Investigating the mechanism of Ni-mediated trifluoromethylthiolation of aryl halides using AgSCF3

The mechanism of the Ni-catalysed trifluoromethylthiolation of aryl chlorides using AgSCF₃ is studied herein. A variety of IPr Ni^II complexes were synthesized via oxidative addition of Ni⁰ to 2-(2-chloro)phenylpyridines. Their reactivity with AgSCF₃ was tested by performing stoichiometric experiments, cyclic voltammetry, and NMR spectroscopic studies. CuSCF₃ was shown to behave similarly to AgSCF₃, while reactions with NMe₄SCF₃ revealed a major stoichiometric side reaction that forms a nickel fluoride complex. NMR kinetic studies revealed there is little correlation between the electron-withdrawing/donating nature of the para substituents on either the phenyl or pyridyl groups with the formation of the corresponding products. Cyclic voltammetry (CV) indicated the feasibilty of Ni^I/Ni^III transitions, and an increased rate of formation of product was observed with increased solvent polarity. Evidence suggests that the mechanism proceeds via inner-sphere electron transfer (ET) from AgSCF₃ to Ni^II, ultimately leading to the formation of the trifluoromethylthiolated product.

Direct metal-carbon bonding in symmetric bis(C-H) agostic nickel(i) complexes

Agostic interactions are examples of σ-type interactions, typically resulting from interactions between C–H σ-bonds with empty transition metal d orbitals. Such interactions often reflect the first step in transition metal-catalysed C–H activation processes and thus are of critical importance in understanding and controlling σ bond activation chemistries. Herein, we report on the unusual electronic structure of linear electron-rich d⁹ Ni(i) complexes with symmetric bis(C–H) agostic interactions. A combination of Ni K edge and L edge XAS with supporting TD-DFT/DFT calculations reveals an unconventional covalent agostic interaction with limited contributions from the valence Ni 3d orbitals. The agostic interaction is driven via the empty Ni 4p orbitals. The surprisingly strong Ni 4p-derived agostic interaction is dominated by σ contributions with minor π contributions. The resulting ligand–metal donation occurs directly along the C–Ni bond axis, reflecting a novel mode of bis-agostic bonding.

Symmetric Ni(i) agostic complexes reveal an unusual mode of bonding that is dominated by direct carbon-to-metal charge transfer.

Multisite Evaluation of Next-Generation Methods for Small RNA Quantification

Small RNAs (smRNAs) are important regulators of many biologic processes and are now most frequently characterized using Illumina sequencing. However, although standard RNA sequencing library preparation has become routine in most sequencing facilities, smRNA sequencing library preparation has historically been challenging because of high input requirements, laborious protocols involving gel purifications, inability to automate, and a lack of benchmarking standards. Additionally, studies have suggested that many of these methods are nonlinear and do not accurately reflect the amounts of smRNAs in vivo. Recently, a number of new kits have become available that permit lower input amounts and less laborious, gel-free protocol options. Several of these new kits claim to reduce RNA ligase-dependent sequence bias through novel adapter modifications and to lessen adapter-dimer contamination in the resulting libraries. With the increasing number of smRNA kits available, understanding the relative strengths of each method is crucial for appropriate experimental design. In this study, we systematically compared 9 commercially available smRNA library preparation kits as well as NanoString probe hybridization across multiple study sites. Although several of the new methodologies do reduce the amount of artificially over- and underrepresented microRNAs (miRNAs), we observed that none of the methods was able to remove all of the bias in the library preparation. Identical samples prepared with different methods show highly varied levels of different miRNAs. Even so, many methods excelled in ease of use, lower input requirement, fraction of usable reads, and reproducibility across sites. These differences may help users select the most appropriate methods for their specific question of interest.

