First Stabilization of 14‐Electron Rhodium(I) Complexes by Hemichelation

Hemichelation is emerging as a new mode of coordination where non‐covalent interactions crucially contribute to the cohesion of electron‐unsaturated organometallic complexes. This study discloses an unprecedented demonstration of this concept to a Group 9 metal, that is, Rh^I. The syntheses of new 14‐electron Rh^I complexes were achieved by choosing the anti‐[(η⁶:η⁶‐fluorenyl){Cr(CO)₃}₂] anion as the ambiphilic hemichelating ligand, which was treated with [{Rh(nbd)Cl}₂] (nbd=norbornadiene) and [{Rh(CO)₂Cl}₂]. The new T‐shaped Rh^I hemichelates were characterized by analytical and structural methods. Investigations using the methods of the DFT and electron‐density topology analysis (NCI region analysis, QTAIM theory) confirmed the closed‐shell, non‐covalent and attractive characters of the interaction between the Rh^I center and the proximal Cr(CO)₃ moiety. This study shows that, by appropriate tuning of the electronic properties of the ambiphilic ligand, truly coordination‐unsaturated Rh^I complexes can be synthesized in a manageable form.     

Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Alkynes through CH Activation

The efficient Rh^I‐catalyzed cycloisomerization of benzylallene‐alkynes produced the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton through a C? H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C? H bond to Rh^I, an ene‐type cyclization to the vinylidenecarbene–Rh^I intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.     

Visible Light Induced Rhodium(I)‐Catalyzed C−H Borylation

An efficient visible light induced rhodium(I)‐catalyzed regioselective borylation of aromatic C?H bonds is reported. The photocatalytic system is based on a single NHC?Rh^I complex capable of both harvesting visible light and enabling the bond breaking/forming at room temperature. The chelating nature of the NHC‐carboxylate ligand was critical to ensure the stability of the Rh^I complex and to provide excellent photocatalytic activities. Experimental mechanistic studies evidenced a photooxidative ortho C?H bond addition upon irradiation with blue LEDs, leading to a cyclometalated Rh^III‐hydride intermediate.     

Rhodium(I)‐Catalyzed Sequential C(sp)C(sp3) and C(sp3)C(sp3) Bond Formation through Migratory Carbene Insertion

A Rh^I‐catalyzed three‐component reaction of tert‐propargyl alcohol, diazoester, and alkyl halide has been developed. This reaction can be considered as a carbene‐involving sequential alkyl and alkynyl coupling, in which C(sp)? C(sp³) and C(sp³)? C(sp³) bonds are built successively on the carbenic carbon atom. The Rh^I‐carbene migratory insertion of an alkynyl moiety and subsequent alkylation are proposed to account for the two separate C? C bond formations. This reaction provides an efficient and tunable method for the construction of all‐carbon quaternary center.     

Enantioselective Rhodium‐Catalyzed Cycloisomerization of (E)‐1,6‐Enynes

An enantioselective rhodium(I)‐catalyzed cycloisomerization reaction of challenging (E)‐1,6‐enynes is reported. This novel process enables (E)‐1,6‐enynes with a wide range of functionalities, including nitrogen‐, oxygen‐, and carbon‐tethered (E)‐1,6‐enynes, to undergo cycloisomerization with excellent enantioselectivity, in a high‐yielding and operationally simple manner. Moreover, this Rh^I‐diphosphane catalytic system also exhibited superior reactivity and enantioselectivity for (Z)‐1,6‐enynes. A rationale for the striking reactivity difference between (E)‐ and (Z)‐1,6‐enynes using Rh^I‐BINAP and Rh^I‐TangPhos is outlined using DFT studies to provide the necessary insight for the design of new catalyst systems and the application to synthesis.     

Mechanochemical Rhodium(III)‐ and Gold(I)‐Catalyzed C−H Bond Alkynylations of Indoles under Solventless Conditions in Mixer Mills

Mechanochemical activations in Rh^III‐ and Au^I‐catalyzed C?H alkynylations lead selectively to C₂‐ and C₃‐alkynylated indoles. The processes show excellent functional group tolerance, do not require additional heating and proceed under solventless conditions. Compared to solvent‐based standard protocols, the reaction times are shorter and the catalyst quantities lower resulting in high product yields under ambient atmosphere in mixer mills.     

