Hydrolytic stability of N-methyl-2,6-dimesityl-4,4′-pyrylogen bis-tetrafluoroborate

Article abstract of DOI:10.1021/jo201180j

The synthesis and characterization of a new mesityl ring-substituted pyrylogen with a substantially decreased rate of reaction with water is reported. Computational and experimental data are presented that suggest that addition of water to the pyrylium ri

Full text of DOI:10.1021/jo201180j

ARTICLE
pubs.acs.org/joc
Hydrolytic Stability of N-Methyl-2,6-dimesityl-4,40-Pyrylogen
Bis-tetrafluoroborate
Tamer T. El-Idreesy*,§ and Edward L. Clennan*,†
§Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
†Department of Chemistry, University of Wyoming, 1000 East University Avenue, Laramie, Wyoming, United States
S Supporting Information
b
ABSTRACT: The synthesis and characterization of a new mesityl ring-
substituted pyrylogen with a substantially decreased rate of reaction
with water is reported. Computational and experimental data are
presented that suggest that addition of water to the pyrylium ring of
this highly sterically shielded pyrylogen is reversible. On the other hand,
experimental data suggest that the overall hydrolysis of this new
sterically shielded pyrylogen, but not the parent pyrylogen, is irrever-
sible. Two potential explanations for this behavior are presented and
discussed. These results provide important new information that can be
used to design and synthesize new electron transfer sensitizers that can be used even in highly aqueous environments.
’ INTRODUCTION
’ RESULTS AND DISCUSSION
In 2008 we introduced pyrylogens, PY2+, as a new type of
electron-transfer sensitzer.1ꢀ7 These new materials incorporated
a pyrylium ring that endowed them with ideal optical and
electrochemical properties and a pyridinium ring that together
ensured formation, upon reaction with a substrate (S), of a
repulsive cation-radical/cation-radical pair whose rapid separa-
tion (kSEP) competitively inhibited return electron transfer (RET
in Scheme 1).
These pyrylogens were shown to behave as electron-trans-
fer sensitzers; unfortunately, however, they also slowly
reacted with adventitious water even in scrupulously dried
acetonitrile.1 Previous studies of pyrylium cation reactions
with water suggested that the reaction was initiated by
nucleophilic attack at position 2.8ꢀ10 Consequently, we pos-
tulated that introducing mesityl (2,4,6-trimethylphenyl)
groups in positions 2 and 6 in the pyrylium ring would increase
hydrolytic stability. The o-methyl groups would force the aryl
rings to adopt a near-perpendicular geometry relative to the
pyrylium core and afford steric hindrance to nucleophilic
attack at these positions. In this paper, we describe the
synthesis and properties of this target molecule, 12+, and
preliminary results on its hydrolytic stability.
The synthesis of 12+ followed the route that was previously
utilized to make other 2,6-diaryl pyrylogens as shown in
Scheme 2.1ꢀ7 However, the steric bulk associated with the 2,4,6-
trimethylphenyl (mesityl) groups reduced the rates of both the
condensation reaction to form the 1,5-diketone and the oxidative
cyclization step and required extended reaction times and harsher
reaction conditions than used for the formation of the analogues in
which the mesityl are replaced by phenyl groups.1,6 For example, the
final oxidative cyclization step in the reaction sequence shown in
Scheme 2 required refluxing at an elevated temperature of 95 °C for
2 days to give a 57% yield of 12+. In contrast, N-allyl-2,6-diphenyl-
4,40-pyrylogen bis-tetrafluoroborate6 formed by oxidative cyclization
in 73% yield by just stirring the reaction mixture at 80 °C for 30 min.
