Direct Transamidation of Thioamides with Amines via Acetophenone-Promoted Enamine Catalysis under Metal-Free Conditions

Herein, we developed a highly selective, efficient, and simple method for direct transamidation of thioamides with amines, promoted by commercially available acetophenone under metal-/solvent-free conditions. The reaction tolerated a wide range of functional groups and substrates, including single- or double-thioamides, benzylamines, or alkyl/cycloalkyl-substituted aliphatic amines. The present protocol can be applied to gram-scale in good yields. In addition, the Pt-/Ni-complexes of double-transamidation products were obtained in good to excellent yields. The investigation of photophysical properties indicated that the fluorescence spectra of Pt-complexes showed an emission band centered at 550-750 nm and exhibited red fluorescence when irradiated by a UV lamp (365 nm).

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure

Sulfur/carbon/sulfur pincer ligands have an interesting combination of strong-field and weak-field donors, a coordination environment that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a proton-responsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N-H bond dissociation free energy, derived from the Bordwell equation, is 56 ± 2 kcal/mol. Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with a large zero field splitting, and X-ray absorption spectra show a high Fe-S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and density functional theory calculations explain this phenomenon through a change between having the doubly occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Metal–ligand cooperativity across two sites of a square planar iron(ii) complex ligated by a tetradentate PNNP ligand

A square planar (PNNP)FeII complex is shown to readily activate two B–H bonds across the Fe–amide linkages in an overall four-electron process facilitated by metal–ligand cooperativity.

Five-Coordinate Low-Spin {FeNO}7 PNP Pincer Complexes

The synthesis and characterization of air-stable cationic mono nitrosonium Fe(I) PNP pincer complexes of the type [Fe(PNP)(NO)Cl]⁺ are described. These complexes are obtained via direct nitroslyation of [Fe(PNP)Cl₂] with nitric oxide at ambient pressure. On the basis of magnetic and EPR measurements as well as DFT calculations, these compounds were found to adopt a low-spin d⁷ configuration and feature a nearly linear bound NO ligand suggesting Fe^INO⁺ rather than Fe^IINO^• character. X-ray structures of all nitrosonium Fe(I) PNP complexes are presented. Preliminary investigations reveal that [Fe(PNP^NH- iPr)(NO)(Cl)]⁺ efficiently catalyzes the conversion of primary alcohols and aromatic and benzylic amines to yield mono N-alkylated amines in good isolated yields.

A Computational Study on the Addition of HONO to Alkynes toward the Synthesis of Isoxazoles; a Bifurcation, Pseudopericyclic Pathways and a Barrierless Reaction on the Potential Energy Surface

Homopropargyl alcohols react with t-BuONO to form acyloximes which can be oxidatively cyclized to yield ioxazoles. The mechanism for the initial reaction of HONO with alkynes to form acyloximes (e.g., 13c) has been explored at the B3LYP/6-31G(d,p) + ZPVE level of theory. The observed chemoselectivity and regioselectivity are explained via an acid-catalyzed mechanism. Furthermore, the potential energy surface revealed numerous surprising features. The addition of HONO (8) to protonated 1-phenylpropyne (18) is calculated to follow a reaction pathway involving sequential transition states (TS6 and TS8), for which reaction dynamics likely play a role. This reaction pathway can bypass the expected addition product 21 as well as transition state TS8, directly forming the rearranged product 23. Nevertheless, TS8 is key to understanding the potential energy surface; there is a low barrier for the pseudopericylic [1,3]-NO shift, calculated to be only 8.4 kcal/mol above 21. This places TS8 well below TS6, making the valley-ridge inflection point (VRI or bifurcation) and direct formation of 23 possible. The final tautomerization step to the acyloxime can be considered to be a [1,5]-proton shift. However, the rearrangement in the case of 17h to 13c is calculated to be barrierless, arguably because the pathway is pseudopericyclic and exothermic.