One-pot access to push-pull oligoenes by sequential [2 + 2] cycloaddition-retroelectrocyclization reactions

Article abstract of DOI:10.1021/jo402440m

The formal [2 + 2] cycloaddition-retroelectrocyclization reaction was employed as the key transformation to obtain donor-substituted, π-conjugated polycyanohexa-1,3,5-trienes (TCHTs and PCHTs) and polycyanoocta-1,3,5,7- tetraenes from donor-substituted te

Full text of DOI:10.1021/jo402440m

Note
pubs.acs.org/joc
One-Pot Access to Push−Pull Oligoenes by Sequential [2 + 2]
Cycloaddition−Retroelectrocyclization Reactions
Govindasamy Jayamurugan,† Aaron D. Finke,† Jean-Paul Gisselbrecht,‡ Corinne Boudon,‡
W. Bernd Schweizer,† and Francois Diederich*,†
̧
†Laboratorium fur Organische Chemie, ETH-Zurich , Honggerberg, HCI, CH-8093 Zurich, Switzerland
̈
̈
̈
̈
‡Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Institut de Chimie−UMR 7177, C.N.R.S., Universite
́
de
Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
S
* Supporting Information
ABSTRACT: The formal [2 + 2] cycloaddition−retroelectrocyclization
reaction was employed as the key transformation to obtain donor-
substituted, π-conjugated polycyanohexa-1,3,5-trienes (TCHTs and
PCHTs) and polycyanoocta-1,3,5,7-tetraenes from donor-substituted
tetracyanobuta-1,3-dienes (TCBDs) and electron-rich alkynes. These
push−pull-substituted oligoene chromophores were also accessed in
good yield from tetracyanoethylene and donor-substituted alkynes by
using a one-pot protocol. All bis-(N,N-dialkylanilino) donor-substituted
push−pull trienes and tetraenes showed better electron-accepting
potency and lower HOMO−LUMO gaps than the corresponding
TCBDs, as evidenced by optical and electrochemical studies.
ligoenes are fully conjugated fragments of polyacetylene
their chemistry is not limited to CA−RE-type transformations.9
O
and constitute some of the simplest “molecular wires”.1
Herein, we demonstrate that the terminal dicyanovinyl moiety
of donor-substituted TCBDs reacts cleanly in the CA−RE
reaction with donor-substituted alkynes to give cyano-rich hexa-
1,3,5-trienes and octa-1,3,5,7-tetraenes. We also show that such
oligoenes can be directly synthesized from TCNE in a one-pot,
multistep CA−RE reaction in good yield.
Despite their presence in natural products2 and potential use in
organic electronic applications,3 the study of oligoenes is
hampered by a number of issues, including product instability
and low solubility. One recent strategy to overcome these
problems was reported by Nuckolls and co-workers, who
showed that decoration of polyenes with cyano groups leads to
lowered HOMO energies and greater chemical stability with
respect to oxidative decomposition.4
Since 2005, expanding on the earlier work by Bruce and
others,5 we have developed a new class of chromophores:
donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs).6
These chromophores are easily synthesized by a click-type
formal [2 + 2] cycloaddition−retroelectrocyclization (CA−RE)
of donor-substituted alkynes and tetracyanoethylene (TCNE).
