Cationic Rh Research Articles

Frontispiece: Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B−H and O−H Bonds Using a Cationic Rh(I)/Diene Catalyst

Frontispiece: Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B−H and O−H Bonds Using a Cationic Rh(I)/Diene Catalyst

Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B-H and O-H Bonds Using a Cationic Rh(I)/Diene Catalyst.

Transition-metal-catalyzed enantioselective transformations of aryl/aryl carbene are inherently challenging due to the difficulty in distinguishing between two arene rings in the reaction process thus remain largely less explored. The few successful examples reported so far, without exception, have all been catalyzed by Rh(II)-complexes. Herein, we describe our successful development of a novel cationic Rh(I)/chiral diene catalytic system capable of efficient enantioselective B-H and O-H insertions with diaryl diazomethanes, allowing the access to a broad range of gem-diarylmethine boranes and gem-diarylmethine ethers in good yields with high enantioselectivities. Notably, previously unattainable asymmetric diarylcarbene insertion into the O-H bond was achieved for the first time. A remarkable feature of this newly designed Rh(I)/diene catalyst bearing two ortho-amidophenyl substitutents is that it can distinguish between two arene rings of the diaryl carbene through a stereochemically selective control of π-π stacking interactions. DFT calculations indicate that the rotation-restricted conformation of Rh(I)/diene complex played an important role in the highly enantioselective carbene transformations. This work provides an interesting and unprecedented stereocontrol mode in diaryl metal carbene transformations.

Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B−H and O−H Bonds Using a Cationic Rh(I)/Diene Catalyst

Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B−H and O−H Bonds Using a Cationic Rh(I)/Diene Catalyst

Enantioselective Synthesis, Crystal Structures, and Stereoisomerism of Substituted o,m,o,p-Tetraphenylenes.

We have achieved the enantioselective synthesis of highly strained, substituted o,m,o,p-tetraphenylenes (≤98% ee) via the cationic Rh(I)/(R)-H8-BINAP complex-catalyzed chemo-, regio-, and enantioselective intermolecular cross-[2+2+2] cycloaddition of teraryl diynes with dimethyl acetylenedicarboxylate. X-ray crystallographic analyses demonstrate the highly bent structures of the para-substituted benzene moieties, and density functional theory calculations reveal the large local strain of the paraphenylene unit. 1H nuclear magnetic resonance analyses and theoretical calculations elucidate the stereoisomerism, indicating that the nonrotatable ortho-disubstituted biphenyl structure results in cis and trans isomers.

Bifunctional Ligands: Evaluating the Role of Acidic Protons in the Secondary Coordination Sphere.

To evaluate bifunctional ligand reactivity involving NH acidic sites in the secondary coordination sphere, complexes where the proton has been substituted with a methyl group (NMe) are often investigated. An alternative strategy involves substitution of the NH group for an O. This contribution considers and compares the merits of these approaches; the synthesis and characterization of cationic square-planar Rh carbonyl complexes bearing diprotic bispyrazole pyridine ligand L1, and the bis-methylated pyrazole pyridine ligand L1Me are described. The syntheses and characterization of the novel monoprotic pyrazole isoxazole pyridine ligand L2 and aprotic bisisoxazole pyridine ligand L3, and their corresponding Rh carbonyl complexes are also described. Comparison of the CO stretching frequencies of the four Rh complexes suggest that substitutions of NH with NMe, as well as with O, lead to significant electronic differences. These electronic differences result in different reactivities with respect to ligand addition/substitution of the Rh carbonyl complexes. Overall, the data suggest that electronic differences arising due to the NH substitutions can be significant and should be considered when the NH group is substituted in investigations of the participation of the NH proton in a reaction.

Asymmetric Cyclopropanation of Primary N-Vinylamides via Carbene Transfer Catalyzed by Cationic Rh(I)/Diene Complexes: An Unexpected Outer-Sphere Mechanism

Asymmetric Cyclopropanation of Primary <i>N</i>-Vinylamides via Carbene Transfer Catalyzed by Cationic Rh(I)/Diene Complexes: An Unexpected Outer-Sphere Mechanism

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides.

Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H+ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to py aziridines at 80 °C but unprecedentedly result N-pico CAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy , as revealed by computational analysis, plays a crucial role in this unusual 1,3-H+ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both N-pico CAAC and py aziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3 SiH. Notably, the py aziridine-Rh(I) is superior than the N-pico CAAC-Rh(I) catalyst.

