Singly Hydrogen Bonded Supramolecular Ligands for Highly Selective Rhodium-Catalyzed Hydrogenation Reactions

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Rational Optimization of Supramolecular Catalysts for the Rhodium-Catalyzed Asymmetric Hydrogenation Reaction

Rational design of catalysts for asymmetric transformations is a longstanding challenge in the field of catalysis. In the current contribution we report a catalyst in which a hydrogen bond between the substrate and the catalyst plays a crucial role in determining the selectivity and the rate of the catalytic hydrogenation reaction, as is evident from a combination of experiments and DFT calculations. Detailed insight allowed in silico mutation of the catalyst such that only this hydrogen bond interaction is stronger, predicting that the new catalyst is faster. Indeed, we experimentally confirmed that optimization of the catalyst can be realized by increasing the hydrogen bond strength of this interaction by going from a urea to phosphine oxide H-bond acceptor on the ligand.

Riding the Wave of Monodentate Ligand Revival: From the A/B Concept to Noncovalent Interactions

The rediscovery of chiral monodentate ligands made in the period 1999-2003 had important consequences in enantioselective transition-metal catalysis, such as the introduction of the A/B concept (i.e., use of monodentate ligand mixtures) and, later, a renewed interest in supramolecular ligands capable of ligand-ligand and ligand-substrate interactions. This Personal Account summarizes the contributions made by our research group in this area in the period 2004-2015, which reflect the abovementioned developments. Within this area, we introduced some original concepts, such as 1) the use of chiral tropos ligand mixtures; 2) the development of new strategies to maximize heterocomplex formation from combinations of simple monodentate ligands; 3) the investigation of new ligand-ligand interactions to achieve selective heterocomplex formation; and 4) the development of highly efficient and synthetically accessible supramolecular ligands.

Combinatorial approach to chiral tris-ligated carbophilic platinum complexes: application to asymmetric catalysis

A straightforward methodology for the synthesis of libraries of chiral tris-ligated cationic platinum complexes and their in situ evaluation as asymmetric carbophilic catalysts in a model domino hydroarylation/cyclization reaction of a 1,6-enyne was developed. A catalyst-generation process based on a combination of a monodentate and a bidentate phosphorus ligand allowed the formation of 108 chiral complexes. One-pot screening of the stereoinduction obtained with this library in a test domino addition/cyclization reaction validated this approach and stressed the key role played by the monodentate ligand partner in obtaining high enantioselectivities. In the case of two challenging substrate/nucleophile combinations, the combinatorial approach resulted in a significant gain in enantioselectivity.

Dinuclear Ru-aqua complexes for selective epoxidation catalysis based on supramolecular substrate orientation effects

Ru-aqua complex {[Ru(II)(trpy)(H2O)]2(μ-pyr-dc)}(+) is a powerful epoxidation catalyst for a wide range of linear and cyclic alkenes. High turnover numbers (TNs), up to 17000, and turnover frequencies (TOF), up to 24120 h(-1) (6.7 s(-1)), have been obtained using PhIO as oxidant. This species presents an outstanding stereospecificity for both cis and trans olefins towards the formation of their corresponding cis and trans epoxides. In addition, it shows different reactivity to cis and trans olefins due to a substrate orientation supramolecular effect transmitted by its ligand scaffold. This effect together with the impressive reaction rates are rationalized using electrochemical techniques and DFT calculations.

Hydrogen-bond-assisted activation of allylic alcohols for palladium-catalyzed coupling reactions

We report direct activation of allylic alcohols using a hydrogen-bond-assisted palladium catalyst and use this for alkylation and amination reactions. The novel catalyst comprises a palladium complex based on a functionalized monodentate phosphoramidite ligand in combination with urea additives and affords linear alkylated and aminated allylic products selectively. Detailed kinetic analysis show that oxidative addition of the allyl alcohol is the rate-determining step, which is facilitated by hydrogen bonds between the alcohol, the ligand functional group, and the additional urea additive.

Structures and interligand interaction pattern of phosphoramidite Pd complexes by NMR spectroscopy: modulations in extended interaction surfaces as stereoselection mode of a privileged class of ligands

During the last decade, phosphoramidites have been established as a so-called privileged class of ligands in various transition metal catalyses. However, the interactions responsible for their favorable properties have hitherto remained elusive. To address this issue, the formation trends, structural features, and interligand interaction patterns of several trans- and cis-[PdLL'Cl2] complexes have been investigated by NMR spectroscopy. The energetic contribution of their interligand interactions has been measured experimentally using the supramolecular balance for transition-metal complexes. The resulting energetics combined with an analysis of the electrostatic potential surfaces reveal that in phosphoramidites not only the aryl groups but the complete (CH)CH3 Ph moieties of the amine side chains form extended quasi-planar CH-π and π-π interaction surfaces. Application of the supramolecular balance has shown that modulations in these extended interaction surfaces cause energetic differences that are relevant to enantioselective catalysis. In addition, the energetics of these interligand interactions are quite independent of the actual structures of the complexes. This is shown by similar formation and aggregation trends of complexes with the same ligand but different structures. The extended quasi-planar electrostatic interaction surface of the (CH)CH3 Ph moiety explains the known pattern of successful ligand modulation and the substrate specificity of phosphoramidites. Thus, we propose modulations in these extended CH-π and π-π interaction areas as a refined stereoselection mode for these ligands. Based on the example of phosphoramidites, this study reveals three general features potentially applicable to various ligands in asymmetric catalysis. First, specific combinations of alkyl and aryl moieties can be used to create extended anisotropic interaction areas. Second, modulations in these interaction surfaces cause energetic differences that are relevant to catalytic applications. Third, bulky substituents with matching complementary interaction surfaces should not only be considered in terms of steric hindrance but also in terms of attractive and repulsive interactions, a feature that may often be underestimated in asymmetric catalysis.

The N-H functional group in organometallic catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst-substrate, ligand-ligand, ligand-catalyst, and catalyst-catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.

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.

The coordination chemistry of the proton

It is well known that an acidic hydrogen atom can form hydrogen bonds to a hydrogen bond acceptor, a Lewis base. It is considerably less known that the proton can coordinate two or more atoms conveniently in bonding modes that cannot be described as hydrogen bonding. Agostic interactions, bridging hydrides, 3-centre-2-electron bonds in boranes, bifurcated hydrogen atoms, they are all elements of the coordination chemistry of the proton and, of course, the hydrogen bond comes in more than one facette as well.

A combinatorial approach to the identification of self-assembled ligands for rhodium-catalysed asymmetric hydrogenation

An effective and efficient means to catalyst discovery is the high-throughput screening of catalyst libraries. However, the current status of this approach suffers from a number of limitations, namely access to structurally diverse and meaningful ligand libraries and the enormous effort required for massive parallel screening of the resulting catalysts. We report an integrated solution to these drawbacks, which combines a diversity-oriented ligand synthesis, a catalyst-generation process driven by self-assembly and, finally, a combinatorial iterative library deconvolution strategy to identify the optimal catalyst. As a test case, rhodium-catalysed asymmetric hydrogenation was studied and, from a library of 120 self-assembling catalysts, highly enantioselective catalysts for the asymmetric hydrogenation of different olefinic substrates were identified within 17 experiments. Comparison of the results of the iterative library deconvolution strategy with those of the classic parallel-screening process confirmed the validity of this approach.