Timo Repo

Facile Heterolytic H2 Activation by Amines and B(C6F5)3

In industrially important reactions, such as hydroformylation and hydrogenation, H2 gas serves as a reducing agent and/or a hydrogen-atom source. Even small improvements in the efficiency of these reactions translate into large monetary savings. The key step in these transformations is the activation of H2 at a transition metal. The nodal character of the energetically accessible d orbitals allows a transition-metal center to react directly with H2 in a concerted reaction with a low activation barrier. However, not only are transitionmetal complexes expensive, but the complete removal of metal impurities from the reaction product is generally required in the production of pharmaceutical intermediates owing to toxicity concerns. Although countless synthetic complexes and enzymes with transition metals at their reactive core are well known, there are significantly fewer examples of H H bond activation facilitated solely by a nonmetal. Several reactions of H2 with compounds containing main-group elements in lowtemperature matrices have been reported; however, H2 activation at nonmetals under mild conditions had only been observed by Power and co-workers in product mixtures of digermenes, digermanes, and primary germanes, until recently, when Stephan and co-workers reported the thermal liberation of H2 from a phosphonium borate salt. The resulting product, (C6H2Me3)2P(C6F4)B(C6F5)2, undergoes the addition of H2 at 25 8C to reform the original salt. [7] In an analogous fashion, mixtures of sterically demanding phosphanes and boranes (“frustrated Lewis pairs”) can also cleave H2 heterolytically to form phosphonium borates [R3PH][HBR’3]. [9] More recently, Bertrand and co-workers reported that selected organic carbenes are just nucleophilic enough to cleave H2 and NH3. [10] Herein we extend the family of “frustrated Lewis pairs” and demonstrate that not only bulky phosphanes and boranes or organic carbenes can cleave H2, but also inexpensive, stable amines in combination with B(C6F5)3. Solutions of stoichiometric mixtures of diisopropylethylamine, diisopropylamine, or 2,2,6,6-tetramethylpiperidine and B(C6F5)3 in toluene were investigated by H, B, and F NMR spectroscopy. The reactions of diisopropylethylamine and diisopropylamine with B(C6F5)3 gave mixtures of the salt 1a or 1b and the zwitterion 2a or 2b as expected (Scheme 1); however, no reaction was observed for 2,2,6,6tetramethylpiperidine, a bulky secondary amine with no a hydrogen atoms. Whereas the reaction between diisopropylamine and B(C6F5)3 is reversible at elevated temperature, the mixture of 1a and 2a from the reaction with diisopropylethylamine is thermally stable (Scheme 1).

A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Frustrated Lewis pairs are compounds containing both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and have been shown to be capable of heterolysis of molecular hydrogen, a property that has led to their use in hydrogenation reactions of polarized multiple bonds. Here, we describe a general approach to the hydrogenation of alkynes to cis-alkenes under mild conditions using the unique ansa-aminohydroborane as a catalyst. Our approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies. Frustrated Lewis pairs have been shown to be capable of heterolysis of strong covalent bonds such as those in molecular hydrogen, and have been used in the hydrogenation of polar multiple bonds. Here, a new type of ansa-aminohydroborane is shown to be active for the partial hydrogenation of alkynes under mild conditions.

New bis(imino)pyridine-iron(II)- and cobalt(II)-based catalysts: synthesis, characterization and activity towards polymerization of ethylene

Abstract The synthesis of a new series of iron(II)- and cobalt(II)-based complexes of the general formula M(N∩N∩N)Cl 2 (M=Fe; M=Co) bearing 2,6-bis(imin)pyridyl ligands [A–NC–Py–CN–A] that carry bulky, alkyl-free aromatic terminals (A=naphthyl, pyrenyl, 2-benzylphenyl, phenyl) or chiral cycloaliphatic auxiliary moieties (A=((−)- cis -myrtanyl) is described. The Fe(II) complexes are exceptionally active (up to 40800 kg PE/(mol M h) towards the polymerization of ethylene in the presence of methylaluminoxane (MAO) as activator. Varying the steric bulkiness of the aromatic groups in the tridentate ligands affects catalytic productivity, molecular weight and for the first time the microstructure of the resulted material. The Fe(II) precatalysts are an order of magnitude more active than the corresponding Co(II) precatalysts.

