Cationic Cp Research Articles

Molar-Ratio-Dependent Coordination Assembly of Organoiridium(III)-Octatungstate Complexes in Aqueous Solution

We scrutinized the speciation of Cp*Ir-containing tungsten oxide clusters (Cp* is pentamethylcyclopentadienyl anion) in aqueous mixtures of [(Cp*IrCl)2(μ-Cl)2] and Na2WO4 in varying molar ratios. 1H nuclear magnetic resonance (NMR) spectroscopy revealed the formation of three distinct Cp*Ir-polyoxotungstate species in the reaction solution, and they were isolated as Na4[(Cp*Ir)2(μ-OH)3]2[(Cp*Ir)2H2W8O30] (1), [(Cp*Ir)2(μ-OH)3]2[(Cp*Ir)2{Cp*Ir(OH2)}2H2W8O30] (2), and [(Cp*Ir)2{Cp*Ir(OH2)}2{Cp*Ir(OH2)2}2H2W8O30](NO3)2 (3) from the mixtures in which iridium concentration is less than, equal to, and more than the tungsten concentration, respectively. These results show the octatungstate [H2W8O30]10- anion is the major polyoxotungstate species in the presence of {Cp*Ir}2+ cations, and it has high nucleophilicity enough to bind up to six {Cp*Ir}2+ cations on its surfaces producing a cationic Cp*Ir-octatungstate complex. The octatungstate anion was also generated from the reaction of [(Cp*IrCl)2(μ-Cl)2] and methylammonium paratungstate-B, (CH3NH3)10[H2W12O42], and was isolated as a methylamine-coordinated complex (CH3NH3)2[(Cp*Ir)2{Cp*Ir(NH2CH3)}2H2W8O30] (4), indicating {Cp*Ir}2+ cations function as a structure-directing agent that converts tungsten species into octatungstate anions in aqueous solution. In addition, the coordination environment of {Cp*Ir}2+ can be further modified by coordination with pyridine forming [{Cp*Ir(NC5H5)}2(μ-OH)2][(Cp*Ir)2{Cp*Ir(NC5H5)}2H2W8O30] (5).

Ligand-Dependant Selective Synthesis of Mono-and Dialkenylcarbazoles through Rhodium(III)-Catalyzed C-H Alkenylation.

The C-H alkenylation of N-acetylcarbazoles proceeds smoothly at the C1-position in the presence of a cationic Cp*Rh(III) catalyst to produce 1-alkenylcarbazoles. The use of a cationic CpERh(III) catalyst enables further alkenylation to give 1,8-dialkenylcarbazoles. The direct alkenylation procedure in combination with the ready removal of the acetyl directing group provides a straightforward synthetic pathway to 1- and/or 8-alkenyl-N-H-carbazole derivatives. One of 1-alkenyl-N-H-carbazoles obtained by the present C-H alkenylation/deacetylation exhibits solvatochromism.

Rhodium(III)‐Catalyzed Regioselective C−H Annulation and Alkenylation of 2‐Pyridones with Terminal Alkynes

AbstractCp*Rh(III)‐catalyzed regioselective C−H annulation and alkenylation of 2‐pyridones with terminal alkynes have been developed. The cationic Cp*Rh(III) catalytic system containing FeCl3 additive enables annulation of 1‐(2‐pyridyl)‐2‐pyridones with terminal alkynes, providing efficient access to 5,7‐diarylated 2‐quinolinones. The reaction pathway can be switched to alkenylation with [Cp*RhCl2]2 as the catalyst, NaOAc as the additive and HOAc as the solvent, affording C6‐alkenylated 1‐(2‐pyridyl)‐2‐pyridones in high yields. These protocols accommodate a wide range of substrates with good functional group compatibility.magnified image

Uptake, Trapping, and Release of Organometallic Cations by Redox-Active Cationic Hosts.

