Noncatalyzed Intramolecular B-N and B-O Cross-Coupling of "Inert" Carboranes Lead to the Formation of an Unusual Oxoborane, via Reversible Cluster C-B Bond Scission

Abstract

Polyhalogenated closo-12-vertex carborane anions are thought to be inert species incapable of participating in direct B-X substitution reactions. Here, we show that this is not true and that such species can be easily coaxed into intramolecular cross-coupling cyclizations without the need for a catalyst. When cage C-tethered O and N-heteroallylic anions are generated, a variety of cyclized products can be formed in high yield under mild conditions. Additionally, we show that even C-tethered neutral nucleophiles, such as the pyridine moiety, undergo facile B-X substitution chemistry and these reactions are not dependent on the countercation. Serendipitously, we also found that when these cyclizations are attempted with acetamide derivatives, an unprecedented cluster C-B bond scission reaction occurs, producing an unprecedented oxoborane stabilized by multicentered bonding. Amazingly this molecule can be protonated, leading to reformation of the C-B bond and cluster reorganization, and this process is reversible.

1

An observation of an unusual side reaction resulting in the formation of the B–Cl substitution at the cage of 1Cl11. The same cyclization is not observed using brominated analogue 1Br11. Unlabeled vertices = B.

2

Intramolecular cross-coupling of “inert” carborane anions without a catalyst.

3

Coupling reactions with a neutral nucleophile (top). Graph showing that these reactions are not dependent on the nature of the countercation (bottom).

4

Intramolecular cross-coupling followed by spontaneous cage C–B bond rupture and formation of oxoborane 12 × 10 (top). Treating 12 × 10 with HNMe₃Cl protonates the oxygen of the BO bond leading to B–OH, which reforms the C–B bond to afford 13 × 10. Subsequent treatment of 13 × 10 with Cs₂CO₃ reforms 12 × 10. Unlabeled cluster vertices in the scheme above are boron. Solid-state structures of 12I10 and 13I10, thermal ellipsoids drawn at the 50% probability level (bottom). Color code: C = gray, H = white, B = brown, N = blue, and O = red. I, Cs, and MeCN are omitted for clarity.

5

Calculated molecular orbitals of interest for 13 _Cl10 (left) and 12 _Cl10 (right). Functional: M06-2x-D3; basis set: Def2-tzvpd; acetonitrile PBF implicit solvent.