Phosphorus halides hydrogen

Amine-phosphorus halide reactions involve hydrogen halide (or amine hydrohalide) formation. They can be complicated through acid catalysis or side reactions. Thus, the possible formation of skeletally stabilized products through transamination reactions in which no hydrohalide products are obtained is of interest. 

The mechanism for the reactions with phosphorus halides can be illustrated using phosphorus tribromide. Initial reaction between the alcohol and phosphorus tribromide leads to a trialkyl phosphite ester by successive displacements of bromide. The reaction stops at this stage if it is run in the presence of an amine which neutralizes the hydrogen bromide that is formed.9 If the hydrogen bromide is not neutralized the phosphite ester is protonated and each alkyl group is successively converted to the halide by nucleophilic substitution by bromide ion. The driving force for cleavage of the C—O bond is the... 

Alkyl halides are almost always prepared from corresponding alcohols by the use of hydrogen halides (HX) or phosphorus halides (PX3) in ether (see Section 5.5.3). Alkyl chlorides are also obtained by the reaction of alcohols with thionyl chloride (SOCI2) in triethylamine (Et3N) or pyridine (see Section 5.5.3). 

Propargyl alcohols may be converted to allenes by several methods, for example, (a) through the intermediate formation of propargyl halides which are not isolated but react directly with cuprous salts and hydrogen halide [60, 72-73] or cyanide [71] (b) typical alcohol reactions with thionyl chloride [74a-d] phosphorus halides [75-77], and miscellaneous reagents (see Scheme 3). 

It is easy to reduce anhydrous rare-earth halides to the metal by reaction of more electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pr mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphorus, and hydrogen at elevated temperature and remove these elements from most other metals. 

Replacement of the hydroxyl group in a phenol by halogen cannot be accomplished by reaction with the hydrogen halides as in the case of alcohols, and reaction with phosphorus halides gives only low yields of halogenobenzenes (except in the case of nitrophenols), the main product being a phosphite or phosphate ester. 

Alcohols and phenols are also weak bases. They can be protonated on the oxygen by strong acids. This reaction is the first step in the acid-catalyzed dehydration of alcohols to alkenes and in the conversion of alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols to alkyl halides by reaction with hydrogen halides. Alkyl halides can also be prepared from alcohols by reaction with thionyl chloride or phosphorus halides. 

Thiophosphoryl chloride dissolves sulphur and phosphorus freely when hot, but only sparingly when cold.2 Since the liquid is immiscible with water the hydrolysis proceeds only on the surface at first, as is usual with phosphorus halides. In this case the products are phosphoric acid, hydrogen chloride and sulphide and a little sulphur. It reacts with ethyl alcohol, giving ethyl chloride and ethyl thiophosphate ... 

Theaction of ammonia on phosphorus halides gives either ammoniates or, by elimination of all hydrogen as ammonium chloride, halonitrides. The amides, which should be formed as intermediate products, seem to be difficult to isolate from the true halides, although the oxylialides (p. 109) and thiohalides (p. 118) readily yield such compounds. Exceptionally, phosphorus diamidotrifluoride, PF3(NH2)2, was prepared as a white mass by the following reaction —... 

Removal of an AT-oxide function utilizes the usual reagents such as zinc and acetic or hydrochloric acids, sodium borohydride, hydriodic acid, phosphoryl chloride, sodium dithionite, phosphorus trichloride or hydrogenation over Raney nickel. When l-hydroxy-2-methyl-5-phenylimidazole (262) reacts with butyllithium and hexachlofodisilane this also induces dehydroxylation (Scheme 152) (80AHC(27)24l). At times, as with other heterocyclic AT-oxides, deoxygenation with phosphorus halides can introduce a halogen atom at C-2 of imidazole (75JCS(P1)275). 

A number of piperazine-2,5-diones (a-amino acid anhydrides ) has been converted to pyrazines by the action of phosphorus halides. Baxter and Spring (312) first described the conversion of isoleucine anhydride (45) with phosphoryl chloride to 2,5-di-s-butyl-3,6-dichloropyrazine (46, X = Cl) and 2,5-di-s-butyl-3-chloropyrazine (46, X = H), and of DL-alanine anhydride (47) similary to 2,5-dichloro-3,6-dimethylpyrazine (48, X = Q) and 3from alanine anhydride does not involve an oxidation step, whereas the formation of 2,5-dichloro-3,6-dimethylpyrazine involves the oxidation of an intermediate dihydropyrazine derivative. Treatment of DL-alanine anhydride with phosphoryl chloride in the presence of a tertiary base (dimethylaniline) gave only the monochloro derivative the intermediate dichlorodihydropyrazine presumably loses hydrogen chloride and gives the stable aromatic 3-chloro-2,5-dimethylpyrazine. 

Alkyl halides are prepared from alcohols by use of hydrogen halides or phosphorus halides. Phosphorus halides are often preferred because they tend less to bring about rearrangement (Sec. 16.4). 

The reactions of thionyl chloride and phosphorus halides, also in their polymer-modifled form, involve the formation of hydrogen halide and cannot, therefore, be applied without complications to acid sensitive compounds. The combination of triphenylphosphine and tetrachloromethane as reagent provides conditions under which, for instance, pelargonic acid can be converted to the acid chloride in good yield according to equation (5). ° This method has been recently applied to polymer-bound triphenylphos-phine. Table 2 lists some examples of acids treated in this manner. 

---