Pentamethylcyclopentadienyl Research Articles

Dual Effects of Cyclopentadienyl Ligands on Rh(III)-Catalyzed Dehydrogenative Arylation of Electron-Rich Alkenes

Despite extensive research on transition metal-catalyzed Fujiwara–Moritani type C–H olefinations, the alkenes used in these transformations are still mainly limited to active acrylate esters and styrenes. Selective aryl C–H olefination with electron-rich alkenes is recognized as a challenging issue. We herein report that simple and readily accessible electron-deficient [CpRh(III)] and [CpCF3Rh(III)] (CpCF3 = C5Me4CF3) complexes are powerful catalysts for dehydrogenative arylation of electron-rich alkenes, including vinyl acetates, enamides, and vinyl ethers. Employing an electron-withdrawing Cp or CpCF3 ligand instead of the privileged Cp* (C5Me5) ligand not only can facilitate the electrophilic aryl C–H rhodation but also can lower the olefin insertion barrier. Both electron-withdrawing and electron-donating directing groups such as -CONR2 and -NHAc could be employed in these reactions, which provides convenient routes toward a series styryl acetates, N-acetylindoles, and aryl methyl ketones.

Multifunctional Cyclopentadienes as a Scaffold for Combinatorial Bioorganometallics in [(η5 -C5 H2 R1 R2 R3 )M(CO)3 ] (M=Re, 99m Tc) Piano-Stool Complexes.

Multifunctional cyclopentadiene (Cp) ligands and their rhenium and 99m Tc complexes were prepared by a versatile synthetic route. The properties of these Cp ligands can be tuned on demand, either during their synthesis (variation of R1 ) or through post-synthetic functionalization with two equal or different vectors (V1 and V2 ). Variation of these groups enables a combinatorial approach in the synthesis of bioorganometallic complexes. This is demonstrated by the preparation of Cp ligands containing both electron-donating and electron-withdrawing groups at the R1 position and their subsequent homo- or heterofunctionalization with biovector models (benzylamine and phenylalanine) under standard amide bond-formation conditions. All ligands can be coordinated to the fac-[Re(CO)3 ]+ and fac-[99m Tc(CO)3 ]+ cores to give tetrafunctional complexes in straightforward and functional-group-tolerant procedures. The 99m Tc complexes were prepared in one step, in 30 min, and under aqueous conditions from generator-eluted [99m TcO4 ]- .

Functionalized Cyclopentadienyl Ligands and Their Substituent Effects on a Rhodium(III)‐Catalyzed Oxidative [4+2] Annulation of Indole‐ and Pyrrole‐1‐Carboxamides with Alkynes

AbstractThe effect of substituents on carbamoylmethyl‐cyclopentadienyl (CpA) ligands on the neutral rhodium(III)‐catalyzed oxidative [4+2] annulation of indole‐ and pyrrole‐1‐carboxamides with alkynes, in which the C−H bond cleavage is the partially rate‐limiting step, was investigated. As a result, in the reactions with terminal alkynes, a rhodium(III) complex with a dimethyl‐substituted CpA ligand (CpA3) showed high catalytic activity and improved the regioselectivity as compared to a commercially available Cp*RhIII complex. On the other hand, in reactions with internal alkynes, a rhodium(III) complex with a diphenyl‐substituted CpA ligand (CpA1 or CpA2) showed high catalytic activity, which is comparable to the activity of the Cp*RhIII complex. Interestingly, a rhodium(III) complex with an electron‐deficient di(ethoxycarbonyl)‐substituted Cp ligand (CpE) that shows high catalytic activity toward the annulation of benzamides with internal alkynes, in which the C−H cleavage is rate‐limiting, showed low catalytic activity toward the reactions with both terminal and internal alkynes. The ligand effects above provide important guidelines for the application of our CpA and CpE ligands to the rhodium(III)‐catalyzed C−H bond functionalization reactions.

A Readily Accessible Class of Chiral Cp Ligands and their Application in RuII -Catalyzed Enantioselective Syntheses of Dihydrobenzoindoles.

Chiral cyclopentadienyl (Cpx ) ligands have a large application potential in enantioselective transition-metal catalysis. However, the development of concise and practical routes to such ligands remains in its infancy. We present a convenient and efficient two-step synthesis of a novel class of chiral Cpx ligands with tunable steric properties that can be readily used for complexation, giving Cpx RhI , Cpx IrI , and Cpx RuII complexes. The potential of this ligand class is demonstrated with the latter in the enantioselective cyclization of azabenzonorbornadienes with alkynes, affording dihydrobenzoindoles in up to 98:2 e.r., significantly outperforming existing binaphthyl-derived Cpx ligands.

