Metal-oxygen complexes

The biochemical properties of many of the dioxygenases have now been elucidated (J, 2). Moreover, much data has been obtained from the study of model systems in which the chemistry of oxygenated metal complexes (9, JO) and the reactions of the various forms of molecular oxygen with the substrates of the dioxygenases (JJ) have been studied. We shall draw on the evidence from these three areas in order to present a coherent mechanistic picture of enzyme-catalyzed dioxygenation. 

Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

Transition-metal complexes such as [Rh(CO)2Cl]2,204 Rh(butadiene)2Cl,205 or Cr(CO)3(NH3)312 have also been used for the deoxygenation of oxepins to give 312,204 205 and benzoxepins to give 4.12,204 Occasionally, substantial amounts of phenolic compounds have been isolated due to the competing NIH shift of the arene oxide.204 1-Benzoxepin and 3-benzoxepin resist oxygen extrusion under these conditions probably due to their inability to form arene oxi-des.133,204... 

Reversible oxygenation of metal complexes. E. Bayer and P. Schretzmann, Struct. Bonding (Berlin), 1967, 2,181-250 (247). 

Activation of molecular oxygen on interaction with transition metal complexes. A. V. Savitskii and V. I. Nelyubin, Russ. Chem. Rev. (Engl. Transl.), 1975,44,110-121 (124). 

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