Alkynes as Electrophilic or Nucleophilic Allylmetal Precursors in Transition-Metal Catalysis

Redox-Neutral Propargylic C–H Functionalization by Using Iron Catalysis

In spite of their rich stoichiometric chemistry, cyclopentadienyliron(II) dicarbonyl complexes are rarely used as catalysts in organic synthesis. Inspired by precedents in the chemistry of cationic olefin complexes and neutral allylmetal species, our group has developed a coupling of alkynes or alkenes with aldehydes and other carbonyl electrophiles to give homopropargylic and homoallylic alcohols, respectively, by using a substituted cyclopentadienyliron(II) dicarbonyl complex as the catalyst. In this article, we first contextualize this development within the conceptual background of C–H functionalization chemistry and relative to key stoichiometric precedents. We then give an account of our group’s discovery and development of the catalytic α-functionalization of alkenes and alkynes with electrophilic reagents.

Introduction

Preliminary Stoichiometric Work

Hydroxyalkylation Development and Scope

Conclusions and Future Directions

Rhodium-Catalyzed Branched and Enantioselective Direct α-Allylic Alkylation of Simple Ketones with Alkynes

Herein, we report the first direct branched-selective α-allylic alkylation of simple ketones with alkynes under rhodium and secondary amine cooperative catalysis. Through a rhodium-hydride-catalyzed allylic substitution pathway, a series of valuable γ,δ-unsaturated ketones are obtained with excellent regioselectivity in an atom-economic and byproduct-free manner. With a chiral BIPHEP ligand, high enantioselectivity has been achieved for this transformation.

Mechanism and origins of stereo- and enantioselectivities of palladium-catalyzed hydroamination of racemic internal allenes via dynamic kinetic resolution: a computational study

DFT calculations were performed to investigate the Pd-catalyzed hydroamination of racemic internal allenes with pyrazoles.

Recent advances in transition metal-free catalytic hydroelementation (E = B, Si, Ge, and Sn) of alkynes

This review describes the recent advances in the transition metal-free hydroelementation of alkynes with various metalloid hydrides.

Palladium-catalyzed diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes under base-free conditions

Herein, we report a highly diastereo- and enantioselective allylic alkylation of oxazolones with 1,3-dienes by palladium-hydride catalyst under base-free conditions.

Rhodium-catalyzed regio- and enantioselective allylic alkylation of pyrazol-5-ones with alkynes

An atom-economical, regio- and enantioselective allylic alkylation of pyrazol-5-ones with alkynes was developed under rhodium catalysis.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes

This review article provides an overview of progress in asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.

Rhodium-Catalyzed Diastereo- and Enantioselective Tandem Spirocyclization/Reduction of 3-Allenylindoles: Access to Functionalized Vinylic Spiroindolines

A highly selective rhodium-catalyzed tandem spirocyclization/reduction of 3-allenylindoles is reported. By employing a Hantzsch ester as reductant, vinylic spiroindolines are obtained in excellent yields as well as diastereo- and enantioselectivity. In addition, the reaction's synthetic utility is highlighted by broad functional group compatibility and exemplified by a gram scale reaction with subsequent assorted transformations.

Palladium(0)/benzoic acid catalysis merges sequences with D2O-promoted labelling of C-H bonds

The combination of a Pd(0) complex with benzoic acid in the presence of D2O enables the synthesis of valuable families of highly deuterated organics through elaborate sequential reactions. The catalytic system can convert 2-butyne fragments into the corresponding d-dienamides, which can then readily deliver labeled polycyclic quinone motifs. Propargylated tryptamines lead to formation of highly enriched tetrahydrocarbolines through the C-H activation of their unprotected indole ring. Mechanistic studies reveal the ordered series of events that regulate the outcome of these complex reactions, which include multiple, sequential and selective H/D scrambling from the cheapest and safest deuterium source.

Rhodium-catalyzed asymmetric intramolecular hydroarylation of allenes: access to functionalized benzocycles

A rhodium-catalyzed regio- and enantioselective cyclization of tethered allenylbenzenes is reported. Employing a RhI/Josiphos catalytic system a diverse set of 6-membered, α-chiral benzocycles were obtained, scaffolds found in many bioactive compounds. Moreover, a gram scale reaction as well as the application of suitable transformations are demonstrated.