Transition States of Binap–Rhodium(I)‐Catalyzed Asymmetric Hydrogenation: Theoretical Studies on the Origin of the Enantioselectivity

By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap–Rh^I‐catalyzed oxidative‐addition and insertion steps of the asymmetric hydrogenation of the enamide 2‐acetylamino‐3‐phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the “lock‐and‐key” motif and leads to the major enantiomeric product via an energetically favorable binap–dihydride–Rh^III–enamide complex. Our theoretical results are consistent with the mechanism that takes place via Rh^III dihydride formation, that is, oxidative addition of H₂ followed by enamide insertion.     

Polymerizations Mediated by Well‐Defined Rhodium Complexes

This Minireview details the current state‐of‐the‐art relating to (co)polymerizations mediated by well‐defined Rh^I‐ethynyl, vinyl, and aryl complexes. In particular, we focus on Rh^I species suitable for the (co)polymerization of phenylacetylenes, arylisocyanides, as well as propargyl esters and amides.     

α‐MsO/TsO/Cl Ketones as Oxidized Alkyne Equivalents: Redox‐Neutral Rhodium(III)‐Catalyzed CH Activation for the Synthesis of N‐Heterocycles

α‐Halo and pseudohalo ketones are used for the first time as C(sp³)‐based electrophiles in transition‐metal‐catalyzed C? H activation and as oxidized alkyne equivalents in Rh^III‐catalyzed redox‐neutral annulations to generate diverse N‐heterocycles. This transformation is efficient and scalable. Due to the mild reaction conditions, a variety of functional groups could be tolerated.     

Facile Synthesis and Versatile Reactivity of an Unusual Cyclometalated Rhodium(I) Pincer Complex

The synthesis of the reactive PN(C^H) ligand 2‐di(tert‐butylphosphanomethyl)‐6‐phenylpyridine ( 1^H ) and its versatile coordination to a Rh^I center is described. Facile C?H activation occurs in the presence of a (internal) base, thus resulting in the new cyclometalated complex [Rh^I(CO)(κ³‐P,N,C‐ 1 )] ( 3 ), which has been structurally characterized. The resulting tridentate ligand framework was experimentally and computationally shown to display dual‐site proton‐responsive reactivity, including reversible cyclometalation. This feature was probed by selective H/D exchange with [D₁]formic acid. The addition of HBF₄ to 3 leads to rapid net protonolysis of the Rh?C bond to produce [Rh^I(CO)(κ³‐P,N,(C?H)‐ 1 )] ( 4 ). This species features a rare aryl C?H agostic interaction in the solid state, as shown by X‐ray diffraction studies. The nature of this interaction was also studied computationally. Reaction of 3 with methyl iodide results in rapid and selective ortho‐methylation of the phenyl ring, thus generating [Rh^I(CO)(κ²‐P,N‐ 1^Me )] ( 5 ). Variable‐temperature NMR spectroscopy indicates the involvement of a Rh^III intermediate through formal oxidative addition to give trans‐[Rh^III(CH₃)(CO)(I)(κ³‐P,N,C‐ 1 )] prior to C?C reductive elimination. The Rh^III–trans‐diiodide complex [Rh^I(CO)(I)₂(κ³‐P,N,C‐ 1 )] ( 6 ) has been structurally characterized as a model compound for this elusive intermediate.     

Investigation and Comparison of the Mechanistic Steps in the [(Cp*MCl2)2] (Cp*=C5Me5; M=Rh,Ir)‐Catalyzed Oxidative Annulation of Isoquinolones with Alkynes

The mechanism of the [(Cp*MCl₂)₂] (M=Rh, Ir)‐catalyzed oxidative annulation reaction of isoquinolones with alkynes was investigated in detail. In the first acetate‐assisted C? H‐activation process (cyclometalated step) and the subsequent mono‐alkyne insertion into the M? C bonds of the cyclometalated compounds, both Rh and Ir complexes participated well. However, the desired final products, dibenzo[a,g]quinolizin‐8‐one derivatives, were only formed in high yield when the Rh species participated in the final oxidative coupling of the C? N bond. Moreover, a Rh^I sandwich intermediate was isolated during this transformation. The iridium complexes were found to be inactive in the oxidative coupling processes. All of the relevant intermediates were fully characterized and determined by single‐crystal X‐ray diffraction analysis. Based on this mechanistic study, a Rh^III→Rh^I→Rh^III catalytic cycle was proposed for this reaction.     

