Tianning Diao

Synthesis of Cyclic Enones via Direct Palladium-Catalyzed Aerobic Dehydrogenation of Ketones

α,β-Unsaturated carbonyl compounds are versatile intermediates in the synthesis of pharmaceuticals and biologically active compounds. Here, we report the discovery and application of Pd(DMSO)(2)(TFA)(2) as a catalyst for direct dehydrogenation of cyclohexanones and other cyclic ketones to the corresponding enones, using O(2) as the oxidant. The substrate scope includes heterocyclic ketones and several natural-product precursors.

Aerobic Dehydrogenation of Cyclohexanone to Phenol Catalyzed by Pd(TFA)2/2-Dimethylaminopyridine: Evidence for the Role of Pd Nanoparticles

We have carried out a mechanistic investigation of aerobic dehydrogenation of cyclohexanones and cyclohexenones to phenols with a Pd(TFA)2/2-dimethylaminopyridine catalyst system. Numerous experimental methods, including kinetic studies, filtration tests, Hg poisoning experiments, transmission electron microscopy, and dynamic light scattering, provide compelling evidence that the initial Pd(II) catalyst mediates the first dehydrogenation of cyclohexanone to cyclohexenone, after which it evolves into soluble Pd nanoparticles that retain catalytic activity. This nanoparticle formation and stabilization is facilitated by each of the components in the catalytic reaction, including the ligand, TsOH, DMSO, substrate, and cyclohexenone intermediate.

Direct aerobic α,β-dehydrogenation of aldehydes and ketones with a Pd(TFA)2/4,5-diazafluorenone catalyst

The direct α, β-dehydrogenation of aldehydes and ketones represents an efficient alternative to stepwise methods to prepare enal and enone products. Here, we describe a new Pd(TFA)(2)/4,5-diazafluorenone dehydrogenation catalyst that overcomes key limitations of previous catalyst systems. The scope includes successful reactivity with pharmaceutically important cyclopentanone and flavanone substrates, as well as acyclic ketones. Preliminary mechanistic studies compare the reactivity of this catalyst to previously reported dehydrogenation catalysts and reveal that cleavage of the α-C-H bond of the ketone is the turnover-limiting step of the catalytic mechanism.

Bis(imino)pyridine Cobalt-Catalyzed Dehydrogenative Silylation of Alkenes: Scope, Mechanism, and Origins of Selective Allylsilane Formation

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ((Mes)PDI)CoCH3 ((Mes)PDI = 2,6-(2,4,6-Me3C6H2-N═CMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.

Aerobic Dehydrogenation of Cyclohexanone to Cyclohexenone Catalyzed by Pd(DMSO)2(TFA)2: Evidence for Ligand-Controlled Chemoselectivity

The dehydrogenation of cyclohexanones affords cyclohexenones or phenols via removal of 1 or 2 equiv of H2, respectively. We recently reported several Pd(II) catalyst systems that effect aerobic dehydrogenation of cyclohexanones with different product selectivities. Pd(DMSO)2(TFA)2 is unique in its high chemoselectivity for the conversion of cyclohexanones to cyclohexenones, without promoting subsequent dehydrogenation of cyclohexenones to phenols. Kinetic and mechanistic studies of these reactions reveal the key role of the dimethylsulfoxide (DMSO) ligand in controlling this chemoselectivity. DMSO has minimal kinetic influence on the rate of Pd(TFA)2-catalyzed dehydrogenation of cyclohexanone to cyclohexenone, while it strongly inhibits the second dehydrogenation step, conversion of cyclohexenone to phenol. These contrasting kinetic effects of DMSO provide the basis for chemoselective formation of cyclohexenones.

