Rh-C Bond Research Articles

Photochemical conversion of CO to C1 and C2 products mediated by porphyrin rhodium(II) metallo-radical complexes

Unimolecular reduction and bimolecular reductive coupling of carbon monoxide (CO) represent important ways to synthesize organic feedstocks. Reductive activation of CO through open-shell pathways, though rare, can help overcome the barriers of many traditional organometallic elementary reactions that are hard to achieve. Herein we successfully achieve the unimolecular reduction of CO to (TPP)RhCH2OSiR1R2R3 (TPP = 5,10,15,20-tetraphenylporphyrin), and the release of products CH3OSiR1R2R3, TEMPO-CH2OSiR1R2R3 and BrCH2OSiR1R2R3 in near-quantitative yield under visible light (420–780 nm), which involves radical formation from Rh-C bond homolysis. Bimolecular CO reductive coupling products, (TPP)RhCOCH2OSiR1R2R3, are then obtained via a radical mechanism. Subsequent treatment with n-propylamine, BrCCl3 or TEMPO under thermal or photochemical conditions afford small-molecule bimolecular reductive coupling products. To the best of our knowledge, homogeneous systems which reductively couple CO under photochemical conditions have not been reported before. Here, the use of an open-shell transition metal complex, that delivers more than one kind of small-molecule CO reductive coupling products bearing different functional groups, provides opportunities for useful CO reductive transformations.

Sequential H/D Exchanges Resulting from Rollover-Cyclometallation during Photoirradiation of Rhodium(III) Complex in Methanol-d4.

We present the photoreaction of newly prepared bis(6,6'-dimethyl-2,2'-bipyridine)(oxalato)rhodium(III) ([Rh(N^N)2(ox)]+) in CD3OD. Photoirradiation of this complex causes the dissociation of ox, followed by the formation of the unprecedented Rh(III) complex with Rh-H and Rh-C s bonds, [Rh(N^N)(C^N)(H)(CD3OD)]+ (C^N = [6,6'-dimethyl-2,2'-bipyridine]-3-yl-κC3,κN1'). This hydride formation and cyclometallation spontaneously proceed owing to the conflict between the steric hindrance arising from the methyl groups of N^N and the driving force for the structural change due to [Rh(N^N)2]+ formation. Although [Rh(N^N)(C^N)(H)(CD3OD)]+ is initially converted to [Rh(N^N)2]+ by photoirradiation, it is immediately regenerated by the rollover cyclometallation of the [Rh(N^N)2]+ complex. [Rh(N^N)(C^N)(H)(CD3OD)]+ undergoes H/D exchange for the H atoms in the Rh-H bond and at the 3, 3'-positions of the N^N ligand during the photoirradiation. DFT calculations predict with reasonable certainty the spontaneous structural change of [Rh(N^N)2]+ to [Rh(N^N)(C^N)(H)(CD3OD)]+ and the subsequent photodriven Rh-C bond rupture leading to the formation of [Rh(N^N)2]+ accompanied by H/D exchange reactions.

Adsorption and gas sensitivity properties of graphene and Rh-decorated graphene towards SF6 decomposed gases: A DFT study

In order to solve the serious threat of SF6 decomposed gases (H2S, SO2, SOF2, SO2F2) to the stable operation of electrical equipment. In this study, the geometric structure, adsorption energy, density of states (DOS), sensitivity based on band structure and recovery time of H2S, SO2, SOF2, SO2F2 on graphene and Rh-modified graphene (Rh-graphene) were investigated computationally by density functional theory (DFT). The geometric structure of Rh-graphene was optimized with an average Rh-C bond length of 1.891 Å. According to the analysis of DOS and band structure, the electrical conductivity has a little effect of the Rh-graphene compared with pristine graphene, but Rh modification can regulate the band gap of graphene. The adsorption energy (Ead) and the sensitivity (S) at work temperature indicate that graphene has ideal adsorption and sensing properties for SO2 and Rh-graphene reflects the excellent applicability as the SO2 and SO2F2 gases sensor with the sensitivity of approximately −99 % and −60 %, respectively. The Graphene/SO2 system owns the minimum recovery time of 2154.3 ps. The recovery time of Rh-graphene/H2S and SOF2 is several orders of magnitude smaller than Rh-graphene/SO2 and SO2F2. The analysis of DOS and band structure was used to explain the sensing mechanism. The study would lay a theoretical foundation for the development of SF6 decomposed gases sensor for electrical equipment insulation.

