Photochemical cycloaromatization of non-benzenoid enediynes

Article abstract of DOI:10.1002/(SICI)1521-3773(19990503)38:9<1267::AID-ANIE1267>3.0.CO;2-F

An efficient photo-Bergman reaction of aliphatic enediynes has been realized. Photolysis of 1 results in the formation of 4 in good yields along with [D₂]3. Enediyne 4, which has never been isolated in the thermal reaction of 1, arises here by a retro-Bergman reaction of the diradical intermediate 2.

Full text of DOI:10.1002/(SICI)1521-3773(19990503)38:9<1267::AID-ANIE1267>3.0.CO;2-F

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Table 1. Photoreaction of 1,2-diethynylcyclopentene derivatives 1a ± 1d in
hexane.
Photochemical Cycloaromatization of
Non-Benzenoid Enediynes**
Toshio Kaneko, Miki Takahashi, and
Masahiro Hirama*
Dedicated to Professor Satoru Masamune
on the occasion of his 70th birthday
The Bergman reaction, the thermal cycloaromatization of
hexa-1,5-diyn-3-enes to afford benzenoid compounds, is well-
Substrate
Reaction time [h]
Yield [%]
[
1, 2]
known.
Some photochemical examples have been report-
1
1
a: R ˆ H
6
18
18
18
4a:
4b:
4c:
3
0
0
[3±5]
ed; however, these are restricted to arene-fused enediynes.
1b: R
1
ˆ TMS
1
A photo-Bergman reaction of aliphatic enediynes could play
an important role in 1,4-didehydrobenzene (p-benzyne)
chemistry because it could be carried out at low temperature
and thus allow characterization of reactive intermediates.[
We describe herein the successful photo-cycloaromatization
of several aliphatic enediynes.
1c: R ˆ Ph
1
1
d: R ˆ Me
4d: 71
6]
1
[8]
derivative 1d (R ˆ Me) gave 4d in high yield (71%; 80%
yield by GC analysis).[ Photoreduction products such as
dienynes, which are formed in the case of benzenoid
enediynes,[ were not detected. The photo-cycloaromatiza-
tion took place in a variety of solvents (but not in MeOH):
n-hexane/1,4-cyclohexadiene (20/1; 4d: 41%), cyclohexane
9]
Based on our previous studies on the synthesis of nine-
membered cyclic enediynes,[7] we first examined the photo-
reaction of 1,2-diethynylcyclopentene derivatives 1. The
photochemical formation of 3 was expected through the 1,4-
didehydrobenzene intermediate 2 (Scheme 1). A degassed
4b]
(
4d: 52%), THF (4d: 35%), CH CN (4d: 21%, 5: 8%),
3
iPrOH (4d: 20%), CH Cl (4d: trace, 6: 20%).
2
2
Photolysis of 1d was then carried out in [D ]THF
8
(
99.5 atom% D). The deuterated product [D ]4d (X ˆ Yˆ
2
D) ([D ]: 88%, [D ]: 12%, [D ]: 0%) was obtained in 14%
2
1
0
yield. These results strongly suggest that the photocyclization
of 1d proceeds through the 4,7-didehydroindane diradical
1
intermediate 2 (R ˆ Me). Irradiation of 1d under a high-
pressure mercury lamp (400 W, Riko UVL-400HA, pyrex) in
hexane did not cause any reaction, not even in the presence of
triplet photosensitizers (toluene, acetophenone, benzophe-
none, anthraphenone, or naphthalene). Thus, neither pyrex-
filtered light nor the triplet energy of these sensitizers is
effective for the photo-cycloaromatization of 1d.