The host-guest chemistry of metal-organic nanocages is typically driven by thermodynamically favorable interactions with their guests such that uptake and release of guests can be controlled by switching this affinity on or off. Herein, we achieve this effect by reducing porphyrin-walled cationic nanoprisms 1a12+ and 1b12+ to zwitterionic states that rapidly uptake organometallic cations Cp*2Co+ and Cp2Co+, respectively. Cp*2Co+ binds strongly (Ka = 1.3 × 103 M-1) in the neutral state 1a0 of host 1a12+, which has its three porphyrin walls doubly reduced and its six (bipy)Pt2+ linkers singly reduced (bipy = 2,2'-bipyridine). The less-reduced states of the host 1a3+ and 1a9+ also bind Cp*2Co+, though with lower affinities. The smaller Cp2Co+ cation binds strongly (Ka = 1.7 × 103 M-1) in the 3e- reduced state 1b9+ of the (tmeda)Pt2+-linked host 1b12+ (tmeda = N,N,N',N'-tetramethylethylenediamine). Upon reoxidation of the hosts with Ag+, the guests become trapped to provide unprecedented metastable cation-in-cation complexes Cp*2Co+@1a12+ and Cp2Co+@1b12+ that persist for >1 month. Thus, dramatic kinetic effects reveal a way to confine the guests in thermodynamically unfavorable environments. Experimental and DFT studies indicate that PF6- anions kinetically stabilize Cp*2Co+@1a12+ through electrostatic interactions and by influencing conformational changes of the host that open and close its apertures. However, when Cp*2Co+@1a12+ was prepared using ferrocenium (Fc+) instead of Ag+ to reoxidize the host, dissociation was accelerated >200× even though neither Fc+ nor Fc have any observable affinity for 1a12+. This finding shows that metastable host-guest complexes can respond to subtler stimuli than those required to induce guest release from thermodynamically favorable complexes.

Relationship Between Energy Landscape Shape and Dynamics Trajectory Outcomes for Methane C–H Activation by Cationic Cp*(PMe3)Ir/Rh/Co(CH3)

Density functional theory (DFT) calculations are routinely used to determine organometallic reaction mechanisms. However, these calculations only represent an average structure and lack mechanistic...

Silicon(II) Cation Cp*Si:+ X–: A New Class of Efficient Catalysts in Organosilicon Chemistry

The catalytic activity of the pentamethylcyclopentadienylsilicon(II) cation Cp*Si:+ was investigated. It was shown that Cp*Si:+ efficiently catalyzes reactions of technical relevance in organosilicon chemistry: Cp*Si:+ proved to be a very efficient nonmetallic catalyst for the hydrosilylation of olefins at low catalyst amounts of <0.01 mol % and for the Piers–Rubinsztajn reaction in order to make controlled silicone topologies. The thermal induction of hydrosilylation which is important for the manufacturing of silicone rubber can be achieved by small amounts of alkoxysilanes.

Direct dynamics analysis of the cationic Cp*(PMe3)Ir(CH3) methane C-H activation mechanism.

For the σ-bond metathesis reaction between methane and cationic Cp*(PMe3)IrIII(CH3), previous static DFT calculations revealed a two-step oxidative addition/reductive elimination mechanism with an intervening IrV-H intermediate. We recently reported quasiclassical direct molecular dynamics simulations where starting from the vibrationally-averaged oxidative addition transition state a minor, but significant, amount of trajectories bypassed the IrV-H intermediate in a dynamical one-step mechanism. These trajectories also revealed that after the reductive coupling transition state is passed methane always dissociates and the C-H σ-complex is skipped. Here we report direct dynamics simulations using a microcanonical temperature sampling method with trajectories initialized and propagated using the Gaussian program. Sets of 30 trajectories were examined with average time steps ranging from 1.5 to 0.25 femtoseconds (fs). These results showed that a step size of ∼1.5 fs is too large and likely overestimates the amount of trajectories skipping the IrV-H intermediate. Trajectories with 0.75 and 0.25 fs time steps gave qualitatively similar profiles compared to our previous results using our program DynSuite with a 1 fs time step. Reverse trajectories with an average time step of 0.25 fs showed complete skipping of the methane σ complex. 30 trajectories were also propagated using a SMD continuum dichloromethane solvent model. These trajectories showed very similar behavior to gas-phase trajectories.

Piano-Stool Rhodium Enalcarbenoids: Application to Catalyst-Controlled Metal-Templated Annulations of Diazoenals and 1,3-Dicarbonyls

An electrophilic piano-stool rhodium(III)-enalcarbenoid resulted from the reaction of diazoenal with the cationic Cp*RhIII in the presence of a 1,3-diketone. The synthetic utility of these transient carbenoids has been demonstrated in the metal-templated [3 + 2] annulation of diazoenals and 1,3-dicarbonyls, thus leading to the enal-functionalized tetrasubstituted furans. The significance of the piano-stool enalcarbenoids has been further exemplified by the mechanistically distinct, complementary Lewis acid templated [2 + 3] annulation of diazoenals and 1,3-dicarbonyls, resulting in the trisubstituted furanyl-enones and acrylates. Mechanistic investigations revealed that these annulations proceed through catalyst-dependent chemoselective activation of diazoenal by the in situ formed metal diketonates. These methodologies gave access to core structures of indeno[1,2-b]furans, tetracyclic OLED, and a pan-AKT inhibitor.

Dynamical Mechanism May Avoid High-Oxidation State Ir(V)-H Intermediate and Coordination Complex in Alkane and Arene C-H Activation by Cationic Ir(III) Phosphine.