The electrochemical and photophysical properties for 12+ are
compared to those for the parent compound, N-methyl-2,6-
diphenyl-4,40-pyrylogen bis-tetrafluoroborate, 22+, and to 2,4,6-
triphenylpyrylium cation, TPP+, in Table 1. Remarkably, despite
the substantially increased electron-donating ability of mesityl in
comparison to phenyl, the electrochemical and photophysical
behaviors are very similar for these two pyrylogens. For example,
the two one-electron reductions of 12+ to its radical cation and
neutral redox partners only differ by 40 mV from those observed
in 22+. However, counterintuitively, despite the greater electron
donating ability of the mesityl in comparison to the phenyl rings,
the reduction to the radical cation in 12+ is easier than in 22+. On
the other hand, as anticipated, the second reduction to form the
neutral redox state is more difficult in 12+ than in 22+.
The structure of 12+, its radical cation, 1•+, and neutral redox,
10 partners were explored with the B3LPY/6-31+G(d,p)
Received: June 13, 2011
Published: July 15, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/jo201180j J. Org. Chem. 2011, 76, 7175–7179
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Scheme 1. Pyrylogen Electron Transfer
Scheme 2. Synthesis of 12+
Figure 1. CAM-B3LYP/6-31+G(d,p) low energy transistion in 12+.
nucleophilic attack. In comparison to 12+ the radical cation, 1•+,
and neutral, 10, redox partners are characterized by nearly coplanar
pyrylium and pyridinium rings (>34403; 40°, 0.25°, and 0.37°),
significantly shorter pyryliumꢀpyridinium inter-ring distances
0
(d44 ; 1.486, 1.434, and 1.386 Å) but larger mesitylꢀpyrylium ring
dihedral angles (>12ab; 42°, 59°, and 64°), respectively.
The two lowest energy bands in the UVꢀvis spectrum for
TPP+ at 401 and 355 nm have been assigned to two perpendi-
cular chromophores polarized in the X and Y directions,
respectively.13 In 12+, on the other hand, TD-DFT calculations14
with the 6-31+G(d,p) basis set suggest that the two lowest energy
bands are both primarily charge-transfer bands from the mesityl
rings to the pyrylogen core. Excitation energies of charge transfer
bands have been reported to be poorly reproduced with time-
dependent density functional calculations as a result of their
incorrect long-range form of the exchange potential.15 That was
certainly the case here with the computed low energy bands 90 and
167 nm bathochromic of the observed bands at 432 and 274 nm,
respectively. (Table 1) In contrast, the Coulomb-attenuated
hybrid exchange-correlation functional (CAM-B3LYP)16 does a
much better job predicting excitation energies of 427 and 343 nm.
The major contributors to the two lowest energy absorbances in this
calculation were also primarily charge-transfer excitations from the
mesityl rings to the pyrylogen core as shown in Figure 1.
Table 1. Photophysical and Electrochemical Data for 12+,
22+, and TPP+
12+
22+
TPP+
E1/2(1)a
E1/2(2)a
λmax (ε)b
0.21
0.17
ꢀ0.35
ꢀ0.39
274 (22000)
432 (8000)
ꢀ0.35
ꢀ1.53
281 (28000)
328 (10900)
440 (20800)
533
277 (18000)
355 (32000)
401 (25000)
465
c
λF
570 (512)
563
d
λP
565
520
Hydrolysis of 12+ in dilute acetonitrile was followed by NMR
and resulted in ring cleavage to give a 4:1 mixture of two geometric
isomers of 1,5-dimesityl-3-(1-methylpyridinium-4-yl)-2-pentene-
1,5-dione, 5+-Z,E in Scheme 3. At the B3LYP/6-31G(d) level, the
Z isomer is approximately 1.9 kcal/mol more stable than the E
isomer. A reasonable/economical pathway to these 1,5-diketones is
also shown in Scheme 3. The unproductive nucleophilic addition of
water to C4 in 12+ to give 6+, which cannot lead to a hydrolysis
product, is also included in Scheme 3 because the Mulliken charge at
C4, in 12+ at both the B3LYP/6-31+G(d,p) and B3LYP/6-311
+G(2d,p) computational levels is substantially greater than at either
C2 or C6. In addition, 6+ is 4.5 kcal/mol more stable than 7+ at the
B3LYP/6-31(G) computational level. On the other hand, only 5+-Z,
E is observed by NMR in hydrolysis reaction mixtures. Conse-
quently, if 6+ is formed faster than 7+, which seems likely given
both the greater charge at C4 than at C2,6 in 12+ and the greater
stability of 6+ than 7+, it must be formed reversibly.
