Promoting non-transition metal alkylation with organic halides in the presence of binary systems based on an organometallic compound and a transition metal compound: VIII. Selectivity of the binary systems action

Selectivity of action of binary systems based on an organometallic compound and a transition metal compound in a direct synthesis of organometallic compounds via alkylation of metals with organic halides has been considered. Study of the side reactions in the course of zinc and cadmium powders alkylation (gaseous hydrocarbons evolution) taken as an example has demonstrated that the increase in activity of the binary system components is accompanied by decrease in its selectivity. The intensity of the side reactions is steeply increased above certain temperature determined by the nature of the reactants and components of the binary system. The surface of the alkylated metal containing adatoms and small clusters of the transition metal promotes the side processes.     

Promoting nontransition metal alkylation with organic halides in the presence of binary systems based on an organometallic compound and a transition metal compound: V. Effect of the nature of the transition metal compound in the binary system on the rate of a steady-state process

The example of alkylation of commercial zinc and cadmium powders with organic halides in the presence of binary systems comprising an organometallic and a transition metal compounds was used to show that the nature of the transition metal compound in the binary system strongly affects the rate of a steady-state process. Therewith, the significant factors are both the nature of the transition metal and the ligand composition. It was found that the activity of the binary systems correlates with the activity of the transition metal compound in the transmetalation reaction with the organometallic component of the binary system (reduction of the transition metal compound).     

Promoting non-transition metal alkylation with organic halides in the presence of binary systems based on an organometallic compound and a transition metal compound: IX. Alkylation of aluminum and concluding remarks

Study of aluminum powder alkylation with ethyl bromide in the presence of ethylaluminum sesquibromide and copper(I) iodide has revealed that binary systems based on an organometallic compound and a transition metal compound are efficient promotors of direct synthesis of organoaluminum compounds as well. Major conclusions drawn in a series of studies on non-transition metals (Zn, Cd, and Al) alkylation with organic halides in the presence of the binary systems are summarized.     

Promoting alkylation of non-transition metals with organic halides in the presence of binary systems based on organometallic compound and transition metal compound: II. Comparative study of efficiency of various agents in the course of destruction of passivating film on the surface of alkylated metal

It has been demonstrated that alkylzinc halides efficiently destroy the passivating film on the zinc-copper pair in the course of its alkylation with ethyl bromide to give ethylzinc bromide. The alkylzinc halides efficiency is comparable to that of ethyl iodide and exceeds that of salts of transition or non-transition metals as well as of ultraviolet irradiation. Addition of alkylzinc halides or metal salts as well as ultraviolet irradiation have practically no effect on the developed reaction. The results have demonstrated that the organometallic component of the binary systems is polyfunctional; this permits a generalization of known features of a number of known methods promoting direct synthesis of organometallic compounds.     

Promoting non-transition metal alkylation with organic halides in the presence of binary systems based on an organometallic compound and a transition metal compound: VII. Additional details of the mechanism

Mechanism of metals alkylation with an organic halide in the presence of binary systems has been defined in more detail. It has been shown that the passivating film on the surface of zinc and cadmium is partially preserved in the course of the process, and the reaction in diethyl ether is decelerated due to the competitive adsorption of the organyl halide and diethyl ether on the surface of the reacting metal. The ratelimiting stages of the studied alkylation process have been elucidated basing on the experimental data on the effect of the reagents (organyl halide and alkylated metal) nature on the rate of the steady-state reaction and modeling of the suggested catalytic cycle.     

Transition metal compound surfaces

Studies of transition metal compound surfaces using modern surface science techniques are reviewed. Studies of the surface structure and composition of model transition metal compound surfaces are emphasized. The growth of the transition metal compound surface from a chemisorbed layer is used as an introduction to investigations of the surface properties of macroscopic single crystals of transition metal compounds. Examples of both binary and tenary compound systems are examined in relation to chemisorbed layer studies. Although only a few systems are chosen to illustrate work in this field, extensive references to other studies and other systems are included.     

