Angewandte
Chemie
DOI: 10.1002/anie.201406807
[4+2] Cycloaddition
Cobalt-Catalyzed Formal [4+2] Cycloaddition of a,a’-Dichloro-ortho-
Xylenes with Alkynes**
Kimihiro Komeyama,* Yuji Okamoto, and Ken Takaki
Abstract: A formal [4+2] cycloaddition of a,a’-dichloro-
ortho-xylenes with various alkynes has been developed using
a low-valent cobalt catalyst. The transformation has a wide
substrate scope and high functional-group tolerance and led to
1,4-dihydronaphthalenes. The formed cycloadducts were easily
aromatized with MnO2 under air. A mechanistic investigation
suggests that the transformation proceeds through a benzyl
cobaltation of alkyne, not the classical Diels–Alder reaction of
ortho-quinodimethanes. This methodology provides a straight-
forward and streamlined access to linearly expanded p-
conjugated aromatics.
O
rganic semiconductors based on linear acenes, such as
anthracenes,[1] tetracenes,[2] pentacenes,[3] hexacenes,[4] and
others,[5] are important materials for organic field-effect
transistors (OFETs), often possessing high charge mobilities
as a result of their fused planar structure. Despite the
prominent role of linear acenes in material chemistry, their
synthetic methods have remained unchanged over the past
decade. Among them, one of the most attractive protocols for
the direct synthesis of acenes is the cycloaddition of zircona-
cyclopentadienes, generated from Negishi reagent ([Cp2Zr],
Cp = cyclopentadienyl) and a,a’-dialkynyl-o-xylenes, with
alkynes, affording linear tricycles [Scheme 1, Eq. (1);
EWG = electron-withdrawing group, Tf = trifluoromethane-
sulfonyl, TMS = trimethylsilyl].[6] Although the method ena-
bles a straightforward approach to linear acenes from readily
available starting materials, the high reactivity of the zirco-
nium reagent limits the functional-group tolerance. In addi-
tion, it requires activated alkynes and the stoichiometric
organozirconium reagent, which is expensive and sensitive to
oxygen and moisture.
Scheme 1. Comparison of various [4+2] cycloaddition-type reactions of
À
substrates containing carbon carbon multiple bonds.
a suitable oxidant to construct naphthalenes.[8] The continu-
ous process would result in a linear expansion of the p-
conjugated system.[9] However, the cycloaddition is often
utilized with electronically biased alkenes, and examples with
unactivated alkynes have not been reported so far. In
contrast, palladium-catalyzed Diels–Alder type reactions of
ortho-[(trimethylsilyl)methyl]benzyl carbonates[10] and a,a’-
dichloro-ortho-xylenes[11] with activated alkenes were dem-
onstrated by Kuwano and Shige, and Renaud, Bruneau, and
Liꢀgault, respectively [Eq. (3)]. However, their protocols
need harsh reaction conditions, and are limited to activated
alkenes, similar to the reaction with o-QDM.
On the other hand, the Diels–Alder reaction of ortho-
quinodimethanes (o-QDMs) with alkenes or alkynes is also
a powerful and versatile method for the construction of
benzene fused with six-membered carbocyclic frameworks,
such as tetralines or 1,4-dihydronaphthalenes [Eq. (2)].[7] In
particular, the latter cycloadducts are easily aromatized with
Our group recently developed an efficient cobalt-cata-
lyzed annulation of aryl iodides with alkynes to give multi-
substituted naphthalenes. This transformation involves
a
sequential insertion of two unactivated alkynes to
À
a cobalt aryl bond generated in situ, followed by a displace-
[*] Prof. Dr. K. Komeyama, Y. Okamoto, Prof. Dr. K. Takaki
Department of Applied Chemistry
[12]
À
ment of the aryl C H bond.
These results obviously
Graduate School of Engineering, Hiroshima University
1-4-1 Kagamiyama, Higashi-Hiroshima City, Hiroshima 739-8527
(Japan)
indicate a high alkynophilicity of the arylated cobalt species
as well as in the Pauson–Khand reaction,[13] Alder-ene
reaction,[14] and alkyne cyclotrimerization.[15] The encourag-
ing reactivity of the cobalt species led us to apply the cobalt-
catalyzed reaction to more challenging substrates, such as
a,a’-halo-o-xylenes, to achieve the versatile [4+2] cycloaddi-
tion with unactivated alkynes [Eq. (4)].
E-mail: kkome@hiroshima-u.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. We are thankful for financial support from
the Electronic Technology Research Foundation of Chugoku.
When a,a’-dichloro-ortho-xylene (1a) was treated with
1.2 equivalents of 1,4-dimethoxybut-2-yne (2a) in the pres-
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!