Molecular hydrogen elimination

The first study of dihydrogen bonding between LiH and the strong proton donor HF has been carried out by Liu and Hoffman [1], The authors have performed RHF and MP2 calculations where dihydrogen bonds have not been localized because of the high exothermicity of molecular hydrogen elimination ... 

The same relationship has been found for molecular hydrogen eliminations from (C2H4)f, (C2HS)+ and (C6H7)+ [443], It has been argued that these results confirm that the reactions are concerted, in the sense that bonding to both of the hydrogen atoms to be eliminated is weakened in the transition state [443, but see 744]. 

FIGURE 11.11 Products formed in the reactions of ground state carbon atoms with unsaturated hydrocarbon molecules under single collision conditions via hydrogen atom elimination. Note that for the reaction of carbon atoms with acetylene, the tricarbon plus molecular hydrogen elimination channel was observed, too. 

The effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

In Schemes 1-3 the photolyses of liquid methanol, ethanol and isopropanol are summarized. They are derived using data on quantum yields, rate constants of radical reactions, and data from deuterium labeling experiments. The evaluation of the quantum yields of the molecular hydrogen-eliminating processes is somewhat uncertain in the case of ethanol (mainly through the uncertainty in is problematic in the case of iso-... 

Both, ligand and molecular hydrogen elimination processes probably occur by a similar mechanism with hydride species as intermediates. Isolation of species like [Os5(CO)i5H2l] and NMR detection of intermediates like [Os7(CO)2oH2C1] in reactions of the corresponding hydrides with iodide and chloride ions agree with this kind of mechanism. 

The basic requirement for cellulose dissolution is that the solvent is capable of interacting with the hydroxyl groups of the AGU, so as to eliminate, at least partially, the strong inter-molecular hydrogen-bonding between the polymer chains. There are two basic schemes for cellulose dissolution (i) Where it results from physical interactions between cellulose and the solvent (ii) where it is achieved via a chemical reaction, leading to covalent bond formation derivatizing solvents . Both routes are addressed in details below. 

Based on our observation in these two systems, it would appear that Cp Cr -alkyls, if rendered electrophilic and/or sufficiently coordinatively unsaturated, will both bind and insert a-olefins. However, the more heavily substituted alkyl ligands thus formed (i.e. CrBl-CH2-CH(R)-P vs. Crni-CH2-CH2-P resulting from ethylene insertion) seem to be very susceptible to facile 3-hydrogen elimination. Rapid chain transfer and very low molecular weights are the results of this tendency. Whether the latter is an innate property of all chromium alkyls or reflects the particular chemical nature of the Cp Cr-moiety remains to be established. To this end, investigation of chromium alkyls with a variety of other ancillary ligands are needed. 

Oxidative addition of molecular hydrogen was considered to be involved in the alkyne hydrogenations catalyzed by [Pd(Ar-bian)(dmf)] complexes (4 in Scheme 4.4) [41, 42]. Although the mechanism was not completely addressed, 4 was considered to be the pre-catalyst, the real catalyst most likely being the [Pd(Ar-bian)(alkyne)] complex 18 in Scheme 4.11. Alkyne complex 18 was then invoked to undergo oxidative addition of H2 followed by insertion/elimination or pairwise transfer of hydrogen atoms, giving rise to the alkene-complex 19. 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

One of the possible catalytic cycles (i.e. for olefin hydrogenation) is described in Figure 12.2. The molecular hydrogen is first complexed to the metal. Then the olefin is complexed and inserted into the M H bond. The alkane is liberated by elimination and the catalyst regenerated. 

Pines and Kolobielski (18) have shown that phenylcyclohexene, although it is not a cyclic diolefin, will also undergo reactions similar to those that cyclic diolefins undergo when treated with base catalysts. When heated to 200-220 with a sodium-benzyl-sodium catalyst, it underwent a hydrogen transfer reaction resulting in the formation of biphenyl and of phenyl-cyclohexane molecular hydrogen was not produced. The mechanism of this reaction may be pictured as an elimination of sodium hydride from one molecule with the hydride ion being accepted by another molecule (A"-E"). 

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