Scheme 6. Successful Synthesis of 1, Which after Thermal Ring Closure, Was Isolated as 11
the VHF in a similar system, we decided to functionalize
compound 2 with four cycloheptatrienyl units in order to
explore this property in the final elimination steps (Scheme 6).
Gratifyingly, a reaction between 2 and 4 molar equiv of
tropylium tetrafluoroborate in the presence of Et3N gave
10 in quantitative yield with no purification needed other
than aqueous workup. This compound was then refluxed
with 2 molar equiv of tritylium tetrafluoroborate in DCE,
which ultimately generated DHA 11 via the VHF 1 as an
intermediate. By a line of careful purifications by dry
column vacuum chromatography, followed by a crystal-
lization from heptanes, DHA 1114 was finally obtained in
pure form. The rather low yield reflects the difficult puri-
fication.
DHA 11 exhibits an absorption maximum at 375 nm in
MeCN (Figure 5), which is red-shifted by ca. 20 nm
relative to that of DHA 7 owing to the extended conjuga-
tion in 11. With this DHA in hand, we were able to study
the thermoswitch properties of the corresponding VHF 1.
First, a sample of DHA 11 in MeCN was quantitatively
ring-opened by light (375 nm) to provide 1 (Figure 5).
This compound exhibits an absorption at λmax 483 nm.
The ring closure of 1 was then followed at 25 °C
(Figure 5), and from the exponential decay (first-order
kinetics) of the VHF absorption at λmax 483 nm, the rate
constant k for ring closure was found to be 9.64 ꢁ 10ꢀ5 sꢀ1
and the half-life t1/2 to be 120 min. In line with the
rationale behind the design, t1/2 is reduced by almost a
factor of 2 relative to that obtained previously at 25 °C for
the VHF of DHA 7.
An Arrhenius plot based on the rate constants obtained
at six different temperatures is shown in Figure 5 (inset),
from which we obtain an activation energy for the ring
closure of 1 of 87.0 ( 0.7 kJ molꢀ1 and a preexponential
factor of 1.71 ꢁ 1011 sꢀ1
.
In summary, a new VHF thermoswitch 1 with two sites
available for ring closure was prepared from a suitable
precursor by subsequent hydride and tropylium remov-
als, while procedures based on subsequent hydride and
proton removals were unsuccessful. The two heptaful-
vene groups in the product result in an enhanced rate of
thermal ring closure in comparison to a related VHF, due
to the higher chance of accessing a reactive s-cis confor-
mation for this thermoswitch. Future work will focus on
enhancing the properties further by incorporating differ-
ent functional groups in the two seven-membered rings.
In addition, we have shown that the tropylium ring is
readily obtained by hydride abstraction from DHA,
furnishing a compound that may be used as an electro-
philic species for further reactions.
(14) 2-(Cyclohepta-2,4,6-trienylidenemethyl)-1,8a-dihydroazulenedi-
carbonitrile (11). To a solution of 2 (0.57 g, 5.40 mmol) in CH2Cl2
(125 mL), compound 3 (4.61 g, 25.9 mmol) was added and the temperature
was lowered to ꢀ78 °C. NEt3 (3.5 mL, 25 mmol) was added dropwise
over 1 h, and the mixture was stirred for 4 h while the temperature was
allowed to slowly reach rt. The mixture was diluted with Et2O (100 mL),
then washed with brine (2 ꢁ 100 mL), dried with MgSO4, filtered, and
concentrated in vacuo. The resulting dark yellow oil (compound 10) was
redissolved in DCE (200 mL), and Ph3CBF4 (4.27 g, 13.0 mmol) was
added. The mixture was stirred at 80 °C for 4 h, then washed with brine
(2 ꢁ 100 mL), dried with MgSO4, filtered, and concentrated in vacuo.
Dry column vacuum chromatography (SiO2, 12.6 cm2, 0ꢀ100% toluene/
heptanes, 10% steps, 40 mL fractions) followed by recrystallization from
boiling heptanes gave 11 (381 mg, 25%) as a yellow solid. Mp
118.2ꢀ120.6 °C. 1H NMR (500 MHz, CDCl3) δ 7.53 (d, J = 7.5 Hz,
2H), 7.41ꢀ7.37 (m, 2H), 7.36ꢀ7.31 (m, 1H), 7.17 (d, J = 16.6 Hz, 1H),
6.94 (d, J = 16.6 Hz, 1H), 6.56 (dd, J = 11.2, 6.2 Hz, 1H), 6.52 (s, 1H),
6.46 (dd, J = 11.2, 6.2Hz, 1H), 6.34ꢀ6.27 (m, 2H), 5.83 (dd, J = 10.1, 3.7
Hz, 1H), 3.73 (dt, J = 3.7, 1.9 Hz, 1H) ppm. 13C NMR (125 MHz,
CDCl3) δ 139.4, 138.9, 135.8, 135.0, 134.6, 131.1, 131.1, 129.4, 129.0,
127.8, 127.3, 121.0, 119.4, 119.3, 115.0, 112.6, 50.9, 43.9 ppm (two signals
missing due to overlap). Calcd for C20H14N2: C, 85.08; H, 5.00; N, 9.92.
Found: C, 84.90; H, 4.96; N, 9.87%.
Acknowledgment. Support from The Danish Council
for Independent Research| Natural Sciences (#10-082088)
is gratefully acknowledged.
Supporting Information Available. Experimental pro-
cedures, X-ray crystallographic data, and spectroscopic
data. This material is available free of charge via the
Org. Lett., Vol. 14, No. 1, 2012
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