Donor-substituted TCBDs are nonplanar and thermally robust;
they feature facile electrochemical reduction, strong charge-
transfer bands in the vis/near-IR (NIR) region, and possess
high third-order optical nonlinearities. A TCBD derivative has
been successfully integrated into silicon−organic hybrid slot
waveguides.7 However, the chemistry of TCBDs themselves has
been relatively unexplored. We recently reported that the
anilino group of donor-substituted TCBDs can be synthetically
elaborated to form new molecular entities.8 To date, there have
only been a few efforts to explore the chemistry of the acceptor
unit of TCBDs. Previous work in our group found that donor-
substituted pentacyanobutadienes, prepared by a CA−RE
reaction between a donor-substituted cyanoalkyne and
TCNE, are reactive with donor-substituted alkynes; however,
First, we tested the CA−RE reactivity of donor-substituted
TCBDs6b 1a and 1b with donor-substituted alkynes 2a−2c
(Table 1). We reasoned that the dicyanovinyl moiety in direct
conjugation with the anilino donor moiety would be less
electrophilic and less reactive due to steric encumbrance.9a,10
Indeed, TCBDs 1a and 1b both reacted cleanly with alkynes 2a
and 2b at room temperature in CHCl3 to give 1,1,6,6-
tetracyano-1,3,5-hexatrienes (TCHTs) 3a−3c in good yield
and exclusively in form of the 3E-isomer. We found that less
electron-rich alkynes, such as 1-ethynyl-4-methoxybenzene and
3-[4-(dimethylamino)phenyl]propiolonitrile, do not undergo a
CA−RE reaction with 1a. However, the synthesis of 1,1,3,6,6-
pentacyano-1,3,5-hexatrienes (PCHTs) 4 could be achieved by
increasing the electron richness of the alkyne upon replacement
of the N,N-dimethylamino with a pyrrolidinyl group.11 Thus,
pyrrolidinyl-substituted cyanoalkyne 2c reacted with TCBD 1a,
but only at high temperatures (120 °C in 1,1,2,2-tetrachloro-
ethane), to give a mixture of PCHT isomers Z-4a and E-4b in
low yield.12
Received: November 2, 2013
Published: December 4, 2013
© 2013 American Chemical Society
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Note
Table 1. Synthesis of 1,1,6,6-Tetracyano-1,3,5-hexatrienes
(TCHTs) 3 and 4 from TCBDs 1 with Electron-Rich
Alkynes 2
Scheme 1. Synthesis of 1,1,8,8-Tetracyano-1,3,5,7-tetraenes
Crystals suitable for X-ray analysis of 3a, 3c, 3d, 3e, Z-4a, E-
4b, and (3Z,5E)-5 were prepared (Section 1SI, Supporting
Information) to confirm the configuration assigned to each
adduct. The central double bond of 3 exclusively has an E-
configuration when it is unsubstituted, as in 3a, 3c, 3d, and 3e.
By contrast, when the central double bond is substituted, as in
hexatrienes Z-4a, E-4b, or in the case of one of the two central
double bonds in (3Z,5E)-5, a mixture of E- and Z-isomers is
formed (Figure 1). The mixture of PCOT isomers 5 is also
stable in solution, and no coalescence in the NMR spectra (500
MHz, 1,1,2,2-tetrachloroethane-d2) occurs when heating up to
100 °C, indicating a high barrier of isomerization. The high
barrier results from the high nonplanarity of the push−pull
skeleton (Figure 1d), which strongly reduces the π-conjugation
along the pentacyanotetraene backbone and raises the
transition state for E/Z isomerization.13
Once we established that TCBDs were reactive in the CA−
RE reaction, we envisaged that a reaction between
anilinoalkynes and TCNE could form TCHTs in one pot via
in situ formation of the TCBD, given an initial stoichiometry of
1 equiv of TCNE to 2 equiv of alkyne (Table 2). Mixing 2
Table 2. One-Pot Synthesis of 1,1,6,6-Tetracyano-1,3,5-
hexatrienes (TCHTs)
The quinoidal character14,15 δr of the donor rings of 3a, 3c,
3d, Z-4a, and E-4b was determined from the X-ray bond
lengths (see Section 2SI, Supporting Information). The
quinoidal character of the aniline rings is moderate, with δr
values ranging from 0.025 to 0.052. By contrast, the quinoidal
character of the methoxyphenyl ring in 3c is extremely low
(0.004), indicative of the lower donor strength of the methoxy
group. The bond lengths and bond length alternations of the
1,3,5-triene and 1,3,5,7-tetraene moieties are typical for
oligoenes.
The four 3E-configured TCHTs 3a, 3c, 3d, and 3e all adopt
a similar conformation in the solid state. The triene moieties
are almost, but not fully, planar; the torsion angles between
each pair of neighboring alkene bonds range from 15° to 17°.