A Readily Accessible Class of Chiral Cp Ligands and their Application in RuII‐Catalyzed Enantioselective Syntheses of Dihydrobenzoindoles

AbstractChiral cyclopentadienyl (Cpx) ligands have a large application potential in enantioselective transition‐metal catalysis. However, the development of concise and practical routes to such ligands remains in its infancy. We present a convenient and efficient two‐step synthesis of a novel class of chiral Cpx ligands with tunable steric properties that can be readily used for complexation, giving CpxRhI, CpxIrI, and CpxRuII complexes. The potential of this ligand class is demonstrated with the latter in the enantioselective cyclization of azabenzonorbornadienes with alkynes, affording dihydrobenzoindoles in up to 98:2 e.r., significantly outperforming existing binaphthyl‐derived Cpx ligands.

Recyclable rhodium-Cp′-heterogenized catalysts for hydrogenation of olefins

Immobilized catalysts were prepared by mixing the ligand Cp′, [C5Me4(n-propyl-Si(OEt)3)], with RhCl3·xH2O for subsequent reaction with silica Aerosil 200 (system A) or by copolymerization with TEOS (system B). Both systems were applied on olefin hydrogenation, showing high activity and robustness, allowing for many recycles (TON = 1.0 × 105 for system A and 2.0 × 104 for system B). The characterization techniques (IR-DRIFTS, 13C-CPMAS, and TEM) suggest a molecular nature for catalytic species. According to the deuteration experiment, a tetra-hapto-Cp′-Rh would be formed on the surface during the catalytic runs.

Pharmaco-genomic investigations of organo-iridium anticancer complexes reveal novel mechanism of action.

Resistance to platinum drugs (used in >50% of cancer chemotherapies) is a clinical problem. Other precious metal complexes with distinct mechanisms of action might overcome this. Half-sandwich organometallic complexes containing arene or cyclopentadienyl (Cp) ligands show promise. We screened two iridium(iii) complexes [Ir(CpXbiph)(ppy)Cl] (ZL49, 1, ppy = phenylpyridine) and [Ir(CpXph)(azpyNMe2)Cl]PF6 (ZL109, 2, azpyNMe2 = N,N-dimethylphenylazopyridine) in 916 cancer cell lines from 28 tissue types. On average, complex 2 was 78× more potent than 1, 36× more active than cisplatin (CDDP), and strongly active (nanomolar) in patient-derived ovarian cancer cell lines. RNA sequencing of A2780 ovarian cells revealed upregulation of antioxidant responses (NRF2, AP-1) consistent with observed induction of reactive oxygen species (ROS). Protein microarrays, high content imaging and cell cycle analysis showed S/G2 arrest, and late-stage DNA damage response without p53 requirement. The triple-negative breast cancer cell line OCUB-M was highly sensitive to 2 as were cell lines with KIT mutations. Complex 2 exhibits a markedly different pattern of antiproliferative activity compared to the 253 drugs in the Sanger Cancer Genome database, but is most similar to osmium(ii) arene complexes which share the same azopyridine ligand. Redox modulation and DNA damage can provide a multi-targeting strategy, allowing compounds such as 2 to overcome cellular resistance to platinum anticancer drugs.

Electronic and Steric Control of the Spin-Crossover Behavior in [(CpR)2Mn] Manganocenes.

A computational study of the spin-crossover behavior in the family [(CpR)2Mn] (R = Me, iPr, tBu) is presented. Using the OPBE functional, the different electronic and steric effects over the metal's ligand field are studied, and trends in the spin-crossover-temperature (T1/2) behavior are presented in terms of the cyclopentadienyl (Cp) ligand functionalization. Our calculations outlined a delicate balance between both electronic and steric effects. While an increase in the number of electron-donating groups increases the spin-crossover temperature (T1/2) to the point that the transition is suppressed and only the low-spin state is observed, steric effects play an opposite role, increasing the distance between the Cp rings, which in turns shifts T1/2 to lower values, eventually stabilizing the high-spin state. Both effects can be rationalized by exploring the electronic structure of such systems in terms of the relevant d-based molecular orbitals.

Transition-Metal Complexes Featuring Dianionic Heavy Group 14 Element Aromatic Ligands.