Theoretical insights into Rh-catalyzed hydroamidation: Mechanism and chemoselectivity

Rh-catalyzed hydroamidation of unactivated alkenes were investigated by a density functional theory study. The catalytic cycle proceeds through β-H elimination, alkene addition, CO2 extrusion, nitrene migratory insertion and protodemetalation. CO2 extrusion is the rate-determining step for the overall reaction. More importantly, the hydroamidation-to-aziridination and hydroamidation-to-amidation chemoselectivities have been investigated and the origins of impossibilities of CC bond aziridination and C(sp2/sp3)H bond amidations have been elucidated. The Rh-nitrene group is more inclined to insert RhC bond rather than CC and C(sp2/sp3)H bonds to form the new CN bond of hydroamidation products. These calculation results provide valuable information for further selective synthesis of amides.

Rhodium(I)-Catalyzed Asymmetric Hydroarylative Cyclization of 1,6-Diynes to Access Atropisomerically Labile Chiral Dienes.

Axially chiral open-chained olefins are an underexplored class of atropisomers, whose enantioselective synthesis represents a daunting challenge due to their relatively low racemization barrier. We herein report rhodium(I)-catalyzed hydroarylative cyclization of 1,6-diynes with three distinct classes of arenes, enabling highly enantioselective synthesis of a broad range of axially chiral 1,3-dienes that are conformationally labile (ΔG≠ (rac)=26.6-28.0 kcal/mol). The coupling reactions in each category proceeded with excellent enantioselectivity, regioselectivity, and Z/E selectivity under mild reaction conditions. Computational studies of the coupling of quinoline N-oxide system reveal that the reaction proceeds via initial oxidative cyclization of the 1,6-diyne to give a rhodacyclic intermediate, followed by σ-bond metathesis between the arene C-H bond and the Rh-C(vinyl) bond, with subsequent C-C reductive elimination being enantio-determining and turnover-limiting. The DFT-established mechanism is consistent with the experimental studies. The coupled products of quinoline N-oxides undergo facile visible light-induced intramolecular oxygen-atom transfer, affording chiral epoxides with complete axial-to-central chirality transfer.

Theoretical Investigations on the Rh(III)-Catalyzed Oxidative C-H Activation/Annulation of Salicylaldehydes with Masked Enynes: Mechanism Insights and Regioselectivity Origins.

The mechanism underlying the rhodium(III)-catalyzed reaction of the C-H alkenylation/annulation reaction of salicylaldehydes with enynes has been thoroughly investigated using DFT calculations. Based on mechanistic studies, our focus primarily lies on the regioselectivity of asymmetric alkynes inserting into the Rh-C bond and the involvement of the auxiliary group OAc- in these reactions. Our theoretical study uncovers that, with acetate assistance, a stepwise SN2' cyclization, 1,3-Rh migration, β-H elimination, and reductive elimination process occur. Furthermore, we also explore the role of substitution at Cα (CH3 vs H) in the reaction. As demonstrated in this work, these findings are applicable to other related reactions.

Mechanistic Insights Into the Rhodium-Catalyzed C–H Alkenylation/Directing Group Migration and [3+2] Annulation: A DFT Study

The mechanism of the rhodium-catalyzed C-H alkenylation/directing group migration and [3+2] annulation of N-aminocarbonylindoles with 1,3-diynes has been investigated with DFT calculations. On the basis of mechanistic studies, we mainly focus on the regioselectivity of 1,3-diyne inserting into the Rh-C bond and the N-aminocarbonyl directing group migration involved in the reactions. Our theoretical study uncovers that the directing group migration undergoes a stepwise β-N elimination and isocyanate reinsertion process. As studied in this work, this finding is also applicable to other relevant reactions. Additionally, the role of Na+ versus Cs+ involved in the [3+2] cyclization reaction is also probed.

Rhodium(I)-Catalyzed Three-Component [4+2+1] Cycloaddition of Two Vinylallenes and CO.

