Radical Stability as a Guideline in C–H Amination Reactions

The stability of N‐centered radicals and radical cations of potential relevance in C–H amidation reactions has been quantified using highly accurate theoretical methods. Combination with available C–H bond energies for substrate fragments allows for the prediction of reaction enthalpies in 1,5‐hydrogen atom transfer (HAT) steps frequently encountered in reactions such as the Hofmann–Löffler–Freytag (HLF) reaction. Protonation of N‐radicals is found to be essential in classical HLF reactions for thermochemically feasible HAT steps. The stability of neutral N‐radicals depends strongly on the type of N‐substituent. Among the electron‐withdrawing substituents, the trifluoroacetyl (TFA) group is the least and the toluenesulfonyl (tosyl) group the most stabilizing. This implies that TFA‐aminyl radicals have the broadest and tosyl‐aminyl radicals the smallest window of synthetic applicability. In how far the intramolecular C–H amidation reactions compete with hydrogen abstraction from common organic solvents can be judged based on a comparison of reaction thermodynamics.

     

过渡金属配合物催化烯烃的自由基反应进展

近年来，通过烯烃官能团化构建有机化合物成为有机合成领域的研究热点之一。当选择种类、晶体结构和性能多样的过渡金属配合物作为催化剂时，这类反应具有高效、高选择性且成本低的特点。本文总结了近五年来利用过渡金属盐及其配合物作为催化剂，经过自由基反应历程实现未活化烯烃官能团化的研究进展，其反应特点是在过渡金属催化下，烯烃生成自由基并与其它底物或试剂偶联成键，从而实现官能团化。其中，催化性能优异的催化剂除了贵重金属铑、钯和钌等的配合物之外，还有普通金属，如铁、镍、铜和钴的盐及其配合物。这些方法拓展了烯烃官能团化的研究领域，为有机合成工作者提供新方法和思路，还为将来的产业化生产提供新方案。     

The Charge Transfer Photochemistry of Coordination Complexes in Aqueous Solution

Irradiation of charge transfer absorption bands of coordination complexes can result in net oxidation reduction or ligand exchange reactions. The photoredox processes generally involve the formation of reactive radicals in the primary photochemical act. The identification of these primary radicals is essential to the specification of the primary photochemical act since radical reactions with substrate, other radicals, etc. can make the net products and yields appear vastly different from the nature of the primary act. For example, in a few documented cases, the metal ion fragment formed in a photoredox process has characteristics of a radical and can couple with ligand radicals giving rise to a net photochemical process which appears to be ligand exchange rather than photoredox. A variety of radicals, organic as well as inorganic, can be generated and studied in aqueous solution. Some information has also been obtained concerning the nature of the photochemical precursors to the radicals formed in photoredox processes.     

Trends in the chemistry of mono- to multimetal π-arene complexes of chromium and manganese

The preparation and the features of the ring activation in π-arene metal (chromium, manganese) complexes are discussed. The bottom-up approach to the synthesis of σ, π-bi- and multimetal complexes, and the synthesis of cyclometallated and multicyclic complexes from a variety of selected routes are also reviewed. The stereochemical features of such complexes, and their applications in carbon–carbon coupling reactions and metal insertion reactions are highlighted. The redox reactivities and selected applications of (multi)metal π-arene complexes in catalysis and organic synthesis are included. Finally, the extension of the multimetal π-arene complexes to larger assemblies is shown. The importance of arene ring substituents in the basic mononuclear units and their use as bidentate metalloligands, with bidentate amines as linkers, in the construction of polymers, metal organic frameworks, networks (also on nanoparticles or silica supports), is highlighted.     

