. Two different transition states of the four-membered rings are generated successively, the transfer of I─O bonds determined the progress of the reaction. Theoretical calculations in this work provide the most basic understanding of ring-opening polymerization of L-lactide by ICl₃ catalysts. © 2019 Wiley Periodicals, Inc.     

How reliable are estimates of hydrolysis constants?

The equilibrium constant for the first hydrolysis reaction of tetravalent plutonium is surrounded by uncertainty. A new method illustrates criteria by which the reliabilities of the numerical estimates can be judged. The new formulas are simple, the method is easy to apply, and the results are easy to compare.     

Hidden Brønsted acid catalysis: pathways of accidental or deliberate generation of triflic acid from metal triflates

The generation of a hidden Br?nsted acid as a true catalytic species in hydroalkoxylation reactions from metal precatalysts has been clarified in case studies. The mechanism of triflic acid (CF(3)SO(3)H or HOTf) generation starting either from AgOTf in 1,2-dichloroethane (DCE) or from a Cp*RuCl(2)/AgOTf/phosphane combination in toluene has been elucidated. The deliberate and controlled generation of HOTf from AgOTf and cocatalytic amounts of tert-butyl chloride in the cold or from AgOTf in DCE at elevated temperatures results in a hidden Br?nsted acid catalyst useful for mechanistic control experiments or for synthetic applications.     

Does the decomposition of peroxydicarbonates and diacyl peroxides proceed in a stepwise or concerted pathway?

M?ller-Plesset perturbation theory and density functional theory calculations are used to study decomposition mechanisms of polymerization initiators, such as diethyl peroxydicarbonate, trifluoroacetyl peroxide, and acetyl peroxide, which possess a general structure of RC(O)OO(O)CR. It is found that the decomposition of initiators with electron-donating R groups follows two favorable stepwise pathways: a two-bond cleavage mechanism in which the O-O single bond and one of R-C bonds of [R-C(O)O-O(O)C-R] break simultaneously followed by decomposition of the R-C(O)O(*) radical and a one-bond cleavage mechanism in which the single O-O bond cleavage produces a carboxyl radical pair and a subsequent decomposition of the carboxyl radicals. It is also found that the initiators with electron-withdrawing R groups follow the two-bond cleavage pathway only. Geometrical and energetic analyses indicate that despite the similar structures of the peroxydicarbonates, quite different decomposition energy barriers are determined by the nature of the R groups.     

Which is the determinant for cellulose degradation in cooperative ionic liquid pairs: dissolution or catalysis?

Catalytic conversion of sustainable cellulose to the value-added chemicals and high quality biofuel has been recognized as a perfect approach for the alleviation of the dependence on the non-renewable fossil resources. Previously, we successfully designed and explored novel and efficient cooperative ionic liquid pairs for this renewable material, which has advantages of high reactor efficiency than current technologies because of the dissolution and in situ catalytic decomposition mechanism. Here, the determinant of this process is further studied by the intensive investigation on the relationship between the cellulose conversion and the properties of ionic liquid catalyst and solvent. Scanning electron microscope (SEM), thermogravimetric analysis (TG) and elemental analysis were used for the comparative characterization of raw cellulose and the residues. The results demonstrate that this consecutive dissolution and in situ catalysis process is much more dependent on the dissolution capability of ionic liquid solvent, while comparatively, the effect of in situ acid catalysis is relatively insignificant.     

