Markus Pernpointner

A New Insight into Gold(I)‐Catalyzed Hydration of Alkynes: Proton Transfer

Hydration and alkoxylation of alkynes are important reactions in organic synthesis, for which mercury(II) salts initially served as catalysts. Due to substantial practical aspects, such as the requirement of strongly acidic reaction media, the quick denaturation of the Hg catalyst and environmental concerns, alternative catalysts were sought for quite some time. In particular, for the addition of water and methanol to unactivated alkynes, Au3] and Pt-containing catalysts 5] were investigated as an alternative but turned out to be less efficient. In 1998, Teles et al. reported a very efficient method for the alkoxylation of alkynes using cationic Au complexes of the type [(PR3)Au] + . In dry methanol, acetals D and E were obtained, whereas in the presence of water, ketones B and C were the only products (Scheme 1). In the case of terminal alkynes, the Markovnikov products were formed with very high selectivity whereas internal alkynes led to product mixtures. Subsequent work by Tanaka et al. and Nolan et al. even further improved that reaction.

Fully Relativistic, Comparative Investigation of Gold and Platinum Alkyne Complexes of Relevance for the Catalysis of Nucleophilic Additions to Alkynes.

For a range of additions to alkynes gold is known to exhibit a much higher catalytic activity than a corresponding platinum compound. In order to approach the origin of this behavior we first investigate the propyne activation by the gold and platinum catalysts AuCl3 and PtCl2(H2O) where both metals possess a d(8) electron configuration and where the catalysts exhibit similar steric effects. Propyne serves as a representative for alkynes. Fully relativistic ab initio calculations of these alkyne-catalyst complexes are presented at the Dirac-Hartree-Fock self-consistent field (DHF-SCF), density functional theory (DFT/B3LYP), and Greens function (GF) level in order to properly account for the large relativistic effects of gold and platinum. For the alkyne/catalyst complexes both the perpendicular and in-plane conformations were studied as these possess very similar ground state energies and may easily transform into each other. Strongly varying orbital populations together with sizable energetic and structural differences of the frontier orbitals are found and can be considered as a major source of the differing catalytic activity. These mainly comprise vanishing LUMO densities at the carbon centers in the platinum complex together with increased LUMO energies making a nucleophilic attack harder than in the gold compound. As Greens function calculations show, DFT/B3LYP seems to overestimate correlation contributions leading to an unphysical energetic lowering of many unoccupied orbitals.

A point-charge model for the nuclear quadrupole moment: Coupled-cluster, Dirac–Fock, Douglas–Kroll, and nonrelativistic Hartree–Fock calculations for the Cu and F electric field gradients in CuF

A point charge model for the nuclear quadrupole moment tensor (PCNQM) is developed in order to determine accurate electric field gradients (EFG) at the relativistic and correlated levels. The symmetric s contributions arising from the Poisson equation are avoided by using an appropriate point charge distribution in three-dimensional space. It is shown that the PCNQM model yields virtually the same EFGs compared to the conventional method of expectation values, if the point charges are set at small displacements from the nucleus (d<10−13 m) and the SCF energy is converged out to 12 significant figures. We further demonstrate that the choice of the point charge ζ is not very critical to the PCNQM perturbation, and that the correlation energy at both the nonrelativistic and relativistic level of theory depends linearly on ζ. This suggests that accurate EFG tensors can be obtained by performing only two correlated calculations for each atom and tensor component. The PCNQM model is tested on one-electron atoms...

Are β‐H‐Eliminations or Alkene Insertions Feasible Elementary Steps in Catalytic Cycles Involving Gold(I) Alkyl Species or Gold(I) Hydrides?

The β-H-elimination in the (iPr)AuEt complex and its microscopic reverse, the insertion of ethene into (iPr)AuH, were investigated in a combined experimental and computational study. Our DFT-D3 calculations predict free-energy barriers of 49.7 and 36.4 kcal mol(-1) for the elimination and insertion process, respectively, which permit an estimation of the rate constants for these reactions according to classical transition-state theory. The elimination/insertion pathway is found to involve a high-energy ethene hydride species and is not significantly affected by continuum solvent effects. The high barriers found in the theoretical study were then confirmed experimentally by measuring decomposition temperatures for several different (iPr)Au(I) -alkyl complexes which, with a slow decomposition at 180 °C, are significantly higher than those of other transition-metal alkyl complexes. In addition, at the same temperature, the decomposition of (iPr)AuPh and (iPr)AuMe, both of which cannot undergo β-H-elimination, indicates that the pathway for the observed decomposition at 180 °C is not a β-H-elimination. According to the calculations, the latter should not occur at temperatures below 200 °C. The microscopic reverse of the β-H-elimination, the insertion of ethene into the (iPr)AuH could neither be observed at pressures up to 8 bar at RT nor at 1 bar at 80 °C. The same is true for the strain-activated norbornene.