A Chemical–Biological Evaluation of Rhodium(I) N‐Heterocyclic Carbene Complexes as Prospective Anticancer Drugs

Rhodium(I) complexes bearing N‐heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry, but there are very few reports of biological properties of these organometallics. A series of Rh^I‐NHC derivatives with 1,5‐cyclooctadiene and CO as secondary ligands were synthesized, characterized, and biologically investigated as prospective antitumor drug candidates. Pronounced antiproliferative effects were noted for all complexes, along with moderate inhibitory activity of thioredoxin reductase (TrxR) and efficient binding to biomolecules (DNA, albumin). Biodistribution studies showed that the presence of albumin lowered the cellular uptake and confirmed the transport of rhodium into the nuclei. Changes in the mitochondrial membrane potential (MMP) were observed as well as DNA fragmentation in wild‐type and daunorubicin‐ or vincristine‐resistant Nalm‐6 leukemia cells. Overall, these studies indicated that Rh^I‐NHC fragments could be used as partial structures of new antitumor agents, in particular in those drugs designed to address resistant malignant tissues.     

Capturing HBCy2: Using N,O‐Chelated Complexes of Rhodium(I) and Iridium(I) for Chemoselective Hydroboration

1,3‐N,O‐chelated complexes of Rh^I and Ir^I cooperatively and reversibly stabilized the B?H bond of HBCy₂ to afford six‐membered metallaheterocycles (M=Rh ( 7 ) or Ir ( 8 )) having a δ‐[M]???H‐B agostic interaction. Treatment of these Shimoi‐type borane adducts 7 or 8 with both an aldehyde and an alkene resulted in chemoselective aldehyde hydroboration and reformation of the 1,3‐N,O‐chelated starting material. The observed chemoselectivity is inverted from that of free HBCy₂, which is selective for alkene hydroboration.     

An Enantioselective CpxRh(III)‐Catalyzed C−H Functionalization/Ring‐Opening Route to Chiral Cyclopentenylamines

A chiral Cp^xRh^III catalyst system in situ generated from a Cp^xRh^I(cod) precatalyst and bis(o‐toluoyl) peroxide as activating oxidant was developed for a C?H activation/ring‐opening sequence between aryl ketoxime ethers and 2,3‐diazabicyclo[2.2.1]hept‐5‐enes. This transformation provides access to densely functionalized chiral cyclopentenylamines in excellent yields and enantioselectivities of up to 97:3 er. The reported method is also well suitable for asymmetric alkenyl C?H functionalizations of α,β‐unsaturated oxime ethers, furnishing skipped dienes with high levels of enantiocontrol.     

New rhodium(I) water‐soluble complexes with 1‐alkyl‐1‐azonia‐3,5‐diaza‐7‐phospha‐adamantane iodides and their catalytic activity

The water‐soluble phosphine ligands, 1,3,5‐triaza‐7‐phosphatricyclo[3.3.1.1^3,7]decane (tpa) and 1‐alkyl‐1‐azonia‐3,5‐diaza‐7‐phosphatricyclo[3.3.1.1^3,7]decane iodides (Rtpa⁺I⁻), with alkyl=methyl(mtpa⁺I⁻), ethyl (etpa⁺I⁻) and n‐propyl, (ptpa⁺I⁻), and mtpa⁺Cl⁻ react with [Rh₂Cl₂(CO)₄] giving the rhodium(I) complexes [RhCl(CO)(tpa)₂], [RhI(CO)(Rtpa⁺I⁻)₂], [RhCl‐­(CO)(mtpa⁺Cl⁻)₃] and [RhI(CO)(Rtpa⁺I⁻)₃]. The properties and reactivities of the complexes have been investigated using ¹H and ³¹PNMR and IR spectroscopies. The five‐coordinate complexes in solutions show dynamic properties. The complexes are catalysts of the water‐gas shift reaction, the hydrogenation of CC and CO bonds, the hydroformylation of alkenes and the isomerization of unsaturated compounds. Copyright © 1999 John Wiley & Sons, Ltd.     

