Parallel Synthesis of Aminomethylphosphine Ligands

Ligand libraries for high throughput screening of homogeneous catalysts

This review describes different approaches to construct ligand libraries towards high throughput screening of homogeneous metal catalysts.

The combinatorial approach to asymmetric hydrogenation: phosphoramidite libraries, ruthenacycles, and artificial enzymes

For a more general implementation of asymmetric catalysis in the production of fine chemicals, the screening for new catalysts and ligands must be dramatically accelerated. This is possible with a high-throughput experimentation (HTE) approach. However, implementation of this technology requires the rapid preparation of libraries of ligands/catalysts and consequently dictates the use of simple ligands that can be readily synthesised in a robot. In this concept article, we describe how the development of new ligands based on monodentate phosphoramidites enabled the development of an integral HTE protocol for asymmetric hydrogenation. This "instant ligand library" protocol makes it possible to synthesise 96 ligands in one day and screen them the next day. Further diversity is possible by using mixtures of monodentate ligands. This concept has already led to an industrial application. Other concepts, still under development, are based on chiral ruthenacycles as new transfer hydrogenation catalysts and the use of enzymes as ligands for transition-metal complexes.

Synthesis of solution-phase phosphoramidite and phosphite ligand libraries and their in situ screening in the rhodium-catalyzed asymmetric addition of arylboronic acids

Herein, we report the automated parallel synthesis of solution-phase libraries of phosphoramidite ligands for the development of enantioselective catalysts. The ligand libraries are screened in situ in the asymmetric rhodium-catalyzed addition of arylboronic acids to aldehydes and imines. It is shown that the described methodology results in the straightforward discovery of leads for highly efficient enantioselective catalysts.

Parallel and combinatorial approaches for synthesis of ligands

Although new detection screening methods must still be developed, the actual main limitation in combinatorial chemistry seems to be the diversity of ligands that can be generated in terms of real structural and chemical diversity. Thus, there is a strong interest for the development of different strategies for the parallel or combinatorial synthesis of ligands. We report here a selection of recent attempts proposing 'open' approaches able to increase the diversity of molecular architecture truly accessible via parallel or combinatorial processes.

Solid-phase synthesis of chiral 3,4-diazaphospholanes and their application to catalytic asymmetric allylic alkylation

Functionalized chiral diazaphospholanes ligate to a variety of transition metals, yielding chiral, catalytically active, metal complexes. Previous work has established that amino acid derivatization of the carboxyl groups of (R,R)-N,N'-phthaloyl-2,3-(2-carboxyphenyl)-phenyl-3,4-diazaphospholane (1) yields phosphines that are excellent ligands for palladium-catalyzed asymmetric allylic alkylation reactions. Alanine functionalization is particularly effective for allylic alkylation of 1,3-dimethylallyl acetate. Standard Merrifield resins and amino acid coupling methods are used to synthesize the bead-attached phosphine having the topology bead-linker-LAla-(R,R)-1-LAla-OMe, as a 1:1 mixture of linkage isomers. Use of this supported phosphine in Pd-catalyzed asymmetric allylic alkylation yields 92% enantiomeric excess, matching prior solution-phase results. A 20-member collection of amino acid-functionalized phosphines on beads with the topology bead-linker-AA(2)-AA(1)-1-AA(1)-AA(2) was synthesized by using parallel solid-state methods and screened for efficacy in allylic alkylation. Resulting enantioselectivities indicate that the AA(1) position has the strongest effect on the reaction. Catalyst activities can vary widely with the nature of the phosphine ligand and the reaction conditions. Meaningful analysis of intrinsic catalytic activities awaits identification of the structure and abundance of the active catalyst.

Recent advances in the discovery of organometallic catalysts using high-throughput screening assays

Combinatorial techniques were developed initially to accelerate the identification of molecules with biological activity. The successes of these techniques inspired the design of high-throughput methods to assist in the discovery of new catalysts. Over the past year, many groups in academia and industry have utilized high-throughput screening assays to reduce the time required to identify catalysts for asymmetric processes, cross-coupling reactions and other metal-catalyzed transformations. The continued success of combinatorial techniques in organometallic chemistry should propagate the development of new and improved methods to facilitate catalyst discovery.

Combinatorial approaches as a component of high-throughput experimentation (HTE) in catalysis research

We consider the application of high-throughput experimentation (HTE), including combinatorial methods, to catalyst discovery and early-phase optimization. While combinatorial- and parallel-testing methods have an already substantial history in catalysis, recent work by several groups promises significant efficiency gains. In molecular catalysis, progress is noted in library design, library synthesis by pooled, parallel, and discrete formats, catalyst testing and reaction optimization; the prime constraint for organometallic catalysts is the limited scope of synthesis procedures for non-peptide-based ligand libraries. Routes described for the synthesis of heterogeneous catalysts include hydrothermal synthesis, arraying of solution precursors, automated impregnation and precipitation, and arraying of solid precursors. The key challenge in applying HTE to heterogeneous catalysis is testing; we distinguish here between Stage 1, or "discovery" testing and Stage 2, optimization testing, and describe techniques with potential in each case. Recent examples from the literature and our own work are used to illustrate these principles and the prospects for HTE applied to catalysis.

Palladium/P,O-Ligand-Catalyzed Suzuki Cross-Coupling Reactions of Arylboronic Acids and Aryl Chlorides. Isolation and Structural Characterization of (P,O)-Pd(dba) Complex

The phenyl backbone-derived P,O-ligands 1 and 2 were investigated for their utility as ligands in palladium/ligand-catalyzed Suzuki reactions. The 2-(2'-dicyclohexylphosphinophenyl)-2-methyl-1,3-dioxolane (ligand 1) in combination with Pd(dba)(2) affords an efficient catalyst for general Suzuki reactions of a wide variety of arylboronic acids and aryl chlorides, bromides, and iodides to afford the desired biaryl products in high isolated yields. Arylboronic acids and aryl chlorides containing electron-poor, electron-rich, and ortho substituents participate effectively. In contrast, the structurally related ligand 2-(2'-dicyclohexylphosphinophenyl)-1,3-dioxolane (ligand 2) was found to be less efficient under similar conditions. The reaction of ligand 1 with Pd(dba)(2) affords the complex LPd(dba) (14, L = 1). The NMR spectroscopic and X-ray crystallographic data of complex 14 establish that ligand 1 functions as a P,O-chelating ligand in complex 14. The reaction of ligand 2 (2 equiv) with Pd(dba)(2) and excess 4-(t)()Bu-C(6)H(4)Br or the ligand displacement reaction of {Pd[P(o-tolyl)(3)](4-(t)()Bu-C(6)H(4))(&mgr;-Br)}(2) with ligand 2 affords the bis-phosphine complex L(2)Pd(4-(t)()Bu-C(6)H(4))(Br) (13, L = 2). The NMR spectroscopic data of complex 13 establish that ligand 2 in complex 13 functions as a nonchelating ligand. Thus, the higher efficiency of ligand 1 over ligand 2 in Pd/L-catalyzed Suzuki arylation of aryl chlorides can be ascribed to the ability of ligand 1 to generate and stabilize mono-phosphine P,O-chelating Pd/L intermediates, which appear to be most suitable for Suzuki arylation reactions involving certain substrates and conditions.