Type A oxides

FIGURE 5.41 Structural possibilities for binary oxides, (a) Type A oxides metal excess/anion vacancies, (i) This shows the two electrons that maintain charge neutrality, localized at the vacancy, (ii) The electrons are associated with the normal cations making them into M. (b) Type B oxides metal excess/interstitials. (i) This shows an interstitial atom, whereas in (ii) the atom has ionized to and the two liberated electrons are now associated with two normal cations, reducing them to M. (c) Type C oxides metal deficiency/interstitial anions. The charge compensation for an interstitial anion is by way of two ions, (d) Type D oxides metal deficiency/cation vacancies. The cation vacancy is compensated by two cations. 

Type A oxides compensate for metal excess with anion vacancies. To maintain the overall neutrality of the crystal, two electrons have to be introduced for each anion vacancy. These can be trapped at a vacant anion site, as depicted in (a)(1). However, it is an extremely energetic process to introduce electrons into the crystal and so we are more likely to find them associated with the metal cations as shown in (a) (ii), which we can describe as reducing those cations from to M. VOi is an example of this type of system. 

Oxidation of similarly substituted 3,6-dihydro-27/-selenines 91 with w-chloroperoxybenzoic acid affords the corresponding selenopyrans 92 as well as its 3-chlorobenzoic acid addition product (Equation 38) <1993CC577>. This reaction and that mentioned in Scheme 4 presumably proceed through initial oxidation at selenium followed by a Pummerer-type a-oxidation. Typical yields of the selenopyrans were relatively modest. The effect of base on this reaction was investigated however, it was not clear whether added base was really helpful in improving yields in the reaction. 

In contrast to the diverse chemistry of allylpalladium complexes, vinylpalladium intermediates have been involved mainly in the vinylpalladation of carbon-carbon unsaturated systems and in the insertion into carbon-hydrogen and heteroatom-hydrogen bonds (Scheme 10). It is found that a vinylpalladium species reacts with carbon electrophiles such as aldehydes, ketones, and nitriles (Scheme 11, type a). Oxidative addition of vinyl-halogen and -pseudohalogen bonds to Pd(0) is the common way to generate the vinylpalladium species. Vinyl-... 

Tracer Type. A discrete quantity of a foreign substance is injected momentarily into the flow stream and the time interval for this substance to reach a detection point, or pass between detection points, is measured. From this time, the average velocity can be computed. Among the tracers that have historically been used are salt, anhydrous ammonia, nitrous oxide, dyes, and radioactive isotopes. The most common appHcation area for tracer methods is in gas pipelines where tracers are used to check existing metered sections and to spot-check unmetered sections. 

Eig. 2. Efficiency to a primary intermediate as % of maximum (zero conversion) efficiency x axis is feed conversion. Parameters are oxidation rate-constant ratios ( 2 / i) for primary intermediate vs feed and reactor type A, plug-flow or batch B, back-mixed. 

The selectivity of the oxidation of 2,6-disubstituted phenols depends on the type of oxidizing agent. For example, with a series of cobalt-containing catalysts of the salcomine type, oxidation of 2,6-dimethylphenol produces three products the poly(phenylene oxide), the diphenoquinone, and... 

Amine oxides, known as A[-oxides of tertiary amines, are classified as aromatic or aliphatic, depending on whether the nitrogen is part of an aromatic ring system or not. This stmctural difference accounts for the difference in chemical and physical properties between the two types. 

The various cerium-containing derivatives available commercially are summari2ed in Table 3. The 1991 prices, per kg of contained Ln oxide, ie, not necessarily Ce oxide, range from about 3 for types A and B, ca 5 for type C, ca 12 type D, and ca 25 for type E. 

The principal mbbers, eg, natural, SBR, or polybutadiene, being unsaturated hydrocarbons, are subjected to sulfur vulcanization, and this process requires certain ingredients in the mbber compound, besides the sulfur, eg, accelerator, zinc oxide, and stearic acid. Accelerators are catalysts that accelerate the cross-linking reaction so that reaction time drops from many hours to perhaps 20—30 min at about 130°C. There are a large number of such accelerators, mainly organic compounds, but the most popular are of the thiol or disulfide type. Zinc oxide is required to activate the accelerator by forming zinc salts. Stearic acid, or another fatty acid, helps to solubilize the zinc compounds. 

