Manganese-phosphorus oxide catalyst

NR is c/ s -1,4-polyisoprcnc, of molecular weight 200,000-500,000, but it also contains a small level of highly important non-rubber constituents. Of these, the most important are the proteins, sugars and fatty acids which are antioxidants and activators of cure. Trace elements present include potassium, manganese, phosphorus, copper and iron which can act as catalysts for oxidation. 

Manganese oxide (ous) catalyst, alphabromination Phosphorus, yellow catalyst, alphachlorination Sulfur chloride catalyst, alumina Cerium oxide catalyst, ammonia... 

The fust reaction takes place in the vapor phase, between 330 and 360°C, at low pressure (between 03 and 04.106 Pa absolute), in the presence of air and steam in a r-butanol/air/steam ratio of about 1 10 to 15/6 to 12, on a catalyst based on mixed oxides of molybdenum, cobalt, bismuth, iron, nickel, and additions of derivatives of alkaline metals, antimony, tellurium, phosphorus, tungsten, tin, manganese, etc. With residence times of 2 to 3 s, the molar yield of methacrolein exceeds 85 per cent for virtually total once-throngh conversion of r-butanoLThe main by-products formed are... 

Reactions with alkynes may lead to the formation of cyclized products. The reaction of iodobenzenes with two equivalents of an alkyne has been shown to give naphthalene derivatives in the presence of cobalt catalyst with a manganese reduc-tant. The process, shown in Scheme 15, is thought to involve oxidative addition of the aryliodide to cobalt followed by double alkyne insertion. The cobalt-catalysed annulation step probably involves an pathway. The cyclopentadienyl-rhodium-catalysed annulation of benzoic acids with alkynes has been used to form isocoumarin derivatives, such as (126). The process is thought to involve cyclorhodation at the ortho-position of a rhodium benzoate intermediate, followed by alkyne insertion to form a seven-membered rhodacycle and reductive elimination The silver-catalysed annulations of diphenylphosphine oxides with alkynes proceed in the absence of rhodium. Benzophosphole oxides such as (127), formed with diphenylethyne, are produced. Here, the proposed mechanism involves homolytic cleavage of the phosphorus-hydrogen bond to give a radical which can add to the alkyne and subsequently cyclize. ... 

The functionalization of C—H bonds remains one of the most difficult challenges in synthetic chemistry, and while a number of catalysts have been developed to promote specific reactions, general approaches are quite rare. Many of these reactions were proposed to occur through radical processes however, the precise mechanism for many reactions remains unknown. Several versions of this chemistry have been used to generate arylphosphonates, and the nature and reactivity of phosphorus-based radicals have been reviewed [396]. Some of the earliest reports on this chemistry were based upon using anionic oxidation [397], peroxides [398], cerium nitrate [399], and manganese(lll) acetate [400]. The following sections will outUne specific examples of successful C—H phosphonations. 

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