Generation and reactivity of o-carborynes

Abstract

o-Carboryne (1,2-dehydro-o-carborane) is a very reactive intermediate that can be generated in situ from 1-Br-2-Li-1,2-C2B10H10, 1-I-2-Li-1,2-C2B10H10 or 1-Me3Si-2-[IPh(OAc)]-1,2-C2B10H10. It exists in two resonance forms, a bonding form vs. a biradical form. Each form demonstrates unique reactivity patterns. o-Carboryne can undergo various types of reactions such as [4 + 2]/[2 + 2] cycloaddition, ene reaction, hydrogen abstraction, and sp(2)/sp(3) C-H bond insertion reaction with a broad spectrum of substrates including alkenes, alkynes, (hetero)aromatics, ethers, and ferrocenes. It is a very useful synthon for generating a variety of functionalized carboranes that have potential applications in medicine, materials science and organometallic/coordination chemistry. This perspective offers an overview of recent advances in this interesting research field.

1,3‐Dehydro‐o‐Carborane: Generation and Reaction with Arenes

“…[14] When the non-nucleophilic base lithium diisopropylamide (LDA) was used, 3 a was obtained in 72 % yield (entry 2). Other less-nucleophilic bases gave relatively lower yields (entries [3][4][5]. Increasing the amount of base did not improve the yield (entry 6).…”

“…The results show that breaking of the cage C À C(sp 3 ) bond is energetically favored over that of the cage B À C(sp 3 ), and 4 a is thermodynamically more stable than 5 a by 42.1 kcal mol À1 . Accordingly, a reaction mechanism is proposed in Scheme 5.…”

“…Accordingly, a reaction mechanism is proposed in Scheme 5. Heterolytic cleavage of the cage CÀC(sp 3 ) bond in 3 a generates the intermediate A. Migration of a proton in A gives the thermodynamic product 4 a. Alternatively, the formation of a cage C À C(sp 3 ) bond affords another intermediate, B (formally [2+2] cycloadduct), which undergoes sigmatropic rearrangement to yield the kinetic product 5 a.…”

Recent advances in transition metal-mediated functionalization of o-carboranes

“…Furthermore, (g 2 -C 2 B 10 H 10 )Ni(PPh 3 ) 2 does not react with anthracene, furan, or thiophene, whereas these 4p systems react readily with in situ generated carboryne to give [4+2] cycloaddition products. 17 This result suggests that carboryne and Ni-carboryne should undergo different reaction pathways.…”

Facial Synthesis ofo‐Carborane‐Substituted Alkenes and Allenes by a Regioselective Ene Reaction of 1,3‐Dehydro‐o‐carborane

“…Such reactions,h owever,a re often accompanied by [2+ +2] cycloadditions, leading to apoor selectivity. [11] An ene reactionc an be rationalized as interactions between the HOMO of the ene and LUMO of the enophile.[12] Thus, am ore polarized enophile should favor an ene reaction. In this regard, our recently reported1 ,3-dehydro-o-carborane (2), [13] featuring ap olarized cage C-B multiple bond, would be an excellent candidate for ene reactions with alkenes and alkynes.…”