Gino P. F. van Strijdonck

Deactivation processes of homogeneous Pd catalysts using in situ time resolved spectroscopic techniquesElectronic supplementary information (ESI) available: EXAFS data and Fourier Transform. See http://www.rsc.org/suppdata/cc/b2/b206758g/

UV-Vis, combined with ED-XAFS shows, for the first time, the evolution of inactive Pd dimers and trimers, that are a possible first stage in the deactivation process of important palladium catalysed reactions, leading to larger palladium clusters and eventually palladium black.

Palladium‐Catalyzed Amination of Aryl Bromides and Aryl Triflates Using Diphosphane Ligands: A Kinetic Study

[Pd(P-O-P)(Ar)]+ complexes with ligands that have wide bite angles are active catalysts for the coupling of aniline derivatives with aryl triflates. Kinetic studies show that for these systems a fast equilibrium that involves coordination of the amine precedes the deprotonation, which is the rate-limiting step of the reaction. This reaction is faster for compounds with a smaller P-Pd-P angle. When halide salts are present, the base sodium tert-butoxide is activated and adds to the palladium complex. This rate-limiting step is preceded by a fast equilibrium that involves decoordination of the halide. The initial reaction rate is faster for compounds with a larger P-Pd-P angle. This is due to the closer proximity of the oxygen to the Pd center, and this assists in the dissociation of the halide.

The effect of ligand donor atoms on the regioselectivity in the palladium catalyzed allylic alkylation.

Abstract The regioselectivity of the palladium catalyzed allylic alkylation was studied systematically using bidentate ligands based on a xanthene backbone, bearing different donor atoms. The nature of the ligand donor atoms has a pronounced influence on the regioselectivity of the reaction. The results can be explained by a mechanism that distinguishes two ‘stages’ in the alkylation reaction. Ligands bearing strong π-acceptor donor atoms induce the formation of branched products (60% for the PP derivative), whereas the use of ligands with weak π-acceptor donor atoms mainly yields linear products (>99% for the NN derivative).

On the influence of the bite angle on the allylic alkylation of (E) and (Z) substrates: loss and retention of double bond stereochemistry.

The bite angle of bidentate phosphane ligands has a pronounced influence on the degree of retention of the double bond geometry of the allylic substrate in the allylic alkylation reaction. To study the effect of the ligand on the regioselectivity, (Z)- and (E)-pent-2-enyl acetate were used as substrates. The alkylation of substrates with an (E) conformation of the double bond results in the preferential formation of the linear (E) product. A larger bite angle of the ligand results in an increase of the regioselectivity to >98% for the Sixantphos ligand. Analogously, the alkylation of (Z) substrates results in the formation of the linear (Z) product. Remarkably, for (Z) substrates, a larger bite angle of the ligand leads to an increased regioselectivity for the formation of the branched product instead of the linear product, up to 47.5% for Sixantphos. The observed regioselectivities are rationalized in terms of: a) a competition between syn-anti isomerization and alkylation, and b) a combination of steric and electronic effects in the transition state of the reaction. For all ligands tested, the reaction is faster for the (E) than for the (Z) substrate. However, competition experiments using the Sixantphos ligand show a relatively fast reaction rate for the (Z) substrate, which indicates that the coordination of the substrate to palladium is the discriminating, but not the rate-determining, step when both substrates are present.

Synthesis and characterisation of bite angle-dependent (η1-allyl)Rh and (η3-allyl)Rh complexes bearing diphosphine ligands. Implications for nucleophilic substitution reactions

Abstract Several novel rhodium allyl complexes have been prepared and their structures have been studied using NMR spectroscopy and X-ray crystallography. Depending on the bite angle of the ligand and the substitution pattern of the allyl group, two different coordination modes (η1 and η3) have been observed for the allyl moiety. The activities of these Rh allyl complexes in the allylic alkylation reaction have been tested. We have shown that both coordination modes give active complexes in this reaction, but that the regioselectivity is dependent on the coordination mode of the allyl group.

The coordination behaviour of large natural bite angle diphosphine ligands towards methyl and 4-cyanophenylpalladium(II) complexes

The structures of neutral and ionic 4-cyanophenylpalladium(II) and methylpalladium(II) complexes containing bidentate phosphine ligands were investigated in solution and in the solid state. Diphosphine ligands with a xanthene and a ferrocene backbone were used. New bis(dialkylphosphino) substituted Xantphos ligands were synthesised. 1H NMR and 31P NMR spectroscopy, conductivity measurements, UV-Vis spectroscopy, and X-ray crystallography were used to elucidate the structures of the complexes. Subtle changes of the phosphine ligands govern the coordination mode of the ligand. A variety of bidentate cis-, and trans-coordination and terdentate P–O–P, P–S–P and P–Fe–P coordination modes of the ligands were observed.

Bite angle effect of bidentate P–N ligands in palladium catalysed allylic alkylation

Two series of new bidentate P–N ligands have been synthesized. Application of these ligands in the palladium catalysed allylic alkylation of crotyl chloride and cinnamyl chloride leads to the preferential formation of the branched product. A larger bite angle of the ligand leads to higher regioselectivity. Stoichiometric alkylation of the complex [Pd(C4H7){p-MeOC6H4CN(CH2)4OPPh2}][O3SCF3] proceeds with 88% regioselectivity to the branched product.

CYCLOTRIVERATRYLENE MODELS FOR 4FE-4S PROTEINS : 3:1 SUBSITE DIFFERENTIATION AND MODULATION OF THE REDOX POTENTIAL

The potential of cyclotriveratrylene (ctv) (2,3,7,8,12,13-hexamethoxy-10,15-dihydro-5H-tribenzo[a, d,g]cyclononene) trithiols as ligands that can easily be functionalised and show subsite differentiation in their complexes with [4Fe–4S] clusters has been explored. The cluster complexes of tris(2-sulfanylethoxy)- and tris(3-sulfanyl-methylbenzyloxy)-functionalised ctvs have been studied by core-extrusion experiments, spectroscopy and electrochemical techniques. With [Fe 4 S 4 Cl 4 ] 2- as starting material a cluster complex was obtained in which the unique Fe and its co-ordinating Cl was turned into the cavity and show no reactivity. Starting with the more bulky [Fe 4 S 4 (SBu t ) 4 ] 2- the unique iron points outwards and is susceptible to substitution reactions. The effects of hydrogen bonding and electron density on the redox potential of the cluster complex have been investigated. The redox potential becomes more negative when the length of the spacer between the ctv and cluster core is increased, which is explained by the longer distance between the cluster and the electron-withdrawing phenoxy moiety of the ctv. The synthesis of ctv derivatives with one thiol and one alcohol functionality per phenyl unit, and comparison with corresponding derivatives where hydrogen bonding is not possible, showed that no significant differences were found. The effects of a substituent in an aromatic amide group that could hydrogen bond to the co-ordinated thiol were investigated. A weak effect, in the direction expected, was found upon substitution of methyl for H.