.
Angewandte
Communications
DOI: 10.1002/anie.201405023
Cage Compounds
1,3-Dehydro-o-Carborane: Generation and Reaction with Arenes**
Da Zhao, Jiji Zhang, and Zuowei Xie*
À
Abstract: Like the importance of benzyne, witnessed in
modern arene chemistry for decades, 1,2-dehydro-o-carborane
(o-carboryne), a three-dimensional relative of benzyne, has
been used as a synthon for generating a wide range of cage,
carbon-functionalized carboranes over the past 20 years.
However, the selective B functionalization of the cage still
represents a challenging task. Disclosed herein is the first
a cage compound having C B bonds with multiple bonding
characters, would be a reactive intermediate for simultaneous
functionalization of both cage carbon and boron vertices.
Such a species might be generated in a similar manner to that
of o-carboryne, which can be produced from 1-X-2-Li-o-
C2B10H10 by LiX salt elimination (X = Br,[2a] I[2c]; Scheme 1).
À
example of 1,3-dehydro-o-carborane featuring a cage C B
bond having multiple bonding characters, and is successfully
generated by treatment of 3-diazonium-o-carborane
tetrafluoroborate with non-nucleophilic bases. This presents
a new methodology for simultaneous functionalization of both
cage carbon and boron vertices.
I
cosahedral carborane has superaromatic character exhibit-
ing extraordinary thermal stability and unusual chemical
reactivity such as aromatic substitution, similar to that of
benzene.[1] 1,2-Dehydro-o-carborane (o-carboryne) is a very
reactive intermediate, which can be regarded as a three-
dimensional relative of 1,2-dehydrobenzene (benzyne)
(Figure 1).[2] They share some common features, however, o-
Scheme 1. Generation of o-carboryne and 1,3-dehydro-o-carborane.
Our initial attempts to obtain 1,3-dehydro-o-carborane
from 1-Li-3-X-o-C2B10H10 (X = Br, I) by LiX elimination
failed since 1-Li-3-X-o-C2B10H10 was thermally stable, even
À
under forced reaction conditions, owing to a very strong B X
bond.[10] In view of the properties of diazonium salts of
carboranes,[11] we thought that 3-(N2 BF4 )-o-C2B10H11 (1)
may serve as a good precursor for 1,3-dehydro-o-carborane as
dinitrogen is an excellent leaving group after deprotonation
+
À
Figure 1. Benzyne, o-carboryne, and 1,3-dehydro-o-carborane.
À
of the cage C H (Scheme 1). Indeed, this is an efficient
carboryne has its own unique properties resulting mainly from
method to generate previously unknown 1,3-dehydro-o-
carborane. Its generation and chemical properties are
reported herein.
steric/electronic featueres.[3] It can undergo [4+2] and [2+2]
cycloadditions,[2a,4] ene reactions,[5] and C H bond-insertion
À
reactions[6] with a variety of organic molecules to afford
a large class of o-carborane derivatives. Thus, o-carboryne is
a very useful synthon for generating a wide range of cage,
carbon-functionalized carboranes which may have potential
applications in medicine,[7] materials science,[8] and organo-
metallic/coordination chemistry.[9] With this in mind, we
speculated that 1,3-dehydro-o-carborane (Figure 1), featuring
3-Diazonium-o-carborane tetrafluoroborate (1) was pre-
pared in 70% yield upon isolation, by treatment of 3-amino-
o-carborane[12] with 1.2 equivalents of in situ generated nitro-
syl fluoride in the presence of boron trifluoride.[13] It was
noted that the stability of 1 is dependent upon the counterion
À
used and BF4 offers the highest thermal stability of the salt
among the anions, such as PF6À and ClÀ, examined.
To test our hypothesis, a benzene suspension of 1 was
treated with 1 equivalent of nBuLi at room temperature for
10 minutes to give the expected [4+2] cycloaddition product
3a in 38% yield upon isolation (entry 1, Table 1). The low
yield resulted from the formation of 3-nBu-o-C2B10H11, which
was generated from the nucleophilic attack of nBuLi.[14]
When the non-nucleophilic base lithium diisopropylamide
(LDA) was used, 3a was obtained in 72% yield (entry 2).
Other less-nucleophilic bases gave relatively lower yields
(entries 3–5). Increasing the amount of base did not improve
the yield (entry 6). The reaction also proceeded well in the
[*] D. Zhao, J. Zhang, Prof. Dr. Z. Xie
Department of Chemistry and State Key Laboratory of Synthetic
Chemistry, The Chinese University of Hong Kong
Shatin, N.T., Hong Kong (China)
E-mail: zxie@cuhk.edu.hk
[**] The work described in this paper was supported by grants from the
Research Grants Council of the Hong Kong Special Administration
Region (Project No. CUHK7/CRF/12G and 403912).
Supporting information for this article is available on the WWW
8488
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 8488 –8491