Oxidation dimeric systems

Calcium Chelates (Salicylates). Several successhil dental cements which use the formation of a calcium chelate system (96) were developed based on the reaction of calcium hydroxide [1305-62-0] and various phenohc esters of sahcyhc acid [69-72-7]. The calcium sahcylate [824-35-1] system offers certain advantages over the more widely used zinc oxide—eugenol system. These products are completely bland, antibacterial (97), facihtate the formation of reparative dentin, and do not retard the free-radical polymerization reaction of acryhc monomer systems. The principal deficiencies of this type of cement are its relatively high solubihty, relatively low strength, and low modulus. Less soluble and higher strength calcium-based cements based on dimer and trimer acid have been reported (82). 

The first representatives of these interesting 7r-systems have recently been described. In the even series (12, n = 1,3,...) the synthesis of the square molecule 184 and the hexagonal one 185 starts from the building block 182. This is first oxidatively dimerized... 

In phenolic oxidative coupling reactions, these phenol-derived radicals do not propagate a radical chain reaction instead, they are quenched by coupling with other radicals. Thus, coupling of two of these resonance structures in various combinations gives a range of dimeric systems, as shown. The... 

Porphyrines and phthallocyanines suffer from oxidative degradation and oxidative dimerization [68]. The improved activity of the zeolitic systems is due to the effective site isolation within the pores, which prevents any bimolecular pathways to catalyst destruction [63]. Therefore, deactivation is more severe for the homogeneous catalysts than for the heterogenized TMPc. FePc itself is a poor catalyst for alkane oxidation with a high initial turn-over, but after less than 45 minutes it becomes completely inactive. On the contrary, FePc encaged in zeolite Y is stable for 24 hours [63]. 

No accounts have reported any derivatives of this system. However, ab initio calculations of this supposed nitrous oxide dimer have been performed (72JA8250). 

Pulse radiolysis studies on unsymmetrical, chemically linked organic systems have shown the expected fall off in rate constant as AG becomes more favorable.81 In these experiments, advantage is taken of the fact that capture of electrons produced by pulse radiolysis is relatively indiscriminate and in some of the pulse events an electron is captured by the component in the dimeric systems which is the weaker oxidant. Following such an event, the experimental observation made is of the system relaxing by intramolecular electron transfer to the stable redox configuration, as shown for (I)->(2) where A is one of a series of polyaromatic or quinone electron acceptors.81... 

Thermal as well as oxidative dimers have also been isolated and characterized from pure fatty acid and TGs oxidized under simulated deep fat frying conditions. These model systems have been employed in order to simplify and control the various parameters affecting the thermal-oxidative reactions and to facilitate the structure elucidation of the decomposition products. 

The dimers studied present two simultaneous hydrogen bonds as indicated in Scheme 3.22 for the homochiral system. The interaction energies of the sulfoxide dimers range between —36.4 and —69.9 kJ mol-1, while the range for the thioper-oxides dimers is from —15.5 to —37.2 kJ mol-1. These differences can be explained... 

A study of the olefin oxidation catalyst system, palladium acetate-MOAc (M = Li or Na), has shown that in the absence of acetate ion, Pd acetate-acetic acid exists as the trimeric species [Pd3(OAc)6].32 Reaction with MOAc is not instantaneous, and u.v.-visible spectra indicate an initial equilibrium involving trimer - dimer (9). When M = Na conversion into dimer is complete at 0.2M-NaOAc. Further addition of... 

Aside from oxidative dimerizations [175-186] vanadium catalysts are able to induce C-C and C-0 bond cleavages. Momose and coworkers developed a catalytic system to fragment a-hydroxy ketones 101 to diesters or ketoesters 102 (Fig. 31) [190]. Using 1 mol% of EtOVOCl2 as a catalyst and oxygen as the terminal oxidant, 55-87% of 102 was obtained. The reaction mechanism is not known, but the reaction is inhibited by 2,6-di-tert-butylcresol (BHT) pointing to the involvement of a radical process. 

To demonstrate their approach, the preparation of the parent system 76 and the so-called bow-type hydrocarbon 78 is discussed here [42]. To prepare the trimer of 47 (which was not among the oxidative dimerization products, see above), the triyne 82 was coupled with ortho-... 