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl‐Tethered Robust Azomethine Ylides

AbstractAzomethine ylides are typically in situ generated synthons for making N‐heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium‐based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3‐H+ transfer. Herein, two thermally robust azomethine ylides with a N‐appended picolyl sidearm are isolated, which cyclize to pyaziridines at 80 °C but unprecedentedly result N−picoCAAC‐CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy, as revealed by computational analysis, plays a crucial role in this unusual 1,3‐H+ shift using a deprotonation‐protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N‐tethered additional donor is priorly unknown. Both N‐picoCAAC and pyaziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3SiH. Notably, the pyaziridine‐Rh(I) is superior than the N‐picoCAAC‐Rh(I) catalyst.

Enhanced adsorption of dyes by functionalized UiO-66 nanoparticles: Adsorption properties and mechanisms

UiO-66-based metal-organic framework (MOF) materials are a class of porous crystalline materials consisting of Zr metal clusters compounded with organic ligands of terephthalic acid group. In this study, functionalized UiO-66-X (X=H, NH2, (COOH)2) nanoparticles were synthesized. The morphology of UiO-66-X nanoparticles were characterized by SEM, while the crystal structure was analyzed using XRD and TGA. Chemical composition of UiO-66-X nanoparticles were analyzed through TFIR and XPS. The adsorption capacities of UiO-66-X nanoparticles for anionic dye of Congo red (CR), and cationic dyes of methylene blue (MB) and rhodamine B (Rh B) dyes were evaluated. Furthermore, the adsorption mechanisms of UiO-66-X nanoparticles for dyes were analyzed and summarized. Results showed that the adsorption capacities of UiO-66 and UiO-66-NH2 on CR were 93.05 mg/g and 94.20 mg/g, respectively. In particular, the adsorption capacities of UiO-66-(COOH)2 on MB and Rh B were significantly increased with the adsorption capacities of 80.66 mg/g and 72.64 mg/g, respectively. Due to the strong π-π effect and hydrogen bonding interactions between UiO-66 and CR dyes. UiO-66-NH2 has stronger hydrophilicity and has electrostatic interactions, Lewis acid-base interactions and hydrogen bonds with CR. The abundant -COOH groups and higher negative Zeta potential (-23.43 mV) in UiO-66-(COOH)2 led to strong electrostatic interactions and Lewis acid-base interactions with the cationic dyes MB and Rh B. Therefore, the adsorption performance of UiO-66-(COOH)2 on cationic dyes was improved.

Enantioselective Synthesis of Axially Chiral Figure-Eight Spirocycloparaphenylenes via Rh-Catalyzed Intermolecular [2 + 2 + 2] Cycloaddition.

We have achieved the enantioselective synthesis of axially chiral figure-eight spiro[9.9]cycloparaphenylene (CPP) tetracarboxylates with up to 75:25 er via the cationic Rh(I)/(R)-H8-BINAP complex-catalyzed chemo-, regio-, and enantioselective intermolecular double [2 + 2 + 2] cycloaddition of an achiral symmetric tetrayne with dialkyl acetylenedicarboxylates followed by reductive aromatization. The spiro[9.9]CPP tetracarboxylates are highly distorted at the phthalate moieties with large dihedral and boat angles and exhibit weak aggregation-induced emission enhancement behavior.

Rhodium-Catalyzed Double Hydroboration of Quinolines

Selective double hydroelementation of N-heteroarenes can be one of the most straightforward and atom-economic routes toward dearomatized N-heterocycles bearing a (chiral) sp3 C–E bond (E = B, Si, etc.) at a specific site. Herein, we describe the development of a one-pot, site- and stereoselective borylative reduction of quinolines leading to a series of tetrahydroquinolines that possess an (enantioenriched) C(sp3)–B bond in the 4-position. A cationic Rh precatalyst [Rh(cod)2]OTf with a phosphine ligand DPEPhos enabled the borylative reduction of a range of quinolines with HBpin to provide the C4-borylated tetrahydroquinolines (THQ) in good to high yields under mild conditions. Mechanistic studies elucidated the Rh-mediated stepwise dearomative reduction cascade: (i) moderately regioselective hydroboration of quinoline resulting in a mixture of 1,2- and 1,4-dihydroquinolines (DHQ), (ii) regioselective hydroboration of the 1,2-DHQ intermediate to give the C4-borylated THQ, (iii) slow isomerization of the 1,4-DHQ to the 1,2-DHQ, and (iv) formation of the cationic Rh-quinoline adduct as a catalytic resting species. The C4-selective asymmetric borylative reduction within the cationic Rh/(S)-BINAP system was accomplished albeit with moderate enantioselectivity on average in moderate to good yields. The installed C(sp3)–Bpin unit in the 4-position was transformed to various functional groups through simple organic reactions.

