New deconjugation reaction of (E)-1-indanylidene methylarene brought by photolysis with protic acid

Article abstract of DOI:10.1016/j.tetlet.2004.05.108

A new photodeconjugation reaction of (E) indanylidene methylarenes (1a-d) carried out by photolysis in the presence of protic acid are reported with 80% to >95% yields. The reaction mechanism is through the protonation of the less stable (Z) isomer to form stable indanyl cation followed by deprotonation.

Full text of DOI:10.1016/j.tetlet.2004.05.108

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 5665–5667
New deconjugation reaction of (E)-1-indanylidene methylarene
brought by photolysis with protic acidq
Tong-Ing Ho* and Tai-Chen Li
Department of Chemistry, Nation Taiwan University, Taipei 106, Taiwan, ROC
Received 9 April 2004; revised 18 May2004; accepted 20 May2004
Available online 10 June 2004
Abstract—A new photodeconjugation reaction of (E) indanylidene methylarenes (1a–d) carried out byphotolysis in the presence of
protic acid are reported with 80% to >95% yields. The reaction mechanism is through the protonation of the less stable (Z) isomer to
form stable indanyl cation followed by deprotonation.
Ó 2004 Elsevier Ltd. All rights reserved.
S
S
S
S
Deconjugation reaction is one of the most interesting
topics for photochemistry, since it involves the conver-
sion of the more stable reactants to the less stable
products. Recentlythere are several reports on the
photodeconjugation reactions. Arnold and Mines have
studied the photodeconjugation reaction of 1-phenyl-
propene1;2 and 1-arylcyclohexene.3 The reaction is
carried out byphotoslyis of a mixture of 1,4-
dicyanobenzene (DCB), biphenyl (BP), collidine and the
reaction (e.g., 2-methyl-1-phenylpropene) in acetonitrile
afford 2-methyl-3-phenylpropene in 90% yield (Eq. 1).
The reaction is initiated byphotosensitized electron
transfer from the reactant to the excited state of 1,4-
dicyanobenzene and involves reactive intermediates
such as the cation radical, anion radical, allyl radical
and allyl anion.
CH3
CH2
hv, HCl
C6H6
ð2Þ
EWG
EWG
Photodeconjugation of an a,b-unsaturated ketone to the
less stable b,c-unsaturated ketone has also been reported
through the Norrish-type hydrogen abstraction reac-
tion.5 Photolysis6 of 3-methyl-1,2-dihydronaphthalene
(3-MDHN) in acidic acetonitrile with 254 nm light at
room temperature, the major product is the deconju-
gated 2-methylene-1,2,3,4-tetrahydronaphthalene (2-
MTHN) with the quantum yield of 0.027 (Eq. 3). The
deconjugation product is formed via protonation and
deprotonation of the ring opening product of an initially
formed intramolecular [2 þ 2] cycloadduct.
H3C
H2C
CH3
CH3
hv, DCB, BP
Ph
Ph
hv (254 nm)
HCl, CH3CN
ð1Þ
collidine, CH3CN
CH3
CH3
ð3Þ
Another type of photodeconjugation is reported by
Kutateladze and co-workers.4 Their photodeconjuga-
tion for 2-aryl-3-methyl-6,7-dihydro-5H-[1,4]dithiepine
is assisted byprotic acids (Eq. 2). This reaction is
achieved bythe combination of electron donating sulfur
and electron-withdrawing phenyl systems.
+H+
-H+
CH2
The course of photochemical reaction is sometimes
sensitive to its acidic environments.7–10 We have pre-
pared a series of (E)-1-indanylidene methylarenes 1a–g
(Scheme 1) and would like to report a new deconjuga-
tion reaction, which is assisted byphotolsyis in the
presence of protic acid. Compounds 1a–d were synthe-
sized by reacting indanyl-(1)-triphenyl-phosphonium
bromide with the corresponding aldehydes and sodium
amide.11 Compounds 1a–g show stilbene type
of absorption at kmax around 330 nm. When 105 mg of
Keywords: Deconjugation; Photochemistry; Protic acid.
q Supplementarydata associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.05.108
* Corresponding author. Tel.: +886-223691801; fax: +886-223636359;
e-mail: hall@ntu.edu.tw
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.05.108

