Phosphites rhodium complexes

Figure 6 Union Carbide chiral bis-phosphite-rhodium complexes used as catalysts for the asymmetric hydroformylation.

The hydridotetrakis(triphenyl phosphite)rhodium complex described below is the first example of a rhodium hydride complex stabilized by phosphite ligands.2... 

Episulfides can be converted to alkenes. " However, in this case the elimination is syn, so the mechanism cannot be the same as that for conversion of epoxides. The phosphite attacks sulfur rather than carbon. Among other reagents that convert episulfides to alkenes are Bu3SnH, certain rhodium complexes, LiAlH4 (this compound behaves quite differently with epoxides, see 10-85), and methyl iodide.Episulfoxides can be converted to alkenes and sulfur monoxide... 

The characterization of the rhodium complexes formed under hydroformylation conditions by NMR techniques and in situ IR spectroscopy showed that there is a relationship between the structure of the [HRh(CO)2 (BINAPHOS)] species and their enantiodiscriminating performance. Thus, (R,S)- and (S,R)-BINAPHOS ligands show high equatorial-axial (ea) coordination preference with the phosphite moiety in the axial position. Meanwhile, the characterization of the (R,R)- and (S,S)-BINAPHOS ligands suggests that there is either a structural deviation of the monohydride complexes from an ideal TBP structure or an equilibrium between isomers [20,34],... 

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. 

Rhodium complexes of phosphorus based ligands are of considerable importance as (pre-)catalysts in hydroformylation which has developed into one of the most important homogeneous catalytic processes [57]. A recent advantage in this field involved the use of phosphinines as ligands whose low-lying r -orbitals provide for similar r-acceptor qualities as for phosphites [13, 58]. 

Based on the precedent of Van Leeuwen and Roobeek, livinghouse and co-workers screened a variety of electron-deficient phosphine/phosphite ligands for the rhodium-catalyzed [4-1-2] reaction, which provided an alternative catalyst system for the formation of 5,6- and 6,6-ring systems [13]. The most notable of these was the tris-(hexafluoro-2-propyl) phosphite-modified rhodium complex, which was applicable to both carbon- and oxygen-tethered substrates, and also provided the first example of a facial-directed diastereoselective intramolecular rhodium-catalyzed [4-i-2] reaction (Eq. 4). 

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-... 

Figure 7 A rhodium complex of chiral phosphine-phosphite ligand (H,S)-BINAPHOS used as a catalyst for asymmetric hydroformylation.

A similar arrangement was found for a rhodium complex based on the phosphine-phosphite BINAPHOS ligand see ref. [26a]. 

The activity of rhodium complexes with phosphine or phosphite ligands is about three to four orders of magnitude higher than that of cobalt catalysts.21-23 [RhH(CO)(PPh3)3] preformed or prepared in situ has proved to be the active species when triphenylphosphine is used as ligand. Despite the high cost of rhodium, the mild reaction conditions and high selectivities make rhodium complexes the catalyst of choice in hydroformylation. 

In 2001, Van den Hoven and Alper reported the unexpected 2(Z)-6(ft)-47/-[l,4]-thiazepin-5-one 215, as the major product, from the reaction of acetylenic thiazole 214 with carbon monoxide and hydrogen in presence of a zwitterionic rhodium complex and triphenyl phosphite. After optimization of the reaction condition, the pressure, and the temperature, up to 90% yield is achieved with good selectivity for thiazepine 215 over thiazole side products 216-218 (Scheme 38) <2001 JA1017>. 

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