Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions.

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc) catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques.

Cyclometalated N-heterocyclic carbene iridium(iii) complexes with naphthalimide chromophores: a novel class of phosphorescent heteroleptic compounds.

A series of cyclometalated N-heterocyclic carbene complexes of the general formula [Ir(C^N)(C^C:)] has been prepared. Two sets of compounds were designed, those where (C^C:) represents a bidentate naphthalimide-substituted imidazolylidene ligand and (C^N) = ppy (3a), F2ppy (4a), bzq (5a) and those where (C^C:) represents a naphthalimide-substituted benzimidazolylidene ligand and (C^N) = ppy (3b), F2ppy (4b), bzq (5b). The naphthalimide-imidazole and naphthalimide-benzimidazole ligands 1a,b and the related imidazolium and benzimidazolium salts 2a,b were also prepared and fully characterized.

On the Triple Role of Fluoride Ions in Palladium-Catalyzed Stille Reactions.

The mechanism of Stille reactions (cross-coupling of ArX with Ar'SnnBu3 ) performed in the presence of fluoride ions is established. A triple role for fluoride ions is identified from kinetic data on the rate of the reactions of trans-[ArPdBr(PPh3 )2 ] (Ar=Ph, p-(CN)C6 H4 ) with Ar'SnBu3 (Ar'=2-thiophenyl) in the presence of fluoride ions. Fluoride ions promote the rate-determining transmetallation by formation of trans-[ArPdF(PPh3 )2 ], which reacts with Ar'SnBu3 (Ar'=Ph, 2-thiophenyl) at room temperature, in contrast to trans-[ArPdBr(PPh3 )2 ], which is unreactive.

Oxidative Addition of Haloheteroarenes to Palladium(0): Concerted versus SN Ar-Type Mechanism.

The kinetics of the oxidative additions of haloheteroarenes HetX (X=I, Br, Cl) to [Pd(0) (PPh3 )2 ] (generated from [Pd(0) (PPh3 )4 ]) have been investigated in THF and DMF and the rate constants have been determined. In contrast to the generally accepted concerted mechanism, Hammett plots obtained for substituted 2-halopyridines and solvent effects reveal a reaction mechanism dependent on the halide X of HetX: an unprecedented SN Ar-type mechanism for X=Br or Cl and a classical concerted mechanism for X=I. These results are supported by DFT studies.

A unique class of neutral cyclometalated platinum(II) complexes with π-bonded benzenedithiolate: synthesis, molecular structures and tuning of luminescence properties.

A unique class of neutral cyclometalated platinum(ii) complexes with π-bonded benzenedithiolate are reported including two X-ray molecular structures. To the best of our knowledge these are the first structures to be reported for cyclometalated platinum complexes with a benzenedithiolate ligand. All of the complexes are luminescent in fluid solution at room temperature and in frozen solvent glasses at 77 K and their emission properties can be tuned through ligand variation.

Evidence for the interaction between (t)BuOK and 1,10-phenanthroline to form the 1,10-phenanthroline radical anion: a key step for the activation of aryl bromides by electron transfer.

Electron paramagnetic resonance and electrochemistry are used to evidence the interaction between 1,10-phenanthroline (Phen) and KO(t)Bu to form the 1,10-phenanthroline radical anion, PhenË™(-), and the (t)BuOË™ radical via an inner-sphere electron transfer. In addition, electrochemistry is also used to explain the formation of aryl radicals from aryl bromides via outer-sphere electron transfer from the key intermediate PhenË™(-).

Three roles for the fluoride ion in palladium-catalyzed Hiyama reactions: transmetalation of [ArPdFL₂] by Ar'Si(OR)₃.

From the kinetic data on the transmetalation/reductive elimination in fluoride-promoted Hiyama reactions, obtained using electrochemical techniques, it has been established that fluoride ions play three roles. F(-) reacts with trans-[ArPdBrL2] (L=PPh3) to form trans-[ArPdFL2], which reacts with Ar'Si(OMe)3 in the rate-determining transmetalation, whereas trans-[ArPdBrL2] does not react with Ar'Si(OMe)3. F(-) reacts with Ar'Si(OMe)3 to deliver the unreactive silicate Ar'SiF(OMe)3(-), thus leading to two antagonistic kinetic effects.

Activation of aryl and heteroaryl halides by an iron(I) complex generated in the reduction of [Fe(acac)₃] by PhMgBr: electron transfer versus oxidative addition.