Transition metal-catalyzed [4+2+1] reactions of dienes (or diene derivatives such as vinylallenes), alkynes/alkenes, and CO (or carbenes) are expected to be the most straightforward approach to synthesize challenging seven-membered ring compounds, but so far only limited successes have been realized. Here, an unexpected three-component [4+2+1] reaction between two vinylallenes and CO was discovered to give highly functionalized tropone derivatives under mild conditions, where one vinylallene acts as a C4 synthon, the other vinylallene as a C2 synthon, and CO as a C1 synthon. It was proposed that this reaction occurred via oxidative cyclization of the diene part of one vinylallene molecule, followed by insertion of the terminal alkene part of the allene moiety in another vinylallene, into the Rh-C bond of five-membered rhodacycle. Then, CO insertion and reductive elimination gave the [4+2+1] cycloadduct. Further experimental exploration of why ene/yne-vinylallenes and CO gave monocyclic tropone derivatives instead of 6/7-bicyclic ring products were reported here.

Computational Investigation of Substituent Effect on the Thermodynamics and Kinetics of β-Hydrocarbyl Elimination from a Rhodium(I) Iminyl Complex

This study investigates the effect of different substituents on the β-hydrocarbyl elimination reaction of the Rh(I) iminyl complexes (H3P)3Rh[N=CAr2]; Ar = para-C6H4X (X = NH2, OMe, Me, H, F, COOH, CHO, CN) and formation of (H3P)3Rh–Ar complexes and para-C6H4X(CN) molecules using MPW1PW91-based quantum mechanical calculations. There are correlations between Hammett constants for the para-substituted (σp) functional groups of electron withdrawing (EDGs) and donating (EDGs) groups with the relative energies, thermodynamic parameters and the rate constant of this reaction. Also, the calculated Wiberg indices of the Rh-C bond values are used to illustrate the Rh–N bond cleavage and Rh–C bond formation. The rate constants of the β-hydrocarbyl eliminations are computed from 300 to 1200 K, in gas phase.

Theoretical study of the influence of solvent polarity on the 31P and 13C NMR parameters of the Ru(PH3)4(η2-benzyne) complex

In this study, using the wB97XD method, quantum chemical calculations were used to analyze the effect of solvent on 31P and 13C NMR parameters of the Ru(PH3)4(η2-benzyne) complex. The solvent effects were examined using the self-consistent reaction field theory (SCRF) based on the Polarizable Continuum Model (PCM). The stability of this complex was studied in the singlet and triplet states. The 31P and 13C NMR chemical shifts of the complex were calculated for both gas and solution phases. One-bond nuclear spin–spin coupling constants of RhC and RuP bonds as well as two-bond nuclear spin–spin coupling constants of P⋯P were estimated. The correlations of these parameters and solvent polarity functions including both the dielectric constant (ε) and refractive index (nD) of the liquid medium were explored further. In addition, the most significant Ramsey terms involved significantly in 1J(RuP), 1J(RuC) and 2J(P⋯P) values were determined. Finally, the correlations between 1J(RuP), 1J(RuC) and the APT net charge on the Ru atom were studied.

Theoretical study of rhodium(III)-catalyzed synthesis of benzoxepine and coumarin.

The mechanisms of the rhodium-catalyzed cycloaddition of 2-vinylphenol with diphenylacetylene and carbon monoxide have been studied using density functional theory calculations at the B3LYP/6-31G (d, p) (Lanl2dz for Rh) level of theory. The SMD solvation model was used in MeCN solvents at M06-2X/6-311 ++ G (d, p) (Lanl2dz (f) for Rh) levels using a single-point calculation to consider the solvent effect. The calculation results show that there are two competitive reaction pathways for the cycloaddition reaction of rhodium-catalyzed synthesis of benzohexine and coumarin. Starting from the precursor reaction complex, the reaction channel is more favorable for the carbon atoms of diphenylacetylene and carbon monoxide to attack the Rh-C bond (the barriers of 9.88 and 10.01kcal/mol) rather than attack the Rh-O bond (the barriers of 15.37 and 30.17kcal/mol), and carbon monoxide in two different reaction channels has a greater energy difference than diphenylacetylene. The results show that the computational study of the rhodium-catalyzed cycloaddition reaction has a high catalytic activity consistent with the high yield of the experiment of Gulías et al.

Substituent-dependent generation of tricyclic frameworks by the rhodium-catalyzed cycloisomerization of homopropargyl allene-alkynes: a theoretical study.

