Rhodium catalysts bisphosphines

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Arylation of alkynes via addition of arylboronic acids to alkynes represents an attractive strategy in organic synthesis. The first addition of arylboronic acids to alkynes in aqueous media catalyzed by rhodium was reported by Hayashi et al.89 They found that rhodium catalysts associated with chelating bisphosphine ligands, such as 1,4-Ws(diphenyl-phosphino)butane (dppb) and 1,1 -/ E(diphenylphospliino)fcrroccnc... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

The cationic rhodium catalysts with bisphosphine-modified CD-s were highly active in the biphasic hydroformylation of 1-octene (Scheme 10.8) [9,11]. In a two-phase system of l-octene/30 % DMF in water, quantitative conversion was obtained with 0.03 mol % of the catalyst at 80 °C and 100 bar syngas within 18 h (TOP = 180 h" ). Selectivity to aldehydes was higher than 99 % with 76 % regioselectivity in favour of the straight-chain product. 

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. 

Catalytic asymmetric 1,6-additions to 2,4-dien-l-ones have been realized with up to 98% ee using a chiral bisphosphine-rhodium catalyst, arylzinc reagents, and a chlorosilane.120... 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

A class of chiral bisphosphines based on 3,4-bis(diphenylphosphino)pyrrolidines (9) has been developed by Degussa and the University of Munich. Rhodium-bisphosphine catalysts of this class can reduce a variety of enamides to chiral amino acid precursors with high enantioselectivities. These catalysts are extremely rapid and can operate with high S/C ratios (10,000-50,000) under moderately high hydrogen pressure (150-750 psig). Contrary to other rhodium catalysts that contain... 

The bisphosphine ligand Biphemp (106a) has been used in rhodium catalysts in the asymmetric isomerization of N,/V-diethyIneryTamine with enantioselectivities up to 99.5% ee. [Rh(Biphemp)]+ catalysts compared favorably to [Rh(BINAP)]+ catalysts in rate and enantioselectivities.120... 

Dehydroamino acids are important synthetic intermediates in that their reduction provides a route to a-amino acids. There has been a large volume of research examining the hydrogenation of such compounds, mediated by asymmetric homogeneous catalysts, to provide amino acids with varying degrees of optical purity. For example, a key step in an industrial procedure for the manufacture of l-DOPA is reported to involve the hydrogenation of (39) in the presence of the bisphosphine rhodium catalyst (40) to provide (41) in 94% optical yield (equation 21). 

Reduction of a carbon-carbon double bond will produce a chiral product if the olefin is (unsymmetrically) geminally disubstituted. Although hundreds of catalysts having chiral ligands have been synthesized and screened with a number of alkene structural types (reviews ref. [65,97-107]), the present discussion will focus on only one the reduction of acetamido cinnamates using soluble rhodium catalysts (reviews ref. [97,100,108-110]). The development of chiral bisphosphine ligands and the herculean effort that led to the elucidation of the mechanism of this reaction make it an important example for study, since we now know that the major enantiomer of the product arises from a minor (often invisible) component of a pre-equilibrium [109,111]. This aspect of chemical reactivity is an important lesson whose importance cannot be overemphasized when we strive to understand the... 

Reactions of internal alkenes can also form terminal amines, and examples of this process are shown in Equation 17.24. These reactions occur in the presence of a rhodium catalyst containing the electron-poor bisphosphine ligand shown in these equations. For example, the reaction of 2-pentene (R = Me in Equation 17.24) with piperidine forms a 4 1 ratio of the linear to branched amines, and the same reaction with a excess of 2-butene (R = H in Equation 17.24) forms a 96 4 ratio of linear to branched products. 

Besides the Wilkinson complex, [(diene)RhCl]2 (diene = 1,5-cycIooctadiene, norbomadiene, etc.), [Rh(P-P)(diene)] X (X = Bp4, C104 , PF, P-P = bisphosphine) and Rh6(CO)i6 of a cluster complex, are used as rhodium catalysts [64]. The selective hydrogenation of an olefin with a Wilkinson complex is shown in eq. (18.32). The carvone absorbed hydrogen readily and quantitatively to afford dihydrocarvone in high yield [64]. 

Selke, R., Holz, J. and Riepe, A., Impressive enhancement of the enantioselectivity for a hydroxy-containing rhodium(I) bisphosphine catalyst in aqueous solution by micelle-forming amphiphiles, Chem. Eur. J., 1998, 4, 769, and references cited therein. For a recent application, see Grassert, I., Schmidt, U., Ziegler, S., Fischer, C. and Oehme, G., Use of rhodium complexes with amphiphilic and nonamphiphilic Ugands for the preparation of chiral of-aminophosphonic acid esters by hydrogenation in micellar media. Tetrahedron Asymmetry, 1998, 9, 4193. 

As described in this chapter, a number of rhodium-based catalysts have been developed and employed widely for the [2 -I- 2 -I- 2] cycloaddition reactions of alkynes. Among these rhodium catalysts, Wilkinson s complex [RhCl(PPh3)3] and cationic rhodium(I)/biaryl bisphosphine complexes are the most frequently employed. As Wilkinson s complex is a stable single-component catalyst, this complex is the most easily usable catalyst in organic synthesis. On the other hand, cationic rhodium(I)/ (axially chiral) biaryl bisphosphine complexes exhibit excellent catalytic activity and selectivity under mild conditions. These chiral complexes, especially, enable the catalytic enantioselective syntheses of various chiral arenes with high enantioselectivity. 

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