Ruthenium carboxylate complexes as easily prepared and efficient catalysts for the synthesis of β-oxopropyl esters

Carboxylate-Capped Analogues of Ru265 Are MCU Inhibitor Prodrugs

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that resides on the inner membrane of the mitochondria and mediates calcium uptake into this organelle. Given the critical role of mitochondrial calcium trafficking in cellular function, inhibitors of this channel have arisen as tools for studying the biological relevance of this process and as potential therapeutic agents. In this study, four new analogues of the previously reported Ru-based MCU inhibitor [ClRu(NH3)4(μ-N)Ru(NH3)4Cl]Cl3 (Ru265) are reported. These compounds, which bear axial carboxylate ligands, are of the general formula [(RCO2)Ru(NH3)4(μ-N)Ru(NH3)4(O2CR)]X3, where X = NO3- or CF3SO3- and R = H (1), CH3 (2), CH2CH3 (3), and (CH2)2CH3 (4). These complexes were fully characterized by IR spectroscopy, NMR spectroscopy, and elemental analysis. X-ray crystal structures of 1 and 3 were obtained, revealing the expected presence of both the linear Ru(μ-N)Ru core and axial formate and propionate ligands. The axial carboxylate ligands of complexes 1-4 are displaced by water in buffered aqueous solution to give the aquated compound Ru265'. The kinetics of these processes were measured by 1H NMR spectroscopy, revealing half-lives that span 5.9-9.9 h at 37 °C. Complex 1 with axial formate ligands underwent aquation approximately twice as fast as the other compounds. In vitro cytotoxicity and mitochondrial membrane potential measurements carried out in HeLa and HEK293T cells demonstrated that none of these four complexes negatively affects cell viability or mitochondrial function. The abilities of 1-4 to inhibit mitochondrial calcium uptake in permeabilized HEK293T cells were assessed and compared to that of Ru265. Fresh solutions of 1-4 are approximately 2-fold less potent than Ru265 with IC50 values in the range of 14.7-19.1 nM. Preincubating 1-4 in aqueous buffers for longer time periods to allow for the aquation reactions to proceed increases their potency of mitochondrial uptake inhibition to match that of Ru265. This result indicates that 1-4 are aquation-activated prodrugs of Ru265'. Finally, 1-4 were shown to inhibit mitochondrial calcium uptake in intact, nonpermeabilized cells, revealing their value as tools and potential therapeutic agents for mitochondrial calcium-related disorders.

Coordination Chemistry of the Parent Dithiocarbamate H2NCS2-: Organometallic Chemistry and Tris-Chelates of Group 9 Metals

Two tris-chelate complexes of cobalt and rhodium and two complexes of Ru(II) of dithiocarbamate, [S₂CNH₂]^-, were synthesized. The complexes were spectroscopically characterized by IR, NMR, UV-vis, and MS and structurally characterized by X-ray diffraction. The structural features of the rhodium complex were compared to those of other tris-chelate Rh(III) dithiocarbamate complexes and are characterized by a change in the ground-state geometry in comparison to expected octahedral tris-chelate complexes. This was confirmed both experimentally by X-ray diffraction and theoretically using DFT calculations. The inversion barriers of Rh(Bz₂dtc)₃, Ir(Bz₂dtc)₃, and Rh(Et₂dtc)₃ were determined using VT-NMR in DMSO. These barriers were found to be surprisingly low for heavy group 9 elements of d⁶ tris-chelate complexes: values of 16.7, 17.1, and 16.4 kcal/mol were calculated, respectively. By comparing structural features, we are able to determine that the activation barrier for the inversion of stereochemistry of Rh(H₂dtc)₃ must have a similarly low value. A modified version of the Bailar twist involving an intermediate with C_3h geometry was proposed as the mechanism of inversion.