Metal-Free sp2-C–H Borylation as a Common Reactivity Pattern of Frustrated 2-Aminophenylboranes

C-H borylation is a powerful and atom-efficient method for converting affordable and abundant chemicals into versatile organic reagents used in the production of fine chemicals and functional materials. Herein we report a facile C-H borylation of aromatic and olefinic C-H bonds with 2-aminophenylboranes. Computational and experimental studies reveal that the metal-free C-H insertion proceeds via a frustrated Lewis pair mechanism involving heterolytic splitting of the C-H bond by cooperative action of the amine and boryl groups. The adapted geometry of the reactive B and N centers results in an unprecedentently low kinetic barrier for both insertion into the sp(2)-C-H bond and intramolecular protonation of the sp(2)-C-B bond in 2-ammoniophenyl(aryl)- or -(alkenyl)borates. This common reactivity pattern serves as a platform for various catalytic reactions such as C-H borylation and hydrogenation of alkynes. In particular, we demonstrate that simple 2-aminopyridinium salts efficiently catalyze the C-H borylation of hetarenes with catecholborane. This reaction is presumably mediated by a borenium species isoelectronic to 2-aminophenylboranes.

Intramolecular Frustrated Lewis Pair with the Smallest Boryl Site: Reversible H2 Addition and Kinetic Analysis

Ansa-aminoborane 1 (ortho-TMP-C6H4-BH2; TMP = 2,2,6,6-tetramethylpiperid-1-yl), a frustrated Lewis pair with the smallest possible Lewis acidic boryl site (-BH2), is prepared. Although it is present in quenched forms in solution, and BH2 represents an acidic site with reduced hydride affinity, 1 reacts with H2 under mild conditions producing ansa-ammonium trihydroborate 2. The thermodynamic and kinetic features as well as the mechanism of this reaction are studied by variable-temperature NMR spectroscopy, spin-saturation transfer experiments, and DFT calculations, which provide comprehensive insight into the nature of 1.

Homo- and copolymerization of strained cyclic olefins with new palladium(II) complexes bearing ethylene-bridged heterodonor ligands

Amorphous and high molar mass polymers of norbornene and phenylnorbornene (endo/exo ratio of 80/20) were prepared by the use of new dicationic palladium(II) single-component catalysts of the general type [Pd(L∩L)(NCCH 3 ) 2 ](BF 4 ) 2 . [(L∩L) = 2-(diphenylarsino)-1-(methylthio)ethane (S∩As, 1) (4), 2-diphenylphosphino)-1-methylthio)ethane (S∩P, 2) (5), 1, 2-bis(diphenylphosphino)ethane (P∩P, 3) (6). With increasing the trans influence of the donor atom in the ligands (P>As>S) the polymerization activity of the catalysts towards polymerization of norbornene increases instantly. Both the molecular weight and the thermal behavior of the polymer can be tailored using ethene as chain transfer agent. Catalyst 4 is also active towards the alternating copolymerization of carbon monoxide and norbornene. The isolated copolymer is highly soluble in toluene, THF and chlorinated solvents. The DSC measurement of the norbornene-carbon monoxide copolymer showed a melting temperature (T m ) of 241°C (ΔH f = 47.3 J/g) and a glass transition temperature (T g ) of 161°C which is about 170°C lower compared to the homopolynorbornene (T g ≃ 330°C).

Palladium(II) complexes bearing ethylene-bridged S∩As and S∩P donor ligands: synthesis, crystal structure and reactivity towards the polymerization of norbornene

