The Effect of Conformational Freedom vs Restriction on the Rate in Asymmetric Hydrogenation: Iridium-Catalyzed Regio- and Enantioselective Monohydrogenation of Dienones

Transition metal-catalyzed asymmetric hydrogenation constitutes an efficient strategy for the preparation of chiral molecules. When dienes are subjected to hydrogenation, control over regioselectivity still presents a large challenge and the fully saturated alkane is often yielded. A few successful monohydrogenations of dienes have been reported, but hitherto these are only efficient for dienes comprised of two distinctly different olefins. Herein, the reactivity of a conjugated carbonyl compound as a function of their conformational freedom is studied, based on a combined experimental and theoretical approach. It was found that alkenes in the (s)-cis conformation experience a large rate acceleration while (s)-trans restrained alkenes undergo hydrogenation slowly. Ultimately, this reactivity aspect was exploited in a novel method for the monohydrogenation of dienes based on conformational restriction ((s)-cis vs (s)-trans). This mode of discrimination conceptually differs from existing monohydrogenations and dienones constructed of two olefins similar in nature could efficiently be hydrogenated to the chiral alkene (up to 99 % ee). The extent of regioselection is even powerful enough to overcome the conventional reactivity order of substituted olefins (di>tri>tetra). This high yielding and atom-economical protocol provides an interesting opportunity to instal a stereogenic center on a carbocycle, while leaving a synthetically useful alkene untouched.

P-Stereogenic Ir-MaxPHOX: A Step toward Privileged Catalysts for Asymmetric Hydrogenation of Nonchelating Olefins

The Ir-MaxPHOX-type catalysts demonstrated high catalytic performance in the hydrogenation of a wide range of nonchelating olefins with different geometries, substitution patterns, and degrees of functionalization. These air-stable and readily available catalysts have been successfully applied in the asymmetric hydrogenation of di-, tri-, and tetrasubstituted olefins (ee's up to 99%). The combination of theoretical calculations and deuterium labeling experiments led to the uncovering of the factors responsible for the enantioselectivity observed in the reaction, allowing the rationalization of the most suitable substrates for these Ir-catalysts.

Rh-Catalyzed Asymmetric Hydrogenation of α,β- and β,β-Disubstituted Unsaturated Boronate Esters

A highly enantioselective hydrogenation of α,β-unsaturated boronate esters catalyzed by Rh-(S)-DTBM-Segphos complex has been developed. Both (Z)-α,β- and β,β-disubstituted substrates can be successfully hydrogenated to afford chiral boronates with excellent enantioselectivities, up to 98 % ee. Furthermore, the obtained chiral boronate esters, as important versatile synthetic intermediates are successfully transformed to the corresponding chiral alcohols, amines and other important derivatives with maintained enantioselectivities.

Catalytic Reductive Aldol and Mannich Reactions of Enone, Acrylate, and Vinyl Heteroaromatic Pronucleophiles

Catalytic reductive coupling of enone, acrylate, or vinyl heteroaromatic pronucleophiles with carbonyl or imine partners offers an alternative to base-mediated enolization in aldol- and Mannich-type reactions. In this review, direct catalytic reductive aldol and Mannich reactions are exhaustively catalogued on the basis of metal or organocatalyst. Stepwise processes involving enone conjugate reduction to form discrete enol or (metallo)enolate derivatives followed by introduction of carbonyl or imine electrophiles and aldol reactions initiated via enone conjugate addition are not covered.

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.

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.

Giving a Second Chance to Ir/Sulfoximine-Based Catalysts for the Asymmetric Hydrogenation of Olefins Containing Poorly Coordinative Groups

This work identifies a family of Ir/phosphite-sulfoximine catalysts that has been successfully used in the asymmetric hydrogenation of olefins with poorly coordinative or noncoordinative groups. In comparison with analogue Ir/phosphine-sulfoximine catalysts previously reported, the presence of a phosphite group extended the range of olefins than can be efficiently hydrogenated. High enantioselectivities, comparable to the best ones reported, have been achieved for a wide range of olefins containing relevant poorly coordinative groups such as α,β-unsaturated enones, esters, lactones, and lactams as well as alkenylboronic esters.

Design, complexing and catalytic properties of phosphorus thiazoles and benzothiazoles: a concise overview

Playing with the versatile reactivity of thiazole and phosphorus derivatives leads to active ligands and complexes in catalysis.

Cobalt-Catalyzed Asymmetric Sequential Hydroboration/Hydrogenation of Internal Alkynes

A highly regio- and enantioselective cobalt-catalyzed hydroboration/hydrogenation of internal alkynes with HBpin and a hydrogen balloon in one pot was developed. A new type of chiral imidazoline iminopyridine (IIP) ligand was introduced for the first time in this novel and efficient strategy. This protocol used relatively simple and available starting materials with good functional group tolerance to construct more valuable chiral secondary organoboronates. The primary mechanistic studies illustrated that the cobalt-catalyzed regioselective hydroboration of alkynes did initially occur followed by HBpin-promoted and cobalt-catalyzed enantioselective hydrogenation of alkenylboronates.

A Ligand-Free Pt3 Cluster Catalyzes the Markovnikov Hydrosilylation of Alkynes with up to 106 Turnover Frequencies

The Pt-catalyzed hydrosilylation of alkynes is the procedure of choice to obtain vinylsilanes, and is claimed to be the most relevant application of Pt in organic synthesis. More than half a century after its discovery, only β-vinylsilanes (anti-Markovnikov addition) are obtained with simple Pt catalysts, whereas α-vinylsilanes (Markovnikov addition) remain elusive compounds. Here the catalysis of the Markovnikov hydrosilylation of terminal alkynes by Pt₃ clusters, in parts-per-million amounts, to give a wide variety of α-vinylsilanes in reasonable isolated yields and with turnover frequencies that can reach up to one million per hour is reported. Moreover, these α-vinylsilanes are reactive in well-stablished C-C bond-forming cascade reactions, in which the corresponding β-isomers are unreactive. Besides its efficiency and synthetic usefulness, this catalytic system is an excellent example of how the atom-by-atom aggregation of a catalytic metal leads to a different selectivity for a given reaction.

Adaptable P-X Biaryl Phosphite/Phosphoroamidite-Containing Ligands for Asymmetric Hydrogenation and C-X Bond-Forming Reactions: Ligand Libraries with Exceptionally Wide Substrate Scope

In this personal review, we present our efforts in the design of ligand libraries for the discovery of suitable metal catalysts for asymmetric hydrogenation and C-X bond-forming reactions.