Claudio Santo

The marked influence of steric and electronic properties of ancillary pyridylthioether ligands on the rate of allene insertion into the palladium–carbon bond

Abstract Neutral methyl- and acyl-palladium chloro complexes containing pyridylthioether ancillary ligands (R′NSR) (R′=H, Me, Cl; R=Me, Et, i-Pr, t-Bu, Ph) have been synthesised and characterised by elemental analysis and spectroscopic methods. The reactivity of these complexes toward allene (allene=DMA=1,1-dimethylpropadiene; TMA=1,1,3,3-tetramethylpropadiene) insertion into the palladium–carbon bond has been studied by 1H-NMR and UV–vis techniques. The rate of reaction appears to be strongly influenced by the steric and electronic properties of the ancillary ligand. The distortion induced by the substituent R′ in position 6 of the pyridine ring on the main coordination plane of the substrate (allowed by sulphur sp3 hybridisation) renders the substrate itself more prone to nucleophilic attack by the allene. The rate of allene insertion can further be enhanced by lowering the basicity of the chelating atoms in the NS moiety which results in an increase of electrophilicity of the palladium core, so that the rate constants measured in the case of the complexes containing the ligand 6-chloro-2-phenylthiomethylpyridine (ClNSPh) are by far the greatest observed so far for similar reactions. Furthermore, on the basis of the indications emerging from the exhaustive study on the behaviour of all the related pyridylthioether methyl complexes, an associative asynchronous bond making mechanism for the rate determining nucleophilic attack by allene is proposed.

A novel mechanism for the fluxional behaviour of [Pd(η2-tetramethylethylenetetracarboxylate)(2-methylthiomethylpyridine)]

Abstract The fluxional behaviour of [Pd(η 2 -tmetc)(NSMe)] (tmetc=tetramethylethylenetetracarboxylate, NSMe=2-methylthiomethylpyridine) in CD 2 Cl 2 is governed by two mechanisms: (i) the concentration independent inversion at sulfur which averages the methylenic signals and collapses the four methyl signals of tmetc to two; (ii) the concentration dependent mechanism occurring via a dimeric intermediate with C S symmetry which collapses the two residual tmetc signals into a singlet.

The mechanism of olefin exchange in platinum(0) pyridyl–methanimine and pyridyl–thioether complexes. A kinetic study

The complexes of type [Pt(η2-ol1)(L–L′)] (ol1 = dimethylfumarate (dmf), naphthoquinone (nq); L–L′ = pyridyl–methanimine (N-N′R′; R′ = But, 4-MeOC6H4), pyridyl–thioether (N-SR; R = But, Ph) ligands) were synthesised and fully characterised by means of spectrometric and spectroscopic techniques and elemental analysis. At variance with analogous palladium complexes which display a more complicated solution behaviour, the fluxional rearrangement of pyridyl–thioether platinum(0) olefin substrates occurs via inversion at sp3 sulfur only (L–L′ = N-SR). Moreover, the exchange olefin reactions between [Pt(η2-ol1)(L–L′)] complexes and the entering olefin ol2 [ol2 = maleic anhydride (ma), fumaronitrile (fn), naphthoquinone (nq) and tetramethylethylenetetracarboxylate (tmetc)] were studied in CHCl3 either under pseudo-first order or second order conditions. On the basis of the ensuing results and of the activation parameters, an associative reaction mechanism is proposed and a novel reactivity scale for the electron poor olefins acting as entering species is determined. The crystal structures of the complexes [Pt(η2-ol)(L–L′)] (ol = fn, ma, dmf) were also determined and compared with those of analogous platinum(0) and palladium(0) species.

Mechanism of the reaction of allyl amination of Pd(II) allyl complexes containing chelating pyridine–chalcogen ligands. A surprisingly low influence of the chalcogen atom

The rates of amine nucleophilic attack on the allyl ligand (k2) and the equilibrium constants (KE) for the displacement of bidentate ligands in Pd(II) allyl complexes of chelating pyridine–chalcogen ethers [Pd(η3-allyl)(RNXPh)]+ (R=H, Me; X=S, Se) are shown to depend strongly on the steric and electronic requirements of the reactants but are hardly affected by the nature of the chalcogen. Results about the reactivity and solution behaviour of these systems help build up a fairly complete mechanistic picture for this important class of reactions involving coordinated allyl species. In particular the reactivity of the complexes bearing pyridine–thioether ligands is close to that of their pyridine–selenoether analogues, probably owing to a balance of σ and π capabilities of the chalcogen atom. The associative nature of the ligand displacement is markedly affected by steric requirements which depend on the allyl bulkiness. The complexes bearing the ligands with methyl substituted pyridine are the more reactive, due to the destabilisation of the complex ground state induced by the distortion of the starting substrate. We also describe the fluxional behaviour of these species in terms of inversion of the chalcogen absolute configuration and apparent rotation of the allyl ligand.

Insertion of 1,1-Me2propadiene across the Pd–C bond of pyridyl–thioether methyl complexes. A mechanistic study

Abstract Novel neutral and cationic palladium methyl complexes containing pyridylthioethers (R′N–SR) as ancillary ligands and different leaving groups were synthesised and characterised by spectroscopic methods and elemental analysis. The reactivity of these complexes toward allene (allene=DMA=1,1-Me 2 propadiene) insertion across the Pd–C bond was studied under different conditions. The influence of steric and electronic parameters, solvents and leaving groups was explored and an overall mechanistic picture is proposed. The distortion promoted by the ancillary ligand geometry together with the nature of the leaving group proved to be of primary importance in determining the reactivity of the complexes under study. The observed rate constants span several orders of magnitude. On the contrary, only a modest effect is observed when solvents with increasing dielectric constants are employed. The complex [Pd(PPh 3 )(Me)(MeN–S t Bu)] + shows a reactivity modulated by the presence of the phosphine which imparts to the substrate a peculiar reactivity owing to the scarce tendency of the phosphine itself to act as a leaving group.

The unexpected case of reactions of halogens and interhalogens with halide substituted Pd(II) σ-butadienyl complexes†

We have experimentally studied and theoretically interpreted the addition under stoichiometric conditions of halogens or interhalogens to σ-butadienyl palladium complexes bearing the heteroditopic thioquinolines as spectator ligands. The observed reactions do not involve the expected extrusion of the butadienyl fragment but rather the unpredictable substitution of the halide coordinated to palladium and in some cases also of that bound to the terminal butadienyl carbon. We have explained this peculiar reactivity with a mechanistic hypothesis based on a sequence of selective processes of oxidative addition and reductive elimination involving Pd(iv) intermediates.