P.W.N.M. van Leeuwen

Rhodium catalyzed hydroformylation

Preface. 1. Introduction to hydroformylation P.W.N.M. van Leeuwen. 2. Hydroformylation with unmodified rhodium catalysts R. Lazzaroni, et al. 3. Rhodium phosphite catalysts P.C.J. Kamer, et al. 4. Phosphines as ligands P.W.N.M. van Leeuwen, et al. 5. Asymmetric hydroformylation C. Claver, P.W.N.M. van Leeuwen. 6. Hydroformylation in organic synthesis S. Castillon, E. Fernandez. 7. Aqueous biphasic hydroformylation J. Herwig, R. Fischer. 8. Process aspects of rhodium-catalyzed hydroformylation P. Arnoldy. 9. Catalyst preparation and decomposition P.W.N.M. van Leeuwen. 10. Novel developments in hydroformylation J.N.H. Reek, et al. Subject index.

Self-assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for molecular hydrogen evolution

The large-scale production of clean energy is one of the major challenges society is currently facing. Molecular hydrogen is envisaged as a key green fuel for the future, but it becomes a sustainable alternative for classical fuels only if it is also produced in a clean fashion. Here, we report a supramolecular biomimetic approach to form a catalyst that produces molecular hydrogen using light as the energy source. It is composed of an assembly of chromophores to a bis(thiolate)-bridged diiron ([2Fe2S]) based hydrogenase catalyst. The supramolecular building block approach introduced in this article enabled the easy formation of a series of complexes, which are all thoroughly characterized, revealing that the photoactivity of the catalyst assembly strongly depends on its nature. The active species, formed from different complexes, appears to be the [Fe2(μ-pdt)(CO)4{PPh2(4-py)}2] (3) with 2 different types of porphyrins (5a and 5b) coordinated to it. The modular supramolecular approach was important in this study as with a limited number of building blocks several different complexes were generated.

Rhodium catalysed asymmetric hydroformylation with diphosphite ligands based on sugar backbones

Abstract Chiral diphosphite ligands (PP) prepared from {(2,2′-biphenyl-1,1′-diyl), (4,4′,6,6′-tetra- t -butyl-2,2′-biphenyl-1,1′-diyl), 4,4′-di- t -butyl-6,6′-dimethoxy-2,2′-biphenyl-1,1′-diyl) and di(2- t -butyl, 6-methylphenyl)} phosphorochloridites and sugar backbones {1,2- O -isopropylidene-D-xylofuranose, methyl-2,3- O -isopropylidene- α -D-mannopyranoside and (methyl-3,6-anhydro)-α-D-mannopyranoside, α-D-glucopyranoside and β-D-galactopyranoside} have been used in the rhodium catalysed asymmetric hydroformylation of styrene. Enantioselectivities up to 64% have been obtained with stable hydridorhodium diphosphite dicarbonyl catalysts (HRhPP(CO) 2 ). High regioselectivities (up to 97%) to the branched aldehyde were found at relatively mild reaction conditions (T = 25–40°C, 9–45 bar of syngas pressure). The solution structures of HRhPP(CO) 2 catalysts have been studied by 31 P and 1 H NMR spectroscopy. Bidentate coordination of the diphosphite ligand to the rhodium centre takes place in a bis-equatorial way. A relation between the trigonal bipyramidal structure and the enantioselectivity of the HRhPP(CO) 2 complex is found. Rigid ligands with unsuitable geometries for bidentate coordination probably coordinate as monodentates and give rise to unstable catalysts and low selectivities during catalysis.

Rhodium-catalysed hydroformylation of branched 1-alkenes; bulky phosphite vs. triphenylphosphine as modifying ligand

The influence of alkyl substituents in 1-alkene substrates in the rhodium-catalysed hydroformylation in the presence of tris(2-tert-butyl-methylphenyl) phosphite has been studied and compared with that observed for the reaction involving the conventional PPh3-modified catalyst. Hindered alkenes underwent hydroformylation at good rates (i.e. 1300 mol (mol Rh)−1 h−1 for 3,3-dimethyl-1-butene as T = 70°C and P = 20 bar (H2CO)); under mild conditions the rates were only slightly affected by the alkyl substituents. The selectivity towards the linear aldehyde increases progressively with substitution, from 66% for 1-octene up to 100% for 3,3-dimethyl-1-butene, and the proportion of isomerized alkenes remained substantial (up to 17.4% for allylcyclohexane). The differences between the two systems are explained in terms of the different kinetics observed for them.

