Metal-free systems

Finally, there is active interest in developing catalyst systems, both ballistic and polymerization, that would promote combustion stability at high pressures (especially in metal-free systems for smokeless applications) and allow processing lattitude for relatively large motors. The ferric-based systems currently being used fall short of these performance measures. Compounds that form complex structures with the metal chelate to reduce its activity to acceptable levels seem to be most promising. Interestingly, the use of an antibiotic has been cited in this context [19],... 

A given ligand system was first prepared on the solid support, and a small collection of catalysts (library 1) generated by adding 11 different metals and in one case no metal (Scheme 23).119 It turned out that the best ee-value (19%) was in fact obtained in the metal-free system. Based on this... 

Co(NH3)5NCCH5 + N3- Co(NH3) N4CCH3 + (5-methyltetrazole) see (6.24) complete in 2 hours at room temperature 25 hours at 150°C for metal-free system 48... 

Early workers appeared to show that electrophilic substitution reactions could not be carried out on porphyrins, and began to question the aromaticity of porphyrins since this classical pre-requisite of aromatic character could not be accomplished. However, they had concentrated on reactions of metal-free systems, and since many electrophilic substitution reactions utilize acidic conditions (nitration, sulfonation), they were actually dealing with the non-nucleophilic porphyrin dication. But, as early as 1929, H. Fischer had realised that diacetylation of deuteroporphyrin-IX (Table 1) had to be carried out on a metal complex, such as the iron (III) derivative chelation with a metal ion which cannot be removed under the acid conditions of the subsequent reaction, effectively eliminates dication formation. A judicious choice of metal complex therefore needs to be made for any particular reaction. For example, though magnesium(II) produces an extremely reactive substrate for electrophilic substitution reactions, it is removed by contact with the mildest of acids and is, consequently, of little use for this purpose. 

Derivatives containing metal tricarbonyl groups thus show peaks corresponding to loss offour carbonyl groups. This decarbonylation process is not observed in the metal-free systems, nor does it occur where the COR group is not directly bonded to the metal carrying 7r-organic moiety. 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

Subsequently, the alkali metal free systems (CsMes)2NdCl(thf) and... 

One other approach to metalated M21)-methyl corroles was reported by Johnson and coworkers in the early 1970s. In this instance, the A-methyl thiaph-lorin 2.206 was used as the starting material. It was heated in the presence of Pd(OAc)2 in acetic acid to give the metalated corrole derivative 2.233 directly (see Scheme 2.1.81). The rate of sulfur extrusion in the presence of the palladium(II) ions was found to be dramatically increased in comparison to that of the analogous metal-free system 2.206 or the N-unsubstituted thiaphlorins 2.102 (Scheme 2.1.22). It was proposed that this rate enhancement reflected the extra stabilization that... 

The octa-alkyl porphycenes are also able to support the formation of metal(II) complexes. Examples of these include the nickel(II) complex of octamethylpor-phycene (3.69) and the nickel(II) and zinc(II) complexes of octaethylporphycene (3.70 and 3.71). As shown by single crystal X-ray diffraction data, incorporation of divalent zinc into octaethylporphycene results in the formation of a nearly planar structure (Figure 3.1.13). Considering the somewhat non-planar nature of the starting metal-free system 3.28, the insertion of this cation apparently serves to enforce a marked increase in ligand planarity. 

The most important figure of merit in r.b.s is the theoretical energy density s,th- As shown in Tables 10-12, where inorganic examples are included for comparison, the metal-free systems seems to have some limitations. However, the top inorganic... 

It should be remembered that the practical energy density Eg in metal-free batteries is often closer to the theoretical value than in the case of conventional systems. This can be rationalized in terms of higher active mass utilizations through thin-layer technology (via thermoplastic binders, for instance), lighter current collectors (at least in bipolar systems), and so on. The lead-acid accumulator has a ratio a = Eg sh)/Es,th 15% thus Eg — 25 Wh/kg. But a metal-free system with s,th = 80 Wh/kg may allow Eg = 40 Wh/kg, if a is 50% in this way. 

Copper-containing mordenite catalysts have also been reported to be active for carbonylation of vapor-phase methanol [170]. Initially, the predominant reaction products were hydrocarbons resulting from methanol-to-gasoline chemistry, but after about 6 h on stream at 350 °C the selectivity of the catalyst changed to give acetic acid as the main product. A recent investigation was carried out with in situ IR and solid-state NMR spectroscopies to probe the mechanism by detecting surface-bound species. The rate of carbonylation was found to be enhanced by the presence of copper sites (compared to the metal-free system), and formation of methyl acetate was favored by preferential adsorption of CO and dimethyl ether on copper sites [171],... 

We recently initiated efforts within our own laboratory to synthesize terpy-based ligands that incorporate appended Lewis acid/base pairs [36]. Motivated by metal-free systems of frustrated Lewis pairs (FLPs) which bind and form adducts with small molecules, we sought to couple the reactivity of FLPs to a transition-metal fragment to afford a metal Lewis acid/base triad (LABT). The combination of both partners with a transition-metal center opens up the possibiUty to synergistically use these systems for further activation and/or redox transformations - currently a hmitation of metal-free FLP systems. 

Cui H-H, Wang J-Y, Hu M-Q, Ma C-B, Wen H-M, Song X-W, Chen C-N (2013) Efficient photo-driven hydrogen evolution by binuclear nickel catalysts of different coordination in noble-metal-free systems. Dalton Trans 42(24) 8684—8691. doi 10.1039/c3dt50140j... 

Because, as shown above, in the oxidation catalysis of alkanes, one can replace a transition metal catalyst by a redox-inactive nontransition metal catalyst, the question arises whether it would be possible to go even further and eliminate completely the use of any metal catalyst, thus establishing a metal-free system capable of oxidizing alkanes under mild conditions. 

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