By means of phosphorus halides

Provided moisture is rigidly excluded, phosphorus trichloride or tribromide and a diazoalkane at low temperatures afford a-halo alkyldihalophosphines 342 

More important is the reaction of phosphorus trichloride with arenediazo-nium salts, presented as fluoroborates, which causes introduction of an aryl group. The reaction was originally used for the preparation of phosphonic acids, the products being hydrolysed  

In spite of the moderate yields (generally 20-50%), the process is of great interest because, unlike the reaction of PC13/A1C13 with aromatic compounds (page 711), it gives products that are free from isomers. Small amounts of diarylphosphinic acids are formed as by-products. 

0-Toluenediazonium tetrafluoroborate does not react, although the m- and / -compounds give good yields 344 ethylbenzenediazonium salts give poor yields.345 Diazonium fluorosilicates may be used in place of the fluoroborates 

Later it was found that the chlorophosphonium tetrafluoroborates formed by the reaction of the diazonium salts can be reduced to chlorophosphines by magnesium or aluminum. In this way phosphorus trichloride yields aryldichloro-phosphines,348,349 and the latter yield diarylchlorophosphines 349 yields are of the same order as on hydrolytic working up. 

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For the mechanism of substitution of halogen for OH by means of phosphorus halides see Gerrard et al.916... 

Replacement of hydrogen by phosphorus by means of phosphorus halides... 

The hydroxyl group in alcohol can be replaced by a halogen atom by means of the halides of phosphorus. A reaction between alcohol and phosphorus trichloride takes place according to the equation,... 

In a similar vein, the keto bridge in 5 can be replaced by oxygen with retention of activity. Reduction of acetophenone derivative 19 by means of sodium borohydride leads to the corresponding alcohol (20). Reaction with phosphorus tribromide with cyanide gives... 

Simple reaction occurs with aryl halides only when the ring is sufficiently substituted with electron-withdrawing functions to allow attack by the nucleophilic phosphorus.53-56 Generally, reaction with aryl halides requires the presence of a Lewis acid catalyst or some other means of reaction initiation. These reactions will be considered in detail in Chapter 6 of this work. Interestingly, while reactions involving vinylic halides seem to correlate with those of aromatic halides (see Chapter 6), acetylenic halides undergo facile reaction with these phosphorus reagents.57 58... 

Interchange of halogen is a means of synthesis of certain acyl halides which cannot be conveniently prepared by other methods. Acetyl fluoride is made from acetyl chloride and sodium hydrogen fluoride in acetic anhydride solution." By passing a stream of hydrogen bromide through oxalyl chloride an 85% yield of oxalyl bromide, (COBr), is obtained. The bromide cannot be made by the action of phosphorus pentabromide on oxalic acid. The method has also been applied to the preparation of acetyl bromide and iodide and other acyl iodides. ... 

The foregoing has made it abundantly clear that the automotive catalyst in itself is a very complex chemical system and becomes even more so when all the subtle interactions with the exhaust environment are taken into account. Relatively minor fuel constituents such as the always present sulfur or small amounts of halides may have a pronounced effect on its overall behavior. By no means has the preceding been a complete account of all the possible interactions. Thus we have omitted the important effects of possible alloy formation between the active metals [18, 19] and the various deactivating influences deriving from automotive lubricants, the most important being the effect of phosphorus [20], Further, quite often unexpected contaminants may do severe harm to the emission hardware [21],... 

Halogen derivatives of aromatic compounds may also be prepared from hydroxyl derivatives by a reaction which is analogous to that used in the case of aliphatic compounds. When phenol, CeHsOH, and similar substances are treated with the halides of phosphorus, the hydroxyl groups are replaced by halogen. The yield of halogen compound is small in most cases, however, and the reaction is seldom used as a means of preparing such compounds. When the hydroxyl group is situated in a side-chain, the reaction takes place, in the main, as in the case of aliphatic compounds. 

An aldehyde or ketone reacts with a phosphorus ylide to yield an alkene in which the oxygen atom of the carbonyl reactant is replaced by the =0 2 of the ylide. Preparation of the phosphorus ylide itself usually involves reaction of a primary alkyl halide with triphenylphosphine, so the ylide is typically primary, RCH = P Ph)3-This means that the disubstituted alkene carbon in the product comes from the carbonyl reactant, while the monosubstituted alkene carbon comes from the ylicle. 

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