Dirhodium Complex Rh2 Research Articles

Reactivity of α-diazo sulfonium salts: rhodium-catalysed ring expansion of indenes to naphthalenes.

In the presence of catalytic amounts of the paddlewheel dirhodium complex Rh2(esp)2, α-diazo dibenzothiophenium salts generate highly electrophilic Rh-coordinated carbenes, which evolve differently depending on their substitution pattern. Keto-moieties directly attached to the azomethinic carbon promote carbene insertion into one of the adjacent C-S bonds, giving rise to highly electrophilic dibenzothiopyrilium salts. This intramolecular pathway is not operative when the carbene carbon bears ester or trifluoromethyl substituents; in fact, these species react with olefins delivering easy to handle cyclopropyl-substituted sulfonium salts. When indenes are the olefins of choice, the initially formed cyclopropyl rings smoothly open with concomitant departure of dibenzothiophene, enabling access to a series of 2-functionalized naphthalenes.

Rhodium-Catalyzed Intramolecular Nitrogen Atom Insertion into Arene Rings.

In this study, we describe the direct insertion of an intramolecular nitrogen atom into an aromatic C-C bond. In this transformation, carbamoyl azides are activated by a Rh catalyst and subsequently directly inserted into the C-C bond of an arene ring to access fused azepine products. This transformation is challenging, owing to the existence of a competitive C-H amination pathway. The use of a paddlewheel dirhodium complex Rh2(esp)2 effectively inhibited the undesired C-H insertion. Density functional theory calculations were performed to reveal the reaction mechanism and origin of the chemoselectivity of the Rh-catalyzed reactions. The novel fused azepine products are highly robust and allow for downstream diversification.

Computational Exploration of Dirhodium Complex-Catalyzed Selective Intermolecular Amination of Tertiary vs. Benzylic C-H Bonds.

The mechanism and origins of site-selectivity of Rh2(S-tfpttl)4-catalyzed C(sp3)-H bond aminations were studied using density functional theory (DFT) calculations. The synergistic combination of the dirhodium complex Rh2(S-tfpttl)4 with tert-butylphenol sulfamate TBPhsNH2 composes a pocket that can access both tertiary and benzylic C-H bonds. The nonactivated tertiary C-H bond was selectively aminated in the presence of an electronically activated benzylic C-H bond. Both singlet and triplet energy surfaces were investigated in this study. The computational results suggest that the triplet stepwise pathway is more favorable than the singlet concerted pathway. In the hydrogen atom abstraction by Rh-nitrene species, which is the rate- and site-selectivity-determining step, there is an attractive π-π stacking interaction between the phenyl group of the substrate and the phthalimido group of the ligand in the tertiary C-H activation transition structure. By contrast, such attractive interaction is absent in the benzylic C-H amination transition structure. Therefore, the DFT computational results clearly demonstrate how the synergistic combination of the dirhodium complex with sulfamate overrides the intrinsic preference for benzylic C-H amination to achieve the amination of the nonactivated tertiary C-H bond.

Enantioselective Synthesis of α-Aryl-β-Aminocyclopropane Carboxylic Acid Derivatives via Rh(II)-Catalyzed Cyclopropanation of Vinylsulfonamides with α-Aryldiazoesters.

The reaction of vinylsulfonamides with donor-acceptor carbenes derived from α-aryldiazoesters, catalyzed by the tert-butyl glycine-derived dirhodium complex Rh2(S-4-Br-NTTL)4, has been reported. This method provides a variety of α-aryl-β-aminocyclopropane carboxylic acid derivatives bearing one quaternary carbon stereogenic center vicinal to the amino-substituted carbon in high yields with excellent diastereo- and enantioselectivities. Vinylsulfonamides showed complementary advantages over the well-developed vinylamides or vinylcarbamates for this Rh(II)-catalyzed cyclopropanation strategy. Moreover, these conformationally restricted α-aryl-β-aminocyclopropyl carboxylic acid derivatives can be readily incorporated into dipeptides.

Immobilization of a chiral dirhodium catalyst on SBA-15 via click-chemistry: Application in the asymmetric cyclopropanation of 3-diazooxindole with aryl alkenes

A novel immobilized chiral dirhodium catalyst, Rh2(S-PTTL)3(S-PTTL-linker)∼SBA-15 (8), has been prepared via click reaction of azide-groups on functionalized SBA-15 with the dirhodium complex Rh2(S-PTTL)3(S-PTTL-alkyne) (6) containing an alkyne moiety. During the synthesis of this complex, one chiral ligand of the parent Rh2(S-PTTL)4 catalyst is exchanged with an analogous chiral ligand system containing an alkyne moiety, which to a great extent maintains the intrinsic catalytic performance of the catalyst. The heterogeneous dirhodium catalyst is characterized by FT-IR and 13C solid-state NMR to validate the successful immobilization. The catalytic performance of the heterogeneous catalyst 8 is investigated in the asymmetric cyclopropanation of 3-diazooxindole with different aryl alkenes that form spiro-cyclopropyloxindoles which serve as precursors for pharmaceuticals. The resulting heterogeneous catalyst shows high catalytic activity and significant enantioselectivity. Importantly, it can be readily recovered and reused at least four times without significant loss of its catalytic performance.

