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Anodic Oxidation of Organoboranes
Schlegel, Guenter,Schaefer, Hans J.
, p. 1400 - 1423 (2007/10/02)
Organoboranes are converted into more easily oxidizable borates by reaction with nucleophiles and the alkyl groups are dimerized by anodic oxidation.The oxidation potentials (Ep) of the borates depend strongly on the nature of the complexing nucleophile, for instance Ep = +0.37 V (vs.SCE) with OH- or +1.65 V with tetrahydrofuran.The dimer yields are optimized with trioctylborane (5) by variation of the electrode material and the elctrolyte.At the platinum anode in sodium hydroxide-methanol/tetrahydrofuran yields of 80percent are obtained for acyclic alkyl groups, and lo wer ones for cycloalkyl groups.They exceed those obtained by the Kolbe electrolysis or the oxidation with neutral hydrogen peroxide and they are comparable to those of the AgNO3 oxidation. - The selective preparation of unsymmetrical products from borates with different alkyl groups is not possible, the dimerization proceeds likely via free radicals that couple statistically.Good yields of unsymmetrical coupling products are achieved, when one olefin is used in excess.With choro-, ethoxy-, acetoxy-, and aryl-substituted alkyl groups the dimers are obtained in 21 - 66percent yield, with bromide the yield are lower and with nitriles the dimerization fails.
