Boron trifluoride reaction with ammonia

The reactions of boron trifluoride adducts of ammonia,and primary, secondary, and tertiary amines with phosphorus pentachloride have been studied and in the first two cases acyclic phosphazenes were obtained. With the ammonia-adduct, a previously characterized phosphazene salt was obtained ... 

Figure 8 The reaction of boron trifluoride (a Lewis acid) with ammonia (a Lewis base) by the sharing of a pair of electrons.

The Lewis concept embraces many reactions that we might not think of as acid-base reactions. The reaction of boron trifluoride with ammonia is an example. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

In another approach to analogs, nortestosterone is first converted to the thioketal by treatment with ethylene dithiol in IUe presence of boron trifluoride. (The mild conditions of this reaction compared to those usually employed in preparing the oxygen ketals probably accounts for the double bond remaining at 4,5.) Treatment of this derivative with sodium in liquid ammonia... 

BF3 + NH3 —> BF3NH3. In the ammonia molecule there is a nonbonding pair of electrons that can react in a variety of ways. Later in the book, you will learn about complexes that can form with ammonia. In this reaction, boron trifluoride is a molecule that is usually represented with only 6 electrons around the central boron, making it a perfect mate for the ammonia molecule. As tempted as you might be to try to apply a double-replacement strategy here, remember that they are both molecular substances. This suggests another approach a combination reaction. 

A convenient synthesis of 4-methyl- and 4-phenylseleno-l,l,l-trihalo-3-alken-2-ones 74 from the reaction of the corresponding 4-methoxy-l,l,l-trihalo-3-alken-2-ones 73 with methyl- or phenylselenols in the presence of boron trifluoride etherate was reported. The reaction of 74 (R = Me) with bromine followed by ammonia led to 3-trihalomethyl-isoselenazoles in good yields (Scheme 26) <2002S2220>. 

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