Investigation of the mechanism of nucleophilic attack on the [trans-BH] anion
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The substitution chemistry of the [trans-B20H18]2- anion was first investigated by Hawthorne and coworkers. The product formed from the reaction of the [trans-B20H18]2- anion with the hydroxide ion is an apical-equatorial isomer of the [B20H170H]4- ion with the hydroxide substituent located on the equatorial belt adjacent to the intercage linkage. Analogous reactions have been reported for the synthesis of alkoxy and ammonio derivatives. Previously, our laboratory reported the synthesis of an unexpected isomer of the [B20H17SC(O)OC(CH3) 3]4- ion, characterized by an apical-apical boron atom intercage connection with the sulfur substituent located on the equatorial belt adjacent to the terminal boron apex. In an effort to investigate the factors which lead to the formation of the unusual isomer, our laboratory initiated an investigation of the reaction of the [trans-B20H18]2- anion with sterically demanding nucleophiles. During the initial investigation, a novel THP solvent-coordinated species, analogous to that proposed by Hawthorne and coworkers for CH3CN and THF, was isolated.
Therefore, the synthetic goals of the current research project are:
- Develop a synthetic route to evaluate the reaction of the [trans-B20H18]2- anion with the original proposed nucleophiles,
- Evaluate the role of nucleophile in the formation of the THP-coordinated product, and
- Evaluate the potential chemistry of the THP-coordinated product.
Synthesis of the proposed nucleophile derivatives has been accomplished and the products have been isolated and characterized. Isomeric designations (ae or a2) have been made and the location of the substituent has been predicted for each of the products. The direct formation of the a2 isomer appears to be the result of the electronics associated with the nucleophiles containing the sulfur atom. The location of the substituent on the equatorial belt adjacent to the terminal boron apex appears to be the result of the carbamate functionality; however, the result should be verified with two-dimensional 11B NMR spectroscopy.
The reactivity of the solvent-coordinated intermediate has been studied and the reaction conditions required for the formation of the anion have been determined. Although the absolute role of the nucleophile has not been determined, the nucleophile is required for the formation of the THP-coordinated compound. The nucleophile does not act as a catalyst in the reaction nor is the reaction base-mediated. The reaction does not occur unless THP is the solvent for the reaction. Therefore, the results suggest that the formation of the solvent-coordinated intermediate is a result of mass action.
Preliminary investigations have been completed using sulfur and nitrogen analogues of THF and THP. The sulfur analogues, like THP, require the presence of a nucleophile for any type of reaction to occur. Although the nitrogen analogues will eventually react with the [trans-B20H18]2- ion in the absence of the nucleophile, the presence of the nucleophile significantly reduces the reaction time. Results suggest that continued investigation of the sulfur and nitrogen derivatives is warranted.
CitationSmits, J. P. (2006). Investigation of the mechanism of nucleophilic attack on the [trans-BH] anion (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.
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