The Construction of Molecular Complexity from Functionalized Alkylidenecyclopropanes (FACPs)

As a special family of cyclopropanes, alkylidenecyclopropanes (ACPs), exhibit outstanding physical and chemical activities, which provide opportunities to participate in fascinating chemical transformations to access cyclopropane-containing units without ring-opening processes and other unavailable compounds through conventional routes with ring-opening processes owing to their strain-driven reactivity and synthetic accessibility. Nowadays, intramolecular reactions of methylenecyclopropanes (MCPs) or ACPs with adjacent functionalities have emerged as a powerful synthetic protocol for the construction of a variety of polycyclic and heterocyclic compounds with different sized skeletons through catalytic methods. Recently, we put forward the concept of functional alkylidenecyclopropanes (FACPs) and in this Minireview, we will summarize the reactions of FACPs after 2016 including several important early works from three aspects: 1) reactions with distal C−C bond cleavage, 2) reactions with proximal C−C bond cleavage (including ring-expansion reactions), and 3) reactions without C−C bond cleavage.     

Reactions between or within molecular crystals

Reactions that occur within or between molecular crystals, in particular those reactions that are activated by mechanical methods, are reviewed. The focus is on processes (whether intrasolid or intersolid) that are controlled primarily by supramolecular bonding, such as template cycloadditions, formation of inclusion compounds, reactions between molecular crystals by the reassembling of noncovalent bonds, and the formation of complexes and coordination compounds. It is proposed that solvent-free mechanochemical methods, for example, cogrinding, milling, and kneading, represent viable "green" routes for the preparation of novel molecular and supramolecular solids.     

α,α‐Diarylprolinol Ethers: New Tools for Functionalization of Carbonyl Compounds

α,α‐Diaryl(dialkyl)prolinol ethers constitute a potent organocatalyst family which has been shown to be very general for a broad range of reactions involving enamine and iminium ion activation or a combination of both. The reactions are characterized by an efficient steric control approach and can lead to a variety of α‐, β‐, γ‐, and α,β‐functionalized carbonyl compounds with excellent stereocontrol. As a full expression of their catalytic activity, these compounds are also excellent promoters of elegant cascade processes and valuable catalysts in water‐compatible systems.     

Displacement Reactions of Phosphorus(V) Compounds and Their Pentacoordinate Intermediates

Reactions of phosphorus(v) compounds involving the mutual interconversion of tetra- and pentacoordinate species are discussed in a critical review emphasizing stereochemical implications of the reaction mechanism. This discussion includes the formation and decomposition of the stable oxyphosphoranes, the Michaelis-Arbusov, Perkov, and Wittig reactions, interconversions of phosphines and their oxides, and the nucleophilic displacements on phosphonium compounds. Reactions of phosphate esters and related compounds receive particular attention. All chemical arguments are derived by considering the effect of factors determining the relative stabilities of phosphorane derivatives, their rates of formation, decomposition and rearrangement by bond deformation or rupture and recombination processes, considerations which are uniformly applied on the basis of concepts developed in a preceding communication^[2]. It is shown that a comprehensive mechanistic interpretation of the foregoing reactions requires substantial addition to available conceptual foundations such that, in many cases, present concepts and mechanisms must be revised.     

Fluorinated Imines in Tandem and Cycloaddition Reactions

The chemistry of fluorinated compounds has experienced extraordinary growth in recent decades due to the many and varied properties which many of the compounds that contain fluorinated groups possess. Among all of them, fluorinated chiral imines, in particular the Ellman's imines, are of great importance since they are some of the most interesting building blocks for the synthesis of a large number of enantioenriched carbocycles and heterocycles with extraordinary biological and synthetic properties. This personal account covers the most significant results obtained in our research group in the last two decades concerning asymmetric tandem reactions, paying special attention to the intramolecular aza-Michael reaction (IMAMR), diversity oriented synthesis (DOS), asymmetric tandem reactions involving a p-tolylsulfinyl group as chiral inducer and cycloaddition processes, in particular, the Pauson-Khand reaction, [2+2+2]-cycloadditions and metathesis reactions, starting mainly from enyne compounds and through the use of fluorinated chiral N-sulfinyl imines and their derivatives as starting materials.     

