Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex.

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal-oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of CuII and CuIII. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical-oxo coupling pathways, which were previously proposed for Ru-based catalysts. [CuII(HL)(OTf)2] [HL = Hbbpya = N,N-bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [CuII(L-)(H2O)]+ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH⧧ value of 4.49 kcal·mol-1, a ΔS⧧ value of -42.6 cal·mol-1·K-1, and a ΔG⧧ value of 17.2 kcal·mol-1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [CuII(L•)(O•)(H2O)]+. From this intermediate onward, formation of the O-O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the CuII center is proposed to be redox-inactive.

Diastereospecific Gold(I)-Catalyzed Cyclization Cascade for the Controlled Preparation of N- and N,O-Heterocycles.

The reaction of oxime-tethered 1,6-enynes with a cationic gold(I) catalyst demonstrates a great potential for the synthesis of a range of heterocycles in a diastereospecific fashion. The control of the configuration of the oxime and the alkene of the enyne moiety is the key to selectively obtain dihydro-1,2-oxazines, isoxazolines or dihydropyrrole-N-oxides as single diastereoisomers. As supported by DFT calculations, these cascade reactions proceed stepwise, by the intramolecular addition of the O or N atom of the oxime onto cyclopropyl gold(I) carbene intermediates. In this study, a rare [3,3]-sigmatropic rearrangement of nitrones is also observed.

Ruthenium-Catalyzed Peri- and Ortho-Alkynylation with Bromoalkynes via Insertion and Elimination.

The alkynylation of naphthols takes place with total regiocontrol at the peri position of the hydroxyl group in the presence of [RuCl2(p-cymene)]2 as the catalyst. This reaction features high functional group tolerance. The related ortho-alkynylation of benzoic acids proceeds under similar conditions and also shows wide functional group tolerance. Both reactions proceed through metalation, insertion of the alkyne, and bromide elimination.

Enantioselective Synthesis of Cyclobutenes by Intermolecular [2+2] Cycloaddition with Non-C2 Symmetric Digold Catalysts.

The enantioselective intermolecular gold(I)-catalyzed [2+2] cycloaddition of terminal alkynes and alkenes has been achieved using non-C2-chiral Josiphos digold(I) complexes as catalysts, by the formation of the monocationic complex. This new approach has been applied to the enantioselective total synthesis of rumphellaone A.

The Trifluoromethyl Anion.

First evidence for the existence of free trifluoromethyl anion CF3 (-) has been obtained. The 3D-caged potassium cation in [K(crypt-222)](+) is inaccessible to CF3 (-) , thus rendering it uncoordinated ("naked"). Ionic [K(crypt-222)](+) CF3 (-) has been characterized by single-crystal X-ray diffraction, solution NMR spectroscopy, DFT calculations, and reactivity toward electrophiles.

Ruthenium-catalyzed nucleophilic fluorination of halobenzenes.

The first π-coordination-catalyzed nucleophilic fluorination of unactivated aryl halides has been demonstrated. Chlorobenzene reacts with alkali metal fluorides (CsF, KF) in the presence of a Cp*Ru catalyst at 120-180 °C to give fluorobenzene.

Easy access to the copper(III) anion [Cu(CF3 )4 ](-).

CuCl or pre-generated CuCF3 reacts with CF3 SiMe3 /KF in DMF in air to give [Cu(CF3 )4 ](-) quantitatively. [PPN](+) , [Me4 N](+) , [Bu4 N](+) , [PhCH2 NEt3 ](+) , and [Ph4 P](+) salts of [Cu(CF3 )4 ](-) were prepared and isolated spectroscopically and analytically pure in 82-99% yield. X-ray structures of the [PPN](+) , [Me4 N](+) , [Bu4 N](+) , and [Ph4 P](+) salts were determined. A new synthetic strategy with [Cu(CF3 )4 ](-) was demonstrated, involving the removal of one CF3 (-) from the Cu atom in the presence of an incoming ligand. A novel Cu(III) complex [(bpy)Cu(CF3 )3 ] was thus prepared and fully characterized, including by single-crystal X-ray diffraction. The bpy complex is highly fluxional in solution, the barrier to degenerate isomerization being only 2.3 kcal mol(-1) . An NPA study reveals a huge difference in the charge on the Cu atom in [Cu(CR3 )4 ](-) for R=F (+0.19) and R=H (+0.46), suggesting a higher electron density on Cu in the fluorinated complex.

Mechanism of trifluoromethylation of aryl halides with CuCF3 and the ortho effect.

A combined experimental (radical clock, kinetic, Hammett) and computational (DFT, MM) study of the trifluoromethylation reaction of aryl halides with CuCF3 reveals a nonradical mechanism involving Ar-X oxidative addition to the Cu(I) center as the rate determining step. The reaction is second order, first order in each reactant with ΔG(⧧) ≈ 24 kcal/mol for PhI (computed ΔG(⧧) = 21.9 kcal/mol). An abrupt change in the gradient on the Hammett plot of log(kR/kH) versus σp for 11 p-RC6H4I substrates produces two correlations (ρ = +0.69 and +1.83), which is temptingly suggestive of two different reaction pathways. Only one mechanism is operational, however, as advocated by a single linear correlation with σp(-) (ρ = +0.91), analysis of the experimental ρ values, close similarity of the transition states varying in R and displaying clear signs of -M interactions, and excellent reproduction of the plot by DFT. The long-known yet previously uncomprehended ortho effect has been quantified, for the first time, using the reaction of CuCF3 with a series of o-RC6H4Br: R(kR/kH) = H (1) < Me (3.5) < MeO (4) < CN (20) < CHO (250) < CO2Me (850) < NO2 (4300) < Ac (7300) < CO2H (150 000). With minor contributions from electronic factors, the ortho effect is largely determined by (i) the stabilizing coordination of the o-substituent to Cu in the transition state with the Cu···O distance varying directly with the barrier and (ii) the steric bulk of the o-substituent that raises the ground state free energy of the haloarene (G(o)ortho - G(o)H or G(o)ortho - G(o)para) by inflicting molecular strain and consequently weakening the Ar-X bond.

The critical effect of the countercation in the direct cupration of fluoroform with [Cu(OR)2 ](-).

Just a spectator or a key player? The alkali-metal counterion (K(+) ) plays a remarkable key role in the recently discovered cupration reaction of fluoroform with dialkoxycuprates. A total of eight Lewis acid and Lewis base centers synergistically interacting with one another arrange in a stable transition state, providing a low-energy pathway for this unique transformation.