Organometallic reaction mechanisms are assumed to be appropriately described by minimum energy pathways mapped out by density functional theory calculations. For the two-step oxidative addition/reductive elimination mechanism for C-H activation of methane and benzene by cationic Cp*(PMe3)IrIII(CH3), we report quasiclassical direct dynamics simulations that demonstrate the IrV-H intermediate is bypassed in a significant amount of productive trajectories initiated from vibrationally averaged velocity distributions of oxidative addition transition states. This organometallic dynamical mechanism is akin to the σ-bond metathesis pathway but occurs on the oxidative addition/reductive elimination energy surface and blurs the line between two- and one-step mechanisms. Quasiclassical trajectories also reveal that the momentum of crossing the reductive elimination structure always induces complete alkane and arene dissociation from the Ir metal center, skipping weak C-H σ and π coordination complexes. This suggests that these weak coordination complexes after reductive elimination are not necessarily on the reaction pathway and likely result from a solvent cage.

Multiple pathways for C–H cleavage in cationic Cp*Rh(iii)-catalyzed C–H activation without carboxylate assistance: a computational study

Multiple pathways for C–H cleavage was uncovered in cationic Cp*Rh(iii)-catalyzed C–H functionalization with different heteroatom-containing species as possible proton acceptors.

Isolation of Cp*CoIII -Alkenyl Intermediate in Efficient Cobalt-Catalyzed C-H Alkenylation with Alkynes.

A general and efficient procedure for C-H alkenylation of arenes with a broad substrate scope catalyzed by Cp*CoIII was demonstrated with alkynes. A highly selective mono-alkenylation and sequential bis-C-H bond functionalization was displayed to exemplify the versatility of the cobalt catalyst. Isolation of cationic Cp*CoIII -alkenyl intermediate was achieved under identical catalytic conditions to further establish the proposed pathway.

(η5‐Pentamethylcyclopentadienyl)iridium Complex Catalyzed Imine Reductions Utilizing the Biomimetic 1,4‐NAD(P)H Cofactor and N‐Benzyl‐1,4‐dihydronicotinamide as the Hydride‐Transfer Agent

AbstractThe interaction between synthetic organometallic complexes and metabolic cofactors has proven to be a newly emerging topic in bioorganometallic chemistry. Thus, the first cationic Cp*Ir‐catalyzed (Cp*=η5‐pentamethylcyclopentadienyl) imine reduction in neutral buffered aqueous medium was examined. The reaction was found to proceed through hydride transfer from NADH as the hydride source at room temperature in air. Cationic Cp*Ir complexes proved to be the most efficient catalysts for this transformation. We also highlighted that the choice of the proton source was essential. The method was subsequently applied to cyclic and noncyclic imines. Eventually, the concept was extended to the reductive alkylation of one amine.

Cationic Cp*–Ruthenium Catalysts for Metal-Catalyzed Living Radical Polymerization: Cocatalyst-Independent Catalysis Tuned by Counteranion

We present the cationic Cp*–ruthenium complexes [Cp*Ru(CH3CN)(PPh3)2A; A–: the counteranion; abbreviated Cp*Ru+A–] as the active, cocatalyst-free, and tunable catalysts for metal-catalyzed living radical polymerization (Mt-LRP). A PF6-based cationic complex in conjunction with a alkyl halide initiator led to controlled polymerization of methyl methacrylate (MMA) giving well-controlled PMMAs with narrow molecular weight distribution (Mw/Mn 80% in 15 h), although cocatalyst was not combined. The catalytic feature was clearly different from the neutral counterpart [Cp*RuCl(PPh3)2: Cp*Ru], resulting in retarded polymerization under the cocatalyst-free condition. Interestingly, the catalytic activity was influenced by the counteranions. For example, the PF6-based complex did not afford catalysis for controlled polymerization of methyl acrylate (MA), but an Sb6-based [Cp*Ru(CH3CN)(PPh3)2SbF6: Cp*Ru+SbF6–] allowed control of the polymerization. Model reacti...

Phosphoramidate-Supported Cp*Ir(III) Aminoborane H2 B=NR2 Complexes: Synthesis, Structure, and Solution Dynamics.

Reaction of aminoboranes H2 B=NR2 (R=iPr or Cy) with the cationic Cp*Ir(III) phosphoramidate complex [IrCp*{κ(2) -N,O-Xyl(N)P(O)(OEt)2 }][BAr(F) 4 ] generates the aminoborane complexes [IrCp*(H){κ(1) -N-η(2) -HB-Xyl(N)P(OBHNR2 )(OEt)2 }][BAr(F) 4 ] (R=iPr or Cy) in which coordination of a P=O bond with boron weakens the B=N multiple bond. For these complexes, solution- and solid-state, as well as DFT computational techniques, have been employed to substantiate B-N bond rotation of the coordinated aminoborane.

