Catalytic enantioselective C-H functionalization of alcohols by redox-triggered carbonyl addition: borrowing hydrogen, returning carbon

Catalytic Enantioselective C-C Coupling of Alcohols for Polyketide Total Synthesis beyond Chiral Auxiliaries and Premetalated Reagents

Catalytic enantioselective hydrogen autotransfer reactions for the direct conversion of lower alcohols to higher alcohols are catalogued and their application to the total synthesis of polyketide natural products is described. These methods exploit a redox process in which alcohol oxidation is balanced by reductive generation of organometallic nucleophiles from unsaturated hydrocarbon pronucleophiles. Unlike classical carbonyl additions, premetalated reagents, chiral auxiliaries and discrete alcohol-to-aldehyde redox reactions are not required. Additionally, chemoselective dehydrogenation of primary alcohols in the presence of secondary alcohols enables C-C coupling in the absence of protecting groups.

Asymmetric Allylic Amination of Alkyl-Substituted Allylic Carbonates with Pyridones Catalyzed by the Krische Iridium Complex

An efficient Ir-catalyzed asymmetric allylic amination reaction of alkyl-substituted allylic carbonates is disclosed. With the Krische iridium complex as the catalyst, asymmetric allylic amination of alkyl-substituted allylic carbonates with pyridones proceeds effectively, affording pyridone derivatives containing a stereocenter α to the nitrogen atom in excellent yields and enantioselectivity (up to 99% yield, 95% ee). This catalytic system broadens the substrate scope of the reaction compared with that of the known catalytic systems. This reaction can also be conducted on a gram scale, further enhancing its potential for synthetic application.

Manganese catalyzed chemo-selective synthesis of acyl cyclopentenes: a combined experimental and computational investigation

Cyclopentenes serve as foundational structures in numerous natural products and pharmaceuticals. Consequently, the pursuit of innovative synthetic approaches to complement existing protocols is of paramount importance. In this context, we present a novel synthesis route for acyl cyclopentenes through a cascade reaction involving an acceptorless-dehydrogenative coupling of cyclopropyl methanol with methyl ketone, followed by a radical-initiated ring expansion rearrangement of the in situ formed vinyl cyclopropenone intermediate. The reaction, catalyzed by an earth-abundant metal complex, occurs under milder conditions, generating water and hydrogen gas as byproducts. Rigorous control experiments and detailed computational studies were conducted to unravel the underlying mechanism. The observed selectivity is explained by entropy-driven alcohol-assisted hydrogen liberation from an Mn-hydride complex, prevailing over the hydrogenation of unsaturated cyclopentenes.

Pd(0)-Catalyzed Asymmetric Intramolecular Grignard-Type Reaction of Vinyl Iodide-Carbonyl

The insertion of carbonyl into C(sp2)-Pd(II) σ-bond (Grignard-type addition) was not established until the 1990s. While this elemental reaction has been well explored since then, its application in Pd(0) asymmetric catalysis remain elusive. Herein, we report the Pd(0)-catalyzed asymmetric intramolecular Grignard-type reaction of vinyl iodide-carbonyl in the presence of HCO2H additive, affording cyclic allylic alcohol with good to excellent enantioselectivity and diastereoselectivity. Mechanistic studies suggested that besides serving as an efficient reductant, HCO2H is also capable of facilitating protonation of the involved secondary alkoxyl-Pd(II), thus completely suppressing the β-H elimination. Moreover, no KIE was found in the competing reaction between vinyl iodide-aldehyde and 1-deuterated one, demonstrating the facile step of aldehyde insertion.

Leveraging the Stereochemical Complexity of Octahedral Diastereomeric-at-Metal Catalysts to Unlock Regio-, Diastereo-, and Enantioselectivity in Alcohol-Mediated C-C Couplings via Hydrogen Transfer

Experimental and computational studies illuminating the factors that guide metal-centered stereogenicity and, therefrom, selectivity in transfer hydrogenative carbonyl additions of alcohol proelectrophiles catalyzed by chiral-at-metal-and-ligand octahedral d6 metal ions, iridium(III) and ruthenium(II), are described. To augment or invert regio-, diastereo-, and enantioselectivity, predominantly one from among as many as 15 diastereomeric-at-metal complexes is required. For iridium(III) catalysts, cyclometalation assists in defining the metal stereocenter, and for ruthenium(II) catalysts, iodide counterions play a key role. Whereas classical strategies to promote selectivity in metal catalysis aim for high-symmetry transition states, well-defined low-symmetry transition states can unlock selectivities that are otherwise difficult to achieve or inaccessible.