Effect of BaNH,CaNH,Mg3N2 on the activity of Co in NH3 decomposition catalysis

Development of active and non-noble metal-based catalyst for H2 production via NH3 decomposition is crucial for the implementation of NH3 as a H2 carrier.Co-based catalysts have received increasing attention because of its high intrinsic activity and moderate cost.In this work,we examined the effect of BaNH,CaNH and Mg3 N2 on the catalytic activity of Co in the NH3 decomposition reaction.The H2 formation rate ranks the order as Co-BaNH>Co-CaNH>Co-Mg3 N2≈Co/CNTs within a reaction temperature range of 300-550℃.It is worth pointing out that the H2 formation rate of Co-BaNH at 500℃reaches20 mmolH2 gcat-1 min-1,which is comparable to those of the active Ru/Al2 O3(ca.17 mmolH2 gcat-1 min1)and Ru/AC(21 mmolH2 gcat-1 min-1)catalysts under the similar reaction conditions.In-depth research shows that Co-BaNH exhibits an obviously higher intrinsic activity and much lower Ea(46.2 kJ mol-1)than other Co-based catalysts,suggesting that BaNH may play a different role from CaNH,Mg3 N2 and CNTs during the catalytic process.Combined results of XRD,Ar-TPD and XAS show that a[Co-N-Ba]-like intermediate species is likely formed at the interface of Co metal and BaNH,which may lead to a more energy-efficient reaction pathway than that of neat Co metal for NH3 decomposition.     

Structure of Au8: planar or nonplanar?

We have studied the structures and stabilities of Au6 and Au8 at the density-functional theory (DFT) and ab initio correlated levels of theory. For Au8, our ab initio calculations predict the lowest Au8 isomer to be planar, in line with the DFT calculations.     

Folding of elongated proteins: conventional or anomalous?

The complexity of the mechanisms by which proteins fold has been shown by many studies to be governed by their native-state topologies. This was manifested in the ability of the native topology-based model to capture folding mechanisms and the success of folding rate predictions based on various topological measures, such as the contact order. However, while the finer details of topological complexity have been thoroughly examined and related to folding kinetics, simpler characteristics of the protein, such as its overall shape, have been largely disregarded. In this study, we investigated the folding of proteins with an unusual elongated geometry that differs substantially from the common globular structure. To study the effect of the elongation degree on the folding kinetics, we used repeat proteins, which become more elongated as they include more repeating units. Some of these have apparently anomalous experimental folding kinetics, with rates that are often less than expected on the basis of rates for globular proteins possessing similar topological complexity. Using experimental folding rates and a larger set of rates obtained from simulations, we have shown that as the protein becomes increasingly elongated, its folding kinetics becomes slower and deviates more from the rate expected on the basis of topology measures fitted for globular proteins. The observed slow kinetics is a result of a more complex pathway in which stable intermediates composed of several consecutive repeats can appear. We thus propose a novel measure, an elongation-sensitive contact order, that takes into account both the extent of elongation and the topological complexity of the protein. This new measure resolves the apparent discrimination between the folding of globular and elongated repeat proteins. Our study extends the current capabilities of folding-rate predictions by unifying the kinetics of repeat and globular proteins.     

Mechanisms of C-C and C-H alkane reductive eliminations from octahedral Pt(IV): reaction via five-coordinate intermediates or direct elimination?

The Pt(IV) complexes P(2)PtMe(3)R [P(2) = dppe (PPh(2)(CH(2))(2)PPh(2)), dppbz (o-PPh(2)(C(6)H(4))PPh(2)); R = Me, H] undergo reductive elimination reactions to form carbon-carbon or carbon-hydrogen bonds. Mechanistic studies have been carried out for both C-C and C-H coupling reactions and the reductive elimination reactions to form ethane and methane are directly compared. For C-C reductive elimination, the evidence supports a mechanism of initial phosphine chelate opening followed by C-C coupling from the resulting five-coordinate intermediate. In contrast, mechanistic studies on C-H reductive elimination support an unusual pathway at Pt(IV) of direct coupling without preliminary ligand loss. The complexes fac- P(2)PtMe(3)R (P(2) = dppe, R = Me, H; P(2) = dppbz, R = Me) have been characterized crystallographically. The Pt(IV) hydrides, fac-P(2)PtMe(3)H (P(2) = dppe, dppbz), are rare examples of stable phosphine ligated Pt(IV) alkyl hydride complexes.