Solid-phase synthesis of an alpha-aminophosphine library

Microwave-Assisted Kabachnik⁻Fields Reaction with Amino Alcohols as the Amine Component

In this paper, the microwave (MW)-assisted catalyst-free and mostly solvent-free Kabachnik-Fields reaction of amino alcohols, paraformaldehyde, and various >P(O)H reagents (dialkyl phosphites, ethyl phenyl-H-phosphinate, and secondary phosphine oxides) is reported. The synthesis of N-2-hydroxyethyl-α-aminophosphonate derivatives was optimized in respect of the temperature, the reaction time, and the molar ratio of the starting materials. A few by-products were also identified. N,N-Bis(phosphinoylmethyl)amines containing a hydroxyethyl group were also prepared by the double Kabachnik-Fields reaction of ethanolamine with an excess of paraformaldehyde and secondary phosphine oxides. The crystal structure of a 2-hydroxyethyl-α-aminophosphine oxide and a bis(phosphinoylmethyl)ethanolamine was studied by X-ray analysis.

Synthesis of platinum, palladium and rhodium complexes of α-aminophosphine ligands

α-Aminophosphine-type ligands are of interest as building blocks of transition metal complexes. This review focuses on the utilization of α-aminophosphines as monodentate and bidentate ligands in platinum, palladium and rhodium complexes. Besides the linear derivatives, the applications of cyclic α-aminophosphines as ligands are also summarized. Various aspects, such as synthesis, structure and applications, as well as the catalytic activity of these complexes are discussed.

Parallel and modular synthesis of P-chirogenic P,O-ligands

A modular synthesis of P-chirogenic α-alkoxyphosphine ligands has been developed, allowing for the variation of two of the three groups on phosphorus. Oxidation and concomitant desymmetrization of a prochiral alkyl- or aryldimethylphosphine borane afforded α-hydroxyphosphines, which were subsequently deprotonated and alkylated in a parallel fashion. The choice of base and temperature for the alkylation step was found to be crucial for the outcome of the reaction. Selected ligands were subsequently screened in palladium catalyzed allylic substitution, affording product in good to excellent yield but moderate enantioselectivity, indicating that further optimization of the ligand structures is desirable to increase the stereoselectivity.

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.