Metal-free β,γ-C(sp3)-H difunctionalization of propanols: DMP-initiated asymmetric spirocyclopropanation

A DMP-initiated metal-free effective β,γ-asymmetric spirocyclopropanation of propanols strategy using oxidative iminium activation is described. This process has been realized by a synergistic amine-catalyzed one-pot cascade oxidation-Michael addition cyclopropanation for "one-pot" access to various spirocyclopropyl propionaldehydes/propanols from diverse 3-arylpropanols and α-brominated active methylene compounds under mild conditions and with high enantioselectivity (ee up to >99%).

Copper(I)-Catalyzed Asymmetric Allylation of Ketones with 2-Aza-1,4-Dienes

Catalytic asymmetric allylation of ketones under proton-transfer conditions is a challenging issue due to the limited pronucleophiles and the electrophilic inertness of ketones. Herein, a copper(I)-catalyzed asymmetric allylation of ketones with 2-aza-1,4-dienes (N-allyl-1,1-diphenylmethanimines) is disclosed, which affords a series of functionalized homoallyl tertiary alcohols in high to excellent enantioselectivity. Interestingly, N-allyl-1,1-diphenylmethanimines work as synthetic equivalents of propanals. Upon the acidic workup, a formal asymmetric β-addition of propanals to ketones is achieved. An investigation on KIE effect indicates that the deprotonation of N-allyl-1,1-diphenylmethanimines is the rate-determining step, which generates nucleophilic allyl copper(I) species. Finally, the synthetic utility of the present method is demonstrated by the asymmetric synthesis of (R)-boivinianin A and (R)-gossonorol.

Total Synthesis of the Proposed Structure of Neaumycin B

The total synthesis of the proposed structure of anti-glioblastoma natural product neaumycin B was achieved in 22 steps (longest linear sequence). The synthesis features HCl-mediated [6,6]-spiroketalization, a combination of Krische iridium-catalyzed crotylation, Marshall palladium-catalyzed propargylation, Fürstner nickel-catalyzed regio- and enantioselective vicinal monoprotected diol formation, Brown crotylation and asymmetric halide-aldehyde cycloaddition, so as to establish the challenging contiguous stereocenters.

Borrowing Hydrogen β-Phosphinomethylation of Alcohols Using Methanol as C1 Source by Pincer Manganese Complex

Herein, we report a manganese-catalyzed three-component coupling of β-H containing alcohols, methanol, and phosphines for the synthesis of γ-hydroxy phosphines via a borrowing hydrogen strategy. In this development, methanol serves as a sustainable C1 source. A variety of aromatic and aliphatic substituted alcohols and phosphines could undergo the dehydrogenative cross-coupling process efficiently and deliver the corresponding β-phosphinomethylated alcohol products in moderate to good yields. Mechanistic studies suggest that this transformation proceeds in a sequential manner including catalytic dehydrogenation, aldol condensation, Michael addition, and catalytic hydrogenation.

Dynamic Kinetic Resolution of 2-(Quinolin-8-yl)Benzaldehydes: Atroposelective Iridium-Catalyzed Transfer Hydrogenative Allylation

An atroposelective Ir-catalyzed dynamic kinetic resolution (DKR) of 2-(quinolin-8-yl)benzaldehydes/1-naphthaldehydes by transfer hydrogenative coupling of allyl acetate is disclosed. The allylation reaction takes place with simultaneous installation of central and axial chirality, reaching high diastereoselectivities and excellent enantiomeric excesses when ortho-cyclometalated iridium-DM-BINAP is used as the catalyst. The racemization of the substrates occurs through a designed transient Lewis acid-base interaction between the quinoline nitrogen atom and the aldehyde carbonyl group.