e
τP
22.7 ( 2
57
35.6 ( 0.2
59
225 ( 3
65g
E(S1)f
E(T1)ϕ
56
54
53g
a In volts versus SCE. b Nanometers (L molꢀ1cmꢀ1). In nanometers
(nm) at 298 K (77 K in EtOH/HCl(g). d In nm at 77 K in EtOH/
HCl(g). e In milliseconds (ms) at 77 K in EtOH/HCl(g). f In kcal/mol.
g See ref 11.
c
computational model.12 The interpyrylium-pyridinium ring dihe-
dral angle (>344030) is very similar for 12+ (40°) and 22+ (43°) and
rather insensitive to the computational level (12+ dihedral angle
41° at B3LYP/6-31G(d) and 40° at B3LYP/6-311+G(2d,p)). In
contrast, the intermesitylꢀpyrylium dihedral angle in 12+ is
significantly larger (43°) than the interphenylꢀpyrylium ring dihedral
angle (16°) in 22+. In addition, examination of space-filling models
indicate that the o-methyls in the mesityl rings significantly shield and
should protect the 2,6-positions on the pyrylium ring from
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Scheme 3. Mechanism of Hydrolysis of 12+
The hydrolysis of 12+ can be accurately followed at pH’s up to
4.5 at 432 nm. At pH 4.5 12+ and 2,4,6-triphenylpyrylium
tetrafluoroborate, TPP+ disappear at comparable rates despite
the greater positive charge on the pyrylogen. The relative hydro-
lytic stability of 12+ and 22+ are compared at pH 2.5 in Figure 2.
The reaction of 22+ is clearly too fast even at pH 2.5 to be
followed by conventional UVꢀvis spectrometry. Nevertheless, a
qualitative analysis of the data reveals two interesting observa-
tions: (1) the mesityl pyrylogen, 12+, disappears approximately
20 time more slowly than the parent pyrylogen, 22+, and (2) the
mesityl pyrylogen, 12+ but not the parent pyrylogen, 22+, is
completely consumed at long reaction times. This suggests the
intriguing possibility that the hydrolysis of 22+ but not mesityl
pyrylogen, 12+, is reversible. The reversibility of the hydrolysis of
simple pyrylium cations have been noted previously.1ꢀ7 The
irreversible hydrolysis of 12+ must be due to the inability of 5+ to
close to regenerate 7+ since as argued above the addition of water
to generate 7+ from 12+ is reversible. There are two possible
reasons for the reluctance of 5+ to cyclize; (1) the population of
5+E, the isomer needed for closure, is too low, or (2) the
increased size of the mesityl ring in comparison to the phenyl
ring sterically destabilizes the transition state for the formation of
7+. Possibility 1 can be ruled out by comparing the ZꢀE energy
difference in 5+ to the ZꢀE energy difference in 5+ in which the
mesityl rings have been replaced by phenyl rings. At the B3LYP/
6-31G(d) computational level the replacement of mesityl by
phenyl actually decreases the population of the “closure-ready”
E-isomer17 despite the fact that hydrolysis of phenyl-substituted
pyrylogen, 22+, is far more reversible than the hydrolysis of 12+.
Parenthetically, this result also suggests that it is likely that the
slow rate of cyclization of the diketone precursor rather than a
steric impediment to hydride abstraction plays a dominant role in
dramatically slowing down the rate of formation of 12+ (vide
supra) in comparison to the rate of formation of 22+ (Scheme 2).
