Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Challenging Task of Ni-Catalyzed Highly Regio-/Enantioselective Semihydrogenation of Racemic Tetrasubstituted Allenes via a Kinetic Resolution Process

The first earth-abundant transition metal Ni-catalyzed highly regio- and enantioselective semihydrogenation of racemic tetrasubstituted allenes via a kinetic resolution process as a challenging task was well established. This protocol furnishes expedient access to a diversity of structurally important enantioenriched tetrasubstituted allenes and chiral allylic molecules with high regio-, enantio-, and Z/E-selectivity. Remarkably, this semihydrogenation proceeded with one carbon-carbon double bond of allenes, which was regioselective complementary to the Rh-catalyzed asymmetric version. Deuterium labeling experiments and density functional theory (DFT) calculations were carried out to reveal the reasonable reaction mechanism and explain the regio-/stereoselectivity.

Nickel-catalyzed acylzincation of allenes with organozincs and CO

Transition metal-catalyzed carbonylative reaction with CO gas are among the central task in organic synthesis, enabling the construction of highly valuable carbonyl compound. Here, we show an earth-abundant nickel-catalyzed three-component tandem acylzincation/cyclization sequence of allene and alkylzinc reagent with 1 atm of CO under mild conditions. This protocol is featured by broad functional group tolerance with high reaction selectivity, providing a rapid and convenient synthetic method for the construction of diverse fully substituted benzotropone derivatives. Mechanistic studies reveal that the installation of a cyano group tethered to allene moiety enables the high regio- and stereoselectivity of this acylzincation of allene, allowing the selective formation of three consecutive C-C bonds in a highly efficient manner.

Iron-Catalyzed Borylation of Propargylic Acetates for the Synthesis of Multisubstituted Allenylboronates

A novel iron-catalyzed borylation of propargylic acetates leading to allenylboronates has been developed. The method allows the preparation of a variety of di-, tri- and tetrasubstituted allenylboronates at room temperature with good functional group compatibility. Stereochemical studies show that an anti-S_N 2' displacement of acetate by boron occurs; this also allows transfer of chirality to yield enantiomerically enriched allenylboronates. The synthetic utility of this protocol was further substantiated by transformations of the obtained allenylboronates including oxidation and propargylation.

Stereoselective Total Synthesis of (-)-Thallusin for Bioactivity Profiling

Chemical mediators are key compounds for controlling symbiotic interactions in the environment. Here, we disclose a fully stereoselective total synthesis of the algae differentiation factor (-)-thallusin that utilizes sophisticated 6-endo-cyclization chemistry and effective late-stage sp² -sp² -couplings using non-toxic reagents. An EC₅₀ of 4.8 pM was determined by quantitative phenotype profiling in the green seaweed Ulva mutabilis (Chlorophyte), underscoring this potent mediator's enormous, pan-species bioactivity produced by symbiotic bacteria. SAR investigations indicate that (-)-thallusin triggers at least two different pathways in Ulva that may be separated by chemical editing of the mediator compound structure.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.

Gold-catalyzed cycloadditions of allenes via metal carbenes

In recent years, gold-catalyzed cycloadditions of allenes, especially those involving a gold carbene intermediate, have received significant interest, as they avoid the utilization of potentially hazardous and inaccessible diazo compounds as starting materials for carbene generation. Cycloaddition reactions consisting of the uncomplicated addition of two or more unsaturated functional groups are one of the most efficient synthetic methodologies for the rapid assembly of carbo- and heterocyclic structures from simple acyclic precursors. In this review, we introduce an overview of the advances in the gold-catalyzed cycloaddition of allenes via a metal carbene intermediate and categorize these reactions according to the reaction types of the cycloadditions.

Stereoselective Synthesis of Allenyl Alcohols by Cobalt(III)-Catalyzed Sequential C-H Bond Addition to 1,3-Enynes and Aldehydes

An efficient and stereoselective CoIII -catalyzed sequential C-H bond addition to 1,3-enynes and aldehydes is disclosed. This transformation represents the first example of sequential C-H bond additions to 1,3-enynes and a second coupling partner and provides the first example of preparing allenes by C-H bond addition to 1,3-enynes. A wide range of aldehydes, C-H bond substrates and 1,3-enynes with large substituents on the alkynes are effective substrates. The allenyl alcohol products can be further converted to dihydrofurans with high stereoselectivity either in situ or under Ag-mediated cyclization conditions. The allenyl silyl group can also be transferred to the adjacent alcohol by a Brook rearrangement. Moreover, a mechanism for the transformation is proposed supported by X-ray structural characterization of a cobaltacycle intermediate.