Scheme 60. Mechanistic speculation on the origins of enantioselection upon oxidative dimerization of 68f in the copper/225 system.

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

The oxidized dimer, [Fe2(TPA)20(0Ac)]3+, 41, was shown to be an efficient catalyst for cyclohexane oxidation using tert-BuOOH as a source of oxygen (69). This catalyst reacts in CH3CN to yield cyclohexanol (9 equiv), cyclohexanone (11 equiv), and (tert-butylperoxy)cyclohexane (16 equiv) in 0.25 h at ambient temperatures and pressures under an inert atmosphere. The catalyst is not degraded during the catalytic reaction as determined by spectroscopic measurements and the fact that it can maintain its turnover efficiency with subsequent additions of oxidant. Solvent effects on product distribution were significant benzo-nitrile favored the hydroxylated products at the expense of (tert-butyl-peroxy)cyclohexane, whereas pyridine had the opposite effect. Addition of the two-electron oxidant trap, dimethyl sulfide, to the catalytic system completely suppressed the formation of cyclohexanol and cyclohexanone, but had no effect on the production of (tert-butylper-oxy)cyclohexane. These and other studies suggested that cyclohexanol and cyclohexanone must arise from an oxidant different from that responsible for the formation of (tert-butylperoxy)cyclohexane. Thus, two modes of tert-BuOOH decomposition were postulated a heterolytic... 

A ruthenium-catalyzed three-component reaction between propargylic alcohols, 1,3-dicarbonyl compounds, and primary amines leading to fully substituted pyrroles was developed <07CEJ9973>. Cyclohexa[a]pyrroles ( azabicyclo[4.3.0] systems ) were formed by a three-component sequence involving allenic ketones, primary amines, and acryloyl chloride <07SL431>. An oxidative dimerization sequence involving arylpyruvates in the presence of ammonia was the key step in an approach to the pyrrole natural product, lukianol A <07S608>. 

Cycloaddition rates range over several orders of magnitude and to predict the likely success of a reaction, when alternative reaction pathways such as nitrile oxide dimerization are possible, it is necessary to understand the reactivity of the system. 

On the other hand, Ito et al. (99) found that the oxidative dimerization of methane to yield ethylene and ethane can be achieved with a high yield and good selectivity on Li-doped MgO catalysts. Since this pioneering work, many oxidic systems have been studied. Anpo et al. (100) found that surface sites of low coordination produced by the incorporation of Li into MgO play a vital role in the methane oxidative coupling reaction. Thus, although it was known that MgO acts as an acid-base catalyst, both the catalytic and photocatalytic activities of the MgO catalysts seem to be associated with the existence of surface ions in low coordination located on MgO microcrystals. 

Another class of nitrogen-bridged expanded porphyrins was first reported in 1993 by Dolphin and coworkers. Referred to as porphocyanines, the first of these tetrapyrrolic expanded porphyrins to be reported was system 9.110. It was prepared via the oxidative dimerization (with loss of ammonia) of the bis(amino-methyl)dipyrrylmethane derivative 9.109, a precursor derived, in turn, from the bis(-cyano)-substituted dipyrrylmethane 9.108 (Scheme 9.2.2). ... 

The reaction of a wide range of imines and other compounds containing the C=N moiety with 14 leads to the formation of azacyclopentenones, e.g. 337 (equation 100). Certain of the product heterocycles undergo oxidative dimerization Moreover, related five-membered ring systems result from analogous reactions of 14 with l-(r-pyrrolidinyl) acenaphthylene and thiazoline-5-thiones. ... 

Photolysis of (244) in the presence of alcohols likewise results in the formation of esters of methyl(phenyl)phosphinic acid. However, O-isotopic labelling studies provide evidence of, at least, the partial involvement of a pentacovalent phosphorus intermediate in this reaction, and so the previous assumption of the intermediacy of (245) may not be the whole story. The phosphole oxide dimers (246) have been shown to undergo regioselective reduction and complexation on treatment with the dimethylsulfide-borane adduct, to form the bicyclic system (247). Treatment of the propadienylphosphine oxide... 

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