Mechanism of Iron-Catalyzed Oxidative α-Amination of Ketones with Sulfonamides.

We report the mechanism of the iron-catalyzed oxidative α-amination of ketones with sulfonamides. Using linear free energy relationships, competition experiments, and identification of reaction intermediates, we have found that the mechanism of this reaction proceeds through rate-limiting electron transfer to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) from an iron enolate in the process of forming an α-DDQ adduct. The adduct then serves as the electrophile for substitution with sulfonamide nucleophiles, accelerated by iron and additional DDQ. This mechanistic study rules out formation of an α-carbocation intermediate and purely radical mechanistic hypotheses.

Iron-Catalyzed Oxidative α-Amination of Ketones with Primary and Secondary Sulfonamides.

We report the iron-catalyzed α-amination of ketones with sulfonamides. Using an oxidative coupling approach, ketones can be directly coupled with free sulfonamides, without the need for prefunctionalization of either substrate. Primary and secondary sulfonamides are both competent coupling partners, with yields from 55% to 88% for deoxybenzoin-derived substrates.

Mechanistic Studies of Palladium-Catalyzed Aminocarbonylation of Aryl Chlorides with Carbon Monoxide and Ammonia.

Mechanistic information on a reliable, palladium-catalyzed aminocarbonylation of aryl chlorides with ammonia is reported. The reaction occurs with ethylene complex 1 as catalyst, and mechanistic information was gained by isolation of catalytic intermediates and kinetic measurements, including the first mechanistic data on the oxidative addition of aryl chloride to a palladium(0) complex in the presence of CO. Arylpalladium and phenacylpalladium halide intermediates were synthesized, and kinetic measurements of the formation and reactions of these intermediates were undertaken to determine the mechanism of the oxidative addition of aryl bromides and chlorides to a Pd(0) dicarbonyl compound in the presence of CO and the mechanism of the reaction of ammonia with a Pd(II) phenacyl complex to form benzamide. The oxidative addition of aryl chlorides and aryl bromides was determined to occur with rate-limiting reaction of the haloarene with a three-coordinate Pd(0) species bearing a bidentate phosphine and one CO ligand. A primary 13C kinetic isotope effect suggested that this step involves cleavage of the carbon-halogen bond. Our data show that the formation of benzamide from the reaction of phenacylpalladium halide complexes with ammonia occurs by a pathway involving reversible displacement of chloride from a phenacylpalladium chloride complex by ammonia, deprotonation of the bound ammonia to form a phenacylpalladium amido complex, and reductive elimination to form the C-N bond. Consistent with this mechanism, the reaction of an aryl palladium amido complex with CO formed the corresponding primary benzamide. A catalyst deactivation pathway involving the formation of a Pd(I) dimer also was elucidated.

One-pot anti-Markovnikov hydroamination of unactivated alkenes by hydrozirconation and amination.

A one-pot anti-Markovnikov hydroamination of alkenes is reported. The synthesis of primary and secondary amines from unactivated olefins was accomplished in the presence of a variety of functional groups. Hydrozirconation, followed by amination with nitrogen electrophiles, provides exclusive anti-Markovnikov selectivity. Most products are isolated in high yields without the use of column chromatography.

Mechanism of C-F reductive elimination from palladium(IV) fluorides.

The first systematic mechanism study of C-F reductive elimination from a transition metal complex is described. C-F bond formation from three different Pd(IV) fluoride complexes was mechanistically evaluated. The experimental data suggest that reductive elimination occurs from cationic Pd(IV) fluoride complexes via a dissociative mechanism. The ancillary pyridyl-sulfonamide ligand plays a crucial role for C-F reductive elimination, likely due to a kappa(3) coordination mode, in which an oxygen atom of the sulfonyl group coordinates to Pd. The pyridyl-sulfonamide can support Pd(IV) and has the appropriate geometry and electronic structure to induce reductive elimination.

Silver-mediated fluorination of functionalized aryl stannanes.

We report a regiospecific silver-mediated fluorination of aryl stannanes. The presented reaction can afford complex fluoroarenes from readily available phenols in three steps. The operational simplicity and the broad substrate scope of the fluorination should render this reaction a useful tool for the synthesis of milligram to gram quantities of functionalized aryl fluorides. Silver-mediated oxidative transformations of aryl nucleophiles that proceed via bimetallic redox processes are a new avenue to develop carbon-heteroatom bond formations.

Ni-Catalyzed Cross-Electrophile Coupling for the Synthesis of Skipped Polyenes.

Skipped polyenes featuring high ( E)-selectivity and varying methyl substitution patterns are synthesized using a nickel-catalyzed cross-coupling reaction between allyl trifluoroacetates and vinyl bromides. The utility of this cross-electrophile coupling is showcased in part by the synthesis of the RST fragment of the marine ladder polyether, maitotoxin. Construction of this fragment is particularly challenging due to the alternating methyl substitution pattern.

Synthetic and Computational Studies on the Rhodium-Catalyzed Hydroamination of Aminoalkenes.

The influence of ligand structure on rhodium-catalyzed hydroamination has been evaluated for a series of phosphinoarene ligands. These catalysts have been evaluated in a set of catalytic intramolecular Markovnikov hydroamination reactions. The mechanism of hydroamination catalyzed by the rhodium(I) complexes in this study was examined computationally, and the turnover-limiting step was elucidated. These computational studies were extended to a series of theoretical hydroamination catalysts to compare the electronic effects of the ancillary ligand substituents. The relative energies of intermediates and transition states were compared to those of intermediates in the reaction catalyzed by the unsubstituted catalyst. The experimental difference in the reactivities of electron-rich and electron-poor catalysts was compared to the computational results, and it was found that the activity for the electron-poor catalysts predicted from the reaction barriers was overestimated. Thus, the analysis of the catalysts in this study was expanded to include the binding preference of each ligand, in comparison to that of the unsubstituted ligand. This information accounts for the disparity between observed reactivity and the calculated overall reaction barrier for electron-poor ligands. The ligand-binding preferences for new ligand structures were calculated, and ligands that were predicted to bind strongly to rhodium generated catalysts for the experimental catalytic reactions that were more reactive than those predicted to bind more weakly.