Main group element-promoted

The oxidative carbonylation of amines to give ureas is at present one of the most attractive ways for synthesizing this very important class of carbonyl compounds via a phosgene-free approach. Ureas find extensive application as agrochemicals, dyes, antioxidants, resin precursors, synthetic intermediates (also for the production of carbamates and isocyanates), and HIV-inhibitors. Many transition metals (incuding Au [244], Co [248,253-255], Cu [242], Mn [249,256-258], Ni [259], Rh [246,247,260-262], Ru [224,260,263] and especially Pd [219,225,226,264-276], and, more recently, W [277-283]) as well as main-group elements (such as sulfur [284-286] and selenium [287— 292]) have been reported to promote the oxidative carbonylation of amines, usually under catalytic conditions. In some cases, carbamates and/or oxamides are formed as byproducts, thus lowering the selectivity of the process. 

An important class of industrial catalysts consists of an active component dispersed in the form of very small particles over high surface area solids. As the field of industrial heterogeneous catalysis has developed, catalyst formulations have evolved such that state-of-the-art catalysts often contain two or more metals and/or main group elements. The additives may promote a desired reaction, prevent undesirable side reactions, or enhance catalyst longevity.Bimetallic nanoparticle catalysts in particular are widely... 

Transition metal complexes act as templates that regulate organic reactions that occur in the coordination sphere (4). Ligands are often activated or stabilized by participation of metal d orbitals, where the central metals are electronically amphoteric in contrast to the main group elements, which normally act as Lewis acids. The bonding scheme of an olefin-transition metal complex is illustrated in Scheme 2. The olefin 7T electrons are donated to a vacant metal orbital to make a a-type bond the metal d elections are back-donated to olefin anti-bonding orbitals with the same symmetry to form a ir-type bond. In this way, the olefin is activated by formal electron promotion from the tt to tt orbital, as... 

Other main group elements have been used as promoters in MCS beds containing copper, tin and zinc. Phosphorus and indium have been used in the MCS reaction. The reaction of silicon (40 g) with MeCl (2 bar) at 300 °C in the presence of Cu (3.2 g), ZnO (0.05 g), In (0.004 g) and P (0.056 g) gave 1.7% of Me3SiCl, 0.017% mixture of MeSiCl3/Me2SiCl2 and 3.7% of polysilanes34. Antimony, just below phosphorus in... 

Main Group Element- and Transition Metal-Promoted Carboxylation... 

These salts can be used for the synthesis of both transition metal and main group element thionyl imides by metathetical reactions, for example, Cp2Ti(NSO)2 and Ph3 As(NSO) (x = 1, 2), respectively. " The M NSO group invariably adopts a cis geometry in these derivatives. A characteristic reaction of thionyl imides is the thermal or base-promoted elimination of SO2 to give the corresponding sulftir dimide. ... 

Metals with low oxidation potentials and high Lewis acidity in their highest oxidation states are superior catalysts and show the following order of reactivity Mo > W > V > Ti. Metals which readily promote homolytic decomposition of alkyl hydroperoxides via one-electron pathways, e.g., Co, Mn, Fe, and Cu, are not effective. Certain main group elements, e.g., B and Sn, exhibit activity, albeit significantly lower than molybdenum. 

All precursors are amorphous up to calcination temperatures of around 600°C. At higher temperatures, in most cases powders with extremely small crystallite sizes of around 20-40 nm are formed (Fig. 7). A further increase in calcination temperature promotes crystal growth. With aluminum nitride, a white powder with a low oxygen and carbon content is obtained [97]. Other main group element precursors exhibit fairly different behaviors Mg and Ca precursors yield metal cyanamide [99]. Calcination of the transition element precursors (Fig. 8) results in the formation of nitrides, carbonitrides, or carbides. For the titanium-containing precursors, TiN/TiC solid solutions can be obtained [96] the quantity of carbon strongly depends on the calcination atmosphere applied (argon, 31 wt% ammonia, 5.1 wt%). 

Zevaco T, Dinjus E (2010) Main group element- and transition metal promoted carboxyla-tions of organic substrates (alkanes, alkenes, alkynes, aromatics, and others). In Aresta M (ed) Carbon dioxide as chemical feedstock. Wiley-VCH Verlag GmbH Co, KGaA, Weinheim... 

After reduction, the oxygen Is spectra are dominated by species bound to main group elements. The possibility cannot be excluded, however, that, in addition to the expected cations of the promoter elements, there may also be some isolated hydroxyl groups present on the activated catalyst surfaces. 

Figure 3.19 illustrates the dependence of Asp on location in the periodic table for each of the first three elements of main groups 13-17. The promotion energy varies with group number in a manner similar to electronegativity (cf. Fig. 3.17). Although Asp is doubtless important, other factors (such as differences in orbital radii) appear to play a role in general spd hybridization.

A number of iodide salts have been proposed as catalyst stabilizers and copromoters, in particular the iodide salts of Main Group IVA, VA, and VIA elements [99-104]. At equivalent iodide catalyst concentrations, these salts appear to have no significant stabilization or promotional benefit over the preferred alkali metal... 

The scheme most commonly used today is the one promoted by lUPAC. This was designed to resolve the confusion between the two A/B schemes. However, the lUPAC system has a serious deficiency. On the A/B schemes, the main groups are numbered 1-8, corresponding to the maximum valency exerted by elements in each group, and (except for He) the number of valence electrons possessed by their atoms (see Chapter 11). To recoverthis correspondence, I shall use the labels in the last column of the table above, where M stands for main , T for transition , and IT for iimer-transition . 

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