The increasing demand to produce enantiomerically pure pharmaceuticals, agrochemicals, flavors, and other fine chemicals has advanced the field of asymmetric catalytic technologies.1,2 Among all asymmetric catalytic methods, asymmetric hydrogenation utilizing molecular hydrogen to reduce prochiral olefins, ketones, and imines, have become one of the most efficient methods for constructing chiral compounds.3 The development of homogeneous asymmetric hydrogenation was initiated by Knowles4a and Horner4b in the late 1960s, after the discovery of Wilkinson’s homogeneous hydrogenation catalyst [RhCl(PPh3)3]. By replacing triphenylphosphine of the Wilkinson’s catalystwithresolvedchiralmonophosphines,6Knowles and Horner reported the earliest examples of enantioselective hydrogenation, albeit with poor enantioselectivity. Further exploration by Knowles with an improved monophosphine CAMP provided 88% ee in hydrogenation of dehydroamino acids.7 Later, two breakthroughs were made in asymmetric hydrogenation by Kagan and Knowles, respectively. Kagan reported the first bisphosphine ligand, DIOP, for Rhcatalyzed asymmetric hydrogenation.8 The successful application of DIOP resulted in several significant directions for ligand design in asymmetric hydrogenation. Chelating bisphosphorus ligands could lead to superior enantioselectivity compared to monodentate phosphines. Additionally, P-chiral phosphorus ligands were not necessary for achieving high enantioselectivity, and ligands with backbone chirality could also provide excellent ee’s in asymmetric hydrogenation. Furthermore, C2 symmetry was an important structural feature for developing new efficient chiral ligands. Kagan’s seminal work immediately led to the rapid development of chiral bisphosphorus ligands. Knowles made his significant discovery of a C2-symmetric chelating bisphosphine ligand, DIPAMP.9 Due to its high catalytic efficiency in Rh-catalyzed asymmetric hydrogenation of dehydroamino acids, DIPAMP was quickly employed in the industrial production of L-DOPA.10 The success of practical synthesis of L-DOPA via asymmetric hydrogenation constituted a milestone work and for this work Knowles was awarded the Nobel Prize in 2001.3k This work has enlightened chemists to realize * Corresponding author. 3029 Chem. Rev. 2003, 103, 3029−3069

New Chiral Phosphorus Ligands for Enantioselective Hydrogenation

We report a new Pd(II)-catalyzed C−H bond amination reaction to form carbazoles, an important motif that is prevalent in a range of systems. The catalytic amination process operates under extremely mild conditions and produces carbazole products in good to excellent yields. Carbazoles possessing complex molecular architecture can also be formed using this reaction, highlighting its potential in natural product synthesis applications. Preliminary mechanistic investigations reveal the reaction proceeds through a Pd(II)/Pd(IV) manifold and that reductive elimination from a high oxidation state Pd(IV) complex facilitates the mild conditions of this transformation.

Oxidative Pd(II)-catalyzed C-H bond amination to carbazole at ambient temperature.

Rhodium-catalyzed asymmetric arylation (RCAA) reactions provide one of the most straightforward and powerful ways to introduce aryl fragments in an enantioselective manner. The discovery of novel chiral ligands and catalytic systems is a major focus in generating optical chiralities for RCAA reactions. In the past decade, the chelating functionalities in ligands have been significantly expanded from traditional phosphorus to interesting diene, bissulfoxide, and their hybrids. Herein we highlight the research on these distinct families of chiral ligands and describe their applications in the RCAA of arylmetals to activated alkenes, aldehydes, ketones and imines, and RCAA-tandem reactions.

Rhodium-Catalyzed Asymmetric Arylation

Coordination and organometallic chemistry of metal-NO complexes.

Recent Advances in Palladium‐Catalyzed Cross‐Coupling Reactions at ppm to ppb Molar Catalyst Loadings

Chelating diphosphines that contain a rhenium stereocenter in the backbone: Applications in rhodium-catalyzed enantioselective ketone hydrosilylations and alkene hydrogenations

Rhenium-Containing Phosphorus Donor Ligands for Palladium-Catalyzed Suzuki Cross-Coupling Reactions: A New Strategy for High-Activity Systems

