Wim Buijs

Three dinuclear copper(II) carboxylates with the paddle-wheel cage structure as intermediates in copper(II) catalyzed oxidations of carboxylic acids. X-ray crystal structures of [tetrakis(diphenyl acetato-μ-O,O′)bis(acetonitrile-N)dicopper(II)] tetrakis(acetonitrile), [] and [tetrakis(1-phenyl-1-carboxylato-μ-O,O′-cyclopentane)-bis(ethanol-O)dicopper(II)] bis (ethanol)

Abstract The synthesis spectroscopic magnetic and structural characterization of three new dinuclear copper(II) carboxylates are described. All compounds exhibit the typical dinuclear paddle-wheel type structure, consisting of two copper(II) ions in square-pyramidal geometry bridged by four carboxylate anions in the xy plane and O- or N-donor ligands at the apex. [tetrakis(diphenyl acetato-μ-O,O′)bis(acetonitrile-N dicopper(II)tetrakis(acetonitrile) (1, CnhCu2N2O8, crystallizes in the triclinic space group P 1 (No.2) with a=10.8941(17) b=11.140), c=13.346(3) A , α=69.9271(17), β=78.658(16), γ=86.819(14)°= Z=1, R=0.0438 and R w =0.0479 for 5188 observed reflections. The −2J value of this compounds is 330(17)cm−1, the triplet state exhibiting a rhombic zero-field splitting with D=0.353(11 cm1 and E=0.0022(2) cm−1, with g=2.03(1), g=2.05(1) and g=2.34(1). The compound [tetrakis(diphenylacetato-μ-O,O′)bis(acetone-O)dicopper(II)] (2). C62H50Cu2O10 crystallizes in the triclinic space group Pt (No. 2). The cell dimensions are a=10.8722(6), b=11.4003(8), c=11.6856(10) A , α=94.216(6), β=104.859(6), γ=111.193(5)°, Z=1, R1=0.0325 and wR2=0.0795 . The −2J value is 303(15) cm1, the triplet state exhibiting a rhombic zero-field splitting with D=0.345(10)cm−1 and E=0.0004cm1, with g1=2.06(1), g1=2.33(1) and g1=2.33(1). The dinuclear compound [tetrakis(1-phenyl-1-carboxylato-μ-O,O′-cyclopentane)bis(ethanol-O)dicopper(II)] bis(ethanol) (3). C56H76Cu2O12, crystallizes in the triclinic space group P 1 (No. 2). The cell dimensions for 3 are a=10.1013(6), b=11.6013(6), c=12.7596(5) A , α=64.908(4), β=70.116(4), γ=83.079(5)°, Z=1, R=0.0311 and R w =0.0253 . The structure forms an extended chain by hydrogen bonding from the axial ethanol, via a lattice ethanol to a carboxylate oxygen of another dicopper unit. The relatively small −2J value of this compound is 284(14) cm−1, the triplet state exhibiting a rhombic zero-field splitting with [D]=0.346(10) cm1 and relatively large [E]=0.0058 cm1, g1=2.04, g1=2.04 and g1=2.33, A1=0.065cm−1.

Structural and mechanistic studies of the copper(II)-assisted ortho-hydroxylation of benzoates by trimethylamine N-oxide

N-benzoyl-2-methylalanine (H2L 1 )i sortho-hydroxylated stereoselectively by trimethylamine N-oxide (TMAO) in the presence of copper(II). The experimental structure of [Cu(L 1 )(TMAO)2] suggests that the oxygen transfer agent TMAO transfers the oxygen atom to copper(II), and (L 1 ) 2 − , coordinated to copper(II) by a carboxylate oxygen and the amide nitrogen donor, is well pre-organized for an oxygen transfer from copper to the ortho carbon atom of the benzene ring. Product analyses as a function of reaction time of the copper(II)-mediated ortho-hydroxylation reaction with H2L 1 and various derivatives support the suggestion of a reactive copper-oxo or copper-hydroxo intermediate, stabilized by a five-membered chelate with hard carboxylate and N-amide donors. The analysis also suggests that there is a pre-equilibrium with a Cu:L = 1:1 ratio, and this might involve Cu/L 2 − /TMAO or dicopper complexes. Depending on the ligand H2L, complexation with the salicylate product may inhibit the ortho-hydroxylation reaction.

Correlation between the reactivities and the computed conformational and electronic properties of N,S,O-mixed-donor crown ethers