2
Scheme 1. Plausible course of the photo-Bergman reaction. R ˆ TBS ˆ
We then irradiated the strained, ten-membered cyclic
tert-butyldimethylsilyl.
enediyne 7[
8, 10]
with a low-pressure mercury lamp (4 Â 20 W)
at room temperature for 3 h. Tetrahydronaphthalene (8) was
obtained, and an appreciable amount of 1,2-diethynylcyclo-
hexene (9)[ was formed in all solvents except iPrOH
1
[8]
solution of 1a (R ˆ H) in hexane was irradiated in a quartz
flask at room temperature with a low-pressure mercury lamp
11]
(
4 Â 20 W) for 6 h. The enediyne 1a disappeared completely,
(Table 2). Compound 9 underwent photolysis under the same
but the aromatic product 4a was obtained only in very low
reaction conditions, and it was consumed completely upon
photolysis in iPrOH for 3 h to give 8 in 3% yield. Photolysis of
1
yield (3%) and no nine-membered enediyne 3 (R ˆ H) was
[
8]
[8]
detected (Table 1). Neither 1b nor 1c possessing the bulky
substituents trimethylsilyl (TMS) and phenyl, respectively, at
the alkyne termini yielded any indane derivatives under the
same reaction conditions; the reactants were recovered
almost quantitatively. However, photolysis of the dipropynyl
7
in [D ]iPrOH was stopped after 30% conversion (5 min),
8
1
and the products were analyzed by 500-MHz H NMR
Table 2. Photolysis of cyclodeca-1,5-diyn-3-ene (7).
[
*] Prof. Dr. M. Hirama, T. Kaneko, M. Takahashi
Department of Chemistry, Graduate School of Science
Tohoku University
and
CREST, Japan Science and Technology Corporation (JST)
Sendai 980 ± 8578 (Japan)
Solvent
Yield of 8 [%][a]
Yield of 9 [%][a]
Fax: (‡81)22-217-6566
hexane[b]
cyclohexane
29
26
12
3
6
20
24
E-mail: hirama@ykbsc.chem.tohoku.ac.jp
[
**] This work was supported in part by the Ministry of Education,
Science, Sports, and Culture of Japan and a fellowship to T.K. from the
Japanese Society for the Promotion of Science for Young Japanese
Scientists.
CH
3
CN
[c]
iPrOH
±
[a] By GC analysis. [b] Irradiated for 5 h. [c] Not detected.
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Angew. Chem. Int. Ed. 1999, 38, No. 9
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1267

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‡
spectroscopy (Scheme 2). The yields of [D ]8 and 9 were 30
and 53%, respectively, based on recovered 7. The formation of
261.1675, found: 261.1678; calcd for C17
found: 219.1238.
H
28OSi [M � tBu]: 219.1205,
2
9
in over 50% yield is highly remarkable because enediyne 9
Received: November 9, 1998 [Z12644IE]
German version: Angew. Chem. 1999, 111, 1347 ± 1349
should arise from the retro-Bergman reaction of the hypo-
thetical diradical intermediate 10, and because 9 has never
been isolated in the thermal reaction of 7.
[
10, 12]
Keywords: cyclizations ´ diradicals ´ electrocyclic reactions
´
enynes ´ photochemistry
[
1] a) R. R. Jones, R. G. Bergman, J. Am. Chem. Soc. 1972, 94, 660 ± 661;
R. G. Bergman, Acc. Chem. Res. 1973, 6, 25 ± 31; T. P. Lockhart, P. B.
Comita, R. G. Bergman, J. Am. Chem. Soc. 1981, 103, 4082 ± 4090;
b) N. Darby, C. U. Kim, J. A. Salaün, K. W. Shelton, S. Takada, S.
Masamune, J. Chem. Soc. Chem. Commun. 1971, 1516 ± 1517; J.
Mayer, F. Sondheimer, J. Am. Chem. Soc. 1966, 88, 602 ± 604.
Scheme 2. Photolysis of 7 in [D
8
]iPrOH for 5 min (30% conversion).
[2] Reviews: a) K. C. Nicolaou, W.-M. Dai, Angew. Chem. 1991, 103,
453 ± 1481; Angew. Chem. Int. Ed. Engl. 1991, 30, 1387 ± 1416;
1
b) M. E. Maier, Synlett 1995, 13 ± 26; c) H. Lhermitte, D. Grierson,
Contemp. Org. Synth. 1996, 3, 93 ± 124; d) J. W. Grissom, G. U.