Pd-Catalyzed Asymmetric Three-Component Allenol Carbopalladation and Allylic Cycloaddition Cascade: A Route to Functionalized Tetrahydrofurans

The first Pd-catalyzed asymmetric three-component reaction of 2,3-allenol, aryl iodides, and 2-arylmethylenemolononitriles has been developed via an allenol carbopalladation and an allylic cycloaddition cascade. This process allows rapid access to substituted tetrahydrofurans bearing diverse functional groups in good yields with high diastereoselectivities and excellent enantioselectivities. The concise total synthesis of a lignan, (-)-2-episesaminone, has been achieved by the elaboration of a functionalized tetrahydrofuran obtained from this reaction.

Dimerizing cascades of enallenamides reveal the visible-light-promoted activation of cumulated C-C double bonds

The visible-light-promoted activation of conjugated C-C double bonds is well developed, while that of cumulated systems is underexplored. We present the feasibility of this challenging approach. The localization of a triplet on an allenamide arm can be favored over that on a conjugated alkene. Allenamides with an arylacryloyl arm dimerize at room temperature in the presence of visible light and an iridium(iii) photocatalyst. Two orthogonal polycyclizations took place and their outcome is entirely dictated by the substitution of the alkene partner. Both cascades afford complex molecular architectures with high selectivity. Products form through the ordered rearrangement of twelve π electrons, providing a [3.2.0] bicyclic unit tethered to a fused tricycle, whose formation included an aryl C-H functionalization step, using disubstituted alkenes. The outcome was reverted with trisubstituted ones, which gave rise to taxane-like bridged tricycles that had two six-membered lactams flanking a cyclooctane ring, which was established through the creation of four alternate C-C bonds.

Catalytic Enantioconvergent Allenylation of Aldehydes with Propargyl Halides

α-Allenol is a versatile synthon in organic synthesis. The catalytic asymmetric synthesis of α-allenols from readily available starting materials remains a prominent challenge, especially when simultaneous control over axial and central chirality is required. Herein, we describe the Cr-catalyzed enantioconvergent allenylation of aldehydes with racemic propargyl halides to rapidly access a wide range of chiral α-allenols with adjacent axial and central chiralities. This method features excellent regio-, diastereo- and enantioselectivity control with broad substrate scope, and provides facile access to all four stereoisomers when allied with a Mitsunobu reaction. Preliminary mechanistic studies support radical-based reaction pathways. The synthetic utility is demonstrated by the application in late-stage functionalization and the formal total synthesis of (+)-varitriol.

Visible light photocatalytic one pot synthesis of Z-arylvinyl halides from E-arylvinyl acids with N-halosuccinimide

An efficient visible light photocatalytic strategy to synthesize thermodynamically less stable Z-arylvinyl halides (with up to >99/1 Z/E ratio and 86% yield) was developed. The reaction combined base-mediated halodecarboxylation of E-arylvinyl acids with N-halosuccinimide and visible light Ir-photocatalyzed isomerization of E-arylvinyl halides in a one pot sequential catalytic process.

Blue light enhanced Heck arylation at room temperature applied to allenes

A blue light enhanced synthesis of 2-vinyl pirrolidines and piperidines through a domino Heck arylation–cyclisation applied to allenyl amines is described. Essential is the role of the light in the aryl migration in the carbo-palladation step.

Room Temperature Allenation of Terminal Alkynes with Aldehydes

A gold-catalyzed room temperature allenation of terminal alkynes (ATA) with aldehydes affording 1,3-disubstituted allenes with diverse functional groups has been developed by identifying a gold(I) catalyst and an amine. The practicality of this reaction has been demonstrated by a ten gram-scale synthesis and the synthetic potentials have been demonstrated via various transformations and formal total synthesis of (-)-centrolobine. Mechanistic studies revealed that the gold catalyst, the aldehyde effect, the fluoroalkyl hydroxyl solvent (TFE or HFIP) and the structure of amine are vital in this room temperature ATA reaction.

