Multivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV-Vis/ATR-IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas-Liquid Phase Reactions.

Operando EPR, XANES/EXAFS, UV-Vis and ATR-IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6-tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least-squares fitting (MCR-ALS) of simultaneously recorded XAS and UV-Vis data sets, it was found that an initially formed (bpy)(NMI)CuI - complex (bpy=2,2'-bipyridine, NMI=N-methylimidazole ) is converted to two different CuII species, a mononuclear (bpy)(NMI)(CH3 CN)CuII -OOH species detectable by EPR and ESI-MS, and an EPR-silent dinuclear (CH3 CN)(bpy)(NMI)CuII (µ-OH)2 ⋅CuII (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)CuII -TEMPO monomers that are also EPR-silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non-reducible CuII leads to slight deactivation at extended reaction times.

Sustainable Co-Synthesis of Glycolic Acid, Formamides and Formates from 1,3-Dihydroxyacetone by a Cu/Al2 O3 Catalyst with a Single Active Sites.

Glycolic acid (GA), as important building block of biodegradable polymers, has been synthesized for the first time in excellent yields at room temperature by selective oxidation of 1,3-dihyroxyacetone (DHA) using a cheap supported Cu/Al2 O3 catalyst with single active CuII species. By combining EPR spin-trapping and operando ATR-IR experiments, different mechanisms for the co-synthesis of GA, formates, and formamides have been derived, in which . OH radicals formed from H2 O2 by a Fenton-like reaction play a key role.

Origins of high catalyst loading in copper(i)-catalysed Ullmann-Goldberg C-N coupling reactions.

A mechanistic investigation of Ullmann-Goldberg reactions using soluble and partially soluble bases led to the identification of various pathways for catalyst deactivation through (i) product inhibition with amine products, (ii) by-product inhibition with inorganic halide salts, and (iii) ligand exchange by soluble carboxylate bases. The reactions using partially soluble inorganic bases showed variable induction periods, which are responsible for the reproducibility issues in these reactions. Surprisingly, more finely milled Cs2CO3 resulted in a longer induction period due to the higher concentration of the deprotonated amine/amide, leading to suppressed catalytic activity. These results have significant implications on future ligand development for the Ullmann-Goldberg reaction and on the solid form of the inorganic base as an important variable with mechanistic ramifications in many catalytic reactions.

Effects of Imidazole-Type Ligands in CuI/TEMPO-Mediated Aerobic Alcohol Oxidation.

Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)CuI(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV-vis/attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of imidazole in terms of ligand and/or base as well as of its substitution pattern on the catalytic performance. For molar ratios of IM/Cu ≥ 2, a (bpy)CuI/II(IM)a(IM)b complex is formed, in which the Cu-N distances and/or angles for the two IM ligands a and b are different. The coordination of a second IM molecule boosts the oxidation of CuI to CuII and, thus, helps to activate O2 by electron transfer from CuI to O2. The rates of CuI oxidation and CuII reduction and, thus, the rates of benzaldehyde formation depend on R of the R-N moiety in the IM ligand. Oxidation is fastest for R = H and alkyl, while reduction is slowest for R = H. The CuI/CuII interplay leads to decreasing total benzaldehyde formation rates in the order R (I+ effect) > R (conjugated system) > R = H.

Molybdenum(0) Dinitrogen Complexes Supported by Pentadentate Tetrapodal Phosphine Ligands: Structure, Synthesis, and Reactivity toward Acids.

The syntheses of two pentadentate tetrapodal phosphine (pentaPod(P)) ligands, P2(Ph)PP2(Ph) and P2(Me)PP2(Ph), are reported, which derive from the fusion of a tripod and a trident ligand. Reaction of the ligand P2(Ph)PP2(Ph) with [MoCl3(THF)3] followed by an amalgam reduction under N2 does not lead to well-defined products. The same reactions performed with the ligand P2(Me)PP2(Ph) afford the mononuclear molybdenum dinitrogen complex [MoN2(P2(Me)PP2(Ph))]. Because of the unprecedented topology of the pentaphosphine ligand, the Mo-P bond to the phosphine in the trans position to N2 is significantly shortened, explaining the very strong activation of the dinitrogen ligand (ν̃NN = 1929 cm(-1)). The reactivity of this complex toward acids is investigated.

Novel Base-Free Catalytic Wittig Reaction for the Synthesis of Highly Functionalized Alkenes.

A highly efficient catalyst system for base-free catalytic Wittig reactions has been developed and optimized. Initially, several potential (pre)catalysts as well as different silanes as reducing agents were screened. A system based on a readily available phosphine oxide as precatalyst and trimethoxy silane as reducing agent proved to be optimal. The effect of various Brønsted acidic additives was studied. Subsequently, the reaction conditions were optimized and standard reaction conditions were determined. Under these conditions the scope of this new protocol was evaluated. Nine activated olefins and 33 aldehydes were converted into 42 highly functionalized alkenes. Notably, aromatic, aliphatic as well as heteroaromatic aldehydes could be converted, giving the desired products in isolated yields up to 99 % and with good to excellent E/Z selectivities. These results underline the remarkable efficiency of this protocol considering the complexity of the reaction mixture and the four reaction steps that proceed in parallel in one pot.

First Base-Free Catalytic Wittig Reaction.

The first base-free catalytic Wittig reaction utilizing readily available Bu3P (5 mol %) as an organocatalyst is reported. The initial Michael addition of the phosphine to a suitable acceptor substituted alkene ultimately results in the formation of an ylide which is subsequently converted with an aldehyde. The presented (1)H NMR studies actually reveal evidence for the Michael addition and proposed ylide formation. Under the optimized reaction conditions various maleates and fumarates were converted with aromatic, heteroaromatic, and aliphatic aldehydes to evaluate the scope and limitations of this unprecedented reaction. Notably, maleates and fumarates react in a stereoconvergent fashion. The corresponding products were obtained in up to 95% isolated yield and E/Z-selectivities up to 99:1.