Characterization of DMSO Coordination to Palladium(II) in Solution and Insights into the Aerobic Oxidation Catalyst, Pd(DMSO)2(TFA)2

Recent studies have shown that Pd(DMSO)(2)(TFA)(2) (TFA = trifluoroacetate) is an effective catalyst for a number of different aerobic oxidation reactions. Here, we provide insights into the coordination of DMSO to palladium(II) in both the solid state and in solution. A crystal structure of Pd(DMSO)(2)(TFA)(2) confirms that the solid-state structure of this species has one O-bound and one S-bound DMSO ligand, and a crystallographically characterized mono-DMSO complex, trans-Pd(DMSO)(OH(2))(TFA)(2), exhibits an S-bound DMSO ligand. (1)H and (19)F NMR spectroscopic studies show that, in EtOAc and THF-d(8), Pd(DMSO)(2)(TFA)(2) consists of an equilibrium mixture of Pd(S-DMSO)(O-DMSO)(TFA)(2) and Pd(S-DMSO)(2)(TFA)(2). The O-bound DMSO is determined to be more labile than the S-bound DMSO ligand, and both DMSO ligands are more labile in THF relative to EtOAc as the solvent. DMSO coordination to Pd(II) is substantially less favorable when the TFA ligands are replaced with acetate. An analogous carboxylate ligand effect is observed in the coordination of the bidentate sulfoxide ligand, 1,2-bis(phenylsulfinyl)ethane to Pd(II). DMSO coordination to Pd(TFA)(2) is shown to be incomplete in AcOH-d(4) and toluene-d(8), resulting in Pd(II)/DMSO adducts with <2:1 DMSO/Pd(II) stoichiometry. Collectively, these results provide useful insights into the coordination properties of DMSO to Pd(II) under catalytically relevant conditions.

Bimetallic C–C Bond-Forming Reductive Elimination from Nickel

Ni-catalyzed cross-coupling reactions have found important applications in organic synthesis. The fundamental characterization of the key steps in cross-coupling reactions, including C-C bond-forming reductive elimination, represents a significant challenge. Bimolecular pathways were invoked in early proposals, but the experimental evidence was limited. We present the preparation of well-defined (pyridine-pyrrolyl)Ni monomethyl and monophenyl complexes that allow the direct observation of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp(3)-sp(3) and sp(2)-sp(2) couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from (pyridine-pyrrolyl)Ni-methyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data are most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the rate-determining step. In contrast, the formation of biphenyl is facilitated by the coordination of a bidentate donor ligand.

Expeditious Approach to Coumarins via Pechmann Reaction Catalyzed by Molecular Iodine or AgOTf

Abstract An efficient and facile route for the synthesis of coumarins via the Pechmann reaction catalyzed by molecular iodine or AgOTf was described.

Binuclear, High-Valent Nickel Complexes: Ni−Ni Bonds in Aryl–Halogen Bond Formation

Metal-metal bonds play a vital role in stabilizing key intermediates in bond-formation reactions. We report that binuclear benzo[h]quinoline-ligated NiII complexes, upon oxidation, undergo reductive elimination to form carbon-halogen bonds. A mixed-valent Ni(2.5+)-Ni(2.5+) intermediate is isolated. Further oxidation to NiIII , however, is required to trigger reductive elimination. The binuclear NiIII -NiIII intermediate lacks a Ni-Ni bond. Each NiIII undergoes separate, but fast reductive elimination, giving rise to NiI species. The reactivity of these binuclear Ni complexes highlights the fundamental difference between Ni and Pd in mediating bond-formation processes.

Structure and Isotope Effects of the β-H Agostic (α-Diimine)Nickel Cation as a Polymerization Intermediate

Single-crystal X-ray characterization of cationic (α-diimine)Ni-ethyl and isopropyl β-agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni-Cα -Cβ angles (75.0(3)° and 74.57(18)°) and short Cα -Cβ distances (1.468(7) and 1.487(5)u2005Å) provide unambiguous evidence for a β-agostic interaction. An inverse equilibrium isotope effect (EIE) for ligand coordination upon cleavage of the agostic bond highlights the weaker bond strength of Ni-H relative to the C-H bond. An Eyring plot for β-hydride elimination-olefin rotation-reinsertion is constructed from variable-temperature NMR spectra with 13 C-labeled agostic complexes. The enthalpy of activation (ΔH≠ ) for β-H elimination is 13.2u2005kcalu2009mol-1 . These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the β-agostic bonds and chain-migration via β-H elimination.