FULL PAPERS
tions. However, care will have to be taken owing to the
moisture and light sensitivity of the system.
cently, it has been used as a dinitrogen equivalent where it
undergoes a [4+2] cycloaddition reaction with aryl keteni-
mines to yield 1,2,4-triazoloACHTUNGTRNEUNG
[1,2-a]cinnolines.[24d–e]
NTADs have also been described to react with various
fulvenes and the high reactivity of both the diene and the di-
enophile is indicated by the formation of a rearranged, di-
merized cycloadduct even at low temperature.[25]
Results and Discussion
DA Reaction between Anthracene Derivatives and 7
Anthracene has been implemented as a diene in the DA re-
action for many years. The first example dates back to the
early twentieth century.[16] Anthracene derivatives are well
documented in the literature to undergo [4+2] cycloaddition
reactions at the 9,10-positions. However, the reported retro-
DA (r-DA) reactions usually require high temperatures.[17]
Thus, the DA cycloadduct between maleimide and various
anthracene derivatives attained equilibrium in xylene at
reflux and the nature of the substituents at the 9-and 10-po-
sitions of the anthracene component had a significant effect
on the position of the equilibrium,[18] as substantiated by a
detailed kinetic study of the DA reaction of various anthra-
cene and maleic anhydride derivatives.[19] More recently, a
detailed investigation of the structural effects controlling the
rate of the r-DA reaction showed that, in the case of anthra-
cene-based cycloadducts, electron-donating groups in the
diene and electron-withdrawing groups in the dienophile in-
creased the rate of adduct formation. However, acceleration
was reported to be relatively small.[20]
rev-DA Reactions
In our search for new systems undergoing rev-DA reactions
at moderate temperatures, and preferably at room tempera-
ture or below, we have employed anthracene and its deriva-
tives as dienes and 7 as a dienophile for the current investi-
gation. In a previous report, the DA reaction between 9,10-
dimethylanthracene and diethyl dicyanofumarate proceeded
smoothly and attained equilibrium within 3 hours at room
temperature with almost 98% conversion into the adduct.
However, under the same conditions, diethyl dicyanofuma-
rate reacted only very slowly to reach equilibrium with an-
thracene in the course of over a week and with 9-methylan-
thracene within a time span of 30 hours.[10c]
In contrast, when a stoichiometric amount of anthracene
(1) was treated with 7 in the absence of light and under an-
hydrous conditions (Scheme 1), the reaction mixture at-
The dienophile can be considered to have Lewis acid
character. Thus, a substituent that alters its ability to act as
a better p acceptor should accelerate the rate of the DA re-
action. Similarly, steric hindrance around the diene would
have a detrimental effect on the rate because it hinders ap-
proach of the dienophile. It thus appeared that azo-based N-
substituted 1,2,4-triazoline-3,5-diones (NTADs) might be ex-
cellent candidates as dienophiles for devising dynamic DA
reactions. The azo-based compounds should also have an
added advantage of requiring less energy for the cleavage of
À
the C N bond (formed in course of forward DA reaction),
Scheme 1. DA adduct formation and equilibrium between dienes (1–6)
and dienophiles (7–10) at 258C.
when the adduct undergoes r-DA reaction, owing to the fact
À1
À
that the C N bond energy is about 40 kJmol lower than
that of a C C bond. Thus, DA adducts of such azo-dieno-
philes with dienes, such as anthracene, should in principle
undergo comparatively facile r-DA reactions.
NTADs are known to be very reactive dienophiles and
have been reported to undergo DA reactions very rapidly
with conjugated dienes.[22] Ene reactions of 7 with various
olefins have been reported.[23] With certain olefins, com-
pound 7 leads to the formation of 1,2-azetidines.[24a–c] Re-
[21]
À
tained equilibrium immediately (less than 2 min) after the
addition of the dienophile to the diene, thus forming 84%
of the adduct (at a concentration of 35 mm). The equilibrium
composition of adduct and starting material did not change
over 12 hours, indicating that equilibration had been
reached immediately after the addition of the dienophile.
Increasing the temperature of the reaction mixture to 508C
shifted the equilibrium towards the starting materials and
reduced the fraction of adduct to 71%.
In the presence of light and moisture other competing
secondary side reactions, such as anthracene derivative di-
merization and partial decomposition of 7 take place (see
the Supporting Information).[26] To prevent these side reac-
tions, the studies must be carried out in the absence of light
and under anhydrous conditions.
Abstract in Bengali:
In an attempt to alleviate such moisture and light sensitiv-
ity, we prepared dienophiles with bulky substituents on the
2420
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 2419 – 2425