Jonas Oxgaard

Thermal decomposition of RDX from reactive molecular dynamics

We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH(2)N(NO(2))](3)] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH(2)N(NO(2))](4)]. Our simulations show that the equilibrium population of CO and CO(2) (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO(2) molecules. The equilibrium populations of N(2) and H(2)O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales.

The Reaction Mechanism of the Enantioselective Tsuji Allylation: Inner-Sphere and Outer-Sphere Pathways, Internal Rearrangements, and Asymmetric C−C Bond Formation

We use first principles quantum mechanics (density functional theory) to report a detailed reaction mechanism of the asymmetric Tsuji allylation involving prochiral nucleophiles and nonprochiral allyl fragments, which is consistent with experimental findings. The observed enantioselectivity is best explained with an inner-sphere mechanism involving the formation of a 5-coordinate Pd species that undergoes a ligand rearrangement, which is selective with regard to the prochiral faces of the intermediate enolate. Subsequent reductive elimination generates the product and a Pd(0) complex. The reductive elimination occurs via an unconventional seven-centered transition state that contrasts dramatically with the standard three-centered C-C reductive elimination mechanism. Although limitations in the present theory prevent the conclusive identification of the enantioselective step, we note that three different computational schemes using different levels of theory all find that inner-sphere pathways are lower in energy than outer-sphere pathways. This result qualitatively contrasts with established allylation reaction mechanisms involving prochiral nucleophiles and prochiral allyl fragments. Energetic profiles of all reaction pathways are presented in detail.

Facile Oxy‐Functionalization of a Nucleophilic Metal Alkyl with a cis‐Dioxo Metal Species via a (2+3) Transition State

Selective, low-temperature hydroxylation of alkanes catalyzed by transition-metal complexes is an important area of study, given its possible applications to natural-gas conversion as well as to more efficient production of bulk chemicals and energy. Several promising electrophilic catalysts that couple C–H activation to facile oxy-functionalization of the resulting electrophilically activated M R intermediates have been reported (Figure 1). To address practical challenges with

Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed by homogeneous Ir(III) complexes

The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(μ-acac-O,O,C3)-(acac-O,O)2]2 and [R-Ir(acac-O,O)2(L)] (R = acetylacetonato, CH3, CH2CH3, Ph, or CH2CH2Ph, and L = H2O or pyridine) Ir(III) complexes was studied by experimental methods. The system is selective for generating the anti-Markovnikov product of linear alkylarenes (61:39 for benzene + propylene and 98:2 for benzene + styrene). The reaction mechanism was found to follow a rate law with first-order dependence on benzene and catalyst, but a non-linear dependence on olefin. 13C-labelling studies with CH313CH2-Ir-Py showed that reversible β-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the 13C-label from the α to β-positions was found to be slower than the C–H activation of benzene (and thus formation of ethane and Ph-d5-Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and β-hydride elimination (kCH: kβ) of ∼0.5. The comparable magnitude of these rates suggests a common rate determining transition state/intermediate, which has been shown previously with B3LYP density functional theory (DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O)2-Sol (where Sol refers to coordinated solvent). This species then undergoes trans to cisisomerization of the acetylacetonato ligand to yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the regioselective and rate determining step), and then activation of the C–H bond of an incoming benzene to generate the product and regenerate the catalyst.

Acid-Catalyzed Nucleophilic Aromatic Substitution: Experimental and Theoretical Exploration of a Multistep Mechanism.

The mechanism for the acid-mediated substitution of a phenolic hydroxyl group with a sulfur nucleophile has been investigated by a combination of experimental and theoretical methods. We conclude that the mechanism is distinctively different in nonpolar solvents (i.e., toluene) compared with polar solvents. The cationic mechanism, proposed for the reaction in polar solvents, is not feasible and the reaction instead proceeds through a multistep mechanism in which the acid (pTsOH) mediates the proton shuffling. From DFT calculations, we found a rate-determining transition state with protonation of the hydroxyl group to generate free water and a tight ion pair between a cationic protonated naphthalene species and a tosylate anion. Kinetic experiments support this mechanism and show that, at moderate concentrations, the reaction is first order with respect to 2-naphthol, n-propanethiol, and p-toluenesulfonic acid (pTsOH). Experimentally determined activation parameters are similar to the calculated values (Delta H exp not equal =105+/-9, Delta H calcd not equal =118 kJ mol(-1); Delta G exp not equal =112+/-18, Delta G calcd not equal =142 kJ mol(-1)).

Ligand Field Strengths of Carbon Monoxide and Cyanide in Octahedral Coordination

Ligand field splittings were extracted from absorption spectra of metal hexacarbonyl and hexacyano complexes over 35 years ago by Gray and coworkers (J. Am. Chem. Soc., 85, 2922 (1963); ibid. 90, 4260, 5713 (1968)). Recent time-dependent density functional theory calculations by Baerends and coworkers (J. Am. Chem. Soc., 121, 10356 (1999)) on M(CO)6 with M = Cr, Mo, W raised questions about the magnitudes of these ligand field splittings. In order to reexamine such effects systematically, we report here the splittings for a series of 3d6 metal hexacarbonyl, hexaisocyano and hexacyano complexes: , Cr(CO)6, and ; and ; and , Cr(CNH)6, and . This work demonstrates that the 3d splittings induced by the ligands in question generally fall in the order CO > CNH > CN−. We agree with Baerends and colleagues that CO exerts a much stronger ligand field than was originally thought.

Oxy-Functionalization of Nucleophilic Rhenium(I) Metal Carbon Bonds Catalyzed by Selenium(IV)

We report that SeO2 catalyzes the facile oxy-functionalization of (CO)5Re(I)-Me(delta-) with IO4(-) to generate methanol. Mechanistic studies and DFT calculations reveal that catalysis involves methyl group transfer from Re to the electrophilic Se center followed by oxidation and subsequent reductive functionalization of the resulting CH3Se(VI) species. Furthermore, (CO)3Re(I)(Bpy)-R (R = ethyl, n-propyl, and aryl) complexes show analogous transfer to SeO2 to generate the primary alcohols. This represents a new strategy for the oxy-functionalization of M-R(delta-) polarized bonds.