Insertion of Carboryne into Aromatic Rings: Formation of Cyclooctatetraenocarboranes

Abstract

1-Iodo-2-lithiocarborane is an efficient precursor to carboryne. It can react with arenes to give different types of dearomatization products, [4+2] cycloaddition and/or cycloinsertion products, dependent upon the substituents on the aromatic rings. The formal cycloinsertion products, cyclooctatetraenocarboranes, is generated from the [2+2] cycloaddition intermediates followed by thermal [3,3] sigmatropic rearrangement. This novel dearomatization of arenes with carboryne also serves as an important method for the synthesis of cyclooctatetraenocarboranes.

Functionalization of Carborane via Carboryne Intermediates

“…Thermal [3,3] sigmatropic rearrangement of the [2 + 2] cycloadducts generates the formal cycloinsertion compounds. Detailed studies also show that steric factors play a role in both regio-and chemoselectivity, and electronic factors have a dramatic influence on the chemoselectivity as only the [4 + 2] cycloaddition was observed with toluene [69].…”

“…In contrast, only the [2 + 2] cycloadduct was obtained in 78 % isolated yield and the formation of the undesired ene product was suppressed for C2-,C3-disubstituted indole III-3ae (Table 3.2, entry 5). Electron-withdrawing (F, Cl, Br, Ph) as well as electron-donating (Me, OMe, iPr) benzenoid substituents were well tolerated in this reaction (Table 3.2, entries [6][7][8][9][10][11][12][13][14]. The molecular structures of III-4aa, III-4ab, III-4ad, III-4ag, and III-4am were further confirmed by single-crystal X-ray analyses (Figs.…”

“…1.3) [54]. Like the critical role of arynes in modern arene chemistry, o-carborynes serve as very useful synthons for functional o-carboranes that have found many applications in medicine [6], materials science [5], and organometallic/coordination …”

“…When anisoles are examined in these reactions, the formal cycloinsertion products, cyclooctatetraenocarboranes, are isolated as the major products in addition to the minor [4 + 2] cycloaddition products (Scheme 1.18) [69]. It is noted that benzyne reacts with anisoles to generate exclusively the [4 + 2] cycloaddition products, which can be further transformed to benzocyclooctatetraene by photolysis followed with gas phase thermolysis process [70].…”

“…Due to its fascinating features such as spherical geometry and hydrophobic molecular surface and remarkable thermal and chemical stability, carboranes have been found numerous applications in polymers, ceramics, catalysts, and medicals [1,5]. Besides serving as an ideal source of boron in boron neutron capture therapy (BNCT), newly merging biomedical and other applications of carborane have been recently recognized [6]. as "deboronation."…”