Lewis acids oxides

Another unusual two-step [3 + 2] cycloaddition involves the ring expansion of tert-bu-tyl-l-vinylcyclopropane-l-carboxylate 148 to the a-ethylidenebutyrolactone 149 (Scheme 14.18) [108]. When the reaction is catalyzed by boron tribromide the monocycHc product 149 is formed, but when the Lewis acidic oxidant VOCl2(OEt) is used, a very unusual dimeric product (150) is formed. 

The strongest Lewis acidic oxides in normal circumstances are alumina and gal-lia, which are oxides of elements at the limit of the metallic character. 

Direct polymerization of benzene through oxidative coupling yields polytp-phenylene) (PPP) an insoluble polymer of low molecular weight.427-429 Kovacic s original synthesis430 using a Lewis acid-oxidant combination [Eq. (13.81)] is the most widely employed and still the most effective procedure for the synthesis of PPP ... 

Nickel-catalysed addition of HCN to butadiene was developed by du Pont for adiponitrile production [81]. A Ni(0)-phosphite complex is used as the catalyst in the presence of Lewis acids. Oxidative addition of HCN to Ni(0), followed by insertion of butadiene, generates 7r-allyl intermediate 187. Reductive elimination of 187 yields 188 and 189, and isomerization of the double bond in 189 to the terminal position gives 4-pentenonitrile (190). Then, insertion of 190 to H—Ni—CN affords adiponitrile (191). 

The first examples of a direct substitution upon the carbon atoms of the dioxetane core have now appeared (Scheme 9) <1997JA245>. Thus, treatment of alkylthiodioxetane 49 with a slight excess of a Lewis acid oxidant such as iV-chlorosuccinimide (NCS) or mercuric acetate in the presence of excess ROH leads to the formation of oxygen-substituted dioxetanes 50 in low to moderate yields. 

Cationic P compounds exhibit several modes of reactivity, including coordination to Lewis acids oxidation by acids, water, and alkyl chlorides and substitution of the stabilizing phosphine ligands by stronger donors (Scheme 17). Some of these P and As cations have also been shown to be useful sources of P and As ions that provide zirconium complexes (12) containing unique square-planar Pn environments (equation 22). ... 

Oxidations in Bronsted Acid Media Oxidation by Lewis Acids Oxidation by Halogens Use of Metal Salts. ... 

The way in which Lewis acids oxidize aromatic compounds is not known clearly. Aromatics which are not easily oxidized, such as benzene, alkylbenzenes, naphthalene, and which give colored solutions as noted above, undoubtedly form charge-transfer complexes with the Lewis acid (see p. 175). Aromatics which are oxidized easily undergo complete electron transfer and form the cation radical, but the final state of the electron acceptor is not too-well known. A Lewis-acid anion radical has never been detected in these systems. Although the initial reaction in oxidations by antimony pentachloride has been represented as in eqn (12), it is not... 

With the exception of a few rare examples from the chemistry of molybdenum, manganese, rhenium, iron, and ruthenium, chelate complexes of O-func-tionalized cyclopentadienyl ligands have only been reported in this category for the very oxophilic group 4 metals, preferentially in their highest and most Lewis acidic oxidation state. Linked alkoxo— or aryloxo—cyclopentadienyl and ether—cyclopentadienyl systems are almost equally abundant for these metals. 

Other Lewis acid/oxidant systems have been employed including ASFj/AsFj [100] and liquid SO2 or sulfuric acid and aluminum chloride [101]. Poly(paraphenylene) has been synthesized by the electrochemical oxidation of benzene in solvents such as liquid SO2 [102] and concentrated sulfuric acid [103], or with the addition of Lewis acids including aluminum chloride [104], CuCl2/LiAsFg [105], and BF30Et2 [106], affording polymeric films. To improve the solubility, poly(paraphenylene) has been sulfonated [107] and alkylated [108] with propyl halides to give materials with enhanced solubility. 

There are at least three possible roles for copper and oxygen in this system 1) the CuCl serves as an HCl scavenger, to reduce the concentration of HCl formed by spontaneous decomposition of Pd(II) and thus promote oxidation 2) the possible intermediates involved in the oxidation of CuCl by oxygen, such as copper peroxo species, might function as Lewis acids, oxidants or nucleophiles to promote oxidation and 3) the reaction might proceed via a Pd-OOH intermediate. 

Figure 13. XANES analysis of the Lewis acid oxidative degradation of TPB on Na-exchanged montmorillonite (SWy-2). (A) The change in the Fe near edge structure over 10 hr. The arrows indicate the direction of change in spectral features. (B) The reduction of Fe (III) measured from the edge positions in (A).

The formation of symmetrical C-C bonds by the oxidative homocoupling of benzene and its electron-rich derivatives to yield poly(p-phenylene)s was achieved in the pioneering studies of Scholl [13, 14] during the early 1920s, followed by Kovacic (1960) [15-17], by applying various Lewis acid/oxidant combinations. The formation of each new C-C bond leads to the elimination of two protons, which are expelled as two HCl molecules (Scheme 13.1). The intramolecular adaptations of these oxidative methods have been applied towards the synthesis... 

In the case of these binary catalysts it was found that the Lewis acid-oxidant ratio had an important effect on benzene polymerization. With the AICI3-CUCI2 system the polymer yield increased considerably as the... 

Poly(pyrene) has also been synthesized by Lewis acid oxidative coupling of the monomer. These polymers are contaminated with low-molecular-weight materials and contain extensive polynuclear structures [119]. 

On the contrary, dehydroxylation of ionic oxides is easier, and results in the generation of Lewis acid sites, which are sufficiently stable to stay as such in relatively mild conditions. The strongest Lewis acidic oxides in normal conditions are alumina and gallia (and silica after very strong pretreatments) that is, oxides of elements at the limit of the metallic character. The same elements also give rise to halides characterized by an even stronger Lewis acidity. 

In Fig. 1.21a, the differential heats of adsorption of CO on H—BEA zeolite and on MFI-Silicalite are reported as a function of the adsorbed amounts. Volumetric isotherms are illustrated in the figure inset. In both cases the adsorption was fully reversible upon evacuation of the CO pressure, as typical of both physical and weak, associative chemical adsorption. For H-BEA a constant heat plateau at 60kJ mol was measured. This value is typical of a specific interaction of CO with coordinative unsaturated Al(III) atoms, as it was confirmed by combining adsorption microcalorimetry and molecular modeling [73, 74, 78, 89] Note that the heat value was close to the heat of adsorption of CO at cus Al(III) sites on transition catalytic alumina, a typical Lewis acidic oxide [55, 73], Once saturated the Al(III) defects, the heat of adsorption started decreasing down to values typical of the H-bonding interaction of CO with the Br0nsted acidic sites (- 30 kJ mol , as reported by Savitz et al. [93]) and with polar defects, either confined in the zeolite nanopores or at the external surface. 

Poly(/ -phenylene) has been synthesized by various procedures, such as oxidative cationic polymerization of benzene with Lewis acid-oxidant systems, electrolytic polymerization of benzene, coupling of aromatic halogen compounds, and condensation of aromatic nuclei with organometallic reagents, such as aryl Grignard reagents or aryllithium compounds [56]. 

---