Palladium-NHC-Catalyzed Allylic Alkylation of Pronucleophiles with Alkynes

The palladium-N-heterocyclic carbene (NHC)-catalyzed allylic alkylation of various pronucleophiles with alkynes has been accomplished under mild conditions. The protocol exhibits broad functional group compatibility and high atom economy. Moreover, the catalytic process avoids the use of external oxidants and acid as additives.

A regioselectivity switch in Pd-catalyzed hydroallylation of alkynes

By exploiting the reactivity of a vinyl-Pd species, we control the regioselectivity in hydroallylation of alkynes under Pd-hydride catalysis. A monophosphine ligand and carboxylic acid combination promotes 1,5-dienes through a pathway involving isomerization of alkynes to allenes. In contrast, a bisphosphine ligand and copper cocatalyst favor 1,4-dienes via a mechanism that involves transmetalation. Our study highlights how to access different isomers by diverting a common organometallic intermediate.

Rhodium-Catalyzed Enantioselective Cyclization of 3-Allenyl-indoles: Access to Functionalized Tetrahydrocarbazoles

A highly selective rhodium-catalyzed cyclization of tethered 3-allenylindoles is reported. In a smooth reaction, 1-vinyltetrahydrocarbazoles are obtained in excellent yields and enantioselectivities. Aside from a great functional group tolerance, this method requires neither the Schlenk technique nor the use of anhydrous solvents. Preliminary mechanistic investigations proved that the reaction proceeds via an intermediary formed spiroindolenine which rapidly undergoes an acid-catalyzed stereospecific migration.

A rhodium catalyzed cycloisomerization and tandem Diels-Alder reaction for facile access to diverse bicyclic and tricyclic heterocycles

A rhodium catalyzed cycloisomerization of 1,6-allenenes successfully prepared six-membered ring exocyclic 1,3-dienes. Further tandem Diels–Alder reaction with various dienophiles provided diverse bicyclic and tricyclic heterocycles. This methodology is characterised by complete atom and step economy, low catalyst loading, and excellent chemo-, regio-, and diastereoselectivity.

A regioselective distal cycloisomerization of 1,6-allenenes was successfully developed to afford six-membered ring exocyclic 1,3-dienes employing a rhodium/diphosphine catalyst system. Deuterium labelling experiments and DFT calculations were performed to provide insights into the reaction mechanism of this unprecedented transformation. In addition, one-pot tandem Diels–Alder reactions with various dienophiles could readily construct diverse bicyclic and tricyclic nitrogen heterocycles, which are ubiquitous core scaffolds for a variety of natural products and bioactives. High efficiency and exclusive chemo and regioselectivities for a broad substrate scope were achieved under mild conditions using a low catalyst loading of 0.5 mol%.

Catalytic Hydrothiolation: Counterion-Controlled Regioselectivity

In this Article, we expand upon the catalytic hydrothiolation of 1,3-dienes to afford either allylic or homoallylic sulfides with high regiocontrol. Mechanistic studies support a pathway in which regioselectivity is dictated by the choice of counterion associated with the Rh center. Non-coordinating counterions, such as SbF6-, allow for η4-diene coordination to Rh complexes and result in allylic sulfides. In contrast, coordinating counterions, such as Cl-, favor neutral Rh complexes in which the diene binds η2 to afford homoallylic sulfides. We propose mechanisms that rationalize a fractional dependence on thiol for the 1,2-Markovnikov hydrothiolation while accounting for an inverse dependence on thiol in the 3,4- anti-Markovnikov pathway. Through the hydrothiolation of an essential oil (β-farnesene), we achieve the first enantioselective synthesis of (-)-agelasidine A.

Palladium-catalyzed trisallylation of benzoxazoles and 2-aryl-1,3,4-oxadiazoles with alkyne

In this report, a palladium catalyzed direct trisallylation of azoles with alkyne via a novel subsequent mono C(sp²)–H allylation/olefin isomerization/y-selective double C(sp³)–H allylation process was developed. A wide range of substituted trisallylated azoles bearing all quaternary carbon centers has been synthesized efficiently and with high atom economy.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

Chemo-, regio-, and enantioselective synthesis of allylic nitrones via rhodium-catalyzed addition of oximes to allenes

The first chemo-, regio-, and enantioselective rhodium-catalyzed addition of oximes to allenes is reported.

