Hydrogenation reaction sequence

To complete the alkene hydrogenation reaction sequence, the first hydrogen transfer must be followed by a second, which results in the reductive elimination of the alkane product. This proceeds through a three-centered transition state. The catalytic cycle is shown in Fig. 22-1 but the process is actually more complicated since the equilibria are dependent on phosphine, alkene, rhodium concentrations, temperature, and pressure. 

Example 2.5 tert-Butyl hydrogen sulfate is required as an intermediate in a reaction sequence. This can be produced by the reaction between isobutylene and moderately concentrated sulfuric acid ... 

Toluene Diisocyanate. Toluene diisocyanate is the basic raw material for production of flexible polyurethane foams. It is produced by the reaction sequence shown below, in which toluene is dinitrated, the dinitrotoluene is hydrogenated to yield 2,4-diaminotoluene, and this diamine in turn is treated with phosgene to yield toluene 2,4-diisocyanate. 

Chloroform slowly decomposes on prolonged exposure to sunlight in the presence or absence of air and in the dark in the presence of air. The products of oxidative breakdown include phosgene, hydrogen chloride, chlorine, carbon dioxide, and water. At 290°C, chloroform vapor is not attacked by oxygen. In contact with iron and water hydrogen peroxide is also produced, probably by the following reaction sequence (2) ... 

Maltol. Otsuka Chemical Co. in Japan has operated several electroorganic processes on a small commercial scale. It has used plate and frame and aimular cells at currents in the range of 4500—6000 A (133). The process for the synthesis of maltol [118-71 -8], a food additive and flavor enhancer, starts from furfural [98-01-1] (see Food additives Flavors and spices). The electrochemical step is the oxidation of a-methylfurfural to give a cycHc acetal. The remaining reaction sequence is acid-catalyzed ring expansion, epoxidation with hydrogen peroxide, and then acid-catalyzed rearrangement to yield maltol, ie ... 

There are several reaction sequences which involve such intramolecular hydrogen abstraction steps. One example is the photolytically intitiated decomposition of N-haloamines in acidic solution, which is known as the Hofinann-Loffier reactionThe reaction leads initially to y-haloamines, but these are usually converted to pyrrolidines by intramolecular nucleophilic substitution ... 

Note 1. An alternative procedure proceeds by oxidation of the 3/5-hydroxy group with chromic acid-sulfuric acid and subsequent elimination of hydrogen chloride by treatment of the intermediate chloroketone with potassium acetate in methanol. Good overall yields are obtained with this reaction sequence in the androstane series. 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

Acetylenes have hijh synthetic utility, and hydrogenation of the triple bond occurs in many reaction sequences (7). Often the goal of this reduction is formation of the cis olefin, which usually can be achieved in very high yields (for an exception, see Ref. 10). Continued reduction gives the paraffin. Experimentally, both the relative and absolute rates of acetylene and olefin hydrogenation have been found to depend on the catalyst, substrate, solvent, reaction conditions, and hydrogen availability at the catalyst surface. Despite these complexities, high yields of desired product usually can be obtained without difficulty. 

The carbocation may rearrange, eliminate a proton to produce an olefin, or crack at a beta position to yield an olefin and a new carbocation. Under an atmosphere of hydrogen and in the presence of a catalyst with hydrogenation-dehydrogenation activity, the olefins are hydrogenated to paraffinic compounds. This reaction sequence could be represented as follows ... 

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

Deoxy-D-jcylo hexose 6-(dihydrogen phosphate) (21) has also been synthesized (2) the reaction sequence makes use of 3-deoxy l 2,5 6-di-O-isopropylidene D-galactofuranose (16), a compound that can be easily prepared from D-glucose (2, 60). The mono-isopropylidene derivative (17) formed by partial hydrolysis of the di-ketal is converted into the 6-tosylate (18) by reaction with one molar equivalent of p-toluenesulfonyl chloride. From this the epoxide (19) is formed by reaction with sodium methoxide. Treatment of the anhydro sugar with an aqueous solution of disodium hydrogen phosphate (26) leads to the 6-phosphate (20)... 

Triethylaluminum is prepared from aluminum powder, ethylene, and hydrogen according to the reaction sequence ... 

The occurrence of multiple-order reactions in the methanol system is shown by the increase of R with N in Figure 2, and the system can be analyzed in terms of the kinematic theory of hydrogen transfer developed above. Consider the situation for CH3OD for which the following reaction sequence applies. 

To test this theory, a mixture of n-hexane and Relabeled 1-hexene was reacted in hydrogen over the catalyst at various space velocities. The specific activity of each of the products (the n-hexenes were lumped together) are shown in Figure 2. The important observation is made at zero conversion. When extrapolated to Infinite space velocity, the benzene has approximately the same specific activity as the hexene, thus clearly indicating that essentially all the benzene is formed in a reaction sequence that involves equilibrium with gaseous n-hexenes. It may then be concluded that olefins are intermediates in the aromatiza-tion process. 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

Au "=0 species are postulated, inter alia, as active intermediates in the oxidation of alkanes with hydrogen peroxide catalyzed by gold(III) and gold(I) complexes [115]. The reaction sequence is proposed in Scheme 2.8. 

This generalized reaction sequence consumes the halide, the stannane, and the reactant X=Y, and effects addition to the organic radical and a hydrogen atom to the X=Y bond. The order of reactivity of organic halides toward stannyl radicals is iodides > bromides > chlorides. 

One of the simplest chemical reactions involving a barrier, H2 + H —> [H—H—H] —> II + H2, has been investigated in some detail in a number of publications. The theoretical description of this hydrogen abstraction sequence turns out to be quite involved for post-Hartree-Fock methods and is anything but a trivial task for density functional theory approaches. Table 13-7 shows results reported by Johnson et al., 1994, and Csonka and Johnson, 1998, for computed classical barrier heights (without consideration of zero-point vibrational corrections or tunneling effects) obtained with various methods. The CCSD(T) result of 9.9 kcal/mol is probably very accurate and serves as a reference (the experimental barrier, which of course includes zero-point energy contributions, amounts to 9.7 kcal/mol). 

---