Kinetic Aspects of Redox Reactions of Nickel(III) Complexes

The synthesis and characterisation of nickel(III) complexes are reviewed. Kinetic aspects of redox reactions including oxidation by Co(OH₂)³⁺₆ and Fe(phen)³⁺₃ are described, as are the reactions involving oxidation of non-metallic substrates. Reference is made to the evaluation of self-exchange rates for the Ni(III)L/Ni(II)L couples, especially for systems where both oxidant and reductant retain an octahedral geometry. Using the reversible reaction between Ni(9-aneN₃)³⁺₂ and Ni(10-aneN₃)³⁺₂ (9-ane = 1,3,7-triazanonane, 10-ane = 1,3,7-triazadecane), data are presented for evaluation of the self-exchange rate constant for the Ni(10-aneN₃)^3+/2+₂ couple [(7.5 ± 1) X 10³ M⁻¹s⁻¹]. This value is compared with that for similar systems where changes in bond lengths on oxidation have been determined by X-ray techniques.     

Synthesis of Group 9 Metal‐Olefin Complexes with Identical Ligand Frameworks and Comparison of their Catalytic Activity in [2+2+2] Cycloaddition and other Addition Reactions

The preparation and catalytic evaluation of the three group 9 (cyclopentadienyl)metal(trimethylvinylsilane) complexes of the type C₅H₅M(H₂CCHSiMe₃)₂ (M=cobalt, rhodium and iridium) are reported. The complexes were investigated in [2+2+2] cycloaddition as well as in hydrogenation, hydroformylation and hydroboration reactions. Despite the identical organic frameworks and structural parameters, the complexes display remarkable reactivity and stability differences.     

Kinetic study of polyurethane synthesis using different catalytic systems of Fe,Cu, Sn,and Cr

This work presents a comparative study between alternative catalytic systems, metal‐β‐diketones complexes (iron, copper, chromium, and tin), and the commercial catalyst dibutyltin dilaurate, DBTDL, in the polyurethanes synthesis obtained from isophorone diisocyanate (IPDI) and polyols as polypropyleneglycol/diethyleneglycol and 1,6‐hexanodiol polyadipate (polyester A‐Mn = 2000 g/mol and polyester B‐Mn = 1000 g/mol) reactions. The polyurethanes synthesis was followed by the IPDI consumption in time, verified by infrared spectroscopy (FTIR) through the decrease of free NCO characteristic band at 2300–2200 cm^?1. The FTIR data was used to determine the polyurethanes formation kinetic behavior. It was verified that for the reactions with polyethers excess, DBTDL catalyst was more effective when compared to metal‐β‐diketones complexes, while for the reactions with polyester, A and B, the metal‐β‐diketones complexes were more effective. © 2009 Wiley Periodicals, Inc. JAppl Polym Sci, 2010     

Organometallic-like C-H bond activation and C-S bond formation on the disulfide bridge in the RuSSRu core complexes

C-S bond formations on the disulfide bridge in the dinuclear Ru(III) complexes have been found in the reaction with unsaturated organic molecules, such as alkenes and ketones. The reactions are initiated by the organometallic-like C-H bond activation. Through the mechanistic study of these novel reactions, the specific nature of the disulfide bridging ligand has been unveiled: (i) the double bond character of the sulfur-sulfur bond, which allows the Diels-Alder-type [4 + 2] cycloaddition reaction with butadiene to form a C(4)S(2) ring, (ii) the C-H bond activation on the S-S bond forming a C-S bond. All of the C-H activation reactions and the ensuing C-S bond formation reactions suggest that the disulfide ligand can act in a manner similar to the transition metal centers of the organometallic complexes.     

