Asymmetric hydrogenation of tri-substituted alkenes with Ir-NHC-thiazole complexes

Ir–Biaryl phosphite–oxazoline catalyst libraries: a breakthrough in the asymmetric hydrogenation of challenging olefins

This work describes our progress in the successful development of phosphite/phosphinite–oxazoline ligands for the Ir-catalyzed asymmetric hydrogenation of challenging olefins.

A leap forward in iridium-NHC catalysis: new horizons and mechanistic insights

This review summarises the most recent advances in Ir-NHC catalysis while revisiting all the classical reactions in which this type of catalyst has proved to be active. The influence of the ligand system and, in particular, the impact of the NHC ligand on the activity and selectivity of the reaction have been analysed, accompanied by an examination of the great variety of catalytic cycles hitherto reported. The reaction mechanisms so far proposed are described and commented on for each individual process. Moreover, some general considerations that attempt to explain the influence of the NHC from a mechanistic viewpoint are presented at the end of the review. The first sections are dedicated to the most widely explored reactions that use Ir-NHCs, i.e., hydrogenation and transfer hydrogenation, for which a general overview that tries to compile all the Ir-NHC catalysts hitherto reported for these processes is provided. The next sections deal with hydrogen borrowing, hydrosilylation, water splitting, dehydrogenation (of alcohols, alkanes, aminoboranes and formic acid), hydrogen isotope exchange (HIE), signal amplification by reversible exchange and C-H bond functionalisation (silylation and borylation). The last section compiles a series of reactions somewhat less explored for Ir-NHC catalysts that include the hydroalkynylation of imines, hydroamination, diboration of olefins, hydrolysis and methanolysis of silanes, arylation of aldehydes with boronic acids, addition of aroyl chlorides to alkynes, visible light driven reactions, isomerisation of alkenes, asymmetric intramolecular allylic amination and reactions that employ heterometallic catalysts containing at least one Ir-NHC unit.

Synthesis and reactions of C-phosphanylated thiazol-2-thiones

Regioselective synthesis of the P(iii) substituted thiazole-2-thione I is presented. Oxidation of I resulted in P(V) chalcogenide thiazole-2-thiones (E = O, S, Se). Desulfurization of I (E = O) using hydrogenperoxide led to the first C-phosphanoyl thiazolium salt II. Deprotonation of II and reaction with cyclooctadiene rhodium(i) chloride dimer yielded thiazole-2-ylidene rhodium(i) complex III.

Comparison Of Asymmetric Hydrogenations Of Unsaturated- Carboxylic Acids And -Esters

As methodology development matures it can be difficult to discern the most effective ways of performing certain transformations from the rest. This review summarizes the most important contributions leading to asymmetric hydrogenations of simple unsaturated-acid and ester substrates, with the objective of highlighting at least the best types of catalysts for each. Achievements in the area are described and these reveal situations where further efforts should be worthwhile, and ones where more research is only likely to give diminishing returns. In general, our conclusions are that the most useful types of catalysts for unsaturated-acids and -esters tend to be somewhat different, simple substrates have been studied extensively, and the field is poised to address more complex reactions. These could be ones involving alternative, particularly cyclic, structures, chemoselectivity issues, and more complex substrate stereochemistries.

Filling gaps in asymmetric hydrogenation methods for acyclic stereocontrol: application to chirons for polyketide-derived natural products

The large volume of research studying hydrogenation catalysis might suggest that stereoselective hydrogenation of alkenes is a solved problem, but we believe the most important parts of asymmetric hydrogenation methodology remain unmastered. The most popular chiral catalysts, Rh- and Ir-diphosphine complexes, do not hydrogenate the largest categories of prochiral alkenes, hindered tri- and tetra-substituted ones, at useful rates unless the substrate has a "classical" coordinating functional group (CFG), for example, amides or homoallylic alcohols, to anchor the substrate to the metal. Therefore, while many methods are available for the asymmetric hydrogenation of alkenes with appropriate CFGs, synthetic chemistry would benefit from chiral hydrogenations of substrates with functional groups that typically do not coordinate in Rh- and Ir-diphosphine complexes. In this Account, we demonstrate the application of chiral analogues of Crabtree's catalyst to asymmetric hydrogenations of coordinating unfunctionalized, trisubstituted alkenes. Crabtree's catalyst, a complex of iridium with 1,5-cyclooctadiene, tris-cyclohexylphosphine, and pyridine, differs from Rh- and Ir-diphosphine complexes, which we broadly refer to as "chiral analogues of Wilkinson's catalyst." Crabtree's catalyst analogues hydrogenate substrates that do not contain functionalities generally recognized as CFGs, and we propose reasons for this chemistry based on the catalytic mechanisms. Thus, chiral analogues of Crabtree's catalyst facilitate many hydrogenations that would not be possible using Rh- or Ir-diphosphine complexes. Directed hydrogenations have been used in acyclic stereocontrol for decades, but the realization that these catalysts can be used for acyclic stereocontrol without the types of directing groups that are necessary for other hydrogenations significantly broadens the scope of hydrogenations for this purpose. Recently, we have prepared chirons for polyketide-derived natural products using an N,carbene-Ir complex (1). This approach has led to catalytic syntheses of several important chirons to facilitate preparations of these ubiquitous natural products.

Phosphine-free chiral metal catalysts for highly effective asymmetric catalytic hydrogenation

In addition to the brilliant chiral phosphorus metal catalysts, chiral phosphine-free metal complexes find increasing application as catalysts for asymmetric hydrogenation. In this account, two types of chiral phosphine-free ligands, N-heterocyclic carbene-based C,N-ligands and diamine-based N,N-ligands, in the homogeneous asymmetric hydrogenation of prochiral ketones, imines and quinolines are reviewed.