Tungsten catalysts rearrangement

Schrock and co-workers note that the chain mechanism is almost certainly correct, but major questions remain unanswered. They are conducting studies with alkyhdene complexes of niobium, tantalum, and tungsten, directed towards understanding in detail how and why metathesis catalysts work. From preliminary results they predict that the olefin co-ordinates to the metal before a metallocyclobutane complex can be formed, that rearrangement of metallocyclobutane is slow relative to the rate of metathesis, and that important chain-termination steps are rearrangement of metallocyclobutane intermediates and bimolecular decomposition of methylene complexes. For these systems, co-catalysts such as the alkyl-aluminium chlorides are not necessary the initial alkyl group on the metal... 

Tantalum oxide on silica, made by treatment of silica with tantalum alkoxides, is a catalyst for the Beckman rearrangement of cyclohexanone to caprolactam (6.9).49 The catalyst lasted at least 10 h in operation. This process eliminates the by-product ammonium sulfate from the present commercial rearrangement in sulfuric acid. Caprolactam is polymerized to produce nylon-6. Alkylations can also be performed with iron oxide catalysts (6.10).50Other alcohols, such as n-propyl and /t-butyl alcohols, also worked. Highly acidic catalysts can also be obtained by supporting tungsten oxide on iron, tin, and titanium oxides.51... 

The rearrangement of a-acetylenic alcohols into a-p ethylenic carbonyl derivates has been extensively studied. Different catalysts have been proposed acid catalysts such as sulfuric, hydrochloric or acetic acids which give rise to unselective rearrangements [1,2] and more recently, oxo derivatives of vanadium, molybdenum or tungsten in liquid phase [3]. 

As shown in the last equation, aromatic compounds, which are unsubsti-stituted in the para position, can be converted to p-aminophenols by reduction in the presence of sulfuric acid. Sulfides of cobalt, tungsten, molybdenum, or platinum are used as catalysts. Presumably, this reaction proceeds by way of rearrangement of the intermediate hydroxylamines, as is the case when aluminum is used in the presence of sulfuric acid. The reduction is carried out at about 150°C and 400-500 lb pressure. The degree of aminophenol production depends upon the catalyst, as can be seen from Table 5-9. 

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