Clark R. Landis

All-Electron Scalar Relativistic Basis Sets for Third-Row Transition Metal Atoms.

A family of segmented all-electron relativistically contracted (SARC) basis sets for the elements Hf-Hg is constructed for use in conjunction with the Douglas-Kroll-Hess (DKH) and zeroth-order regular approximation (ZORA) scalar relativistic Hamiltonians. The SARC basis sets are loosely contracted and thus offer computational advantages compared to generally contracted relativistic basis sets, while their sufficiently small size allows them to be used in place of effective core potentials (ECPs) for routine studies of molecules. Practical assessments of the SARC basis sets in DFT calculations of atomic (ionization energies) as well as molecular properties (geometries and bond dissociation energies for MHn complexes) confirm that the basis sets yield accurate and reliable results, providing a balanced description of core and valence electron densities. CCSD(T) calculations on a series of gold diatomic compounds also demonstrate the applicability of the basis sets to correlated methods. The SARC basis sets will be of most utility in calculating molecular properties for which the core electrons cannot be neglected, such as studies of electron paramagnetic resonance, Mössbauer and X-ray absorption spectra, and topological analysis of electron densities.

Natural bond orbital methods

Natural bond orbital (NBO) methods encompass a suite of algorithms that enable fundamental bonding concepts to be extracted from Hartree‐Fock (HF), Density Functional Theory (DFT), and post‐HF computations. NBO terminology and general mathematical formulations for atoms and polyatomic species are presented. NBO analyses of selected molecules that span the periodic table illustrate the deciphering of the molecular wavefunction in terms commonly understood by chemists: Lewis structures, charge, bond order, bond type, hybridization, resonance, donor–acceptor interactions, etc. Upcoming features in the NBO program address ongoing advances in ab initio computing technology and burgeoning demands of its user community by introducing major new methods, keywords, and electronic structure system/NBO communication enhancements.

NBO 6.0: natural bond orbital analysis program.

We describe principal features of the newly released version, NBO 6.0, of the natural bond orbital analysis program, that provides novel “link‐free” interactivity with host electronic structure systems, improved search algorithms and labeling conventions for a broader range of chemical species, and new analysis options that significantly extend the range of chemical applications. We sketch the motivation and implementation of program changes and describe newer analysis options with illustrative applications.

NATURAL BOND ORBITALS AND EXTENSIONS OF LOCALIZED BONDING CONCEPTS

We provide a brief overview of “natural” localized bonding concepts, as implemented in the current natural bond orbital program (NBO 5.0), and describe recent extensions of these concepts to transition metal bonding. [Chem. Educ. Res. Pract. Eur.: 2001, 2, 91-104]

Enantioselective Hydroformylation of N-Vinyl Carboxamides, Allyl Carbamates, and Allyl Ethers Using Chiral Diazaphospholane Ligands

Rhodium complexes of diazaphospholane ligands catalyze the asymmetric hydroformylation of N-vinyl carboxamides, allyl ethers, and allyl carbamates; products include 1,2- and 1,3-aminoaldehydes and 1,3-alkoxyaldehydes. Using glass pressure bottles, short reaction times (generally less than 6 h), and low catalyst loading (commonly 0.5 mol %), 20 substrates are successfully converted to chiral aldehydes with useful regioselectivity and high enantioselectivity (up to 99% ee). Chiral Roche aldehyde is obtained with 97% ee from the hydroformylation of allyl silyl ethers. Commonly difficult substrates such as 1,1- and 1,2-disubstituted alkenes undergo effective hydroformylation with 89-97% ee and complete conversion for six examples. Palladium-catalyzed aerobic oxidative amination of allyl benzyl ether followed by enantioselective hydroformylation yields the β(3)-aminoaldehyde with 74% ee.

Origin of Pressure Effects on Regioselectivity and Enantioselectivity in the Rhodium-Catalyzed Hydroformylation of Styrene with (S,S,S)-BisDiazaphos

