Nucleophilic addition to organotransition metal cations containing unsaturated hydrocarbon ligands

Cyclopentadienyl ring activation in organometallic chemistry and catalysis

The cyclopentadienyl (Cp) ligand is a cornerstone of modern organometallic chemistry. Since the discovery of ferrocene, the Cp ligand and its various derivatives have become foundational motifs in catalysis, medicine and materials science. Although largely considered an ancillary ligand for altering the stereoelectronic properties of transition metal centres, there is mounting evidence that the core Cp ring structure also serves as a reservoir for reactive protons (H⁺), hydrides (H^-) or radical hydrogen (H^•) atoms. This Review chronicles the field of Cp ring activation, highlighting the pivotal role that Cp ligands can have in electrocatalytic H₂ production, N₂ reduction, hydride transfer reactions and proton-coupled electron transfer.

Chiral Arene Ligand as Stereocontroller for Asymmetric C-H Activation

Development of chiral ligands is the most fundamental task in metal-catalyzed asymmetric synthesis. In the last 60 years, various kinds of ligands have been sophisticatedly developed. However, it remains a long-standing challenge to develop practically useful chiral η⁶ -arene ligands, thereby seriously hampering the asymmetric synthesis promoted by arene-metal catalysts. Herein, we report the design and synthesis of a class of readily tunable, C₂ -symmetric chiral arene ligands derived from [2.2]paracyclophane. Its ruthenium(II) complexes have been prepared and successfully applied in the enantioselective C-H activation to afford a series of axially chiral isoquinolones (up to 99 % yield and 96 % ee). This study not only lays chemists' longstanding doubts about whether it is possible to use chiral arene ligands to stereocontrol ruthenium(II)-catalyzed asymmetric C-H activation, but also opens up a new avenue to achieve asymmetric C-H activation.

Teaching cyclopentadienyl how to leave: a case study of [CpIr(COD)Br]+ complex

This work was motivated by a recent report that describes the substitution of the cyclopentadienyl ring in [CpIr(COD)Br]+ with P(OMe)3 in mild conditions. We have shown that the first step...

Enantioselective Synthesis of Isocarbostyril Alkaloids and Analogs Using Catalytic Dearomative Functionalization of Benzene

Enantioselective total syntheses of the anticancer isocarbostyril alkaloids (+)-7-deoxypancratistatin, (+)-pancratistatin, (+)-lycoricidine, and (+)-narciclasine are described. Our strategy for accessing this unique class of natural products is based on the development of a Ni-catalyzed dearomative trans-1,2-carboamination of benzene. The effectiveness of this dearomatization approach is notable, as only two additional olefin functionalizations are needed to construct the fully decorated aminocyclitol cores of these alkaloids. Installation of the lactam ring has been achieved through several pathways and a direct interconversion between natural products was established via a late-stage C-7 cupration. Using this synthetic blueprint, we were able to produce natural products on a gram scale and provide tailored analogs with improved activity, solubility, and metabolic stability.

Recent advances in chemical dearomatization of nonactivated arenes

Dearomatization reactions provide a synthetic connection between readily available, simple aromatic starting materials and more saturated intermediates of greater molecular complexity and synthetic utility. The last decade has witnessed a steady increase in the development of dearomative methods, providing new synthetic approaches to high-value building blocks and natural products. This review highlights advances both in the area of dearomatization methodologies for the most chemically inert arenes and in synthetic applications of such strategies.

Nickel-Catalyzed Dearomative trans-1,2-Carboamination

We describe the development of an arenophile-mediated, nickel-catalyzed dearomative trans-1,2-carboamination protocol. A range of readily available aromatic compounds was converted to the corresponding dienes using Grignard reagents as nucleophiles. This strategy provided products with exclusive trans-selectivity and high enantioselectivity was observed in case of benzene and naphthalene. The utility of this methodology was showcased by controlled and stereoselective preparation of small, functionalized molecules.

Synthesis of (+)-Pancratistatins via Catalytic Desymmetrization of Benzene

A concise synthesis of (+)-pancratistatin and (+)-7-deoxypancratistatin from benzene using an enantioselective, dearomative carboamination strategy has been achieved. This approach, in combination with the judicious choice of subsequent olefin-type difunctionalization reactions, permits rapid and controlled access to a hexasubstituted core. Finally, minimal use of intermediary steps as well as direct, late stage C-7 hydroxylation provides both natural products in six and seven operations.

Olefin-Supported Rhenium(III) Terminal Oxo Complexes Generated by Nucleophilic Addition to a Cyclopentadienyl Ligand

The reactivity of the oxo Re^V β-diketiminate, OReCl₂ (BDI), with various cyclopentadienide (Cp) sources has been investigated. As a result, we have developed a route to a new class of terminal oxo complexes of Re^III supported by olefin moieties of substituted cyclopentadienes. The success of this pathway is due to the electrophilic nature of the Cp ligand in the cation, [ORe(η⁵ -Cp)(BDI)]⁺ (3⁺ ), which allows for nucleophilic attack by a variety of reagents under mild conditions. In contrast, ^t BuNC was found to attack at the oxo moiety to produce isocyanate by oxygen atom transfer.