Enantioconvergent transformations of secondary alcohols through borrowing hydrogen catalysis

Direct substitution of readily available alcohols is recognized as a key research area in green chemical synthesis. Starting from simple racemic secondary alcohols, the achievement of catalytic enantioconvergent transformations of the substrates will be highly desirable for efficient access to valuable enantiopure compounds. To accomplish such attractive yet challenging transformations, the strategy of the enantioconvergent borrowing hydrogen methodology has proven to be uniquely effective and versatile. This review aims to provide an overview of the impressive progress made on this topic of research that has only thrived in the past decade. In particular, the conversion of racemic secondary alcohols to enantioenriched chiral amines, N-heterocycles, higher-order alcohols and ketones will be discussed in detail.

Enantioselective Transfer Hydrogenative Cycloaddition Unlocks the Total Synthesis of SF2446 B3: An Aglycone of Arenimycin and SF2446 Type II Polyketide Antibiotics

The first total synthesis and structure validation of an arenimycin/SF2446 type II polyketide is described, as represented by de novo construction of SF2446 B3, the aglycone shared by this family of type II polyketides. Ruthenium-catalyzed α-ketol-benzocyclobutenone [4 + 2] cycloaddition, which occurs via successive stereoablation-stereoregeneration, affects a double dynamic kinetic asymmetric transformation wherein two racemic starting materials combine to form the congested angucycline bay region with control of regio-, diastereo-, and enantioselectivity. This work represents the first application of transfer hydrogenative cycloaddition and enantioselective intermolecular metal-catalyzed C-C bond activation in target-oriented synthesis.

Ruthenium-Catalyzed C-C Coupling of Terminal Alkynes with Primary Alcohols or Aldehydes: α,β-Acetylenic Ketones (Ynones) via Oxidative Alkynylation

The first metal-catalyzed oxidative alkynylations of primary alcohols or aldehydes to form α,β-acetylenic ketones (ynones) are described. Deuterium labelling studies corroborate a novel reaction mechanism in which alkyne hydroruthenation forms a transient vinylruthenium complex that deprotonates the terminal alkyne to form the active alkynylruthenium nucleophile.

O-Acetyl 1,3-Propanediol as an Acrolein Proelectrophile in Enantioselective Iridium-Catalyzed Carbonyl Allylation

O-Acetyl 1,3-propanediol serves as an acrolein proelectrophile in π-allyliridium-C,O-benzoate-catalyzed carbonyl allylations mediated by racemic α-substituted allylic acetates. Using the iridium catalyst modified by (R)-SEGPHOS, a variety of 3-hydroxy-1,5-hexadienes are formed with uniformly high levels of regio-, anti-diastereo-, and enantioselectivity.

Three-component regioselective carboamidation of 1,3-enynes via rhodium(III)-catalyzed C-H activation

Rhodium-catalyzed regio- and stereoselective three-component carboamidation of 1,3-enynes has been realized using indoles and dioxazolones as the functionalizing reagents. A wide range of multi-substituted skipped 1,4-dienes have been constructed in good yields and excellent stereoselectivity. The stereoselectivity is under substrate control. 1,3-Enynes bearing a relatively bulky alkyne terminus reacted with Z-selectivity. In contrast, a sterically less hindered alkyne terminus tends to predominantly give the E-configured skipped diene.

Iridium-, Ruthenium-, and Nickel-Catalyzed C-C Couplings of Methanol, Formaldehyde, and Ethanol with π-Unsaturated Pronucleophiles via Hydrogen Transfer

In this Perspective, the use of methanol and ethanol as C1 and C2 feedstocks in metal-catalyzed C-C couplings to π-unsaturated pronucleophiles via hydrogen auto-transfer is surveyed. In these processes, alcohol oxidation to form an aldehyde electrophile is balanced by reduction of an π-unsaturated hydrocarbon to form a transient organometallic nucleophile. Mechanistically related reductive couplings of paraformaldehyde mediated by alcohol reductants or formic acid also are described. These processes encompass the first catalytic enantioselective C-C couplings of methanol and ethanol and, more broadly, illustrate how the native reducing ability of alcohols enable the departure from premetalated reagents in carbonyl addition.

