Addition/elimination in the rhodium(II) perfluorobutyrate catalyzed hydrosilylation of 1-alkenes. Rhodium hydride promoted isomerization and hydrogenation

Cross-Linked Crystals of Dirhodium Tetraacetate/RNase A Adduct Can Be Used as Heterogeneous Catalysts

Due to their unique coordination structure, dirhodium paddlewheel complexes are of interest in several research fields, like medicinal chemistry, catalysis, etc. Previously, these complexes were conjugated to proteins and peptides for developing artificial metalloenzymes as homogeneous catalysts. Fixation of dirhodium complexes into protein crystals is interesting to develop heterogeneous catalysts. Porous solvent channels present in protein crystals can benefit the activity by increasing the probability of substrate collisions at the catalytic Rh binding sites. Toward this goal, the present work describes the use of bovine pancreatic ribonuclease (RNase A) crystals with a pore size of 4 nm (P3₂21 space group) for fixing [Rh₂(OAc)₄] and developing a heterogeneous catalyst to perform reactions in an aqueous medium. The structure of the [Rh₂(OAc)₄]/RNase A adduct was investigated by X-ray crystallography: the metal complex structure remains unperturbed upon protein binding. Using a number of crystal structures, metal complex accumulation over time, within the RNase A crystals, and structures at variable temperatures were evaluated. We also report the large-scale preparation of microcrystals (∼10-20 μm) of the [Rh₂(OAc)₄]/RNase A adduct and cross-linking reaction with glutaraldehyde. The catalytic olefin cyclopropanation reaction and self-coupling of diazo compounds by these cross-linked [Rh₂(OAc)₄]/RNase A crystals were demonstrated. The results of this work reveal that these systems can be used as heterogeneous catalysts to promote reactions in aqueous solution. Overall, our findings demonstrate that the dirhodium paddlewheel complexes can be fixed in porous biomolecule crystals, like those of RNase A, to prepare biohybrid materials for catalytic applications.

Reactivity of a fluorine-containing dirhodium tetracarboxylate compound with proteins

Dirhodium complexes of general formula [Rh₂(O₂CR)₄]L₂ are a well-known class of bimetallic compounds that are used as efficient catalysts for a variety of reactions and have been shown to be potent antibacterial and anticancer agents. The catalytic and biological properties of these complexes largely depend on the nature of the bridging carboxylate ligands. Trifluoroacetate (tfa)-containing dirhodium compounds have been used to build artificial metalloenzymes upon reaction with peptides and have been shown to be more cytotoxic than dirhodium tetraacetate. However, there is no structural information on the interaction between these compounds and proteins. Here, cis-Rh₂(μ-O₂CCH₃)₂(μ-O₂CCF₃)₂ ([cis-Rh₂(OAc)₂(tfa)₂]) has been synthesized and its reaction with bovine pancreatic ribonuclease (RNase A) and hen egg white lysozyme (HEWL) was analyzed using a combination of different techniques, including Fluorine-19 nuclear magnetic resonance spectroscopy and macromolecular X-ray crystallography, with the aim to unveil the differences in the reactivity of tfa-containing dihrodium complexes with proteins when compared to [Rh₂(OAc)₄]. [cis-Rh₂(OAc)₂(tfa)₂] and [Rh₂(OAc)₄] bind the N atoms of His side chains of RNase A at the axial position; however the fluorine-containing compound rapidly loses its tfa ligands, while [Rh₂(OAc)₄] can retain the acetate ligands upon protein binding. The reactivity of [cis-Rh₂(OAc)₂(tfa)₂] with HEWL is slightly distinct when compared to that of [Rh₂(OAc)₄] under the same experimental conditions; however, both [cis-Rh₂(OAc)₂(tfa)₂] and [Rh₂(OAc)₄] degrade when soaked within HEWL crystals. These results provide a structural-based guide for the design of new heterogenous chiral dirhodium/peptide and dirhodium/protein adducts with application in the fields of organic synthesis and asymmetric catalysis.

Digging into protein metalation differences triggered by fluorine containing-dirhodium tetracarboxylate analogues

Catalytic and biological properties of dirhodium tetracarboxylates ([Rh2(μ-O2CR)4L2], L=axial ligand, R=CH3-, CH3CH2-, etc) largely depend on the nature of the bridging carboxylate equatorial μ-O2CR ligands, which can be easily exchanged...