Rhodium(III‐Catalyzed C8‐Selective C−H Alkenylation and Alkylation of 1, 2, 3, 4‐Tetrahydroquinolines with Styrenes and Allylic Alcohols

AbstractRh(III)‐catalyzed chelation‐assisted C8‐selective C−H alkenylation and alkylation of 1,2,3,4‐tretrahydroquinolines with styrenes and allylic alcohols have been realized. The cationic Rh(III) catalytic system in combination with a catalytic amount of copper acetate uses oxygen as the terminal oxidant and catalyzes the stereoselective C8‐alkenylation of 1,2,3,4‐tetrahydroquinolines with styrenes to give the corresponding products in 56–93% yields with wide substrate scope and broad functional group compatibility. The reaction can be scaled up. Moreover, this protocol can be extended to the C8‐alkylation of 1,2,3,4‐tetrahydroquinolines with allylic alcohols, providing access to various 8‐(3‐oxoalkyl)‐1,2,3,4‐tetrahydroquinolines in 77–90% yields. The selection of N‐directing group is essential for catalysis, and the readily installable and removable N‐(2‐pyrimidyl) group proves to be optimal choice. Preliminary mechanistic studies are performed to gain insights into the reaction mechanism.magnified image

Phosphonium Ylides vs Iminophosphoranes: The Role of the Coordinating Ylidic Atom in cis-[Phosphine-Ylide Rh(CO)2] Complexes.

The coordinating properties of two families of ylides, namely, phosphonium ylides and iminophosphoranes, differently substituted at the ylidic center (CH2- vs NiPr-), have been investigated in structurally related cationic phosphine-ylide Rh(CO)2 complexes obtained from readily available phosphine-phosphonium salt precursors derived from an ortho-phenylene bridge. However, while the Rh(CO)2 complex bearing the P+-CH2- donor moiety proved to be stable, the P═NiPr donor end appeared to induce lability to one of the CO groups. All of the RhI carbonyl complexes in both ylide series were fully characterized, including through X-ray diffraction analysis. Based on the experimental and calculated infrared (IR) CO stretching frequencies in Rh(CO)2 complexes, we evidenced that the phosphonium ylide ligand is a stronger donor than the iminophosphorane ligand. However, we also found that the difference in the intrinsic electronic properties can be largely compensated by the introduction of an iPr substituent on the N atom of the iminophosphorane, hence pointing to the noninnocent role of the peripheral substituent and opening novel possibilities to tune the properties of metal complexes containing ylide ligands.

Structures, Thermal Properties, and Reactivities of Cationic Rh-cod Complexes in Solid State (cod = 1,5-Cyclooctadiene).

Cationic rhodium complexes with 1,5-cyclooctadiene (cod) ligands are important organometallic compounds that are useful as precatalysts; however, their solid-state structures and thermal properties have not been adequately investigated. In this study, we synthesized [Rh(cod)L]X (L = cod, C6H6, PhMe; X = SbF6, (FSO2)2N (= FSA), CF3BF3, CB11H12) and investigated their phase behaviors, crystal structures, and reactivities. The phase transitions of these salts result in disordered solid-state structures. Moreover, the structural disorder increases with a decrease in the cation symmetry in the SbF6 salts; [Rh(cod)(PhMe)]SbF6 exhibits a rotator phase, and the cations in other salts exhibit a dynamic rotational disorder. In contrast, a lower crystal symmetry with less cation disorder is observed for FSA salts. The thermal stabilities and reactivities of these salts were further investigated. FSA salts with arene ligands produce anion-coordinated complexes upon melting, and SbF6 salts with arene ligands produce [Rh(cod)L'2]SbF6 (L' = MeCN and SMe2) via an in situ single-crystal-to-single-crystal ligand-exchange reaction.

Rose bengal-decorated rice husk-derived silica nanoparticles enhanced singlet oxygen generation for antimicrobial photodynamic inactivation.

The online version contains supplementary material available at 10.1007/s10853-023-08194-z.

Cationic Rhodium(I)-Catalyzed Asymmetric Cyclohydroformylation of 1,6-Enynes with Formaldehyde.

We report a rhodium(I)-catalyzed asymmetric cyclohydroformylation reaction of 1,6-enynes with formaldehyde. The reaction of 1,6-enynes with formaldehyde in the presence of a cationic Rh(I) catalyst, such as [Rh(cod)2 ]+ OTf- , and a chiral biaryl diphosphine led to asymmetric cyclohydroformylation to produce aldehydes with higher-order structures highly enantioselectively. This transformation procedure is applicable to a variety of enynes, with wide compatibility in various atoms liking between the alkyne and alkene parts, substituents at the alkyne terminus, and substituents at the alkene part, being converted to newly formed aldehydes in 14% to 90% yields with 50% to 98% ee. The products were further transformed with various nucleophiles to alcohols, an amine, and a diene without loss of chirality at their γ-position.

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.