Insertion reactions involving palladium complexes with nitrogen ligands I. Reactivity towards carbon monoxide of methylpalladium(II) complexes containing bidentate α-diimine ligands: crystal structures of four methylpalladium(II) and acylpalladium(II) complexes

Abstract Neutral compounds of the type (2,2′-bipyridine)Pd(CH3)(Cl) and 6-R′C5H3N-2-C(H)=NR)Pd(CH3)(Cl) ( R = i Pr ; R ′ = H : i Pr  PyCa ) ( R = t Bu ; R ′ = H : t Bu-PyCa ) ( R = i Pr ; R ′ = CH 3 : i Pr -6- Me  PyCa ) (R = CH2CH2C6H5;R′ = H: PhePyCa) ( R = t Bu ; R ′ = CH 3 : t Bu -6- Me  PyCa ) ( R = i Pr ; R′ = C ( H ) O : i Pr  IPA ) have been synthesized starting from (1,5-cyclooctadiene)Pd(CH3)(Cl) and ionic compounds of the type [(N-N)Pd(CH3)]BF4 (NN = bipy, iPrPyCa, tBuPyCa or iPr-6-MePyCa) by treatment of the methyl(chloro)palladium compounds with silver tetrafluoroborate. All products have been characterized by spectroscopic methods. Reaction of the compounds with carbon monoxide gives that neutral acyl compounds (NN)Pd(C(O)CH3)(Cl) and ionic acyl compounds [(NN)Pd(C(O)CH3)]BF4 (NN = bipy, iPrPyca, tBuPyCa, iPr-6-MePyCa or PhePyCa). The crystal structures of (iPr-6-MePyCa)Pd(CH3)(Cl), (iPrIPA)Pd(CH3)(Cl), (bipy)Pd(C(O)CH3)(Cl) and (iPrPyCa)Pd(C(O)CH3(Cl), have been determined. Comparison of data for the PdNCC5H4N moieties of the complexes concerned show that the geometries are almost identical, the largest r.m.s. deviation (between (iPr-6-MePyCa)Pd(CH3)(Cl) and (bipy)Pd(C(O)CH3)(Cl)) being 0.2 A. The PdC bond distances in the two acetylpalladium complexes are 0.05 A shorter than those in the two methylpalladium complexes. The PdN bond distances for the nitrogen atoms situated trans to the organic group are shorter by 0.11 A in the case of acetyl ligands. An unprecedented influence of the steric properties of the ligands on the half-life for the CO insertion is observed; substituents adjacent to the nitrogen donor atoms cause strongly acceleration.

Influence of various P/N and P/P ligands on the palladium-catalysed reductive carbonylation of nitrobenzene

Abstract A series of bidentate phosphorus—nitrogen ligands was synthesised for the palladium-catalysed reductive carbonylation of nitrobenzene in order to combine the favourable influence of the phosphorus atom on the stability of the catalyst complex with the stimulating effect of the nitrogen atom on the catalytic activity. The nitrogen atom of the P/N ligand was either incorporated in an imine function, yielding the N -(2′-diphenylphosphinobenzylidene)—R—amine ligands (R = phenyl, 4-chlorophenyl, 2,4-dimethoxyphenyl, 2,4-dimethylphenyl, tert -butyl), or in a heteroaromatic ring system which gave 2-(2′-(diphenylphosphino)ethyl)pyridine and 8-(diphenylphosphino)quinoline. Complexes of the type Pd(ligand) 2 (BF 4 ) 2 were prepared for these ligands. Additionally, a series of bidentate phosphorus ligands was tested: dppm, dppe, dppp, dppb, dppf, 1,2-bis(diphenylphosphino)benzene, 1,8-bis(diphenylphosphino)naphthalene, bis(2-diphenylphosphino-phenyl)ether, and 9,9-dimethyl-4,6-bis(diphenylphosphino)xanthene. The P/N ligands containing the imine function did not yield any conversion of the nitrobenzene in combination with Pd. On the use of the second type of P/N ligand, moderately active palladium catalysts were obtained. This different behaviour is ascribed to the relatively low π * -level of the imine-containing ligands. Oxidation of the phosphorus donor atom by the nitro substrate inactivated the catalysts derived from the P/N ligands as well as from a series of P/P ligands. For the bidentate phosphorus ligands the bite angle and flexibility of the ligand turned out to be of crucial influence due to the different geometries required for the Pd(II) and Pd(O) intermediates of the catalytic cycle.

