Lanthanide polyoxocationic complexes: experimental and theoretical stability studies and Lewis acid catalysis

The [ε-PMo(V)(8)Mo(VI)(4)O(36)(OH)(4){Ln(III)(H(2)O)}(4)](5+) (Ln=La, Ce, Nd, Sm) polyoxocations, called εLn(4), have been synthesized at room temperature as chloride salts soluble in water, MeOH, EtOH, and DMF. Rare-earth metals can be exchanged, and (31)P NMR spectroscopic studies have allowed a comparison of the affinity of the reduced {ε-PMo(12)} core, thus showing that the La(III) ions have the highest affinity and that rare earths heavier than Eu(III) do not react with the ε-Keggin polyoxometalate. DFT calculations provide a deeper insight into the geometries of the systems studied, thereby giving more accurate information on those compounds that suffer from disorder in crystalline form. It has also been confirmed by the hypothetical La→Gd substitution reaction energy that Ln ions beyond Eu cannot compete with La in coordinating the surface of the ε-Keggin molybdate. Two of these clusters (Ln=La, Ce) have been tested to evidence that such systems are representative of a new efficient Lewis acid catalyst family. This is the first time that the catalytic activity of polyoxocations has been evaluated.     

X=Y-ZH systems as potential 1.3-dipoles : Part 14. bronsted and Lewis acid catalysis of cycloadditions of arylidene imines of α-amino acid esters

Cycloadditions of arylidene imines of α-amino acid esters to a range of dipolarophiles show substantial rate enhancements in the presence of Bronsted and Lewis acids. For Bronsted acids the rate is related to the pK_a of the acid and cycloadditions to reactive dipolarophiles occur at room temperature. For the Lewis acids studied the rate acceleration decreases in the order Zn(OAc)₂ > AgOAc> LiOAc> MgOAc₂ but is also anion dependent with LiBr> LiOAc and AgOAc> AgOTs. The Lewis acid catalysed processes are believed to be examples of cycloadditions of metallo-1,3-dipoles. In both Bronsted and Lewis acid catalysed processes the cycloadditions are regio- and stereo-specific.     

Solid state 13C NMR can distinguish between bronsted and Lewis acid adducts of amines

The solid ¹³C NMR spectra of solid complexes of phenethylamine with SnCl_4, Me₃SnCl, AlCl_3, and HCl are presented and discussed as a means of distinguishing between Bronsted and Lewis acid adducts of alkyl amines. Solution ¹³C NMR results are discussed in terms of species present in complex equilibria and their detection using the two NMR methods.     

A theoretical study of the reaction of Ti+ with propane

Detailed quartet and doublet potential energy surfaces for the Ti⁺ + C₃H₈ → TiC₃H₆⁺ + H₂ and Ti⁺ + C₃H₈ → TiC₂H₄⁺ + CH₄ elimination reactions have been studied using density functional theory with B3LYP functional and ab initio coupled cluster CCSD(T) methods. Several H₂ elimination and CH₄ elimination reaction paths have been examined including the IRC following. In particular, the mechanisms involving, respectively, the H₂TiC₃H₆⁺ and CH₃TiHC₂H₄⁺ intermediates have been studied. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Time-dependent reactive scattering for the system H- + D2 <--> HD + D- and comparison with H- + H2 <--> H2 + H-

This work presents results of quantum mechanical calculations of reaction probabilities for the ion-neutral molecule collisions H- + D2 <--> HD + D-. Time-dependent wave packet propagations for total angular momentum J not equal to 0, including the full Coriolis coupling, are performed. The calculated state-to-state reaction probabilities using product Jacobi coordinates are compared with energy-resolved reaction probabilities calculated with the flux-operator using reactant Jacobi coordinates and with time-independent calculations. Differences between nearly converged integral cross sections and those using the J-shifting method and centrifugal sudden approximation and comparison with experimental results will be presented.     

Testing the veracity of claims of Lewis acid catalysis

Are reactions employing Lewis acids really catalysed by those Lewis acids, or by “hidden Brønsted acids”, i.e. Brønsted acids generated in situ by hydrolysis? Testing of a series of reactions using Sc(III), Fe(III), In(III) and Y(III) by addition of 2,6-di-t-butyl-4-methylpyridine reveal that all are likely to follow the latter pathway. A reaction claimed to be catalysed by CBr₄ through halogen bonding is also likely to be Brønsted acid catalysed.     