Promoting non-transition metal alkylation with organic halides in the presence of binary systems based on an organometallic compound and a transition metal compound: VI. Effect of various additives on the rate of the steady-state process

The effect of various additives such as transition and non-transition metal oxides, hydroxides, and salts on the rate of steady-state alkylation of commercial zinc powder with ethyl bromide in the presence of ethylzinc bromide–copper(I) iodide binary system has been studied. Most of the examined compounds either do not affect the reaction rate or reduce it. Only addition of zinc(II) bromide, zinc(II) hydroxide, and water appreciably accelerates the process. Mechanism of action of the additives is discussed.     

Preparation of Organozinc and Organocadmium Compounds from the Metals and Alkyl Halides in the Presence of Stimulating Systems Based on a Derivative a Transition Metal and an Organometallic Compound

Full and mixed alkyl derivatives of zinc and cadmium were prepared from these metals and organic halides in the presence of stimulating systems necessarily containing a transition metal derivative and an organometallic compound capable of reducing this derivative under the process conditions. Such stimulating systems make it possible to introduce selectively organic halides (iodides, bromides, chlorides) into the reaction with zinc and cadmium to obtain the corresponding mixed organometallic compounds.     

Enantioselective transition metal catalysis directed by chiral cations

Enantioselective transition metal catalysis directed by chiral cations is the amalgamation of chiral cation catalysis and organometallic catalysis. Thus far, three strategies have been revealed: ligand scaffolds incorporated on chiral cations, chiral cations paired with transition metal ‘ate’-type complexes, and ligand scaffolds incorporated on achiral anions. Chiral cation ion-pair catalysis has been successfully applied to alkylation, cycloaddition, dihydroxylation, oxohydroxylation, sulfoxidation, epoxidation and C–H borylation. This development represents an effective approach to promote the cooperation between chiral cations and transition metals, increasing the versatility and capability of both these forms of catalysts. In this review, we present current examples of the three strategies and suggest possible inclusions for the future.

Enantioselective transition metal catalysis directed by chiral cations is the amalgamation of chiral cation catalysis and organometallic catalysis.     

Transition metal catalysis and nucleophilic fluorination

Transition metal catalyzed transformations using fluorinating reagents have been developed extensively for the preparation of synthetically valuable fluorinated targets. This is a topic of critical importance to facilitate laboratory and industrial chemical synthesis of fluorine containing pharmaceuticals and agrochemicals. Translation to (18)F-radiochemistry is also emerging as a vibrant research field because functional imaging based on Positron Emission Tomography (PET) is increasingly used for both diagnosis and pharmaceutical development. This review summarizes how fluoride sources have been used for the catalytic nucleophilic fluorination of various substrates inclusive of aryl triflates, alkynes, allylic halides, allylic esters, allylic trichloroacetimidates, benzylic halides, tertiary alkyl halides and epoxides. Until recently, progress in this field of research has been slow in part because of the challenges associated with the dual reactivity profile of fluoride (nucleophile or base). Despite these difficulties, some remarkable breakthroughs have emerged. This includes the demonstration that Pd(0)/Pd(II)-catalyzed nucleophilic fluorination to access fluoroarenes from aryl triflates is feasible, and the first examples of Tsuji-Trost allylic alkylation with fluoride using either allyl chlorides or allyl precursors bearing O-leaving groups. More recently, allylic fluorides were also made accessible under iridium catalysis. Another reaction, which has been greatly improved based on careful mechanistic work, is the catalytic asymmetric hydrofluorination of meso epoxides. Notably, each individual transition metal catalyzed nucleophilic fluorination reported to date employs a different F-reagent, an observation indicating that this area of research will benefit from a larger pool of nucleophilic fluoride sources. In this context, a striking recent development is the successful design, synthesis and applications of a fluoride-derived electrophilic late stage fluorination reagent. This new class of reagents could greatly benefit preclinical and clinical PET imaging.     

Development of organometallic (organo‐transition metal) pharmaceuticals

This paper is aimed at introducing the organometallic chemist to the fascinating area of organometallic pharmaceuticals. It commences by identifying the properties of organometallic (transition metal) compounds that lend themselves to medical applications. Next, the specialized techniques and methods that are used to assess the medicinal properties of compounds are summarized, and although these techniques are not restricted to organometallic compounds, all examples are concerned with organometallic compounds. The design and evaluation of organometallic compounds for medicinal applications are described in context with the diseases they have been evaluated against, and areas are identified that may have most potential for organometallic pharmaceuticals. Some new results, including the first example of an organo‐osmium compound that might exhibit effective anticancer properties, are also described. Copyright © 2005 John Wiley & Sons, Ltd.     

