Boronic acids reaction with halogens

Heating with metal oxides at elevated temperatures produces anhydrous borates. Reactions with halogens in the presence of carbon at temperatures above 500°C give boron trdialides. Heating a mixture of boric acid, ammonia and calcium phosphate in an electric furnace produces boron nitride. 

No examples of such reactions have been disclosed. Displacement of halogens on the parent heterocycle through metal-catalyzed processes have surprisingly not been reported to our knowledge on the neutral heterocycle. Recently, Suzuki-Miyaura cross-coupling reactions of imidazolium bromide 113 with various boronic acids or esters were reported <2005T6207> to proceed in good yield, without deprotonation at the C-3 position (Scheme 35). 

Lewis acids such as boron trihalides will also form stronger bonds with fluoride than with the other halogens, and can be used to abstract fluoride selectively from halogenated compounds. Some examples of Friedel-Crafts alkylations with fluoro-haloalkanes in which only fluoride is displaced are sketched in Scheme 4.14. As shown by the last example, however, hydride migrations can readily occur under such strongly acidic reaction conditions. 

Very dangerous fire hazard by spontaneous chemical reaction. Moderately explosive when exposed to flame. Explosive reaction with dichlorine oxide, silver nitrate, concentrated nitric acid, nitrogen trichloride, oxygen. Reacts with mercury(II) nitrate to form an explosive product. Ignition or violent reaction with air, boron trichloride, Br2, CI2, aqueous halogen solutions, iodine, metal nitrates, NO, NCI3, NO3, N2O,... 

Bombykol is a well-known pheromone, first isolated from Bombyx mori L. Bombykol the three related isomers were synthesized by cross-coupling reaction. Three alkenylboronates or boronic acids (35 and 36) required for the coupling are prepared by starting from two alkynes. The stereoselective syntheses of ( )- and (Z)-l-alkenylboronic acids or esters have been discussed in Section 2.2.1. Halogenation of alkenylboronic acid 34 with either iodine or bromine provides ( )- and (Z)-haloalkenes [71]. The palladium-and base-assisted coupling of the Cj-boronate 32 or 33 with the C,-halide 35 or 36 stereoselectively provides bombykol and its three geometrical isomers (Scheme 2-29) [72]. 

The neurotoxic quaterpyridine natural product nemertelline was successfully synthesized by S. Rault et al. using a Suzuki cross-coupiing as the key step. The boronic acid coupling partner, required for the Suzuki reaction, was prepared by first subjecting 3-amino-2-chloropyridine to the conditions of the Sandmeyer reaction followed by a lithium-halogen exchange and trapping the lithiopyridine derivative with triisopropylborate. 

There is a lot of information on the mercuration of heterocycles in the old literature, but it is seldom used nowadays due to the major disadvantages of toxicity and associated waste management it can be, however, a very useful reaction. The advantages of mercuration are that it can be carried out in hydroxylic and acidic solvents and in the presence of air, and that mercury in the product is easily replaced by ipso-substitution with other electrophiles, such as halogens, and gives boronic acids by reaction with borane. 

---