Catalyst ligand precursors

The main advantage of these coordinating anions is that they stabilize the active species. This is particularly obvious in the case of palladium complexes, whose tendency to decompose into black metal is well documented. Imidazolium-based ionic liquids can generate in situ formation of metal-imidazolylidene carbene complexes by a deprotonation of the imidazolium cation. The ease of deprotonation depends on the nucleophilicity of the anions. In this case, NAILs may act as both solvents and catalyst ligand precursors [13],... 

The method of Ito et at. 50) as applied by Bakos et at. 12) to the reduction of acctylacetone to either ( —)-(2R,4J )- or ( + H25,45)-2,4-pentanediol will serve to illustrate how a chiral heterogeneous catalyst has been used to prepare a chiral homogeneous ligand precursor. 

Depending on the coordinative properties of the anion and on the degree of the cation s reactivity, the ionic liquid can be regarded as an innocent solvent, as a ligand (or ligand precursor), as a co-catalyst, or as the catalyst itself... 

Some of the above-mentioned catalysts or precursors are commercially available, such as the Corey catalyst (,S) - 3,3 - d i p h e n yI -1 - met h y 1 tctralr yd ro- 3 H - py r-rolo[l,2-c] [l,3,2]oxazaborole (Me-CBS). The amino alcohol (5)-(—)-2-amino-3-methyl-l,l -diphenylmethan-l-ol, used as the ligand in the Itsuno catalyst is also readily available. The ligand used to prepare the oxazaphospholidine or oxazaphosphinamide complex (from Wills) can be synthesized easily from commeri-cally available material. The preparation of the Bolm (3-hydroxysulfoximine catalyst will be described in this chapter (Figure 11.2). 

Macrocyclization. Trost and Warner17 have effected efficient cyclization to ten-and fifteen-membered rings by use of Pd(0) supported on a polystyrene bearing phosphine ligands. Precursors with an epoxy vinyl terminal group proved particularly suitable. Thus, 1 in the presence of such a Pd(0) catalyst cyclizes to 2 and 3 in 71% yield. The products are isomeric at the double bond, since both are oxidized to the ketone 4. The same catalyst system converted 1 (n = 9) into two macrocyclic isomers in 66% yield. Both cyclizations are concentration dependent. The temperature is also critical, the reaction being particularly clean at 65°. 

This screening concept was also applied to liquid/liquid systems. As a test reaction, the isomerization of allylic alcohols to carbonyls with water-soluble catalysts in a biphasic heptane/water system was chosen [109,113]. The catalysts (metal precursor, Rh, Ru, Pd, Ni ligands, sulfonated phosphane or disphosphane ligands) were injected the liquid carrier 2 (water). The substrates (different allylic alcohols) were injected into liquid carrier 1 (heptane) ... 

Crabtree and Chianese have extended the scope of Hoveyda s ligand by making the imidazolium salt 39 in two steps from l,l/-diamino-2,2/-binaphthyl (Fig. 10) [80]. They prepared neutral rhodium and iridium complexes with that ligand precursor and applied these complexes in the asymmetric hydrosilylation of acetophenone. Moderate enantioselectivities were obtained with the iridium derivative (up to 60% ee) whilst the rhodium catalysts only gave low enantioselectivities. 

The primary aldehyde product is reduced to the desired butanol, or it is subjected to a base-catalyzed aldol condensation and then hydrogenated to give 2-ethylhexanol. The phthalic ester of the latter is used as a plasticiser in PVC. The first process was based on a Co2(CO)s catalyst, a precursor of HCo(CO)4. The pressure is high, ca. 200-300 bar, in order to maintain the catalyst s stability. In the 60s Shell developed a process using phosphine ligands which allowed the use of lower pressures. The catalyst is less active but it directly produces alcohols with a somewhat higher linearity. 

Hicks and Brookhart reported the amination of a tropolone triflate using the BINAP/Pd-catalyst as a strategy to prepare new ligand precursors [106]. For example, the coupling the sterically hindered 2,5-difso-propylaniHne proceeded in 86% isolated yield, Eq. (123). 

Hydrocyanation of aldehydes opens access to the synthetically valuable cyanohydrins, precursors for hydroxycarboxylic acids, a-hydroxyketones and /S-ami-noalcohols. Applying the principles of homogeneous catalysis to this reaction it is possible to obtain cyanohydrins in the optically active form, depending on how well the catalyst-ligand system is adapted to the substrate. 

One advantage of these catalysts is the stability of their protonated precursors toward air and moisture. The ligand precursor imidazolium salts are simple to prepare and are now commercially available. Complete conversion of 4-chlorotoluene was observed for a reaction with morpholine conducted without degassing the reagent grade DME86. 

Thus eleven N-amino-N-heterocycles were synfhesized. Together with the commercially available unsubstituted N-amino-pyrrole, twelve different N-amino-azoles became available as ligand precursor. Their properties - with regard of their intended use as steering groups in the ligand backbone of the olefin polymerization catalysts - differ in terms of steric bulk of the substituents, symmetry, and electronic properties, as can be seen from inspecting Chart 3.1. 

In all these systems, one of the most important steps is the reduction of the transition metal to a low-valency state in which the metal possesses unfilled ligand sites. These low-valency transition metal species are believed to be the real catalysts or precursors of the real catalysts. 

Some of the steps in the above sequence of reactions are reduction steps in which the transition metal is reduced to a low valency state possessing unfilled ligand sites. The reduction steps are very important as the low-valency transition metal species are believed to be the real catalysts or precursors of real catalysts. For heterogeneous catalysts, the reactions are, in fact, more complicated than those shown above. Radicals formed in these reactions may be removed by different processes such as combination, disproportionation, or reaction with solvent. Unlike heterogeneous catalysts, the soluble catalysts appear to have well defined structures. For example, the soluble catalyst system that is obtained by the reaction of triethyl aluminum and bis(cyclopentadienyl)titanium dichloride is known by elemental and X-ray analysis to have a halogen-bridged structure (I) ... 

---