Rh(III)-Catalyzed Diastereoselective C-H Bond Addition/Cyclization Cascade of Enone Tethered Aldehydes.

The Rh(III)-catalyzed cascade addition of a C-H bond across alkene and carbonyl π-bonds is reported. The reaction proceeds under mild reaction conditions with low catalyst loading. A range of directing groups were shown to be effective as was the functionalization of alkenyl in addition to aromatic C(sp2)-H bonds. When the enone and aldehyde electrophile were tethered together, cyclic β-hydroxy ketones with three contiguous stereocenters were obtained with high diastereoselectivity. The intermolecular three-component cascade reaction was demonstrated for both aldehyde and imine electrophiles. Moreover, the first x-ray structure of a cationic Cp*Rh(III) enolate with interatomic distances consistent with an η3-bound enolate is reported.

ChemInform Abstract: Rhodium‐Catalyzed Direct ortho‐Alkenylation of Phenyl Sulfones with Alkynes Utilizing Sulfonyl Function as Modifiable Directing Group.

Abstract The ortho-selective alkenylation of phenyl sulfones with alkynes proceeds effectively in the presence of a cationic Cp*-rhodium(III) catalyst together with an appropriate carboxylic acid involving regioselective C–H bond cleavage directed by the sulfonyl function. An (ortho-alkenylated phenyl) methyl sulfone prepared by this hydroarylation method undergoes palladium-catalyzed α-arylation and subsequent diastereoselective cyclization to directly produce the corresponding thiochromane 1,1-dioxide derivatives.

Chiral Cationic Cp(x)Ru(II) Complexes for Enantioselective Yne-Enone Cyclizations.

The cyclopentadienyl (Cp) group is a ligand of great importance for many transition-metal complexes used in catalysis. Cationic CpRu(II) complexes with three free coordination sites are highly versatile catalysts for many atom-economic transformations. We report the synthesis of a family of Cp(x)Ru(II) complexes with chiral Cp ligands keeping the maximum number of available coordination sites. The cationic members are efficient and selective catalysts for yne-enone cyclizations via formal hetero-Diels-Alder reactions. The transformation proceeds in <1 h at -20 °C and provides pyrans in up to 99:1 er. Unsaturated ester or Weinreb-amide substrates directly yield the iridoid skeleton.

Mechanistic studies on the Rh(III)-mediated amido transfer process leading to robust C-H amination with a new type of amidating reagent.

Mechanistic investigations on the Cp*Rh(III)-catalyzed direct C-H amination reaction led us to reveal the new utility of 1,4,2-dioxazol-5-one and its derivatives as highly efficient amino sources. Stepwise analysis on the C-N bond-forming process showed that competitive binding of rhodium metal center to amidating reagent or substrate is closely related to the reaction efficiency. In this line, 1,4,2-dioxazol-5-ones were observed to have a strong affinity to the cationic Rh(III) giving rise to dramatically improved amidation efficiency when compared to azides. Kinetics and computational studies suggested that the high amidating reactivity of 1,4,2-dioxazol-5-one can also be attributed to the low activation energy of an imido-insertion process in addition to the high coordination ability. While the characterization of a cationic Cp*Rh(III) complex bearing an amidating reagent was achieved, its facile conversion to an amido-inserted rhodacycle allowed for a clear picture on the C-H amidation process. The newly developed amidating reagent of 1,4,2-dioxazol-5-ones was applicable to a broad range of substrates with high functional group tolerance, releasing carbon dioxide as a single byproduct. Additional attractive features of this amino source, such as they are more convenient to prepare, store, and use when compared to the corresponding azides, take a step closer toward an ideal C-H amination protocol.

Rhodium-catalyzed direct ortho-alkenylation of phenyl sulfones with alkynes utilizing sulfonyl function as modifiable directing group

The ortho-selective alkenylation of phenyl sulfones with alkynes proceeds effectively in the presence of a cationic Cp*-rhodium(III) catalyst together with an appropriate carboxylic acid involving regioselective C–H bond cleavage directed by the sulfonyl function. An (ortho-alkenylated phenyl) methyl sulfone prepared by this hydroarylation method undergoes palladium-catalyzed α-arylation and subsequent diastereoselective cyclization to directly produce the corresponding thiochromane 1,1-dioxide derivatives.

Cp∗Co(III)-catalyzed oxidative C–H alkenylation of benzamides with ethyl acrylate

The utility of Cp∗Co(III)-catalysts was expanded to oxidative C–C bond-forming reaction. In situ-generated Cp∗Co(III)-catalyst, rather than a preformed cationic Cp∗Co(III)-complex, was effective. Oxidative alkenylation of benzamides and acetanilide with ethyl acrylate proceeded with silver acetate as a stoichiometric oxidant, giving products in up to 93% yield.