Reactions of CB₉H₁₀^- with Electrophiles, Including the Regioselective Mono- and Dihalogenation of the Lower Belt

Abstract

The 11B NMR spectrum of CpFe(CO)2(CB9H10) dissolved in dichloromethane shows the presence of two isomers in a 75:25 ratio. In the predominant isomer, the C 4 v -symmetry CB9H10 - anion is coordinated to the iron atom via the B10−H10 bond (the antipodal B−H bond). In the less abundant isomer, the CB9H10 - anion is coordinated to the iron atom via the B6−H6 bond, one of four equivalent B−H bonds (the lower-belt B−H bonds) adjacent to B10−H10. In spite of the preference of the electrophilic cation [CpFe(CO)2]+ for the antipodal B−H bond, mono- and dihalogenation with the electrophilic reagents (i) anhydrous hydrogen fluoride, (ii) N-chlorosuccinimide, (iii) N-bromosuccinimide, and (iv) elemental iodine occurred almost exclusively at lower-belt boron atoms (>95% regioselectivity). The new anions 6-CB9H9X- (X = F, Cl, Br, I) were isolated as cesium salts. Little or no isomeric 10-CB9H9X- was observed. Dihalo derivatives were predominantly 6,8-CB9H8X2 -, with varying amounts of 6,7-CB9H8X2 - present. Small amounts of 6,10-CB9H8X2 - were present in some reaction mixtures. In contrast to the nearly exclusive lower-belt halogenations, electrophilic H/D exchange of CB9H10 - with 20% aqueous DCl occurred equally rapidly at the antipodal and lower-belt boron atoms, forming 6,7,8,9,10-CB9H5D5 -. The order of deuteration rates for 6-CB9H9F- under the same conditions was B8 > B10 > {B7,B9}. No deuterium exchange was observed even after 1 month for 6-CB9H9I- under the same conditions. All of the carborane cluster anions were characterized by a combination of 1D and 2D NMR and mass spectroscopy. The anions 6-CB9H9F-, 6,7-CB9H8F2 -, and 6,8-CB9H8F2 - are the first examples of 10-vertex closo-borane or -heteroborane clusters containing B−F bonds. The structure of Ag(C6H6)2(6,8-CB9H8F2) was determined by X-ray crystallography: orthorhombic, space group P212121, a = 7.945(2) Å, b = 11.274(2) Å, c = 20.756(4) Å, V = 1859.2(7) Å3, Z = 4, T = −100 °C, least-squares refinement on F 2, R 1(I>2σ(I)) = 0.078, wR 2(all data) = 0.197. The difluorinated carborane cluster has the same overall structure as the parent ion CB9H10 -, but B6 and B8 each bear a single, terminal fluorine atom. The silver(I) ion is coordinated to two benzene molecules in distorted η2 fashion and to hydrogen atoms H9 and H10 from the same 6,8-CB9H8F2 - anion.

Silver Phosphanes Partnered with Carborane Monoanions: Synthesis, Structures and Use as Highly Active Lewis Acid Catalysts in a Hetero-Diels–Alder Reaction

“…[42] Results and Discussion Synthesis and structures: Silver(i) salts of monoanionic carborane anions are readily prepared and are air stable and useful synthons for subsequent reactions. For example, they can be used in silver salt metathesis reactions to introduce a carborane anion into a metal×s coordination sphere [23,25,29,43,44] or to generate synthetically useful salts. [28] In addition they often have significant solubility in aromatic solvents compared with other cations, facilitating more complete characterisation of the cation/anion pair.…”

Polyhedral monocarbaborane chemistry

“…For example, it has been reported that halogenation of these two anions occurs also initially at the 6-position [2,20,21].…”

Four Decades of Organic Chemistry of closo-Boranes: A Synthetic Toolbox for Constructing Liquid Crystal Materials. A Review

“…Iodination as well as other halogenation reactions of ten-vertex analog 8 exhibits high regioselectivity for the 6 position and the desired 10-iodo-1-carba-closo-decaborate (42) is formed in less than 2% yield 130 . It has been proposed that the latter may be obtained by thermal rearrangement of the kinetic 6-isomer 43 isolated in 51% yield 130 .…”

“…It has been proposed that the latter may be obtained by thermal rearrangement of the kinetic 6-isomer 43 isolated in 51% yield 130 .…”