Allyl Alcohol as an Acrolein Equivalent in Enantioselective C-C Coupling: Total Synthesis of Amphidinolides R, J, and S

The first systematic study of catalytic enantioselective 1,2-additions to acrolein is described. Specifically, using allyl alcohol as a tractable, inexpensive acrolein proelectrophile, iridium-catalyzed acrolein allylation is achieved with high levels of regio-, anti-diastereo-, and enantioselectivity. This process delivers 3-hydroxy-1,5-hexadienes, a useful compound class that is otherwise challenging to access via enantioselective catalysis. Two-fold use of this method unlocks concise total syntheses of amphidinolide R (9 vs 23 steps, LLS) and amphidinolide J (9 vs 23 or 26 steps, LLS), which are prepared in fewer than half the steps previously possible, and the first total synthesis of amphidinolide S (10 steps, LLS).

Iridium-Catalyzed Enantioconvergent Borrowing Hydrogen Annulation of Racemic 1,4-Diols with Amines

We present an enantioconvergent access to chiral N-heterocycles directly from simple racemic diols and primary amines, through a highly economical borrowing hydrogen annulation. The identification of a chiral amine-derived iridacycle catalyst was the key for achieving high efficiency and enantioselectivity in the one-step construction of two C-N bonds. This catalytic method enabled a rapid access to a wide range of diversely substituted enantioenriched pyrrolidines including key precursors to valuable drugs such as aticaprant and MSC 2530818.

Economical Access to Diverse Enantiopure Tetrahydropyridines and Piperidines Enabled by Catalytic Borrowing Hydrogen

We disclose herein a catalytic borrowing hydrogen method that enables an unprecedented, economical one-pot access to enantiopure tetrahydropyridines with minimal reagent use or waste formation. This method couples a few classes of readily available substrates with commercially available 1,3-amino alcohols, and delivers the valuable tetrahydropyridines of different substitution patterns free of N-protection. Such transformations are highly challenging to achieve, as multiple redox steps need to be realized in a cascade and numerous side reactions including a facile aromatization have to be overcome. Highly diastereoselective functionalizations of tetrahydropyridines also result in a general access to enantiopure di- and tri-substituted piperidines, which ranks the topmost frequent N-heterocycle in commercial drugs.

Ruthenium-catalysed α-prenylation of ketones using prenol

Prenol and isoprenoids are common structural motifs in biological systems and possess diverse applications. An unprecedented direct catalytic prenylation of ketones using prenol is attained. This C-C bond formation reaction requires only a ruthenium pincer catalyst and a base, and H₂O is the only byproduct.

Catalysis Driven Six-Step Synthesis of Apratoxin A Key Polyketide Fragment

Apratoxin A is a potent anticancer natural product whose key polyketide fragment constitutes a considerable challenge for organic synthesis, with five prior syntheses requiring 12 to 20 steps for its preparation. By combining different redox-economical catalytic stereoselective transformations, the key polyketide fragment could be rapidly prepared. Followed by a site-selective protection of the diol, this strategy enables the preparation of the apratoxin A fragment in only six steps, representing the shortest route to this polyketide.

Iron-Catalyzed α-Methylation of Ketones Using Methanol as the C1 Source under Photoirradiation

A mild, environmentally benign approach for α-methylation of ketones utilizing methanol as the C1 source under visible light has been developed. The reaction conditions were favorable for a wide range of ketones with both aromatic and aliphatic backbones, allowing for good-to-excellent yields of the respective products. The tentative mechanism is postulated after preliminary mechanistic and kinetic experiments.

Enantioselective Metal-Catalyzed Reductive Coupling of Alkynes with Carbonyl Compounds and Imines: Convergent Construction of Allylic Alcohols and Amines

The use of alkynes as vinylmetal pronucleophiles in intermolecular enantioselective metal-catalyzed carbonyl and imine reductive couplings to form allylic alcohols and amines is surveyed. Related hydrogen auto-transfer processes, wherein alcohols or amines serve dually as reductants and carbonyl or imine proelectrophiles, also are cataloged, as are applications in target-oriented synthesis. These processes represent an emerging alternative to the use of stoichiometric vinylmetal reagents or Nozaki-Hiyama-Kishi (NHK) reactions in carbonyl and imine alkenylation.

Total Synthesis of Bipolamine I

Indolizidine alkaloids have been the target of chemical and biological studies for decades, most recently highlighted by the isolation of the curvulamine and bipolamine polypyrrole-containing subclass. Herein we report a stereoselective 15-step synthesis of bipolamine I, a distinct member of the broader family, and through this work develop an intermediate that will serve to access other polypyrrole natural products and key analogues going forward.