Gas pressure influences the regioselectivity and enantioselectivity of aryl alkene hydroformylation as catalyzed by rhodium complexes of the BisDiazaphos ligand. Deuterioformylation of styrene at 80 degrees C results in extensive deuterium incorporation into the terminal position of the recovered styrene. This result establishes that rhodium hydride addition to form a branched alkyl rhodium occurs reversibly. The independent effect of carbon monoxide and hydrogen partial pressures on regioselectivity and enantioselectivity were measured. From 40 to 120 psi, both regioisomer (b:l) and enantiomer (R:S) ratios are proportional to the carbon monoxide partial pressure but approximately independent of the hydrogen pressure. The absolute rate for linear aldehyde formation was found to be inhibited by carbon monoxide pressure, whereas the rate for branched aldehyde formation is independent of CO pressure up to 80 psi; above 80 psi one observes the onset of inhibition. The carbon monoxide dependence of the rate and enantioselectivity for branched aldehyde indicates that the rate of production of (S)-2-phenyl propanal is inhibited by CO pressure, while the formation rate of the major enantiomer, (R)-2-phenyl propanal, is approximately independent of CO pressure. Hydroformylation of alpha-deuteriostyrene at 80 degrees C followed by conversion to (S)-2-benzyl-4-nitrobutanal reveals that 83% of the 2-phenylpropanal resulted from rhodium hydride addition to the re face of styrene, and 83% of the 3-phenylpropanal resulted from rhodium hydride addition to the si face of styrene. On the basis of these results, kinetic and steric/electronic models for the determination of regioselectivity and enantioselectivity are proposed.

Highly enantioselective hydroformylation of aryl alkenes with diazaphospholane ligands.

Asymmetric, rhodium-catalyzed hydroformylation of terminal and internal aryl alkenes with diazaphospholane ligands is reported. Under partially optimized reaction conditions, high enantioselectivity (>90% ee) and regioselectivities (up to 65:1 alpha:beta) are obtained for most substrates. For terminal alkenes, both enantioselectivity and regioselectivity are proportional to the carbon monoxide partial pressure, but independent of hydrogen pressure. Hydroformylation of para-substituted styrene derivatives gives the highest regioselectivity for substrates bearing electron-withdrawing substituents. A Hammett analysis produces a positive linear correlation for regioselectivity.

Stopped-Flow NMR: Determining the Kinetics of (rac-(C2H4(1-indenyl)2)ZrMe)(MeB(C6F5)3)-Catalyzed Polymerization of 1-Hexene by Direct Observation

Stopped-flow NMR measurements suitable for determination of reaction kinetics on time scales of 100 ms or longer have been achieved by adaptation of a commercial NMR flow probe with a high-efficiency mixer and drive system. Studies of metallocene-catalyzed alkene polymerization at room temperature have been complicated by high rates, imprecise knowledge of the distribution of different catalyst species with time, and the high sensitivity of the catalysts to low concentrations of impurities. Application of the stopped-flow NMR method to the study of the kinetics of 1-hexene polymerization in the presence of (EBI)ZrMe[MeB(C(6)F(5))(3)] demonstrates that NMR spectroscopy provides an efficient method for direct and simultaneous measurement of substrate consumption and catalyst speciation as a function of time. Kinetic modeling of the catalyst and substrate concentration time courses reveal efficient determination of initiation, propagation, and termination rate constants. As first suggested by Collins and co-workers (Polyhedron 2005, 24, 1234-1249), a kinetic model in which Zr-HB(C(6)F(5))(3) forms rapidly upon beta-hydride elimination but reacts relatively slowly with alkene to reinitiate chain growth is supported by these data.

Regioselective rhodium-catalyzed hydroformylation of 1,3-dienes to highly enantioenriched β,γ-unsaturated aldehyes with diazaphospholane ligands.

Regioselective and enantioselective rhodium-catalyzed hydroformylation of 1,3-dienes with chiral bisdiazaphospholane ligands yields β,γ-unsaturated aldehydes that retain a C═C functionality for further conversion. The reaction conditions are mild, featuring low catalyst loadings (0.5 mol %), pressures readily obtained in glass bottles, and convenient reaction times (1.5-12 h). Optimized reaction conditions produce high enantioselectivity (>90% ee), regioselectivity (88-99%), and conversion to β,γ-unsaturated aldehydes (99%) for ten 1,3-dienes encompassing a variety of substitution patterns.

Cationic rhodium hydrogenation catalysts containing chelating diphosphine ligands: effect of chelate ring size

Abstract Earlier studies on the [(1,2-bis(diphenylphosphino)ethane)rhodium] p+ -catalyzed hydrogenation of 1-hexene and methyl-( Z )-α-acetamidocinnamate have been extended to catalysts containing larger chelating diphosphine ligands, i.e., Ph 2 P(CH 2 ) n PPh 2 , where n = 3, 4 and 5. Comparisons include measurements of equilibrium constants for the binding of the olefinic substrates to the catalysts and of the catalytic hydrogenenation rates. Some related measurements also are reported for the corresponding catalyst systems containing the chiral ligand, 4R,5R-bis(diphenylphosphinomethyl)-2,2,-dimethyldioxalane (DIOP) and non-chelating PPh 3 ligands.