METAMORPhos: Adaptive Supramolecular Ligands and Their Mechanistic Consequences for Asymmetric Hydrogenation

Chemoselective Staudinger Reactivity of Bis(azido)phosphines Supported with a π-Donating Imidazolin-2-iminato Ligand

Synthesis and characterization of new P(III) and P(V) bis(azido)phosphines/phosphoranes supported by an N,N'-bis(2,6-diisopropylphenyl) imidazolin-2-iminato (IPrN) ligand and their reactivity with various secondary and tertiary phosphines result in the formation of chiral and/or asymmetric mono(phosphinimino)azidophosphines via the Staudinger reaction. The reaction of IPrNP(N3)2 (2) or IPrNP(S)(N3)2 (4S) with an excess of tertiary phosphine resulted in the chemoselective formation of IPrNP(N3)(NPMe3) (7) or IPrNP(S)N3(NPR3) (5R), respectively. The chemoselective Staudinger reactivity was also observed in reactions using a secondary phosphine (HPCy2) to produce IPrNP(S)N3[NP(H)Cy2] (6a), which exists in equilibrium with a tautomeric IPrNP(S)N3[N(H)PCy2] form (6b), as confirmed by 31P-31P nuclear Overhauser effect spectroscopy (NOESY). Density functional theory (DFT) calculations point to a combination of energetically unfavorable lowest unoccupied molecular orbitals (LUMOs) and the accumulation of increasing negative charge at the terminal azido-nitrogen upon a single azide-to-phosphinimine conversion that gave rise to the observed chemoselectivity.

Novel Chiral Bis-Phosphoramides as Organocatalysts for Tetrachlorosilane-Mediated Reactions

The formation of novel chiral bidentate phosphoroamides structures able to promote Lewis base-catalyzed Lewis acid-mediated reactions was investigated. Two different classes of phosphoroamides were synthetized: the first class presents a phthalic acid/primary diamine moiety, designed with the aim to perform a self-assembly recognition process through hydrogen bonds; the second one is characterized by the presence of two phosphoroamides as side arms connected to a central pyridine unit, able to chelate SiCl₄ in a 2:1 adduct. These species were tested as organocatalysts in the stereoselective allylation of benzaldehyde and a few other aromatic aldehydes with allyl tributyltin in the presence of SiCl₄ with good results. NMR studies confirm that only pyridine-based phosphoroamides effectively coordinate tetrachlorosilane and may lead to the generation of a self-assembled entity that would act as a promoter of the reaction. Although further work is necessary to clarify and confirm the formation of the hypothesized adduct, the study lays the foundation for the design and the synthesis of chiral supramolecular organocatalysts.

Formic acid as a hydrogen storage material – development of homogeneous catalysts for selective hydrogen release

Liquid energy: formic acid is an ideal candidate for catalytic release and storage of hydrogen.

Intermolecular C–H activation with an Ir-METAMORPhos piano-stool complex – multiple reaction steps at a reactive ligand

Reaction of phenylacetylene with an Ir^III(METAMORPhos) complex generates a unique four-membered IrPNC metallacyclic species with an exocyclic CC fragment.

Dehydrogenation of formic acid by Ir-bisMETAMORPhos complexes: experimental and computational insight into the role of a cooperative ligand

The synthesis of Ir-complexes with three bisMETAMORPhos ligands is reported. The activity of these systems towards HCOOH dehydrogenation and the dual role of the ligand during catalysis is discussed, using spectroscopic and computational methods.

The synthesis and tautomeric nature of three xanthene-based bisMETAMORPhos ligands (La–Lc) is reported. Coordination of these bis(sulfonamidophosphines) to Ir(acac)(cod) initially leads to the formation of Ir^I(L^H) species (1a), which convert via formal oxidative addition of the ligand to Ir^III(L) monohydride complexes 2a–c. The rate for this step strongly depends on the ligand employed. Ir^III complexes 2a–c were applied in the base-free dehydrogenation of formic acid, reaching turnover frequencies of 3090, 877 and 1791 h^–1, respectively. The dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by ¹H and ³¹P NMR spectroscopy and DFT calculations. Besides functioning as an internal base, bisMETAMORPhos also assists in the pre-assembly of formic acid within the Ir-coordination sphere and aids in stabilizing the rate-determining transition state through hydrogen-bonding.

Rhodium-catalyzed asymmetric hydrogenation of olefins with PhthalaPhos, a new class of chiral supramolecular ligands

A library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (PhthalaPhos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2-acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1-yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3-phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect.

Hybrid bidentate phosphorus ligands in asymmetric catalysis: privileged ligand approach vs. combinatorial strategies

In this perspective the development of chiral phosphorus ligands for asymmetric catalysis is discussed, with a special focus on hybrid bidentate phosphorus ligands, in particular phosphine-phosphoramidites. An attempt is made to compare privileged ligand and combinatorial approaches to ligand development--for which the class of phosphine-phosphoramidite ligands is well suited--highlighting differences, similarities and their complementary use.