Bi-directional alkyne tandem isomerization via Pd(0)/carboxylic acid joint catalysis: expedient access to 1,3-dienes

An in situ formed palladium hydride catalyst enables the sequential dual isomerization of propargylamide derivatives to 1-amido-1,3-dienes with high chemo- and regioselectivity. The reaction shows ample functional group tolerance, delivering a valuable class of products, including highly deuterated ones, from readily available reagents. The reaction occurs through a complex mechanism studied by DFT modelling.

Nickel(0)-Catalyzed Hydroalkylation of 1,3-Dienes with Simple Ketones

We developed a highly regioselective addition of 1,3-dienes with simple ketones by nickel-hydride catalyst bearing DTBM-SegPhos ligand. A wide range of aromatic and aliphatic ketones directly coupled with 1,3-dienes, providing synthetically useful γ,δ-unsaturated ketones in high yield and regioselectivity. The asymmetric version of the reaction was also realized in high enantioselectivity by using novel chiral ligand DTBM-HO-BIPHEP. The utility of this hydroalkylation was demonstrated by facile product modification and enantioselective synthesis of ( R)-flobufen.

Catalytic Hydrothiolation: Regio- and Enantioselective Coupling of Thiols and Dienes

We report a Rh-catalyzed hydrothiolation of 1,3-dienes, including petroleum feedstocks. Either secondary or tertiary allylic sulfides can be generated from the selective addition of a thiol to the more substituted double bond of a diene. The catalyst tolerates a wide range of functional groups, and the loading can be as low as 0.1 mol %.

Monomerisation of [Rh2(1,3-Bis-(Diphenylphosphino)-Propane)2(μ2-Cl)2] Detected by Pulsed Gradient Spin Echo Spectroscopy and 31P Nmr Monitoring of Metathesis Experiments

A pulsed gradient spin echo experiment on [Rh2(1,3-bis-(diphenylphosphino)-propane)2(μ2-Cl)2] complex has been conducted in order to shed light on the supposed monomerisation process of [Rh2(diphosphine)2(μ2-Cl)2] complexes in solution. Such a process should generate a 14-electron [Rh(diphosphine)Cl] complex, which has only been postulated to date. Metathesis experiments on [Rh2(1,3-bis-(diphenylphosphino)-propane)2(μ2-Cl)2] and [Rh2(bis[2-(diphenylphosphino)phenyl]ether)2(μ2-Cl)2] complexes, analysed by 31P NMR, reveal that monomerisation of [Rh2(diphosphine)2(μ2-Cl)2] complexes is not restricted to the case of 1,3-bis-(diphenylphosphino)propane.

Intermolecular Metal-Catalyzed Reductive Coupling of Dienes, Allenes, and Enynes with Carbonyl Compounds and Imines

Metal-catalyzed reductive coupling has emerged as an alternative to the use of stoichiometric organometallic reagents in an increasingly diverse range of carbonyl and imine additions. In this review, the use of diene, allene, and enyne pronucleophiles in intermolecular carbonyl and imine reductive couplings are surveyed, along with related hydrogen autotransfer processes.

Regio- and Enantioselective Rhodium-Catalyzed Allylic Alkylation of Racemic Allylic Alcohols with 1,3-Diketones

Highly regio- and enantioselective rhodium-catalyzed allylic alkylation of 1,3-diketones with racemic secondary allylic alcohols is reported. In the presence of a Rh-catalyst derived from the Carreira (P, olefin)-ligand and TFA as an additive, chiral branched α-allylated 1,3-diketones could be obtained in good to excellent yields, with excellent regio- and enantioselectivity ( b/ l > 19/1, 86-98% ee). The direct utilization of allyl alcohols as electrophiles represents an improvement from the viewpoint of an atom economy. Both aryl- and aliphatic-substituted allyl alcohols are suitable substrates with excellent reaction outcomes. This reaction features mild conditions, broad substrate scope, and readily available substrates.

Asymmetric α-Allylation of Aldehydes with Alkynes by Integrating Chiral Hydridopalladium and Enamine Catalysis

A palladium-catalyzed asymmetric α-allylation of aldehydes with alkynes has been established by integrating the catalysis of enamine and chiral hydridopalladium complex that is reversibly formed from the oxidative addition of Pd(0) to chiral phosphoric acid. The ternary catalyst system, consisting of an achiral palladium complex, a primary amine, and a chiral phosphoric acid allows the reaction to tolerate a wide scope of α,α-disubstituted aldehydes and alkynes, affording the corresponding allylation products in high yields and with excellent levels of enantioselectivity.