Carbon-phosphorus bond formation addition-elimination

All the phosphates can be viewed as sources of phosphoric acid or poly-phosphoric acid and most of the development work in this area has concentrated on identifying the best co-additives to employ. In simple systems such as APP blended with polyhydric compounds like penta-erythritol it has been established that, above 200°C, the components react to form phosphoric ester bonds. Further elimination of water and ammonia leads to a carbon-phosphorus char. Without the pentaerythritol no char is formed. The char structure can be further improved by the addition of melamine which in this case can be viewed as a blowing agent, increasing the char volume. The formation of the char can therefore be seen as a sequence of overlapping reactions which must occur in a particular order for optimum performance [2]. 

The carbanion centre of (6.33) is planar and so, in principle, can be approached from either side by a suitable electrophile. However, as can be seen from structure (6.33) in Figure 6.7, one side is more hindered as a result of the bulky alkyl group, R. An electrophile will therefore approach from the side of the hydrogen atom as shown. In this case, the electrophile is formaldehyde and the product of addition is the lithium alcoho-late (6.34). Rotation by 180° around the carbon-carbon bond allows the formation of the phosphorus-oxygen bond of (6.35) and subsequent elimination of the phosphine oxide to give the allylic alcohol (6.36). 

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