Manganese-Catalyzed Dehydrogenative Silylation of Alkenes Following Two Parallel Inner-Sphere Pathways

We report on an additive-free Mn(I)-catalyzed dehydrogenative silylation of terminal alkenes. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate which undergoes rapid Si–H bond cleavage of the silane HSiR₃ forming the active 16e^– Mn(I) silyl catalyst [Mn(dippe)(CO)₂(SiR₃)] together with liberated butanal. A broad variety of aromatic and aliphatic alkenes was efficiently and selectively converted into E-vinylsilanes and allylsilanes, respectively, at room temperature. Mechanistic insights are provided based on experimental data and DFT calculations revealing that two parallel reaction pathways are operative: an acceptorless reaction pathway involving dihydrogen release and a pathway requiring an alkene as sacrificial hydrogen acceptor.

Isomerization of Functionalized Olefins Using the Dinuclear Catalyst [PdI(μ-Br)(PtBu3)]2: A Mechanistic Study

In a combined experimental and computational study, the isomerization activity of the dinuclear palladium(I) complex [Pd^I(μ‐Br)(P^tBu₃)]₂ towards allyl arenes, esters, amides, ethers, and alcohols has been investigated. The calculated energy profiles for catalyst activation for two alternative dinuclear and mononuclear catalytic cycles, and for catalyst deactivation are in good agreement with the experimental results. Comparison of experimentally observed E/Z ratios at incomplete conversion with calculated kinetic selectivities revealed that a substantial amount of product must form via the dinuclear pathway, in which the isomerization is promoted cooperatively by two palladium centers. The dissociation barrier towards mononuclear Pd species is relatively high, and once the catalyst enters the energetically more favorable mononuclear pathway, only a low barrier has to be overcome towards irreversible deactivation.

Unusual Structural Features in the Adduct of Dirhodium Tetraacetate with Lysozyme

The structures of the adducts formed upon reaction of the cytotoxic paddlewheel dirhodium complex [Rh₂(μ-O₂CCH₃)₄] with the model protein hen egg white lysozyme (HEWL) under different experimental conditions are reported. Results indicate that [Rh₂(μ-O₂CCH₃)₄] extensively reacts with HEWL:it in part breaks down, at variance with what happens in reactions with other proteins. A Rh center coordinates the side chains of Arg14 and His15. Dimeric Rh–Rh units with Rh–Rh distances between 2.3 and 2.5 Å are bound to the side chains of Asp18, Asp101, Asn93, and Lys96, while a dirhodium unit with a Rh–Rh distance of 3.2–3.4 Å binds the C-terminal carboxylate and the side chain of Lys13 at the interface between two symmetry-related molecules. An additional monometallic fragment binds the side chain of Lys33. These data, which are supported by replicated structural determinations, shed light on the reactivity of dirhodium tetracarboxylates with proteins, providing useful information for the design of new Rh-containing biomaterials with an array of potential applications in the field of catalysis or of medicinal chemistry and valuable insight into the mechanism of action of these potential anticancer agents.

Polysiloxanes Grafted with Mono(alkenyl)Silsesquioxanes-Particular Concept for Their Connection

Herein, a facile and efficient synthetic route to unique hybrid materials containing polysiloxanes and mono(alkyl)silsesquioxanes as their pendant modifiers (T₈@PS) was demonstrated. The idea of this work was to apply the hydrosilylation reaction as a tool for the efficient and selective attachment of mono(alkenyl)substituted silsesquioxanes (differing in the alkenyl chain length, from -vinyl to -dec-9-enyl and types of inert groups iBu, Ph at the inorganic core) onto two polysiloxanes containing various amount of Si-H units. The synthetic protocol, determined and confirmed by FT-IR in situ and NMR analyses, was optimized to ensure complete Si-H consumption along with the avoidance of side-products. A series of 20 new compounds with high yields and complete β-addition selectivity was obtained and characterized by spectroscopic methods.

Protein interactions of dirhodium tetraacetate: a structural study

The interactions between the cytotoxic paddlewheel dirhodium complex [Rh2(μ-O2CCH3)4] and the model protein bovine pancreatic ribonuclease (RNase A) were investigated by high-resolution mass spectrometry and X-ray crystallography. The results indicate that [Rh2(μ-O2CCH3)4] extensively reacts with RNase A. The metal compound binds the protein via coordination of the imidazole ring of a His side chain to one of its axial sites, while the dirhodium center and the acetato ligands remain unmodified. Data provide valuable information for the design of artificial dirhodium-containing metalloenzymes.

