Anny Jutand

Mechanistic and kinetic studies of palladium catalytic systems

Abstract It is established that new reactive anionic palladium(0) complexes species are formed in which palladium(0) is ligated by either chloride ions: Pd(0)(PPh3)2Cl− (when generated by reduction of PdCl2(PPh3)2) or by acetate ions: Pd(0)(PPh3)2(OAc)− (when generated in situ in mixtures of Pd(OAc)2 and PPh3). The reactivity of such anionic palladium(0) complexes in oxidative addition to aryl iodides strongly depends on the anion born by the palladium(0). The structure of the arylpalladium(II) complexes formed in the oxidative addition also depends on the anion. Indeed, intermediate anionic pentacoordinated arylpalladium(II) complexes are formed: ArPdI(Cl)(PPh3)2− and ArPdI(OAc)(PPh3)2−, respectively, whose stability depends on the chloride or acetate anion brought by the palladium(0). ArPdI(Cl)(PPh3)2− is rather stable and affords trans ArPdI(PPh3)2 at long times via a neutral pentacoordinated solvated species ArPdI(S)(PPh3)2 involved in an equilibrium with the chloride ion. ArPdI(OAc)(PPh3)2− is quite unstable and rapidly affords the stable trans ArPd(OAc)(PPh3)2 complex. Consequently, the mechanism of the PdCl2(PPh3)2-catalyzed cross-coupling of aryl halides and nucleophiles has been revisited. The nucleophilic attack does not proceed on the trans ArPdI(PPh3)2 complexes as usually postulated but on the intermediate neutral pentacoordinated species ArPdI(S)(PPh3)2 to afford a pentacoordinated anionic complex ArPdI(Nu)(PPh3)2− in which the aryl group and the nucleophile are adjacent, a favorable position for the reductive elimination which provides the coupling product Ar–Nu. The mechanism of the Heck reactions catalyzed by mixtures of Pd(OAc)2 and PPh3 has also been revisited. The nucleophilic attack of the olefin proceeds on ArPd(OAc)(PPh3)2 and not on the expected trans ArPdI(PPh3)2 complex which is never formed when the oxidative addition is performed from Pd(0)(PPh3)2(0Ac)−. This work emphasizes the crucial role played by the anions born by the precursors of palladium(0) complexes and rationalize empirical findings dispersed in literature concerning the specificity of palladium catalytic systems.

Absolute determination of electron consumption in transient or steady state electrochemical techniques

Abstract A method elaborated from original work by Lingane is discussed and used to determine the absolute value of the electron stoichiometry corresponding to any electrochemical wave, under the exact conditions of its observation. The method is based on comparison of the currents measured by a pair of microscale techniques such as chronoamperometry, cyclic voltammetry, RDE or steady state methods at microelectrodes. From an evaluation of the uncertainties associated with the use of each possible pair, it is concluded that the best choice consists in chronoamperometry and a steady state method at a microelectrode. The experimental validity of the method is examined in five different cases pertaining to organic or organometallic electrochemistry, in which the determination of the absolute value of the number of electrons exchanged leads to important mechanistic or synthetic consequences.

Au–Pd Core–Shell Nanoparticles Catalyze Suzuki–Miyaura Reactions in Water through Pd Leaching†

CNRS; ENS; UPMC in France[UMR 8640]; NSFC in China[20620130427]; Chinese Scholarship Council

Role of dba in the reactivity of palladium(0) complexes generated in situ from mixtures of Pd(dba)2 and phosphines

Abstract The ligand dba plays a crucial role both on the structure and on the reactivity of palladium(0) complexes generated in situ in mixtures of Pd(dba)2 and phosphine ligands. Whatever the ligand, the major complex is always Pd(dba)L2 where L is a monodentate phosphine ligand or Pd(dba)(L–L) where L–L is a bidentate phosphine ligand. In all cases, the most reactive species in the oxidative addition with phenyl iodide is the lowest ligated complex PdL2 or Pd(L–L) in equilibrium with the major complex and dba. However, Pd(dba)(L–L) also reacts with phenyl iodide. The presence of the major complexes ligated by dba diminishes the concentration of the more reactive species PdL2 or Pd(L–L) and consequently controls the rate of the overall reaction. The overall reactivity is governed both by the intrinsic reactivity of the reactive species and its concentration, two factors which can be antagonistic and a non-linear Hammett correlation of the oxidative addition with the basicity of the phosphine is observed for monodentate ligands.