Rhodium catalysed hydroformylation of higher alkenes using amphiphilic ligands

A variety of amphiphilic ligands has been synthesised comprising PhzArP (Ar= 3-hydroxyphenyl, 4-carboxyphenyl), Ph,Ar, _ .P ( Ar = 4-PhCH,X, X = NE& NMePh, NPh,; n = l-2) and Ph,Ar, -,P ( Ar = 3/4-pyridyl; n = l-2). In the hydroformylation of act-1-ene (80°C 20 bar syngas, toluene) the ligands were shown to be comparable with triphenylphosphine. Tumover frequencies of 2.2 X lo3 (mol aldehyde . mol Rh ’ . h’ ) were found for most ligands with an act1 -ene concentration of 0.84 M. The pyridylphosphines were up to two times faster. The selectivity of the hydroformylation is not affected by the modifications and in all cases aldehydes were formed with a n/b ratio of 2.8. Ph,P( 4-C,H&OOH) showed low catalytic activity under standard conditions. Preliminary experiments have shown that the new ligands in their protonated, water-soluble form do not produce active hydroformylation catalysts.

The bite angle makes the catalyst

Catalytic reactions are described for metal complexes containing bidentate phos- phine ligands that enforce wide bite angles in the complexes. The calculated natural bite angles are in the range 100-1208. Three applications will be discussed: (i) nickel catalysed hydro- cyanation, for which the first active phosphine catalyst was found, (ii) palladium catalysed allylic alkylation, for which the selectivity also strongly depends on the bite angle, and (iii) rhodium catalysed hydroformylation, which leads to highly linear products.

Copper-catalysed asymmetric 1,4-addition of organometallic reagents to 2-cyclohexenone using novel phosphine-phosphite ligands

Abstract A series of novel non-symmetrical phosphine-phosphite ligands 1 – 10 have been tested in the copper-catalysed asymmetric addition of diethylzinc and triethylaluminium to 2-cyclohexenone. In all cases, excellent conversion and regioselectivities for the 1,4-product were found. Good enantiomeric excesses (up to 62%) were obtained when triethylaluminium was used as the alkylating reagent.

On the Influence of the Bite Angle of Bidentate Phosphane Ligands on theRegioselectivity in Allylic Alkylation

The natural bite angle of bidentate phosphane ligands influences the isomer distribution (syn and anti) in (1-methylallyl)(bisphosphane)Pd OTf complexes. It was found (31P- and 1H-NMR studies) that the syn/anti ratio changes from 12 (dppp) to 1.3 (sixantphos). Molecular orbital calculations [PM3(tm) level] indicate that for ligands inducing a large bite angle, the phenyl rings of the ligand embrace the allyl moiety, thus influencing the syn/anti ratio. This bite-angle effect on the syn/anti ratio is transferred to the regioselectivity in stoichiometric allylic alkylation. Ligands inducing large bite angles direct the regioselectivity towards the formation of the branched product 2. Catalytic alkylation of (E)-2-butenyl acetate showed that for ligands with a small bite angle the regioselectivity of the catalytic and stoichiometric alkylation are in good agreement. This correspondence is worse for ligands with a larger bite angle, which is rationalised in terms of the relative rates of syn/anti isomerisation and alkylation. The ligand with the largest bite angle (sixantphos) gives the most active catalytic species.