H Oxidation

Heavy water, see Hydrogen[ H] oxide Heazlewoodite, see rn-Nickel disulfide Hematite, see Iron(III) oxide Hermannite, see Manganese silicate Hessite, see Silver telluride Hieratite, see Potassium hexafluorosilicate Hydroazoic acid, see Hydrogen azide Hydrophilite, see Calcium chloride Hydrosulfite, see Sodium dithionate(III)... 

X = NH2, Y = H). Oxidation (54) of tetracyclines usiag the Udenfriend reagent has yielded 9-hydroxytetracyclines and disubstituted (C-7 and C-9) products (48) can also be obtained. Substituent assignments are made from nmr spectral iaterpretations. The 7- and 9-methyl tetracyclines have been prepared and reported to retain biological activity (55). 

Compound 123 (R = R = H) oxidatively adds chloroform and gem-dichloro-alkanes (940M4153). With methyl iodide, trans addition occurs and species 147 is formed. With dichloromethane, the route is three-fragment and four-electron, and the result is species 148 (R = H). Dichloromelhylbenzene and methyl dichloro-ethanoate give 148 (R = Ph and COOMe, respectively). Simultaneously, the... 

The coupling of two DIT molecules to form T4—or of an MIT and DIT to form T3—occurs within the thyroglobulin molecule. A separate coupfing enzyme has not been found, and since this is an oxidative process it is assumed that the same thyroperoxidase catalyzes this reaction by stimulating free radical formation of iodotyrosine. This hypothesis is supported by the observation that the same drugs which inhibit H oxidation also inhibit coupfing. The formed thyroid hor-... 

Zurcher, M. and Pfander, H., Oxidation of carotenoids II. Ozonides as products of the oxidation of canthaxanthin, Tetrahedron, 55, 2307, 1999. 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

The P-H oxidative addition, acrylonitrile insertion, and C-H reductive elimination steps were observed directly with the dcpe catalyst, and the potential intermediates Pt(diphos)(PHMes )(CH2CH2CN) (7, diphos = dppe, dcpe) were shown not to undergo P-C reductive elimination. The generality of this proposed mechanism for less bulky phosphine substrates, or for Pt catalysts supported by monodentate ligands, remains to be investigated [9]. 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

The reaction rate appears to be limited by the P-H oxidative addition because of tight binding of the olefin in the complexes Pt(Me-Duphos)(olefin). Increasing the reaction temperature to speed up this step, however, reduced the enantiomeric excess [13]. 

After formation of Pd(0) from the Pd(II) precursor, oxidative addition of the P-H bond could give a hydride complex. Insertion of the alkyne into either the Pd-P or Pd-H bond, followed by reductive eUmination, gives the product Consistent with this proposal, treatment of Pt(PEt3)3 with PH(0)(0Et)2 gave the P-H oxidative addition product 14, which reacted with phenylacetylene to give primarily (>99 1) the Markovnikov alkenylphosphonate (Scheme 5-18, Eq. 2). 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with diaUcyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive eUmination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

As mentioned, in the case of most electroless processes, deposition tends to be accompanied by H2 gas evolution. The efficiency of this reaction tends to be <100%, most notably in certain electroless Pd solutions where competing reactions involving H oxidation appear to occur, e.g. ... 

The oxidative addition of alkane C-H bonds to Pt(II) has also been observed in these TpRa -based platinum systems. As shown in Scheme 19, methide abstraction from the anionic Pt(II) complex (K2-TpMe2)PtMe2 by the Lewis acid B(C6F5)3 resulted in C-H oxidative addition of the hydrocarbon solvent (88). When this was done in pentane solution, the pentyl(hydrido)platinum(IV) complex E (R = pentyl) was observed as a... 

The authors point out that the dependence of the site of electrophilic attack on the ligand trans to the hydride in the model systems may be important with respect to alkane activation. If the information is transferable to Pt-alkyls, protonation at the metal rather than the alkyl should be favored with weak (and hard ) a-donor ligands like Cl- and H20. These are the ligands involved in Shilov chemistry and so by the principle of microscopic reversibility, C-H oxidative addition may be favored over electrophilic activation for these related complexes. 

R-H oxidative addition more favored Ligand loss Constrained ° Linear Pt(0) or geometry a square planar Pt(II) K < 1 1 > R-H oxidative addition more favored... 

Generally high barrier to R-H oxidative addition, which is normally uphill ... 

Miyafuji and Katsuki95 reported the desymmetrization of meso-tetrahydrofuran derivatives via highly enantioselective C-H oxidation using Mn-salen catalysts. The optically active product lactols (up to 90% ee) are useful chiral building blocks for organic synthesis (Scheme 8-48). 

Applying more drastic conditions (100 bar of CO 175 °C), [Rh6(CO)i6] also catalyzes cyclocarbonylation of 2-alkynylphenols. The mechanism involves here also an 0 - H oxidative addition initial step. The a-methylene lactone is further reduced since rhodium catalyzes the water-gas-shift reaction and water is present in the medium [145]. 

The first reports on c-alkane metal complexes date back to the 1970s, the work of Perutz and Turner on photochemically generated unsaturated metal carbonyls of Group 6 [4], which is well before the C-H oxidative addition studies of alkanes. The enthalpy gain of formation of c-alkane metal complexes... 

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