Regio- and Enantioselective Iridium-Catalyzed Amination of Alkyl-Substituted Allylic Acetates with Secondary Amines

Robust air-stable cyclometalated π-allyliridium C,O-benzoates modified by (S)-tol-BINAP catalyze the reaction of secondary aliphatic amines with racemic alkyl-substituted allylic acetates to furnish products of allylic amination with high levels of enantioselectivity. Complete branched regioselectivities were observed despite the formation of more highly substituted C-N bonds.

Diastereoconvergent synthesis of anti-1,2-amino alcohols with N-containing quaternary stereocenters via selenium-catalyzed intermolecular C–H amination

We report a diastereoconvergent synthesis of anti-1,2-amino alcohols bearing N-containing quaternary stereocenters using an intermolecular direct C–H amination of homoallylic alcohol derivatives catalyzed by a phosphine selenide. Destruction of the allylic stereocenter during the selenium-catalyzed process allows selective formation of a single diastereomer of the product starting from any diastereomeric mixture of the starting homoallylic alcohol derivatives, eliminating the need for the often-challenging diastereoselective preparation of starting materials. Mechanistic studies show that the diastereoselectivity is controlled by a stereoelectronic effect (inside alkoxy effect) on the transition state of the final [2,3]-sigmatropic rearrangement, leading to the observed anti selectivity. The power of this protocol is further demonstrated on an extension to the synthesis of syn-1,4-amino alcohols from allylic alcohol derivatives, constituting a rare example of 1,4-stereoinduction.

We report a diastereoconvergent synthesis of anti-1,2-amino alcohols bearing N-containing quaternary stereocenters using an intermolecular direct C–H amination of homoallylic alcohol derivatives catalyzed by a phosphine selenide.

Understanding Halide Counterion Effects in Enantioselective Ruthenium-Catalyzed Carbonyl (α-Aryl)allylation: Alkynes as Latent Allenes and Trifluoroethanol-Enhanced Turnover in The Conversion of Ethanol to Higher Alcohols via Hydrogen Auto-transfer

Crystallographic characterization of RuX(CO)(η3-C3H5)(JOSIPHOS), where X = Cl, Br, or I, reveals a halide-dependent diastereomeric preference that defines metal-centered stereogenicity and, therefrom, the enantioselectivity of C-C coupling in ruthenium-catalyzed anti-diastereo- and enantioselective C-C couplings of primary alcohols with 1-aryl-1-propynes to form products of carbonyl anti-(α-aryl)allylation. Computational studies reveal that a non-classical hydrogen bond between iodide and the aldehyde formyl CH bond stabilizes the favored transition state for carbonyl addition. An improved catalytic system enabling previously unattainable transformations was developed that employs an iodide-containing precatalyst, RuI(CO)3(η3-C3H5), in combination with trifluoroethanol, as illustrated by the first enantioselective ruthenium-catalyzed C-C couplings of ethanol to form higher alcohols.

Iron-Catalyzed Alkyne-Based Multicomponent Synthesis of Pyrimidines under Air

An iron-catalyzed sustainable, economically affordable, and eco-friendly synthetic protocol for the construction of various trisubstituted pyrimidines is described. A wide range of trisubstituted pyrimidines were prepared using a well-defined, easy to prepare, bench-stable, and phosphine-free iron catalyst featuring a redox-noninnocent tridentate arylazo pincer under comparatively mild aerobic conditions via dehydrogenative functionalization of alcohols with alkynes and amidines.

Allenes and Dienes as Chiral Allylmetal Pronucleophiles in Catalytic Enantioselective C=X Addition: Historical Perspective and State-of-The-Art Survey

The use of allenes and 1,3-dienes as chiral allylmetal pronucleophiles in intermolecular catalytic enantioselective reductive additions to aldehydes, ketones, imines, carbon dioxide and other C=X electrophiles is exhaustively catalogued together with redox-neutral hydrogen auto-transfer processes. Coverage is limited to processes that result in both C-H and C-C bond formation. The use of alkynes as latent allylmetal pronucleophiles and multicomponent C=X allylations involving allenes and dienes is not covered. As illustrated in this review, the ability of allenes and 1,3-dienes to serve as tractable non-metallic pronucleophiles has evoked many useful transformations that have no counterpart in traditional allylmetal chemistry.

