Mechanochemical Activation of Aryl Diazonium Salts: Synthesis of Polycyclic (Hetero)Aromatics

Although the synthesis of polycyclic (hetero)aromatics via the [4 + 2] benzannulation process has been thoroughly explored, the restricted availability of energy sources (including thermal, light, and electrical energy) mandates the utilization of substantial quantities of organic solvents, inevitably leading to environmental pollution, resource wastage, and low reaction efficiency. Herein, we report a new method for the synthesis of polycyclic (hetero)aromatics from diazonium salts and alkynes under ball-milling conditions. This mechanochemical approach requires only substoichiometric amounts of DMSO as a liquid-assisted grinding additive and furnishes the desired product in a short time.

Mechanochemical Synthesis of α-halo Alkylboronic Esters

α-halo alkylboronic esters, acting as ambiphilic synthons, play a pivotal role as versatile intermediates in fields like pharmaceutical science and organic chemistry. The sequential transformation of carbon-boron and carbon-halogen bonds into a broad range of carbon-X bonds allows for programmable bond formation, facilitating the incorporation of multiple substituents at a single position and streamlining the synthesis of complex molecules. Nevertheless, the synthetic potential of these compounds is constrained by limited reaction patterns. Additionally, the conventional methods often necessitate the use of bulk toxic solvents, exhibit sensitivity to air/moisture, rely on expensive metal catalysts, and involve extended reaction times. In this report, a ball milling technique is introduced that overcomes these limitations, enabling the external catalyst-free multicomponent coupling of aryl diazonium salts, alkenes, and simple metal halides. This approach offers a general and straightforward method for obtaining a diverse array of α-halo alkylboronic esters, thereby paving the way for the extensive utilization of these synthons in the synthesis of fine chemicals.

Organic Reactions Enabled by Mechanical Force-Induced Single Electron Transfer

Mechanochemical reactions, achieved through milling, grinding, or other mechanical actions, have emerged as a solvent-free alternative to traditional solution-based chemistry. Mechanochemistry not only provides the opportunity to eliminate bulk solvent use, reducing waste generation, but also unveils a new reaction strategy which enables the realization of reactions previously inaccessible in solution. While the majority of organic reactions facilitated by mechanical force traditionally follow two-electron transfer pathways similar to their solution-based counterparts, the field of mechanochemically induced single-electron transfer (SET) reactions has witnessed rapid development. This review outlines examples of mechanochemical reactions facilitated by the SET process, focusing on the reagents that initiate SET, thereby positioning mechanochemistry as a burgeoning field within the realm of single-electron chemistry.

Prediction of Degradation Profiles for Various Sartans under Solvent-Free Mechanochemical Conditions

For the approval of a drug, the stability data must be submitted to regulatory authorities. Such analyses are often time-consuming and cost-intensive. Forced degradation studies are mainly carried out under harsh conditions in the dissolved state, often leading to extraneous degradation profiles for a solid drug. Oxidative mechanochemical degradation offers the possibility of generating realistic degradation profiles. In this study, a sustainable mechanochemical procedure is presented for the degradation of five active pharmaceutical ingredients (APIs) from the sartan family: losartan potassium, irbesartan, valsartan, olmesartan medoxomil, and telmisartan. High-resolution mass spectrometry enabled the detection of impurities already present in untreated APIs and allowed the elucidation of degradation products. Significant degradation profiles could already be obtained after 15-60 min of ball milling time. Many of the identified degradation products are described in the literature and pharmacopoeias, emphasizing the significance of our results and the applicability of this approach to predict degradation profiles for drugs in the solid state.

Mechanical-Force-Induced Non-spontaneous Dehalogenative Deuteration of Aromatic Iodides Enabled by Using Piezoelectric Materials as a Redox Catalyst

The development of green and efficient deuteration methods is of great significance for various fields such as organic synthesis, analytical chemistry, and medicinal chemistry. Herein, we have developed a dehalogenative deuteration strategy using piezoelectric materials as catalysts in a solid-phase system under ball-milling conditions. This non-spontaneous reaction is induced by mechanical force. D2O can serve as both a deuterium source and an electron donor in the transformation, eliminating the need for additional stoichiometric exogenous reductants. A series of (hetero)aryl iodides can be transformed into deuterated products with high deuterium incorporation. This method not only effectively overcomes existing synthetic challenges but can also be used for deuterium labelling of drug molecules and derivatives. Bioactivity experiments with deuterated drug molecule suggest that the D-ipriflavone enhances the inhibitory effects on osteoclast differentiation of BMDMs in vitro.

Mechanochemical C-H Arylation and Alkylation of Indoles Using 3 d Transition Metal and Zero-Valent Magnesium

Although the 3 d transition-metal catalyzed C-H functionalization have been extensively employed to promote the formation of valuable carbon-carbon bonds, the persistent problems, including the use of sensitive Grignard reagents and the rigorous operations (solvent-drying, inert gas protection, metal pre-activation and RMgX addition rate control), still leave great room for further development of sustainable methodologies. Herein, we report a mechanochemical technology toward in-situ preparation of highly sensitive organomagnesium reagents, and thus building two general 3 d transition-metal catalytic platforms that enables regioselective arylation and alkylation of indoles with a wide variety of halides (including those containing post transformable functionalities and heteroaromatic rings). This mechanochemical strategy also brings unique reactivity and high step-economy in producing functionalized N-free indole products.

