Isoquinoline formation by a novel photochemical intermolecular cycloaddition of 1,1-diphenylethene derivatives to 1,4-dicyanobenzene

Article abstract of DOI:10.1016/S0040-4039(00)01078-9

Phenanthrene-sensitized photoreaction of 1,1-diphenylethene (DPE) derivatives and 1,4-dicyanobenzene (DCNB) in benzene leads to a novel intermolecular cycloaddition to give isoquinoline derivatives as a novel synthetic method. The feasibility of this reac

Full text of DOI:10.1016/S0040-4039(00)01078-9

Tetrahedron Letters 41 (2000) 6467±6471
Isoquinoline formation by a novel photochemical
intermolecular cycloaddition of 1,1-diphenylethene derivatives
to 1,4-dicyanobenzene
Hideki Ishii,a,y Yuko Imai,a Takashi Hirano,a,* Shojiro Maki,a
Haruki Niwaa,* and Mamoru Ohashib
aDepartment of Applied Physics and Chemistry, The University of Electro-Communications, Chofu,
Tokyo 182-8585, Japan
bDepartment of Materials Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan
Received 12 April 2000; revised 22 June 2000; accepted 23 June 2000
Abstract
Phenanthrene-sensitized photoreaction of 1,1-diphenylethene (DPE) derivatives and 1,4-dicyanobenzene
(DCNB) in benzene leads to a novel intermolecular cycloaddition to give isoquinoline derivatives as a novel
synthetic method. The feasibility of this reaction was dependent upon the side chain structure of DPE
derivatives, which would regulate the stability of radical cation DPE+Á in a highly polar exciplex, and/or a
contact radical ion pair, by hyperconjugation or neighboring group participation. # 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: photochemistry; electron transfer; isoquinolines; cycloadditions.
Inter- and intramolecular cyclization reactions of a radical ion species via photoinduced
electron transfer (PET) reaction have been widely investigated from the synthetic and mechanistic
viewpoints.1,2 In particular, the PET-induced cycloaddition reactions of arene radical ions to
alkenes have been studied for the construction of a polycyclic skeleton.3 A substrate possessing an
electron-rich 1,1-diphenylethene moiety is advantageous in application to a radical cation±cyclization
reaction, because of its low oxidation potential and moderate reactivity of the corresponding
radical cation.4,5 Recently, we reported that the PET-induced intramolecular cyclization reactions
of 1,1-diphenyl-1,6-heptadiene derivatives in acetonitrile lead eciently to products bearing the
6/6/6-fused ring system.6 Diene 1, for instance, gave 2 in good yields. In this reaction, a radical
* Corresponding author. E-mail: hirano@pc.uec.ac.jp
y
Present address: Graduate School of Engineering, Tokyo Institute of Technology, Nagatsuta, Yokohama, Kanagawa
226-8502, Japan.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01078-9

6468
cation of a 1,1-diphenylethene derivative was generated under phenanthrene (Phen)-sensitized
irradiation conditions in the presence of 1,4-dicyanobenezene (DCNB) as an electron acceptor.
We also investigated solvent dependency on the radical cation reactivity of 1,1-diphenylethene
derivatives, and found a novel photoreactivity in benzene, leading to the formation of
isoquinoline derivatives such as 3, which were obtained by a formal [4+2] cycloaddition of a
1,1-diphenylethene moiety to DCNB. We report herein, this novel photochemical reactivity of
1,1-diphenylethene derivatives with DCNB in benzene, which was dependent upon the side chain
structure on the 1,1-diphenylethene moiety.
A solution of 1 (1Â10^2 M), DCNB (5Â10^2 M), and Phen (1Â10^2 M) in benzene was
irradiated by using a 100-W high-pressure Hg lamp with a CuSO4 ®lter solution (l>334 nm)
under Ar at room temperature. After irradiation, isoquinoline 3 was isolated by preparative TLC
in 23% yield (conversion 15%), accompanied by the recovery of Phen (ca. 100%). The formation
of cyclized product 2 was not observed in this reaction at all. The structure of 3 was established
by the spectroscopic data.7 Direct irradiation (l>280 nm) of a benzene solution of 1 and DCNB
in the absence of Phen also gave 3, although a competing photochemical decomposition of 1
lowered the yield of 3.
Under similar conditions, the phenanthrene-sensitized photoreactions of 1,1-diphenylethene
derivatives 4a±k with DCNB in benzene were investigated as summarized in Table 1. The results
indicate that the eciency to yield isoquinoline products is dependent upon the structure of the
side chain R of 1,1-diphenylethene derivatives. In the case of 1,1-diphenylethene (4a; R=H) and
1,1-diphenylpropene (4b; R=CH3; entries 1 and 2), the formation of 5a and 5b was negligible,
although 4a and 4b were consumed during the photoreactions. It is of interest that the yields of
the isoquinoline products increased with increase in the alkyl chain length, as shown in the case
of 4c (R=Et; 17%) and 4d (R=pentyl; 28%; entries 3 and 4). Furthermore, dienes 4e, 4f, and 4g,
bearing butenyl, pentenyl, and hexenyl groups, gave isoquinolines 5e (48%), 5f (59%), and 5g
(34%), respectively (entries 5±7), in good yields. Pentenyl-substituted 4f gave the best yield of
isoquinoline product 5f in this research. Thus, we investigated the e€ect of methyl-substitution on
the structure 4f by using 4h and 4i (entries 8 and 9). The results showed a decrease in the yields of
5h (27%) and 5i (10%). As another structural modi®cation, we investigated the photoreactions
of alcohol 4j and ether 4k (entries 10 and 11) as well as alcohol 6. Although 4j and 4k yielded
isoquinolines 5j (48%) and 5k (41%), alcohol 6 gave only intramolecular cyclized product 7
(40%) rather than the corresponding isoquinoline product, as reported previously.5

