Chiral rhodium complexes

The relay catalysis combining a mechanistically distinct transition metal catalyst and an organocatalyst for one cascade has stimulated intensive interest in recent years, as it could potentially enable highly efficient and/or unprecedented transformations in a one-pot operation. Indeed, excellent transformations have been established by this approach. One elegant example is relay catalysis using a rhodium complex/chiral Brpnsted acid binary system (Scheme 5.87) [88]. It is believed that the first catalytic... 

SCHEME 5.87 Relay catalysis using a rhodium complex/chiral Biytnsted acid binary... 

Fisher, C., and H. S. Mosher Asymmetric Homogenous Hydrogenation with Phosphine-Rhodium-Complexes Chiral Both at Phosphorus and Carbon. Tetrahedron Letters 1977,2487. 

C2.7.6.2 CHIRAL HYDROGENATION OF ALKENES CATALYSED BY A RHODIUM COMPLEX... 

Metal Complex. Complexation gas chromatography was first introduced by V. Schurig in 1980 (118) and employs transition metals (eg, nickel, cobalt, manganese or rhodium) complexed with chiral terpenoid ketoenolate ligands such as 3-ttifluoroacetyl-lR-camphorate (6),... 

The strategy of the catalyst development was to use a rhodium complex similar to those of the Wilkinson hydrogenation but containing bulky chiral ligands in an attempt to direct the stereochemistry of the catalytic reaction to favor the desired L isomer of the product (17). Active and stereoselective catalysts have been found and used in commercial practice, although there is now a more economical route to L-dopa than through hydrogenation of the prochiral precursor. 

The influence of the concentration of hydrogen in [BMIM][PFg] and [BMIM][BF4] on the asymmetric hydrogenation of a-acetamidocinnamic acid catalyzed by rhodium complexes bearing a chiral ligand has been investigated. FFydrogen was found to be four times more soluble in the [BFJ -based salt than in the [PFg] -based one. 

Recently a novel chiral ferrocene-based amidinato ligand and its rhodium complexes have been described. The chiral N,N -bis(ferrocenyl)-substituted formamidine (N,N -bis[(S)-2- (lR)-l-(diphenylphosphino)ethyl ferrocen-l-yl]for-mamidine was prepared from commercially available (IR)-l-(dimethylamino) ethyl ferrocene by a multistep procedure in an overall yield of 29%. Deprotonation of the ligand with -butyllithium followed by addition of [RhCl2(COD)2] as illustrated in Scheme 167 yielded the corresponding (formamidinato)rhodium(l)... 

These examples are part of a broader design scheme to combine catalytic metal complexes with a protein as chiral scaffold to obtain a hybrid catalyst combining the catalytic potential of the metal complex with the enantioselectivity and evolvability of the protein host [11]. One of the first examples of such systems combined a biotinylated rhodium complex with avidin to obtain an enantioselective hydrogenation catalyst [28]. Most significantly, it has been shovm that mutation-based improvements of enantioselectivity are possible in these hybrid catalysts as for enzymes (Figure 3.7) [29]. 

Chelucci et al. [41] synthesized further chiral terpyridines derived from (-)-yd-pinene, (-i-)-camphor, and (-l-)-2-carene and tested their ability to chelate copper or rhodium for the asymmetric cyclopropanation of styrene. The copper catalysts were poorly efficient and selective in this reaction. The corresponding rhodium complexes led to the best result (64% ee) with the ligand derived from (-l-)-2-carene (ligand 33 in Scheme 17). 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Asymmetric hydrosilylation can be extended to 1,3-diynes for the synthesis of optically active allenes, which are of great importance in organic synthesis, and few synthetic methods are known for their asymmetric synthesis with chiral catalysts. Catalytic asymmetric hydrosilylation of butadiynes provides a possible way to optically allenes, though the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (2S,4S)-PPM turned out to be the best catalyst for the asymmetric hydrosilylation of butadiyne to give an allene of 22% ee (Scheme 3-20) [59]. 

Chiu s group [193] used this domino process for an entry to pseudolaric acid 6/2-27, starting from 6/2-28, to yield 6/2-29 and 6/2-30 as an almost l l-mixture of di-astereomers (Scheme 6/2.5). Attempts to improve the stereoselectivity by using chiral rhodium complexes did not change the picture very much. The pseudolaric acids A, B and C are diterpenoids, which were isolated from the root bark of Pseudo-larix kaempferi Gordon (Pinaceae), and are components of the traditional Chinese medicine called tujinpi. They reveal antifungal activity and cytotoxicities at submicromolar levels [194]. 

Rhodium complexes based on the chiral ligand (120) have been used in the asymmetric hydrogenation of functionalized chelating olefins in methanol and water. The results are compared to those obtained using the corresponding non-sulfonated catalysts in water all sulfonated... 

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

Rhodium complexes with chiral dithiolato and dithiother ligands have been studied in rhodium-catalyzed asymmetric hydroformylation. In all instances, enantioselectivities were low.391-393 Catalysis with compounds containing thiolate ligands has been reviewed.394... 

An alternative approach to hydroboration has utilized a chiral B-H source with either achiral or chiral rhodium complexes.58 The enantiomerically pure reagent (21) is derived from ephedrine. Notably in the reactions with BINAP, a higher enantiomeric excess is produced from (R)-BINAP (6) compared to the Y-form (Scheme 13). 

The rhodium complexes with hydroxyphospholane ligand 125663 or 126660 catalyze the asymmetric hydrogenation of a-acetamidoacrylates with ee values in excess of 98%. System 125 is also very effective in the asymmetric hydrogenation of P-acetamidoacrylates (up to 99.6% ee).664 The planar-chiral heterocyclic ligand 127 complexed with rhodium(I) catalyzes the hydrogenation of a-acetamidoacrylates in excellent yields and ee values from 79-96% under mild conditions.665... 

C-Chiral diphosphinites based on cyclohexane and cyclopentane rings (31) (cf. structure 17) have been used in 1 1 rhodium complexes at 50 atm H2 in unspecified solvents (259, 260). 

A rhodium complex (NBD)Rh(PPh3)(DIOS)+ was active via H2Rh(PPh3)(DIOS)+, but there was no optical induction with unsaturated acid substrates, which possibly displace the chiral ligand (275). The DIOS-type ligands have not yet been resolved at the sulfur. The extent... 

Supported cationic rhodium(I) phosphine complexes, chiral at a men-thyl moiety, effected hydrogenation of ketones, but the 2-butanol produced from methylethylketone was optically inactive (348). Polystyrene-or silica gel-supported DIOP systems, however, are particularly effective for production of optically active alcohols (up to 60% ee) via asymmetric hydrosilylation of ketones (10, 284, 296, 366, 368 see also Section III, A,4). 

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