Organocatalytic multicomponent alpha-methylenation/Diels-Alder reactions: a versatile route to substituted cyclohexenecarbaldehyde derivatives

This article describes the design and optimization of an effective organocatalytic three-component domino alpha-methylenation/Diels-Alder reaction to produce vinyl-substituted cyclohexenecarboxaldehydes in a highly regioselective fashion. In these one-pot transformations, 2-formyl-1,3-butadienes (4) were prepared in situ from alpha,beta-unsaturated aldehydes and formalin and were subsequently trapped with a variety of buta-1,3-dienes. The outcomes of the reactions were dependent on the electronic properties of the dienes. 1-Vinylcyclohexenecarbaldehydes 6 were formed by use of acyclic electron-rich dienes, while the initially formed cycloadducts of 4 with cyclopentadiene underwent Cope rearrangements, leading to the formation of tetrahydro-3H-indene-5-carbaldehyde compounds 7. The mechanisms involved in these reactions were deduced from experimental findings. Furthermore, the method was also extended to one-pot domino methylenation/Diels-Alder reactions of dihydrofurans and dihydropyrans to yield spirocyclic lactols 22. In these reactions, the unstable intermediate hydroxyethyl and hydroxypropyl acroleins behaved as dienophiles, undergoing cycloaddition reactions with dienes with good yields and selectivities. The wide variety of functionalized 1-vinylcyclohex-3-enecarbaldehydes 6, 4-vinylcyclohex-1-enecarbaldehydes 7, and spiro lactols 22 generated through the use of these organocatalytic domino processes as a diversity-oriented synthesis provided useful intermediates for the construction of novel odorants.     

纳米二氧化硅负载FeCl3作为一种高效多相催化剂用于合成1,2,4-三嗪衍生物（英文）

The one‐pot synthesis of a series of 1,2,4‐triazines from the reactions of semicarbazide or thiosemi‐carbazide with various α,β‐dicarbonyl compounds under reflux conditions in a EtOH‐H2O (9:1) mixture as solvent and catalyzed by nano‐sized silica supported FeCl3 (FeCl3@SiO2) was investigat‐ed. The FeCl3 content of the catalyst was measured by atomic absorption to get the adsorption ca‐pacity. The reactions gave high yields of the product and the catalyst was easily separated and re‐used for successive reaction runs without significant loss of activity.     

Redox transformations in desorption electrospray ionization

Redox changes occur in some circumstances when organic compounds are analyzed by desorption electrospray ionization mass spectrometry (DESI-MS). However, these processes are limited in scope and the data presented here suggest that there are only limited analogies between the redox behavior in DESI and the well-known solution-phase electrochemical processes in standard electrospray ionization (ESI). Positive and negative ion modes were both investigated and there is a striking asymmetry between the incidence of oxidation and of reduction. Although in negative ion mode DESI experiments, some aromatic compounds were ionized as odd-electron anion radicals, examples of full reduction were not found. By contrast, oxidation in the form of oxygen atom addition (or multiple oxygen atom additions) was observed for several different analytes. These oxidation reactions point to chemically rather than electrochemically controlled processes. Data is presented which suggests that oxidation is predominantly caused by reaction with discharge-created gas-phase radicals. The fact that common reducing agents and known antioxidants such as ascorbic acid are not modified, while a saturated organic acid like stearic acid is oxidized in DESI, indicates that the usual electrochemical redox reactions are not significant but that redox chemistry can be induced under special experimental conditions.     

Carbopalladation Cascades Using Carbon–Carbon Triple Bonds: Recent Advances to Access Complex Scaffolds

Alkynes are one of the most versatile functional groups as synthetic handles. They allow for a direct access to partially or fully substituted alkenes through difunctionalization reactions. A prominently utilized transformation for these sequences is the carbopalladation of alkynes, which can be followed by various termination steps such as aromatizations, dearomatizations, cross‐coupling reactions, or pericyclic processes, amongst others. This Minireview provides an overview of the recent literature published in the field of carbopalladation chemistry, both with a focus on methodology as well as its application in the syntheses of complex molecular scaffolds, natural products, and functional molecules.     

