Deciphering complexity in Pd-catalyzed cross-couplings

Understanding complex reaction systems is critical in chemistry. While synthetic methods for selective formation of products are sought after, oftentimes it is the full reaction signature, i.e., complete profile of products/side-products, that informs mechanistic rationale and accelerates discovery chemistry. Here, we report a methodology using high-throughput experimentation and multivariate data analysis to examine the full signature of one of the most complicated chemical reactions catalyzed by palladium known in the chemical literature. A model Pd-catalyzed reaction was selected involving functionalization of 2-bromo-N-phenylbenzamide and multiple bond activation pathways. Principal component analysis, correspondence analysis and heatmaps with hierarchical clustering reveal the factors contributing to the variance in product distributions and show associations between solvents and reaction products. Using robust data from experiments performed with eight solvents, for four different reaction times at five different temperatures, we correlate side-products to a major dominant N-phenyl phenanthridinone product, and many other side products.

Synthesis, polymorphism, and shape complementarity-induced co-crystallization of hexanuclear Co(II) clusters capped by a flexible heteroligand shell

Polymorphism and co-crystallization have gradually gained attention as new tools in the development of modern crystalline functional materials. However, the study on the selective self-assembly of metal clusters into multicomponent crystals is still in its infancy. Herein, we present the synthesis and characterization of two new heteroleptic hydroxido-acetato and acetato Co(II) clusters [Co6(OH)2(OAc)4(pyret)6] (1) and [Co6(OAc)6(pyret)6] (2) incorporating auxiliary 2-pyrrolidinoethoxylate (pyret) ligands. On this occasion, we revealed that the commonly used thermal procedure for dehydration of cobalt(II) acetate leads to a reagent comprising substantial contamination by cobalt hydroxido moieties. Comprehensive structural analysis of new compounds demonstrated intriguing crystal structure diversity of hydroxido-acetato cluster 1, which represents a rare example of both conformational and packing polymorphism in one compound, originating from the flexibility of organic O,N-ligands in the secondary coordination sphere. Furthermore, both clusters exhibit an interesting propensity for the selective formation of co-crystals 1·2 driven mainly by van der Waals forces and specific shape complementarity between co-formers.

Light-Induced Dyotropic Rearrangement of Diarylethenes: Scope, Mechanism, and Prospects

Irreversible two-photon photorearrangement of 1,2-diarylethenes is a unique process providing access to complex 2a1 ,5a-dihydro-5,6-dithiaacenaphthylene (DDA) heterocyclic core. This reaction was serendipitously discovered during studies on photoswitchable diarylethenes and was initially considered as a highly undesired process. However, in recent years, it has been recognized as an efficient photochemical reaction, interesting by itself and as a promising synthetic method for the synthesis of challenging molecules. Herein, we discuss the state-of-the-art in studies on this notable process.

Automated Library Generation and Serendipity Quantification Enables Diverse Discovery in Coordination Chemistry

Library generation experiments are a key part of the discovery of new materials, methods, and models in chemistry, but the question of how to generate high quality libraries to enable discovery is nontrivial. Herein, we use coordination chemistry to demonstrate the automation of many of the workflows used for library generation in automated hardware including the Chemputer. First, we explore the target-oriented synthesis of three influential coordination complexes, to validate key synthetic operations in our system; second, the generation of focused libraries in chemical and process space; and third, the development of a new workflow for prospecting library formation. This involved Bayesian optimization using a Gaussian process as surrogate model combined with a metric for novelty (or serendipity) quantification based on mass spectrometry data. In this way, we show directed exploration of a process space toward those areas with rarer observations and build a picture of the diversity in product distributions present across the space. We show that this effectively "engineers" serendipity into our search through the unexpected appearance of acetic anhydride, formed in situ, and solvent degradation products as ligands in an isolable series of three Co(III) anhydride complexes.

