Cornelis J. Elsevier

Selective Homogeneous Palladium(0)-Catalyzed Hydrogenation of Alkynes to (Z)-Alkenes.

Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)-alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO(2)Me; R(1), R(2)=4-OCH(3), 4-CH(3), 2,6-(CH(3))(2).

Hydrogenation of carboxylic acids with a homogeneous cobalt catalyst

A direct route from acids to alcohols Making alcohols via hydrogen addition to C=O bonds is among the most widely applied reactions in chemistry. The transformation has also garnered renewed interest for generating commodity chemicals from biomass. Korstanje et al. now show that a cobalt compound can catalyze hydrogenation of the C=O bonds in carboxylic acids. These constitute a particularly challenging substrate class, given the propensity of many other catalysts to degrade under acidic conditions. The cobalt catalyst tolerates a versatile substrate range, and the Earth abundance of the metal bodes well for long-term utility. Science, this issue p. 298 Earth-abundant cobalt catalyzes a broadly useful chemical conversion of acids to alcohols. The reduction of esters and carboxylic acids to alcohols is a highly relevant conversion for the pharmaceutical and fine-chemical industries and for biomass conversion. It is commonly performed using stoichiometric reagents, and the catalytic hydrogenation of the acids previously required precious metals. Here we report the homogeneously catalyzed hydrogenation of carboxylic acids to alcohols using earth-abundant cobalt. This system, which pairs Co(BF4)2·6H2O with a tridentate phosphine ligand, can reduce a wide range of esters and carboxylic acids under relatively mild conditions (100°C, 80 bar H2) and reaches turnover numbers of up to 8000.

Influence of ligands and anions on the insertion of alkenes into palladium-acyl and palladium-carbomethoxy bonds in the neutral complex (dppp)Pd(C(O)CH3)Cl and the ionic complexes [(PP)PdR(L)]+SO3CF3− (PP = dppe, dppp, dppb; R C(O)CH3, L CH3CN, PPh3; R C(O)OCH3, L PPh3)

Abstract Insertions of alkenes in Pd-acetyl bonds of (dppp)Pd(C(O)CH3)Cl and [(PP)Pd(C(O)CH3)L)]+ SO3CF3− (PP = dppe, dppp, dppb; L  CH3CN, PPh3) have been studied as a function of the ligand, the anion and the alkene. The neutral acetyl complex (dppp)Pd(C(O)CH3)Cl underwent insertion only with norbornadiene and norbornene, while the ionic acetyl complexes [(PP)Pd(C(O)CH3)(L)]+ SO3CF3− (PP = dppe, dppp, dppb) reacted with norbornadiene, norbornene, styrene, cis-stilbene, 1-pentene, 3,3-dimethyl-1-butene, vinyltrimethylsilane, methyl vinyl ketone, methyl acrylate, diethyl fumarate, and diethyl maleate. The insertion was observed to give an intermediate in which there was intramolecular coordination of the ketone oxygen atom to the palladium centre. In monosubstituted alkenes the acetyl group migrates to the unsubstituted carbon atom. The insertion products underwent β-elimination to give (trans) unsaturated ketones and a palladium hydride. The rate of this elimination was higher for complexes containing ligands LL with smaller bite angles (dppe > dppp), and the rate of insertion showed the reverse order. The carbomethoxy complexes [(PP)Pd(C(O)OCH3)(PPh3)]+ SO3CF3− (PP = dppe, dppp, dppb) were prepared from (PP)Pd(SO3CF3)2 with CO, CH3OH and PPh3. The carbomethoxy complex reacted with norbornadiene to give a carbomethoxy oxgyen-coordinated intermediate. The carbomethoxy complexes were less reactive than the analogous acetyl complexes towards alkenes.

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo- and stereoselectivity when using a zerovalent palladium(NHC)(maleic anhydride)-complex as precatalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuterium-labeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR, we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations, we propose a mechanism in which hydrogen transfer from coordinated formate anion to zerovalent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.