Abstract Palladium(II) complexes of the general type [Pd(L∩L)Cl2] (4–6) and their corresponding dicationic complexes [Pd(L∩L)(NCCH3)2](BF4)2 (7–9) (L∩L=[(1-(thiomethyl)-2-(diphenylarsino)ethane] (S∩As, 1) (4, 7) [1-(thiomethyl)-2-(diphenylphosphino)ethane] (S∩P, 2) (5, 8) and [1,2-bis(diphenylphosphino)ethane] (P∩P, 3) (6, 9) have been synthesized and utilized as catalysts for the polymerization reaction of bicyclo[2.2.1]hept-2-ene (norbornene). The dichloride complexes can be activated with methylaluminoxane (MAO), while the corresponding dicationic compounds were used as single component catalysts for the polymerization reactions. Catalytic performance of the new type of catalyst can be altered by modifying the donor atoms in the bidentate ligand. The activity of the Pd(II) complexes towards the polymerization reaction increases with increasing the trans-influence of the donor atoms (P∩P>S∩P>S∩As). The influence of donor atoms on the length of the PdCl bonds was elucidated by X-ray structure analysis carried out on the palladium(II) complex [Pd(S∩As)Cl2] (4).

Zirconocene-MAO-catalyzed homo- and copolymerizations of linear asymmetrically substituted dienes with propene: A novel strategy to functional (co)poly(α-olefin)s

The polymerization behavior of three linear asymmetrically substituted dienes, i.e. 6-phenyl-1,5-hexadiene (A), 7-methyl-1,6-octadiene (B) and R(+)-5,7-dimethyl-1,6-octadiene (isocitronellene, C) is reported in order to study the effect of substitution at one vinylic group. Homopolymerization of these monomers with the catalyst system rac-Et[Ind] 2 ZrCl 2 /MAO resulted in no reaction products in the case of monomer A and in polymers with M w = 3.5 kg/mol and M w = 14.0 kg/mol with the monomers B and C, respectively. According to NMR analysis, the vinylene end group of isocitronellene remained untouched during polymerization, which excludes the possibility of cyclopolymerization or crosslinking. Copolymerization of isocitronellene with propene resulted in incorporation of the diene (15.6 mol-%) equal to the stoichiometric ratio in the monomer feed, with a relatively high catalyst activity. The degree of incorporated isocitronellene is inversely related to the polymerization temperature, providing control over crystallinity (isotacticity) and molecular weight of the copolymer. The isocitronellene homo- and copolymers could be epoxidized quantitatively and brominated to an extent of 90%. Perfluorohexyl iodide was grafted onto the isocitronellene/propene copolymer by radical reaction (conversion 80%), yielding a poly(a-olefin) with fluorinated side chains.

A novel alkaline oxidation pretreatment for spruce, birch and sugar cane bagasse

Alkaline oxidation pretreatment was developed for spruce, birch and sugar cane bagasse. The reaction was carried out in alkaline water solution under 10 bar oxygen pressure and at mild reaction temperature of 120-140°C. Most of the lignin was solubilised by the alkaline oxidation pretreatment and an easily hydrolysable carbohydrate fraction was obtained. After 72 h hydrolysis with a 10 FPU/g enzyme dosage, glucose yields of 80%, 91%, and 97%, for spruce, birch and bagasse, respectively, were achieved. The enzyme dosage could be decreased to 4 FPU/g without a major effect in terms of the hydrolysis performance. Compared to steam explosion alkaline oxidation was found to be significantly better in the conditions tested, especially for the pretreatment of spruce. In hydrolysis and fermentation at 12% d.m. consistency an ethanol yield of 80% could be obtained with both bagasse and spruce in 1-3 days.

The Role of Salts and Brønsted Acids in Lewis Acid-Catalyzed Aqueous-Phase Glucose Dehydration to 5-Hydroxymethylfurfural

The effect of salts and Brønsted acids on the Lewis acid (CrCl3⋅6 H2O)‐catalyzed glucose dehydration to 5‐hydroxymethylfurfural (HMF) in aqueous media are described. We show that the reaction with bromide salts in place of chlorides leads to higher HMF yields. The influence of salts can be attributed to the anions in solution, specifically to the bromide anions enhancing the fructose dehydration step. Additionally, we demonstrate that the reaction kinetics are governed strongly by acidity. Although the fructose dehydration step is accelerated by the addition of Brønsted acids, even on a catalytic scale, a significant retardation of the glucose conversion rate results in a substantial drop in HMF yields. The suppression in glucose‐to‐fructose isomerization rate with increasing acidity is ascribed to the restrained formation of the chromium–glucose chelate complex during the reaction.