Size-selective Lewis acid catalysis in a microporous metal-organic framework with exposed Mn2+ coordination sites

Treatment of selected aldehydes and ketones with cyanotrimethylsilane in the presence of the microporous metal-organic framework Mn3[(Mn4Cl)3BTT8(CH3OH)10]2 (1, H3BTT = 1,3,5-benzenetristetrazol-5-yl) leads to rapid conversion to the corresponding cyanosilylated products. The transformation is catalyzed by coordinatively unsaturated Mn2+ ions that serve as Lewis acids and lead to conversion yields of 98 and 90% for benzaldehyde and 1-naphthaldehyde, the highest thus far for a metal-organic framework. Larger carbonyl substrates cannot diffuse through the pores of 1, and conversion yields are much lower for these, attesting to the heterogeneity of the reaction and its dependence on guest size. The Mukaiyama-aldol reaction, known to require much more active Lewis catalysts, is also catalyzed in the presence of 1, representing the first such example for a metal-organic framework. Conversion yields obtained for the reaction of selected aldehydes with silyl enolates reach 63%, on par with those obtained with zeolites. Size selectivity is demonstrated for the first time with this reaction through the use of larger silyl enolate substrates.     

Regioselective alkenylation of imidazoles by nickel/Lewis acid catalysis

Nickel/Lewis acid binary catalysis is found effective to direct regioselective alkenylation of imidazoles through C-H bond activation and stereoselective insertion of alkynes. Use of P(t-Bu)₃ as a ligand allows exclusive regioselective C(2)-alkenylation, while PCyp₃ is found effective for C(5)-alkenylation of C(2)-substituted imidazoles. The reaction demonstrates a broad scope of imidazoles and internal alkynes to give trisubstituted ethenes highly regio- and stereoselectively in modest to good yields.     

Molecular rearrangement of epoxide derived from sesquiterpenes by Lewis acid catalysis

The molecular rearrangement under mild conditions of two epoxides derived from the sesquiterpenic himachalenes has been studied using BF₃·Et₂O as the Lewis acid catalyst. Both reactions are efficient and selective affording in each case one new enantiomerically pure ketone isolated in good yield. Their structures have been fully characterized by spectroscopic methods. Mechanisms for the BF₃-catalyzed reactions are proposed.     

A new and efficient method for the facile synthesis of N-acyl sulfonamides under Lewis acid catalysis

The N-acylation of sulfonamides with carboxylic acid anhydrides in the presence of Lewis acids is described. Several Lewis acids such as BF₃·Et₂O, ZnCl₂, MoCl₅, TiCl₄, B(C₆F₅)₃, Sc(OTf)₃ and I₂ were found to catalyze the reaction efficiently to furnish the N-acylated products in good yields under solvent-free conditions. The reactions of various sulfonamides were studied with different carboxylic acid anhydrides including the less reactive benzoic and pivalic anhydrides, in the presence of 3 mol % ZnCl₂ as the catalyst. Carboxylic acids were also successfully used as acylating agents via the mixed anhydride method.     

Quantum mechanical rate constants for H + O2 <--> O + OH and H + O2 --> HO2 reactions

Canonical rate constants for both the forward and reverse H + O(2) <--> O + OH reactions were calculated using a quantum wave packet-based statistical model on the DMBE IV potential energy surface of Varandas and co-workers. For these bimolecular reactions, the results show reasonably good agreement with available experimental and theoretical data up to 1500 K. In addition, the capture rate for the H + O(2) --> HO(2) addition reaction at the high-pressure limit was obtained on the same potential using a time-independent quantum capture method. Excellent agreement with experimental and quasi-classical trajectory results was obtained except for at very low temperatures, where a reaction threshold was found and attributed to the centrifugal barrier of the orbital motion.     

The modulation of face-face π-π interactions in Lewis acid catalysis

The enantiomeric excess observed for the exo-adduct from the Lewis acid catalysed Diels-Alder reaction between cyclopentadiene and methacrolein can be increased up to 21% by simple modification of the electronics of the aromatic ring in a series of stilbene-derived diol ligands, suggesting that the proposed face-face π-π interaction between the catalyst and the dienophile can be modulated by altering the electron density on the aromatic ring.     

Electronic effects of linker substitution on Lewis acid catalysis with metal-organic frameworks

Functionalized linkers can greatly increase the activity of metal-organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σ(m) values of the linker substituents X and the rate k(X) of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group.     

Enantioselective fluoride ring opening of aziridines enabled by cooperative Lewis acid catalysis

The enantioselective ring opening of aziridines using a latent source of HF is described. A combination of two Lewis acids, (salen)Co and an achiral Ti(IV) cocatalyst, provided optimal reactivity and enantioselectivity for the trans β-fluoroamine product. The use of a chelating aziridine protecting group was crucial. Acyclic and cyclic meso N-picolinamide aziridines underwent fluoride ring opening in up to 84% ee, and the kinetic resolution of a piperidine-derived aziridine was performed with k_rel=6.6. The picolinamide group may be readily removed without epimerization of the fluoroamine. Preliminary studies revealed a bimetallic mechanism wherein the chiral (salen)Co catalyst delivers the nucleophile and the Ti(IV) cocatalyst activates the aziridine.     

