Arene Insertion with Pincer-Supported Molybdenum-Hydrides: Determination of Site Selectivity, Relative Rates, and Arene Complex Formation.

The synthesis of phosphino(oxazoline)pyridine-supported molybdenum(0) cycloocta-1,5-diene complexes is described. Exposure of these complexes to dihydrogen in the presence of an arene resulted in insertion of the substrate into the molybdenum hydride bond and afforded the corresponding molybdenum cyclohexadienyl hydrides. For mono- and disubstituted arenes, the site selectivity for insertion of the most substituted bond increases with increasing size of the substituent from methyl to ethyl, iso-propyl, and tert-butyl. In contrast, 1,3,5-trisubstituted arenes underwent insertion with exclusive site selectivity. Relative rates of insertion were determined by competition experiments and established faster insertions for electron-rich arenes. Introduction of electron-withdrawing trifluoromethyl groups on the arene resulted in decreased relative rates of insertion and an increased rate for H2 reductive elimination, favoring formation of the corresponding molybdenum η6-arene complex. Studies on the reductive elimination of the cyclohexadienyl ligand with the hydride enabled the synthesis of an enantioenriched cyclohexa-1,3-diene. This study provides new insights into the ligand requirements for catalytic arene hydrogenation and a new strategy for selective arene reduction.

Identification of Cyclohexadienyl Hydrides as Intermediates in Molybdenum-Catalyzed Arene Hydrogenation.

Treatment of phosphino(imino)pyridine (PIP) molybdenum cyclooctadiene (COD) complexes [(PIP)Mo(COD)] with dihydrogen in the presence of benzene selectively furnished the molybdenum cyclohexadienyl hydrides [(PIP)MoH(η5 -C6 H7 )], which are precatalysts for the hydrogenation of benzene to cyclohexane. [(PIP)MoH(η5 -C6 H7 )] arises from a rarely observed insertion of benzene into a molybdenum-hydride bond, a key step in the molybdenum-catalyzed homogeneous hydrogenation of arenes. The reaction with toluene afforded a single isomer of the corresponding molybdenum cyclohexadienyl hydride while para-xylene predominantly formed the molybdenum η6 -arene complex with the insertion product being a minor component. Addition of carbon monoxide to a cyclohexane-d12 solution of [(PIP)MoH(η5 -C6 H7 )] liberated cyclohexadiene, providing experimental support for a higher kinetic barrier for the subsequent steps en route to cycloalkanes.

Molybdenum-Catalyzed Asymmetric Hydrogenation of Fused Arenes and Heteroarenes.

The synthesis of enantioenriched molybdenum precatalysts for the asymmetric hydrogenation of substituted quinolines and naphthalenes is described. Three classes of pincer ligands with chiral substituents were evaluated as supporting ligands in the molybdenum-catalyzed hydrogenation reactions, where oxazoline imino(pyridine) chelates were identified as optimal. A series of 2,6-disubstituted quinolines was hydrogenated to enantioenriched decahydroquinolines with high diastereo- and enantioselectivities. For quinoline derivatives, selective hydrogenation of both the carbocycle and heterocycle was observed depending on the ring substitution. Spectroscopic and mechanistic studies established molybdenum η6-arene complexes as the catalyst resting state and that partial hydrogenation arises from dissociation of the substrate from the coordination sphere of molybdenum prior to complete reduction. A stereochemical model is proposed based on the relative energies of the respective coordination of the prochiral faces of the arene determined by steric interactions between the substrate and the chiral ligand, rather than through precoordination by a heteroatom.

Asymmetric Hydrogenation of Unfunctionalized Tetrasubstituted Acyclic Olefins.

Asymmetric hydrogenation has evolved as one of the most powerful tools to construct stereocenters. However, the asymmetric hydrogenation of unfunctionalized tetrasubstituted acyclic olefins remains the pinnacle of asymmetric synthesis and an unsolved challenge. We report herein the discovery of an iridium catalyst for the first, generally applicable, highly enantio- and diastereoselective hydrogenation of such olefins and the mechanistic insights of the reaction. The power of this chemistry is demonstrated by the successful hydrogenation of a wide variety of electronically and sterically diverse olefins in excellent yield and high enantio- and diastereoselectivity.

Iridium-Catalyzed Asymmetric Hydrogenation of Benzo[b]thiophene 1,1-Dioxides.

An efficient iridium-catalyzed asymmetric hydrogenation of substituted benzothiophene 1,1-dioxides is described. The use of iridium complexes with chiral pyridyl phosphinite ligands provides access to highly enantiomerically enriched sulfones with substituents at the 2- and 3-position. Sulfones of this type are of interest as core structures of agrochemicals and pharmaceuticals. Moreover, they can be further reduced to chiral 2,3-dihydrobenzothiophenes.

A unified lead-oriented synthesis of over fifty molecular scaffolds.

Controlling the properties of lead molecules is critical in drug discovery, but sourcing large numbers of lead-like compounds for screening collections is a major challenge. A unified synthetic approach is described that enabled the synthesis of 52 diverse lead-like molecular scaffolds from a minimal set of 13 precursors. The divergent approach exploited a suite of robust, functional group-tolerant transformations. Crucially, after derivatisation, these scaffolds would target significant lead-like chemical space, and complement commercially-available compounds.

Iridium-catalyzed asymmetric hydrogenation of 3,3-disubstituted allylic alcohols in ethereal solvents.

Ir-phosphinomethyl-oxazoline complexes have been identified as efficient, highly enantioselective catalysts for the asymmetric hydrogenation of 3,3-disubstituted allylic alcohols and related homoallylic alcohols. In contrast to other N,P ligand complexes, which require weakly coordinating solvents, such as dichloromethane, these catalysts perform well in more ecofriendly THF or 2-MeTHF. Their synthetic potential was demonstrated with the formal total synthesis of four bisabolane sesquiterpenes.

Recent advances and applications of iridium-catalysed asymmetric allylic substitution.

Since their discovery in 1997, iridium-catalysed asymmetric allylic substitutions have been developed into a broadly applicable tool for the synthesis of chiral building blocks via C-C and C-heteroatom bond formation. The remarkable generality of these reactions and the high levels of regio- and enantioselectivity that are usually obtained in favour of the branched products have been made possible by a thorough investigation of the catalyst system and its mode of action. Therefore, today the Ir-catalysed asymmetric allylic substitution is a powerful reaction in the organic chemist's repertoire and has been used extensively for several applications. This article aims to provide an overview of the development of iridium catalysts derived from an Ir salt and a chiral phosphoramidite and their application to the enantioselective synthesis of natural products and biologically relevant compounds.

Catalyst control in sequential asymmetric allylic substitution: stereodivergent access to N,N-diprotected unnatural amino acids.

The sequential use of Cu-catalyzed asymmetric allylic alkylation, olefin cross-metathesis, and Ir-catalyzed asymmetric allylic amination allows the concise, stereodivergent synthesis of complex chiral amines with complete regiocontrol and good diastereoselectivity, exemplified by the synthesis of a pair of diastereoisomeric unnatural branched amino acid derivatives.