Gilbert Meyer

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.

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).

Unexpected bell-shaped effect of the ligand on the rate of the oxidative addition to palladium(0) complexes generated in situ from mixtures of Pd(dba)2 and para-substituted triarylphosphines

As with PPh3, mixtures of Pd(dba)2 and n L (L = para-Z-substituted triphenylphosphines, n ≥ 2) in DMF lead to the formation of Pd(dba)L2 and PdL3 in equilibrium with PdL2. The equilibrium between Pd(dba)L2 and PdL3 is more in favor of PdL3 when the phosphine is less electron rich. In other words, the exchange of the dba ligand by a phosphine from Pd(dba)L2 to form PdL3 is more favored when the phosphine is less electron rich. The less ligated complex PdL2 is the reactive species in the oxidative addition with phenyl iodide. It was therefore expected that the rate of the oxidative addition would increase when the phosphine is more electron rich. However, surprisingly, when the palladium(0) complex is generated from mixtures of Pd(dba)2 and n L (n ≥ 2), the oxidative addition does not follow a linear Hammett correlation and the reactivity of the palladium(0) complex exhibits a maximum value. This is due to two antagonist effects. Indeed, the overall reactivity in the oxidative addition is governed by two factors: the intrinsic reactivity of PdL2 and its concentration. When the phosphine becomes more electron rich, the complex PdL2 becomes more nucleophilic and its intrinsic reactivity in the oxidative addition increases. However, when the phosphine becomes more electron rich, the concentration of PdL2 decreases because the equilibrium between the palladium(0) complexes becomes more in favor of Pd(dba)L2. These results emphasize the crucial role of the dba ligand on the reactivity of palladium(0) complexes generated in situ in mixtures of Pd(dba)2 and phosphines.

Rate and Mechanism of the Oxidative Addition of Phenyl Iodide to Pd0 Ligated by Triphenylarsine: Evidence for the Formation of a T-Shaped Complex [PhPdI(AsPh3)] and for the Decelerating Effect of CH2=CH−SnBu3 by Formation of [Pd0(η2-CH2=CH−SnBu3)(AsPh3)2]

The oxidative addition of phenyl iodide to the palladium(o) generated from [Pd0(dba)2] and n equivalents of AsPh3 (the most efficient catalytic precursor in Stille reactions) proceeds from [(solv)Pd0(AsPh3)2] (solv= solvent). However, the latter is present only in trace concentrations because it is involved in an equilibrium with the major, but nonreactive, complex [Pd0(dba)(AsPh3)2]. As regards the phosphine ligands, dba has a decelerating effect on the rate of the oxidative addition by decreasing the concentration of the reactive species. Relative to PPh3, the effect of AsPh3 is to increase the rate of the oxidative addition of PhI by a factor ten in DMF and seven in THF, independent of the value of n, provided that n > or = 2. In contrast to PPh3, the addition of more than two equivalents of AsPh3 to [Pd0(dba)2] (dba= trans,trans-dibenzylideneacetone) does not affect the kinetics of the oxidative addition because of the very endergonic displacement of dba from [Pd0(dba)(AsPh3)2] to form [Pd0(AsPh3)3]. The complex trans-[PhPdI(AsPh3)2], formed in the oxidative addition, is involved in a slow equilibrium with the T-shaped complex [PhPdI(AsPh3)] after appreciable decomplexation of one AsPh3. Under catalytic conditions, that is, in the presence of a nucleophile, such as CH2=CH-SnBu3 which is able to coordinate to [Pd0(AsPh3)2], a new Pd0 complex is formed: [Pd0(eta2-CH2=CHSnBu3)(AsPh3)2]; however, this complex does not react with PhI. Consequently, CH2=CH-SnBu3 slows down the oxidative addition by decreasing the concentration of the reactive species [(solv)Pd0(AsPh3)2]. This demonstrates that a nucleophile may be not only involved in the transmetallation step, but may also interfere in the kinetics of the oxidative addition step by decreasing the concentration of reactive Pd0.

Oxidative Addition of Allylic Carbonates to Palladium(0) Complexes: Reversibility and Isomerization

The oxidative addition of a cyclic allylic carbonate to the palladium(0) complex generated from a [Pd(dba)2]+2 PPh3 mixture affords a cationic pi-allylpalladium(II) complex with the alkyl carbonate as the counter-anion. This reaction is reversible and proceeds with isomerization of the allylic carbonate at the allylic position. The equilibrium constant has been determined in DMF. The influence of the precursor of the palladium(0) is discussed.

A zerovalent nickel-2,2′-bipyridine complex: an efficient catalyst for electrochemical homocoupling of ortho-substituted halides and their heterocoupling with meta- and para-substituted halides

Abstract Electroreduction of ortho-substituted halides ArX in a nickel-2,2′-bipyridyl system and N-methylpyrrolidone as solvent gave the homocoupling product ArAr. The nickel-2,2′-bipyridyl system is also a catalyst for the electrochemical heterocoupling of ArX with meta- or para-substituted halides Ar′X′.

Preparative scale synthesis ofO-glycosides and of a disaccharide by electrochemical oxidation of phenylS-glycosides

SummaryO-glycosides were synthesized by electrochemical oxidation of phenylS-glycosides in the presence of primary alcohols in acetonitrile. Similarly, a β-linked disaccharide was obtained selectively by oxidation of phenylS-glycoside in the presence of a sugar alcohol. Electrosyntheses were performed under controlled potential or at constant current, in an undivided cell, on a large scale. 1 to 60 g of phenylS-glycosides in 0.5 to 1 dm3 of acetonitrile were converted with chemical yields in the range of 65–75%.

Synthèse de biaryles dissymétriques par électroréduction d'halogénures aromatiques catalysée par des complexes du nickel associé à la 2,2′-bipyridine

Abstract Nickel-2-2′-bipyridine complexes are useful catalysts in the preparation of unsymmetrical biaryls by electroreduction of a mixture of two aryl halides which are para -substituted with electron acceptors and electron donors, respectively. The syntheses were carried out in N -methylpyrrolidone by constant current electrolyses in an undivided cell fitted with a sacrificial magnesium anode.

Electrosynthesis of Aromatic Aldehydes by Palladium-Catalyzed Carbonylation of Aryl Iodides in the Presence of Formic Acid

The palladium-catalyzed electrocarbonylation of aryl halides performed in the presence of formic acid under one atmosphere of carbon monoxide affords aromatic aldehydes in good to high yields.