Chiral monodentate phosphorus ligands for rhodium-catalyzed asymmetric hydrogenation

Phosphoramidites: privileged ligands in asymmetric catalysis

Asymmetric catalysis with transition-metal complexes is the basis for a vast array of stereoselective transformations and has changed the face of modern synthetic chemistry. Key to this success has been the design of chiral ligands to control the regio-, diastereo-, and enantioselectivity. Phosphoramidites have emerged as a highly versatile and readily accessible class of chiral ligands. Their modular structure enables the formation of ligand libraries and easy fine-tuning for a specific catalytic reaction. Phosphoramidites frequently show exceptional levels of stereocontrol, and their monodentate nature is essential in combinatorial catalysis, where a ligand-mixture approach is used. In this Review, recent developments in asymmetric catalysis with phosphoramidites used as ligands are discussed, with a focus on the formation of carbon-carbon and carbon-heteroatom bonds.

Multi-component cycloaddition approaches in the catalytic asymmetric synthesis of alkaloid targets

Cycloaddition reactions are attractive strategies for the rapid formation of molecular complexity in organic synthesis, as multiple bonds are formed in a single process. To this end, several research groups have been actively involved in the development of catalytic methods to activate readily accessible pi-components to achieve cycloadditions. However, the use of C-N pi-components for the formation of heterocycles by these processes is less well developed. It has been previously demonstrated that the combination of different isocyanates with two alkynes yields pyridones of several types by metal-catalyzed [2 + 2 + 2] cycloadditions. The potential of this chemistry has been extended to alkenes as C-C pi-components, allowing the formation of sp(3)-stereocenters. In this tutorial review directed towards [n + 2 + 2] cycloadditions of heterocumulenes, alkynes and alkenes, the recent advances in the catalytic asymmetric synthesis of indolizidine, quinolizidine and azocine skeletons are discussed.

Recent advances in enantioselective copper-catalyzed 1,4-addition

A comprehensive overview of recent literature from 2003 concerning advances in enantioselective copper catalysed 1,4-addition of organometallic reagents to alpha,beta-unsaturated compounds is given in this critical review. About 200 ligands and catalysts are presented, with a focus on stereoselectivities, catalyst loading, ligand structure and substrate scope. A major part is devoted to trapping and tandem reactions and a variety of recent synthetic applications are used to illustrate the practicality and current state of the art of 1,4-addition of organometallic reagents. Finally several mechanistic studies are discussed (162 references).

Solid-state phosphorus-31 NMR spectroscopy of a multiple-spin system: an investigation of a rhodium-triphosphine complex

Phosphorus-31 NMR spectra of solid [tris(dimethylphenylphosphine)](2,5-norbornadiene) rhodium(I) hexafluorophosphate have been acquired at several applied magnetic field strengths. The phosphorus nuclei of the three phosphine ligands are spin-spin coupled to each other and to 103Rh, resulting in complex NMR spectra; however, the three phosphorus chemical shift (CS) tensors were determined through the analysis of NMR spectra of slow magic angle spinning and stationary samples. Spectra of spinning samples in rotational resonance and two-dimensional 31P NMR spectra were particularly useful for determining the magnitudes of the indirect spin-spin couplings, and to probe their signs. Despite being in similar environments, the three phosphorus nuclei of the phosphine ligands have distinct CS tensors. In particular, the spans of these tensors, delta11-delta33, range from 80 to 176 ppm. The phosphorus CS tensors have been assigned to specific sites determined by X-ray crystallography, based on a combination of the experimental results and the results of quantum chemical calculations of the phosphorus shielding and 2J(31P,31P) values. The effect of coordination of dimethylphenylphosphine with rhodium has been investigated by comparing calculated phosphorus CS tensors for the uncoordinated ligand with those obtained for the ligands in the complex.

Chiral diphosphine and monodentate phosphorus ligands on a spiro scaffold for transition-metal-catalyzed asymmetric reactions

The preparation of chiral compounds in enantiomerically pure form is a challenging goal in modern organic synthesis. The use of chiral metal complex catalysis is a powerful, economically feasible tool for the preparation of optically active organic compounds on both laboratory and industrial scales. In particular, the metals coordinated by one or more chiral phosphorus ligands exhibit amazing enantioselectivity and reactivity. Many chiral phosphorus ligands have been synthesized and used in transition-metal-catalyzed asymmetric reactions in past decades. However, a large number of reactions still lack effective chiral ligands, and the enantioselectivities in many reactions are substrate-dependent. The development of effective chiral phosphorus ligands, especially ligands having novel chiral backbones, is still an important task in the area of asymmetric catalysis. Molecules containing a spirocyclic framework are ubiquitous in nature. The synthesis of molecules with this spiro structure can be traced back to 100 years ago. However, the use of this spirocyclic framework to construct chiral phosphorus ligands is a recent event. This Account outlines the design and synthesis of a new family of chiral spiro phosphorus ligands including spiro diphosphines and spiro monodentate phosphorus ligands with 1,1'-spirobiindane and 9,9'-spirobifluorene backbone and their applications in transition-metal-catalyzed asymmetric hydrogenation and carbon-carbon bond formation reactions. The chiral spiro diphosphine lgands SDP with a 1,1'-spirobiindane backbone and SFDP with a 9,9'-spirobifluorene backbone, and the spiro monophosphorus ligands including phosphoramidites, phosphites, phosphonites, and phospholane with a 1,1'-spirobiindane backbone were synthesized in good yields from enantiomerically pure 1,1'-spirobiindane-7,7'-diol and 9,9'-spirobifluoren-1,1'-diol. The ruthenium complexes of chiral spiro diphosphine ligands proved to be very effective catalysts for asymmetric hydrogenations of ketones, alpha-arylaldehydes and alpha,beta-unsaturated acids. The rhodium complexes of chiral spiro monophosphorus ligands are highly enantioselective for the asymmetric hydrogenations of alpha- and beta-dehydroamino acid derivatives, alpha-arylethenyl acetamides and non- N-acyl enamines. The spiro monophosphorus ligands were demonstrated to be highly efficient for the Rh-catalyzed asymmetric addition of arylboronic acids to aldehydes and N-tosylarylimines, Pd-catalyzed asymmetric allylation of aldehydes with allylic alcohols, Cu-catalyzed asymmetric ring opening reactions with Grignard reagents, and Ni-catalyzed asymmetric hydrovinylation of styrene derivatives with ethylene. The chiral spiro phosphorus ligands show high enantioselectivities for a wide range of transition-metal-catalyzed asymmetric reactions. In most of these transformations, the enantioselectivities of spiro phosphorus ligands are superior to those obtained by using the corresponding phosphorus ligands with other backbones. These results arise from the intriguing chiral inducement of spiro structures of the ligands.