The mechanism of the reactions of aryl/heteroaryl halides with aryl Grignard reagents catalyzed by [Fe(III)(acac)3] (acac=acetylacetonate) has been investigated. It is shown that in the presence of excess PhMgBr, [Fe(III)(acac)3] affords two reduced complexes: [PhFe(II)(acac)(thf)n] (n=1 or 2) (characterized by (1)H NMR and cyclic voltammetry) and [PhFe(I)(acac)(thf)](-) (characterized by cyclic voltammetry, (1)H NMR, EPR and DFT). Whereas [PhFe(II)(acac)(thf)n] does not react with any of the investigated aryl or heteroaryl halides, the Fe(I) complex [PhFe(I)(acac)(thf)](-) reacts with ArX (Ar=Ph, 4-tolyl; X=I, Br) through an inner-sphere monoelectronic reduction (promoted by halogen bonding) to afford the corresponding arene ArH together with the Grignard homocoupling product PhPh.

Oxidative addition to palladium(0) diphosphine complexes: observations of mechanistic complexity with iodobenzene as reactant.

Using a combination of electrochemical and NMR techniques, the oxidative addition of PhX to three closely related bis-diphosphine P2Pd(0) complexes, where the steric bulk of just one substituent was varied, has been analysed quantitatively. For the complex derived from MetBu2P, a rapid reaction ensued with PhI following an associative mechanism, and data was also obtained by cyclic voltammetry for PhOTs, PhBr and PhCl, revealing distinct relative reactivities from the related (PCx3)2Pd complex (Cx = cyclohexyl) previously studied. The corresponding EttBu2P complex reacted more slowly with PhI and was studied by NMR spectroscopy.

Mechanism of palladium-catalyzed Suzuki-Miyaura reactions: multiple and antagonistic roles of anionic "bases" and their countercations.

In Suzuki-Miyaura reactions, anionic bases F(-) and OH(-) (used as is or generated from CO3(2-) in water) play multiple antagonistic roles. Two are positive: 1) formation of trans-[Pd(Ar)F(L)2] or trans-[Pd(Ar)-(L)2(OH)] (L = PPh3) that react with Ar'B(OH)2 in the rate-determining step (rds) transmetallation and 2) catalysis of the reductive elimination from intermediate trans-[Pd(Ar)(Ar')(L)2]. Two roles are negative: 1) formation of unreactive arylborates (or fluoroborates) and 2) complexation of the OH group of [Pd(Ar)(L)2(OH)] by the countercation of the base (Na(+), Cs(+), K(+)).

Autocatalytic intermolecular versus intramolecular deprotonation in C-H bond activation of functionalized arenes by ruthenium(II) or palladium(II) complexes.

The activation of the C-H bond of 1-phenylpyrazole (2) and 2-phenyl-2-oxazoline (3) by [Ru(OAc)2(p-cymene)] is an autocatalytic process catalyzed by the co-product HOAc. The reactions are indeed faster in the presence of acetic acid and water but slower in the presence of a base K2CO3. A reactivity order is established in the absence of additives: 2-phenylpyridine>2-phenyl-2-oxazoline>1-phenylpyrazole (at RT).

Near-infrared room temperature emission from a novel class of Ru(II) heteroleptic complexes with quinonoid organometallic linker.

A novel class of luminescent octahedral ruthenium complexes 2-4 displaying a π-bonded quinonoid ligand is described. Remarkably, the presence of this organometallic ligand affects their UV-vis properties and transforms them into panchromatic absorbers. Furthermore, it turns on room temperature NIR emissions.

Discriminating role of bases in diketonate copper(I)-catalyzed C-O couplings: phenol versus diarylether.

The mechanism of the formation of phenol from PhI and CsOH catalysed by copper(i) ligated to the 1,3-diketonate ket'(-) generated from 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD) has been investigated by DFT calculations associated with experimental techniques: cyclic voltammetry, (1)H NMR, and ESI-MS. Weak halogen bonding between the negatively charged O atom of [(ket')Cu(I)-OH](-) and PhI leads to an oxidative addition that gives (ket')Cu(III)(Ph)-OH. The latter undergoes a faster reductive elimination that delivers (ket')Cu(I)(PhOH) from which PhOH is released.

Ï€-Bonded dithiolene complexes: synthesis, molecular structures, electrochemical behavior, and density functional theory calculations.