The bromocyclopentadienyl complex [(eta5-C5H4Br)Re(CO)3] is converted to racemic [(eta5-C5H4Br)Re(NO)(PPh3)(CH2PPh2)] (1 b) similarly to a published sequence for cyclopentadienyl analogues. Treatment of enantiopure (S)-[(eta5-C5H5)Re(NO)(PPh3)(CH3)] with nBuLi and I2 gives (S)-[(eta5-C5H4I)Re(NO)(PPh3)(CH3)] ((S)-6 c; 84 %), which is converted (Ph3C+ PF6 -, PPh2H, tBuOK) to (S)-[(eta5-C5H4I)Re(NO)(PPh3)(CH2PPh2)] ((S)-1 c). Reactions of 1 b and (S)-1 c with Pd[P(tBu)3]2 yield [{(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(mu-X)}2] (10; X = b, Br, rac/meso, 88 %; c, I, S,S, 22 %). Addition of PPh3 to 10 b gives [(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(PPh3)(Br)] (11 b; 92 %). Reaction of (S)-[(eta5-C5H5)Re(NO)(PPh3)(CH2PPh2)] ((S)-2) and Pd(OAc)(2) (1.5 equiv; toluene, RT) affords the novel Pd3(OAc)4-based palladacycle (S,S)-[(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(mu-OAc)2Pd(mu-OAc)2Pd(mu-PPh2CH2)(Ph3P)(ON)Re(eta5-C5H4)] ((S,S)-13; 71-90 %). Addition of LiCl and LiBr yields (S,S)-10 a,b (73 %), and Na(acac-F6) gives (S)-[(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(acac-F6)] ((S)-16, 72 %). Reaction of (S,S)-10 b and pyridine affords (S)-[(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(NC5H5)(Br)] ((S)-17 b, 72 %); other Lewis bases yield similar adducts. Reaction of (S)-2 and Pd(OAc)2 (0.5 equiv; benzene, 80 degrees C) gives the spiropalladacycle trans-(S,S)-[{(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)}2Pd] (39 %). The crystal structures of (S)-6 c, 11 b, (S,S)- and (R,R)-132 C7H8, (S,S)-10 b, and (S)-17 b aid the preceding assignments. Both 10 b (racemic or S,S) and (S)-16 are excellent catalyst precursors for Suzuki and Heck couplings.

Synthesis, Structure, and Reactivity of Palladacycles That Contain a Chiral Rhenium Fragment in the Backbone: New Cyclometalation and Catalyst Design Strategies

The title compounds are accessed by sequences starting with racemic and enantiomerically pure [(eta5-C5H5)Re(NO)(PPh3)(CH3)]. Reactions with chlorobenzene/HBF4, PPh2H, and tBuOK give the phosphido complex [(eta5-C5H5)Re(NO)(PPh3)(PPh2)] (3). Reactions with Ph3C+ BF4-, PPh2H, and tBuOK give the methylene homologue [(eta5-C5H5)Re(NO)(PPh3)(CH2PPh2)] (9). Treatment of 3 or 9 with nBuLi or tBuLi and then PPh3Cl gives the diphosphido systems [(eta5-C5H4PPh2)Re(NO)(PPh3)((CH2)nPPh2)] (n = 0/1, 5/11). Reactions of 5 and 11 with [Rh(NBD)Cl]2/AgPF6 (NBD = norbornadiene) give the rhenium/rhodium chelate complexes [(eta5-C5H4PPh2)Re(NO)(PPh3)((mu-CH2)nPPh2)Rh(NBD)]+ PF6- (n = 0/1, 6+/12+ PF6-; 30-32% overall from commercial Re2(CO)10). The crystal structures of 6+ PF6- and 12+ PF6- are compared to those of 3 and 9, and other rhodium complexes of chelating bis(diphenylphosphines). The chiral pockets defined by the PPh2 groups show unusual features. Four alkenes of the type (Z)-RCH=C(NHCOCH3)CO2R' are treated with H2 (1 atm) and (R)-6+ PF6- or (S)-12+ PF6- (0.5 mol%) in THF at room temperature. Protected amino acids are obtained in 70-98% yields and 93-82% ee [(R)-6- PF6-] or 72-60% ee [(S)-12+ PF6-]. Pressure and temperature effects are defined, and turnover numbers of > 1600 are realized.

A New Family of Chelating Diphosphines with a Transition Metal Stereocenter in the Backbone: Novel Applications of “Chiral-at-Rhenium” Complexes in Rhodium-Catalyzed Enantioselective Alkene Hydrogenations

Klemenz Kromm

Papers

Chelating diphosphines that contain a rhenium stereocenter in the backbone: Applications in rhodium-catalyzed enantioselective ketone hydrosilylations and alkene hydrogenations

Rhenium-Containing Phosphorus Donor Ligands for Palladium-Catalyzed Suzuki Cross-Coupling Reactions: A New Strategy for High-Activity Systems

Synthesis, Structure, and Reactivity of Palladacycles That Contain a Chiral Rhenium Fragment in the Backbone: New Cyclometalation and Catalyst Design Strategies

A New Family of Chelating Diphosphines with a Transition Metal Stereocenter in the Backbone: Novel Applications of “Chiral-at-Rhenium” Complexes in Rhodium-Catalyzed Enantioselective Alkene Hydrogenations

A New Family of Chelating Diphosphines with Transition-Metal and Carbon Stereocenters in the Backbone: A Second-Generation Rhenium-Containing System