Polycyclic compounds having biological activities can be modified by employing different substituents. Substituent-dependent generation of tricyclic frameworks by the rhodium-catalyzed cycloisomerization of homopropargyl allene-alkynes was investigated by using density functional theory calculations. A mechanistic study revealed that all the three reactions, A (Z = Me, X = tBu), B (Z = Me, X = Me), and C (Z = H, X = Me), underwent initial oxidative cyclization, from which the divergent cycloisomerization was triggered when different substituents were employed. It was found from our calculations that reactions A and B underwent alkyne insertion into the Rh-C(sp2) bond and resulted in a key metal carbenoid intermediate. From the metal carbenoid species, reaction A bearing a more hindered tert-butyl group prefers 1,2-hydrogen migration to produce a 6/5/4 tricyclic product, while reaction B bearing a less hindered methyl group prefers 1,2-carbon migration leading to ring expansion and producing a 6/5/5 tricyclic product. The origins of the distinct selectivity were disclosed, which are beneficial for the development of novel related reactions. For reaction C (Z=H, Z=H or Z=H, Z=Me), the β-hydrogen elimination was found to be favored over the alkyne insertion into the Rh-C(sp2) bond and thus the monocyclic product is delivered. The E/Z effects involved in the above two cases were also discussed.

A resorcinarene-based tetrabenzoimidazolylidene complex of rhodium.

A tetrabenzoimidazolium-resorcinarene cavitand was used for the preparation of a tetra-benzoimidazolylidene of rhodium, which is unprecedented in the field of poly-NHC metal complexes. Both the experimental and computational analyses of the molecule reveal a distorted vase conformation as the most stable one, although several non-interconverting conformational isomers due to the restricted rotation about the Rh-C(carbene) bond coexist in the product. There is a fluxional behaviour involving the vase-kite interconversion. The main interactions between the arms of the cavitand are mostly concentrated on the terminal organometallic fragments attached to NHC, along with those between -CH3 and a N-heterocyclic carbene ring from benzoimidazoles.

Rhodium(I)-Catalyzed Enantioselective Cyclization of Enynes by Intramolecular Cleavage of the Rh-C Bond by a Tethered Hydroxy Group.

Rhodium(I)-catalyzed enantioselective intramolecular cyclization of enynes having a hydroxy group in the tether was investigated, and various cyclic compounds possessing a chiral quaternary carbon center were obtained in high yields with high ees. In this cyclization, a Rh-C(sp2 ) bond in the rhodacyclopentene intermediate, which was formed by enantioselective oxidative cycloaddition of enynes to a chiral rhodium(I) complex, was intramolecularly cleaved by σ-bond metathesis of a tethered O-H bond in the substrate. Furthermore, it was found that the cyclic compounds were obtained with high ees even when the starting materials having a racemic secondary alcohol moiety were used in this reaction.

Theoretical study on the intramolecular oxyamination involved in Rh(III)-catalyzed cyclization of unsaturated alkoxyamines

The unexpected oxyamination reaction of O, ω-unsaturated alkoxyamines was found experimentally. The mechanistic issues were studied by DFT calculations. It is suggested that the reaction undergoes [3 + 2] cyclic addition, ON bond cleavage, CN reductive elimination, and the RhN unit protonation, generating the product and regenerating the active catalyst. The nitrene Rh(V) species containing a RhC bond rather than a RhO bond was suggested to be involved in the reaction mechanism. Why the substrate A with X = O but not X = C undergoes oxyamination reaction was rationalized based on the suggested reaction mechanism.

PH-Dependent chromogenic properties of an orthogonal metal-C (aryl) bonded azobenzene-rhodium(III) porphyrin conjugate

An orthogonally arranged conjugate comprised of two rhodium(III) tetrakis-4-tolyporphyrins linked with an azobenzene moiety through a covalent Rh-C (aryl) bond has been facilely synthesized and characterized. Although photochemical isomerization of the azobenzene component was not observed, addition of trifluoroacetic acid (TFA) to the conjugate in a toluene solution causes significant colorimetric changes. UV-vis absorption, resonance light scattering (RLS), [Formula: see text]H-NMR and IM-MS spectra were employed to investigate the pH-dependent chromogenic behavior of the triad, which gives a new pathway for construction of pH-dependent assemblies of porphyrin-azobenzene complexes.

