Mechanistic Investigations, Complexes, and Ligands

The mechanism of the hydroformylation has been intensively investigated by Drent and Budzelaar [6], who analyzed the competition between alternative reactions once a Pd-acyl complex was formed from a Pd-hydride species. The reaction with a second olefin leads to ketones (hydroacylation) and polyketones (copolymerization), respectively, whereas upon hydrogenolysis of the Pd-acyl bond, an aldehyde is released and thus a catalytic hydroformylation cycle is finally closed. Because of the high hydrogenation activity of palladium complexes, the aldehydes formed may be immediately converted into the corresponding alcohols. The type of the actually observed reaction pathway is mainly determined by [6] 

1) Only weakly coordinating ligands allow the CO insertion into the Pd-C bond and subsequent reactions [6]. The chemoselectivity for the hydroformylation decreases with increasing acid strength. The addition of HCl or HOAc blocked entirely the reaction. The following order of activity with respect to the acid has been established in the hydroformylation of propene and 1-octene  

Addition of urea, which forms hydrogen bonds with the anion, enhances the hydroacylation over the hydroformylation route as a result of decreased coordination strength [8]. 

3) Halide anions affect the rate of the hydroformylation of internal olefins as well as its chemo- and regioselectivity [7]. The rate of hydroformylation of thermally equilibrated internal higher alkenes increased by a factor of 6-7 by the addition of substoichiometric amounts (with respect to palladium) of Cl or Br and about a factor of 3-4 with I . Moreover, the selectivity toward the formation of the alcohol was dramatically increased. Highest yields of alcohols were noted with the assistance of iodide. Only traces of alkanes were formed. Up to now, a general explanation of the effect could not be given, but it seems that it is also dependent on the diphosphine ligand used. 

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Phosphorus ligands are crucial for the stabilization of the systems and the complex cis-[PtCl2(PPh3)2] (16) is most often employed, but complexes with chelating diphosphines also have been studied extensively. The stability of the related alkyl- and acylplatinum(II) complexes has favored extensive mechanistic investigations based on studies of the reactivity of model complexes. 

More attention has been devoted to aromatic and heteroaromatic substrates since first reported in 1983 [40]. The results are shown in Table 2 [25, 41-51]. All these reactions were run with nickel complexes associated with a phosphane or bpy ligand. Depending on the experimental conditions, the polymers were either precipitated during the electrolysis or deposited as films at the surface of the electrode. The method is also convenient to prepare copolymers from a mixture of two aryl dihalides. A mechanistic investigation on the nickel-bpy catalyzed polymerisation has been reported very recently [52]. 

These mechanistic investigations are supported by theoretical studies on the model compound [(PH3)3RhH] and they confirm the formation of a formate unit coordinating in a -binding mode in the presence of three phosphine ligands [47]. Recent ab initio calculations pointed out that the -formate unit is the most stable coordination mode in complexes, but the species incorporating the -bonded formate seems to be the more reactive intermediate [48]. 

A main question - not yet really considered - concerns the inertness of ionic liquids. Not only are the anions potential ligands, especially for neutral and cationic metal complexes one has also to take into consideration what is known for cations like (imid)azolium formation of carbene complexes via deprotonation is a rather facile process especially if ligands of sufficient basicity are present, e. g., -OR, -NR2. Therefore, several of the impressive catalytic results [113] deserve mechanistic investigation to find out whether they are really limited to the ionic liquid effects. For example, solvent and complexation effects are likely to enhance one another in the Heck coupling reactions that were run in the presence of Structure 18, Scheme 11 [115]. 

Of the transition metals listed above, chromium complexes are the most frequently used. Mechanistic investigations reveal that the transfer of a ligand within a carbene - alkene - metal complex rather than a free carbene is involved in these reactions. The best yields result if equimolar amounts of the alkene and the metal complex are used, the application of a large excess of alkene (ca. 10 mol) is not necessary. 

Two mechanistically plausible scenarios for nucleophilic attack on the q benzyl palladium species seem feasible. Formation of the C N bond could occur either via external attack of the amine through inversion of configuration at the carbon stereocenter, or alternatively the amine could coordinate to palladium followed by an internal attack on the q benzyl ligand. Mechanistic investigations [15] using stoi chiometric amounts ofthe enantio and diastereomerically pure q benzyl palladium complex [ (R) Tol BINAP [q 1 (2 naphthyl)ethyl Pd](OTf) (8) revealed that the re action with aniline produced predominantly (R) N1 (2 naphthyl)ethylaniline R) 9), consistent with external nucleophilic attack (Scheme 11.3) [15]. However, it was noted that the catalytic reaction of [ (1 ) Tol BINAP Pd(OTf)2] with vinyl arenes and amines produced preferentially the opposite enantiomeric (S) amine hydroamination... 

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