Electroreductive dechlorination of γ-Hexachlorocyclohexane catalyzed by Rh2(dpf)4 in nonaqueous media, where dpf = N,N′-Diphenylformamidinate (1-) ion

The electroreductive dechlorination of γ-hexachlorocyclohexane (lindane) was investigated in non-aqueous media using as a catalyst the dirhodium complex Rh2(dpf)4, where dpf=the anion of N, N′-diphenylformamidinate. Rh2(dpf)4 undergoes two reversible one-electron redox processes in pyridine and/or N,N′-dimethylformamide (DMF), corresponding respectively to Rh24+/Rh25+ and Rh24+/Rh23+. Electrochemical and spectroelectrochemical measurements indicate that lindane reacts with the Rh23+ center in singly reduced [Rh2(dpf)4]−, giving a transient carbon-bonded dirhodium complex along with Cl−. Further catalytic dechlorination of lindane subsequently occurs with the activity of Rh2(dpf)4 as a catalyst depending upon the solvent conditions. Rh2(dpf)4 shows a higher catalytic reduction current in pyridine than in DMF for the degradation of lindane. This catalytic current can be further enhanced if 6% water (v/v) is added to the pyridine solution. Bulk controlled-potential electrolysis combined with GC–MS verified that although the addition of water to the neat pyridine increased the catalytic activity of Rh2(dpf)4, neat pyridine is still a better solvent for the formation of more extensively dechlorinated products of lindane. In neat pyridine, lindane can be converted stepwise to tetrachlorocyclohexene, dichlorobenzene isomers, chlorobenzene and the fully dechlorinated product benzene, the last of which becomes predominant at the end of the electrolysis. In pyridine having 6% added water, other than those dechlorinated products seen above, trichlorobenzene isomers are formed upon lindane degradation. Meanwhile, the relative concentrations of dichlorobenzene isomers and chlorobenzene become higher, while the yield of benzene becomes lower than the corresponding results obtained in neat pyridine. Based on the measured distribution of the products under the different solvent conditions, an overall pathway for the degradation of lindane is proposed.

Synthesis of complex hexacyclic compounds via a tandem Rh(II)-catalyzed double-cyclopropanation/Cope rearrangement/Diels-Alder reaction.

Treatment of (E)-1-(methoxymethylene)-1,2,3,4-tetrahydronaphthalene with styryl diazoacetates in the presence of catalytic amounts of the dirhodium complex Rh2(S-DOSP)4 provides a highly enantioenriched hexacyclic product with 10 new stereogenic centers. The transformation proceeds by a cascade sequence starting with a double cyclopropanation of a benzene ring, followed by a Cope rearrangement of a divinylcyclopropane and then an intramolecular Diels–Alder cycloaddition.

ChemInform Abstract: Carbenoid‐Mediated N—O Bond Insertion and Its Application in the Synthesis of Pyridines.

Highly efficient synthesis of 3-hydroxypyridines has been developed based on carbenoid-mediated N–O bond insertion. Treatment of δ-diazo oxime ethers with dirhodium complex Rh2(tfacam)4 rapidly provides a variety of pyridines in 5–10 minutes in good to excellent yields.

Enantioselective Synthesis of Trifluoromethyl-Substituted Cyclopropanes

The reaction of 1-aryl-2,2,2-trifluorodiazoethanes with alkenes, catalyzed by the adamantylglycine-derived dirhodium complex Rh2(R-PTAD)4, generates trifluoromethyl-substituted cyclopropanes with high diastereoselectivity (>94%) and enantioselectivity (88->98%).

Phenylselenenylalkanes, their adducts with the dirhodium complex Rh2(MTPA)4 and ligand exchange mechanisms in solution as studied by 1H, 13C and 77Se NMR spectroscopy

AbstractTwelve secondary phenylselenenylalkanes and ‐cycloalkanes were studied by 1H, 13C and 77Se NMR spectroscopy in the presence of the chiral dirhodium complex Rh2[(R)‐MTPA]4 [Rh–Rh; MTPA‐H = (R)‐(+)‐methoxytrifluoromethylphenylacetic acid, Mosher's acid]. The 1 : 1 and 2 : 1 adducts were identified in solution at low temperatures. Two different mechanisms of ligand exchange, ‘switch’ and ‘replacement,’ were characterized and their energy barriers estimated and steric congestion during the exchange transitions is discussed. Coordination‐induced shifts Δδ(77Se) are generally negative (shielding). For menthone bis(phenylselenoacetal) (7), these values indicate that a selection of the two selenium atoms occurs showing that 7 prefers complexation at the equatorial selenium atom whereas the axial selenium atom is hardly involved. Copyright © 2003 John Wiley & Sons, Ltd.

Dirhodium tetraacylate complexes and monovalent ligands. Adduct formation in solution as monitored by NMR spectroscopy

Evidences of adduct formation between the dirhodium complex Rh2[(R)-MTPA4] (Rh–Rh; MTPA-H = Mosher acid) and various monovalent ligands L were obtained in solution by using NMR spectroscopy. Strongly binding ligands, e.g. selenoethers or nitriles, prefer L → Rh–Rh ← L adducts whereas weaker ligands, e.g. olefins with low steric hindrance, tend to form oligomers with bridging L molecules: ⋯Rh–Rh⋯L⋯Rh–Rh⋯. Larger steric congestion forces olefins to avoid bridge situations or even prohibits adduct formation at all.

Chiral Discrimination of Some Annelated Xanthine Derivatives by the Dirhodium Method

Complete 1H and 13C signal assignments and conformation analyses of the title compounds (racemic mixtures) were performed. Most 1H and 13C NMR signals were resolved in the presence of the enantiomerically pure dirhodium complex Rh2(MTPA)4 (Rh*) allowing for clear and simple chiral recognition. A detailed interpretation of the signal shifts (Δδ) and dispersions (Δν) in the diastereomeric complexes suggested that the π-system around the C-4/C-5 bond of the central imidazole ring (B) is a binding site of the xanthine derivatives. This assumption was based on HOMO−LUMO considerations including back-donation and on the fact that diastereotopic dispersions for protons within methylene groups are very different. Signal shifts are in the range of 0.5 to 1 ppm for carbon atoms close to the binding site and fade away at the periphery of the molecule. Dispersions are spread all over the molecule.