Regioselective Iridium-Catalyzed Asymmetric Monohydrogenation of 1,4-Dienes

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.

Synthesis of Chiral Tetrahydro-3-benzazepine Motifs by Iridium-Catalyzed Asymmetric Hydrogenation of Cyclic Ene-carbamates

A highly efficient N,P-ligated iridium complex is presented for the simple preparation of chiral tetrahydro-3-benzazepine motifs by catalytic asymmetric hydrogenation. Substrates bearing both 1-aryl and 1-alkyl substituents were smoothly converted to the corresponding hydrogenated product with excellent enantioselectivity (91-99% ee) and in isolated yield (92-99%). The synthetic value of this transformation was demonstrated by a gram-scale hydrogenation and application in the syntheses of trepipam and fenoldopam.

Scalable Birch reduction with lithium and ethylenediamine in tetrahydrofuran

[Figure: see text].

Site- and Enantioselective Iridium-Catalyzed Desymmetric Mono-Hydrogenation of 1,4-Dienes

The control of site selectivity in asymmetric mono-hydrogenation of dienes or polyenes remains largely underdeveloped. Herein, we present a highly efficient desymmetrization of 1,4-dienes via iridium-catalyzed site- and enantioselective hydrogenation. This methodology demonstrates the first iridium-catalyzed hydrogenative desymmetriation of meso dienes and provides a concise approach to the installation of two vicinal stereogenic centers adjacent to an alkene. High isolated yields (up to 96 %) and excellent diastereo- and enantioselectivities (up to 99:1 d.r. and 99 % ee) were obtained for a series of divinyl carbinol and divinyl carbinamide substrates. DFT calculations reveal that an interaction between the hydroxy oxygen and the reacting hydride is responsible for the stereoselectivity of the desymmetrization of the divinyl carbinol. Based on the calculated energy profiles, a model that simulates product distribution over time was applied to show an intuitive kinetics of this process. The usefulness of the methodology was demonstrated by the synthesis of the key intermediates of natural products zaragozic acid A and (+)-invictolide.

Design, scope and mechanism of highly active and selective chiral NHC-iridium catalysts for the intramolecular hydroamination of a variety of unactivated aminoalkenes

Chiral, cationic NHC-iridium complexes are introduced as catalysts for the intramolecular hydroamination reaction of unactivated aminoalkenes. The catalysts show high activity in the construction of a range of 5- and 6-membered N-heterocycles, which are accessed in excellent optical purity, with various functional groups being tolerated with this system. A major deactivation pathway is presented and eliminated by using alternative reaction conditions. A detailed experimental and computational study on the reaction mechanism is performed providing valuable insights into the mode of action of the catalytic system and pointing to future modifications to be made for this catalytic platform.

Kinetic resolution of racemic allylic alcohols via iridium-catalyzed asymmetric hydrogenation: scope, synthetic applications and insight into the origin of selectivity

Asymmetric hydrogenation is one of the most commonly used tools in organic synthesis, whereas, kinetic resolution via asymmetric hydrogenation is less developed. Herein, we describe the first iridium catalyzed kinetic resolution of a wide range of trisubstituted secondary and tertiary allylic alcohols. Large selectivity factors were observed in most cases (s up to 211), providing the unreacted starting materials in good yield with high levels of enantiopurity (ee up to >99%). The utility of this method is highlighted in the enantioselective formal synthesis of some bioactive natural products including pumiliotoxin A, inthomycin A and B. DFT studies and a selectivity model concerning the origin of selectivity are presented.

Asymmetric hydrogenation is one of the most commonly used tools in organic synthesis, whereas, kinetic resolution via asymmetric hydrogenation was less developed.

Enantioselective Synthesis of cis-Decalin Derivatives by the Inverse-Electron-Demand Diels-Alder Reaction of 2-Pyrones

A novel strategy for the synthesis of cis-decalins by an ytterbium-catalyzed asymmetric inverse-electron-demand Diels-Alder reaction of 2-pyrones and silyl cyclohexadienol ethers is reported here. A broad range of synthetically important cis-decalin derivatives with multiple contiguous stereogenic centers and functionalities are obtained in good yields and stereoselectivities. A full set of diastereomeric substituted cis-decalin motifs are readily accessible by tuning the absolute configurations of substituted silyl cyclohexadienol ethers (R or S) as well as the ligands (R or S). The synthetic potential is showcased by the enantioselective total synthesis of 4-amorphen-11-ol, and further demonstrated by the first total synthesis of cis-crotonin.