The mesityl substituted pyrylogen, 12+, can also be reduced
with zinc dust to the radical cation with a λmax at approximately
570 nm. For comparison, N-benzyl-2,6-diphenyl-4,40-pyrylogen
radical cation absorbs at 619 nm7 and the radical cation of 22+ at
Figure 2. UVꢀvis hydrolysis data for 12+ (top) and 22+ (bottom).
Figure 3. B3LYP/6-31+G(d,p) molecular orbitals involved in the
transition responsible for the lowest energy absoeption in the radical
cations of 12+ and 22+.
614 nm. The molecular orbitals primarily involved in this low
energy transition for the radical cations of 12+ and 22+ are shown
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in Figure 3. They look topographically similar for the two
pyrylogen radical cations, but both MOs 110 and 111 in the
mesityl-substituted compound are more localized on the pyrylo-
gen core. The hypsochromic shift in the mesityl in comparison to
the phenyl pyrylogen radical cation is reproduced by both TD-
DFT and CAM calculations. (With the B3LYP/6-31+G(d,p)
geometries: TD-DFT 1•+ 546 nm and 2•+ 596 nm; CAM-B3LYP
(dd, J = 6.0, 18.8 Hz, 2H, CH2), 4.08 (tt, J = 6.0, 7.6, Hz, 1H, CH), 6.82
(s, 4H, mesityl CH), 7.28ꢀ7.30 (m, 2H, pyridinium CH), 8.53ꢀ8.55
(m, 2H, pyridinium CH). 13C NMR (CDCl3): δ = 18.9, 21.0, 34.6, 49.6,
123.5, 128.6, 132.6, 138.7, 149.9, 152.8, 207.7.
Synthesis of N-3-(1-Methylpyridinium-4-yl)-1,5-dimesityl-
pentane-1,5-dione Tetrafluoroborate, 4+. Trimethyloxonium
tetrafluoroborate (1.82 g, 12.30 mmol) was added to a solution of 1,5-
dimesityl-3-(4-pyridyl)pentane-1,5-dione (4.52 g, 10.94 mmol) in di-
chloromethane/acetonitrile mixture, and the mixture was stirred at
room temperature for 24 h. The solvent was removed under reduced
pressure, and then the residue was triturated with ether to give the
product as yellowish-orange solid which can be used without further
purification (5.60 g, 10.87 mmol, 99%). IR (KBr; cmꢀ1): 3276, 3060,
1
•+ 540 nm and 2•+ 579 nm.) This result is consistent with the
anticipated increase in the MO energies of the aromatic ring by
the inductively donating methyl groups. In fact, in the case of the
phenyl-substituted pyrylogen radical cation the transition 86 f
87 appears to be associated with a modest amount of charge
transfer from the pyrylogen core out to the phenyl rings. This
does not occur in the mesityl-substituted radical cation presum-
ably because of the greater twist out of the plane of the pyrylogen
core of the mesityl in comparison to the phenyl rings. The slightly
greater predicted hypsochromic shift as the computation method
is changed from TD-DFT to CAM for 2•+ (596 nm f579 nm)
than for 1•+ (546 nm f540 nm) might also be indicative of the
greater charge transfer character of the transition in the phenyl-
substituted pyrylogen radical cation.
1
2953, 2925, 2261, 1703, 1643, 1609, 1449, 1060, 985, 858. H NMR
(CD3CN): δ 2.06 (s, 12H, 4 CH3), 2.27 (s, 6H, 2 CH3), 3.33 (m, 4 H, 2
CH2), 4.22 (m, 1H, CH), 4.25 (s, 3H, CH3), 6.88 (s, 4H, mesityl CH),
8.11 (d, J = 6.8 Hz, 2H, pyridinium CH), 8.56 (d, J = 6.8 Hz, 2H,
pyridinium CH). 13C NMR (CD3CN): δ = 18.1, 20.2, 35.5, 47.7, 48.6,
127.7, 128.4, 132.6, 138.3, 138.9, 144.6, 164.8, 207.7.
