Cu(II)-Catalyzed 6π-Photocyclization of Dienynes

Article abstract of DOI:10.1021/acs.joc.6b02537

The first 6π-photocyclization of dienynes was developed, which provides a new and effective protocol for the synthesis of the phenyl ring in excellent yields with nice functional group tolerance. In this transformation, the Cu(OTf)₂ catalyst pl

Full text of DOI:10.1021/acs.joc.6b02537

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Cu(II)-catalyzed 6pi-photocyclization of dienynes
Ruiwen Jin, Jinjin Chen, Yiyong Chen, Wangsheng Liu, Dawen Xu, Yawei Li, Aishun Ding, and Hao Guo
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02537 • Publication Date (Web): 16 Nov 2016
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The Journal of Organic Chemistry
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Cu(II)-catalyzed 6π-photocyclization of dienynes
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Ruiwen Jin,† Jinjin Chen,‡ Yiyong Chen,† Wangsheng Liu,† Dawen Xu,† Yawei Li,†
Aishun Ding,† Hao Guo*,†
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† Department of Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433, P.
R.
China.
Tel:
+86-21-55664361,
Fax:
+86-21-55664361,
E-mail:
Hao_Guo@fudan.edu.cn
‡
Department of Child Health Care, Shanghai Children’s Hospital Affiliated to
Shanghai Jiao Tong University, 1400 Beijing Xi Road, Shanghai 200040, P. R. China
Table of Contents
Keywords: 6π-photocyclization, photochemistry, dienyne, alkyne, phenanthrene
ABSTRACT: The first 6π-photocyclization of dienynes was developed, which
provides a new and effective protocol for the synthesis of phenyl ring in excellent
yields with nice functional group tolerance. In this transformation, the Cu(OTf)2
catalyst plays a key role in the conversion of alkyne moiety into alkene-type moiety,
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The Journal of Organic Chemistry
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which means that the dienyne reactant is converted into a triene-type substrate. Thus,
this reaction proceeds via a Cu(II)-catalyzed 6π-photocyclization of triene-type
derivatives.
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6π-Photocyclization1 is a kind of classical photochemical reaction, which is quite
different from thermocyclizations that have been well established by many metal
catalysts2 or halonium ions.3 It has a rich history. Since Srimivansan reported the first
example in 1963,4 numerous achievements have been reported in this field.5 In
modern organic chemistry, such a kind of reactions provide attractive methods for the
preparation of cyclohexa-1,3-diene derivatives, which have been found wide
applications in the synthesis of various medicines,6 functionalized materials,7 and
building blocks for organic synthesis.8 Although it has been studied for quite a long
time, the investigation of 6π-photocyclization is still limited to triene substrates.9 To
the best of our knowledge, there are indeed a few photo-induced cyclizations of
dienynes,10 however, it has been proved that such reactions did not proceed via
6π-photocyclization process.10 It is well known that the 6π-photocyclization reaction
proceeds via two key steps: the formation of a six-membered cyclic transition state
and the following intramolecular 1,5-H shift.5a In a conjugated triene system, it is
quite easy to meet the above requirements (Scheme 1a).5 When a dienyne substrate is
irradiated under ultraviolet light, an unstable allene intermediate might be generated
in situ.10a Subsequently, it could in principle aromatize via 1,5-H shift. However, such
a 1,5-H shift is a high barrier process. Instead, isomerization via solvent involved
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The Journal of Organic Chemistry
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protonation-deprotonation will occur to realize the whole transformation, which is a
totally different process from 6π-photocyclization (Scheme 1a).10a Considering that
6π-photocyclization of dienynes will result in the formation of a new phenyl ring,
which is totally different from that of trienes and very useful in organic synthesis,
development of 6π-photocyclization of dienynes is highly desired. It is well known
that Lewis acid could coordinate with an alkyne moiety. As described by the standard
Dewar-Chatt-Duncanson model,11 electrons in one filled π orbital of the alkyne will
fill into an empty orbital of the metal center. Thus, in the resulting metal-alkyne
complex, the C-C triple bond will process some C-C double bond properties. Based
on this consideration, we questioned that whether it is possible to convert the dienyne
derivatives into triene-type substrates under the catalysis of a proper Lewis acid? If
this idea works, the 6π-photocyclization of dienynes might proceed via triene-type
species and afford a new aromatic system as the final product.
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In
our
previous
works,
we
found
that
the
cyclization
of
2-((4-methoxyphenyl)ethynyl)-1,1'-biphenyl 1a could be catalyzed by the merging of
Cu(OTf)2 and Eosin Y via visible light-induced single electron transfer (SET)
process.12 In the mechanism explorations, we found that when the single electron
transfer mediate Eosin Y was removed from the catalysts system, the same cyclization
could also take place under high energy UV irradiation.12 Such a transformation must
proceed via a totally different mechanism. Thus, we further studied this UV-induced
photocyclization. Herein, we wish to report this observation on the Cu(II)-catalyzed
6π-photocyclization of dienynes, which provides a novel protocol for the synthesis of
3
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