Fluxionality in the Tropolone Hinokitiol Chelate

Tropolonate complexes of Ru(II), Ru(III), and Os(II) with hinokitiol, also termed β-thuljaplicin, or 4-isopropyltropolone, readily formed by chloride and triarylphosphine substitution in RuCl₂(PPh₃)₃ and MHCl(CO)(PR₃)₃ (M = Ru, R = Ph; M = Os, R = Ph and p-tolyl). The resulting colorful complexes have variable and strong charge transfer bands and also have a surprising combination of stereochemical selectivity and lability. For the Os(II) d⁶ examples, the tropolone chelate has a fluxionality with a barrier of only 90 kJ/mol for the R = aryl examples, as determined by variable-temperature ³¹P NMR. Chlorination with N-chlorosuccinimide results in MCl(CO)(hino)(PPh₃)₂ (M = Ru and Os). Together these results quantify the fluxionality of this important chelate which in turn has consequences for its biochemistry.

Synthesis of α-Acyloxyketone Derivatives via the Platinum-Catalyzed Migration of Propargylic Esters

The synthesis of α-acyloxyketones via the migration of a propargylic ester followed by the intramolecular nucleophilic addition of the resulting allene was achieved using a cationic platinum catalyst. The optimized conditions for this transformation were determined to be 3 mol% of Pt(cod)Cl2, 3 mol% of AgNTf2, and 3 eq of water in toluene at 100 °C, and these conditions were successfully applied to the synthesis of a wide variety of α-aryl-α-acyloxyketones. The mechanism of this reaction was evaluated in detail based on the results of isotope labeling experiments using H2(18)O.

Cationic mononuclear ruthenium carboxylates as catalyst prototypes for self-induced hydrogenation of carboxylic acids

Carboxylic acids are ubiquitous in bio-renewable and petrochemical sources of carbon. Hydrogenation of carboxylic acids to yield alcohols produces water as the only byproduct, and thus represents a possible next generation, sustainable method for the production of these alternative energy carriers/platform chemicals on a large scale. Reported herein are molecular insights into cationic mononuclear ruthenium carboxylates ([Ru(OCOR)]⁺) as prototypical catalysts for the hydrogenation of carboxylic acids. The substrate-derived coordinated carboxylate was found to function initially as a proton acceptor for the heterolytic cleavage of dihydrogen, and subsequently also as an acceptor for the hydride from [Ru–H]⁺, which was generated in the first step (self-induced catalysis). The hydrogenation proceeded selectively and at high levels of functional group tolerance, a feature that is challenging to achieve with existing heterogeneous/homogeneous catalyst systems. These fundamental insights are expected to significantly benefit the future development of metal carboxylate-catalysed hydrogenation processes of bio-renewable resources.

Many structurally diverse carboxylic acids are available from renewable and petrochemical sources. Here, the authors report a method to reduce carboxylic acids to alcohols using hydrogen, with high selectivity, wherein the carboxylates form active and hydrogen-accepting parts of the catalyst system.

cis,cis,cis-(Acetato-κ(2) O,O')bis-[1,2-bis-(diphenyl-phosphan-yl)ethane-κ(2) P,P']ruthenium(II) 0.75-trifluoro-methane-sulfonate 0.25-chloride

In the title Ru(II) carboxyl-ate compound, [Ru(C2H3O2)(C26H24P2)2](CF3O3S)0.75Cl0.25, the distorted tris-bidentate octa-hedral stereochemistry about the Ru(II) atom in the complex cation comprises four P-atom donors from two 1,2-bis-(diphenyl-phosphan-yl)ethane ligands [Ru-P = 2.2881 (13)-2.3791 (13) Å] and two O-atom donors from the acetate ligand [Ru-O = 2.191 (3) and 2.202 (3) Å]. The disordered counter-anions are located on the same site in the structure in a 3:1 ratio, the expanded formula comprising four complex cations, three trifluoro-methane-sulfonate anions and one chloride anion, with two such formula units in the unit cell.