The H NMR spectrum of pyridazine shows two symmetrical quartets of an A2X2 or A2B2 type dependent on the solvent and concentration. The satellites have been used to obtain all coupling constants. Spectra of C-substituted pyridazines, methylthio- and methylsulfonyl-pyridazines, both as neutral molecules and as cations, N-1 and N-2 quater-nized species, pyridazinones, hydroxypyridazinones, A-oxides and 1,2-dioxides have been reviewed (b-73NMR88> and are summarized in Tables 6, 7 and 8. 

A very useful procedure for introducing a cyano group into a pyridazine ring is the Reissert-type reaction of the A/-oxide with cyanide ion in the presence of an acyl halide or dimethyl sulfate. The cyano group is introduced into the a-position with respect to the A-oxide function of the starting compound. The yields are, however, generally poor. In this way, 6-cyanopyridazines (111) can be obtained from the corresponding pyridazine 1-oxides (Scheme 33). 

Dihydropyrazines are relatively stable, although they are easily oxidized. They are usually formed via the addition of organometallic reagents to the pyrazine ring. Similarly, 2,3-dihydropyrazines are usually easily oxidized to pyrazines and are formed during type A synthesis (see Section 2.14.3.2). 

Nitrile A-oxides, under reaction conditions used for the synthesis of isoxazoles, display four types of reactivity 1,3-cycloaddition 1,3-addition nucleophilic addition and dimerization. The first can give isoxazolines and isoxazoles directly. The second involves the nucleophilic addition of substrates to nitrile A-oxides and can give isoxazolines and isoxazoles indirectly. The third is the nucleophilic addition of undesirable nucleophiles to nitrile A-oxides and can be minimized or even eliminated by the proper selection of substrates and reaction conditions. The fourth is an undesirable side reaction which can often be avoided by generating the nitrile A-oxide in situ and by keeping its concentration low and by using a reactive acceptor (70E1169). 

The reaction of benzonitrile A-oxide with a- and )3-azidostyrenes (351) and (352) gave 3,5- and 3,4-diphenylisoxazoles (353) and (354), respectively, in good yields (74JOC1221). Clearly, the X function has a pronounced directional effect on this type of 1,3-cycloaddition... 

However, the thermolysis of diacylfuroxans (429) yielded two types of nitrile Af-oxides. An uncrowded diacylfuroxan such as (429a) rearranged to the a- acyloximino nitrile A-oxide (430) the diacylfuroxan with bulky substituents such as in (429b) gave rise to the half molecule acyl nitrile Af-oxide (431). Both types of nitrile Af-oxides (431) and (430) have been trapped with DMAD and hexafluoro-2-butyne to give isoxazoles in good yield. These reactions are shown in Scheme 97. 

The first commercial grades were introduced by Phillips Petroleum in 1968 under the trade name Ryton. These were of two types, a thermoplastic branched polymer of very high viscosity which was processed by PTFE-type processes and an initially linear polymer which could be processed by compression moulding, including laminating with glass fibre, and which was subsequently oxidatively cross-linked. 

That these elements have in the past been considered unfamiliar has been due largely to the difficulties involved in preparing the pure metals and also to their rather diffuse occurrence. Like their predecessors in Group 3, they are classified as type-a metals and are found as silicates and oxides in many silicaceous materials. These are frequently resistant to weathering and so often accumulate in beach deposits which can be profitably exploited. 

AHC(S1), p. 485 84CHEC-I(5)345], except that for both types of compounds the A-oxide forms 117b and 118b predominate in proton donor solvents (Scheme 47). 

Tetraazafulvalenes bearing two pyrazole subunits could be prepared by an original way. Tlius, treatment of benzylidene acetophenone with iso-pentylnitrite leads to an A, A -dihydroxy-bipyrazolyl-A, A -oxide, which in turn can be oxidized to TAF of type 100 (72CC961, 79JOC3211). Another type of oxidative dimerization was observed by the reaction of the electron-rich l-methyl-2,4-bis(dimethylamino)imidazole with silver salts (83TL3563). A bis-cation was isolated in 30% yield in the presence of sodium tetrafluo-roborate an unsymmetrical structure 101 was predicted from its NMR data (Scheme 40). 

Br R" = R" = Br, R" = H R" = R" = R" = Br). This type of oxidative addition has also been studied for a series of thienyl ligands formed from 2-bromothiophene or 2-bromo-3-methylthiophene [80JOM(188)121]. 

---