Room Temperature Fluoranthene Synthesis through Cationic Rh(I)/H8-BINAP-Catalyzed [2 + 2 + 2] Cycloaddition: Unexpected Acceleration due to Noncovalent Interactions

The fluoranthene skeleton is a structure often found in natural products and fluorescent materials, and thus developing an operationally simple method for diversity-oriented fluoranthene synthesis is an important research topic in organic synthesis. However, existing synthetic methods require harsh reaction conditions or limited substrate applicability or both. Here, we report the room temperature synthesis of substituted fluoranthenes and azafluoranthenes through the cationic Rh(I)/H8-BINAP complex-catalyzed [2 + 2 + 2] cycloaddition using 1,8-dialkynylnaphthalenes. DFT calculations reveal that the H8-BINAP ligand stabilizes the intermediates, allowing the room temperature reaction. Among the substrates examined, 1,8-bis(phenylethynyl)naphthalene and diarylacetylenes show high reactivity, contrary to the reactivity predicted by their steric bulkiness. Theoretical and experimental mechanistic studies have demonstrated that the noncovalent interactions of the phenyl groups on both the substrates and the ligand accelerate the present sterically demanding reactions by stabilizing the transition states rather than inducing steric repulsions.

Effect of liquid confinement on regioselectivity in the hydrosilylation of alkynes with cationic Rh(I) N-heterocyclic carbene catalysts.

Polymeric mesoporous monoliths were prepared via ring-opening metathesis polymerization (ROMP) from norbornene (NBE), 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene (DMN-H6), tris(norborn-2-enylmethylenoxy)methylsilane and the 1st-generation Grubbs catalyst [RuCl2(PCy3)2(CHC6H5)] in the presence of 2-propanol and toluene and surface grafted with 1-(2-((norborn-5-ene-2-carbonyl)oxy)ethyl)-3-ethyl-1H-imidazol-3-ium tetrafluoroborate. Subsequently, a supported ionic-liquid-phase (SILP) system was created by immobilizing the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] with the cationic catalyst [Rh((1-pyrid-1-yl)-3-mesitylimidazol-2-ylidene)(COD)+BF4-] (Rh-1; COD = 1,4-cyclooctadiene) dissolved therein. The regio- and stereoselectivity of Rh-1 dissolved in the IL and supported on the mesoporous monolith, referred to as Rh@SILPROMP, in the hydrosilylation of 1-alkynes with HSiMe2Ph was studied and compared to that of the homogeneous catalyst Rh-1 under biphasic conditions using methyl tert-butyl ether (MTBE) as a second organic phase. Different amounts of IL were used, which allowed for the creation of SILPs with different layer thicknesses. Rh@SILPROMP provided by far better β-(Z) selectivity for both aromatic and aliphatic 1-alkynes in comparison to Rh-1 used under biphasic conditions. The highest β-(Z) selectivity was obtained with the thinnest IL layer. No leaching of the IL or rhodium from the SILP system into the organic phase was observed, resulting in virtually metal-free hydrosilylation products. The data obtained with Rh@SILPROMP were also compared with those from previous studies with Rh-1 in the same IL supported on polyurethane-derived mesoporous monolithic supports (Rh@SILPPUR) and on mesoporous SBA-15 (Rh@SILPSBA-15). For the first time, the use of a liquid confinement created by both a SILP and the support itself to tune the transition state of an organometallic catalyst by non-covalent interactions and thus stereo- and regioselectivity is outlined.

Catalytic activation of remote alkenes through silyl-rhodium(III) complexes.

The tandem isomerization-hydrosilylation reaction is a highly valuable process able to transform mixtures of internal olefins into linear silanes. Unsaturated and cationic hydrido-silyl-Rh(III) complexes have proven to be effective catalysts for this reaction. Herein, three silicon-based bidentate ligands, 8-(dimethylsilyl)quinoline (L1), 8-(dimethylsilyl)-2-methylquinoline (L2) and 4-(dimethylsilyl)-9-phenylacridine (L3), have been used to synthesize three neutral [RhCl(H)(L)PPh3] (1-L1, 1-L2 and 1-L3) and three cationic [Rh(H)(L)(PPh3)2][BArF4] (2-L1, 2-L2 and 2-L3) Rh(III) complexes. Among the neutral compounds, 1-L2 could be characterized in the solid state by X-ray diffraction showing a distorted trigonal bipyramidal structure. Neutral complexes (1-L1, 1-L2 and 1-L3) failed to catalyze the hydrosilylation of olefins. On the other hand, the cationic compound 2-L2 was also characterized by X-ray diffraction showing a square pyramidal structure. The unsaturated and cationic Rh(III) complexes 2-L1, 2-L2 and 2-L3 showed significant catalytic activity in the hydrosilylation of remote alkenes, with the most sterically hindered (2-L2) being the most active one.