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Bernd F Straub

Publications and source records attributed to Bernd F Straub.

7 recordsLinked to original sources
PubMed2005

Mechanism of enyne metathesis catalyzed by Grubbs ruthenium-carbene complexes: a DFT study.

The complete catalytic cycle of the reaction of alkenes and alkynes to dienes by Grubbs ruthenium carbene complexes has been modeled at the B3LYP/LACV3P**+//B3LYP/LACVP level of theory. The core structures of the substrates and the catalyst were used as models, namely, ethene, ethyne, hept-1-en-6-yne, (Me(3)P)(2)Cl(2)Ru=CH(2), and [C(2)H(4)(NMe)(2)C](Me(3)P)Cl(2)Ru=CH(2). Insight into the electronically most preferred mechanistic pathways was gained for both intermolecular as well as for intramolecular enyne metathesis. Alkene metathesis is predicted to proceed fast and reversible, while the insertion of the alkyne substrate is slower, irreversible, and kinetically regioselectivity determining. Ruthenacyclobut-2-ene structures do not exist as local minima in the catalytic cycle. Instead, vinylcarbene complexes are formed directly. The alkyne insertion step and the cycloreversion of 2-vinyl ruthenacyclobutanes feature comparable predicted overall barriers in intermolecular enyne metathesis. For intramolecular enyne metathesis, a noncyclic alkene fragment of the enyne substrate is first incorporated into the Grubbs catalyst by an alkene metathesis reaction. The subsequent insertion of the alkyne fragment then proceeds intramolecularly. Alkene association, cycloaddition, and cycloreversion to the diene product complex close the catalytic cycle. Rate enhancement by an ethene atmosphere (Mori's conditions) originates from a constantly higher overall alkene concentration that is necessary for the rate-limiting [2 + 2] cycloreversion step to the diene product complex.

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PubMed2004

Mechanism of Reppe's nickel-catalyzed ethyne tetramerization to cyclooctatetraene: a DFT study.

In this B3 LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C(2)H(2))(3) yields monoethyne nickelacyclopentadiene in the rate-determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the Ni--C sigma bond. A highly strained monoethyne trans-nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis-structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(eta(2)-C(2)H(2))(eta(2)-C(8)H(8)), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, C--C bond formation steps at Ni(2)(micro-cot) fragments have been predicted to feature prohibitively high overall reaction barriers.

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PubMed2002

Pd0 mechanism of palladium-catalyzed cyclopropanation of alkenes by CH2N2: a DFT study.

Pathways for the reaction of ethene with diazomethane to cyclopropane and dinitrogen catalyzed by Pd(0) complexes have been investigated at the B3LYP level of theory. The computed Gibbs free activation energy of 71.7 kJ mol(-1) for the most favorable catalytic cycle is by far lower than previously reported computed barriers for Pd(II)-catalyzed pathways of this reaction and is now in the range of experimental expectations. Pd(eta(2)-C(2)H(4))(2) is predicted to be the resting state of the catalyst and the product of a Pd(OAc)(2) precatalyst reduction. The Pd(0) ethene complex is in equilibrium with Pd(eta(2)-C(2)H(4))(kappaC-CH(2)N(2)), from which N(2) is eliminated in the rate-determining step. The resulting carbene complex (eta(2)-C(2)H(4))Pd=CH(2) reacts without intrinsic barrier with CH(2)N(2) to Pd(eta(2)-C(2)H(4))(2) and N(2) and with ethene to the palladacyclobutane (eta(2)-C(2)H(4))Pd(II)[kappaC(1),kappaC(3)-(CH(2))(3)]. The N(2) elimination from Pd(eta(2)-C(2)H(4))(2)(kappaC-CH(2)N(2)) to (eta(2)-C(2)H(4))(2)Pd=CH(2) leads to an overall Gibbs free activation energy of 84.2 kJ mol(-1). The intramolecular rearrangement of (eta(2)-C(2)H(4))(2)Pd=CH(2) to the palladacyclobutane (eta(2)-C(2)H(4))Pd(II)[kappaC(1),kappaC(3)-(CH(2))(3)] and the subsequent reductive elimination of cyclopropane are facile. At the BP86 level of theory, Pd(0) preferentially coordinates three ligands. Pd(eta(2)-C(2)H(4))(3) is predicted to be the resting state, and the N(2) elimination from the model complex Pd(eta(2)-C(2)H(4))(2)(kappaC-CH(2)N(2)) is the rate-determining transition state leading to an overall Gibbs free activation energy of 69.4 kJ mol(-1).

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PubMed2002

[P,P-Di-tert-butyl-N-trimethylsilyl-P-(trimethylsilylamino)phosphine imidato-kappa2N,N']bis(pyridine-kappaN)lithium(I).

In the title compound, [Li(C(14)H(36)N(2)PSi(2))(C(5)H(5)N)(2)], the bulky chelating monoanionic P,P-di-tert-butyl-N-trimethylsilyl-P-(trimethylsilylamino)phosphine imidate ligand and two pyridine ligands bind to Li in a pseudo-tetrahedral arrangement with twofold symmetry. The Li-N(phosphine) distance is 2.048 (5) A, while the Li.P distance is 2.520 (6) A.

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