Gunawardena, D. Klingberg, D. Huang, Tetrahedron 1996, 52, 6453 ±
We have established a highly effective photo-Bergman
reaction of the aliphatic, ten-membered cyclic enediyne 7. The
photoreaction of 7 also showed remarkable formation of 9,
which should be a retro-Bergman product from 10. Thus, 7
appears to be ideal for flash photolysis and for the character-
ization of the 1,4-didehydrobenzene diradical intermediate
6
516.
[
[
3] I. D. Cambell, G. Eglinton, J. Chem. Soc. C 1968, 2120 ± 2121.
4] a) N. J. Turro, A. Evenzahav, K. C. Nicolaou, Tetrahedron Lett. 1994,
35, 8089 ± 8092; b) A. Evenzahav, N. J. Turro, J. Am. Chem. Soc. 1998,
1
20, 1835 ± 1841.
[
5] R. L. Funk, E. R. R. Young, R. M. Williams, M. F. Flanagan, T. L.
Cecil, J. Am. Chem. Soc. 1996, 118, 3291 ± 3292.
10. A mechanistic study along this line will be reported in due
course.
[
6] Recent studies on 1,4-didehydrobenzene: W. R. Roth, H. Hopf, T.
Wasser, H. Zimmermann, C. Werner, Liebigs. Ann. 1996, 1691 ± 1695;
R. Marquardt, A. Balster, W. Sander, E. Kraka, D. Cremer, J. G.
Radziszewski, Angew. Chem. 1998, 110, 1001 ± 1005; Angew. Chem.
Int. Ed. 1998, 37, 955 ± 958; J. Hoffner, M. J. Schottelius, D. Feichting-
er, P. Chen, J. Am. Chem. Soc. 1998, 120, 376 ± 385; P. R. Schreiner, J.
Am. Chem. Soc. 1998, 120, 4184 ± 4190; P. G. Wenthold, R. Squires,
W. C. Lineberger, J. Am. Chem. Soc. 1998, 120, 5279 ± 5290.
7] K. Iida, M. Hirama, J. Am. Chem. Soc. 1994, 116, 10310 ± 10311; K.
Iida, M. Hirama, J. Am. Chem. Soc. 1995, 117, 8875 ± 8876; I. Sato, Y.
Akahori, K. Iida, M. Hirama, Tetrahedron Lett. 1996, 37, 5135 ± 5138;
S. Kawata, F. Yoshimura, J. Irie, H. Ehara, M. Hirama, Synlett 1997,
Experimental Section
Photolysis of 1d: A solution of 1d (50.0 mg, 0.182 mmol) in hexane
(
2
50 mL) was irradiated in a quartz flask (30 mm (inner diameter) Â
30 mm) with four GL-20 low-pressure mercury lamps (National Electric
Co. Ltd.) at room temperature under an argon atmosphere for 18 h. The
solution was concentrated and purified by flash column chromatography on
silica gel (eluent: hexane containing 0 ± 1% of ethyl acetate). The solid
residue 4d was recrystallized from hexane; yield: 35.8 mg (0.129 mmol,
[
[
71%).
2
50 ± 252; M. Hirama, Pure Appl. Chem. 1997, 69, 525 ± 530; T. Mita, S.
GC analysis of the photoreaction products of 7 was carried out with 1,3,5-
trimethylbenzene as an internal standard.
1H NMR analysis of the photoreaction of 7: A 5.0mm solution of 7 in
Kawata, M. Hirama, Chem. Lett. 1998, 959 ± 960.
8] UV spectra of enediynes. 1a (n-hexane): lmax (lge) ˆ 270 (4.05), 259
(4.16), 250 nm (sh, 4.05); 1b (n-hexane): lmax (lge) ˆ 294 (4.23),
280 nm (4.15); 1c (n-hexane): lmax (lge) ˆ 350 (sh, 4.11), 328 (4.32),
[
D
8
]iPrOH (99 ‡ atom% D) in a quartz NMR tube (5 mm inner diameter)
was placed in a quartz jacket filled with methanol and irradiated for 5 min
with four GL-20 low-pressure mercury lamps (National Electric Co. Ltd.)
at 258C under an argon atmosphere. The H NMR spectra were taken
260 (4.40), 225 nm (4.31); 1d (n-hexane): l
(lge) ˆ 275 (4.15),
max
263 nm (4.22); 7 (CH CN): l (lge) ˆ 281 (3.76), 270 (sh, 3.83), 267
3
max
1
(3.84), 258 nm (sh, 3.75).
directly for the reaction mixture on a Varian INOVA-500 instrument. The
yields of 8 and 9 and the amount of recovered 7 were determined using the
[9] Turro and co-worker independently found that the photolysis of 1,2-
bis(1-pentynyl)cyclopentene as well as of 1,2-bis(1-phenylacetylenyl)-
cyclopentene yielded the indane derivatives; see citation [39] in
reference [4b].
