Jacky Kress

A simple route to molybdenum–carbene catalysts for alkene metathesis

A convenient method for the synthesis of high oxidation state tetra-co-ordinate molybdenum–carbene complexes such as Mo(NBut)(CHBut)[OCH(CF3)2]2, which are catalytically active in alkene metathesis reactions, is reported.

Molecular complexes for efficient metathesis of olefins. The oxo-ligand as a catalyst–cocatalyst bridge and the nature of the active species

Three families of molecular complexes for the homogeneous metathesis of olefins are described; the role of the oxo-or imido-ligand as bridge for binding the Lewis acid and the nature of the active species in meta-thesis are discussed.

Catalytic oxidation and ammoxidation of propylene. Modelling studies on well-defined molybdenum complexes

Abstract Molybdenum (VI)-allyloxo and -allylamido complexes possessing ancillary oxo and imido ligands have been synthesised and their subsequent decomposition studied in the perspective of modelling the postulated surface intermediates of the catalytic oxidation and ammoxidation of propylene. In solution and under mild conditions, these complexes undergo two types of reactions corresponding effectively to certain elemental steps of the heterogeneous processes, i.e. oxidative dehydrogenation of the allyl groups and allyl migration to oxo and imido functions. The mechanisms and the relative rates of these reactions are compared with that proposed for the surface catalytic chemistry. The possibility of alternate reaction pathways is discussed such as for CN bond formation and the involvement of an allylideneamine intermediate in the catalytic cycle.

Kinetics of initiation and propagation of the metathesis polymerization of the exo diels-alder adduct of cyclopentadiene and maleic anhydride initiated by the tungsten-carbene complex W[H2](OCH2CMe3)2Br2

Abstract The title reaction was followed by 1H NMR at 277 – 314 K. The initiator (I) adds one molecule of monomer (M) to give a tungsten-carbene species P1 which then adds further monomer molecules to give Pn (n > 1). P1 and Pn (n > 1) can be distinguished by their (OCH2CMe3)2 signals at 4.44, 4.39 and 4.46, 4.41 ppm respectively; the two neopentoxy ligands are non-equivalent in each case. [P1] builds up to a maximum (65% of [I0]) and then declines. At 294 K the initiator is all consumed after 40 min and [M] then decays according to a first-order law (t 1 2 = 74 min for [Ptotal] = [I0] = 5.0 × 10−2 mol dm−3). The activation energy for the propagation reaction is about 51 kJ mol−1. The initiation rate constant is 3.8 ± 0.2 times as large as the propagation rate constant. The 13C NMR spectrum of the exo, exo-dimethyl ester polymer derived from the polymer of M indicated that the polymer contained 28% cis double bonds. The endo adduct of cyclopentadiene and maleic anhydride was also found capable of adding to the living chain; this is the first report that the endo monomer can undergo metathesis polymerization.

The detection of ‘living’ propagating tungsten–carbene complexes in the ring-opening polymerization of bicycloalkenes

The tungsten–carbene complex W(CHCMe3)(OCH2CMe3)2Br2, when mixed with GaBr3, adds various bicyclo[2.2.1]hept-2-enes to form ‘living’ propagating carbene complexes which may be characterized by 1H n.m.r. spectroscopy, and used to make block copolymers.

Cyclization of living polyalkenamers via intramolecular secondary metathesis. Dimerization of cycloheptene into cyclotetradeca-1,8-diene initiated by well-defined tungsten-carbene catalysts

Abstract Tungsten-carbene complexes of the type W ( = CRR′) (OR″)2X2 · GaX3 [CRR′ = C(CH 2 ) 3 C H2, C(CH 2 ) 4 C H2, CHt-Bu or CHn-Bu; OR″ = OCH2t-Bu, OCD2t-Bu or Oi-Pr; X = Br or Cl] (I) have been used as catalysts to initiate ring-opening metathesis polymerization (ROMP) of cyclopentene, cycloheptene and cyclooctene (M). These reactions were followed by 1H and 13C NMR spectroscopy at variable temperature. Living polyalkenamers of the type W ( =CHP) (OR″)2X2 · GaX3 [CHP = CHCmH2mCH= (CHCmH2mCH)n − 1 =CRR′, m = 3 (cyclopentene), 5 (cycloheptene), 6 (cyclooctene); n ≥ 1] (PC) were obtained in a first stage. These products undergo secondary metathesis reactions. In particular, intramolecular metathesis between the carbon-carbon double bonds and the tungsten-carbene chain-end function, within a given polymer chain, leads to regeneration of monomer or to cyclic oligomers. The thermodynamic equilibrium between these species is reached at varying rates depending on catalyst, monomer and temperature. For cyclopentene, only monomer was found at room temperature, while polypentenamers are the major species at low temperature. For cycloheptene, the corresponding polymerization equilibrium also exists, but is displaced more towards polyalkenamers. Moreover, the cyclic dimer, cyclotetradeca-1,8-diene (D), was obtained in high proportions under appropriate conditions, and its most stable trans, trans isomer was isolated from the reaction mixtures. For cyclooctene, conversion into polyoctenamers is rapid and complete at room temperature. In a third stage, catalysts I slowly convert olefins M, D and P into saturated polymers at room temperature. This reaction is also induced by GaBr3 alone. Complexes of the type W ( = CRR′) (OR″)2X2 react similarly to their GaBr3 adducts, although much more slowly.

Isomerization of 2-methyl-3-butyn-2-ol into prenal: mechanistic observations on tricomponent catalyst systems of the type Ti(OR)4–CuCl–R′CO2H. Crystal structure of [CuCl(Me2C(OH)CCH)]4

Abstract The mechanism of the title catalytic system was investigated, in particular by synthesizing several compounds that model the postulated metal–oxo–propargyloxo intermediates, such as [Ti(μ-O)(OBu t )(O 2 CTol)] 2 (Tol= p -MeC 6 H 4 ), Ti(OCMe 2 CCH) 4 and [CuCl(Me 2 C(OH)CCH)] 4 . The molecular structure of [CuCl(Me 2 C(OH)CCH)] 4 was determined by X-ray crystallography and heterobimetallic Ti–Cu complexes comprising the 2-methyl-3-butyn-2-oxo fragment are reported.

Metathesis polymerisation of norbornene and its derivatives. Relative stability of tungstacyclobutane intermediates and kinetics of their ring opening to form tungsten–carbene complexes

A family of tungstacyclobutane intermediates in the catalytic ring-opening polymerisation of norbornenes have been studied by 1H n.m.r.; their rates of rearrangement into tungsten–carbene complexes are reported and compared.

X-ray crystal structure of W(OAlBr3)(CH2t-Bu)3Br, a model for catalyst−cocatalyst interaction

Abstract The crystal structure of the acid-base adduct obtained from WO(CH2t-Bu)3Br and AlBr3 is described. The interaction between the Lewis acid and the oxo ligand of the tungsten compound is discussed on the basis of bond distances and bond angles in the BrWOAl linkage. The role of the WOAl linkage in homogeneous catalysis and of such interactions modelling the catalyst-support interactions in heterogeneous catalysis are briefly considered.

Tungsten (IV) carbenes for the metathesis of olefins. Direct observation and identification of the chain carrying carbene complexes in a highly active catalyst system

On addition of aluminium halides to the carbene complexes W(CHBut)(OCH2But)2X2 conversion into highly active catalysts for olefin metathesis occurs; the nature of the initiating and propagating species in olefin metathesis has been identified by n.m.r. spectroscopy.