Transition metal catalysts vitamin

Use of transition metal catalysts opens up previously unavailable mechanistic pathways. With hydrogen peroxide and catalytic amounts of methyl trioxorhe-nium (MTO), 2-methylnaphthalene can be converted to 2-methylnaphtha-l,4-qui-none (vitamin K3 or menadione) in 58 % yield and 86 % selectivity at 81 % conversion (Eq. 10) [43, 44]. Metalloporphyrin-catalyzed oxidation of 2-methylnaphtha-lene with KHSOs can also be used to prepare vitamin K3 [45]. The MTO-catalyzed process can also be applied to the synthesis of quinones from phenols [46, 47]. In particular, several benzoquinones of cardanol derivatives were prepared in this manner [48], The oxidation is thought to proceed through the formation of arene oxide intermediates [47]. 

In early vitamin and carotenoid syntheses, the oxidation of j6-isophorone was accomplished by epoxidation of /1-isophorone followed by tbe ring-opening of the epoxide and oxidation (Fig. 16.13). A significant improvement of the three-step synthesis of KIP was made by the one-step liquid phase oxidation of j6-isophorone using oxygen or an oxygen-containing gas in the presence of a transition metal catalyst. In 1975, the aerobic oxidation of j6-isophorone in tbe presence of Cu(II)-or V(III)-acetylacetonate was published, in which KIP was obtained in a yield of up to 55%. ... 

Ascorbic acid (Vitamin C H A) has been widely used in pharmaceutical, chemical, cosmetic and food industry because of its bioactive and antioxidant properties [1]. Cu (II) ion is important nutrients found in many food and pharmaceutical products. Traces of transition metal ions like as Cu(II) ion also acts as catalyst in the oxidation... 

Functionalised carbenes can anchor free carbenes to the metal site, introduce hemilabil-ity, provide a means to immobilise transition metal carbene catalysts, introduce chirality, provide a chelate ligand or bridge two metal centres. NHC can be attached to carbohydrates and camphor, derived from amino acids and purines, and they can be used as organocata-lysts mimicking vitamin B1 or as weak solvent donors in lanthanide chemistry. There are many possibilities which are still only scarcely explored. 

In addition to being important in industry, transition metal ions play a vital role in living organisms. For example, complexes of iron provide for the transport and storage of oxygen, molybdenum and iron compounds are catalysts in nitrogen fixation, zinc is found in more than 150 biomolecules in humans, copper and iron play a crucial role in the respiratory cycle, and cobalt is found in essential biomolecules such as vitamin Bi2-... 

It appears from Figs. 9.5 and 9.6 that there is a huge variation in colour stability between meat from different sources. A range of intrinsic factors influence the oxidative balance in raw meat and thereby the colour stability of the meat (Bertelsen et al., 2000). Thus the oxidative stability of muscles is dependent on the composition, concentrations, and reactivity of (i) oxidation substrates (lipids, protein and pigments), (ii) oxidation catalysts (prooxidants such as transition metals and various enzymes) and (iii) antioxidants, e.g., vitamin E and various enzymes. For review see Bertelsen et al. (2000). 

The plenary lectures at the 14th I.C.C.C. Meeting at Toronto have been published, as have the papers presented at the Bressanone Conference. Several other useful reviews of reactions involving metal ions have been published. The oxidation-reduction of the cobalt centre in vitamin Big has been discussed and recent developments in the bioinorganic chemistry of this complex have also been described. -Electron-transfer catalysts involving metalloenzyme systems have also been reviewed by Williams. The role of transition metals in homogeneous catalysis has been described and homo-lytic oxidation and reduction reactions of organic compounds by metallic ions have been reviewed. ... 

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