C-C and C-N Couplings Following Hydride Addition on Isocyanide Cyclopolyenyl Dimolybdenum Complexes to Give Tethered Aldimine and Aminocarbene Derivatives

Reaction of [Mo₂ Cp₂ (μ-κ¹ :κ¹ ,η⁶ -PMes*)(CO)₂ ] with S or Se followed by protonation with [H(OEt₂ )₂ ](BAr'₄ ) gave the cationic derivatives [Mo₂ Cp₂ {μ-κ²_P,E :κ¹_P ,η⁵ -EP(C₆ H₃ tBu₃ )}(CNR)(CO)₂ ](BAr'₄ ) (E=S; R=tBu, iPr, Ph, 4-C₆ H₄ OMe, Xyl; or E=Se; R=tBu; Ar'=3,5-C₆ H₃ (CF₃ )₂ ). Reaction of the latter with K[BHsBu₃ ] yielded the aldimine complexes [Mo₂ Cp₂ {μ-κ²_P,E :κ²_P,N ,η⁴ -SP(C₆ H₃ tBu₃ (CHNR))}(CO)₂ ] and their aminocarbene isomers [Mo₂ Cp₂ {μ-κ²_P,E :κ²_P,C ,η⁴ -SP(C₆ H₃ tBu₃ (NRCH))}(CO)₂ ] (R ≠ Xyl), following C-C and C-N couplings, respectively. Monitoring of these reactions revealed that the initial H^- attack takes place at a Cp ligand to give cyclopentadiene intermediates [Mo₂ Cp{μ-κ²_P,S :κ¹_P ,η⁵ -SP(C₆ H₃ tBu₃ )}(η⁴ -C₅ H₆ )(CNR)(CO)₂ ], which then undergo C-H oxidative addition to give the hydride isomers [Mo₂ Cp₂ {μ-κ²_P,S :κ¹_P ,η³ -SP(C₆ H₃ tBu₃ )}(H)(CNR)(CO)₂ ]. In turn, the latter rearrange to give the aldimine and aminocarbene complexes. DFT calculations revealed that the hydride intermediates first undergo migratory insertion of the isocyanide ligand into the Mo-H bond to give unobservable formimidoyl intermediates, which then evolve either by nucleophilic attack of the N atom on the C₆ ring (C-N coupling) or by migratory insertion of the formimidoyl ligand into the C₆ ring (C-C coupling). Our data suggest that increasing the size of the substituent R at the isocyanide ligand destabilizes the aldimine isomer to a greater extent, thus favoring formation of the aminocarbene complex.

Selective Aromatic C-H Hydroxylation Enabled by η6-Coordination to Iridium(III)

We report an aromatic C-H hydroxylation protocol in which the arene is activated through η⁶-coordination to an iridium(III) complex. η⁶-Coordination of the arene increases its electrophilicity and allows for high positional selectivity of hydroxylation at the site of least electron density. Through investigation of intermediate η⁵-cyclohexadienyl adducts and arene exchange reactions, we evaluate incorporation of arene π-activation into a catalytic cycle for C-H functionalization.

Mechanistic insights from mass spectrometry: examination of the elementary steps of catalytic reactions in the gas phase

Real-time mass spectrometric monitoring of speciation in a catalytic reaction while it is occurring provides powerful insights into mechanistic aspects of the reaction, but cannot be expected to elucidate all details. However, mass spectrometers are not limited just to analysis: they can serve as reaction vessels in their own right, and given their powers of separation and activation in the gas phase, they are also capable of generating and isolating reactive intermediates. We can use these capabilities to help fill in our overall understanding of the catalytic cycle by examining the elementary steps that make it up. This article provides examples of how these simple reactions have been examined in the gas phase.

A metal-mediated boron-centred isomerisation reaction via C–H activation

The terminal borylene ligand is activated by Lewis bases to couple with terminal carbonyl ligands reversibly, initially forming a kinetic coupling product, which isomerises via a borylene migratory insertion to a cyclopentadienyl ligand C–H bond.

Quantitative Determination of the Regioselectivity of Nucleophilic Addition to η-Propargyl Rhenium Complexes and Direct Observation of an Equilibrium Between η-Propargyl Rhenium Complexes and Rhenacyclobutenes

PMe(3) adds selectively to the central carbon of the η(3)- propargyl complex [C(5)Me(5)(CO)(2)Re(η(3)-CH(2)C≡CCMe(3))][BF(4)] (1-t-Bu) to form the metallacyclobutene [C(5)Me(5)(CO)(2)Re(CH(2)C(PMe(3))=CCMe(3))][BF(4)] (7). The rate of rearrangement of the metallacyclobutene 7 to η(2)-alkyne complex [C(5)Me(5)(CO)(2)Re(η(2)-Me(3)PCH(2)C≡CCMe(3))][BF(4)] (8) is is independent of phosphine concentration, consistent with a dissociative mechanism proceeding via η(3)-propargyl complex 1-t-Bu. The rate of this rearrangement is 480 times slower than the rate of exchange of PMe(3) with the labeled metallacyclobutene 7-d(9). This rate ratio provides an indirect measurement of the regioselectivity for addition of PMe(3) to the central carbon of η(3)-propargyl complex 1-t-Bu to give 7 compared to addition to a terminal carbon to give 8. The addition of PPh(3) to 1-t-Bu gives the metallacyclobutene [C(5)Me(5)(CO)(2)Re(CH(2)C(PPh(3))=CCMe(3))][BF(4)] (11). Low temperature (1)H NMR spectra provide evidence for an equilibrium between metallacyclobutene 11 and η(3)-propargyl complex 1-t-Bu (K(eq) ≈ 44 M(-1) at -46 °C and ΔG° (0 °C) = -1.2 ± 0.2 kcal mol(-1)).