The promise and challenge of iron-catalyzed cross coupling

Transition metal catalysts, particularly those derived from the group VIII-X metals, display remarkable efficiency for the formation of carbon-carbon and carbon-heteroatom bonds through the reactions of suitable nucleophiles with organic electrophilic partners. Within this subset of the periodic table, palladium and nickel complexes offer the broadest utility, while additionally providing the deepest mechanistic insight into thus-termed "cross-coupling reactions". The mammoth effort devoted to palladium and nickel catalysts over the past 30 years has somewhat obscured reports of alternative metal complexes in this arena. As cross-coupling reactions have evolved into a critical support for modern synthetic chemistry, the search for alternative catalysts has been taken up with renewed vigor.When the current generation of synthetic chemists reflects back to the origins of cross coupling for inspiration, the well-documented effect of iron salts on the reactivity of Grignard reagents with organic electrophiles surfaces as a fertile ground for alternative catalyst development. Iron possesses the practical benefits more befitting an alkali or alkaline earth metal, while displaying the unique reactivity of a d-block element. Therefore the search for broadly applicable iron catalysts for cross coupling is an increasingly important goal in modern synthetic organic chemistry.This Account describes the evolution of iron-catalyzed cross coupling from its inception in the work of Kochi to the present. Specific emphasis is placed on reactivity and synthetic applications, with selected examples from acyl-, alkenyl-, aryl-, and alkyl halide/pseudohalide cross coupling included. The typical reaction partners are Grignard reagents, though organomanganese, -copper, and -zinc derivatives have also been used in certain cases. Such iron-catalyzed processes occur very rapidly even at low temperature and therefore are distinguished by broad functional group compatibility. Furthermore, recent advances in carbon-heteroatom bond formation and studies relevant to the general reactivity of in situ generated and structurally defined "low-valent" iron catalysts are presented.The preparative aspects of iron-catalyzed cross coupling are encouraging, but the inclination to classify these processes within the characteristic reaction manifold is premature, as mechanistic studies have evolved at a comparatively slow pace. A typical protocol for cross coupling employs an Fe(+2) or Fe(+3) precatalyst, which is reduced in situ by the organometallic nucleophile. The nature of the resulting active component(s) is still best described, more than 30 years later, in Kochi's original terms as a "reduced form of soluble iron". Despite huge gaps in our current knowledge, three distinct mechanisms have been formulated, largely based on empirical evidence: a "canonical" cross-coupling process, a manifold wherein alkylation of an organoiron intermediate replaces transmetalation as a key step, and finally a proposal reliant on the formation of nucleophilic ate complexes. Conjecture and speculation abound, but precisely what constitutes the catalytic cycle in iron-catalyzed cross coupling remains an extremely challenging unanswered question.     

Alkaline earth metal catalysts for asymmetric reactions

The group 2 alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) are among the most common elements on Earth, abundant in both the sea and the Earth's crust. Although they are familiar in our daily lives, their application to organic synthesis has, so far, been limited. Some particularly useful properties of these elements include (i) low electronegativity, (ii) a stable oxidation state of +2, meaning that they can potentially form two covalent bonds with anions, and (iii) the ability to occupy a variety of coordination sites due to their large ionic radius. Furthermore, the alkaline earth metals, found between the group 1 and group 3 elements, show mild but significant Lewis acidity, which can be harnessed to control coordinative molecules via a Lewis acid-base interaction. Taken together, these characteristics make the metals Ca, Sr, and Ba very promising components of highly functionalized acid-base catalysts. In this Account, we describe the development of chiral alkaline earth metal catalysts for asymmetric carbon-carbon bond-forming reactions. Recently prepared chiral alkaline earth metal complexes have shown high diastereo- and enantioselectivities in fundamental and important chemical transformations. We chose chiral bisoxazoline (Box) derivatives bearing a methylene tether as a ligand for chiral modification. These molecules are very useful because they can covalently coordinate to alkaline earth metals in a bidentate fashion through deprotonation of the tether portion. It was found that chiral calcium-Box complexes could successfully promote catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions with high diastereo- and enantioselectivities. Both the calcium-Box complexes and chiral strontium-bis-sulfonamide and chiral barium-BINOLate complexes could catalyze asymmetric 1,4-addition reactions with high enantioselectivities. Furthermore, we designed a calcium-neutral coordinative ligand complex as a new type of chiral alkaline earth metal catalyst. We found that pyridinebisoxazolines (Pybox) worked well: they served as excellent ligands for calcium compounds in 1,4-addition reactions and Mannich reactions. Moreover, they were successful in 1,4-additions in concert with enantioselective protonation, affording the desired products in good to high enantioselectivities. Our results demonstrate that alkaline earth metals are very useful and attractive catalysts in organic synthesis. Moreover, their ubiquity in the environment is a distinct advantage over rare metals for large-scale processes, and their minimal toxicity is beneficial in both handling and disposal.     