The synthesis and X-ray molecular structure of the first metal-stabilized o-dithiobenzoquinone [Cp*Ir-o-(η(4)-C(6)H(4)S(2))] (2) are described. The presence of the metal stabilizes this elusive intermediate by π coordination and increases the nucleophilic character of the sulfur atoms. Indeed, the π-bonded dithiolene complex 2 was found to react with the organometallic solvated species [Cp*M(acetone)(3)][OTf](2) (M = Rh, Ir) to give a unique class of binuclear dithiolene compounds [Cp*Ir(C(6)H(4)S(2))MCp*][OTf](2) [M = Rh (3), Ir (4)] in which the elusive dithiolene η-C(6)H(4)S(2) acts as a bridging ligand toward the two Cp*M moieties.

Iron-catalyzed reductive radical cyclization of organic halides in the presence of NaBH4: evidence of an active hydrido iron(I) catalyst.

Iron made'em: iron(II) complexes such as FeCl(2) and [FeCl(2)(dppe)(2) ] (dppe=1,2-bisdiphenylphosphinoethane) are efficient precatalysts for the radical cyclization of unsaturated iodides and bromides in the presence of NaBH(4). Cyclic voltammetry studies suggests that the reaction occurs through a radical mechanism via an anionic hydrido iron(I) species as the key intermediate for the activation of the substrates by electron transfer.

Mechanistic origin of antagonist effects of usual anionic bases (OH-, CO3(2-)) as modulated by their countercations (Na+, Cs+, K+) in palladium-catalyzed Suzuki-Miyaura reactions.

The mechanism of the reaction of trans-ArPdBrL(2) (Ar=p-Z-C(6)H(4), Z=CN, H; L=PPh(3)) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4), Z'=H, CN, MeO), which is a key step in the Suzuki-Miyaura process, has been established in N,N-dimethylformamide (DMF) with two bases, acetate (nBu(4)NOAc) or carbonate (Cs(2)CO(3)) and compared with that of hydroxide (nBu(4)NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M(+) (e.g.

Autocatalysis for C-H bond activation by ruthenium(II) complexes in catalytic arylation of functional arenes.

Kinetic data for the C-H bond activation of 2-phenylpyridine by Ru(II)(carboxylate)(2)(p-cymene) I (acetate) and I' (pivalate) are available for the first time. They reveal an irreversible autocatalytic process catalyzed by the coproduct HOAc or HOPiv (acetonitrile, 27 °C). The overall reaction is indeed accelerated by the carboxylic acid coproduct and water.

Kinetic data for the transmetalation/reductive elimination in palladium-catalyzed Suzuki-Miyaura reactions: unexpected triple role of hydroxide ions used as base.

The mechanism of the reaction of trans-[ArPdX(PPh(3))(2)] (Ar=p-Z-C(6)H(4); Z=CN, F, H; X=I, Br, Cl) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4); Z'=CN, H, OMe) has been established in DMF in the presence of the base OH(-) in the context of real palladium-catalyzed Suzuki-Miyaura reactions. The formation of the cross-coupling product ArAr' and [Pd(0)(PPh(3))(3)] has been followed through the application of electrochemical techniques. Kinetic data have been obtained for the first time, with determination of the observed rate constant, k(obs), of the overall reaction.

Dual effect of halides in the Stille reaction: in situ halide metathesis and catalyst stabilization.

Halide anions can increase or decrease the transmetallation rate of the Stille reaction through in situ halide metathesis. Although the influence of the halogen present in oxidative addition complexes on the transmetallation rate with organostannanes was already known, the application of in situ halide metathesis to accelerate cross-coupling reactions with organometallic reagents is not described in the literature yet. In addition a second unprecedented role of halides was discovered.

On the origin of copper(I) catalysts from copper(II) precursors in C-N and C-O cross-couplings.

Cu(II) precursors ligated to phenanthrolines are reduced in situ by alcohols or amines in the presence of a base (Cs(2)CO(3)) to generate Cu(I) species which are active catalysts in C-N and C-O cross-coupling reactions. The conversion Cu(II) --> Cu(I) has been evidenced and monitored by UV-vis and NMR spectroscopy.

Activation of aryl halides by Cu(0)/1,10-phenanthroline: Cu(0) as precursor of Cu(I) catalyst in cross-coupling reactions.

The activation of aryl iodides and bromides by Cu(0)/1,10-phenanthroline in acetonitrile (S) generates Cu(I)(phenanthroline)S(2)(+) which can be a catalyst for cross-coupling reactions.