Synthesis of N-Methyl-2,6-dimesityl-4,40-pyrylogen Biste-
trafluoroborate, 12+. A solution of tetrafluoroboric acid (11.6 g of
48% solution in H2O, 63.41 mmol) in 10 mL acetic anhydride was gently
added under a nitrogen atmosphere to a mixture of N-3-(1-methylpyr-
idium-4-yl)-1,5-dimesitylpentane-1,5-dione tetrafluoroborate (2.0 g,
3.88 mmol) and triphenylmethanol (8.3 g, 31.92 mmol) in 40 mL of
an acetic anhydride/acetic acid mixture and then heated in oil bath at
95 °C for 2 days. The reaction mixture was cooled to room temperature,
and then diethyl ether was added to precipitate the product. The solid
was recrystallized from acetonitrile/acetic acid to give the pure product
as yellowish-brown solid (1.30 g, 2.23 mmol, 57%). IR (KBr; cmꢀ1):
’ CONCLUSION
In summary, we have successfully synthesized a new pyrylogen
with a substantially reduced rate of reaction with water. The
reactivity of this pyrylogen with water is approximately equal to
that of TPP+, suggesting that it can be used successfully under
many of the conditions developed for TPP+ reactions. On the
other hand, we believe that many of the reactions with TPP+
reported in water rely on the substantial amount of pyrylium cation
that is in equilibrium with the diketone hydrolysis product.18
Unfortunately, in the case of 12+ ring-opening is irreversible which
will restrict the use of this pyrylogen in highly aqueous environ-
ments. Future studies are focused on increasing the water resis-
tance of pyrylogens without simultaneously preventing ring
closure of the hydrolysis product. Of course, the ideal pyrylogen,
which will be difficult to design and synthesize, will not react at all
with water and will open up the possibility of using dicationic
sensitizers, and the advantages they offer, in this valuable envir-
onmentally green solvent.
1
3068, 2975, 1626, 1609, 1515, 1449, 1190, 1065, 849, 768. H NMR
(TFA-CDCl3, 1:2): δ 2.34 (s, 12H, 4 CH3), 2.44 (s, 6H, 2 CH3), 4.58
(s, 3H, CH3), 7.17 (s, 4H, mesityl CH), 8.54 (s, 2H, pyrylium CH), 8.63
(d, J = 6.4 Hz, 2H, pyridinium CH), 9.01 (d, J = 6.4 Hz, 2H, pyridinium
CH). 13C NMR (TFA-CDCl3, 1:2): δ 19.6, 20.7, 48.8, 123.3, 125.5,
127.7, 130.2, 138.2, 145.7, 146.6, 148.5, 160.2, 179.9. Mass spectrometry
(ESI): M•+ = 409.33 (calculated 409.24)
Electrochemical Reduction of Pyrylogen (Cyclic Voltammetry).
An acetonitrile solution 2 ꢁ 10ꢀ3 M in pyrylogen, 0.1 M in
Bu4N+ClO4ꢀ, and 2 ꢁ 10ꢀ3 M in ferrocene as an internal standard
was placed in a five-neck electrochemical flask containing a glassy carbon
electrode, silver wire reference electrode, and a platinum wire electrode.
The cyclic voltammogram was obtained for both the deoxygenated and
oxygen-saturated solutions. Deoxygenation of the solution was achieved
by bubbling argon for 20 min while the oxygen saturated solution was
obtained by bubbling oxygen in the solution for 20 min prior to the
electrochemical experiment. In each case, 100 mV and 500 mV were
used as scan rates.
