J. Am. Chem. Soc. 2001, 123, 7703-7704
7703
Communications to the Editor
A Contribution to the Design of Molecular Switches:
Novel Acid-Mediated Ring-Closing-Photochemical
Ring-Opening of 2,3-Bis(heteroaryl)quinones
(Heteroaryl ) Thienyl, Furanyl, Pyrrolyl)
Xiaohu Deng and Lanny S. Liebeskind*
Emory UniVersity, Department of Chemistry
1515 Pierce DriVe, Atlanta, Georgia 30322
ReceiVed March 8, 2001
ReVised Manuscript ReceiVed June 13, 2001
Data storage and molecular device applications have stimulated
much interest in organic photochromism.1,2 Among the many
known photochromic systems, derivatives of 1,2-bis(heteroaryl)-
ethene (heteroaryl ) thienyl, furanyl, indolyl, thiazolyl, among
others) have received the most attention because of their remark-
able fatigue resistance and thermal and chemical stability.3,4 Novel
molecular devices based on the 1,2-bis(thienyl)ethene system were
also developed.5
Figure 1. 2,3-Bis(heteroaryl)quinone molecular switch. Yields of the
closed form isolated after flash chromatography. b45% 1-opened was
recovered. c9-closed slowly reverted to the opened form at room
temperature. After column chromatography, 41% 9-opened was recovered.
To date, light is the only trigger that will induce the
transformation of 1,2-bis(heteroaryl)ethene systems from the
switch-opened to the switch-closed state. This restriction limits
the range of possible molecular switches that can be designed, it
precludes the efficient preparation of large quantities of the switch-
closed form of 1,2-bis(heteroaryl)ethenes, and it renders difficult
a thorough study of switch-closed structure-activity relationships.
Working from the assumption that a quinone moiety incorporated
into the molecular switch could impart novel redox and polar
characteristics to the switching process, 2,3-bis(thienyl)-, (fura-
nyl)-, and (pyrrolyl)quinones were synthesized. These constitute
a new class of molecular switch that transforms into the switch-
closed state efficiently in the presence of strong protic or Lewis
acids such as AlCl3, FeCl3, and triflic acid (Figure 1).6 Visible
light induces reformation of the switch-opened state. A key
intermediate was characterized spectroscopically, which helped
elucidate the mechanism of the acid-induced ring closure.
The synthesis of 2,3-substituted naphthoquinones 1-10 and
benzoquinone 11 was straightforward (see Supporting Informa-
tion), comprising the Stille cross-coupling7,8 of an appropriately
substituted 3-(tri-n-butylstannyl)heterocycle with either 2,3-di-
bromobenzo- or naphthoquinone. The halo-substituted 2,3-bis-
(thienyl)quinones 5-7 were prepared from 1 by bromination with
NBS or iodination with I2/HIO3.
of AlCl3, FeCl3, or CF3SO3H, a rapid conversion of the red,
switch-opened form to a green intermediate ensued. The blue
switch-closed form was isolated in high yield after aqueous
workup.9
To test the generality of this unique acid-promoted molecular
switching reaction, compounds 1-11 were exposed to 3 equiv
of AlCl3 in CH2Cl2 at room temperature in the dark for 0.5 h
followed by an aqueous workup (Figure 1). With the exception
of the modest transformations of 1 (35%; 45% opened form
recovered) and 9 (50%; 41% opened form recovered), switch-
opened compounds 2-8, 10, and 11 were transformed into the
switch-closed states in very good yields (75-95%). All switch-
closed compounds reverted cleanly to the switch-opened state on
photolysis with visible light (150 W tungsten lamp). When kept
in the dark, all but the 2,3-bis(pyrrolyl)quinone-derived product
were readily isolable as stable, pure compounds after chroma-
tography. The reversion of switch-closed 2,3-bis(pyrrolyl)quinone
9 to the opened form on standing in the dark mimics the thermal
instability of Irie’s related 1,2-bis(indolyl)ethene system.10 Ab-
sorption spectral data for 1-11, opened and closed, are shown
in Table 1. As with other 1,2-bis(heteroaryl)ethene systems, the
switched-closed compounds showed strong, substituent-dependent
absorptions between 400 and 800 nm.
To better understand the mechanism of this unique process,
switch-opened 11 was treated with CF3SO3H in CD2Cl2 and
monitored by NMR. Switch-opened 11 and at least 2 equiv of
CF3SO3H generated a stable intermediate, 12, which was trans-
formed to switch-closed 11 upon aqueous workup or on treatment
with Et3N. Conversion was complete within 5 min at room
temperature. The same stable intermediate, 12, was generated
upon treatment of switch-closed 11 with 2 equiv of CF3SO3H.
Scheme 1 shows a plausible mechanism for acid-induced
conversion of the switch-opened to the switch-closed 2,3-bis-
(thienyl)quinone 11, proceeding by way of the bis-sulfonium salt
intermediate 12. This mechanism, taking place through analogous
bis-onium salts, would apply to the acid-induced closure of all
In a fashion similar to the 1,2-bis(thienyl)- and other 1,2-bis-
(heteroaryl)ethenes, photolysis of 2,3-bis(heteroaryl)quinones
2-11 through a quartz NMR tube (350 nm) slowly established a
photostationary state where only 5-30% conversion to the switch-
closed state was achieved. Surprisingly, the 5-unsubstituted
compound 1 gave no apparent ring closure on photolysis.
In dramatic contrast to the photochemical switching process,
when 2,3-bis(thienyl)quinone 3 was exposed to at least 2 equiv
(1) Feringa, B. L.; Jager, W. F.; Lange, B. D. Tetrahedron 1993, 49, 8267-
8310.
(2) Ward, M. D. Chem. Ind. 1997, 640-645.
(3) Irie, M. Chem. ReV. 2000, 100, 1685-1716.
(4) Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985-996.
(5) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem. Eur. J.
1995, 1, 285-293.
(9) In the photochemical ring closing process, orbital symmetry control
leads to a trans disposition of the two methyl groups. The Lewis acid-mediated
process is stepwise, not concerted, but also leads to the sterically more
favorable trans methyl relationship, which was verified by an X-ray crystal-
lographic determination of switch-closed 2.
(10) Nakayama, Y.; Hayashi, K.; Irie, M. J. Org. Chem. 1990, 55, 2592-
2596.
(6) For a related pH-dependent transformation see Shimizu, I.; Kokado,
H.; Inoue, E. Bull. Chem. Soc. Jpn. 1969, 42, 1726-1729 and 1730-1734.
(7) Farina, V.; Krishnamurthy, V.; Scott, W. J. In Organic Reactions;
Paquette, L., Ed.; John Wiley & Sons: New York, 1997; Vol. 50, pp 1-652.
(8) Arduni, A.; Casnati, A. In Macrocyclic Synthesis: A Practical
Approach; Parker, D. Ed.; Oxford: New York, 1996; p 145.
10.1021/ja0106220 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/11/2001