10] K. C. Nicolaou, G. Zuccarello, Y. Ogawa, E. J. Schweiger, T. Kuma-
zawa, J. Am. Chem. Soc. 1988, 110, 4866 ± 4868; K. C. Nicolaou, G.
Zuccarello, C. Riemer, V. A. Estevez, W.-M. Dai, J. Am. Chem. Soc.
residual protons of [D
8
]iPrOH as an internal standard.
1
1
d: Pale yellow crystals; m.p. 39.5 ± 408C (n-hexane); H NMR (200 MHz,
[
CDCl
2
3
): d ˆ 0.04 (6H, s), 0.87 (9H, s), 2.05 (6H, s), 2.38 ± 2.45 (2H, m),
.67 ± 2.79 (2H, m), 4.37 ± 4.51 (1H, m); 1 C NMR (125 MHz, CDCl
3
3
): d ˆ
�
4.82 (CH
3 3 3 2
), 4.84 (CH ), 18.08 (C), 25.81 (CH ), 46.58 (CH ), 71.05 (CH),
1
992, 114, 7360 ± 7371.
7
1
6.28 (C), 91.98 (C), 126.05 (C); IR (neat): nÄ ˆ 2958, 2932, 2860, 2224, 1473,
�
1
[11] G. M. Pilling, F. Sondheimer, J. Am. Chem. Soc. 1971, 93, 1970 ± 1977.
[12] We confirmed that the thermal reaction of 7 proceeded smoothly at
437,1363, 1257, 1216, 1195, 1104, 1069, 1007 cm ; MS (EI, 70 eV): m/z
‡
‡
‡
(
(
%): 276 (1.1) [M ‡2H], 275 (3.9) [M ‡H] 274 (16) [M ], 217 (100), 143
5
78C and did not give even trace amounts of 9. The enediyne 9 did not
8.9); elemental analysis calcd for C17
H
26OSi: C 74.39, H 9.55; found: C
‡
undergo cycloaromatization at the same temperature; this reaction
did not occur at an appreciable rate until at least 1008C.
74.60, H 9.67; HR-MS (EI, 70 eV) calcd for C17
H
26OSi [M ]: 274.1753,
found 274.1752.
1
4
(
3
6
1
d: Pale yellow amorphous solid; H NMR (500 MHz, CDCl
3
): d ˆ 0.09
3
6H, s), 0.90 (9H, s), 2.22 (6H, s), 2.81 (2H, dd, J(H,H) ˆ 15.5, 6.0 Hz),
3
3
.06 (2H, dd, J(H,H) ˆ 15.5, 6.5 Hz), 4.63 (1H, quint, J(H,H) ˆ 6.2 Hz),
.96 (2H, s); 13C NMR (125 MHz, CDCl
): d ˆ � 4.73 (CH ), 18.25 (C),
9.73 (CH ), 25.93 (CH ), 42.21 (CH ), 74.17 (CH), 125.70 (CH), 134.49
3
3
3
3
2
(
1
C), 138.55 (C); IR (neat): nÄ ˆ 3010, 2960, 2860, 1497, 1464, 1379, 1255,
�
1
‡
‡
199, 1104, 1069 cm ; MS (EI, 70 eV): m/z (%): 276 (2.0) [M ], 261 [M �
‡
Me] (3.5), 219 (100) [M � tBu], 145 (14); HR-MS (EI, 70 eV) calcd for
‡
‡
C
17
H
28OSi [M ]: 276.1909, found: 276.1901; calcd for C17
H
28OSi [M � Me]:
1
268
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Angew. Chem. Int. Ed. 1999, 38, No. 9