Rhodium complexes monodentate

During the late 1960s, Homer et al. [13] and Knowles and Sabacky [14] independently found that a chiral monodentate tertiary phosphine, in the presence of a rhodium complex, could provide enantioselective induction for a hydrogenation, although the amount of induction was small [15-20]. The chiral phosphine ligand replaced the triphenylphosphine in a Wilkinson-type catalyst [10, 21, 22]. At about this time, it was also found that [Rh(COD)2]+ or [Rh(NBD)2]+ could be used as catalyst precursors, without the need to perform ligand exchange reactions [23]. 

Few data are available for rhodium complexes based on monodentate ligands. In a recent study, the rate of COD hydrogenation in Rh(MonoPhos)2(COD)BF4 was determined as 0.071 min-1, which is somewhat slower than the corresponding DuPhos complex [20]. 

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. 

When the rhodium complexes are coordinated by monodentate ligands they frequently show high activity and moderate selectivity for the linear aldehyde [89], but rhodium catalyst that contain bidentate ligands with wide bite angles frequently show a lower activity but an increased selectivity for the linear product [42]. 

Another attempt to apply the oxo reaction to the synthesis of fine chemicals was made by the hydroformylation of styrene. The chiral catalyst, a rhodium complex of a surfactant phosphane 12, gave no optical induction (76). This result agrees with the known poor enantioselection ability of other complexes with monodentate chiral phosphanes. 

Phosphoramidites, a ligand class that has only recently been introduced into asymmetric hydrogenation, in the form of hybrid chelate ligands [29], induce excellent enantioselectivity as monodentate ligands. Thus de Vries, Feringa, and co-workers could reduce standard substrates in >96% ee with a rhodium complex based upon the binaphtholphosphoramidite 3d, once the solvent and reaction temperature had been optimized [30],... 

A few experiments have been performed to evaluate the potential of the P-menthyl-substituted class of phosphe-tanes 76 in rhodium-mediated alkene-hydrogenation reactions <1998S1539>. Within this series, monodentate ligands show low catalytic activity and poor enantioselectivity when employed in the hydrogenation of model dehydroamino acid derivatives. This observation is clearly consistent with the observed lability of their rhodium complexes, due to steric hindrance. 

The less-hindered phenyl-substituted monodentate phosphetanes 79, however, give stable rhodium complexes and moderate to high enantioselectivities (up to 86% ee) in rhodium-catalyzed hydrogenation of functionalized alkenes <2001S2095>. 

The diphenylphosphino group can be substituted by a phenylmercaptyl moiety [154] that would coordinate in chelate fashion to a rhodium centre, but only as a monodentate ligand (with the carbene) in the case of iridium. Interestingly, the rhodium complex, iso-structural to the P functionalised complex, is absolutely inactive in the hydrogenation of itaconate whereas the corresponding phosphino functionalised catalyst was reported to be highly active [154],... 

A Pauson-Khand type reaction of enynes, where the CO source is an aldehyde, has been reported by Morimoto and coworkers. This CO-transfer carbonylation system was carried ont with monomeric or dimeric rhodium complexes supported by monodentate or chelating phosphine ligands (e g. [RhCl(cod)]2/dppe or dppp [RhCl(CO)PPh3]). This reaction is snccessfiil for a series of enynes and aldehydes (Scheme 28). 

Knowles [1] and Homer [2] independently discovered homogeneous asymmetric catalysts based on rhodium complexes bearing a chiral monodentate tertiary phosphine. Continued efforts in this field have produced hundreds of asymmetric catalysts with a plethora of chiral ligands [7], dominated by chelating bisphosphines, that are highly active and enantioselective. These catalysts are beginning to rival biocatalysis in organic synthesis. The evolution of these catalysts has been chronicled in several reviews [8 13]. 

Reek et al.73 developed a new bicarbazolediol-74 (BICOL)-based, chiral monodentate phosphoramidite ligand, in which the Ai-sites in the bicarbazole skeleton permitted the easy introduction of metal centers. As a model reaction, the Rh-catalyzed asymmetric hydrogenation of methyl 2-acetamidocinnamate was evaluated. Using a ligand to rhodium ratio of 2.2, the enantioselectivity induced by the rhodium complex (Figure 10.7) was 93% at full conversion,... 

The first rhodium-catalyzed enantioselective arylation was described by Miyaura in 1998 (Scheme 8.5) [13]. The reaction between 1-naphthaldehyde (13) and phenylboronic acid (14) was catalyzed by a rhodium complex of the monodentate phosphine fSj-MeO-MOP (15), and resulted in the formation of diarylmethanol... 

---