Unprecedented Dearomatized Spirocyclopropane in a Sequential Rhodium(III)‐Catalyzed C−H Activation and Rearrangement Reaction

An unprecedented dearomatized spirocyclopropane intermediate was discovered in a sequential Cp*Rh^III‐catalyzed C?H activation and Wagner–Meerwein‐type rearrangement reaction. How the oxidative O?N bond is cleaved and the role of HOAc were uncovered in this study. Furthermore, a Cp*Rh^III‐catalyzed dearomatization reaction of N‐(naphthalen‐1‐yloxy)acetamide with strained olefins was developed, affording a variety of spirocyclopropanes.     

Rhodium(III)‐Catalyzed Annulation of 2‐Alkenyl Anilides with Alkynes through C−H Activation: Direct Access to 2‐Substituted Indolines

A Rh^III complex featuring an electron‐deficient η⁵‐cyclopentadienyl ligand catalyzed an unusual annulation between alkynes and 2‐alkenyl anilides to form synthetically appealing 2‐substituted indolines. Formally, the process can be viewed as an allylic amination with concomitant hydrocarbonation of the alkyne. Mechanistic experiments indicate that this transformation involves an unusual rhodium migration with a concomitant 1,5‐H shift.     

A Very Short Uranium(IV)–Rhodium(I) Bond with Net Double‐Dative Bonding Character

Reaction of [U{C(SiMe₃)(PPh₂)}(BIPM)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)_0.5]₂ (BIPM=C(PPh₂NSiMe₃)₂; TMEDA=Me₂NCH₂CH₂NMe₂) with [Rh(μ‐Cl)(COD)]₂ (COD=cyclooctadiene) affords the heterotrimetallic U^IV?Rh^I₂ complex [U(Cl)₂{C(PPh₂NSiMe₃)(PPh[C₆H₄]NSiMe₃)}{Rh(COD)}{Rh(CH(SiMe₃)(PPh₂)}]. This complex has a very short uranium–rhodium distance, the shortest uranium–rhodium bond on record and the shortest actinide–transition metal bond in terms of formal shortness ratio. Quantum‐chemical calculations reveal a remarkable Rh U^IV net double dative bond interaction, involving Rh^I 4d ‐ and 4d_xy/xz‐type donation into vacant U^IV 5f orbitals, resulting in a Wiberg/Nalewajski–Mrozek U?Rh bond order of 1.30/1.44, respectively. Despite being, formally, purely dative, the uranium–rhodium bonding interaction is the most substantial actinide–metal multiple bond yet prepared under conventional experimental conditions, as confirmed by structural, magnetic, and computational analyses.     

CO Gas‐free Intramolecular Cyclocarbonylation Reactions of Haloarenes Having a C‐Nucleophile through CO‐Relay between Rhodium and Palladium

We describe transfer carbonylation reactions of 2‐bromoarenes that contain a carbon‐nucleophile using aldehydes as a substitute for CO, leading to the formation of indanone derivatives. The transformation proceeds efficiently under Rh^I/Pd⁰‐hybrid catalytic conditions consisting of two discrete transition metals, rhodium and palladium, which catalyze the decarbonylation of aldehydes and the subsequent carbonylation of bromoarenes using the resulting carbonyl moiety, respectively. The majority of the abstracted CO is transferred directly to the product via a CO‐relay process from rhodium to palladium.     

Chiral Carboxylic Acid Enabled Achiral Rhodium(III)‐Catalyzed Enantioselective C−H Functionalization

Reported is an achiral Cp^xRh^III/chiral carboxylic acid catalyzed asymmetric C?H alkylation of diarylmethanamines with a diazomalonate, followed by cyclization and decarboxylation to afford 1,4‐dihydroisoquinolin‐3(2H)‐one. Secondary alkylamines as well as nonprotected primary alkylamines underwent the transformation with high enantioselectivities (up to 98.5:1.5 e.r.) by using a newly developed chiral carboxylic acid as the sole source of chirality to achieve enantioselective C?H cleavage by a concerted metalation‐deprotonation mechanism.