The synthesis of dilithio-stannoles and -plumboles, dianionic aromatic compounds containing tin and lead atoms in their π-skeletons, opened a new field of transition-metal chemistry. Since the discovery of ferrocene (Cp2Fe), which is composed of anionic aromatic ligands (Cp: cyclopentadienyl) and Fe(II), ferrocene-type sandwich complexes have long played important roles in many fields of chemistry. During the last few decades, the electronic and structural properties of the Cp ligand have been modified by introducing electron-donating, electron-withdrawing, and sterically encumbered substituents on the skeletal carbon atoms to obtain desirable properties of the resulting sandwich complexes. In terms of modifying the Cp ligand, we focused our attention on introducing a heavy group 14 atom into the π-skeleton. This idea was originally inspired by a question of whether or not aromaticity was retained after the replacement of a skeletal carbon atom by a heavy group 14 atom. After we succeeded in the synthesis of aromatic dilithio-stannoles and -plumboles, revealing that the concept of conventional aromaticity was expanded to lead-containing π-systems, we undertook the present project on applying these dianionic aromatic heavy Cp analogues as ligands for transition-metal complexes. The combination of a stannole and Cp*Ru units accomplished the creation of a neutral triple-decker complex and an anionic ruthenocene, which was not be accessible using Cp and its related ligands that are composed of only carbon atoms. The anionic ruthenocene reacted with electrophiles to afford ruthenocene-type sandwich complexes, and the structures of the stannole skeletons were highly dependent on the substituents on the tin atoms, in sharp contrast to the planar Cp ligand. The dianionic plumbole ligand was also found to function as an η5-coordinating ligand in an anionic ruthenocene, which is noteworthy in terms of incorporating the heaviest group 14 atom into a π-ligand to produce a ferrocene-type sandwich complex. The anionic ruthenocene bearing the plumbole ligand reacted with electrophiles to afford ruthenocene-type plumbole complexes, which have oxidation potentials lower than those of the corresponding tin analogues, demonstrating the effect of introduction of a lead atom heavier than a tin atom. In the reactions of dilithiostannoles with group 4 metals, the resulting complexes were found to have exotic electronic structures that cannot be constructed by the Cp ligand. The transition-metal complexes derived from dilithio-stannoles and -plumbole therefore exhibit remarkable differences as well as similarities to the traditional Cp-based transition-metal complexes. These results spotlight the introduction of heavy group 14 atoms into carbon-based π-skeletons, which can perturb the electronic properties of conventional transition-metal complexes and open a new chemistry of transition-metal complexes.

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

The reactivity of the oxo ReV 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 ReIII 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.

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

AbstractThe reactivity of the oxo ReV 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 ReIII 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, tBuNC was found to attack at the oxo moiety to produce isocyanate by oxygen atom transfer.

Palladium-Catalyzed Decarboxylation of Benzyl Fluorobenzoates

The decarboxylation of benzyl fluorobenzoates has been developed by using the palladium catalyst prepared in situ from Pd(η3-allyl)Cp and bulky monophosphine ligand XPhos. The catalytic reaction afforded a range of fluorinated diarylmethanes in good yields with broad functional-group compatibility. The substrates were readily synthesized by condensation of the corresponding benzoic acid with benzyl alcohol. Therefore, the transformation is formally regarded as a cross-coupling reaction between fluorine-containing benzoic acids and benzyl alcohols.

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-κ2P,E :κ1P ,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-κ2P,E :κ2P,N ,η4 -SP(C6 H3 tBu3 (CHNR))}(CO)2 ] and their aminocarbene isomers [Mo2 Cp2 {μ-κ2P,E :κ2P,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-κ2P,S :κ1P ,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-κ2P,S :κ1P ,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.

Combined Experimental and Computational Study on Ruthenium(II)-Catalyzed Reactions of Diynes with Aldehydes and N,N-Dimethylformamide.

Cycloaddition reactions of 1,6-diynes bearing methyl terminal groups with p-anisaldehyde were conducted using a cationic ruthenium catalyst with a η5-pentamethylcyclopentadienyl ligand in THF at room temperature to afford dienyl ketones via ring opening of the initially formed fused pyrans. (Z)-Stereoisomers of dienyl ketones were selectively obtained using the ruthenium catalyst, whereas previously reported rhodium catalysts produced (E)-isomers. These (E)- and (Z)-selectivities are kinetically controlled as the control experiments showed that the E/Z-isomerization of (E)-dienylketone occurs at 70 °C for 10 h to afford an E/Z-ratio of almost 1:1. The origin of this characteristic stereoselectivity for the ruthenium catalyst was attributed to the direct ring opening of the CpRu+-coordinated pyran complex intermediates on the basis of theoretical calculations [PCM (THF) M06L/SDD-6-311++G(d,p)//B3LYP/LanL2DZ-6-31G(d)] and control experiments. The (Z)-selectivity increased when the bulkiness of the diyne terminal substituents increased. Notably, the reaction of 1,6-diynes bearing tert-butyl terminal groups with various α,β-unsaturated aldehydes exclusively afforded (Z)-dienyl ketones even at 70 °C when a cationic ruthenium complex with a smaller η5-cyclopentadienyl (Cp) ligand was used as the catalyst. The same Cp complex was found to be also efficient for the hydrocarbamoylative cyclization of sterically demanding 1,6-diynes bearing tertiary or quaternary carbon tethers with N,N-dimethylformamide.

