Kara J. Stowers

Insights into directing group ability in palladium-catalyzed C-H bond functionalization.

This paper describes a detailed investigation of factors controlling the dominance of a directing group in Pd-catalyzed ligand-directed arene acetoxylation. Mechanistic studies, involving reaction kinetics, Hammett analysis, kinetic isotope effect experiments, and the kinetic order in oxidant, have been conducted for a series of different substrates. Initial rates studies of substrates bearing different directing groups showed that these transformations are accelerated by the use of electron-withdrawing directing groups. However, in contrast, under conditions where two directing groups are in competition with one another in the same reaction flask, substrates with electron-donating directing groups react preferentially. These results are discussed in the context of the proposed mechanism for Pd-catalyzed arene acetoxylation.

Aerobic Pd-catalyzed sp3 C-H olefination: A route to both N-heterocyclic scaffolds and alkenes

This communication describes a new method for the Pd/polyoxometalate-catalyzed aerobic olefination of unactivated sp(3) C-H bonds. Nitrogen heterocycles serve as directing groups, and air is used as the terminal oxidant. The products undergo reversible intramolecular Michael addition, which protects the monoalkenylated product from overfunctionalization. Hydrogenation of the Michael adducts provides access to bicyclic nitrogen-containing scaffolds that are prevalent in alkaloid natural products. Additionally, the cationic Michael adducts undergo facile elimination to release α,β-unsaturated olefins, which can be further elaborated via C-C and C-heteroatom bond-forming reactions.

Mechanistic Comparison Between Pd-Catalyzed Ligand Directed C-H Chlorination and C-H Acetoxylation

This communication describes detailed investigations of the mechanism of the Pd-catalyzed C-H chlorination and acetoxylation of 2-o-tolylpyridine. Under the conditions examined, both reactions proceed via rate-limiting cyclopalladation. However, substrate and catalyst order as well as Hammett data indicate that the intimate mechanism of cyclopalladation differs significantly between PdCl2-catalyzed chlorination and Pd(OAc)2-catalyzed acetoxylation.

Nitrate as a redox co-catalyst for the aerobic Pd-catalyzed oxidation of unactivated sp3-C–H bonds

This paper describes a new method for the catalytic aerobic oxygenation of unactivated sp(3)-C-H bonds. This transformation utilizes Pd(OAc)(2) as a catalyst in conjunction with NaNO(3) as a redox co-catalyst. Both oxime ether and pyridine derivatives are effective directing groups for these reactions. The oxygen incorporated into the product derives from the solvent (acetic acid). Preliminary results show that the addition of simple NaCl to the reaction mixture results in aerobic chlorination under analogous conditions.

Methyl ester synthesis catalyzed by nanoporous gold: from 10−9 Torr to 1 atm

The oxidative coupling reaction of aldehydes with methanol occurs in the vapor phase over a support-free nanoporous gold (npAu) catalyst over a wide pressure range—from 10−9 Torr to 1 atm. The dependence of the aldehyde-to-ester reaction rate on the oxygen, methanol and aldehyde partial pressures suggests that the rate-limiting step for coupling is the reaction of the aldehyde with surface sites saturated with adsorbed methoxy. Stable catalyst activity is achieved for aldehyde–methanol coupling in flowing reactant mixtures at 70 °C. While the conditioned npAu catalyst exhibits high selectivity for methanol–aldehyde coupling, its activity for the self-coupling reaction of methanol to methyl formate is reduced by the exposure to the alcohol–aldehyde mixture in a manner that is consistent with the buildup of spectator species. The activity for methanol self-coupling can be regenerated by extended exposure to flowing methanol, CO and O2 at 70 °C. Overall, the observed catalytic esterification is consistent with model studies of both the npAu catalyst and single crystal gold in ultrahigh vacuum.

Stability and efficiency of CO2 capture using linear amine polymer modified carbon nanotubes

Increasing atmospheric CO2 concentration has become a cause for concern. The design of efficient and stable solid amine adsorbents for CO2 capture, as well as for controlled release, is urgently needed. Polymeric amine modified multiwalled carbon nanotubes (CNT) are a promising material for meeting these goals. In this study, linear polyethylenimine (LPEI) was supported on CNT and was characterized and studied for reversible CO2 adsorption. These results are compared to branched polythylenimine (BPEI) as well as other materials. Upon treatment with 3 M NaOH or 5 M HNO3, the CO2 adsorption was decreased by at least 15%. The CO2 adsorption by LPEI (1.89 mmol g−1) was lower than that of BPEI (2.43 mmol g−1) when coated on CNT supports. The desorption temperature was noticeably lower on CNT–LPEI versus CNT–BPEI for complete CO2 removal. CO2 adsorption capacity was strongly dependent on the packing height of the material in the column reactor. The stability study showed that CO2 adsorption of CNT–LPEI and CNT–BPEI decrease by 9.5% and 61.7%, respectively, after steam treatment, indicating that LPEI was more stable under humid conditions.