Rate constants, activation free enthalpies and activation entropies of the electrochemical reduction of 1,4-diazines in DMF

The standard exchange current densities of nine 1,4-diazines at gold electrodes were measured in DMF in the temperature range of +25 up to −59°C. From the resulting free enthalphy of activation of pyrazine as a function of temperature an activation entropy of −1.5 k is calculated. It is compared with the activation entropy of −1.1 k following from the theory of Marcus. The relative change of the free enthalpy of activation with temperature is only a property of the temperature dependences of refractive index and dielectric constant of the solvent. It is used to obtain the free enthalpies of activation for the standard temperature of 298 K.

The experimental values for the compounds are compared with theoretical values, calculated from the inner and outer reorganization energies λ_i and λ₀. For λ_i the bond lengths and force constants were obtained from Hückel calculations, for λ_o different approximations according to Marcus and Peover were used. The calculated and the measured values coincide within less than 10%.

The relation between the free solvation enthalpy and the outer reorganization energy is discussed.     

High-pressure pulse radiolysis as a tool in the study of transition metal reaction mechanisms

The unique combination of high-pressure and pulse radiolysis kinetic techniques enables detailed mechanistic insight for a large variety of chemical processes to be gained. Typical examples for transition metal reactions are presented. These include ligand substitution, electron transfer, reactions of complexes with uncommon oxidation states, and reactions with radicals, including the formation and decomposition reactions of complexes with metal-carbon sigma bonds. The volumes of activation and volume profiles obtained form the basis of a critical analysis of different plausible reaction mechanisms.     

Electronic structure and reactivity of three-coordinate iron complexes

[Reaction: see text]. The identity and oxidation state of the metal in a coordination compound are typically thought to be the most important determinants of its reactivity. However, the coordination number (the number of bonds to the metal) can be equally influential. This Account describes iron complexes with a coordination number of only three, which differ greatly from iron complexes with octahedral (six-coordinate) geometries with respect to their magnetism, electronic structure, preference for ligands, and reactivity. Three-coordinate complexes with a trigonal-planar geometry are accessible using bulky, anionic, bidentate ligands (beta-diketiminates) that steer a monodentate ligand into the plane of their two nitrogen donors. This strategy has led to a variety of three-coordinate iron complexes in which iron is in the +1, +2, and +3 oxidation states. Systematic studies on the electronic structures of these complexes have been useful in interpreting their properties. The iron ions are generally high spin, with singly occupied orbitals available for pi interactions with ligands. Trends in sigma-bonding show that iron(II) complexes favor electronegative ligands (O, N donors) over electropositive ligands (hydride). The combination of electrostatic sigma-bonding and the availability of pi-interactions stabilizes iron(II) fluoride and oxo complexes. The same factors destabilize iron(II) hydride complexes, which are reactive enough to add the hydrogen atom to unsaturated organic molecules and to take part in radical reactions. Iron(I) complexes use strong pi-backbonding to transfer charge from iron into coordinated alkynes and N 2, whereas iron(III) accepts charge from a pi-donating imido ligand. Though the imidoiron(III) complex is stabilized by pi-bonding in the trigonal-planar geometry, addition of pyridine as a fourth donor weakens the pi-bonding, which enables abstraction of H atoms from hydrocarbons. The unusual bonding and reactivity patterns of three-coordinate iron compounds may lead to new catalysts for oxidation and reduction reactions and may be used by nature in transient intermediates of nitrogenase enzymes.     