Improving the Stability of Maleimide–Thiol Conjugation for Drug Targeting

Maleimides are essential compounds for drug conjugation reactions via thiols to antibodies, peptides and other targeting units. However, one main drawback is the occurrence of thiol exchange reactions with, for example, glutathione resulting in loss of the targeting ability. A new strategy to overcome such retro-Michael exchange processes of maleimide–thiol conjugates by stabilization of the thiosuccinimide via a transcyclization reaction is presented. This reaction enables the straightforward synthesis of stable maleimide–thiol adducts essential in drug-conjugation applications.     

Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

The group additivity method for Arrhenius parameters is applied to hydrogen addition to alkenes and alkynes and the reverse β‐scission reactions, an important family of reactions in thermal processes based on radical chemistry. A consistent set of group additive values for 33 groups is derived to calculate the activation energy and pre‐exponential factor for a broad range of hydrogen addition reactions. The group additive values are determined from CBS‐QB3 ab‐initio‐calculated rate coefficients. A mean factor of deviation of only two between CBS‐QB3 and experimental rate coefficients for seven reactions in the range 300–1000 K is found. Tunneling coefficients for these reactions were found to be significant below 400 K and a correlation accounting for tunneling is presented. Application of the obtained group additive values to predict the kinetics for a set of 11 additions and β‐scissions yields rate coefficients within a factor of 3.5 of the CBS‐QB3 results except for two β‐scissions with severe steric effects. The mean factor of deviation with respect to experimental rate coefficients of 2.0 shows that the group additive method with tunneling corrections can accurately predict the kinetics and is at least as accurate as the most commonly used density functional methods. The constructed group additive model can hence be applied to predict the kinetics of hydrogen radical additions for a broad range of unsaturated compounds.     

Reversible Topotactic Redox Reactions of Solids by Electron/Ion Transfer

Solids having suitable structural and electronic properties are able to form intercalation compounds by reversible redox reactions at room temperature via topotactic electron/ion transfer processes. The host lattices range from inorganic solids with different structural dimensionality to organic molecular solids. Similarly, depending on the host lattice type, the guest species may vary from protons and metal ions to large inorganic and organic molecular ions. The possibilities of a systematic “tailoring” of new stable or metastable compounds, the controlled modification of physical properties of solids, and the technical application of electronic/ionic conductors, provide a wide and attractive field for academic and applied research in an interdisciplinary area that involves solid state chemistry and physics, molecular chemistry, electrochemistry, and interface science.     

Domino Hydroformylation-Wittig Reactions

Methallyl and homomethallyl alcohols can undergo stereoselective hydroformylation-Wittig and hydroformylation-Wittig-hydrogenation reactions in one-pot domino processes. This sequential transformation allows the formation of C−C bonds and the generation of a new stereogenic center, and gives preparatively interesting compounds [Eq. (a)]. The reaction products are obtained in satisfactory to good yields and in diastereoselectivities of 90:10 to >98:2. CDG=catalyst-directing group; R′=H; R″=OEt, Me.     

Solid State Metathesis Reactions in Various Applications

Solid state metathesis reactions have been studied in fused silica tubes, by differential thermal analysis, and by X‐ray powder diffraction. A selection of reactions between metal (La, Nb, and Ni) chlorides and lithium nitride or lithium acetylide were investigated to get more insight into reaction pathways and intermediate reaction stages that may be adopted on course of the formation of metal nitrides or carbides. Intermediate compounds are considered to be important because they can control the reactivity of a system. Such compounds were traced by changing the molar ratios of reaction partners away from the salt‐balanced binary metal nitride or carbide target compositions. New preparative perspectives are discovered when metal chlorides were reacted with lithium nitridoborate or lithium cyanamide. Due to their reductive nature towards several d‐block metal chlorides, (BN₂)^3‐ and (CN₂)^2‐ react to form metals or metal nitrides plus X‐ray amorphous BN, and probably C₃N₄. With lanthanum chloride they can react to form nitridoborates and nitridocarbonates. The metathesis reaction between lithium cyanamide and cyanuric chloride (C₃N₃Cl₃) instead of metal chloride was studied for the synthesis of C₃N₄.     

