Vinyl-Oxygen Compounds

Over Raney Ni, enol ethers appear to be saturated with little hydrogenolysis. Dihy-dropyran is hydrogenated almost quantitatively to tetrahydropyran over Raney Ni at room temperature and 0.28 MPa H2 (eq. 13,45).89 Over platinum, however, 1 -pentanol is formed along with tetrahydropyran.90 

Enol esters are much more labile to hydrogenolysis than enol ethers. Thus, 1-acetoxy-cyclohexene and ethyl 3-acetoxycrotonate are almost quantitatively hydrogenolyzed over Adams platinum in ethanol or acetic acid (eqs. 13.46 and 13.47).91 92 

Over Pd-CaC03, however, 1-acetoxycyclohexene was hydrogenated with little hydrogenolysis to give cyclohexyl acetate in good yield.91 In contrast to the cyclo- 

Jacobs and Scott studied the hydrogenation of various unsaturated y- and 8-lactones over Adams platinum in ethanol.93 Similar to enol esters, these enol lactones are readily hydrogenolyzed over platinum, as seen in the examples shown in eq. 13.48. In contrast, hydrogenolysis takes place only slightly with an a,P-unsaturated y-lactone 50 (eq. 13.49). The saturated lactones were not reduced at all under the reaction conditions. 

The phosphoric acid ester of an enol ether is even more readily hydrogenolyzed than an enol acylate. When the (Z)-enol phosphate 51 was subjected to hydrogenation in ethyl acetate at 0.3 MPa H2, the hydrogenolysis products were isolated in 95% yield not only over platinum oxide but also over 5% Pd-C and 5% Rh-Al203 (eq. 13.50).94 The hydrogenolysis of an enol phosphate has been utilized in conversion of one of two oxo groups to the methylene as shown in eq. 13.51 95 

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Relative activities of allylic and vinylic oxygenated compounds... 

The mercapto form is much more strongly favored than is the hydroxy form for the corresponding oxygen compounds. A pertinent comparison in this respect is the greatly reduced inclination of enethiols to tautomerize to the corresponding thiocarbonyl compounds, in contrast to the facile ketonization of vinyl alcohols. 

One interesting difference between vinyl compounds and cyclic oxygen compounds is that with some of the latter, e.g., tetrahydrofuran, all kinds of chain-breaking reactions... 

Abstract The purpose of this chapter is to present a survey of the organometallic chemistry and catalysis of rhodium and iridium related to the oxidation of organic substrates that has been developed over the last 5 years, placing special emphasis on reactions or processes involving environmentally friendly oxidants. Iridium-based catalysts appear to be promising candidates for the oxidation of alcohols to aldehydes/ketones as products or as intermediates for heterocyclic compounds or domino reactions. Rhodium complexes seem to be more appropriate for the oxygenation of alkenes. In addition to catalytic allylic and benzylic oxidation of alkenes, recent advances in vinylic oxygenations have been focused on stoichiometric reactions. This review offers an overview of these reactions... 

Metal-Oxygen Compounds. Clear examples of the addition of transition metal alkoxides to acetylenes are not known however, the addition of trialkyltin alkoxides has been reported. Triethyltin methoxide, for example, reacts with dimethyl acetylenedicarboxylate to give the vinyl tin derivative XVI (63). 

Under the same reaction conditions, acetaldehyde and butyraldehyde displayed near-complete conversion (greater than 95%). The photocatalytic oxidation of the alcohol 1-butanol displayed similarly high conversion levels, although conversion of methanol was somewhat lower. The oxygenated compounds methyl-t-butyl ether (MTBE), methyl acrylate, 1,4 dioxane, and vinyl acetate displayed conversion levels ranging from 92% to 100%. The lowest conversion levels of the oxygenated compounds studied were seen with the ketones used [acetone and 2-butanone (methylethylketone)], which displayed conversions of approximately 80%. The initial conversion levels seen with -hexane were similar... 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

Free radical polymerization is disturbed by oxygen, as shown in Chapter 1, while cationic polymerization shows no effect from oxygen. Epoxy or vinyl-ether compounds are used for cationic polymerization. Typical cationic polymers are shown in Figure 4.10. Photoinitiators are photoacid-generating compounds, di-(p-toluene)iodonium hexafluoroantimonate and triphenyl-sulfonium hexafluorophosphate, as shown in Figure 4.11. 

An interesting route to PER is the so-called chlorinolysis (= chlorination and pyrolysis) of Ci -C3-hydrocarbons or chlorinated hydrocarbons through high-temperature chlorination. This process allows the preparation of PER from the residues of other processes, e.g. from vinyl chloride, allyl chloride and even oxygenated compounds. 

Ketones are emitted directly to the atmosphere, and their sources were discussed in detail in chapter I. In the U.K. acetone and butanone comprise about 1% and 5%, respectively, of the total anthropogenic emissions of oxygenated compounds, and 1.6% and 1.1%, respectively, of the total anthropogenic emissions of nonmethane volatile organic compounds. Ketone emissions from solvents (both industrial and personal) are substantial emissions from both gasoline- and diesel-fueled vehicles also contribute. Ketones are also formed extensively in the atmosphere in the oxidation of other compounds. Acetone, for example is formed in the OH-initiated oxidation of propane, iio-butane, iso-pentane, and neopentane and from a number of higher hydrocarbons. It is also formed in the oxidation of terpenes. The distribution, sources, and sinks of acetone in the atmosphere have been analyzed by Simpson et al. (1994). Methyl vinyl ketone is an important first generation product in the OH-initiated oxidation of isoprene. 

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

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