Electrosynthéses de composés carbonylés catalysées par des complexes du nickel: Activation du monoxyde de carbone ou de composés acylés

The complex Ni⁰bpy generated from Nibpy²⁺ is an efficient catalyst for the electrochemical conversion of organic halides into symmetrical or unsymmetrical ketones. Carbonylation reactions are achieved from either carbon monoxide or carbonyl metal complexes or carbon dioxide. Acylation reactions are obtained by coupling halocarbons with acid chlorides or anhydides. All syntheses are conducted at normal pressure and room temperature at a constant current intensity in an individed cell fitted with a sacrificial anode.     

Phenolic antioxidants – radical‐scavenging and chain‐breaking activity: A comparative study

Fifty phenolic antioxidants (AH) (42 individual compounds and 8 binary mixtures of two antioxidants) were chosen for a comparative analysis of their radical‐scavenging (H‐donating) and chain‐breaking (antioxidant) activity. Correlations between experimental (antiradical and antioxidant) and predictable (theoretical) activities of 15 flavonoids, 15 hydroxy cinnamic acid derivatives, 5 hydroxy chalcones, 4 dihydroxy coumarins and 3 standard antioxidants (butylated hydroxytoluene, hydroquinone, DL ‐α‐tocopherol) were summarized and discussed. The following models were applied to explain the structure‐activity relationships of phenolic antioxidants of natural origin: (a) model 1, a DPPH assay used for the determination of the radical‐scavenging capacity (AH + DPPH? → A? + DPPH‐H); (b) model 2, chemiluminescence of a model substrate RH (cumene or diphenylmethane) used for the determination of the rate constant of a reaction with model peroxyl radicals (AH + RO₂? → ROOH + A?); (c) model 3, lipid autoxidation used for the determination of the chain‐breaking antioxidant efficiency and reactivity (AH + LO₂? → LOOH + A?; A? + LH (+O₂) → AH + LO₂?); and (d) model 4, theoretical methods used for predicting the activity (predictable activity). The highest lipid oxidation stability was found for antioxidants with a catecholic structure and for their binary mixtures with DL ‐α‐tocopherol, as a result of synergism between them.     

炔卤化合物在有机合成中的应用进展

综述了过渡金属催化下的炔卤化合物的碳-碳键、碳-氮键形成的反应。利用炔卤这种独特的反应物可以简便地构建各种炔基取代的有机分子。这些功能化的分子在有机合成中有着重要的作用。     

离子液体在有机反应中的应用

室温离子液体,由含氮的有机阳离子和无机阴离子组成,由于离子液体是很多化合物的溶剂,也能够溶解作为催化剂的过渡金属络合物,其阴离子还是潜在的配位体,故它们能起催化剂的功能,本文着重讨论这方面的发展状况。     

Photochemische Untersuchungen von Vitamin B12-Modellverbindungen mit Azid bzw. Thiolat als Axialliganden – Precursorverbindungen für koordinativ ungesättigte Cobalt(II)-Komplexe

Photochemistry of Azido and Thiolato Vitamin-B₁₂ Model Complexes as Precursor Compounds for Coordinatively Unsaturated Cobalt(II) Complexes The photolysis of [LCo(chelat)B] complexes ( 1–3 ) (L = azide, N; thiolate, RS⁻; chelat = dimethylglyoxime, dmg; N,N′-o-phenylene-bis(salicylidenimine), salphen; N,N′-ethylene-bis(salicylidenimine), salen; B = pyridine, imidazole, triphenylphosphine) leads upon the homolytic cleavage of the Co L bond to both coordinatively unsaturated reactive cobalt(II) chelates [Co(chelat)B] and ligand radicals L^•. The efficiency of these photochemical redox reactions is described in relation to the structure of the cobalt(III) chelates, the wavelength of irradiation, the light-intensity as well as the solvents and substrates used during the photochemical experiments. Further, sensitization experiments using [Ru(bipy)₃]Cl₂ as sensitizer are described and the redox potentials of the investigated complexes are discussed.     

