Chelating phosphine rhodium

Figure 3. General mechanism for asymmetric catalytic hydrogenation using chelating phosphine rhodium complexes...

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

Table 10 Impact of the chelating phosphine on levels of enantioselectivity in rhodium-catalyzed intramolecular hydrosilylation with [Rh(P-P)(acetone)2]+.

The hydroformylation of ally and vinyl acetals yields some useful intermediates. Allyl acetate undergoes partial double bond migration prior to hydroformylation with a cobalt catalyst in the absence of phosphine (equation 20).2-5 Rhodium catalysts containing chelating phosphines are more selective to the linear aldehyde.31... 

For cis-chelate complexes of rhodium and bisphosphines as catalysts, indeed relatively low ratios of n/i aldehyde products were reported (12, 13). Using a 1 1 mixture of H CO at atmospheric pressure, Sanger reported n/i ratios ranging from 3 to 4 for propylene hydroformylation (12). However, his catalyst systems were produced by adding less than 2 mol of bisphosphine per mole tris(triphenyl-phosphine)rhodium carbonyl hydride. When an excess of the chelating bisphosphines was used by Pittman and Hirao (13), low n/i ratios close to 1 were produced from 1-pentene using a mixture of H2/CO at 100-800 psi between 60° and 120°C. 

Advances in Chemistry, Editors Alyea, E. C., and Meek, D. W., American Chemical Society, 1981, 196, 78, paper of "31p NMR Studies of Equilibria and Ligand Exchange in Triphenyl phosphine Rhodium Complex and Related Chelated Bis-Phosphine Rhodium Complex Hydroformylation Catalyst Systems," by Kastrup, R. V., Merola, J. S., and Oswald, A. A., in press. 

Regardless of the initial source of rhodium, that is, whether it is added just as a halide or as a phosphine complex, under the reaction conditions 4.1 is formed. With phosphine complexes, the phosphine is converted to a phospho-nium (PRj" or 11 PR, ) counter-cation. As chelating phosphines bind more strongly than the monodentate ones, the induction time for the formation of 4.1 with chelating phosphines is longer. 

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). 

Rhodium catalysts supported via chelating phosphines as described above have been used as Asymmetric Hydrogenation catalysts." ... 

Addition of a catalytic amount of Pt(dba)2 (dba = dibenzylidene acetone) to reactions with a variety of C-arylaldimines and B2cat 2 in benzene or THF at 25°C yielded HBcat and a mixture of diboration, 4a-j, boration, 5a-j, and hydroboration products, 6a-j (Equation 5). Addition of one equivalent of a phosphine such as PPh3 or PCy3 to the Pt catalyst severely reduced activity, while Wilkinson s catalyst, RhCl(PPh3)3, gave much less of the diboration product. Both cationic and neutral rhodium complexes with chelating phosphines were also inactive. 

The two square planar species of line 2 are diastereomers. They contain the same optically active chelating phosphine chiraphos, but the rhodium atom is coordinated to different sides of the prochiral olefin (re/si sides). The two diastereomers of line 2 are rapidly interconverting. In this equilibrium the isomer shown on the left-hand side (si-coordination of the olefin) is the minor isomer and the isomer shown on the right-hand side (re-coordination of the olefin) is the major isomer [91, 92]. 

More recently, a rran.y-chelating asymmetric ferrocenyl phosphine (abbreviated as TRAP) has been developed [63]. The rhodium complex of TRAP catalyzes asymmetric Michael additions of a-cyanocarboxylates in high enantioselectivity (72-89 % ee) (eq (18)) [64]. Because these Michael reactions proceed via a A -bound enolato complex [65], the reaction center on the enolato ligand is far from the metal center. Thus, a CLv-chelating phosphine cannot control the direction of electrophilic attack. The reaction with c/.y-chelating phosphines gives only a poor enantiomeric excess (BINAP, 17 % ee DIOP, 12 % ee CHIRAPHOS, 3 % ee). 

Later, in the 1990s a remarkable asymmetric catalyst with the chiral chelating phosphine DuPHOS (3.49) (Figure 3.19) was developed by M. J. Burk at DuPont. This cationic DuPHOS/rhodium complex showed very high ee values of 99% in asymmetric hydrogenation of enamides. As can be seen the choice of chiral phosphine chelating ligand had a dramatic effect on the ee values. 

Fig. 7 Proposed mode of action of rhodium complexes of chelating phosphines tethered to p-cylodextrins. Adapted from Ref. [47].

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