Smart Alkoxyamines: A New Tool for Smart Applications

In 1986, Rizzardo et al. discovered the nitroxide-mediated polymerization which relies on the reversibility of homolysis of the C-ON bond of alkoxyamine R¹R²NOR³, a unique property of these molecules. This discovery has generated a tremendous endeavor in the field of polymer chemistry. Alkoxyamines have been used as initiators/controllers for nitroxide-mediated polymerization. Moreover, photoexcitable alkoxyamines that dissociate under light at different wavelengths have also been developed for polymer chemistry. Over the past few years, alkoxyamines have started to be used in materials sciences. In many cases (e.g., self-healing polymers), the development of smart materials requires the use of smart building blocks, that is, molecules or systems whose properties and/or structures change upon external stimuli. Alkoxyamines exhibit a unique property: reversible homolysis (i.e., homolysis of the C-ON bond into alkyl R³• and nitroxyl R¹R²NO• radicals and reformation via the coupling of these two species). Until now, this property has been controlled only by changes in temperatures or by light irradiation. Chemical and/or biochemical control of the homolysis event would open new gates for the application of these molecules in different fields such as biology and medicine. Thus, the concept of smart alkoxyamines is discussed and exemplified via the activation of alkoxyamines using chemical or/and biochemical changes amplifying the polar, steric, and stabilization effects. In situ activation is also discussed. It is shown that (i) increasing the electron-withdrawing properties of the alkyl fragment weakens the C-ON bond and thus favors homolysis but is opposite for the nitroxyl fragment; (ii) increasing the steric hindrance on the nonactive site affords dramatic conformation changes which weaken the C-ON bond; and (iii) increasing the stabilization of the released alkyl radical weakens the C-ON bond. Solvent effects and intramolecular hydrogen bonding are also discussed. Reactions used to highlight our purpose are either reversible or nonreversible and used under conditions that are as mild as possible (temperatures below 40 °C and atmospheric pressure). For example, a several (thousands of millions of) millions of orders of magnitude enhancement of the homolysis rate constant is observed upon enzymatic hydrolysis at 37 °C, meaning that a shift from a stable alkoxyamine (t_1/2 = 42 000 milleniums) to a highly labile alkoxyamine (t_max = 1500 s for 35% conversion) is achieved. Applications of this concept are discussed for safe NMP initiators and for theranostic agents.

Elucidation of a Sequential Iminium Ion Cascade Reaction Triggered by a Silica Gel-Promoted Aza-Peterson Reaction

In a recent methodological study investigating the synthesis of N-alkoxyazomethine ylides, an unexpected aminal byproduct was generated during our attempt to isolate O-benzyl-N-((trimethylsilyl)methyl)hydroxylamine. After a strategic investigation, silica gel was discovered to be the cause of the byproduct formation. Through the mechanistic insight from control and trapping experiments, we propose the formation of a methaniminium ion via a novel aza-Peterson reaction, which ultimately triggers a sequential iminium ion cascade sequence. Herein, we discuss the elucidation of this cascade reaction mechanism and the constraints for the byproduct formation.

The Reims Journey Towards Discovery and Understanding of Pd-Catalyzed Oxidations

This review recounts the development by the authors of the Pd-catalyzed procedures devoted to various kinds of oxidation. Starting with reactions assisted with UV light, the research has explored reactions under light-free conditions: allylic oxidation, alcohol oxidation, etherification, Wacker oxidation and dehydrogenations with, always, accompanying efforts towards mechanism determination.

Mikhail Kucherov: "The Experiment Confirmed my Hypothesis"

Named reactions are key points in the development of chemistry. Any competent chemist easily identifies the reaction named after Wittig, or Grignard, Diels-Alder, or Friedel-Crafts, Michael, or Favorsky. But how much do we can say about scientist who discovered it? This Essay is devoted to the transition-metal-catalyzed hydration of acetylenic hydrocarbons discovered by Russian chemist Mikhail Kucherov. This reaction is one of the most straightforward methods for the synthesis of carbonyl compounds. With it, in industry for a long time acetaldehyde was essentially manufactured from accessible unsaturated raw material-acetylene. This reaction is one of the first steps in the establishment of homogenous metal complex catalysis in organic synthesis. Herein, we described the history of this discovery and the role of many scientists in the development of research in this field. We would also like to show the life of Russian scientists in the latter half of the 19th century.

Photorearrangement of dihetarylethenes as a tool for the benzannulation of heterocycles

A general strategy for the preparative benzannulation of aromatic heterocycles via photocyclization of 1,2-dihetarylethenes was proposed for the first time. The strategy includes two steps, namely, modular assembly of dihetarylethenes from widely available 3-hetarylacetic acids and 2-bromo-1-hetarylethanones, and subsequent preparative photorearrangement (using a UV lamp at 365 nm as the light source). This approach is efficient for the annulation of a wide range of heterocycles and provides C-, N-, O- or S-substituents in the benzoheterocycles obtained. The photochemical step is a metal-, acid-, and oxidant-free reaction, which requires non-inert conditions, and can be easily monitored by NMR spectroscopy. Applicability of the proposed strategy was tested in the synthesis of a wide range of substituted carbazoles and benzo[b]thiophenes as well as on a gram-scale benzannulation of 3-indoleacetic acid. Our study disclosed how to overcome two notable obstacles to the successful photorearrangement of dihetarylethenes: undesired reactions associated with photogenerated singlet oxygen, and the instability of desired products. The first problem was successfully solved by the addition of DABCO, while development of an in situ alkylation protocol to trap unstable photoproducts allowed us to overcome the second issue.

Comment on ‘Solution growth and thermal treatment of crystals lead to two new forms of 2-((2,6-dimethylphenyl)amino)benzoic acid’ by R. Hu, Y. Zhoujin, M. Liu, M. Zhang, S. Parkin, P. Zhou, J. Wang, F. Yu and S. Long, RSC Adv., 2018, , 15459

The two polymorphs reported by Long et al. are discussed point by point, suggesting that the newly reported crystal structure is in fact the same structure previously reported, differing in how the structure refinement was performed.