P.C.J. Kamer

Supraphos: A Supramolecular Strategy To Prepare Bidentate Ligands

We report a new strategy for the preparation of chelating bidentate ligands, which involves just the mixing of two monodentate ligands functionalized with complementary binding sites. In the current example, the assembly process is based on selective metal-ligand interactions, using phosphite zinc(II) porphyrins 1-6 and the nitrogen donor ligands b-i. From only 16 monodentate ligands, a library of 60 palladium catalysts based on 48 bidentate ligand assemblies has been prepared. The relatively small catalyst library gave a large variety in the selectivity of the alkylation of rac-1,3-diphenyl-2-propenyl acetate. Importantly, small variations in the building blocks lead to large differences in the enantioselectivity imposed by the catalyst (up to 97% ee).

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.

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.

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.

Dendrimers in catalysis

Dendrimers are well-defined hyperbranched macromolecules, with the larger ones having characteristic globular structures. These novel materials have been investigated intensively in recent decades, and among other potential applications, they are suitable as soluble supports for homogeneous transition metal complex catalysts. These catalysts offer the advantage of easy separation from products and recycling as well as the potential advantages of unique catalytic properties, including high activity, selectivity, and stability. In this chapter, the current state of research in dendrimer catalysis is reviewed, with emphasis on the importance of the location of the catalyst in the dendritic framework (e.g., at the core or at the periphery). Several approaches to the separation of dendrimer catalysts are evaluated, and unique dendritic effects in catalysis are discussed.

NOVEL P-CHIRAL BIDENTATE PHOSPHINE LIGANDS : SYNTHESIS AND USE IN ASYMMETRIC CATALYSIS

Abstract Two C2-symmetrical P-chiral diphosphines, (R,R)-(−)-1,1′-bis[phenyl-(2-methoxyphenyl)phosphino]ferrocene (BPAF), and (R,R)-(−)-1,1′-bis(phenyl-1-naphthyl-phosphino)ferrocene (BPNF), were synthesized via a stereocontrolled nucleophilic substitution sequence. The catalytic performance of the new ligands was investigated in several allylic alkylation reactions; with BPNF asymmetric inductions up to 73% e.e. for dimethyl (1,3-diphenyl-2-propenyl)propanedioate were obtained.

NMR spectra and crystal structure of a regioselective hydroformylation catalyst complex RhH(diphosphite)(CO)2

Abstract The crystal structure and characterization in solution are described for a very regioselective hydroformylation catalyst, RhH(CO) 2 (diphosphite) with as the diphosphite. The complex is formed under COH 2 pressure from Rh(CO) 2 Acac and has a distorted TBP geometry with both phosphorus atoms in equatorial positions.