Metal–ligand–Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation. The [(SCS)Pd]₂ complex featuring a non-innocent indenediide-based ligand was found to be a very efficient and versatile catalyst for the Conia-ene reaction, when associated with Mg(OTf)₂. The reaction operates at low catalytic loadings under mild conditions with HFIP as a co-solvent. It works with a variety of substrates, including those bearing internal alkynes. It displays complete 5-exo vs. 6-endo regio-selectivity. In addition, except for the highly congested ^tBu-substituent, the reaction occurs with high Z vs. E stereo-selectivity, making it synthetically useful and complementary to known catalysts.

An original multi-cooperative catalytic approach was developed by combining metal–ligand cooperation and Lewis acid activation.     

Reflected shock tube and theoretical studies of high-temperature rate constants for OH+CF3H<-->CF3+H2O and CF3+OH-->products

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm, using either 36 or 60 optical passes corresponding to total path lengths of 3.25 or 5.25 m, respectively, has been used to study the bimolecular reactions, OH+CF3H-->CF3+H2O (1) and CF3+H2O-->OH+CF3H (-1), between 995 and 1663 K. During the course of the study, estimates of rate constants for CF3+OH-->products (2) could also be determined. Experiments on reaction -1 were transformed through equilibrium constants to k1, giving the Arrhenius expression k1=(9.7+/-2.1)x10(-12) exp(-4398+/-275K/T) cm3 molecule(-1) s(-1). Over the temperature range, 1318-1663 K, the results for reaction 2 were constant at k2=(1.5+/-0.4)x10(-11) cm3 molecule(-1) s(-1). Reactions 1 and -1 were also studied with variational transition state theory (VTST) employing QCISD(T) properties for the transition state. These a priori VTST predictions were in good agreement with the present experimental results but were too low at the lower temperatures of earlier experiments, suggesting that either the barrier height was overestimated by about 1.3 kcal/mol or that the effect of tunneling was greatly underestimated. The present experimental results have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range of 252-1663 K. The three parameter expression k1=2.08x10(-17) T1.5513 exp(-1848 K/T) cm3 molecule(-1) s(-1) describes the rate behavior over this temperature range. Alternatively, the expression k1,th=1.78x10(-23) T3.406 exp(-837 K/T) cm3 molecule(-1) s(-1) obtained from empirically adjusted VTST calculations over the 250-2250 K range agrees with the experimental evaluation to within a factor of 1.6. Reaction 2 was also studied with direct CASPT2 variable reaction coordinate transition state theory. The resulting predictions for the capture rate are found to be in good agreement with the mean of the experimental results and can be represented by the expression k2,th=2.42x10(-11) T-0.0650 exp(134 K/T) cm3 molecule(-1) s(-1) over the 200-2500 K temperature range. The products of this reaction are predicted to be CF2O+HF.     

The formation of Lewis acid/base stabilised phosphanyltrielanes - A theoretical and experimental study

Theoretical investigations on the thermochemistry and the reaction mechanism of the formation of Lewis acid/base stabilised phosphanyltrielanes D · H₂EPH₂ · A (D = Lewis base, A = Lewis acid) were conducted. The reactions of EH₃ · D with A · PH₃ to form D · H₂EPH₂ · A and H₂ (E = B, Al, Ga; D = NH₃; A = BH₃, Cr(CO)₅) are all exothermic, regardless of whether donors and acceptors are present or absent. The lithium chloride elimination reactions between EH₂Cl · D and A · PH₂Li to give D · H₂EPH₂ · A and LiCl are endothermic for donor/acceptor stabilised compounds, if formation of gaseous LiCl is considered. If solid lithium chloride is considered all reactions are strongly exothermic. Studies of the transition state for H₂-elimination reactions between EH₃ · D and A · PH₃ to yield D · H₂EPH₂ · A and H₂ were only successful for E = Al, Ga. In these cases the reaction proceeds via a transition state featuring a five or six-coordinate group 13 element. Different donor molecules do not influence the activation energy of such H₂-elimination reactions, but nevertheless they have an effect on the reaction energy. The synthesis of the Cr(CO)₅ substituted phosphanyltrielanes [(CO)₅Cr(H₂PBH₂ · NMe₃)] (3a) and [(CO)₅Cr(H₂PAlH₂ · NMe₃)] (3b), as well as of the dinuclear complex [(CO)₈Cr₂(μ-HPBH₂ · NMe₃)₂] (4) are described, the latter as a subsequent reaction product of the photolysis of 3a. All compounds were characterised spectroscopically and by X-ray structure analysis.