Selective C-H Iodination of Weinreb Amides and Benzamides through Iridium Catalysis in Solution and under Mechanochemical Conditions

The acid mediated ortho-iodination of Weinreb amides using a readily available catalyst is described. The selective ortho-iodination of Weinreb amides, challenging substrates in directed C-H activations, and also of benzamides is achieved. The process works under mild conditions and tolerates air and moisture, having a great potential for industrial applications. The methodology can be applied under mechanochemical conditions maintaining the reaction outcome and selectivity.

Synthesis of bioactive evodiamine and rutaecarpine analogues under ball milling conditions

Mechanochemical reactions achieved by processes such as milling and grinding are promising alternatives to traditional solution-based chemistry. This approach not only eliminates the need for large amounts of solvents, thereby reducing waste generation, but also finds applications in chemical and materials synthesis. The focus of this study is on the synthesis of quinazolinone derivatives by ball milling, in particular evodiamine and rutaecarpine analogues. These compounds are of interest due to their diverse bioactivities, including potential anticancer properties. The study examines the reactions carried out under ball milling conditions, emphasizing their efficiency in terms of shorter reaction times and reduced environmental impact compared to conventional methods. The ball milling reaction of evodiamine and rutaecarpine analogues resulted in yields of 63-78% and 22-61%, respectively. In addition, these compounds were tested for their cytotoxic activity, and evodiamine exhibited an IC50 of 0.75 ± 0.04 μg mL-1 against the Ca9-22 cell line. At its core, this research represents a new means to synthesise these compounds, providing a more environmentally friendly and sustainable alternative to traditional approaches.

Direct thermodynamic characterization of solid-state reactions by isothermal calorimetry

Despite the growing importance of solid-state reactions, their thermodynamic characterization has largely remained unexplored. This is in part due to the lack of methodology for measuring the heat effects related to reactions between solid reactants. We address here this gap and report on the first direct thermodynamic study of chemical reactions between solid reactants by isothermal calorimetry. Three reaction classes, cationic host-guest complex formation, molecular co-crystallization, and Baeyer-Villiger oxidation were investigated, showcasing the versatility of the devised methodology to provide detailed insight into the enthalpy changes related to various reactions. The reliability of the method was confirmed by correlation with the values obtained via solution calorimetry using Hess's law. The thermodynamic characterization of solid-state reactions described here will enable a deeper understanding of the factors governing solid-state processes.

Mechanochemical Synthesis of Aryl Fluorides by Using Ball Milling and a Piezoelectric Material as the Redox Catalyst

Aryl fluorides are important structural motifs in many pharmaceuticals. Although the Balz-Schiemann reaction provides an entry to aryl fluorides from aryldiazonium tetrafluoroborates, it suffers from drawbacks such as long reaction time, high temperature, toxic solvent, toxic gas release, and low functional group tolerance. Here, we describe a general method for the synthesis of aryl fluorides from aryldiazonium tetrafluoroborates using a piezoelectric material as redox catalyst under ball milling conditions in the presence of Selectfluor. This approach effectively addresses the aforementioned limitations. Furthermore, the piezoelectric material can be recycled multiple times. Mechanistic investigations indicate that this fluorination reaction may proceed via a radical pathway, and Selectfluor plays a dual role as both a source of fluorine and a terminal reductant.

Electrochemical Cross-Dehydrogenative Coupling of Isochroman and Unactivated Ketones

An unprecedented electrochemical cross-dehydrogenative coupling reaction between isochroman and unactivated ketones to directly synthesize α-substituted isochromans has been developed. This strategy provides a facile and efficient procedure to the direct activation of C(sp³)-H bond adjacent to the O atom of isochroman. The method features high atom economy, chemical oxidant-free, and mild conditions, in which methanesulfonic acid (MsOH) acts as both electrolyte and catalyst, making the process more convenient and environmentally friendly. Gram-scale experiment and synthesis of antitumor active compounds demonstrate the great potential of this protocol for practical applications.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Mechanoenzymology in the Kinetic Resolution of β-Blockers: Propranolol as a Case Study

Recent advances in biotechnology, protein engineering, and enzymatic immobilization have made it possible to carry out biocatalytic transformations through alternative non-conventional activation strategies. In particular, mechanoenzymology (i.e., the use of the mechanical force produced by milling or grinding to activate a biotransformation) has become a new area in so-called "green chemistry", reshaping key fundaments of biocatalysis and leading to the exploration of enzymatic transformations under more sustainable conditions. Significantly, numerous chiral active pharmaceutical ingredients have been synthesized via mechanoenzymatic methods, boosting the use of biocatalysis in the synthesis of chiral drugs. In this regard and aiming to widen the scope of the young field of mechanoenzymology, a dual kinetic resolution of propranolol precursors was explored. The biocatalytic methodology mediated by Candida antarctica Lipase B (CALB) and activated by mechanical force allowed the isolation of both enantiomeric precursors of propranolol with high enantiomeric excess (up to 99% ee), complete conversion (c = 50%), and excellent enantiodifferentiation (E > 300). Moreover, the enantiomerically pure products were used to synthesize both enantiomers of the β-blocker propranolol with high enantiopurity.

Mechanochemical Synthesis of Ketones via Chemoselective Suzuki-Miyaura Cross-Coupling of Acyl Chlorides

The direct synthesis of ketones via acyl Suzuki-Miyaura cross-coupling of widely available acyl chlorides is a central transformation in organic synthesis. Herein, we report the first mechanochemical solvent-free method for highly chemoselective synthesis of ketones from acyl chlorides and boronic acids. This acylation reaction is conducted in the solid state, in the absence of potentially harmful solvents, for a short reaction time and shows excellent selectivity for C(acyl)-Cl bond cleavage.