Anionic Boron-Oxygen Compounds

Boron oxides and their derivatives are technologically important and are relatively inexpensive to produce since, as noted earlier, essentially all boron in Nature is in oxygenated form. Boron-oxygen compounds contain predominantly trigonal planar B03, and to a lesser degree tetrahedral B04 units, as in the borate anions in Fig. 5-25. 

Nucleophilic attack hy the hydroperoxide anion produces initially a tetrahedral borale (26) in which one alkyl group, here (-)-Ipc 31. migrates from boron to oxygen, in the course of which a hydroxide anion is eliminated and a stable boron-oxygen bond develops in compound 27. This step proceeds with retention of configuration at the migrating carbon atom. The same step are repealed with the second residue to give trialkoxyborane 28. Finally, hydrolysis results in release of alcohol 4, two molecules of (-)-Ipc alcohol 30. and one equivalent of borale 29. 

Trialkyl boron was first claimed as a new anionic initiator for the polymerization of vinyl compounds (264), although it was rather improbable in view of the low ionic character of the boron-carbon bond. The error was quickly corrected when it was shown that free radicals were involved (265, 266) and that oxygen, peroxides, silver salts and copper salts were co-catalysts (262, 267). Aluminum alkyls can also initiate radical polymerizations in the presence of oxygen (267,262) but, as in the case of zinc, cadmium or boron alkyls, the products were not stereoregular. Thus, complexing between catalyst and monomer probably does not occur. 

Simple sharing of one oxygen atom by two B03 units would give [02B0B02]4 this so-called pyroborate anion has been shown to exist in Co2B2Os. Also, the compound referred to as boron acetate, prepared by the reaction... 

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