Rhodium/Copper Cocatalyzed Highly trans-Selective 1,2-Diheteroarylation of Alkynes with Azoles via C-H Addition/Oxidative Cross-Coupling: A Combined Experimental and Theoretical Study.

Transition metal-catalyzed addition of diaryl alkynes with arylating reagents for the synthesis of tetraarylethylenes generally encounters rigorous reaction conditions and relies on the use of prefunctionalized substrates such as organic halides or surrogates and organometallic reagents. In this work, we establish a highly trans-selective 1,2-diheteroarylation of alkynes with azoles via a rhodium/copper cocatalyzed C-H addition/oxidative coupling process. Moreover, the diheteroarylation developed herein could open a door for the synthesis of heteroarene-doped tetraarylethylenes, and the photoluminescence (PL) spectra in THF-water mixtures and solid powder verify that these tetra(hetero)arylethylenes are aggregation-induced emission (AIE) active, building a new AIE molecule library. With a combination of experimental and theoretical methods, the reaction mechanism for addition/oxidative cross-coupling of internal alkynes with azoles has been investigated. Theoretical calculations reveal that the metalation/deprotonation of azole could occur with either rhodium or copper species. When azolylrhodium is formed, an alkyne could insert into the Rh-C bond. Another azolyl group could then transfer to rhodium from azolylcopper compound. The subsequent intramolecular trans-nucleophilic addition generates the second C-C bond. Meanwhile, the putative pathway for the formation of the hydroheteroarylated byproduct has also been explained by theoretical calculations.

Mechanism of rhodium(III)-catalyzed formal C(sp3)[sbnd]H activation/spiroannulation of α-arylidene pyrazolones with alkynes: A computational study

The rhodium-catalyzed formal C(sp3)H activation/spiroannulation of α-arylidene pyrazolones with alkynes was investigated by means of density functional theory calculations. The calculations indicate that the spiroannulation through the proposed CC reductive elimination is kinetically unfeasible. Instead, the CC coupling from the eight-membered rhodacycle was proposed to account for the experimental results. The overall catalytic cycle consists of six steps: (1) the keto-enol isomerization; (2) the OH deprotonation, (3) the C(sp2)H bond cleavage; (4) the migratory insertion of alkyne into the RhC bond; (5) the CC coupling and (6) the regeneration of the active catalyst.

Mechanism, Regio-, and Diastereoselectivity of Rh(III)-Catalyzed Cyclization Reactions of N-Arylnitrones with Alkynes: A Density Functional Theory Study.

Nitrones have been used for rhodium-catalyzed cyclization C-H bond activation and O atom transfer of arylnitrones with alkynes by Chang et al. ( J. Am. Chem. Soc. 2015 , 137 , 4908 - 4911 ). Density functional theory method has been used to study the mechanism, regio-, and diastereoselectivity of type reactions. The results elucidated that the reaction pathway for Rh(III)-catalyzed cyclization of N-arylnitrones with alkyne contains a C-H bond activation, an alkyne insertion into Rh-C bond, a reductive elimination to form a Rh(I) complex, an oxidative addition leading to N-O cleavage, an imine insertion into the Rh-C bond, and the final protonolysis to regenerate the products and the active catalyst. The regioselectivity of this reaction with asymmetric alkyne is controlled by the electronic effect in alkyne insertion type instead of steric effects. The distortion-interaction analysis is also used to explain the regioselectivity. The diastereoselectivity is controlled by the imine insertion step. In this step, the sterically less hindered transition state is favored, leading to stereoselective product formation.

Rational Design of Emissive NIR-Absorbing Chromophores: Rh(III) Porphyrin-Aza-BODIPY Conjugates with Orthogonal Metal-Carbon Bonds.

The facile synthesis of Group 9 Rh(III) porphyrin-aza-BODIPY conjugates that are linked through an orthogonal Rh-C(aryl) bond is reported. The conjugates combine the advantages of the near-IR (NIR) absorption and intense fluorescence of aza-BODIPY dyes with the long-lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge-transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh(III) conjugates exhibit strong aza-BODIPY-centered fluorescence at around 720 nm (ΦF =17-34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet-oxygen quantum yield (ΦΔ =19-27 %, λex =690 nm) have been observed. Nanosecond pulsed time-resolved absorption spectroscopy confirms that relatively long-lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.