5666
T.-I. Ho, T.-C. Li / Tetrahedron Letters 45 (2004) 5665–5667
Table 1. The conversion and yields of the photodeconjugation of
compounds 1a–g in the presence of 0.27 M hydrochloric acid using
350 nm light at room temperature (25 °C)
S
S
S
S
S
Reactant
Irrad. time Conversion Product
(%)
Yield
(%)
(h)
1a
1b
2a
2b
3a
1f
1a
1a
1b
1c
1d
1e
1f
0.67
93
92
93
94
90
75
––
––
2a
3a
>95
90a
90
0.67
0.67
0.67
1.33
0.67
0.67
2.5
2b
2c
>95
80
2d
2e
8
Z þ E
––
––
1g
No reaction
a Irradiation without HCl.
2c
2d
1c
1d
1g
(E)-2,3,20,30-tetrahydro-[1,10]biindenylidene 1g in the
presence of 0.27 M HCl resulted in the isolation of the
starting reactant only. It has been reported13 that the Z
form of 1g is not stable due to it’s steric hindrance. Thus
the Z compound formation is essential for the photo-
deconjugation process for the indanylidene system.
When compound 1a is photolyzed in acetonitrile in the
absence of hydrochloric acid, the reaction afford the Z
compound 3a in 90% yield (Eq. 5) as compared with the
photolysis in the presence of HCl (Eq. 4).
O
O
S
S
S
1e
2e
3e
Scheme 1.
S
S
ð5Þ
hv (350 nm)
0.67h
(E)-2-(1-indanylidenemethyl)-3-methylthiophene 1a is
dissolved in 105 mL of acetonitrile (4.4 Â 10À3 M) con-
taining 2.34 mL of hydrochloric acid (0.27 M) and the
solution is outgasssed with argon. Then the solution is
irradiated at 350 nm for 40 min with a Rayonet reactor.
The solution is neutralized with sodium hydroxide
solution and the product is extracted with ethyl acetate
to afford >95% yield (96.5 mg isolated at conversion of
93%) of 2-(inden-3-ylmethyl)-3-methylthiophene12 2a
(Eq. 4).
90%
1a
3a
When the Z compound 3a is treated 0.27 M HCl in the
dark the reaction products contain one to one mixture
of the deconjugation product 2a and the E compound 1a
(Eq. 6).
S
ð6Þ
dark, H+
+
1a
2a
50%
50%
3a
S
S
ð4Þ
CH3CN, 0.27 M HCl
The product 2a can also be converted to the starting
reactant 1a byheating 2a in the presence of hydrochloric
acid at 70 °C for 5 h. (Eq. 7).
hv (350 nm)
>95%
2a
1a
Similarlyphotolsyis of compounds 1b–d with 0.27 M
hydrochloric acid afforded 2b–d, respectively, in 80–95%
yields (Table 1).
CH3CN, HCl
ð7Þ
2a
90%
1a
70oC, 5h
Thus the mechanism for the deconjugation process can
be summarized in Scheme 2. Photolysis of 1a produces
the unstable Z compound 3a. The regioselective pro-
tonation of the (Z)-3a produces the 1-indanyl carbo-
cation which can undergo nonselective deprotonation
back to the (E)-1a and to the deconjugated 2a in one to
Irradiation of 2-(E)-(1-(2,3,4-trihydronaphthylidene)-
methyl)-3-methylthiophene 1e in the presence of 0.27 M
HCl afford 2-(1-(3,4-dihydronaphthyl)methyl)-3-methyl-
thiophene 2e in only 8% yield. Photolysis of (E)-1,2-di-
phenyl-propene 1f with 0.27 M HCl leads to mixture of
E, Z isomers without anydeconjugation products. Thus
it is clear that only the indanylidene system can undergo
efficient photodeconjugation. The six-member ring sys-
tem (1e) afford veryfew deconjugation product. The
open system (1f) does not react at all. We can get some
insight into the mechanism of this new photodeconju-
gation system from the fact that photolysis of
14
one ratio. The relativelystable indanyl cation has been
proposed as a reactive intermediate.
The high theoretical proton affinityfor 1-methylindene
has been calculated (209.8 K cal/mol) bydensityfunc-
tional theory.15 And it was predicted that indanyl cation
is a persistent carbenium ion. There is one report16