Activation of Molecular Hydrogen by Bis(η5 ,η1 -pentafulvene)-titanium Complexes - Efficient Formation of Titanium(III)hydrides

The synthesis and crystal structures of two dinuclear titanocene hydride complexes are reported. Both complexes, namely bis(η5-(di-para-tolylmethyl)cyclopentadienyl)titanium hydride dimer, [(η5-C20H19)2Ti(μ-H)]2 (2a), and bis(η5-2-adamantylcyclopentadienyl)-titanium hydride dimer, [(η5-C15H19)2Ti(μ-H)]2 (2b), are formed via activation of molecular hydrogen by the corresponding bis(η5,η1-pentafulvene)titanium complexes 1a and 1b at ambient temperatures and pressures in high yields. The hydride complexes 2a and 2b exhibit planar [Ti2H2] cores and, as a result of the heterolytic cleavage of molecular hydrogen, substituted Cp Ligands were formed during the reaction.

Ken Tanaka

Ken Tanaka

A Trinuclear Radical‐Bridged Lanthanide Single‐Molecule Magnet

Assembly of the triangular, organic radical-bridged complexes Cp*6 Ln3 (μ3 -HAN) (Cp*=pentamethylcyclopentadienyl; Ln=Gd, Tb, Dy; HAN=hexaazatrinaphthylene) proceeds through the reaction of Cp*2 Ln(BPh4 ) with HAN under strongly reducing conditions. Significantly, magnetic susceptibility measurements of these complexes support effective magnetic coupling of all three LnIII centers through the HAN3-. radical ligand. Thorough investigation of the DyIII congener through both ac susceptibility and dc magnetic relaxation measurements reveals slow relaxation of the magnetization, with an effective thermal relaxation barrier of Ueff =51 cm-1 . Magnetic coupling in the DyIII complex enables a large remnant magnetization at temperatures up to 3.0 K in the magnetic hysteresis measurements and hysteresis loops that are open at zero-field up to 3.5 K.

General Enantioselective C-H Activation with Efficiently Tunable Cyclopentadienyl Ligands.

Cyclopentadienyl (Cp) ligands enable efficient steering of various transition-metal-catalyzed transformations, in particular enantioselective C-H activation. Currently only few chiral Cp ligands are available. Therefore, a conceptually general approach to chiral Cp ligand discovery would be invaluable as it would enable the discovery of applicable Cp ligands and to efficiently and rapidly vary and tune their structures. Herein, we describe the three-step gram-scale synthesis of a structurally diverse and widely applicable chiral Cp ligand collection (JasCp ligands) with highly variable and adjustable structures. Their modular nature and their amenability to rapid structure variation enabled the efficient discovery of ligands for three enantioselective RhIII -catalyzed C-H activation reactions, including one unprecedented transformation. This novel approach should enable the discovery of efficient chiral Cp ligands for various further enantioselective transformations.

General Enantioselective C−H Activation with Efficiently Tunable Cyclopentadienyl Ligands

AbstractCyclopentadienyl (Cp) ligands enable efficient steering of various transition‐metal‐catalyzed transformations, in particular enantioselective C−H activation. Currently only few chiral Cp ligands are available. Therefore, a conceptually general approach to chiral Cp ligand discovery would be invaluable as it would enable the discovery of applicable Cp ligands and to efficiently and rapidly vary and tune their structures. Herein, we describe the three‐step gram‐scale synthesis of a structurally diverse and widely applicable chiral Cp ligand collection (JasCp ligands) with highly variable and adjustable structures. Their modular nature and their amenability to rapid structure variation enabled the efficient discovery of ligands for three enantioselective RhIII‐catalyzed C−H activation reactions, including one unprecedented transformation. This novel approach should enable the discovery of efficient chiral Cp ligands for various further enantioselective transformations.

4+2] or [4+1] Annulation: Changing the Reaction Pathway of a Rhodium‐Catalyzed Process by Tuning the Cp Ligand

AbstractA change in reaction pathway was achieved for the first time by tuning the cyclopentadienyl (Cp) ligand used for the rhodium‐catalyzed cyclization of benzamides with conjugated enynones. Depending on the Cp ligand, the reaction pathway switched between [4+2] and [4+1] annulation. Electronic effects turned out to be crucial for the product distribution. The dichotomy was attributed to the alteration of the Lewis acidity of the resultant Cp‐bound rhodium species.