Pure and Applied Chemistry - November 1991 [antikvár]

The selective functionalisation of saturated hydrocarbons: Gif and all that Derek H. R. Barton and Dario Doller Department of Chemistry, Texas A and M University, College Station, TX 77843, USA The Gif family of systems for the selective oxidation of saturated hydrocarbons are briefly described. The mechanism of the reaction is analysed in terms of four intermediates. The first is well characterised as an vFe a-carbon bond species and the last has been fully identified, at least in the case of cyclohexane, as a hydroperoxide. The utility of dynamic I3C NMR spectroscopy in iron containing systems is demonstrated. Gif type chemistry is closely related to the unusual enzyme methane monooxygenase. PARTI. DEREK H. R. BARTON The invention of chemical reactions which will selectively functionalise saturated hydrocarbons under mild conditions (for example at room temperature and at neutral pH) represents a noble challenge for the chemists working in the last two decades of this century1. Already the porphyrin based iron containing enzymes, such as the P450 enzymes, have received much attention2. Their reactivity, at least in model systems, is radical like and considered to involve an FeIV oxenoid species liganded to a porphyrin radical cation. From the practical point of view the most promising reaction is the epoxidation of olefins by model systems3. A long known reaction which satisfies in part our definition is the Fenton hydrogen peroxide Fen oxidation system4. However, this generates hydroxyl radicals, which are very reactive and thus unselective. When we began to study the selective oxidation of saturated hydrocarbons in 1980, we were aware of a short communication from the late Prof. Tabushi5a, who was attempting a biomimetic type oxidation of adamantane 1. In order to solubilise this hydrocarbon he used pyridine. Oxidation with oxygen, a thiol and an Fe11 salt, considered a surrogate for a P450 enzyme, gave, like other such models, very little oxidation. However, the selectivity observed was unusual, since there was more secondary than tertiary substitution. Much radical chemistry by Tabushi and others has shown that adamantane5b, as expected, is substituted mainly at the tertiary position. We repeated and confirmed these experiments and modified the system. There was little oxidation, but the selectivity continued to be unusual. Life existed on Earth under anaerobic conditions long before the blue-green algae started to make oxygen. Under reducing conditions the atmosphere was full of hydrocarbons, especially methane; there was much hydrogen sulfide, from reduction of sulfate; the abundant element iron was present as metallic iron (cf. bog iron) and in the seas as Fe11. A form of life took advantage of the new aerobic conditions to oxidise the iron to Fe111 and deposit it as pure ferric oxide in vast mountain ranges in Australia and Brazil. It seemed to me, on reflection, that the new form of life would have obtained far more energy from concerting the oxidation of iron and hydrocarbons together than from just making ferric oxide. The hydrocarbon oxidation products (C02) would have left no geological trace. Abstract R2 R1 1. R'=R2=H 2. R',R2=0 1567