T.-I. Ho, T.-C. Li / Tetrahedron Letters 45 (2004) 5665–5667
5667
Acknowledgements
S
S
S
The authors would like to acknowledge the financial
support from the National Science Council of ROC.
H+
hv (350 nm)
+
indanyl cation
1a
3a
S
S
-H+
+
References and notes
(50%)
(50%)
1a
2a
1. Arnold, D. R.; Mines, S. A. Can.J.Chem.
2312.
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1987, 65,
1989, 67,
Scheme 2. Proposed mechanism for the photodeconjugation of 1a.
saying that in strong acid environment the indanyl cat-
ion is stable than indene. Haw and co-workers17 have
also measured the NMR spectra for indanyl cation.
Thus in our hydrochloric acid environment, the indanyl
cation is obtained from protonation at the less stable Z-
alkene, which is derived from E-alkene byphotoiso-
merization and undergoes nonselective deprotonation at
room temperature. The related six-member ring cation
(2,3,4-trihydro-1-naphthyl cation) is less stable and is
not easilyachievable at room temperature. This
accounts for the low yield (8% yield for 2e). However
higher deconjugation product can be achieved for
compound 2e when the 3e (which is obtain byphotolysis
of 1e) is heated at high temperature (Eq. 8).
3. Mangion, D.; Kendall, J.; Arnold, D. R. Org.Lett. 2003,
3, 45.
4. (a) Wan, Y.; Kurchan, A. J.Org.Chem. 2001, 66, 1894;
(b) Wan, Y.; Barnhurst, L. A.; Kutateladze, A. G. Org.
Lett. 1999, 1, 937.
5. Yang, N. C.; Jorgenson, M. J. Tetrahedron Lett. 1964,
1203.
6. Duguid, R. J.; Morrison, H. J.Am.Chem.Soc. 1991, 113,
1265.
7. Mallory, F. B.; Mallory, C. W. In Organic Reaction;
Wiley: New York, 1983; Vol. 30, p 1.
8. (a) Hoffmann, N.; Pete, J.-P. Tetrahedron Lett. 1998, 39,
5027; (b) Hoffmann, N.; Pete, J.-P. J.Org.Chem. 1997,
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4555.
1996, 61,
9. Mori, T.; Wada, T.; Inoue, Y. Org.Lett. 2000, 2,
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122, 8575; (b) Ho, J. H.; Ho, T. I.; Liu, R. S. H. Org.Lett.
2001, 3, 409; (c) Ho, J. H.; Ho, T. I. Chem.Commun. 2002,
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11. McCullough, J. K. Tetrahedron Lett. 1982, 23, 21.
12. H NMR (500 MHz, CDCl3): d 7.45 (d, J ¼ 7:3 Hz, 1H),
7.35 (d, J ¼ 7:4 Hz, 1H), 7.28 (t, J ¼ 7:0 Hz, 1H), 7.20 (t,
J ¼ 7:3 Hz, 1H), 7.04 (d, J ¼ 5:1 Hz, 1H), 6.28 (d,
J ¼ 5:2 Hz, 1H), 6.12 (t, J ¼ 1:6 Hz, 1H), 3.96 (d,
J ¼ 1:6 Hz, 2H), 3.33 (d, J ¼ 1:9 Hz, 2H), 2.20 (s, 3H);
13C NMR (125 MHz, CDCl3): d 144.7, 144.5, 142.5, 134.7,
133.6, 129.9, 129.7, 126.0, 124.7, 123.7, 121.7, 118.9, 37.61,
26.5, 13.7; MS (70 eV, EI): 226 (Mþ, 32), 211 (4), 205 (8),
111 (100); HRMS (C15H14S): calcd 226.0816, found
226.0816.
13. (a) Warrener, R. N.; Pitt, I. G.; Russell, R. A. Aust.J.
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15. Nicholas, J. B.; Haw, J. F. J.Am.Chem.Soc. 1998, 120,
11804.
16. Pittman, C. U., Jr.; Miller, W. G. J.Am.Chem.Soc. 1973,
95, 2947.
S
S
S
dark, H+
3h, 70oC
+
3e
1e
60%
40%
2e
ð8Þ
The acyclic carbenium ion is even more unstable and no
reaction is observed for compound 1f. The lack of de-
conjugation for the compound 1g has been ascribed to
the fact that Z form of 1g can not be achieved photo-
chemically, thus the unstable Z isomers of indanylidene
arenes (1a–d) are precursors to the indanyl cations.
In summary, we have presented a new deconjugation
reaction of (E)-indanylidene methylarene system, which
is assisted byphotolysis with protic acid. The reaction
mechanism is through the regioselective protonation of
the Z isomer to form relativelystable indanyl cation.
Then deprotonation from the indanyl cation can lead to
the deconjugation product. Since the deconjugated
product can be converted back to the starting material
under acidic condition at higher temperature (>70 °C) in
high yield (>90%), our system may be developed into a
new thermallyreversible photochromic sytem for
compounds 1a–d.
17. Xu, T.; Haw, J. F. J.Am.Chem.Soc.
10188.
1994, 116,