Olefin system hydrogenation

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

Calculated transition-state geometries (Fig. 5) show a clear similarity with the alkyl -I- olefin system described in the previous section. Transition states are somewhat earlier, since these reactions are much more exothermic this can be seen, e.g., from the noticeably different C- H bond lengths of the hydrogen transfer transition state. 

When the catalyst was used for simple olefin systems, it was not as active as with the amino acid precursors. Table III shows the relative rates for a variety of substrates, special care being taken in each case to purge oxygen. The slow rate of a-phenylacrylic acid was unexpected, but, it may be the result of a stable olefin-rhodium complex similar to the one Wilkinson (15) experienced with ethylene. Such a contention is consistent with the increased speed of hydrogenation with increased pressure. 

New chiral centers are produced by addition reactions to other trigonal centers as well. Hydrogenation of 3-methyl-3-hexene gives 3-methylhexane. Clearly the addition of hydrogen to one face of the planar olefinic system gives one enantiomer and addition to the opposite face gives the opposite enantiomer. Likewise reaction of styrene with chlorine or bromine (X2) or potassium permanganate produces products with a new chiral center. Formation of the two possible enantiomers results from addition to either face of the olefin. 

The other main group elements which form peroxo complexes are d6 and d8 systems in group VIII including iridium, palladium and platinum. The ji-peroxo complexes do not generally catalyse the epoxidation of olefins with hydrogen peroxide,95,96 but it has been found that trifluoromethyl-substituted Pd(II) and Pt(II) hydroperoxides will perform such a transformation.97... 

There are very few examples of direct hydroxylation of olefins using hydrogen peroxide, since these methods are limited to polymer applications or derivatization of natural products. Vinyl monomers have been hydroxy-lated in an alcoholic medium using acidic hydrogen peroxide 111 normally the acid is methanesulfonic. Natural rubber has also been hydroxylated, and simultaneously depolymerized by employing a hydrogen peroxide/UV system.112 The product distribution can be altered by varying the irradiation time. 

Figure 3.32 Cleavage of olefins with hydrogen peroxide/ vanadium ( V) systems.

Heterogeneous systems have also been developed for the cleavage of olefins with hydrogen peroxide.174,175 Titanium-containing zeolites can be used to cleave olefins.176 Adam and co-workers have recently shown that acetophenone, an oxidation product from the Ti-zeolite catalysed oxidation of a-methyl-styrene, derives from 2-hydroxyperoxy-2-phenylpropan-l-ol as an intermediate (which they detected and isolated) (Figure 3.34).177... 

In this context it is interesting to note the recent reports of fluorous catalysis without fluorous solvents [68]. The thermomorphic fluorous phosphines, P[(CH2)m(CF2)7CF3]3 (m=2 or 3) exhibit ca. 600-fold increase in n-octane solubility between -20 and 80 °C. They catalyze the addition of alcohols to methyl propiolate in a monophasic system at 65 °C and can be recovered by precipitation on cooling (Fig. 7.20) [68]. Similarly, perfluoroheptadecan-9-one catalyzed the epoxidation of olefins with hydrogen peroxide in e.g. ethyl acetate as solvent [69]. The catalyst could be recovered by cooling the reaction mixture, which resulted in its precipitation. 

We used for the further tests the system hydrogen fluoride/ boron trifluoride as catalyst and olefins as means of alkylation. 

C. Venturello, E. Alneri. M. Ricci, A new, effective catalytic system for epoxidation of olefins by hydrogen peroxide under phase-transfer conditions, J. Org. Chem. 48, 3831— 3833 (1983). 

Hine and coworkers showed that the dimethylamino group is by far one of the best known double-bond stabilizing substituents. This may be evaluated quantitatively from experimental enthalpies of formation and of hydrogenation presented in Table 17 and may be seen from the value of Hammett s ffp(NMe2)-constant which, at —0.83, has one of the largest negative values observed. Both enthalpy values mentioned show a stabilization of the enamine due to conjugation by — 5 to — 6 kcal mol with respect to allylamines which may be corrected in relation to olefinic systems to about —2.5 kc mol ... 

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