Phosphorus-based mechanisms

When unsubstituted, C-5 reacts with electrophilic reagents. Thus phosphorus pentachloride chlorinates the ring (36, 235). A hydroxy group in the 2-position activates the ring towards this reaction. 4-Methylthiazole does not react with bromine in chloroform (201, 236), whereas under the same conditions the 2-hydroxy analog reacts (55. 237-239. 557). Activation of C-5 works also for sulfonation (201. 236), nitration (201. 236. 237), Friede 1-Crafts reactions (201, 236, 237, 240-242), and acylation (243). However, iodination fails (201. 236). and the Gatterman or Reimer-Tieman reactions yield only small amounts of 4-methyl-5-carboxy-A-4-thiazoline-2-one. Recent kinetic investigations show that 2-thiazolones are nitrated via a free base mechanism. A 2-oxo substituent increases the rate of nitration at the 5-position by a factor of 9 log... 

The objective of this work Is to establish a reaction mechanism between sodium perborate and several organophosphorus esters. By analogy we can then describe Its probable effects upon other phosphorus-based Insecticides. We conclude that the reactivity of sodium perborate toward five model compounds Is attributable to the nucleophilic reactions of hydroperoxyl anion, HO2 , produced by perborate dissociation In water. On this basis we predict that sodium perborate solutions will be effective chemical detoxicants for phosphorus ester Insecticides. 

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

Levchik, S.V., Camino, G., Costa, L., and Levchik, G.F. 1995. Mechanism of action of phosphorus-based flame retardants in nylon 6.1. Ammonium polyphosphate. Fire Mater. 19 1-10. 

The phosphorus-based nerve gases and insecticides act by deactivating acetylcholinesterase. Note that all of these compounds have a good leaving group on the phosphorus. They react readily with the nucleophilic hydroxy group of the enzyme to form a phosphate triester in a reaction that is very similar, both in its mechanism and its product, to the reaction of an acyl chloride with an alcohol to form an ester ... 

Another kinetically equivalent possibility must be considered the free imidazole in these catalysts may be acting as a nucleophile at phosphorus, not as a base. This is not yet excluded experimentally, but it seems unlikely. No intermediate phosphoimidazole is detectable, but of course it might be hydrolyzing rapidly. However, molecular models for such a reaction of VII with VIII are very strained, and essentially impossible with the naphthalene substrate to be described below. Only with the extra water molecule of the general-base mechanism do the models fit well. 

Lowther, W. T., Zhang, Y., Sampson, P. B., et al. (1999) Insights into the mechanism of Escherichia coli methionine aminopeptidase from the structural analysis of reaction products and phosphorus-based transition-state analogues. Biochemistry, 38(45), 14810-14819. 

Scheme 5.1 Mechanism of HAT with phosphorus-based H-donor.

Condensed phase. In condensed-phase modification, the flame retardant alters the decomposition chemistry so that the transformation of the polymer to a char residue is favored. This result could be achieved with additives that catalyze char rather than flammable product formation or by designing polymer structures that favor char formation. Carbonization, which occurs at the cost of flammable product formation, also shields the residual substrate by interfering with the access of heat and oxygen. Phosphorus-based additives are typical examples of flame retardants that could act by a condensed phase mechanism. 

Phosphorus-based olefinations continue to be the most widely used methods for the synthesis of alkenes. An understanding of most facets of the mechanism of the Wittig reaction seems to have been achieved and this has been summarised in a substantive review. Some of the principles established in these mechanistic studies can be applied to phosphorus-based olefinations other than the Wittig reaction. However, substantive mechanistic studies of phosphine oxide-based and phosphonate-based oiefinations are urgently required. A combination of the variety of phosphorus-based methods and the improved understanding of their mechanisms now aliows a substantiai degree of controi of both reactivity and stereochemistry in olefin synthesis. However, studies are required of the applications of established structure-reactivity relationships in ylides and of the various carbon and nitrogen ylide-anions recently reported. 

Only an extremely condensed history of matches will be given in this chapter. It appears to be least confusing to separate the art of fire-making into three branches mechanical (to which may be added chemi cal but not pyrochemical methods), developing into the flint lighter phosphorus-based devices culminating in the modern SAW match and finally, chlorate-based mixtures leading to the present day safety match. 

Reaction of Mo(7r-C5H5)X(CO)g with phosphorus bases normally results in loss of carbon monoxide, but when the entering group is PBu 2Ph or PBu 3 then carbon monoxide or cyclopentadienyl groups are displaced in a ratio dependent on solvent and temperature. Loss of carbon monoxide is dissociative in mechanism here. Dissociative loss, rather than the associative loss often found for M(7t-C5H5)(CO)2 compounds, can be attributed to the steric crowding around the molybdenum in the present compound. ... 

Varley, R.J. Liu, W. Simon, G.P. (2006). Investigation of the reaction mechanism of different epoxy resins using a phosphorus-based hardener. Journal of Applied Polymer Science, Vol.99, No.6 (March 2006), pp. 3288-3299, ISSN 0021-8995. 

Phosphorus-based retardants possess the second largest market [1] and comprise a large number of forms associated with different mechanisms. Among these, a basic mechanism is achieved through the reactions between chemicals produced... 

Flame retardancy of m-PPO is improved by incorporating thermally stable phosphorus based additives, such as red phosphorus and organic phosphoms compounds like triphenyl phosphate, triphenyl phosphine and triphenyl phosphine oxide. The mechanism would appear to be gas phase activity rather than reactions in the condensed phase for all except red phosphoms where both are seen. 

Preventative or secondary antioxidants act at the initiation stage of the radical chain mechanism to prevent the formation of radical products. Their mechanism involves the decomposition of hydroperoxides to form stable nonradical products. In the absence of peroxide scavengers, hydroperoxides thermally or photolyti-cally decompose to radical products and accelerate decomposition. The most common secondary antioxidants are sulfur-based thiosynergist or phosphorus-based phosphites. ... 

Transition metal-catalyzed C—P cross-coupling affords powaful tool for the preparation of aryl phosphorus compounds [198]. The scope of the phosphorus-based nucleophiles used in crosscoupling with aryl electrophiles in the presence of transition metal (Pd-, Cu-, and Ni-based) catalysts is very broad. Figure 20.5 summarizes the common phosphoms nucleophiles used for crosscoupling reactions (for mechanism, see Scheme 20.2). 

If conditions are right, phosphorus-based molecules can voIatiHze and are oxidized, producing active radicals in the flame. On the other hand, phosphorus flame retardants tend to react with the polymer or to oxidize to phosphoric acid in the condensed phase. This favors mostly condensed-phase mechanisms. It is challenging to design a phosphorus-based flame retardant that will volatilize into the flame at relatively low temperatures but wiU not be lost dnring polymer processing. 

Upon heating, melamine-based salts dissociate, and re-formed melamine volatilizes in a manner similar to pure melamine. However, in the case of melamine salts, a larger portion of melamine undergoes progressive condensation than does pure melamine therefore, the condensed-phase contribution of the salts is larger. If the anion contains phosphorus, the phosphoric acid released will phos-phorylate many polymers and produce a flame retardant effect similar to that of other typical phosphorus-based additives (see above). Melamine condensates and phosphoric acid react further at temperatures above 600° C, where triazine rings are opened and cross-linked. A (PON) type of structure known as phosphorus oxynitride is formed. Phosphorus oxynitride is very thermally stable and in some polymers can contribute to condensed-phase mechanisms. ... 

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