Select Reactions of Organoboranes and Organostannanes

Potassium aryl- and alkenyltrifluoroborates and tributyl(aryl)stannanes were found to undergo halodemetallation using trihalide salts. The hydrolysis of organotrifluoroborates was observed to be promoted by iron(III) chloride, zinc dust, and zinc oxide. A variety of potassium aryltrifluoroborates and an alkenyltrifluoroborate were found to undergo bromodeboronation at room temperature in a (1:1) THF/water solution in 30-40 minutes using pyridinium tribromide, providing good yields of aryl and vinyl bromides. Activated arenes underwent dibromination, but this was reduced by adding potassium bromide to the reaction. Potassium aryltrifluoroborates and an alkenyltrifluoroborate were observed to undergo iododeboronation in a (1:2) DMF/water solution at 80 degrees Celsius using cesium triiodide. Activated aryl substrates provide the highest yields of the aryl iodide products. Aryltrifluoroborates with electron withdrawing substituents provided low product yields. Potassium pyridinyl-4-trifluoroborate did not undergo iododeboronation. Unsubstituted aryltrifluoroborates provided modest product yields. A route to (Z)-1,2-dibromoalkenes via the bromodeboronation of stereo-defined potassium alkenylditrifluoroborates using tetrabutylammonium tribromide was investigated. The reaction was found to produce good product yields with very high (Z) stereoselectivity for functionally unsubstituted potassium alkneylditrifluoroborates, but functionalized alkenylditrifluoroborates could not be obtained due to the lack of reactivity of the functionalized alkyne precursors in the diborylation reaction required for their synthesis. Iron(III) chloride, zinc dust, and zinc oxide were found to promote the efficient hydrolysis of potassium organotrifuoroborates in a (1:1) THF/water solution. Excellent yields of the corresponding alkyl-, aryl-, and alkenylboronic acids were obtained in most cases. The hydrolysis reactions are thought to occur due the formation of insoluble fluoride salts. v Iron(III) chloride/sodium iodide was found to be an effective combination to carry out iododeboronation reactions of potassium aryl- and alkenyltrifluoroborates in dry acetonitrile at 80 degrees Celsius in about 19 hours. Good yields of the corresponding iodide products were obtained in most trials. Iron(III) chloride/sodium iodide was also found to be an effective combination to carry out iododestannation reactions of a variety of tributyl(aryl)stannanes at room temperature in a (1:1) THF/water solution. The reactions generally require 40-60 minutes providing excellent yields of the aryl iodide products in most cases.