C.F. Roobeek

Hydroformylation of less reactive olefins with modified rhodium catalysts

The otherwise unreactive olefins (2-methyl-1-hexene, limonene, cyclohexene, methylene cyclohexane) are hydroformylated under mild conditions (90°C, 10 bar) in the presence of phosphite-modified rhodium catalysts. The high rates observed are attributed to the steric and electronic properties of these phosphite ligands and their ability to stabilize unsaturated rhodium species. Examples of these ligands are tris(o-t-butylphenyl) phosphite and tris(hexafluoroisopropyl) phosphite, which are, respectively, sterically demanding and strongly electron-withdrawing.

Influence of ligands and anions on the insertion of alkenes into palladium-acyl and palladium-carbomethoxy bonds in the neutral complex (dppp)Pd(C(O)CH3)Cl and the ionic complexes [(PP)PdR(L)]+SO3CF3− (PP = dppe, dppp, dppb; R C(O)CH3, L CH3CN, PPh3; R C(O)OCH3, L PPh3)

Abstract Insertions of alkenes in Pd-acetyl bonds of (dppp)Pd(C(O)CH3)Cl and [(PP)Pd(C(O)CH3)L)]+ SO3CF3− (PP = dppe, dppp, dppb; L  CH3CN, PPh3) have been studied as a function of the ligand, the anion and the alkene. The neutral acetyl complex (dppp)Pd(C(O)CH3)Cl underwent insertion only with norbornadiene and norbornene, while the ionic acetyl complexes [(PP)Pd(C(O)CH3)(L)]+ SO3CF3− (PP = dppe, dppp, dppb) reacted with norbornadiene, norbornene, styrene, cis-stilbene, 1-pentene, 3,3-dimethyl-1-butene, vinyltrimethylsilane, methyl vinyl ketone, methyl acrylate, diethyl fumarate, and diethyl maleate. The insertion was observed to give an intermediate in which there was intramolecular coordination of the ketone oxygen atom to the palladium centre. In monosubstituted alkenes the acetyl group migrates to the unsubstituted carbon atom. The insertion products underwent β-elimination to give (trans) unsaturated ketones and a palladium hydride. The rate of this elimination was higher for complexes containing ligands LL with smaller bite angles (dppe > dppp), and the rate of insertion showed the reverse order. The carbomethoxy complexes [(PP)Pd(C(O)OCH3)(PPh3)]+ SO3CF3− (PP = dppe, dppp, dppb) were prepared from (PP)Pd(SO3CF3)2 with CO, CH3OH and PPh3. The carbomethoxy complex reacted with norbornadiene to give a carbomethoxy oxgyen-coordinated intermediate. The carbomethoxy complexes were less reactive than the analogous acetyl complexes towards alkenes.

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.

The hydroformylation of butadiene catalysed by rhodium-diphosphine complexes

Abstract Bidentate phosphine ligands bridged by two carbon atoms strongly promote the rhodium-catalysed hydrofonnylation of 1,3-dienes. Particularly interesting is a conversion under mild conditions of butadiene to pentanal with over 90% selectivity and less than 1% branched products. A mechanism involving η3-allyl rhodium species is proposed.

On the mechanism of the nickel-catalysed regioselective cyclodimerization of isoprene

Abstract A study of various Ni-ligand catalysed oligomerizations of isoprene has shown that with π-acidic P ligands the selectivity to cyclodimers amount to 97%. A new type of ligand is introduced, viz. fluoroalkyl phosphites having π-acceptor properties comparable to those of, e.g. PCl3. With tris(hexafluoroisopropyl) phosphite the main product is 1,4-dimethy1-4-vinylcyclohexene. A detailed explanation based on a two-step mechanism is given. As to the first step, for a series of ligands having similar steric properties the changes in product distribution as a function of the electronic ligand parameter are explained in terms of a gradual change in HOMO-LUMO interactions between Ni and the olefins, with strong π-acidic ligands promoting the head-to-head coupling of the isoprene molecules. The second step, involving reductive elimination of a cyclodimer from the metal, shows an increasing selectivity towards substituted cyclohexenes for the head-to-head and tail-to-tail intermediates with increasing π-acidity of the ligand. The qualitative orbital treatment presented as an explanation is also applicable to reactions found for other metallacyclopentanes.

Photogeneration of reactive titanium and zirconium species: a CIDNP study

Abstract Chemically induced dynamic nuclear polarization (CIDNP) studies of the photolytically initiated reactions of Cp′2Ti(CH3)2 (Cp′ = methylcyclopentadienyl) and Cp2Zr(CH3)2 (Cp = cyclopentadienyl) have revealed that homolysis of the metalmethyl bond takes place, followed by a rapid recombination. The metal radical is shown to be one of the patterns in the polarizing radical pair. Only very fast scavengers such as nitroxides, dioxygen and thiophenol can compete with the recombination reaction. Dimethylzirconocene undergoes a photochemically-initiated ethylene insertion reaction in which the olefin is inserted into the zirconiummethyl bond via an unprecedented radical process.

Photodecomposition of platinum and palladium alkyls: A cidnp study

Abstract Several alkyl-palladium and -platinum compounds of the general formulae L2MR2 and L2MRX have been subjected to UV irradiation. A reaction occurs in chlorinated solvents such as chloroform to yield a metal chloride and a radical pair consisting of the alkyl stemming from the organometallic compound and a solvent-derived radical. The radical pair reaction is revealed by CIDNP. The decomposition products containing one more carbon atom than expectedaccording to this mechanism can be accounted for by a novel type of rearrangement, viz. an intramolecular carbene migration. In hydrocarbon solvents alkylplatinums undergo very little reaction, but alkylpalladiums decompose homolytically.

Intramolecular rearrangement of platinum and palladium carbenoids

Abstract Compounds of the general formula (diphos)M(R)(R′), where diphos represents 1,2-bis(diphenylphosphino)ethane, M is Pd or Pt, R is an alkyl group, and R′ is a dichloro- or trichloromethyl group which serves as the carbene precursor, exhi- bit an intramolecular rearrangement whereby the carbene moiety is inserted into the platinum—carbon or palladium—carbon bond. When R′ = CHCl 2 the product is an α-chloroalkylmetal chloride (M = Pd, Pt), and when R′ = CCl 3 the initially formed α,α-dichloroalkylplatinum chloride ((diphos)PtClCCl 2 R) eliminates hydrogen chloride to give α-chlorovinylplatinum chloride, (diphos)PtCl- (CClCHR″), as the final product.

Photolysis of methyl(triphenylphospine)gold(I)

Abstract Methyl(triphenylphosphine)gold(I) reacts with chloroform under UV irradiation to give methyl and solvent-derived radicals in triplet pairs showing CIDNP. An S H 2 substitution by methyl radicals at the gold centre is observed.

Platinum hydroformylation catalysts containing diphenylphosphine oxide ligands

Platinum complexes of the general formula Pt(H)(Ph2PO)(Ph2POH)(PPh3)(1a) catalyse the hydroformylation of hept-1-ene and, more significantly, hept-2-ene, yielding products of high linearity (90 and 60%, respectively); the intermediate alkyl and acyl complexes (1c–e) which most often escape direct observation in a catalytic system, have been successfully isolated and identified.