Chiral Rh -complexes

Lee et al. [103] synthesized a chiral Rh-complex with a bisphosphine-contain-ing cation as ligand (Fig. 41.8, 2) to improve the immobilization of the transition-metal complex within the ionic liquid. 

Figure 12. Asymmetric hydrogenation of a-enamides catalyzed by chiral Rh complexes in scC02...

Beside [2+2+2] cycloaddition, [4+2] and [5+1] cycloadditions represent other approaches for the construction of six-membered ring systems. In parhcular, the intermolecular and intramolecular [4+2] cycloadditions of diene and alkyne have been extensively studied, and a variety of transition-metal complexes-including those of Fe, Ni and Rh-have been reported as efficient catalysts. The first enanh-oselective reaction was achieved with a chiral Rh complex, although the substrates were limited to dienynes with a substituent on the diene terminus [36]. Later, Shibata and coworkers developed an intramolecular and enanhoselective [4+2] cycloaddition using an Ir-BDPP (l,3-bis(diphenylphosphino)pentane) complex (Scheme 11.24) [37], where dienynes with an unsubstituted diene terminus were transformed into bicyclic cyclohexa-1,4-diene with up to 98% ee. 

Hamada, T., Torii, T, Onishi, T, Izawa, K. and Ikariya, T. Asymmetric Transfer Hydrogenation of a-Aminoalkyl-a -Chloromethyl Ketones with Chiral Rh Complexes. J. Org. Chem. 2004, 69, 7391-7394. 

Only limited successful examples of asymmetric hydrogenation of acrylic acids derivatives have included the use of chiral Rh complexes (Scheme 1.17). The diamino phosphine (28) utilizes selective ligation of the amino unit to a Rh center and also exerts electrostatic interaction with a substrate. Its Rh complex catalyzes enantioselective hydrogenation of 2-methylcinnamic acid in 92% optical yield [116], Certain cationic Rh complexes can attain highly enantioselective hydrogenation of trisubstituted acrylic acids [ 1171. 2-(6 -Methoxynaphth-2 -yl)acrylic acid is hydrogenated by an (.S ..S )-BIPNOR- Rh complex in methanol at 4 atm to give (.S)-naproxen with 98% ee but only in 30% yield [26]. 

Semicorrinato)copper catalysts have also been used for intramolecular cyclopropanation reactions of alkenyl diazo ketones (eq 9 and eq 10). In this case the (semicorrinato)copper catalyst derived from complex (5) proved to be superior to related methylene-bis(oxazoline)copper complexes. Interestingly, analogous allyl diazoacetates react with markedly lower enantioselectivity under these conditions, in contrast to the results obtained with chiral Rh complexes which are excellent catalysts for intramolecular cyclopropanations of allyl diazoacetates but give poor enantioselectivities with alkenyl diazo ketones (see Dirhodium(II) Tetrakis(methyl 2-pyrrolidone-5(S -carboxylate ) Moderate enantioselectivities in the reactions... 

Chiral Rh complexes of hydroxyalkyl ferrocenyl phosphine BppfOH catalyze the asymmetric hydrogenation of aminomethyl aryl ketones to 2-amino-1-aryl ethanols. Both occur in high chemical and optical yield ... 

The investigated supported complexes 22 and 23, outlined in Fig. 8, were used for hydrogenations of alkenes, nitriles and a,jS-unsaturated ketones. Furthermore, 23 was used in the reduction of different heterocycles like benzoth-iophene, quinoline, indole, dibenzothiophene and acridine. The supported chiral Rh complexes, depicted in Fig. 9, were used for hydrogenation reactions with prochiral olefins. 

Another more recent publication reported the grafting of chiral Rh complexes on silica gel via hydrogen bonding [50]. Different chiral ligands (Fig. 13) were converted to Rh trifluoromethanesulphonate complexes and immobilized applying the same immobilization technique mentioned above, the anionic part tethered non-covalently to the surface of the silica particles. Those complexes were applied in the hydrogenation of (E)-a-phenylcinnamicacid to... 

Attempts to improve the solubility and immobilization of chiral hydrogenation catalysts in ionic liquids were presented by Lee and coworkers [112]. They synthesized a chiral Rh-complex carrying the dicationic bisphosphine ligand depicted in Fig. 5.3-7. Immobilization of the tricationic complex in [BMIM][SbFe] showed better immobilization results in contact with iPrOH compared to the non-modified complex Me-BDPMI in the Rh-catalyzed asymmetric hydrogenation of N-acetylphenylethenamine (Scheme 5.3-11). The ionic catalyst solution was reused three times without loss of activity. In the fourth mn conversion decreased but high conversions could still be realized by increasing the reaction time. 

Sento, T., Shimazu, S., Ichikuni, N., Uematsu, T. (1998) New clay-supported chiral Rh-complexes interlayer modification with stmctural tuning guests and as5munetric hydrogenation, Chem. Lett. 1191-1192. 