Catalytic Electrophilic Thiocarbocyclization of Allenes

An efficient approach via catalytic electrophilic thiocarbocyclization of allenes to construct indene-based sulfides with excellent regioselectivities is disclosed. The reactions were carried out at low temperatures by selenide catalysis in the presence of TMSOTf. Not only electrophilic arylthio reagents but also electrophilic alkylthio reagents worked well under these conditions. Furthermore, the method could be applied to intermolecular azidothiolation of allenes.

A palladium-catalyzed approach to allenic aromatic ethers and first total synthesis of terricollene A

A palladium-catalyzed C-O bond formation reaction between phenols and allenylic carbonates to give 2,3-allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A variety of synthetically useful functional groups are tolerated and the synthetic utility of this method has been demonstrated through a series of transformations of the allene moiety. By applying this reaction as the key step, the total syntheses of naturally occurring allenic aromatic ethers, eucalyptene and terricollene A (first synthesis; 4.5 g gram scale), have been accomplished.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Polycyclic Compounds from Allenes via Palladium-Mediated Intramolecular Carbopalladation/Nucleophilic Substitution Cascade Processes

In recent decades transition metals have made a substantial contribution to the development of novel synthetic processes, with palladium catalysis being, arguably, at the forefront of this research. The efficiency of Pd-promoted C–C or C–X bond formation along with a variety of other transformations renders this metal an indispensable tool in synthetic organic chemistry. Of particular interest are Pd-catalysed multicomponent cascade reactions as they often allow the creation of complex structures from relatively simple starting materials, mimicking in this sense biochemical processes. Allenes as partners in Pd-promoted cascades involving carbopalladation/nucleophilic substitutions have been extensively studied in recent years. Many tactical variants have been explored showing a high level of efficiency and chemoselectivity with predictable outcomes. This short review is focused on intramolecular processes of this type because they provide access to relatively complex polycyclic products, possessing structural features often found in natural products and related compounds. Various approaches are discussed with the intention to demonstrate their applicability and synthetic potential.

1 Introduction

2 Intramolecular Palladium-Promoted Cascades of Allenes

3 Class I Cyclisations

4 Class II Cyclisations

5 Class III Cyclisations

6 Class IV Cyclisations

7 Conclusion

Asymmetric sequential annulation/aldol process of 4-isothiocyanato pyrazolones and allenones: access to novel spiro[pyrrole-pyrazolones] and spiro[thiopyranopyrrole-pyrazolones]

A catalytic asymmetric sequential annulation/aldol reaction of 4-isothiocyanato pyrazolones and allenyl ketones has been developed, which furnished a series of spiro[pyrrole-pyrazolone] heterocycles and structurally novel spiro[thiopyranopyrrole-pyrazolone] derivatives in good yields with high to excellent enantioselectivities. Notably, parallel resolution of racemic spiro[pyrrole-pyrazolones] was achieved by a catalyst-controlled asymmetric intramolecular vinylogous aldol process. Structure diversity of the product was further enhanced by ready transformations.

A cooperative zinc/catalytic indium system for the stereoselective sequential synthesis of (E)-1,3-dienes from carbonyl compounds

A novel cooperative system based on zinc/catalytic indium for the stereoselective synthesis of highly functionalized (E)-1,3-dienes from aldehydes and ketones has been developed.

Palladium-Catalyzed Fluoroarylation of gem-Difluoroenynes to Access Trisubstituted Trifluoromethyl Allenes

Various new transformations of gem-difluoroalkenes leading to trifluoromethyl substituted compounds have been well established in the past years. However, the development of new transformations of gem-difluoroenynes lags much behind. Herein is reported the fluoroarylation of 1,1-difluoro-1,3-enynes with aryl halides in the presence of silver fluoride affording trisubstituted trifluoromethyl allenes under the catalysis of palladium. The reaction features mild conditions, high functional-group tolerance, and high regioselectivity.

Synthesis of α-CF3-substituted E-dehydroornithine derivatives via copper(i)-catalyzed hydroamination of allenes

Novel α-CF₃-substituted E-dehydroornithine derivatives have been synthesized via the Cu(i)-catalyzed hydroamination of α-CF₃-α-allenyl-α-aminocarboxylates/phosphonates with different amines.

