Kinetics and mechanism of dissociation of ethylenedibiguanidenickel(II) and copper(II) and of bisbiguanide-nickel(II) in acid

Summary The kinetics of dissociation of the ethylenedibiguanidenickel(II) and copper(II), [M(EndibigH₂)]²⁺, where M = Ni^II or Cu^II, and bisbiguanide-nickel(II), [Ni(BigH)₂]²⁺, complexes in acid media forming the aquo-metal ions and the protonated ligand as the ultimate products have been studied by the stopped-flow technique. The reactions occur in two consecutive steps, the first being faster than the second, forming aquometal ions and protonated ligands as the ultimate products. For each step the rate is acid dependent and may be expressed by: k_x = k _x [H⁺] + k _x [H⁺]², where k_x is the observed rate constant and x = f or s for fast and slow steps respectively. Both paths (k _x and k _x ) contribute in the ethylenedibiguanidenickel(II) system, whereas k _x paths are virtually absent in the corresponding copper(II) complex; k _x paths are absent in the bisbiguanidenickel(II) system. A likely mechanism involves protonation of the bound ligand which facilitates its dissociation. A comparison of the S values indicates considerable solvent participation in the transition state, suggesting an assisted dissociative mechanism, which also accounts for the low H values. The observed lability order is [Ni(EndibigH₂)]²⁺ < [Ni(BigH)₂]²⁺ =ca. [Cu(EndibigH₂)]²⁺, and [Cu(BigH)₂]²⁺ dissociates under comparable conditions at a rate too fast to be measured by the stopped-flow method.     

Kinetics and mechanism of dissociation of copper(II) complexes of biguanide and someN 1-substituted biguanides in aqueous acetate buffer media

The kinetics fo dissociation of thebis complexes [Cu(LH)₂]²⁺ formed by Cu^II with biguanide andN ¹-substituted methyl, phenyl, dimethyl and diethyl biguanides into the mono biguanide complexes in aqueous NaOAc-HOAc buffer media have been studied by stopped-flow spectrophotometry. The results, under pseudo-first-order conditions, indicate k_obs=k_o+k_H[H⁺]. For the different complexes k_o values are comparable, but k_H values differ appreciably; log k_H versus log K _d ^H is linear withca. unit slope K _d ^H being the equilibrium constant for the process:

Dynamics of solvent and rotational relaxation of coumarin 153 in room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate confined in Brij-35 micelles: a picosecond time-resolved fluorescence spectroscopic study

The dynamics of solvent and rotational relaxation of Coumarin 153 (C-153) in ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and in the ionic liquid confined in Brij-35 micellar aggregates have been investigated using steady-state and time-resolved fluorescence spectroscopy. We observed slower dynamics in the presence of micellar aggregates as compared to the pure IL. However, the slowing down in the solvation time on going from neat IL to IL-confined micelles is much smaller compared to that on going from water to water-confined micellar aggregates. The increase in solvation and rotational time in micelles is attributed to the increase in viscosity of the medium. The slow component is assumed to be dependent on the viscosity of the solution and involves large-scale rearrangement of the anions and cations while fast component is assumed to originate from the initial response of the anions during excitation. The slow component increases due to the increase in the viscosity of the medium and increase in fast component is probably due to the hydrogen bonding between the anions and polar headgroup of the surfactant. The dynamics of solvent relaxation was affected to a small extent due to the micelle formation.     

Diverse strategies for transition metal catalyzed distal C(sp3)–H functionalizations

Transition metal catalyzed C(sp³)–H functionalization is a rapidly growing field. Despite severe challenges, distal C–H functionalizations of aliphatic molecules by overriding proximal positions have witnessed tremendous progress. While usage of stoichiometric directing groups played a crucial role, reactions with catalytic transient directing groups or methods without any directing groups are gaining more attention due to their practicality. Various innovative strategies, slowly but steadily, circumvented issues related to remote functionalizations of aliphatic molecules. A systematic compilation has been presented here to provide insights into the recent developments and future challenges in the field. The Present perspective is expected to open up a new dimension and provide an avenue for deep insights into the distal C(sp³)–H functionalizations that could be applied routinely in various pharmaceutical and agrochemical industries.

Transition metal catalyzed C(sp³)–H functionalization is a rapidly growing field.     

Expanding medicinal chemistry into 3D space: metallofragments as 3D scaffolds for fragment-based drug discovery

Fragment-based drug discovery (FBDD) is a powerful strategy for the identification of new bioactive molecules. FBDD relies on fragment libraries, generally of modest size, but of high chemical diversity. Although good chemical diversity in FBDD libraries has been achieved in many respects, achieving shape diversity – particularly fragments with three-dimensional (3D) structures – has remained challenging. A recent analysis revealed that >75% of all conventional, organic fragments are predominantly 1D or 2D in shape. However, 3D fragments are desired because molecular shape is one of the most important factors in molecular recognition by a biomolecule. To address this challenge, the use of inert metal complexes, so-called ‘metallofragments’ (mFs), to construct a 3D fragment library is introduced. A modest library of 71 compounds has been prepared with rich shape diversity as gauged by normalized principle moment of inertia (PMI) analysis. PMI analysis shows that these metallofragments occupy an area of fragment space that is unique and highly underrepresented when compared to conventional organic fragment libraries that are comprised of orders of magnitude more molecules. The potential value of this metallofragment library is demonstrated by screening against several different types of proteins, including an antiviral, an antibacterial, and an anticancer target. The suitability of the metallofragments for future hit-to-lead development was validated through the determination of IC₅₀ and thermal shift values for select fragments against several proteins. These findings demonstrate the utility of metallofragment libraries as a means of accessing underutilized 3D fragment space for FBDD against a variety of protein targets.

Fragment-based drug discovery (FBDD) using 3-dimensional metallofragments is a new strategy for the identification of bioactive molecules.