PuPHOS and CamPHOS Ligands in the Intermolecular Catalytic Pauson–Khand Reaction

P-Stereogenic Amino-Phosphines as Chiral Ligands: From Privileged Intermediates to Asymmetric Catalysis

Among chiral phosphines, P-stereogenic phosphines provide unparalleled activity and selectivity and have thus emerged as "state-of-the-art" ligands for asymmetric hydrogenation and other industrially relevant processes. However, the synthesis of this type of ligand implies lengthy multistep sequences, which are a hurdle for many laboratories. There is a lack of methods for the rapid construction of P-stereogenic phosphine ligands. In this respect, P-stereogenic synthons that can be rapidly incorporated into a given ligand scaffold are highly desirable. Over the last 10 years, our group has unveiled that P-stereogenic aminophosphines can be rapidly assembled in a convenient fashion from the corresponding primary aminophosphine and/or the corresponding phosphinous acid.Using cis-1-amino-2-indanol as chiral auxiliary, we devised a multigram synthesis of tert-butylmethylaminophosphine borane and tert-butylmethylphosphinous acid borane, which are key intermediate synthons. Primary aminophosphine works as nucleophilic intermediates at nitrogen. From this synthon, aminodiphosphine (MaxPHOS) and secondary imino phosphoranes (SIP) ligands were synthesized. These ligands exhibit a tautomeric equilibrium between the PH and NH forms, and because of that, they do not undergo oxidation in air. NH/PH tautomerism does not jeopardize their configurational stability, and most importantly, in the presence of a metal source, the equilibrium is shifted toward the NH form, thus allowing coordination through phosphorus. Rh-MaxPHOS and Rh-SIP complexes have been used in asymmetric hydrogenation and [2 + 2 + 2] cycloaddition reactions with outstanding results. On the other hand, P-stereogenic phosphinous acid, upon activation, serves as an electrophilic reagent with amine nucleophiles, allowing S_N2 reactions at phosphorus with complete inversion of configuration. This coupling technology exhibits a great potential because it allows the incorporation of the P*-phosphine fragment in numerous ligand structures, provided there is an amino group with which to react. In a mild and efficient process, phosphinous acid has been coupled to hydrazine to yield C₂ diphosphines and to chiral benzoimidazole-amines to yield P-stereogenic benzoimidazole-phosphine ligands. The most powerful ligand system, however, arises from the condensation of three independent fragments: our phosphinous acid borane, an amino acid, and an amino alcohol, which yields a library of phosphino-oxazoline ligands named MaxPHOX. The corresponding Ir-MaxPHOX catalyst library was applied with excellent results in the asymmetric hydrogenation of α,β-unsaturated esters, 2-aryl allyl phthalimides, unfunctionalized tetrasubstituted alkenes, cyclic enamides, and N-aryl and N-methyl imines. It also has found application in asymmetric isomerization of alkenes.Overall, we developed key P-stereogenic building blocks that can be incorporated stereospecifically to ligand scaffolds and demonstrated that integration of the P*-aminophosphine fragment in a given catalytic system provides structural diversity that can be a critical contribution to obtaining optimal results and selectivity.

Access to Highly Functionalized Cyclopentenones via Diastereoselective Pauson-Khand Reaction of Siloxy-Tethered 1,7-Enynes

A diastereoselective Co₂(CO)₈-mediated Pauson-Khand reaction (PKR) of siloxy-tethered 1,7-enynes for the synthesis of cyclopentaoxasilinones has been developed. This transformation can be performed on a multigram scale and is characterized by a broad substrate scope, functional group compatibility, and high chemo- and diastereoselectivity. Oxidation of the resulting cyclopentaoxasilinones delivers stereoenriched β-alkylated cyclopentenones, which are inaccessible by intermolecular PKRs. This research provides a practical solution to the challenges associated with the classical intermolecular PKR.

Copper(I)-Catalyzed Kinetic Resolution of exo-3-Oxodicyclopentadienes and endo-3-Oxodicyclopentadiene

The first example of highly efficient kinetic resolution of exo-3-oxodicyclopentadienes and endo-3-oxodicyclopentadiene has been developed by means of Cu(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylide. Compared with the existing methodologies for those synthetically important optically active convex molecules, the current protocol provides an alternative but more practical approach from the readily available racemic starting materials, which is free from the repetitive reduction/oxidation steps in the enzymatic resolution or the indispensable stoichiometric amount of chirality-induction reagents.

Enantioselective Synthesis of Chiral Cyclopent-2-enones by Nickel-Catalyzed Desymmetrization of Malonate Esters

The enantioselective synthesis of highly functionalized chiral cyclopent‐2‐enones by the reaction of alkynyl malonate esters with arylboronic acids is described. These desymmetrizing arylative cyclizations are catalyzed by a chiral phosphinooxazoline/nickel complex, and cyclization is enabled by the reversible E/Z isomerization of alkenylnickel species. The general methodology is also applicable to the synthesis of 1,6‐dihydropyridin‐3(2H)‐ones.

Enantioselective synthesis of 5,7-bicyclic ring systems from axially chiral allenes using a Rh(I)-catalyzed cyclocarbonylation reaction

A transfer of chirality in an intramolecular Rh(I)-catalyzed allenic Pauson-Khand reaction (APKR) to access tetrahydroazulenones, tetrahydrocyclopenta[c]azepinones and dihydrocyclopenta[c]oxepinones enantioselectively (22-99% ee) is described. The substitution pattern of the allene affected the transfer of chiral information. Complete transfer of chirality was obtained for all trisubstituted allenes, but loss of chiral information was observed for disubstituted allenes. This work constitutes the first demonstration of a transfer of chiral information from an allene to the 5-position of a cyclopentenone using a cyclocarbonylation reaction. The absolute configuration of the corresponding cyclocarbonylation product was also established, something that is rarely done.

Synthesis and Diels-Alder cycloaddition reaction of norbornadiene and benzonorbornadiene dimers

Dimeric forms of norbornadiene and benzonorbornadiene were synthesized starting with known monobromide derivatives. The Diels–Alder cycloaddition reaction of dimers with TCNE and PTAD was investigated and new norbornenoid polycyclics were obtained. All compounds were characterized properly using NMR spectroscopy.