31P Nuclear Magnetic Resonance Spectroscopy for Monitoring Organic Reactions and Organic Compounds

31P NMR spectroscopy is a consolidated tool for the characterization of organophosphorus compounds and, more recently, for reaction monitoring. The evolution of organic synthesis, mainly due to the combination of elaborated building blocks with enabling technologies, generated great challenges to understand and to optimize the synthetic methodologies. In this sense, 31P NMR experiments also became a routine technique for reaction monitoring, accessing products and side products yields, chiral recognition, kinetic data, intermediates, as well as basic organic parameters, such as acid-base and hydrogen-bonding. This review deals with these aspects demonstrating the essential role of the 31P NMR spectroscopy. The recent publications (the last ten years) will be explored, discussing the experiments of 31P NMR and the strategies accomplished to detect and/or quantify distinct organophosphorus molecules, approaching reaction mechanism, stability, stereochemistry, and the utility as a probe.

Nickel-Catalyzed Sodium Hypophosphite-Participated Direct Hydrophosphonylation of Alkyne toward H-Phosphinates

The traditional methods for the synthesis of phosphinate esters use phosphorus trichloride (PCl₃) as the phosphorous source, resulting in procedures that are often highly polluting and energy intensive. The search for an alternative approach that is both mild and environmentally friendly is a challenging, yet highly rewarding task in modern chemistry. Herein, we use an inorganic phosphorous-containing species, NaH₂PO₂, to serve as the source of phosphorous that participates directly in the nickel-catalyzed selective alkyne hydrophosphonylation reaction. The transformation was achieved in a multicomponent fashion and at room temperature, and most importantly, the H-phosphinate product generated is an advanced intermediate which can be readily converted into diverse phosphinate derivatives, including those bearing new P-C, P-S, P-N, P-Se, and P-O bonds, thus providing a complimentary method to classic phosphinate ester synthesis techniques.

Natural Product-Derived Phosphonic Acids as Corrosion Inhibitors for Iron and Steel

Organic acids, typically derived from an oil-based value chain, are frequently used as corrosion inhibitors in industrial metal working fluids. The criteria for selection of these corrosion inhibitors have changed in the last decades, and are today not only performance-driven, but influenced by ecological considerations, toxicity and regulatory standards. We present scalable semisynthetic approaches to organic corrosion inhibitors based on phosphonic acids from renewable resources. They have been evaluated by chip filter assay, potentiodynamic polarization measurements, electrochemical impedance measurements and gravimetry for corrosion protection of iron and steel in an aqueous environment at slightly alkaline pH. The efficacy of several phosphonic acids tested was found to be strongly dependent on structural features influencing molecular self-assembly of protective layers, and the solubility of salts formed with di- and trivalent cations from the media or formed during corrosion. A carboxyphosphonic acid (derived from castor oil) was found to have remarkable anticorrosive effects in all media tested. We attribute the anticorrosion properties of this carboxyphosphonic acid to the formation of particularly stable protective layers on the metal surface. It might thus serve as a commercially attractive substitute for current acidic corrosion inhibitors, derived from renewable resources.

Routescore: Punching the Ticket to More Efficient Materials Development

Self-driving laboratories, in the form of automated experimentation platforms guided by machine learning algorithms, have emerged as a potential solution to the need for accelerated science. While new tools for automated analysis and characterization are being developed at a steady rate, automated synthesis remains the bottleneck in the chemical space accessible to self-driving laboratories. Combining automated and manual synthesis efforts immediately significantly expands the explorable chemical space. To effectively direct the different capabilities of automated (higher throughput and less labor) and manual synthesis (greater chemical versatility), we describe a protocol, the RouteScore, that quantifies the cost of combined synthetic routes. In this work, the RouteScore is used to determine the most efficient synthetic route to a well-known pharmaceutical (structure-oriented optimization) and to simulate a self-driving laboratory that finds the most easily synthesizable organic laser molecule with specific photophysical properties from a space of ∼3500 possible molecules (property-oriented optimization). These two examples demonstrate the power and flexibility of our approach in mixed synthetic planning and optimization and especially in downselecting promising candidates from a large chemical space via an a priori estimation of the synthetic costs.

Tandem cyclization/arylation of diaryliodoniums via in situ constructed benzoxazole as a directing group for atom-economical transformation

Linear diaryliodoniums often undergo only single arylation and leave equivalent aryl iodide as waste.