Conversion of Primary Alcohols and Butadiene to Branched Ketones via Merged Transfer Hydrogenative Carbonyl Addition-Redox Isomerization Catalyzed by Rhodium

The first examples of rhodium-catalyzed carbonyl addition via hydrogen autotransfer are described, as illustrated in tandem butadiene-mediated carbonyl addition-redox isomerizations that directly convert primary alcohols to isobutyl ketones. Related reductive coupling-redox isomerizations of aldehyde reactants mediated by sodium formate also are reported. A double-labeling crossover experiment reveals that the rhodium alkoxide obtained upon carbonyl addition enacts redox isomerization without dissociation of rhodium at any intervening stage.

Developing a Methodology for Catalytic Asymmetric Crotylation of Aldehydes

Asymmetric crotylation has firmly earned a place among the set of valuable synthetic tools for stereoselective construction of carbon skeletons. For a long time the field was heavily dominated by reagents bearing stoichiometric chiral auxiliaries, but now catalytic methods are gradually taking center stage, and the area continues to develop rapidly. This account focuses primarily on preformed organometallic reagents based on silicon and, to some extent, boron. It narrates our endeavors to design new and efficient chiral Lewis base catalysts for the asymmetric addition of crotyl(trichloro)silanes to aldehydes. It also covers the development of a novel protocol for kinetic resolution of racemic secondary allylboronates to give enantio- and diastereomerically enriched linear homoallylic alcohols. As a separate topic, cross-crotylation of aldehydes by using enantiopure branched homoallylic alcohols as a source of crotyl groups is discussed. Finally, the synthetic credentials of the developed methodology are illustrated by total syntheses of marine natural products, in which crotylation plays a key role in setting up stereogenic centers.

1 Introduction

2 Pyridine N-Oxides as Lewis Base Catalysts

3 Bipyridine N,N′-Dioxides as Lewis Base Catalysts

4 Chiral Allylating Reagents

5 Synthetic Applications

6 Concluding Remarks

Redox-enabled direct stereoconvergent heteroarylation of simple alcohols

The direct transformation of racemic feedstock materials to valuable enantiopure compounds is of significant importance for sustainable chemical synthesis. Toward this goal, the radical mechanism has proven uniquely effective in stereoconvergent carbon-carbon bond forming reactions. Here we report a mechanistically distinct redox-enabled strategy for an efficient enantioconvergent coupling of pyrroles with simple racemic secondary alcohols. In such processes, chirality is removed from the substrate via dehydrogenation and reinstalled in the catalytic reduction of a key stabilized cationic intermediate. This strategy provides significant advantage of utilizing simple pyrroles to react with feedstock alcohols without the need for leaving group incorporation. This overall redox-neutral transformation is also highly economical with no additional reagent nor waste generation other than water. In our studies, oxime-derived iridacycle complexes are introduced, which cooperate with a chiral phosphoric acid to enable heteroarylation of alcohols, accessing a wide range of valuable substituted pyrroles in high yield and enantioselectivity.

Synthesizing complex structures of high enantiomeric excess from racemic feedstock is an enduring challenge. Here, the authors couple racemic secondary alcohols with pyrroles to form enantioenriched 2-substituted heteroarenes, via a borrowing hydrogen mechanism using the combination of an iridium catalyst and chiral phosphoric acid.

Catalytic Diastereo- and Enantioconvergent Synthesis of Vicinal Diamines from Diols through Borrowing Hydrogen

We present herein an unprecedented diastereoconvergent synthesis of vicinal diamines from diols through an economical, redox-neutral process. Under cooperative ruthenium and Lewis acid catalysis, readily available anilines and 1,2-diols (as a mixture of diastereomers) couple to forge two C-N bonds in an efficient and diastereoselective fashion. By identifying an effective chiral iridium/phosphoric acid co-catalyzed procedure, the first enantioconvergent double amination of racemic 1,2-diols has also been achieved, resulting in a practical access to highly valuable enantioenriched vicinal diamines.