Chemical Reduction of Mesityl Pyrylogen (Reduction with
Zn Metal): Synthesis of N-Methyl-2,6-dimesityl-4,40-pyrylo-
gen Tetrafluoroborate Radical Cation, 1+. A 3 mL acetonitrile
solution containing 8 ꢁ 10ꢀ5 M of the pyrylogen dication was placed
into a side arm attached to a quartz cuvette. This solution was then
subjected to three freezeꢀpumpꢀthaw cycles and then tilted in order to
pour the solution into the cuvette containing 200 mg (3.06 mmol) of
zinc dust. The UVꢀvis spectrum was recorded at different time intervals.
The solution turned faint violet upon mixing with zinc and the color
continued to intensify to reach its maximum after about 2 min then faded
until it became colorless after 8 min. The violet color is due to the
formation of the pyrylogen radical cation having an absorption max-
imum around 572 nm and the fading due to its conversion to the neutral
species with an absorption maximum at 382 nm. At the end of the
reaction (8 min and longer), the concentration of the neutral pyrylogen
reaches a steady state and the solution remained colorless.
’ EXPERIMENTAL SECTION
Proton and carbon NMR were obtained on a 400 MHz NMR and are
referenced to TMS. Pyridine-4-carboxaldehyde (97%), triphenylmetha-
nol (97%), trimethyloxonium tetrafluoroborate, tetrafluoroboric acid
(48% by weight in water), 20,40,60-trimethylacetophenone, ferrocene,
trifluoroacetic acid, and tetrabutyl ammonium perchlorate were used
without further purification. HPLC grade acetonitrile (99.9%) was
distilled and stored over activated 3 Å molecular sieves.
Synthesis of 1,5-Dimesityl-3-(4-pyridyl)pentane-1,5-dione,
3. 20,40,60-Trimethylacetophenone (18.96 g; 117.04 mmol) was added to
2.48 g (44.29 mmol) of potassium hydroxide in dilute ethanol followed by
4.18 mL (44.42 mmol) of pyridine-4-carboxaldehyde. The mixture was
refluxed for 45 h, cooled, poured on water, and extracted with chloroform.
The combined organic phase was washed with brine and dried over
anhydrous magnesium sulfate, and the solvent was removed under
reduced pressure. Crystallization of the crude residue from dilute ethanol
afforded the pure product as a yellowish-white solid (12.60 g, 30.51 mmol,
69%). IR (KBr; cmꢀ1): 3382, 3069, 3024, 2908, 2734, 1934, 1700, 1610, 1597,
1408, 1364, 988, 985, 853, 812, m.p. 114ꢀ116. 1H NMR (CDCl3): δ 2.05 (s,
12H, 4 CH3), 2.28 (s, 6H, 2 CH3), 3.11 (dd, J = 7.6, 18.8 Hz, 2H, CH2), 3.26
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’ HYDROLYSIS
(4) Clennan, E. L.; Warrier, A. K. S. J. Sulfur Chem. 2009, 30, 212.
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(7) El-Idreesy, T. T.; Clennan, E. L. Photochem. Photobiol. Sci. 2010,
Hydrolysis of the mesityl pyrylogen was carried out in dilute
acetonitrile and was found to give a 1:4 mixture of two geometrical
isomers. The identity of the products wereconfirmed by NMR and
9, 796.
1
mass spectrometry. H NMR (CD3CN, major isomer): δ 2.21
(8) Berson, J. A. J. Am. Chem. Soc. 1952, 74, 358.
(s, 6H, 2 CH3), 2.24 (s, 6H, 2 CH3), 2.29 (s, 3H, CH3), 2.30
(s, 3H, CH3), 4.31 (s, 3H, CH3), 4.84 (s, 2H, CH2), 6.92 (s, 2H,
aromatic CH), 6.95 (s, 2H, aromatic CH), 7.04 (s, 1H, =CH), 8.11
(d, J = 6.8 Hz, 2H, aromatic CH), 8.66 (d, J = 6.8 Hz, 2H, aromatic
(9) Williams, A. J. Am. Chem. Soc. 1971, 93, 2733.
