Phosphites as ligands

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

Complexes with alkyl and aryl phosphites as ligands 114... 

Phosphites as ligand will displace the equilibrium in Eq. (47) to the left while phosphines should favor the cr-allyl form, and hence promote the formation of VCH. This is indeed observed (Table III). 

The first addition is relatively easy and numerous catalysts have been reported for this reaction 9). In practice, nickel(O) complexes associated with phosphites as ligands, are the most convenient catalysts. 

Only limited data are available for the kinetics of oxo synthesis with the water-soluble catalyst HRh(CO)(TPPTS)3. The hydroformylation of 1-octene was studied in a two-phase system in presence of ethanol as a co-solvent to enhance the solubility of the olefin in the aqueous phase [115]. A rate expression was developed which was nearly identical to that of the homogeneous system, the exception being a slight correction for low hydrogen partial pressures. The lack of data is obvious and surprising at this time, when the Ruhrchemie/ Rhone-Pou-lenc process has been in operation for more than ten years [116]. Other kinetic studies on rhodium-catalyzed hydroformylation have been published, too. They involve rhodium catalysts such as [Rh(nbd)Cl]2 (nbd = norbomadiene) [117] or [Rh(SBu )(CO)P(OMe)3]2 [118], or phosphites as ligands [119, 120]. 

Complexes with monodentate tertiary phosphines, arsines and stibines Complexes with alkyl and aryl phosphites as ligands Complexes with bidentate tertiary phosphines, arsines and stibines... 

Recent research has shown that this mechanism does not hold for all catalyst and substrate combinations. For instance, the rhodium-catalysed hydrogenation with electron-rich bidentate phosphine (2.11) is thought to proceed via initial oxidative addition of hydrogen to a catalyst-substrate complex. Furthermore, enantioselectivity arises from reaction of the major catalyst-ligand complex, in a lock-and-key process, when using monodentate phosphites as ligands. 

Beller, M. and Zapf, A. (1998) Phosphites as ligands for efficient catalysis of Heck reactions. 

Steric and electronic properties of a hgand can drastically influence the rate and selectivity of the hydroformylation reaction (see Chapter 3, Phosphites as ligands). A systematic study though is absent for several... 

Phosphites are easier to synthesize and less prone to oxidation than phosphines. They are much cheaper than most phosphines and a wide variety can be obtained commercially as they are used as anti-oxidants. Disadvantages of the use of phosphites as ligands include several side reactions hydrolysis, alcoholysis, trans-esterification, Arbusov... 

In a related process, formation of benzofuranone from carbonylation of 2-iodobenzyl alcohol in supercritical carbon dioxide was recently reported by Kayaki et al. The rate of reaction in supercritical CO2, using palladium chloride in combination with trialkyl or triaryl phosphites as ligands, was reported to be higher than that in typical organic solvents. 

Because of the immense importance of phosphites as ligands, not only in rhodium-catalyzed hydroformylation, several recent reviews have dealt with these compounds and provided quite complete collections of individuals [2, 15], In contrast to these overviews, in this chapter we will focus on some issues that are seldom in the focus. This concerns the synthesis of alcohols that are required as the alcohol component for the synthesis of phosphorous acid triesters. This has never been considered in detail. However, their availability is an important criterion for chemists dealing with large-scale applications and therefore has economic consequences. Some general synthesis protocols of phosphites together with some typical examples will also be considered. The complexation behavior of phosphites with rhodium will also be discussed briefly. Some remarks about the stability of ligands and Rh catalysts will close this chapter. 

Up to 69% yield of linear nonanal was achieved in the rhodium-catalyzed hydroformylation of 2-pentene or isomeric n-octenes with electronically nonsymmet-ric acylphosphite-phosphites as ligands (Scheme 5.18) [100]. Phosphites with... 

Meanwhile, networks of dominating reactions have been analyzed and models exist, vahdated by experiments, suggesting the optimal reactions conditions [115]. Usually, common Rh catalysts with phosphites as ligands are used. Recently, also rhodium nanoparticles have been successfully submitted to the reaction [116]. 

Gold(I) complexes with phosphites as ligands are more active catalysts, due to their more electron-withdrawing character. The complexes 19 [31] and its cationic derivative 20 [42, 59, 60], bearing tris(2,6-di-ferf-butylphenyl)phosphite are among the most electrophilic cationic gold(I) catalysts reported to date. 

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