The nucleophilic coupling of the secondary nitrogen atoms of the mixed-donor crown ethers [16]aneS4N, [15]aneS4N, [15]aneO4N, [15]aneO2S2N and [12]aneS3N to an epoxide proceeds in an unexpected manner. The degree of coupling appears to be inversely related to the number of thioethers in the crown ether. Two possible explanations were considered: (i) steric factors affecting the accessibility of the lone pair on the secondary nitrogen and (ii) electronic factors affecting the nucleophilicity of the nitrogen. In order to discriminate between these possibilities, molecular modeling was applied. Firstly, suitable conformers of the mixed-donor crown ethers were generated by a conformational search on a molecular mechanics level (MM3). Next, these conformers underwent a (semi-empirical) PM3 geometry optimisation to obtain information on the electronic properties. Results of the molecular-modeling study indicate that both steric and electronic factors work hand-in-hand, rendering a plausible explanation for the observed phenomena: with an increasing number of thioethers in the crown ether, the orientation of the nitrogen lone pair becomes directed more towards the inside of the macrocycle so that the accessibility for reaction is decreased. Apart from this, the nucleophilicity of the secondary nitrogen is also reduced seriously with increasing number of thioethers, as can be deduced from the location, size and level of the HOMOs. These are located mainly on the sulfur atoms in the (oxa)azathia crown ethers, thereby lowering the reactivity of the nitrogen atom. In the case of [15]aneO4N, the HOMO is located on the nitrogen lone pair only, which provides an explanation for the observed higher reactivity of this macrocycle.

Copper-catalyzed oxidative decarboxylation of aliphatic carboxylic acids

Abstract The copper(11)-catalyzed oxidative decarboxylation of aliphatic carboxylic acids proceeds through an initial inner sphere one- or two-electron transfer, yielding a carboxyl radical or cation respectively. The carboxyl radical resulting from one-electron transfer rapidly decarboxylates, after which the aliphatic radical may react with dioxygen -if present- to a ketone, alcohol or aldehyde. This occurs by Cu(1,11) or H+-catalyzed decomposition of the intermediate (hydro) peroxide. The aliphatic radical can also be oxidized by mononuclear Cu(11) to a carbocation, which yields an ester by reaction with a carboxylic acid (anion), or an alkene by p-H elimination. The carboxylate cation, formed by initial two-electron transfer, can perform an electrophilic attack on an x-C of another carboxylic acid, yielding a new carboxylic acid with an ester group. It can also undergo p-H elimination, yielding an alkene carboxylic acid.

The Role of the Apical Donor in the Decomposition of Copper(II) Benzoate under DOW-Phenol Conditions

The synthesis and characterization of copper(II) benzoates with the apical donors pyridine, 2-CH3-pyridine, 2,4-(CH3)2-pyridine, 2,6-(CH3)2-pyridine, 2-fluoropyridine, 2-chloropyridine, 2-bromopyridine, 3-bromopyridine, 2,5-dibromopyridine, 3,5-dibromopyridine, and aniline, starting from copper(II) benzoate, is reported. Single-crystal X-ray structures of the products with four apical ligands show the usual paddle-wheel structure of copper(II) carboxylates; in the case of aniline no paddle-wheel dicopper(II) benzoate could be isolated. The products of thermal decomposition of the pure copper(II) compounds were analyzed by HPLC, LC-MS, and GCFID, and the expected DOW-phenol products were found in all cases other than that of aniline. This supports the assumption that a paddle-wheel dicopper(II) benzoate is required for the DOW-phenol reaction. Generally, high ortho-selectivities (to phenyl benzoate and phenol; the selectivity increases with increasing basicity) are obtained, in good agreement with earlier findings on the role of the base. Small but significant steric effects are observed in the series of methylated pyridine donors and the monohalogenated pyridine donors used as apical ligands; with the two dibromopyridine donors there are large steric effects and the DOW-phenol reaction is partially suppressed. With halogenated pyridine donors as apical ligands, a Cu[I]-catalyzed process occurs, leading to dehalogenation.

School Chemistry vs. Chemistry in Research: An Exploratory Experiment.

This report describes a study which explores, from the out-of-school student viewpoint, why students are not studying chemistry anymore. In a 2-day stay at a research institution three groups of graduating high school students from different schools, together with their chemistry teacher, were confronted hands-on with molecular modeling in industry and in university. Each of these volunteer students had agreed to write an essay on “School Chemistry Vs. Chemistry in Research.” These essays were evaluated together by the students, the teacher, and the researcher in a meeting at their school. The opinion of the students show that school chemistry does not convey todays chemistry in research and in industry. At the computer screen the students demonstrated their skill in performing molecular modeling experiments. Moreover, at the computer screen, chemistry was fun and easier to understand. Now we begin to see the solution: our students are also our teachers.

Catalytic Dehalogenation of Halogenated Pyridine Donors under DOW-Phenol Conditions

Various halogenated pyridines (2-fluoropyridine, 2-chloropyridine, 2-bromopyridine, 3-bromopyridine, 2,5-dibromopyridine, and 3,5-dibromopyridine) are subject to catalytic reduction and substitution under DOW-phenol conditions; copper(I) benzoate is shown to play a key role in these processes. In the absence of copper(I), the halogenated pyridines do not react with benzoic acid, while after precipitation of copper(I) halide the dehalogenation process stops. Marked differences are observed between the copper(I)-catalyzed dehalogenation of the halopyridines and the previously reported dehalogenation of halogenated aromatics. While the copper(I)-catalyzed substitution of haloaryl compounds by benzoate makes only a minor contribution to the overall dehalogenation process, substitution of halopyridine compounds is at least as important as the reduction reaction. Furthermore, the reaction with halopyridine compounds is regioselective, in contrast to that with halogenated aryl compounds.