Allylic alcohol synthesis by Ni-catalyzed direct and selective coupling of alkynes and methanol

Methanol is an abundant and renewable chemical raw material, but its use as a C1 source in C-C bond coupling reactions still constitutes a big challenge, and the known methods are limited to the use of expensive and noble metal catalysts such as Ru, Rh and Ir. We herein report nickel-catalyzed direct coupling of alkynes and methanol, providing direct access to valuable allylic alcohols in good yields and excellent chemo- and regioselectivity. The approach features a broad substrate scope and high atom-, step- and redox-economy. Moreover, this method was successfully extended to the synthesis of [5,6]-bicyclic hemiacetals through a cascade cyclization reaction of alkynones and methanol.

Tandem Catalytic Indolization/Enantioconvergent Substitution of Alcohols by Borrowing Hydrogen to Access Tricyclic Indoles

An efficient tandem catalysis method is achieved for the direct conversion of alcohol-containing alkynyl anilines to valuable chiral 2,3-fused tricyclic indoles. This method relies on a tandem indolization followed by enantioconvergent substitution of alcohols via borrowing hydrogen to construct two rings in one step, enabled by relay and cooperative catalysis of a chiral iridium complex with a chiral phosphoric acid. Highly diastereoselective transformations of the tricyclic indole products also provide efficient access to a diverse array of complex polycyclic indoline compounds.

(Cyclopentadienone)iron Complexes: Synthesis, Mechanism and Applications in Organic Synthesis

(Cyclopentadienone)iron tricarbonyl complexes are catalytically active, inexpensive, easily accessible and air-stable that are extensively studied as an active pre-catalyst in homogeneous catalysis. Its versatile catalytic activity arises exclusively due to the presence of a non-innocent ligand, which can trigger its unique redox properties effectively. These complexes have been employed widely in (transfer)hydrogenation (e. g., reduction of polar multiple bonds, Oppenauer-type oxidation of alcohols), C-C and C-N bond formation (e. g., reductive aminations, α-alkylation of ketones) and other synthetic transformations. In this review, we discuss the remarkable advancement of its various synthetic applications along with synthesis and mechanistic studies, until February 2021.

From Hydrogenation to Transfer Hydrogenation to Hydrogen Auto-Transfer in Enantioselective Metal-Catalyzed Carbonyl Reductive Coupling: Past, Present, and Future

Atom-efficient processes that occur via addition, redistribution or removal of hydrogen underlie many large volume industrial processes and pervade all segments of chemical industry. Although carbonyl addition is one of the oldest and most broadly utilized methods for C-C bond formation, the delivery of non-stabilized carbanions to carbonyl compounds has relied on premetalated reagents or metallic/organometallic reductants, which pose issues of safety and challenges vis-à-vis large volume implementation. Catalytic carbonyl reductive couplings promoted via hydrogenation, transfer hydrogenation and hydrogen auto-transfer allow abundant unsaturated hydrocarbons to serve as substitutes to organometallic reagents, enabling C-C bond formation in the absence of stoichiometric metals. This perspective (a) highlights past milestones in catalytic hydrogenation, hydrogen transfer and hydrogen auto-transfer, (b) summarizes current methods for catalytic enantioselective carbonyl reductive couplings, and (c) describes future opportunities based on the patterns of reactivity that animate transformations of this type.

Ethanol: Unlocking an Abundant Renewable C2 -Feedstock for Catalytic Enantioselective C-C Coupling

With annual production at >85 million tons/year, ethanol is the world's largest-volume renewable small molecule carbon source, yet its use as a C2 -feedstock in enantioselective C-C coupling is unknown. Here, the first catalytic enantioselective C-C couplings of ethanol are demonstrated in reactions with structurally complex, nitrogen-rich allylic acetates incorporating the top 10 N-heterocycles found in FDA-approved drugs.

Hydrogen-Borrowing Alkylation of 1,2-Amino Alcohols in the Synthesis of Enantioenriched γ-Aminobutyric Acids

For the first time we have been able to employ enantiopure 1,2-amino alcohols derived from abundant amino acids in C-C bond-forming hydrogen-borrowing alkylation reactions. These reactions are facilitated by the use of the aryl ketone Ph*COMe. Racemisation of the amine stereocentre during alkylation can be prevented by the use of sub-stoichiometric base and protection of the nitrogen with a sterically hindered triphenylmethane (trityl) or benzyl group. The Ph* and trityl groups are readily cleaved in one pot to give γ-aminobutyric acid (GABA) products as their HCl salts without further purification. Both steps may be performed in sequence without isolation of the hydrogen-borrowing intermediate, removing the need for column chromatography.