Unravelling mechanistic insights in the platinum-catalysed dihydroalkoxylation of allenes

The mechanism of the platinum-catalysed dihydroalkoxylation of allenes to give acetals has been studied experimentally and by computational methods. Our findings further explain divergent reactivity encountered for platinum- and gold-vinyl intermediates after the first nucleophilic attack onto the coordinated allene, as well as provide new details on the catalytic cycle with platinum, uncovering enol ethers as resting states of the catalytic cycle, a SEOx process via Pt(IV)–H as the final protodemetallation step after the second nucleophilic attack when neutral platinum complexes are used, and a fast acid promoted addition of methanol to enol ethers when cationic platinum complexes are employed.

Allenation of Terminal Alkynes with Aldehydes and Ketones

So far, over 150 natural products and pharmaceuticals containing an allene moiety have been identified. During the last two decades, allenes have also been demonstrated as synthetically versatile starting materials in organic synthesis. In comparison to alkenes and alkynes, allenes are unique unsaturated hydrocarbons due to their axial chirality, which could be transformed to central chirality via chirality transfer to provide an irreplaceable entry to chiral molecules. Thus, methods for allene synthesis from readily available chemicals are of great interest. In 1979, Crabbé et al. reported the first CuBr-mediated allenation of terminal alkynes (ATA) reaction to form monosubstituted allenes from 1-alkynes and paraformaldehyde in the presence of diisopropylamine. During the following 30 years, the ATA reactions were limited to paraformaldehyde. This Account describes our efforts toward the development of ATA reactions in the last ten years. First, we improved the yields and scope greatly for the synthesis of monosubstituted allenes by modifying the original Crabbé recipe. Next we developed the ZnI₂-promoted or CuI-catalyzed ATA reactions for the synthesis of 1,3-disubstituted allenes from terminal alkyne and normal aldehydes. Furthermore, we first realized the CdI₂-promoted ATA reaction of ketones with pyrrolidine as the matched amine for the preparation of trisubstituted allenes. Due to the toxicity of CdI₂, we also developed two alternative approaches utilizing CuBr/ZnI₂ or CuI/ZnBr₂/Ti(OEt)₄. The asymmetric version of ATA reactions for the synthesis of optically active 1,3-disubstituted allenes has also been achieved in this group with two strategies. One is called "chiral ligand" strategy, using terminal alkynes, aldehydes, and nonchiral amine with the assistance of a proper chiral ligand. The other is the "chiral amine" strategy, applying terminal alkynes, aldehydes, and chiral amines such as ( S)- or ( R)-α,α-diphenylprolinol or ( S)- or ( R)-α,α-dimethylprolinol. Optically active 1,3-disubstituted allenes containing different synthetically useful functionalities such as alcohol, amide, sulfamide, malonate, carboxylate, and carbohydrate units could be prepared without protection with the newly developed CuBr₂-catalyzed chiral amine strategy. Recently, we have applied these enantioselective allenation of terminal alkyne (EATA) reactions to the syntheses of some natural allenes such as laballenic acid, insect pheromone, methyl ( R)-8-hydroxyocta-5,6-dienoate, phlomic acid, and lamenallenic acid, as well as some non-allene natural γ-butyrolactones such as xestospongienes (E, F, G, and H), ( R)-4-tetradecalactone, ( S)-4-tetradecalactone, ( R)-γ-palmitolactone, and ( R)-4-decalactone.

Base-promoted direct synthesis of functionalized N-arylindoles via the cascade reactions of allenic ketones with indoles

A convenient Cs2CO3-promoted cascade benzannulation reaction of allenic ketones with indoles was achieved for the synthesis of functionalized N-arylindole derivatives under transition-metal-free conditions. A series of readily available starting materials can undergo the process successfully. It represents a practical method for the construction of N-arylindole scaffolds with high atom economy.

Enantioselective Allylation Using Allene, a Petroleum Cracking Byproduct

Allene (C₃H₄) gas is produced and separated on million-metric-ton scale per year during petroleum refining but is rarely employed in organic synthesis. Meanwhile, the addition of an allyl group (C₃H₅) to ketones is among the most common and prototypical reactions in synthetic chemistry. Herein, we report that the combination of allene gas with inexpensive and environmentally benign hydrosilanes, such as PMHS, can serve as a replacement for stoichiometric quantities of allylmetal reagents, which are required in most enantioselective ketone allylation reactions. This process is catalyzed by copper salts and commercially available ligands, operates without specialized equipment or pressurization, and tolerates a broad range of functional groups. Furthermore, the exceptional chemoselectivity of this catalyst system enables industrially relevant C3 hydrocarbon mixtures of allene with methylacetylene and propylene to be applied directly.