Transition Metal Catalysts with Phosphine Ligands

1 Transition Metal Catalysts with Phosphine Ligands 

Hydroformylation is an important industrial process carried out using rhodium phosphine or cobalt carbonyl catalysts. The major industrial process using the rhodium catalyst is hydroformylation of propene with synthesis gas (potentially obtainable from a renewable resource, see Chapter 6). The product, butyraldehyde, is formed as a mixture of n- and iso- isomers the n-isomer is the most desired product, being used for conversion to butanol via hydrogenation) and 2-ethylhexanol via aldol condensation and hydrogenation). Butanol is a valuable solvent in many surface coating formulations whilst 2-ethylhexanol is widely used in the production of phthalate plasticizers. 

Since butyraldehyde has a low boiling point (75 °C) separation of catalyst from both reactants and product is straightforward. Most of the rhodium remains in the reactor but prior to recovery of propene and distillation of crude product the gaseous effluents from the reactor are passed through a demister to remove trace amounts of catalyst carried over in the vapour. This ensures virtually complete rhodium recovery. 

Although rhodium recovery is efficient it is difficult to separate it from heavies that are formed in small amounts. Over time these heavies tend to result in some catalyst deactivation. One solution to this problem has been developed by Ruhrchemie/Rhone-Poulenc. In this process sulfonated triphenyl phosphine is used as the ligand, which imparts water solubility to the catalyst. The reaction is two-phase, a lower aqueous phase containing the catalyst and an upper organic phase. Fortunately the catalyst appears to sit at the interface enabling reaction to proceed efficiently. At the end of 

Cobalt catalysts such as HCo(CO)4 are widely used for hydroformyla-tion of higher alkenes, despite the higher temperatures and pressures required. The main reason for this is that these catalysts are also efficient alkene isomerization catalysts, allowing a mix of internal and terminal alkenes to be used in the process. Catalyst recovery is more of a problem here, involving production of some waste and adding significantly to the complexity of the process. A common recovery method involves treating the catalyst with aqueous base to make it water soluble, followed by separation and subsequent treatment with acid to recover active catalyst (4.3). 

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As the existence of this volume attests, transition metal complexes with phosphine ligands have proven to be useful and versatile catalysts for homogeneous reactions. Recently, binuclear metal complexes, particularly those with bridging phosphine ligands, have begun to attract interest because of their potential as catalysts and because of their novel structural... 

The concept makes use of the complimentary strengths and weaknesses of the two unconventional media. While ionic liquids are known to be excellent solvents for many transition metal catalysts, the solubility of most transition metal complexes in scC02 is poor (if not modified with e. g. phosphine ligands with fluorous "ponytails" [64]). However, product isolation from scC02 is always very simple, while from an ionic catalyst solution it may become more and more complicated depending on the solubility of the product in the ionic liquid and on the product s boiling point. 

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

Non-phosphine type ligands are studied time by time with the aim to obtain water-soluble transition metal complexes with catalytic properties. However, with the exception of a few specific reaction types (e.g. oxidations) these catalysts cannot cope with tertiary phosphines - with the ligands on Figure 20 this has been found once again. 

Although the most versatile hydrogenation catalysts are based on tertiary phosphines there is a continuous effort to use transition metal complexes with other type of ligands as catalysts in aqueous systems some of these are listed in Table 3.3. 

Transition-metal-catalyzed synthesis of poly(arylene)s via carbon-carbon coupling reactions was started by Yamamoto et al. three decades ago [52,53] since then various carbon-carbon bond formation processes with transition-metal catalysts have been applied to polycondensation [54-57]. In recent years, Buchwald et al. and Hartwig et al. developed Pd-catalyzed amination and etherification of aromatic halides by using bulky, electron-rich phosphine ligands [58-60], and this chemistry has been applied to polycondensation for... 

Asymmetric hydroboration of prochiral alkenes has been achieved using transition metal catalysts and chiral phosphines as ligands to obtain enantiomerically pure alkyl boronates <1997CC173>. Catalysts such as Rh(COD)2+BF4 , Rh(COD)2+Cl, Rh+BF4 , etc., in combination with chiral phosphines like DIOP 71, BINAP 72, CHIRAPHOS 73, DIPAMP 74, BDPP 75, ferrocene-based diphosphines 76<1999TL4977>, etc., have been employed for the asymmetric hydroboration of prochiral alkenes with moderate to high ee (DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane BINAP = 2,2-bis(diphenyl-phos-phanyl)-l,1-binaphthyl CHIRAPHOS = 2,3-bis(diphenylphosphino)butane DIPAMP = l,2-bis[(2-methoxyphe-nyl)phenylphosphino]ethane BDPP = 2,4-bis(diphenylphosphino)pentane). 

The catalysts for transfer hydrogenations are usually late transition metal complexes with tertiary phosphine ligands or bidentate nitrogen ligands, and the donors are usually organic compounds whose oxidation potential is sufficiently low to tolerate hydrogen transfer under mild conditions. Suitable donors are secondary alcohols such isopropanol. This alcohol is the most convenient since it is stable, non-toxic, environmentally friendly, easy to handle (bp 82°C), inexpensive and dissolves many organic compounds. 

The reaction is catalyzed by transition metal complexes coordinated with phosphine ligands. Since chiral phosphine ligands are the chiral auxiliaries most extensively studied for transition metal catalyzed asymmetric reactions, one can use the accumulated knowledge of the chiral phosphine ligands for the asymmetric reaction. The asymmetric 1,4-addition of aryl- and 1-alkenylboronic acids to enones proceeded with high enantioselectivity in the presence of a chiral phosphine-rhodium catalyst (Table 2). 

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