Stefan Warsink

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo- and stereoselectivity when using a zerovalent palladium(NHC)(maleic anhydride)-complex as precatalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuterium-labeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR, we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations, we propose a mechanism in which hydrogen transfer from coordinated formate anion to zerovalent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.

Biscarbene palladium(II) complexes. reactivity of saturated versus unsaturated N-heterocyclic carbenes.

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by (1)H and (13)C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C((saturated NHC)) is distinct from that of Pd-C((unsaturated NHC)). The Pd-C((saturated NHC)) bonds are fairly stable toward reagents such as CF(3)COOH, AgBF(4) and I(2), whereas Pd-C((unsaturated NHC)) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)(2)PdCl(2) > (sat-NHC)(unsat-NHC)PdCl(2 )> (unsat-NHC)(2)PdCl(2).

Synthesis and characterization of PdII-methyl complexes with N-heterocyclic carbene-amine ligands

A number of palladium(ii) complexes with a heteroditopic NHC-amine ligand and their precursor silver(i) carbene complexes have been efficiently prepared and their structural features have been investigated. The heteroditopic coordination of this ligand class was unequivocally shown by NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere rapid reductive elimination of the acyl-imidazolium salt was observed.

Intramolecularly Stabilised Group 10 Metal Stannyl and Stannylene Complexes: Multi‐pathway Synthesis and Observation of Platinum‐to‐Tin Alkyl Transfer

Reaction of [PdClMe(P^N)2] with SnCl2 followed by Cl-abstraction leads to apparent Pd-C bond activation, resulting in methylstannylene species trans-[PdCl{(P^N)2SnClMe}][BF4] (P^N = diaryl phosphino-N-heterocycle). In contrast, reaction of Pt analogues with SnCl2 leads to Pt-Cl bond activation, resulting in methylplatinum species trans-[PtMe{(P^N)2SnCl2}][BF4]. Over time, they isomerise to methylstannylene species, indicating that both kinetic and thermodynamic products can be isolated for Pt, whereas for Pd only methylstannylene complexes are isolated. Oxidative addition of RSnCl3 (R = Me, Bu, Ph) to M(0) precursors (M = Pd or Pt) in the presence of P^N ligands results in diphosphinostannylene pincer complexes trans-[MCl{(P^N)2SnCl(R)}][SnCl4R], which are structurally similar to the products from SnCl2 insertion. This showed that addition of RSnCl3 to M(0) results in formal Sn-Cl bond oxidative addition. A probable pathway of activation of the tin reagents and formation of different products is proposed and the relevancy of the findings for Pd and Pt catalysed processes that use SnCl2 as a co-catalyst is discussed.

(N-Benzoyl-N′,N′-diphenyl­thio­ureato-κ2S,O)(η4-cyclo­octa-1,5-diene)rhodium(I)

The title complex, [Rh(C20H15N2OS)(C8H12)], exhibits an essentially square-planar coordination environment around the RhI atom, which bears a bidentate cyclooctadiene ligand as well as a monoanionic bidentate benzoylthioureate ligand. The RhI atom, the S- and O-donor atoms and the alkene centroids of the cyclooctadiene ligand do not deviate by more than 0.031 Å from their least mean-squares plane.

Crystal structure of bis(triphenylphosphine)(carbonyl)(N-benzoyl-N'-(2-methyl-4-hydroxyphenyl)thioureato-κS,κN)rhodium(I), C52H43N2O3P2RhS

Abstract C52H43N2O3P2RhS, triclinic, P1̅ (no. 2), a = 10.6260(3) Å, b = 11.7305(3) Å, c = 19.6582(5) Å, α = 76.489(1)°, β = 82.027(1)°, γ = 64.848(1)°, V = 2154.4 Å3, Z = 2, Rgt(F) = 0.028, wRref(F2) = 0.0722, T = 100 K.

trans-Bis[1-(2-benzamido­eth­yl)-3-(2,4,6-trimethyl­phen­yl)imidazol-2-yl­idene]dichloridopalladium(II)

In the title compound, [PdCl2(C21H23N3O)2], the PdII atom is located on an inversion centre and is coordinated in a slightly distorted square-planar environment by the chloride and N-heterocyclic carbene (NHC) ligands in mutual trans positions. There are several hydrogen-bonding interactions, the most significant of which is a hydrogen bond between the amide moiety of the NHC and the chloride ligand. These hydrogen-bond interactions form a three-dimensional network.

trans-Carbonyl­chloridobis[tris­(4-meth­oxy­phen­yl)phosphane-κP]rhodium(I)

The title complex, [RhCl(C21H21O3P)2(CO)], is a rhodium analogue to Vaska’s complex with para-methoxy substituents on the six phosphanyl–aryl units. Two independent molecules are present in the unit cell, with their metal atoms both located on an inversion centre. This causes the chloride and carbonyl ligands to exhibit a positional disorder in a 0.5:0.5 ratio. The two RhI atoms exhibit a distorted square-planar geometry. There are a few weak intramolecular C—H⋯X interactions (X = O, Cl). Interestingly, no significant intermolecular interactions are found between the two independent molecules.

trans-Carbonyl­chloridobis(tri-o-tolyl­phosphane-κP)rhodium(I)

In the title compound, [RhCl(C21H21P)2(CO)], the coordination geometry around the RhI atom is slightly distorted square-planar with the phosphane ligands in trans positions with respect to each other. The chloride and carbonyl ligands show positional disorder, and the RhI atom lies on a center of inversion. The effective cone angle ΘE for the title compound is 169.0 (3)°. There are no significant intermolecular interactions.