Silyl radical initiated radical cascade addition/cyclization: synthesis of silyl functionalized 4H-pyrido[4,3,2-gh]phenanthridin-5(6H)-ones

A cyclization cascade initiated by the addition of a silyl radical to an electron-deficient carbon-carbon double bond of N-arylacrylamides, followed by intramolecular cyano group insertion and homolytic aromatic substitution has been reported. In the presence of di-lauroyl peroxide (LPO), under metal-free conditions, several readily available hydrosilanes were successfully used as the source of silyl radicals and a series of silyl functionalized 4H-pyrido[4,3,2-gh]phenanthridin-5(6H)-ones were obtained in moderate to good yields.

Isomerization of Internal Alkynes to Iridium(III) Allene Complexes via C-H Bond Activation: Expanded Substrate Scope, and Progress towards a Catalytic Methodology

The synthesis of a series of allene complexes (POCOP)Ir(η²-RC=(.)=CR') 1b-4b (POCOP = 2,6-bis(di-tert-butylphosphonito)benzene) via isomerization of internal alkynes is reported. We have demonstrated that the application of this methodology is viable for the isomerization of a wide variety of alkyne substrates. Deuterium labeling experiments support our proposed mechanism. The structures of the allene complexes 1b-4b were determined using spectroscopic data analysis. Additionally, the solid-state molecular structure of complex 2b was determined using single crystal X-ray diffraction studies and it confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, we report a preliminary step towards developing a catalytic methodology; the allene may be liberated from the metal center by exposure of the complex to an atmosphere of carbon monoxide.

Bis(imino)pyridine cobalt-catalyzed dehydrogenative silylation of alkenes: scope, mechanism, and origins of selective allylsilane formation

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ((Mes)PDI)CoCH3 ((Mes)PDI = 2,6-(2,4,6-Me3C6H2-N═CMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.

Perspective on dirhodium carboxamidates as catalysts

Dirhodium compounds are emerging as highly efficient catalysts for diverse reactions, and those with carboxamidate ligands have the broadest applications. The unique features of these compounds are their structural rigidity, ease of ligand exchange, open diaxial sites for coordination with Lewis bases, and their low oxidation potential. As consequences of this, dirhodium carboxamidates are efficient and effective catalysts for metal carbene reactions, Lewis acid-catalyzed processes, and chemical oxidations. With chiral carboxamidate ligands these dirhodium compounds show exceptional enantiocontrol for intramolecular cyclopropanation and carbon-hydrogen insertion reactions of diazoacetates, and they are also highly efficient and selective for hetero-Diels-Alder reactions. Their limitations lie in their moderate reactivities for metal carbene generation and Lewis acid catalysis and in the cost of the precious metal rhodium.

Dirhodium(II) Tetrakis(perfluorobutyrate)-Catalyzed 1,4-Hydrosilylation of .ALPHA.,.BETA.-Unsaturated Carbonyl Compounds

The use of dirhodium(II) catalysts in the 1,4-hydrosilylation of alpha,beta-unsaturated ketones and aldehydes was explored. Dirhodium(II) tetrakis(perfluorobutyrate), Rh2(pfb)4, proved to be the catalyst of choice for this process, providing the corresponding silyl enol ethers in high yields.

Transition-metal-promoted hydroboration of alkenes: a unique reversal of regioselectivity

When hydroboration of 1-octene is carried out in the presence of catalytic amounts of rhodium trichloride followed by the usual oxidation (hydrogen peroxide in aqueous alkali), only minor proportions of 1-octanol (2.4%) are formed accompanied by very significant amounts of 2- (17.4%), 3- (36.9%), and 4-octanol (43.3%). These product compositions are obtained in good overall yield when the borane-THF complex is slowly added to a stirred solution of 1-octene in THF solvent containing the rhodium trichloride. Isomerization of 1-octene to 2-, 3-, and 4-octene in the presence of rhodium trichloride alone is far too slow to account for the foregoing results. The mechanism likely involves multiple and reversible addition/elimination of a Rh-activated B-H species across the double bonds.