The accuracy of current density functionals for the calculation of electric field gradients: A comparison with ab initio methods for HCl and CuCl

The performance of current density functionals is analyzed in detail for the electric field gradients (EFG) of hydrogen chloride and copper chloride by comparison with ab initio methods and available experimental data. The range of density functionals applied shows good agreement with coupled cluster H and Cl field gradients for HCl, as has been demonstrated previously for other main-group element containing compounds. However, the performance of most density functionals is very poor for the Cu EFG in CuCl (EFG for Cu -0.44 a.u. at the coupled-cluster singles and doubles with perturbative triples [CCSD(T)] level, compared to, e.g., +0.54 a.u. at the B-LYP level). Only the “half-and-half” hybrid functionals give field gradients with the correct sign. The reason for the poor performance of the density functional theory is analyzed in detail comparing density functional with ab initio total electronic densities ρ(r). Due to the conservation of the number of particles, a change in the valence part of the elec...

Accurate nuclear quadrupole moments of the gallium isotopes 69Ga and 71Ga within the PCNQM model

Abstract Accurate non-relativistic, Douglas–Kroll and Dirac–Fock correlated calculations of the electric field gradient of 69 Ga and 71 Ga in GaF are presented using the point-charge model of the nuclear quadrupole moment (PCNQM). The resulting nuclear quadrupole moments of 165.0(8) mb for 69 Ga and 104.0(8) mb for 71 Ga should be more accurate than the current standard values. Since the PCNQM method allows for an arbitrary complexity of the wavefunction without calculating the corresponding expectation values the error of the picture change using the Douglas–Kroll wavefunction can be estimated.

A Theoretical DFT‐Based and Experimental Study of the Transmetalation Step in Au/Pd‐Mediated Cross‐Coupling Reactions

In this work a combined theoretical and experimental investigation of the cross-coupling reaction involving two metallic reaction centers, namely gold and palladium, is described. One metal center (Au) hereby is rather inert towards change in its oxidation state, whereas Pd undergoes oxidative insertion and reductive elimination steps. Detailed mechanistic and energetic studies of each individual step, with the focus on the key transmetalation step are presented and compared for different substrates and ligands on the catalytic Pd center. Different aryl halides (Cl, Br, I) and aryl triflates were investigated. Hereby the nature of the counteranion X turned out to be crucial. In the case of X=Cl and L=PMe3 the oxidative addition is rate-determining, whereas in the case of X=I the transmetalation step becomes rate-determining in the Au/Pd-cross-coupling mechanism. A variety of Au-Pd transmetalation reaction scenarios are discussed in detail, favoring a transition state with short intermetallic Au-Pd contacts. Furthermore, without a halide counteranion the transmetalation from gold(I) to palladium(II) is highly endothermic, which confirms our experimental findings that the coupling does not occur with aryl triflates and similar weakly coordinating counteranions--a conclusion that is essential in designing new Au-Pd catalytic cycles. In combination with experimental work, this corrects a previous report in the literature claiming a successful coupling potentially catalytic in both metals with weakly coordinating counteranions.

The one-particle Green’s function method in the Dirac–Hartree–Fock framework. II. Third-order valence ionization energies of the noble gases, CO and ICN

In this paper we present the third-order extension of the four-component one-particle propagator method in the non-Dyson version of the algebraic diagrammatic construction (ADC) for the calculation of valence ionization energies. Relativistic and electron correlation effects are incorporated consistently by starting from the Dirac-Hamiltonian. The ADC equations derived from the Feynman diagrams can hereby be used in their spin-orbital form and need not be transformed to the spin-free version as required for a nonrelativistic treatment. For the calculation of the constant self-energy contribution the Dyson expansion method was implemented being superior to a perturbational treatment of sigma(infinity). The Dirac-Hartree-Fock- (DHF-) ADC(3) was applied to the calculation of valence photoionization spectra of the noble gas atoms, carbon monoxide and ICN now also reproducing spin-orbit features in the spectrum. Comparison with DHF-ADC(2), nonrelativistic ADC(3), and experimental data was made in order to demonstrate the characteristics and performance of the method.

Nuclear quadrupole moments for Al-27 and Ga-69 derived from four-component molecular coupled cluster calculations

In this work we investigate different approaches for calculating electric field gradients in order to provide accurate theoretical values for the nuclear quadrupole moments (NQM) for aluminum and gallium. Electron correlation is included in a fully four-component framework at the CCSD(T) level. The resulting NQM for 27Al (146.0±0.4 mb) is in good agreement with earlier work, while the value for 69Ga (171±2 mb) is higher than suggested on basis of previous molecular calculations.

Application of the scaled-opposite-spin approximation to algebraic diagrammatic construction schemes of second order

With the concept of scaled-opposite-spin (SOS), a pragmatic semi-empirical approximation has been introduced to the extended algebraic diagrammatic construction scheme of second order (ADC(2)-x) that leads to a significant saving in computational effort. The parameters included were fitted with respect to a benchmark set of electronically excited states in standard organic molecules that include some doubly-excited states, as well. Like the original, unscaled ADC(2)-x scheme it can be used to identify electronically excited states with high double excitation character, however at reduced computational cost. At the same time, it is possible to reduce the overestimation of doubly-excited configurations that is inherent to ADC(2)-x. Additionally, a scheme for the strict variant (ADC(2)-s) was derived directly from SOS-MP2 by application of the intermediate state formalism and compared to an existing version of SOS-ADC(2)-s.