Reversible PtII-CH3 deuteration without methane loss: metal-ligand cooperation vs. ligand-assisted PtII-protonation

Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)Pt^II(CH₃)₂, reacts with CD₃OD at 25 °C to undergo complete deuteration of Pt-CH₃ fragments in ∼5 h without loss of methane to form (dpk)Pt^II(CD₃)₂ in virtually quantitative yield. The deuteration can be reversed by dissolution in CH₃OH or CD₃OH. Kinetic analysis and isotope effects, together with support from density functional theory calculations indicate a metal-ligand cooperative mechanism wherein DPK enables Pt-CH₃ deuteration by allowing non-rate-limiting protonation of Pt^II by CD₃OD. In contrast, other model di(2-pyridyl) ligands enable rate-limiting protonation of Pt^II, resulting in non-rate-limiting C-H(D) reductive coupling. Owing to its electron-poor nature, following complete deuteration, DPK can be dissociated from the Pt^II-centre, furnishing [(CD₃)₂Pt^II(μ-SMe₂)]₂ as the perdeutero analogue of [(CH₃)₂Pt^II(μ-SMe₂)]₂, a commonly used Pt^II-precursor.

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of ³¹ P NMR, Ni K-edge XAS, Ni K_β XES, and DFT calculations. These complexes are best described as square planar d¹⁰ complexes with π-backbonding acting as the dominant contributor to M-L bonding to the π-ligand. The degree of backbonding correlates with ² J_PP from NMR and the energy of the Ni 1s→4p_z pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*_ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d¹⁰ electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*_ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.

Role of Phosphine Sterics in Strained Aminophosphine Chelate Formation

The preparation of four-membered aminophosphine (PN) chelates from common metal precursors has largely evaded realization because of the ring strain associated with these species. We report a straightforward approach to the synthesis of such PN metallacycles using simple α-PN ligands analogous to the popular class of small-bite-angle diphosphinomethane ligands. It is demonstrated that bulky phosphine substituents are important to the formation of these chelates.

Platinum-mediated B–H methoxylation of bis(pyrazolyl)borate

Mechanistic investigations into the role of Pt(ii) in mediating methoxylation of B–H fragments of bis(pyrazolyl)borate.

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.

Oxaziridine cleavage with a low-valent nickel complex: competing C–O and C–N fragmentation from oxazanickela(ii)cyclobutanes

Nickel(0) was found to oxidatively add to oxaziridines to form oxazanickelacyclobutanes which cleave to yield complicated mixtures, including nickel imido and aldehyde complexes.

1,3-N,O-Complexes of late transition metals. Ligands with flexible bonding modes and reaction profiles

1,3-N,O-Chelating ligands are ubiquitous in nature owing to their occurrence as α-chiral amino acids in metalloproteins.

Dehydrogenation of cyclic amines by a coordinatively unsaturated Cp*Ir(iii) phosphoramidate complex

The reaction of a Cp*Ir(iii) phosphoramidate complex with secondary amines gives amine, imine-bound Cp*Ir(iii) hydride complexes resulting from amine dehydrogenation.

Hepatic Temporal Gene Expression Profiling in Helicobacter hepaticus-Infected A/JCr Mice

Helicobacter hepaticus infection of A/JCr mice is a model of infectious liver cancer. We monitored hepatic global gene expression profiles in H. hepaticus infected and control male A/JCr mice at 3 months, 6 months, and 1 year of age using an Affymetrix-based oligonucleotide microarray platform on the premise that a specific genetic expression signature at isolated time points would be indicative of disease status. Model based expression index comparisons generated by dChip yielded consistent profiles of differential gene expression for H. hepaticus infected male mice with progressive liver disease versus uninfected control mice within each age group. Linear discriminant analysis and principal component analysis allowed segregation of mice based on combined age and lesion status, or age alone. Up-regulation of putative tumor markers correlated with advancing hepatocellular dysplasia. Transcriptionally down-regulated genes in mice with liver lesions included those related to peroxisome proliferator, fatty acid, and steroid metabolism pathways. In conclusion, transcriptional profiling of hepatic genes documented gene expression signatures in the livers of H. hepaticus infected male A/JCr mice with chronic progressive hepatitis and preneoplastic liver lesions, complemented the histopathological diagnosis, and suggested molecular targets for the monitoring and intervention of disease progression prior to the onset of hepatocellular neoplasia.