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V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,
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1
CH). H NMR (CD3CN, minor isomer visible signals): δ 2.16
(s, 3H, CH3), 2.19 (s, 6H, 2 CH3), 2.27 (s, 6H, 2 CH3), 4.10
(d, J = 0.8 Hz, 2H, CH2), 4.28 (s, 3H, CH3), 7.84 (d, J = 6.8 Hz,
2H, aromatic CH), 8.51 (d, J = 6.8 Hz, 2H, aromatic CH). Mass
spectrometry (EI): M•+ 426.33 (calculated 426.24)
’ KINETICS OF THE HYDROLYSIS REACTION
The hydrolysis of mesityl pyrylogen, 12+, phenyl pyrylogen, 22+,
and the TPP+ was conducted in various phosphate buffer solutions
at different pH values (pH = 2.5, 3, 4.5, and 6.5). Identical
concentrations of each compound in acetonitrile were prepared,
and identical volumes of each solution was added to the buffers.
The hydrolysisreaction was followed by recordingUVꢀvis spectra
at different time intervals. The change in absorbance was mon-
itored at 432, 442, and 404 nm for 12+, 22+, and TPP+, respectively.
These wavelengths were chosen because they represent the long
wavelength maximum for each compound and are devoid of
interfering absorbance from the hydrolysis products. The pseudo-
first-order rate constant for hydrolysis of 12+ at pH 2.5 under these
carefully controlled conditions was 0.042 ( 0.001 minꢀ1. The
phenyl pyrylogen, 22+, reacted too fast to allow determination
of a rate constant over the preferred 3-half-lives using the conven-
tional UVꢀvis method, however, it is clear that it disappeared
approximately 20 time more rapidly than the mesityl-substituted
pyrylogen, 12+.
(17) The E isomer of 5+ and its phenyl analogue exist in two
rotomers that differ primarily by rotation around the vinylꢀCH2 bond.
The rotomer with a CꢀH bond eclipsed with the double bond is the
most stable by approximately 0.96 kcal/mol. The Z isomer of 5+ is 1.9
kcal/mol more stable than the most stable E isomer at the B3LYP/6-
31G(d) computational level. On the other hand in the phenyl analogue
of 5+ the Z-isomer is actually 2.2 kcal/mol more stable than the most
stable E-isomer. The phenyl analogue, 5+-Ph, behaves very similar to 5+.
The E isomer of 5+-Ph exists in two rotomers that differ primarily by
rotation around the vinylꢀCH2 bond. The rotomer with a CꢀH bond
nearly eclipsed with the double bond is the most stable by 0.29 kcal/mol.
(18) Gavi~na, P.; Lavernia, N. L.; Mestres, R.; Miranda, M. A.
Tetrahedron 1996, 4911.
’ ASSOCIATED CONTENT
S
Supporting Information. Computational details; UVꢀvis,
b
fluorescence, UVꢀvis for reduction, and cyclic voltammogram for
12+; UVꢀvis spectra for hydrolysis reactions of 12+ and 22+; proton
and carbon NMR. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: (T.T.E.) tamertawhid@yahoo.com; (E.L.C.) clennane@
uwyo.edu.
’ ACKNOWLEDGMENT
We thank the USA National Science Foundation (ELC) and
the Bibliotheca Alexandrina-Center for Special Studies and
Programs (BA/CSSP) under Grant No. BA2007/2008 (TTE)
for their generous support of this research.
’ REFERENCES
(1) Clennan, E. L.; Liao, C.; Ayokosok, E. J. Am. Chem. Soc. 2008,
130, 7552.
(2) Warrier, A. K. S.; Clennan, E. L. ARKIVOC 2009, 8, 95.
(3) Clennan, E. L.; Warrier, A. K. S. Org. Lett. 2009, 11, 685.
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