Focus on Chemistry of the 10-Dioxane-nido-7,8-dicarba-undecahydrido Undecaborate Zwitterion; Exceptionally Easy Abstraction of Hydrogen Bridge and Double-Action Pathways Observed in Ring Cleavage Reactions with OH- as Nucleophile.

Mário Bakardjiev,Dmytro Bavol,Zdeňka Růžičková,Suzan El Anwar,Bohumír Grűner

doi:10.3390/molecules25040814

Abstract

Ring cleavage of cyclic ether substituents attached to a boron cage via an oxonium oxygen atom are amongst the most versatile methods for conjoining boron closo-cages with organic functional groups. Here we focus on much less tackled chemistry of the 11-vertex zwitterionic compound [10-(O-(CH2-CH2)2O)-nido-7,8-C2B9H11] (1), which is the only known representative of cyclic ether substitution at nido-cages, and explore the scope for the use of this zwitterion 1 in reactions with various types of nucleophiles including bifunctional ones. Most of the nitrogen, oxygen, halogen, and sulphur nucleophiles studied react via nucleophilic substitution at the C1 atom of the dioxane ring, followed by its cleavage that produces six atom chain between the cage and the respective organic moiety. We also report the differences in reactivity of this nido-cage system with the simplest oxygen nucleophile, i.e., OH−. With compound 1, reaction proceeds in two possible directions, either via typical ring cleavage, or by replacement of the whole dioxane ring with -OH at higher temperatures. Furthermore, an easy deprotonation of the hydrogen bridge in 1 was observed that proceeds even in diluted aqueous KOH. We believe this knowledge can be further applied in the design of functional molecules, materials, and drugs.

Highlights

Eleven vertex 7,8-Dicarba-nido-dodecahydroundecaborate(1−) ion [1] belongs to the most studied boron cluster anions due to its easy availability from ortho-carborane [2], open pentagonal C2 B3 plane with three stereochemically distinctive sites for substitution and an extra hydrogen atom sitting on it [3,4]
Substitution of the cyclic ether by the anionic nucleophile can lead to Nu- substituted boron cluster and to the elimination of the cyclic ether. (C) Hofmann-type dealkylation at the oxonium atom, when the anionic nucleophile Nu– acts as a base abstracting the α-proton of the ring
Unlike in previously reported reactions of compounds that contain a cyclic ether attached via an oxonium atom to other boron cages, the zwitterion 1 exhibits some different features that apparently are a consequence of its nido-structure

Summary

Introduction

Eleven vertex 7,8-Dicarba-nido-dodecahydroundecaborate(1−) ion [1] belongs to the most studied boron cluster anions due to its easy availability from ortho-carborane [2], open pentagonal C2 B3 plane with three stereochemically distinctive sites for substitution and an extra hydrogen atom sitting on it [3,4]. We reported that the dioxane derivative 1 can be produced in high yield by reaction of the neutral carborane nido-C2 B9 H13 with dioxane used as solvent [8] This is a metal free, straightforward, and clean reaction in which the dioxane ring is attached to the nido-cluster by an oxonium atom to the open-face boron site B(10) that is position located opposite the cluster carbon atoms. Enough, the nidosystem from this family of compounds, namely compound 1, has largely remained outside the stream of main interest, despite its potential for the introduction of open-cage boron polyhedra to functional molecules and materials This is even more surprising in the light of recent advances in the use of [7,8-C2 B9 H12 ]– as a pharmacophore in drug design [30,38,39,40]. The compound, and its derivatives arising from ring cleavage, can potentially serve as new charge-compensated [41,42] or common type of ligands in the synthesis of metallacarboranes

Methods

Results

Conclusion