Total Synthesis of K777: Successful Application of Transition-Metal-Catalyzed Alkyne Hydrothiolation toward the Modular Synthesis of a Potent Cysteine Protease Inhibitor

We report the total synthesis of K777 and a series of analogues via alkyne hydrothiolation catalyzed by Wilkinson's complex (ClRh(PPh3)3). The alkyne hydrothiolation reactions proceeded with excellent regio- and diastereoselectivity to generate the desired E-linear vinyl sulfides in high yield. The use of Ellman's auxiliary generates the requisite propargyl amines in excellent enantiomeric excess (ee) and obviates the use of L-homophenylalanine, an expensive unnatural amino acid. The vinyl sulfone derivatives exhibit a large difference in rate toward Michael addition. Kinetic data are consistent with rate-limiting nucleophilic attack to generate the carbanion intermediate.

Capturing HBCy2: Using N,O-Chelated Complexes of Rhodium(I) and Iridium(I) for Chemoselective Hydroboration

1,3-N,O-chelated complexes of Rh(I) and Ir(I) cooperatively and reversibly stabilized the B-H bond of HBCy2 to afford six-membered metallaheterocycles (M=Rh (7) or Ir (8)) having a δ-[M]⋅⋅⋅H-B agostic interaction. Treatment of these Shimoi-type borane adducts 7 or 8 with both an aldehyde and an alkene resulted in chemoselective aldehyde hydroboration and reformation of the 1,3-N,O-chelated starting material. The observed chemoselectivity is inverted from that of free HBCy2 , which is selective for alkene hydroboration.

Exclusive Csp(3)-Csp(3) vs Csp(2)-Csp(3) Reductive Elimination from Pt(IV) Governed by Ligand Constraints

Selective reductive elimination of ethane (Csp(3)-Csp(3) RE) was observed following bromide abstraction and subsequent thermolysis of a Pt(IV) complex bearing both Csp(3)- and Csp(2)-hybridized hydrocarbyl ligands. Through a comparative experimental and theoretical study with two other Pt(IV) complexes featuring greater conformational flexibility of the ligand scaffold, we show that the rigidity of a meridionally coordinating ligand raises the barrier for Csp(2)-Csp(3) RE, resulting in unprecedented reactivity.

Synthesis of 2-Nickela(II)oxetanes from Nickel(0) and Epoxides: Structure, Reactivity, and a New Mechanism of Formation

2-Nickelaoxetanes have been frequently invoked as reactive intermediates in catalytic reactions of epoxides using nickel, but have never been isolated or experimentally observed in these transformations. Herein, we report the preparation of a series of well-defined nickelaoxetanes formed via the oxidative addition of nickel(0) with epoxides featuring ketones. The stereochemistry of the products is retained, which has not yet been reported for nickelaoxetanes. Theoretical calculations support a bimetallic ring-opening/closing pathway over a concerted oxidative addition. Initial reactivity studies of a nickelaoxetane demonstrated protonolysis, oxidatively induced reductive elimination, deoxygenation, and elimination reactions when treated with the appropriate reagents.

Inhibition of aberrant complement activation by a dimer of acetylsalicylic acid

We here report synthesis for the first time of the acetyl salicylic acid dimer 5,5'-methylenebis(2-acetoxybenzoic acid) (DAS). DAS inhibits aberrant complement activation by selectively blocking factor D of the alternative complement pathway and C9 of the membrane attack complex. We have previously identified aurin tricarboxylic and its oligomers as promising agents in this regard. DAS is much more potent, inhibiting erythrocyte hemolysis by complement-activated serum with an IC50 in the 100-170 nanomolar range. There are numerous conditions where self-damage from the complement system has been implicated in the pathology, including such chronic degenerative diseases of aging as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and age-related macular degeneration. Consequently, there is a high priority for the discovery and development of agents that can successfully treat such conditions. DAS holds considerable promise for being such an agent.

Fluorine

Welcome to this themed issue of Dalton Transactions entitled ‘Fluorine’.

Isocyanate deinsertion from κ1-O amidates: facile access to perfluoroaryl rhodium(i) complexes

Perfluorophenyl-subsituted amidates undergo unprecedented C₆F₅ migration to Rh(i) on heating, providing a new route to Rh-C₆F₅ organometallic fragments.