Bulky phosphite ligands

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

Very bulky phosphite ligands (see Fig. 6.3) also yield unstable rhodium complexes (sterically hindered phosphites are commercially available as antioxidants for polyalkenes). The cone angle of the ligand shown is as high as 175° and the complex formed with rhodium has the formula HRh(CO)sL apparently there is not enough space for the coordination of a second phosphite. (In a trigonal bipyramidal complex there is room for two bulky ligands at the two axial positions but this would leave an equatorial position for a a-bonded hydride... 

Very bulky phosphite ligands (see Fig. 6.3) [12,13,30,36] also yield unstable rhodium complexes (sterically hindered phosphites are commercially available as antioxidants for polyalkenes). The cone angle of the ligand shown is as high as... 

The employment of bulky phosphite ligands , e.g., tri(o-t-butylphenyl)phos-phite, yields high reactivity catalysts for the hydroformylation of very low reactivity j8-alkyl-a-olefins, e.g. isoprene. Diorganophosphites such as 3 overcome many of the problems associated with the employment of simple triarylphosphites under hydroformylation reaction conditions such as reaction of the phosphite with the product aldehyde, trans-esterification and organometallic degradation reactions. 

The use of a bulky phosphite ligand P(0-o- BuPh)3 allowed a much milder reaction regime and significantly increased the rate of the hydroformylation [119]. 

The hydroformylation of vegetable oils (soybean, high oleic safflower, safflower, and linseed) using Rh(CO)2(acac) as the catalyst precursor in the presence of PPhs or (PhOlsP was studied. The ligand (PhO)3P resulted in a lesser reactivity compared to TPP in contrast to the rates of bulky phosphite ligands reported in the literature [16]. 

Phosphites are easier to make and may be more stable than phosphines (provided that water and alcohols are absent). Interestingly, the breakthroughs in the area of phosphites originate from industry. Pursuing the effect of bulky phosphite ligands reported by van Leeuwen," Bryant and co-workers at Union Carbide Corporation (UCC, now Dow) turned to bidentates such as 6, 7, and which has led to an impressive number of ligands tested, first... 

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