Reactivity of the [B20H18]2- Ion with Carbon Nucleophiles for Potential Application in BNCT
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Boron neutron capture therapy is a field of interdisciplinary medicine. The well-known binary nuclear reaction of the boron isotope, boron-10, with a thermal neutron is used as the basis of all research in this field. When a neutron comes into contact with the boron-10 nucleus an unstable atom is created which undergoes fission to produce a Li3+ ion, a He2+ ion, and a gamma photon. The process releases 2.28 MeV of kinetic energy, which is distributed between the He2+ ion and the Li3+ ion. The kinetic energy of the resulting ions is the valuable product towards the therapeutic benefits of this reaction. When this reaction is carried out in the cell mass of a tumor, the energetic ions cause ionization tracking and cellular damage resulting in toxicity within a radius of 10 micrometers. As a result of both the localization of the boron-10 nuclei in the tumor cells and the localized distribution of kinetic energy resulting from the neutron capture reaction, the irradiated tumor cells should be destroyed without damage to bordering normal cells. A boron target compound is necessary for boron-10 enrichment and subsequent irradiation with thermal neutrons. The [trans-B20H18]2- anion has been of particular interest due to its large boron content per unit charge and an electron deficient region within its structure. This region is characterized by dual three-center two-electron bonds which are susceptible to nucleophilic attack. While nucleophilic attack on the [trans-B20H18]2- anion has been reported for the hydroxide ion, alkoxide ions, amide ions, and thiolate ions, no literature reports have been made regarding the susceptibility of the electron-deficient bonds to nucleophilic attack by carbanions. Therefore, the specific aims of the proposed research are to: 1) develop a synthetic route to evaluate the reactivity of the [trans-B20H18]2- anion with the selected carbon nucleophiles to produce a series of substituted isomers, of the [B20H17X]4- ion where X is the selected carbon nucleophile. 2) investigate the ability of the resulting reduced polyhedral borane anions to undergo oxidation and restore the three center, two electron bonds that are susceptible to further nucleophilic attack, producing substituted isomers of the [B20H17X]2- ion where X is the selected carbon nucleophile In order for this anion and derivatives of the anion to fulfill their potential use towards BNCT, a method must be available by which their accumulation is possible in therapeutic concentrations inside the tumor. Compounds which have the potential to bind to intracellular proteins remain within the tumor in in vivo biodistribution experiments. Therefore, the synthesis of a series of boron-rich compounds with the potential to bind to intracellular proteins is the goal of this research.