Lewis acid catalyzed decomposition

Complex 24 was used in thermal and Lewis acid-catalyzed decomposition experiments. Intramolecular C—H reductive elimination from 24 to form exo-2-phenyl-aminonorbornane was demonstrated with labeling experiments [7]. 

Lewis acid-catalyzed decomposition of iodonium ylide 49 in the presence of alkenes results in the formation of y-lactones 50 (87IZV2873) (Eq. 20). 

Cyclopentenones (6, 67-68). Details of the Lewis acid catalyzed decomposition of /3,y-unsaturated diazo ketones to form cyclopentenones have been published. Similar decomposition of y,<5-unsaturated diazo ketones to /Ly-unsaturated cyclo-hexenones is possible, but yields are significantly lower.5... 

Several authors have studied the reaction products in the Lewis acid catalyzed decomposition of phenyl and alkyl azides.179-185 Hoegerlee and Butler have found that phenyl azide forms a hydrocarbon-soluble complex at —70° with triethylaluminum, diethylchloroaluminum, and ethyldichloro-aluminum. 1 Upon warming to room temperature, this complex slowly decomposes into an intermediate phenylimine-aluminum compound (25) which then rearranges into a variety of amidoalkylaluminum reaction products (RP) (eq 4). 

The oxygenation of aromatic compounds by Lewis acid-catalyzed decomposition of peroxydicarbonates (19-21) may be formulated as indicated in the following reaction. Saville s rule is applicable to these reactions. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

Decomposition potential (voltage) — The onset voltage for electrochemical decomposition of the electrolytic solution or the electrodes. The decomposition can take place due to either oxidation or reduction, or both. The decomposition potentials define the electrochemical window of the system. Its value depends on the salt, solvent, electrode material, temperature, and the existence of materials that can catalyze decomposition reactions, such as Lewis acids. Exact decomposition voltages are hard to reproduce as the onset current of the process is very sensitive to the experimental conditions (e.g., scan rate, temperature, type of electrode, etc.). Decomposi-... 

Lewis acids have been widely used to catalyze Diels-Alder reactions when thermal conditions were not efficient [43]. A limitation of the Lewis acid catalyzed Diels-Alder cycloaddition reaction has often been found to be due to the sensitivity of the substrates to the strongly acidic media. For instance, when considering the addition of phenylacetylene derivatives to 1-silyloxypyrrole, it was found that the Lewis acids (AICI3, BF3, TiCU) led to decomposition of starting materials, while the thermal processes afforded only negligible amounts of the desired cycloadduct [44]. The successful preparation of the cycloadduct product was achieved with lithium perchlorate in ether. This approach did not produce a very acidic reaction medium, but considerably lowered the LUMO pyrrole energy, almost as much as protonation by itself (Table 14). The final effect was that the reaction became a strongly LUMO diene controlled Diels-Alder reaction. 

The attempted high temperature addition of butadiene to enone (83) led to massive decomposition. However, the low temperature Lewis acid catalyzed procedure 104) gave an excellent yield of (184) (Scheme 43). The structure of the latter was readily deduced from the pmr spectrum in which H-1 appeared as a doublet Ju = 4.0 Hz. This value, which is uncharacteristically large for fran -diequatorial protons (see Scheme 4), is usually observed when a trigonal centre is present at C-4 (25). Accordingly reduction with lithium aluminum hydride gives the diol (185) in which Ji2 < 1 Hz. 

The hydrocarbon resins can be produced by a simple thermal polymerization process (48-50) or by Lewis acid catalyzed reaction (51). The thermal process is carried out at a high temperature in the range of 200-280°C and a reactor pressure above 300 psig. At temperatures below 200°C, the Diels-Alder polymers are formed. They are not desirable in most resins because they are insoluble in aromatic solvents. If reaction temperature exceeds 280°C, decomposition of the resins would occur. 

According to Figure 3, hydroperoxides are reduced to alcohols, and the sulfide group is oxidized to protonic and Lewis acids by a series of stoichiometric reactions. The sulfinic acid (21), sulfonic acid (23), sulfur trioxide, and sulfuric acid are capable of catalyzing the decomposition of hydroperoxides to nonradical species. 

The thermal decompositions are catalyzed by Bronsted and Lewis acids [68]. In general, when M is electron poor and Lewis acidic, the thermal decompositions occur efficiently and at low temperatures (typically between 100 and 200 °C, but sometimes at lower temperature). The addition of a catalytic amount of a Lewis or Bronsted acid (i.e., AICI3 or HCl) has been observed to accelerate the ehmination of isobutylene and the formation of three-dimensional network structures [64,124-126]. Pioneering studies on pyrolyses of various metal alkoxides by Bradley and others have also shown that alkene eliminations represent a primary decomposition pathway [104]. 

The homolytic decomposition of hydroperoxides was proved to be catalyzed by Bronsted as well as Lewis acids (for example, BF3, A1C13, SbCls) [230]. Experimental data on acid catalysis of the homolytic decomposition of hydroperoxides are collected in Table 10.9. 

An alternative approach in the asymmetric catalysis in 1,3-dipole cycloaddition has been developed by Suga and coworkers. The achiral 1,3-dipole 106 was generated by intramolecular reaction of an Rh(ii) carbene complex with an ester carbonyl oxygen in the Rh2(OAc)4-catalyzed diazo decomposition of <9-methoxycarbonyl-o -diazoacetophenone 105 (Scheme 12). The asymmetric induction in the subsequent cycloaddition to G=G and G=N bond was achieved by chiral Lewis acid Sc(iii)-Pybox-/-Pr or Yb(iii)-Pybox-Ph, which can activate the dipolarophile through complexation. With this approach, up to 95% ee for G=0 bond addition and 96% ee for G=G bond addition have been obtained, respectively. ... 

In Section III, A the catalytic action of A1C13 and BBr3 on the thermal decomposition of thiatriazoles was mentioned. This effect is evidently connected with complex formation between a thiatriazole and a Lewis acid since the catalytic activity is lost on addition of compounds that complex more effectively with the Lewis acid.19 It is remarkable that titanium tetrachloride, in contrast to this, does not catalyze decomposition, but instead forms a thermally stable, orange 1 1 complex with 5-phenylthiatriazole.19 The complex is sensitive to atmospheric moisture and is hydrolyzed in high yield to the starting thiatriazole on addition of water. 

Two factors are important in determining the relative values of kd and fce-the oxidation potential of the catalyst and its Lewis acidity. In general, the ease with which transition metal complexes catalyze the decomposition of hydroperoxides is related to their redox potentials (see Table V). Hydroperoxides are strong oxidants but weak reducing agents. Hence, reaction (312) is the slower,... 

This reaction is catalyzed by Lewis acids such as acidic metal oxides.448 Electron-attracting substituents in the aromatic ring, in addition to enhancing the rate of epoxidation, decrease the rate of heterolytic decomposition of the hydroperoxide.434... 

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