Chlorine with boronic acids

An interesting iridium-catalysed 5-CH boronation of 2,3-dimethylpyrazine was reported incidentally in a paper mainly devoted to the reaction of pyridines. The product 89 was used in a Suzuki coupling <06AG(I)489>. Selective mono coupling of 2,6-dichloropyrazine with boronic acids, followed by amine displacement of the second chlorine has been used to prepare potential anti-cancer compounds <06JMC407>. A full paper has been published on the chelation-driven selective Suzuki coupling of the pyridinium ylides 90 <06TL6457>. 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

The boronic acid ester B was synthesized by transesterification of the corresponding pinacolester A with (lR,2R)-l,2-dicyclohexyl-l,2-dihydroxyethane. Stereoselective chlorination of B was carried out with (dichloromethyl) lithium and zinc chloride. Reaction of the obtained chloroboronic ester C with lithio 1-decyne followed by oxidation of the intermediate D with alkaline hydrogen peroxide afforded the propargylic alcohol E. Treatment with acid to saponify the tert-butyl ester moiety and to achieve ring closure, produced lactone F. Finally, Lindlar-hydrogenation provided japonilure 70 in an excellent yield and high enantiomeric purity. 

The thiophene derivatives are relatively stable in the pure state. The fluorine atoms of the trifluoromethylmercapto group can be quantitatively substituted with chlorine by boron trichloride with HjOg, oxidation to the sulfone takes place. Further substitution is achieved in both cases in the presence of perfluorosulfonic acids ... 

Reaction with hydrochloric acid or chlorine in the presence of carbon at elevated temperatures produces boron trichloride ... 

The coupling of the same boronic acid was also achieved with 4-chlorobenzoyl chloride (6.5.), Running the reaction under anhydrous conditions the desired 2-(4 chlorobenzoyl)thiophene was obtained in good yield.7 The opening step in this process is the selective oxidative addition of the palladium into the carbonyl-chlorine bond giving an acylpalladium complex, which on subsequent transmetalation and reductive elimination gives the observed product. 

The substitution of chlorine by the direct action of bromine is rarely effected. Aluminium bromide, cupric bromide in alcoholic solution or boron tribromide under pressure, convert many alkyl chlorides into alkyl bromides. Mono-chloroacetic acid heated to 150° in a sealed tube with hydrobromic acid or potassium bromide yields mono-bromoacetic acid. 

The epioxazoline is successively treated with chlorine, sodium iodide, cuprous oxide and boron trifluoride to produce the exomethylene compound (V). Methoxylation using chlorine with a methoxide followed by reaction with 1-methyl-lH-tetrazole-5-thiol and partial de-blocking using phosphorous pentachloride converts exomethylene (V) into the nucleus (VI). Acylation of the nucleus with an appropriately blocked side-chain (VII) followed by final de-blocking using aluminum chloride gives moxalactam acid (VIII). 

The various forms of boron nitride are all relatively inert to chemical attack under normal conditions. For example, hexagonal boron nitride does not react with oxygen, chlorine, or steam up to 700 °C. Reaction with steam begins at 900 °C. Rapid reactions are observed with hot alkali or fused alkali carbonate, but reactions are slow with most acids and alcohols, and boron nitride is not wetted by most molten metals and glasses. The cubic forms have similar reactivity but rates are slower because of their dense structures. 

Boron Trichloride. Boron trichloride is prepared commercially by the chlorination of boron carbide (equation 15). Direct chlorination of boric acid or a sodium borate in the presence of carbon is an alternative method. Most of the boron trichloride produced is converted to filaments of elemental boron by chemical vapor deposition (CVD) on tungsten wire in a hydrogen atmosphere. Numerous laboratory preparations of boron trichloride have been reported. One of the most convenient is the halogen exchange reaction of aluminum chloride with boron trifluoride or a metal fluoroborate. 

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