Stereoelective polymerization of racemic α-olefins: Influence of conversion and of nature of catalyst components

The dependence of stereoelectivity on conversion has been investigated by polymerizing racemic 4-methyl-1-hexene (I) and 3,7-dimethyl-1-octene (II) in the presence of catalysts prepared from TiCl₄ and bis[(S)-2-methylbutyl]-zinc(III) or tris [(S)-2-methylbutyl]aluminum (IV). The influence of the nature of transition metal halide has also been examined by polymerization of racemic I and II with catalysts obtained from III and transition metal halides as well as from TiCl₄ and optically active organometallic compounds containing phenyl groups. Stereoelectivity remains practically constant or increases slightly with increasing conversion, but it is affected by the nature of transition metal halide. Moreover a detectable variation of stereoelectivity was observed by changing the chemical structure of the alkyl group in the optically active organometallic component of the catalyst. In any case, however, stereoelectivity is not higher than 5–10%. Evidence in favor of asymmetric induction by the catalytic complex is reported and discussed.     

A modular approach to main-chain organometallic polymers

A highly efficient route to a new class of organometallic polymers containing difunctional N-heterocyclic carbenes has been developed. Bis(imidazolium) halides and divalent group X metals were copolymerized to afford organometallic polymers in up to quantitative yields and with molecular weights up to 10(6) Da, depending on the structure of the N-heterocyclic carbene and the incorporated transition metal. Enhanced solubilities were demonstrated through post-polymerization ligation with phosphines. Finally, selective end-group functionalization and excellent molecular weight control was achieved through the inclusion of monofunctional chain transfer agents during the polymerization.     

New directions in supramolecular transition metal catalysis

Supramolecular chemistry has grown into a major scientific field over the last thirty years and has fueled numerous developments at the interfaces with biology and physics, clearly demonstrating its potential at a multidisciplinary level. Simultaneously, organometallic chemistry and transition metal catalysis have matured in an incredible manner, broadening the pallet of tools available for chemical conversions. The interface between supramolecular chemistry and transition metal catalysis has received surprisingly little attention. It provides, however, novel and elegant strategies that could lead to new tools in the search for effective catalysts, as well as the possibility of novel conversions induced by metal centres that are in unusual environments. This perspective describes new approaches to transition metal catalyst development that evolve from a combination of supramolecular strategies and rational ligand design, which may offer transition metal catalysts for future applications.     

C‐Alkylation of Ketones and Related Compounds by Alcohols: Transition‐Metal‐Catalyzed Dehydrogenation

Transition‐metal‐catalyzed C‐alkylation of ketones and secondary alcohols, with alcohols, avoids use of organometallic or environmentally unfriendly alkylating agents by means of borrowing hydrogen (BH) or hydrogen autotransfer (HA) activation of the alcohol substrates. Water is formed as the only by‐product, thus making the BH process atom‐economical and environmentally benign. Diverse homogeneous and heterogeneous transition‐metal catalysts, ketones, and alcohols can be used for this transformation, thus rendering the BH process promising for replacing those procedures that use traditional alkylating agents. This Minireview summarizes the advances during the last five years in transition‐metal‐catalyzed BH α‐alkylation of ketones, and β‐alkylation of secondary alcohols with alcohols. A discussion on the application of the BH strategy for C?C bond formation is included.     