Structural and kinetic effects of chloride ions in the palladium-catalyzed allylic substitutions

Addition of ligands to (Pd(h 3 -RCH/CH/CH2)(m-Cl))2 or chloride ions to cationic ((h 3 -RCHCHCH 2 )PdL 2 ) � BFinduces the formation of neutral complexes h 1 -RCH/CH/CH2/PdClL2 (R � /H with L � /(4-Cl/C6H4)3P, (4-CH3/C6H4)3P, (4-CF3/C6H4)3P or L2� /1,2-bis(diphenylphosphino)butane (dppb), 1,1?-bis(diphenylphosphino)ferrocene (dppf); R � /Ph with L� /(4-Cl/C6H4)3P), instead of the expected cationic complexes ((h 3 -RCH/CH/CH2)PdL2) � Cl � . In the presence of chloride ions, the reaction of morpholine with the cationic complexes ((h 3 -allyl)Pd(PAr3)2) � BF � (Ar� /4-Cl/C6H4, 4-CH3/C6H4) goes slower and involves both cationic ((h 3 -allyl)Pd(PAr3)2) � and neutral h 1 -allyl-PdCl(PAr3)2 complexes as reactive species in equilibrium with Cl � . The cationic complex is more reactive than the neutral one. However, their relative contribution in the reaction strongly depends on the chloride concentration, which controls their relative concentration. The neutral h 1 -allyl-PdCl(PAr3)2 may become the major reactive species at high chloride concentration. Consequently, (Pd(h 3 -allyl)(m-Cl))2 associated with ligands or cationic ((h 3 -allyl)PdL2) � BF � ; used indifferently as precursors in palladium-catalyzed allylic substitutions, are not equivalent. In both situations, the mechanism of the Pd-catalyzed allylic substitution depends on the concentration of the chloride ions, delivered by the precursor or purposely added, that determines which species, ((h 3 -allyl)PdL2) � or/and h 1 -allyl-PdClL2 are involved in the nucleophilic attack with consequences on the rate of the reaction and probably on its regioselectivity. Consequently, the chloride ions of the catalytic precursors (Pd(h 3 -

Electrosyntheses of disaccharides from phenyl or ethyl 1-thioglycosides

Constant current electrolyses of the glycosyl donors phenyl and ethyl 2,3,4,6-tetra-O-benzyl-1-thio-beta-D-glucopyranoside in dry acetonitrile in the presence of various primary and secondary sugar alcohols, performed in an undivided cell, gave beta-linked disaccharide derivatives selectively in good yields. Phenyl 2,3,4,6-tetra-O-benzoyl-1-thio-beta-D-glucopyranoside gave the beta-glucosides exclusively in good to moderate yields.

Nonlinear optical properties of asymmetric polyphenyls: Efficiency versus transparency trade-off

Abstract First-order hyperpolarizabilities β of a sequence of dimethylaminocyanopolyphenyl oligomers are measured by using the electricfield induced second harmonic generation technique. High β values (up to 55 × 10 −30 esu at zero frequency) are reported to be in-keeping with transparency in the visible and near UV spectral range. The behaviour of β related to the number of phenyl units is compared to calculated hyperpolarizabilities and discussed in terms of trade-off between intramolecular charge transfer and the non-coplanarity between the benzene rings.

Efficient palladium-catalyzed synthesis of unsymmetrical donor—acceptor biaryls and polyaryls

Abstract 4,4′-Unsymmetrically substituted biphenyls can be synthesized by cross-coupling reactions of substituted aromatic organometallic reagents and aromatic halides catalyzed by palladium complexes. This two-step method from commercially available aromatic halides has been used for the synthesis of a series of donor/acceptor para -substituted biphenyls, DC 6 H 4 C 6 H 4 A, where D is an electron donor group and A an electron acceptor group, which are of interest as liquid crystal precursors and as having potential in non-linear optics. Biaryls in which the donor-phenyl moiety is replaced by a 2-furyl or 2-thienyl can be synthesized similarly. The method can also be used for the convergent synthesis of previously unreported unsymmetrically substituted polyparaphenylenes ( n = 3,4).

Evidence of the Reversible Formation of Cationic π‐Allylpalladium(II) Complexes in the Oxidative Addition of Allylic Acetates to Palladium(0) Complexes

New insight into Pd-catalyzed allylic substitution: A cationic π-allylpalladium(II) complex is generated by the oxidative addition of allylic acetate to the palladium(0) complex generated from a mixture of Pd(dba)2+2 PPh3. This reaction is reversible and proceeds through at least two successive equilibria to afford free ions in DMF and ion pairs in THF (see diagram).

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. Halide anions stabilize the [Pd(0)(L)(2)] catalyst in Stille reactions, by means of [Pd(0)X(L)(2)](-) formation (X=Cl, I), hereby preventing its leaching from the catalytic cycle. Both arene (iodobenzene) and azaheteroarene (2-halopyridine, halopyrazine, 2-halopyrimidine) substrates were used.