Metalation hetero atoms

A. J. Head, W. D. Good. Combustion of Liquid/Solid Organic Compounds with Non-metallic Hetero-atoms. In Experimental Chemical Thermodynamics, vol. 1 Combustion Calorimetry, S. Sunner, M. Mansson, Eds. IUPAC-Pergamon Press Oxford, 1979 chapter 9. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

The first phosphazene polymers containing carbon (79), sulfur (80,81), and even metal atoms (82) in the backbone have been reported. These were all prepared by the ring-opening polymerization of partially or fully chloro-substituted (or fluoro-substituted) trimers containing one hetero atom substituting for a ring-phosphoms atom in a cyclotriphosphazene-type ring. 

We shall discuss first reactions in which hydrogen or a metallic ion (or in one case phosphorus or sulfur) adds to the hetero atom, and then reactions in which carbon adds to the hetero atom. Within each group, the reactions are classified by the nature of the nucleophile. Additions to isocyanides, which are different in character, are treated at the end. 

Interestingly, when the chloro analog was transmetallated and treated with 3-ethoxy cyclohexen-l-one, the expected enone (XI) was not observed, but an enone with a mass of 34 units greater than (XI) was noticed. It also indicated the enone carried the chloro analog. It was presumed that the hetero atoms in the heterocycle present in the starting material (VIII) had performed a directed metallated lithiation providing a different enone bearing the chloro moiety. 

Hetero-atomic clusters, moreover, may be derived from the binary structures mainly through the introduction of late transition or earlier post-transition elements. Examples of ternary alloys containing such structures are the alkali metal salts of centred clusters In10Me10 (Me = Ni, Pd, Pt), Tl12 Me12- (Me = Mg, Zn, Cd, Hg), etc. The crystal structure of the phase Na T Cdi x)27 (0.24 < x < 0.33)... 

The validity of reaction Scheme 1 is not limited to heterolyses of covalent bonds to carbon. It covers also cases where both X and Y are hetero-atoms (e.g. oxidation of halides and pseudohalides by OH, cf. Ref [40] or where X is a hetero atom and Y is a metal (cf. Ref [41])... 

The HO-energy of a ir-system can be changed, e.g. by occupied orbitals of hetero-atoms which are relative donors or acceptors for the HO s. This can also be true of interactions of the LUMO s of the tr-system with vacant AO s of heteroatoms. The consequences e.g. for the preference of certain conformers relative to the type and position of perturbations in tricarbonyl-chromium benzene complexes (organo-metallic example) are described in Figure 2 of Scheme 2.1-4 together with the consequences for the reactivities in benzene derivatives (example of organic chemistry) due to rektive donor- or acceptor-perturbations (see also Scheme 2.1-2 Fig. 2). 

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, followed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subsequent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided. 

Often, selectivity for a vinylidene-mediated pathway is heavily dictated by substrate structure. It is especially true in the case of hetero-atom substituted alkynes that Jt-alkyne/vinylidene rearrangement is driven by a reduction in steric interactions at the metal center. 

In the following we have broadly divided the available C(l) nucleophiles according to structure and reactivity. C(l)-Nucleophiles based on metallation at an sp -center are categorized in terms of the presence and, as appropriate, the nature of a hetero atom (O or N) substituent at C(2). C(l)-Lithiated glycals correspond to nucleophilic sp -centers, and variations on this theme (concerning the nature of the metal component) are covered in depth. 

It is reasonable to assume, that the counterion prefers to coordinate to the carbon atom with the highest electron density. Hence, according to this crude picture, donor-substituted allylmetallics are to be regarded as 3-metalated hetero-substituted vinyl compounds, acceptor-substituted ones on the other hand as 1-metalated hetero-substituted allyl compounds. 

Reactions in Which Hydrogen or a Metallic Ion Adds to the Hetero Atom A. Attack by OH (Addition of HjO)... 

For catalytic application it is necessary to incorporate hetero-atoms into the silica framework. Several samples have been synthesised using different aluminia precursors. The metal content was determined by X-ray fluorescence analysis, UV-VIS spectra, IR spectra and solid state NMR spectroscopy, respectively. X-ray fluorescence analysis provides information about the metal content of the samples. By variation of the metallic precursor concentration the metal content of the product could be enhanced up to 10 % w/w. 

Many of the crown ethers have considerable specificity with regard to the metal with which they complex. Ring size as well as the number and kind of hetero atoms are very important in this connection. 18-Crown-6 is especially effective for potassium ... 

This section has dealt with the oxidation of CO to C02, especially as it enters into the water-gas shift reaction (26a). A reasonable view of the homogeneous catalysis of this reaction, whether in basic or acidic media, is emerging in which CO formation proceeds from nucleophilic attack of water or OH" on an activated carbonyl followed by either reductive decarboxylation or hetero-atom -elimination yielding, respectively, a reduced metal or a metal hydride species. 

From the point of view of the hetero-atom itself, there are two more known below selenium in the Periodic Table. Each deserves some special comment. The next atom, directly below selenium, is tellurium. It is more metallic, and its compounds have a worse smell yet. I heard a story about a German chemist, many years ago, who was carrying a vial of dibutyl telluride in his pocket in a passenger coach from here to there in Germany, back at about the turn of the century. It fell to the floor and broke. No one could remain in the car, and no amount of decontamination could effectively make the smell tolerable. Scratch one railway coach. But the compound, 2C-TE, would be readily makeable. Dimethyl ditelluride is a known thing. 

Y = hetero atom E= electrophile M = metal Scheme 1. sp3C-H activation by the a-heteroatom effect. 

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