Ojima, I., Kogure, T. and Terasaki, S. (1978) Effective biomimetic route to D-(+)-pantothenate using asymmetric hydrogenation catalyzed by a chiral Rh complex in the key steps, J. Org. Chem. 43,3444-3446. 

On the other hand, the dirhodium bridge caged within a lantern structure is thought to be essential to the success of dirhodium complexes in which two rhodium atoms are surrounded by four ligands in a nominal symmetry. Both computational studies and characterization of dirhodium car-benoid intermediates suggested that the intermediate adopts a Rh—Rh=C framework. In another word, two rhodium atoms are bound to one carbene center, and the bonding scenario obeys the three-center orbital paradigm. As such, metal carbenoids derived from chiral Rh complexes and donor/ acceptor diazo compounds are routinely utilized. 

When 1,4-dienes are utilized, the activity of allylic C—H bonds to undergo carbene insertion reactions is further enhanced. For instance, a range of chiral Rh complexes is able to convert 1,4-cyclohexadiene into the corresponding C—H bond insertion product in a high yield and ee in the presence of methyl phenyldiazoacetate la (Scheme 1.6, eqn (1)). The synthetic utility of this methodology was further demonstrated by total synthesis of natural products (+)-indatraline (Scheme 1.6, eqn (2)) and (+)-cetiedil (Scheme 1.6, eqn (3)). ... 

In 1983, James and co-workers disclosed the first example of enantioselec-tive intramolecular hydroacylation reaction of racemic a,a-disubstituted aliphatic aldehyde 1 when they investigated aldehyde decarbonylation reaction using a chiral Rh complex as the catalyst (Scheme 8.2). The enantioen-riched product 2 (69% ee) could be obtained via kinetic resolution, but only a low conversion was achieved. One possible reason for the low reactivity was the presence of a quaternary stereogenic center at the a-position of the carbonyl group in the substrate. 

Enamides can also be efficiently hydrogenated with chiral Rh-complexes. The Rh complexes of Ph-BPE,f > BICP, TangPhos, SIPHOS,f and others have been shown to catalyze the hydrogenation of a mixture of (E) and (Z) P-methyl-a-phenylenamides 161 with excellent enantioselectivities. The hydrogenation of 2- and 3-substituted A -acetylindoles with the Ph-TRAP-Rh system was also possible with high ee. (R,S,S,R)-D10P was shown to be an excellent catalyst system for a variety of aromatic enamides. ... 

Tanaka et al. developed a Rh-catalyzed asymmetric one-pot transesterification and [2+2+2] cyclotrimerization using nonracemic ligand 415 in the synthesis of enantio-enriched 3,3-disubstituted phthalides (R R ) (Scheme 2-39)P The chiral Rh complex with 415 efficiently desymmetrized dipropargyl alcohols 413 (R = R -OC-) in the reaction with 412 to give phthalides 414 (R = R -C=C-) in up to 87% yield and 93% ee. Also, the kinetic resolution of racemic tertiary propargylic alcohols... 

The greatest advantage of this approach is the circumvention of the use of optically active tertiary propargylic alcohols. The chiral Rh complex could efficiently desymmetrize the achiral alcohol to give the 3,3-disubstituted phthalides products in up to 87% isolated yield and 93% ee. The kinetic resolution of racemic tertiary propargylic alcohols provided disubstituted phthalides with up to 89% 3deld and up to 93% ee. 

Asymmetric Hydroamination with a Chiral Rh Complex Using Ligand 64 1247... 

Structure of major complex between ethyl a-acetamidocinnamate and chiral Rh complex... 

For the synthesis of each enantiomer of both norephedrine and norpseudoephedrine, the appropriate and well-defined chiral Rh-complexes, (5,5)- or (R,/ )-Cp RhCl (TsDPEN), and have been employed. The ATH processes are carried out under mild conditions (room temperature, 15 minutes) using HCOiH/EtaN as a hydrogen source, and they are accompanied by dynamic kinetic resolution. Using this methodology, enantiomeric excess up to >99% ee can be achieved. ... 

Close attention has been devoted in recent years to the homogeneous catalytic reduction of N-acylaminoacrylic acids with the aid of chiral Rh-complexes. In some cases exceptionally high optical yields have been achieved in these reactions. Phosphine-Rh catalysts of the DIOP type, i. e. with chiral carbon skeleton, have been used 108, 109, 133,142,145, 146, 172, 193, 194, 234), as have catalysts with phosphine oxide ligands 409), ferrocenyl-phosphine-Rh complexes 171), bisphosphine-Rh complexes with a chiral pyrrolidine ring 3, 4), systems with chiral phosphines 216—221) and bisphosphines (222), or with a chiral P- and C-skeleton 130). 

Chiral Rh-complexes have recently been successfully employed for the heterogenous hydrogenation of acylaminoacrylic acids, by coupling them to a polymeric carrier 408). 

---