In contrast, the PCHT isomers Z-4a and E-4b adopt drastically
different conformations in the solid state. The internal
cyanoethene-1,2-diyl moiety in Z-4a is almost orthogonal to
the external dicyanovinyl groups (θ = 103−106°), in order to
minimize the torsion (and thus maximize the conjugation
efficiency) between the anilino rings and the dicyanovinyl
groups. In contrast to Z-4a, which adopts an elongated shape,
E-4b is almost folded onto itself; again, the nearly orthogonal
orientation of the central double bond acts to maximize the π-
conjugation between the anilino rings and dicyanovinyl groups.
The spectral and electrochemical data for all new compounds
were collected. The UV/vis spectra demonstrate the effect of
adding additional alkene spacers between the dicyanovinyl
entry
R
conditions
product (yield)
3a (80%)
1
2
3
NMe2 (2a)
NBu2 (2d)
OMe (2e)
CHCl3, 50 °C, 36 h
CHCl3, 50 °C, 36 h
(CHCl2)2, 120 °C, 60 h
3d (85%)
3e (30%) + 1b (60%)
equiv of alkynes 2a and 2d with 1 equiv of TCNE gave TCHTs
3a and 3d, respectively, in good yield. Consequent to its
decreased electron-donation ability, methoxy-substituted alkyne
2e was less reactive, requiring higher temperatures and only
formed TCHT 3e in low yield, along with a significant amount
(60%) of TCBD 1b. Again, in all cases, only the 3E-isomer was
obtained.
We previously reported the preparation of PCHT 7 via a
multistep sequence starting from the reaction of 2f with TCNE
to form 6, followed by a CA−RE reaction of 6 with 2a (Scheme
1) to give 7 as a 1:1 mixture of 3E/3Z isomers.9 Because 7 has a
nondonor-conjugated terminal dicyanovinyl group suitable for
further elaboration via CA−RE chemistry, we reacted TCNE
first with 1 equiv of 2f, followed by 2 equiv of 2a, in a one-pot
procedure to give pentacyanoocta-1,3,5,7-tetraene (PCOT) 5
in good yield. Adduct 5 was isolated as a mixture of 3Z,5E/
1
3E5E isomers in a ratio of 71:29 as measured by H NMR;
unfortunately, despite multiple attempts by chromatographic
methods, the isomers could not be separated.
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Note
Figure 2. UV/vis absorption spectra of compounds (a) 3a−3e, and
(b) Z-4a, E-4b, and 5 in CH2Cl2 at 298 K; c ∼ 10−5 M.
The necessity of the anilino donor for ICT was confirmed by
monitoring the UV/vis spectra upon protonation with
CF3COOH and subsequent neutralization with NEt3 (Section
4SI, Supporting Information). TCHT derivatives 3a−3d all
display reversible loss of ICT bands upon protonation with
CF3COOH; the ICT band returns quantitatively upon
neutralization with NEt3. The UV/vis spectrum of methoxy-
substituted TCHT 3e is unaffected by protonation or
neutralization. PCHTs Z-4a and E-4b also react with
CF3COOH to generate much simpler spectra compared to
those of the parent compounds, with λmax = 460 nm and λmax
=
470 nm for Z-4a and E-4b, respectively; however, neutralization
with NEt3 does not regenerate the original spectra, indicating
that additional chemistry occurs in the process. PCOT
derivative 5, in contrast to the PCHTs, displays reversible
ICT band disappearance upon protonation and subsequent
neutralization.
The electrochemistry of the new oligoene derivatives was
measured in CH2Cl2 + 0.1 M n-Bu4NPF6 (Section 5SI,
Supporting Information; all redox potentials described herein
are reported versus Fc/Fc+ = 0 V). The TCHT and PCHT
derivatives all display two well-resolved, reversible one-electron
reductions at potentials more positive than that of the TCBD
derivative 8 (2,3-bis[4-(dimethylamino)phenyl]buta-1,3-diene-
1,1,4,4-tetracarbonitrile)6a by ca. 130 mV. This indicates that
the additional alkene spacer facilitates reduction of the TCHT.