Design and Synthesis of Chiral Zn2+ Complexes Mimicking Natural Aldolases for Catalytic C–C Bond Forming Reactions in Aqueous Solution

Extending carbon frameworks via a series of C–C bond forming reactions is essential for the synthesis of natural products, pharmaceutically active compounds, active agrochemical ingredients, and a variety of functional materials. The application of stereoselective C–C bond forming reactions to the one-pot synthesis of biorelevant compounds is now emerging as a challenging and powerful strategy for improving the efficiency of a chemical reaction, in which some of the reactants are subjected to successive chemical reactions in just one reactor. However, organic reactions are generally conducted in organic solvents, as many organic molecules, reagents, and intermediates are not stable or soluble in water. In contrast, enzymatic reactions in living systems proceed in aqueous solvents, as most of enzymes generally function only within a narrow range of temperature and pH and are not so stable in less polar organic environments, which makes it difficult to conduct chemoenzymatic reactions in organic solvents. In this review, we describe the design and synthesis of chiral metal complexes with Zn²⁺ ions as a catalytic factor that mimic aldolases in stereoselective C–C bond forming reactions, especially for enantioselective aldol reactions. Their application to chemoenzymatic reactions in aqueous solution is also presented.     

Photochemistry of group 6 Fischer carbene complexes: beyond the photocarbonylation reaction

The seminal report by Hegedus in 1982, showing that alkoxychromium(0) carbenes reacted with imines under bright Colorado sunlight to yield β-lactams, marked the beginning of a key reaction in organometallic chemistry. Very little was known about the mechanism of this reaction. In fact, Hegedus proposed the reversible generation of a chromium-coordinated ketene, which would react with nucleophiles. This coordinated species would show all the advantages of ketenes without their shortcomings, namely, dimerization, formation of undesired adducts, and so forth. The quest for the detection of these species and the pursuit of the mechanism of the photocarbonylation (a reaction exclusive to Cr(0) and Mo(0) carbene complexes, not W(0) carbene complexes) remained unabated over the next 15 years. In fact, all attempts to experimentally determine the mechanism of this useful reaction have been fruitless. At the same time, the photocarbonylation of Cr(0) carbenes matured into a valuable synthetic reaction, allowing access to several families of organic compounds. Unfortunately, reactions other than photocarbonylation remained elusive. We used a combination of experimental and computational methodologies to study the photocarbonylation of Cr(0) carbene complexes and the subsequent reaction of the photogenerated ketenes with nucleophiles. In parallel, we discovered new photochemical processes and succeeded in making photoreactive the so-called "unreactive" W(0) carbene complexes. In this Account, we discuss the disentangling of the mechanisms of these transformations, thereby shedding some light onto the photochemistry of group 6 metal (Fischer) carbene complexes. The original designation of the electronic transitions of group 6 carbene complexes was reassigned, and the photocarbonylation step was analyzed again, resulting in the sequence S(0)-T(1)-S(0), which is far removed from conventional organic photochemistry. The T(1) species is a chromacyclopropanone; its unpaired electrons are primarily localized in the metal fragment and in the former carbene carbon atom. The T(1)-S(0) intersystem crossing occurs with the participation of the solvent through an unusual loose-bolt radiationless mechanism. The photogenerated S(0) species reacts with imines to form the final β-lactams in a mechanism that resembles the organic Staudinger reaction, but here the metal is present during the entire reaction coordinate. The selectivity of these reactions is defined by the nucleophilic attack on the O-bonded metallaketene instead of the subsequent conrotatory ring closure, a distinct departure from the organic reaction. Appropriate modification of the substituents of the carbene ligand or in the coordination sphere of the complex results in new photoprocesses; these include 1,2-metalladyotropic rearrangements as well as α-fragmentations in which W(0) carbene complexes become photoreactive. Moreover, the inclusion of additional metal centers usually results in new reactions, such as the formation of fulvenes by η(5)→ η(3) photoslippage, or in the complete inhibition of the photoreactivity. The photochemistry of group 6 metal-carbene complexes thus offers unexplored territory for pursuing new reactions and reaction mechanisms.