Cycloborates



Our approach to the generation of new organocycloborates involves an unprecedented boron-centered cyclisation reaction of ω-bromoalkyl(diorgano)boranes occurring via Grignard reagents.[1] This synthetic method permits to increase the size of the heterocycle and vary the class of organoborate, by simply modifying the alkyl chain length of the α,ω-bromoalkene or the dialkylborane precursor, respectively.


Synthesis of Cycloborates
ω-Bromoalkyl(diorgano)boranes (1) are obtained via hydroboration of the appropriate terminal bromoalkenes with suitable diboranes(6) {(R2BH)2}. Treatment of the trialkylboranes with activated magnesium turnings furnishes the desired organocycloborates in high yield. The proposed mechanism for this transformation involves initial formation of the expected Grignard reagent upon reaction of the ω-bromoalkyl(diorgano)boranes with Mg turnings. Subsequent nucleophilic attack of the alkyl group on the electrophilic trialkylboron centre yields the anionic boron compound that is isolated as the diethylether-solvated magnesium bromide salt.



Novel Spiroborates
All new compounds have been characterized by means of multinuclear NMR spectroscopy, and the structures of the anions in the solid state have been determined by X-ray crystallography.[2]



Fig. 1: Molecular structures of novel spiroborates (Cations are omitted for clarity.)



Selected references:


  1. H. Braunschweig, G. D'Andola, T. Welton, A. J. P. White, Chem. Commun. 2004, 1738–1739. [pdf]
  2. H. Braunschweig, G. D'Andola, T. Welton, A. J. P. White, Chem. Eur. J. 2006, 1, 600–606. [pdf]


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