6469
Table 1
Phenanthrene-sensitized photoreactions of 1,1-diphenylethylene derivatives and DCNB
The isoquinoline derivative would be formed by the formal [4+2] cycloaddition of a 1,1-
diphenylethylene (DPE) derivative to a cyano group of DCNB in a highly polar exciplex and/or a
contact radical ion pair generated after a PET process in a nonpolar solvent benzene,8 followed
by air oxidation (Scheme 1). It is noteworthy that the participation of the cyano group of DCNB
as a dienophile in this reaction is a new reactivity of cyano-substituted arenes in the PET
reactions.9 Phen acts as a sensitizer, which has been introduced as a redox carrier in the redox-
photosensitized reactions,10 and the phenanthrene-sensitization helps to clear formation of

6470
Scheme 1.
isoquinoline derivatives by suppressing the photochemical decomposition of DPE. It is likely that
the formation of the exciplex and/or the contact radical ion pair of DPE and DCNB occurs via a
termolecular complex of the singlet excited Phen with DPE and DCNB in benzene.8 In the case of
1,1-diphenyl-1,6-heptadiene derivatives, the photochemical reaction manner of DPE with DCNB
in benzene is in contrast with the manner in acetonitrile, which gives an intramolecular cyclization
product of DPE, via the formation of free radical ions.6
The yields of isoquinoline products were dependent on the side chain structure (R) of the DPE
derivative, which would a€ect the properties of the radical cation DPE+Á in the highly polar
exciplex and/or the contact radical ion pair of DPE and DCNB. The better yields of 5c and 5d,
more than that of 5b, would be attributable to stabilization of the cationic character of DPE+Á by
hyperconjugation.11 Radical cations 4c+Á and 4d+Á, whose hyperconjugative interaction is larger
+
^Á
.
than that of 4b , may have a long enough lifetime to react with radical anion DCNB before
dissociation of the exciplex and/or the contact radical ion pair. The e€ect of the terminal alkenyl
groups in 4e, 4f, and 4g was remarkable for increasing the yields of isoquinoline derivatives. In
these cases, the terminal alkenyl group may stabilize the cationic center of 1,1-diphenylethene-
part intramolecularly, as a neighboring group participation. In the case of 4f, for instance, the
pentenyl group stabilizes the cationic center of 4f+Á, and the stability of 4f+Á would be decreased by
steric hindrance of the methyl substitution, as observed in 4h and 4i. Similarly, neighboring group
participation of an oxygen atom would work in 4j+Á and 4k+Á. In the case of alcohol 6, however,
radical cation 6+Á prefers the intramolecular nucleophilic attack of the hydroxy group, rather than
the intermolecular reaction with DCNB^Á.5 This explanation, based on the neighboring group
participation, is similar to an intramolecular coordination e€ect reported for the radical cation
species of group 14 organometallic compounds, which have a heteroatom in an appropriate
position.12
In summary, the phenanthrene-sensitized photoreaction of a DPE derivative and DCNB in
benzene leads to the formation of isoquinoline derivatives as a novel synthetic method.13 The
isoquinoline formation would proceed by the formal [4+2] cycloaddition of a DPE derivative to
the cyano group of DCNB in a highly polar exciplex and/or a contact radical ion pair, followed

6471
by air oxidation. The feasibility of this reaction was dependent upon the side chain structure R on
the 1,1-diphenylethene moiety, which would regulate the stability of the radical cation DPE+Á by
hyperconjugation or neighboring group participation.
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13. A similar isoquinoline formation was also observed, when 1,2- and 1,3-dicyanaobenezene were used instead of 1,4-
dicyanaobenezene. Study on the PET-induced isoquinoline formation of a 1,1-diphenylethene derivative with
various cyano-substituted arenes is now in progress.