Synthesis of P-Substituted 5- and 6-Membered Benzo-Phostams: 2,3-Dihydro-1H-1,2-benzazaphosphole 2-Oxides and 2,3-Tetrahydro-1H-1,2-benzazaphosphinine 2-Oxides

Several approaches were developed for the preparation of phosphorus-substituted 5- and 6-membered benzophostams. Carbodiimide-promoted cyclization of zwitterionic aminophosphinates derived from a nitrobenzene precursor accomplished the cyclization in good yields. Alternatively, a novel copper-catalyzed cross-coupling between a phosphonamide and a bromobenzene precursor produced the heterocycles in moderate to good yields. Three different methods are compared for the synthesis of the P-ethoxy-substituted 5-membered benzophostam.

A practical catalytic reductive amination of carboxylic acids

We report reductive alkylation reactions of amines using carboxylic acids as nominal electrophiles. The two-step reaction exploits the dual reactivity of phenylsilane and involves a silane-mediated amidation followed by a Zn(OAc)2-catalyzed amide reduction. The reaction is applicable to a wide range of amines and carboxylic acids and has been demonstrated on a large scale (305 mmol of amine). The rate differential between the reduction of tertiary and secondary amide intermediates is exemplified in a convergent synthesis of the antiretroviral medicine maraviroc. Mechanistic studies demonstrate that a residual 0.5 equivalents of carboxylic acid from the amidation step is responsible for the generation of silane reductants with augmented reactivity, which allow secondary amides, previously unreactive in zinc/phenylsilane systems, to be reduced.

Let's Make White Phosphorus Obsolete

Industrial and laboratory methods for incorporating phosphorus atoms into molecules within the framework of Green Chemistry are in their infancy. Current practice requires large inputs of energy, involves toxic intermediates, and generates substantial waste. Furthermore, a negligible fraction of phosphorus-containing waste is recycled which in turn contributes to negative environmental impacts, such as eutrophication. Methods that begin to address some of these drawbacks are reviewed, and some key opportunities to be realized by pursuing organophosphorus chemistry under the principles of Green Chemistry are highlighted. Methods used by nature, or in the chemistry of other elements such as silicon, are discussed as model processes for the future of phosphorus in chemical synthesis.

Synthesis of 12-Membered Tetra-aza Macrocyclic Pyridinophanes Bearing Electron-Withdrawing Groups

The number of substituted pyridine pyridinophanes found in the literature is limited due to challenges associated with 12-membered macrocycle and modified pyridine synthesis. Most notably, the electrophilic character at the 4-position of pyridine in pyridinophanes presents a unique challenge for introducing electrophilic chemical groups. Likewise, of the few reported, most substituted pyridine pyridinophanes in the literature are limited to electron-donating functionalities. Herein, new synthetic strategies for four new macrocycles bearing the electron-withdrawing groups CN, Cl, NO₂, and CF₃ are introduced. Potentiometric titrations were used to determine the protonation constants of the new pyridinophanes. Further, the influence of such modifications on the chemical behavior is predicted by comparing the potentiometric results to previously reported systems. X-ray diffraction analysis of the 4-Cl substituted species and its Cu(II) complex are also described to demonstrate the metal binding nature of these ligands. DFT analysis is used to support the experimental findings through energy calculations and ESP maps. These new molecules serve as a foundation to access a range of new pyridinophane small molecules and applications in future work.

Enhancement of the Antioxidant Activity and Neurotherapeutic Features through Pyridol Addition to Tetraazamacrocyclic Molecules

Alzheimer's and other neurodegenerative diseases are chronic conditions affecting millions of individuals worldwide. Oxidative stress is a consistent component described in the development of many neurodegenerative diseases. Therefore, innovative strategies to develop drug candidates that overcome oxidative stress in the brain are needed. To target these challenges, a new, water-soluble 12-membered tetraaza macrocyclic pyridinophane L4 was designed and produced using a building-block approach. Potentiometric data show that the neutral species of L4 provides interesting zwitterionic behavior at physiological pH, akin to amino acids, and a nearly ideal isoelectric point of 7.3. The copper(II) complex of L4 was evaluated by X-ray diffraction and cyclic voltammetry to show the potential modes of antioxidant activity derived, which was also demonstrated by 2,2-diphenyl-1-picrylhydrazyl and coumarin carboxylic acid antioxidant assays. L4 was shown to have dramatically enhanced antioxidant activity and increased biological compatibility compared to parent molecules reported previously. L4 attenuated hydrogen peroxide (H2O2)-induced cell viability loss more efficiently than precursor molecules in the mouse hippocampal HT-22 cell model. L4 also showed potent (fM) level protection against H2O2 cell death in a BV2 microglial cell culture. Western blot studies indicated that L4 enhanced the cellular antioxidant defense capacity via Nrf2 signaling activation as well. Moreover, a low-cost analysis and high metabolic stability in phase I and II models were observed. These encouraging results show how the rational design of lead compounds is a suitable strategy for the development of treatments for neurodegenerative diseases where oxidative stress plays a substantial role.