Birch-Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer

The direct reduction of arenes and heteroarenes by visible-light irradiation remains challenging, as the energy of a single photon is not sufficient for breaking aromatic stabilization. Shown herein is that the energy accumulation of two visible-light photons allows the dearomatization of arenes and heteroarenes. Mechanistic investigations confirm that the combination of energy-transfer and electron-transfer processes generates an arene radical anion, which is subsequently trapped by hydrogen-atom transfer and finally protonated to form the dearomatized product. The photoreduction converts planar aromatic feedstock compounds into molecular skeletons that are of use in organic synthesis.

Ir-Catalyzed Double Asymmetric Hydrogenation of 3,6-Dialkylidene-2,5-diketopiperazines for Enantioselective Synthesis of Cyclic Dipeptides

An Ir/spiro[4,4]-1,6-nonadiene-based phosphine-oxazoline ligand (SpinPHOX) complex-catalyzed double asymmetric hydrogenation of 3,6-dialkylidene-1,4-dimethylpiperazine-2,5-diones has been developed, providing efficient and practical access to a wide variety of chiral 3,6-disubstituted-2,5-diketopiperazines in high yields with exclusive cis-diastereo- and excellent enantioselectivities (>99% de, up to 98% ee). The synthetic utilities of the protocol have been demonstrated in a gram scale synthesis of 6a and efficient construction of chiral products 8, 14, and 17 as well as a 2-butenyl-bridged bicyclic diketopiperazine 10 and hydroxydiketopiperazine 11. With an analogous achiral Ir catalyst, the hydrogenation of enantiopure monohydrogenated intermediate 7a gave cis-6a as the only product, indicating that the second-step hydrogenation of the titled transformation is a chiral substrate controlled process. The reaction profile study for asymmetric hydrogenation (AH) of 5a revealed that the concentration of the monohydrogenation intermediate 7a remained at a low level (<8%) during the course of hydrogenation. The hydrogenation of 5a to 6a proceeded significantly faster than that of its half-hydrogenated intermediate ( S)-7a, indicating that the titled reaction involves primarily a processive mechanism, in which a single catalyst molecule performs consecutive hydrogenation of the two C═C double bonds in substrate 5a without dissociation of the partially reduced 7a. The present protocol represents a rare example of asymmetric catalytic consecutive hydrogenation of heterocycles and provides an alternative way for efficient construction of cyclic dipeptides.

Tandem Peterson olefination and chemoselective asymmetric hydrogenation of β-hydroxy silanes

Here, we report the first Ir-N,P complex catalyzed tandem Peterson olefination and asymmetric hydrogenation of β-hydroxy silanes. This reaction resulted in the formation of chiral alkanes in high isolated yields (up to 99%) and excellent enantioselectivity (up to 99% ee) under mild conditions. Modification of the reaction conditions provides a choice to transform either an olefin or the β-hydroxy silane in a chemoselective manner. Additionally, based on this method, an expedient enantioselective synthesis of (S)-(+)-α-curcumene, from a simple ketone, was accomplished in two steps with 75% overall yield and 95% ee.

Scalable and safe synthetic organic electroreduction inspired by Li-ion battery chemistry

Reductive electrosynthesis has faced long-standing challenges in applications to complex organic substrates at scale. Here, we show how decades of research in lithium-ion battery materials, electrolytes, and additives can serve as an inspiration for achieving practically scalable reductive electrosynthetic conditions for the Birch reduction. Specifically, we demonstrate that using a sacrificial anode material (magnesium or aluminum), combined with a cheap, nontoxic, and water-soluble proton source (dimethylurea), and an overcharge protectant inspired by battery technology [tris(pyrrolidino)phosphoramide] can allow for multigram-scale synthesis of pharmaceutically relevant building blocks. We show how these conditions have a very high level of functional-group tolerance relative to classical electrochemical and chemical dissolving-metal reductions. Finally, we demonstrate that the same electrochemical conditions can be applied to other dissolving metal-type reductive transformations, including McMurry couplings, reductive ketone deoxygenations, and epoxide openings.

Ir-Catalyzed Asymmetric and Regioselective Hydrogenation of Cyclic Allylsilanes and Generation of Quaternary Stereocenters via the Hosomi-Sakurai Allylation

A number of cyclic dienes containing the allylsilane moiety were prepared by a Birch reduction and subjected to iridium-catalyzed regioselective and asymmetric hydrogenation, which provided chiral allylsilanes in high conversion and enantiomeric excess (up to 99 % ee). The compounds were successively used in the Hosomi-Sakurai allylation with various aldehydes employing TiCl₄ as Lewis acid, providing adducts with two additional stereogenic centers in excellent diastereoselectivity.