Rhodium-catalyzed hydroformylation phosphites

In order to assess structural influences on the catalytic performance of mono-cyclic PEG-modifled phosphites, the monocyclic phosphites 14a,c, 17a-e, and 19b,c were used in the rhodium-catalyzed hydroformylation of 1-octene (Scheme 6). The catalysts were prepared in situ by adding a solution of [Rh(acac)(CO)2] as a catalyst precursor in toluene to the phosphite ligands... 

Generally, one would expect that increasing steric hindrance in the catalytically active rhodium complex would result in lower reaction rates. In this respect, the results of Van Leeuwen and Roobeek seemed at first to be contradictory. They used the very bulky tris(ortho tert-butylphenyl)phosphite la (Chart 6.1) as a ligand and found high reaction rates in the rhodium catalyzed hydroformylation of other-... 

Chart 6.2 Intermediates in the tris (2-tert-butyl-4-methyl phenyl) phosphite modified Rhodium catalyzed hydroformylation. 

Slagt. V.F., van Leeuwen, P.W.N.M. and Reek, f.N.H. (2003) Midticomponent porphyrin assemblies as functional bidentate phosphite ligands for regioselective rhodium-catalyzed hydroformylation. Angew. Chem., Int. Ed., 42, 5619-5623. 

A rhodium-catalyzed hydroformylation was also carried out with the bicyclic terpenes [S-pinene and camphene [44, 45]. The influence of phosphine and phosphite ligands (L) on the formation of the different product isomers of the hydroformylation of [S-pinene was studied by adding different mono- and bidentate ligands. However, the basicity of the ligands proved to be relevant for favoring the c/.v-isomer. A steric influence of the ligands to favor one isomer was not found (Scheme 14). 

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]. 

Rhodium-catalyzed hydroformylation using catalysts modified with alkylphosphines and arylphosphines was reported by Wilkinson s group [12]. Phosphine ligand variation hardly affected the rate and selectivity under the circumstances used (70 °C and 100 bar). Pruett (Union Carbide Corporation) found that phosphites can also be used, and the type of phosphite had a profound effect on rates and selectivities [13]. 

Parlevliet has shown that calix[4]arene based monophosphites can exist in different conformations [45]. By using the different conformations as ligands in the rhodium catalyzed hydroformylation of 1-octene he showed that the exact conformation influenced the performance of the catalyst. Some of the conformations behaved more like triphenyl phosphite, whereas others showed catalytic results like bulky monophosphites, giving high rates with moderate selectivity for the linear aldehyde. 

The catalyst discovered by Takaya et al. using (R,S) BINAPHOS, a phosphine-phosphite hgand of Ci symmetry [14] has provided ee s as high as 96% as well as total conversions and high regioselectivities. The origin of the stereodifferentiation in rhodium catalyzed hydroformylation has been discussed in theoretical reports [15, 16]. 

Phosphites have been extensively studied for their use as ligands in rhodium-catalyzed hydroformylation (see Chapter 3). The first publication on the use ofphosphites is from Pruett and Smith, from Union Carbide [31]. The first exploitation of bulky monophosphites was reported by van Leeuwen and Roobeek [32]. They found that very high rates can be obtained for internal and terminal alkenes, but selectivities were low for linear alkenes. The bulky phosphites not only gave higher rates than less bulky phosphites, but they are also more resistant to hydrolysis. Bryant and coworkers [33] introduced even more stable, bulky phosphites by the... 

The electronic properties of phosphinines are located between those of phosphines and phosphites, with somewhat more similarity to phosphites. In general, phosphinines are characterized by considerable it-acceptor properties and thus beneficial for several catalytic applications [32]. A theoretical study on Rh-phosphinine complexes provided evidence that the directionality of the ir-backdonation rather than the overall acceptor ability was responsible for the high catalytic activity [33]. In the rhodium-catalyzed hydroformylation of styrene with phosphinines as ligands, superior results in terms of conversion and TOP (turnover frequency) were noted in comparison to the use of PPhj or standard mono-triarylphosphites [34]. Atropisomeric 2-arylphosphmines have been prepared by Muller s group [35]. In the hydroformylation of trans-2-octene, a clear preference for the formation of 2-methyloctanal was noted. 

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. 

A similar situation arises when three different oxy groups are linked to the phosphorus. As a consequence, it becomes stereogenic. In combination with atropi-somerism, diastereomers are formed as found with acylphosphite phosphites 1 [44]. Attempts to separate them in order to elucidate their individual contribution to rhodium-catalyzed hydroformylation were unsuccessful. However, the different effects of diastereomers on catalyst formation and hydroformylation results could be evidenced by individual synthesis of all three configurationally stable tris(binaphthol) phosphites 2 [108]. 

Also in the rhodium catalyzed hydroformylation of vinyl carbonates with biden-tate phosphine-phosphite complexes, the a-aldehyde was favored a/p = 92 8), which opens up the possibility of running the reaction in a stereoselective manner [41]. 

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... 

A set of different reaction protocols allows a meaningful comparison of different phosphite ligands on the rhodium-catalyzed hydroformylation of methyl oleate (Scheme 5.21). 

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