Recent advances on boron doped diamond (BDD) electrode as cathode in organic and inorganic preparative electrotransformations

Although anodic oxidation reactions mediated by boron-doped diamond (BDD) electrodes have been broadly studied, research on cathodic conversion remains still scarce despite the favorable characteristics of this electrode for such application. A detailed survey of the recent literature about processes in which boron-doped diamond electrode has been employed in cathodic reactions is presented, including inorganic species (O₂, CO₂, nitrate, metaborate) and organic compounds (carbonyl, nitro groups and halogenated compounds).     

Bis(pyridine)iodonium tetrafluoroborate (IPy2BF4): a versatile oxidizing reagent

The use of bis(pyridine)iodonium tetrafluoroborate (IPy(2)BF(4)) as an oxidizing agent towards different types of alcohols is reported. The observed reactivity involves different reaction pathways, as a function both of the structures of the starting materials and of the experimental conditions. Interestingly, the title iodine-containing compound is capable of a tuneable reaction with simple cycloalkanols, providing straight and selective access either to omega-iodocarbonyl compounds or to ketones, a previously unreported and chemoselective range of oxidation potential. Furthermore, appropriate conditions for the preparation of aldehydes and esters from primary alcohols by easily performed experimental procedures were also established. The beta-scission reactions of cycloalkanols and the alpha-oxidation processes of primary, secondary and benzylic alcohols are discussed.     

Design and Use of Electrophilic Thiocyanating and Selenocyanating Reagents: An Interesting Trend for the Construction of SCN- and SeCN-Containing Compounds

Organothiocyanate and organoselenocyanate compounds are of paramount importance in organic chemistry as they are key intermediates to access sulfur- and selenium-containing compounds. Therefore, among the different synthetic pathways to get SCN- and SeCN-containing molecules, original methodologies using electrophilic reagents have recently been explored. This Minireview will showcase the recent advances that have been made. In particular, the design of several electrophilic sources and their applications for the thiocyanation and the selenocyanation of various classes of compounds will be highlighted and discussed.     

Catalytic Isofunctional Reactions—Expanding the Repertoire of Shuttle and Metathesis Reactions

Transfer hydrogenation, alkene metathesis, and alkyne metathesis possess great value to the synthetic chemistry community. One of the key features of these processes is their reversibility, which can be attributed to the presence of the same number and type of functional groups in both the reactants and products, making these reactions isofunctional. These classic reactions have recently inspired the development of novel shuttle and metathesis reactions that offer great promise for synthetic chemistry. This Review describes and systematically categorizes both recent and older examples of shuttle and metathesis reactions other than transfer hydrogenation and alkene/alkyne metathesis.     

Chemoselective Synthesis of Substituted Imines,Secondary Amines,and β‐Amino Carbonyl Compounds from Nitroaromatics through Cascade Reactions on Gold Catalysts

Substituted imines, α,β‐unsaturated imines, substituted secondary amines, and β‐amino carbonyl compounds have been synthesized by means of new cascade reactions with mono‐ or bifunctional gold‐based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, α,β‐unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO₂ system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three‐step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO₂ can also be used as a bifunctional catalyst to obtain substituted β‐amino carbonyl compounds from nitroaromatics and α,β‐unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold‐catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.     

Multifaceted Behavior of 2-Cyanobenzaldehyde and 2-Acylbenzonitriles in the Synthesis of Isoindolinones,Phthalides and Related Heterocycles

2-Cyanobenzaldehye (also called 2-formylbenzonitrile) and related 2-acylbenzonitriles belong to a class of bifunctional aromatic compounds that emerged as useful starting materials in developing efficient cascade-type reactions leading to different heterocyclic compounds. The variety of sometimes unpredictable mechanisms that rise from these structurally simple starting materials renders this class of compounds unique to afford, through divergent cascade reactions, heterocycles like isoindolinones, phthalides (also known with the name of isobenzofuranones), imidates, and in less extent isoindolin-1-imine, six- and seven-membered heterocycles, porphyrins, also in asymmetric way exploiting different organocatalytic activation modes. To give a picture, this chemistry can be associated to Goldberg variations which stem from a common motive in a pressing and surprising way. In this Review, emphasis is also given to the synthetic methods for the access to substituted 2-cyanobenzaldehydes and 2-acylbenzonitriles which allowed to enlarge the scope of the described cascade reactions.