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

P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

Iridium-Catalyzed Asymmetric Hydrogenation of Unfunctionalized Cycloalkenes to Access Chiral 2-Aryl Tetralins

The transition-metal catalyzed asymmetric hydrogenation of unfunctionalized alkenes is challenging. Herein, we report an efficient iridium-catalyzed asymmetric hydrogenation of unfunctionalized cycloalkenes, delivering chiral 2-aryl tetralins in excellent yields and with moderate to excellent enantioselectivities. The reaction can be performed on a gram-scale with a low catalyst loading (S/C = 1000), and the reduced product was obtained without erosion of the enantioselectivity. Deuterium experiments indicated that the C═C bond in the substrate is hydrogenated directly without isomerization.

Unlocking the Asymmetric Hydrogenation of Tetrasubstituted Acyclic Enones

Asymmetric hydrogenation (AH) of tetrasubstituted olefins generating two stereocenters is still an open topic. There are only a few reports on the AH of tetrasubstituted olefins with conjugated functional groups, while this process can create useful intermediates for the subsequent elaboration of relevant end products. Most of the tetrasubstituted olefins successfully submitted to AH belong to a small number of functional classes; remarkably, the AH of tetrasubstituted acyclic enones still represents an unsolved challenge. Herein, we disclose a class of air-stable Ir-P,N catalysts, prepared in three steps from commercially available amino alcohols, that can hydrogenate, in minutes, a wide range of electronically and sterically diverse acyclic tetrasubstituted enones (including exocyclic ones) with high yields and high enantioselectivities. The factors responsible for the excellent selectivities were elucidated by combining deuterogenation experiments and theoretical calculations. The calculations indicated that the reduction follows an IrI /IrIII mechanism, in which enantioselectivity is controlled in the first migratory insertion of the hydride to the most electrophilic olefinic Cβ and the formation of the hydrogenated product via reductive elimination takes place prior to the coordination of dihydrogen and the subsequent oxidative addition. The potential of the new catalytic systems is demonstrated by the derivatization of hydrogenation products.

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.

A Powerful P-N Connection: Preparative Approaches, Reactivity, and Applications of P-Stereogenic Aminophosphines

For more than five decades, P-stereogenic aminophosphine chalcogenides and boranes have attracted scientific attention and are still in the focus of ongoing research. In the last years, novel transition metal-based synthesis methods have been discovered, in addition to the long-known use of chiral auxiliaries. Enantiomerically pure compounds with N-P+ -X- (X=O, S, BH3 ) motifs served as valuable reactive building blocks to provide new classes of organophosphorus derivatives, thereby preserving the stereochemical information at the phosphorus atom. Over the years, intriguing applications in organocatalysis and transition metal catalysis have been reported for some representatives. Asymmetric reductions of C=C, C=N, and C=O double bonds were feasible with selected P-stereogenic aminophosphine oxides in the presence of hydrogen transfer reagents. P-stereogenic aminophosphine boranes could be easily deprotected and used as ligands for various transition metals to enable catalytic asymmetric hydrogenations of olefins and imines. This review traces the emergence of a synthetically and catalytically powerful functional compound class with phosphorus-centered chirality in its main lines, starting from classical approaches to modern synthesis methods to current applications.

Copper-Catalyzed Chemoselective Asymmetric Hydrogenation of C=O Bonds of Exocyclic α,β-Unsaturated Pentanones

A highly chemoselective earth-abundant transition metal copper catalyzed asymmetric hydrogenation of C=O bonds of exocyclic α,β-unsaturated pentanones was realized using H2 . The desired products were obtained with up to 99 % yield and 96 % ee (enantiomeric excess) (99 % ee, after recrystallization). The corresponding chiral exocyclic allylic pentanol products can be converted into several bioactive molecules. The hydrogenation mechanism was investigated via deuterium-labelling experiments and control experiments, which indicate that the keto-enol isomerization rate of the substrate is faster than that of the hydrogenation and also show that the Cu-H complex can only catalyze chemoselectively the asymmetric reduction of the carbonyl group. Computational results indicate that the multiple attractive dispersion interactions (MADI effect) between the catalyst with bulky substituents and substrate play important roles which stabilize the transition states and reduce the generation of by-products.