Quantitative computational thermochemistry of transition metal species

The correlation consistent Composite Approach (ccCA), which has been shown to achieve chemical accuracy (+/-1 kcal mol-1) for a large benchmark set of main group and s-block metal compounds, is used to compute enthalpies of formation for a set of 17 3d transition metal species. The training set includes a variety of metals, ligands, and bonding types. Using the correlation consistent basis sets for the 3d transition metals, we find that gas-phase enthalpies of formation can be efficiently calculated for inorganic and organometallic molecules with ccCA. However, until the reliability of gas-phase transition metal thermochemistry is improved, both experimentally and theoretically, a large experimental training set where uncertainties are near +/-1 kcal mol-1 (akin to commonly used main group benchmarking sets) remains an ambitious goal. For now, an average deviation of +/-3 kcal mol-1 appears to be the initial goal of "chemical accuracy" for ab initio transition metal model chemistries. The ccCA is also compared to a more robust but relatively expensive composite approach primarily utilizing large basis set coupled cluster computations. For a smaller training set of eight molecules, ccCA has a mean absolute deviation (MAD) of 3.4 kcal mol-1 versus the large basis set coupled-cluster-based model chemistry, which has a MAD of 3.1 kcal mol-1. However, the agreement for transition metal complexes is more system dependent than observed in previous benchmark studies of composite methods and main group compounds.     

A new method of calculation of the melting temperatures of crystals of Group 1A metal halides and francium metal

A new method of calculation of melting temperatures of binary ionic crystals has been suggested. The method is based on finding a matrix relation between the ionic radii (the lattice energy U) and melting temperature of ionic crystals of the MX type, where M is a Group 1A metal, and X is a halogen. From the equation for the lattice energy U, a new equation has been derived for calculation of the melting temperature of ionic crystals with the use of only the ionic radii and the degree of bond ionicity ?: T _m = f(U, ?). The average error of determination of T _m for alkali-metal halides is 2.80%. The melting temperatures of francium halides and alkali-metal astatides (including FrAt) have been calculated. It has been shown that the accuracy of calculation of the melting temperature of ionic crystals depends on the degree of bond ionicity: the error increases with an increase in the covalent contribution. On the basis of the melting temperatures of metal halide crystals, a method has been developed for the calculation of the melting temperatures of corresponding metals. The melting temperature of francium has been calculated to be 24.861 ± 0.517°C.     

Binary mixtures of metal compounds as flame retardants for organic polymers

Studies have been made of the effect on the flammability of thermoplastic polymers of the partial or total replacement of one metal compound by another in the presence also of a suitable halogen compound; particular attention has been paid to systems where the primary flame retardant is antimony(III) oxide. With each binary metal compound system investigated, ten different compositions have been chosen so as to provide a symmetrical arrangement of points within a triangular design; resulting calculated values of the limiting oxygen index for each polymer-flame retardant system for a given polymer are shown as a graphical contour analysis. Comprehensive studies of several systems show that both iron(III) oxide and aluminium oxide monohydrate can significantly enhance the flame-retardant action of antimony(III) oxide but that several other metal compounds, although not as effective as Sb₂O₃, may nevertheless be used as adequate partial replacements for it. The Fe₂O₃-SnO₂-H₂O system can also act as an effective flame retardant under certain conditions. The SnOZnO system perhaps best illustrates the importance of the polymer substrate and of the total additive loading as factors controlling the flame-retardant effectiveness. For all the systems studied, however, ABS is a much better substrate than HDPE. The results of a reasonably detailed study of the flame retardance conferred by several different compositions of a binary metal compound mixture give a much more reliable indication of the effects on polymer flammability of the constituent metal compounds than are obtained simply by replacement of a given concentration of one compound by another.     

Synthesis of symmetrical organic carbonates via significantly enhanced alkylation of metal carbonates with alkyl halides/sulfonates in ionic liquid

[reaction: see text] We report a new phosgene-free method for the synthesis of symmetrical organic carbonates via alkylation of metal carbonate with various alkyl halides and sulfonates in 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], as an ecofriendly reaction media. Alkylation of metal carbonate in various ionic liquids with 1-bromo-3-phenylpropane (1a) as a model reactant has thoroughly been investigated. Potassium and cesium carbonates appeared to be the most suitable metal carbonate due to their high solubility in ionic liquids. Besides good to excellent yields, this simple and convenient methodology is devoid of highly toxic and harmful chemicals such as phosgene and carbon monoxide, which is an additional advantage.     

Triple-decker transition metal complexes bridged by a single carbocyclic ring

Triple-decker organometallic complexes of transition metals have attracted interest since 1972, when Werner and Salzer prepared the tris(cyclopentadienyl)dinickel cation, the first example of this class of compound. This paper reviews the literature for those triple-decker complexes which contain a single carbocyclic ligand bridging two metal centres.