Palladium complexes containing rigid bidentate nitrogen ligands as catalysts for carboncarbon bond formation

Abstract Zerovalent Pd(Ar-BIAN)(dimethyl fumarate) and divalent PdCl2(Ar-BIAN) complexes containing the rigid bidentate nitrogen ligand bis(arylimino)acenaphtene (Ar-BIAN; Ar = C6H5,p-MeC6H4,p-MeOC6H4) are efficient catalysts for the cross coupling reaction of various organic halides (including acyl-, allyl-, aryl-, benzyl-, vinyl- and 1,2-dienylhalides) with organomagnesium, -zinc and -tin reagents. Coupling reactions of organic halides with one equivalent of organomagnesium and -zinc reagents, in the presence of 1 mol % of a Pd(Ar-BIAN) catalyst, generally proceed smoothly in THF at 20 °C, giving complete conversion of the starting halide within 1–16 hours. Good isolated yields of carbon-carbon coupled products are obtained and the ratio cross/homo coupling varies between 98/2 and 0/100, depending on the substrates used. Reactions employing organotin reagents proceed best in DMF or HMPA and need longer reaction times and/or higher temperatures, as compared to organomagnesium and -zinc reagents, to go to completion. The selectivity for cross coupling is high (generally >99 %) and high isolated yields of cross coupled products can be obtained. In the presence of carbon monoxide (1–5 bar) ketones can be formed with excellent selectivity and in good yields, as was demonstrated for the carbonylative coupling of benzyl bromide with tetramethyltin or (p-tolyl)trimethyltin. Comparison of some Pd(Ar-BIAN) catalyzed reactions with Pd-phosphine catalyzed reactions reveals that these reactions complement each other, for example, when a Pd(Ar-BIAN) catalyst was employed, the coupling of 2-methylallyl chloride with phenyltributyltin was much faster, whereas the coupling of iodobenzene with vinyltrimethyltin was much slower, as compared to the Pd(PPh3)n catalyzed reactions.

Platinum catalysed 3,4- and 1,4-diboration of α,β-unsaturated carbonyl compounds using bis-pinacolatodiboron

Bis-pinacolatodiboron reacts with α,β-unsaturated carbonyl compounds to give 1,4- and unprecedented 3,4-additions in the presence of a second generation Pt(0) catalyst at ambient temperature.

Supercritical carbon dioxide as solvent and temporary protecting group for rhodium-catalyzed hydroaminomethylation.

Supercritical carbon dioxide (scCO2) acts simultaneously as solvent and temporary protecting group during homogeneously rhodium-catalyzed hydroaminomethylation of ethyl methallylic amine. Cyclic amines are formed as the major products in scCO,, whereas the cyclic amide is formed preferentially in conventional solvents. Multinuclear high-pressure NMR spectroscopy revealed that this selectivity switch is mainly due to reversible formation of the carbamic acid in the solvent CO2, which reduces the tendency for intramolecular ring closure at the Rh-acyl intermediate. These results substantiate the general concept of using scCO2 as a protective medium for amines in homogeneous catalysis and demonstrate for the first time its application for selectivity control.

Biscarbene palladium(II) complexes. reactivity of saturated versus unsaturated N-heterocyclic carbenes.

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by (1)H and (13)C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C((saturated NHC)) is distinct from that of Pd-C((unsaturated NHC)). The Pd-C((saturated NHC)) bonds are fairly stable toward reagents such as CF(3)COOH, AgBF(4) and I(2), whereas Pd-C((unsaturated NHC)) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)(2)PdCl(2) > (sat-NHC)(unsat-NHC)PdCl(2 )> (unsat-NHC)(2)PdCl(2).

Catalytic and stoichiometric C-C bond formation employing palladium compounds with nitrogen ligands

Abstract Carbon–carbon bond formation reactions catalyzed by palladium compounds containing bidentate nitrogen ligands is treated, with a focus on rigid ligands such as the bis(imino)acenaphthenes (bian) and bis(imino)phenanthrenes (bip), and ligands with larger bite-angles such as diazafluorenes and derivatives thereof. The emphasis is on recent work in this area by the author, but reference is made to relevant early and recent studies by others. After introducing the topic and the specific nitrogen ligands involved, several stoichiometric and catalytic C–C coupling protocols are treated. These involve intermediate palladium compounds in zero- di- and tetravalent oxidation states, which have been isolated and identified. Similarities and differences between catalysis by palladium compounds containing bidentate nitrogen ligands and systems containing mono- or bidentate phosphine ligands have been exemplified.

A Well-Defined Pd Hybrid Material for the Z-Selective Semihydrogenation of Alkynes Characterized at the Molecular Level by DNP SENS.

Direct evidence of the conformation of a Pd-N heterocyclic carbene (NHC) moiety imbedded in a hybrid material and of the Pd-NHC bond were obtained by dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) at natural abundance in short experimental times (hours). Overall, this silica-based hybrid material containing well-defined Pd-NHC sites in a uniform environment displays high activity and selectivity in the semihydrogenation of alkynes into Z-alkenes (see figure).