The weaker methoxy donor groups in 3c and 3e lead to more
positive reduction potentials than those of the anilino-
substituted TCHTs. PCHTs Z-4a and E-4b display first
reduction potentials at −0.73 and −0.71 V, respectively,
which are more positive than those of the anilino-TCHTs by
Figure 1. ORTEP of (a) 3a, (b) Z-4a, (c) E-4b, and (d) (3Z,5E)-5. T
= 100 K. Ellipsoids at 50% probability.
termini (Figure 2). Anilino-substituted TCHTs 3a−3d have
similar UV/vis spectra, featuring two major peaks at λmax = 350
nm and λmax = 560 nm. The latter peak corresponds to an
intramolecular charge-transfer (ICT) band with extinction
coefficients in the range of similar push−pull systems (log ε =
4−4.5). TCHT 3e has a less pronounced transition at λmax
=
420 nm due to its weaker methoxy donor moiety. PCHTs Z-4a
and E-4b and PCOT 5 have more complex UV/vis spectra
(Figure 2b), but the ICT band energies are relatively
unaffected.
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The Journal of Organic Chemistry
Note
2 H; H−C(3,5)), 7.43 ppm (d, J = 9.0 Hz, 2 H; H−C(2,6)); 13C
NMR (100 MHz, CDCl3) δ = 25.6 (C(3′,4′)), 47.7 (C(2′,5′)), 62.4
(CC−CN), 86.7 (CC−CN), 102.0 (C(1)), 106.9 (CN), 111.7
about 200 mV, on account of the additional electron-accepting
cyano group. Interestingly, the presence of one extra alkene
spacer in PCOT 5 does not lead to an expected facilitation of
the first reduction compared to the PCHTs; instead, a two-
electron reduction at even more negative potential (−0.86 V)
was observed. This again is a consequence of the profound
nonplanarity of the push−pull-substituted tetraene. In this case,
the polyene appears to stabilize the second reduction. In fact,
this trend holds true for the whole series described in this
study: the potential difference between the first and second
reduction potentials of the TCHTs is ca. 300 mV; for the
PCHTs, the potential difference is 180 mV; and the two
reductions for the PCOT occur simultaneously. Thus,
increasing the polyene length acts to stabilize the second
reduction rather than the first.
In conclusion, the chemical reactivity of the acceptor
dicyanovinyl moieties of TCBDs was elaborated in terms of
their participation in CA−RE reactions with donor-substituted
alkynes. We prepared a series of donor-substituted, cyano-rich
trienes and tetraenes via a one-pot reaction between TCNE and
donor-substituted alkynes. These new chromophores feature
facilitated reductions compared to their TCBD counterparts
and strong intramolecular charge-transfer bands and are easily
accessible from simple starting materials. The utility of these
polyene chromophores in nonlinear optical applications will be
investigated.
̃
(C(3,5)), 135.3 (C(2,6)), 149.7 ppm (C(4)); IR (ATR) ν = 2956 (w),
2914 (w), 2858 (m), 2232 (m), 2211 (m), 2123 (m), 1592 (s), 1523
(m), 1481 (m), 1458 (m), 1399 (m), 1350 (m), 1287 (m), 1180 (s),
1159 (m), 1115 (w), 959 (m), 820 cm−1 (s); HR-EI-MS m/z (%)
+
196.0988 (100, [M]+, calcd for C13H12N2 : 196.1000), 195.0916 (99,
[M − H]+), 140.0495 (53), 126.0339 (27), 68.9947 (21).
(E)-2,5-Bis[4-(dimethylamino)phenyl]hexa-1,3,5-triene-
1,1,6,6-tetracarbonitrile (3a). A solution of 2a (105 mg, 0.72
mmol) and 1a6b (220 mg, 0.8 mmol) in CHCl3 (30 mL) was stirred at
25 °C for 24 h and evaporated. FC (SiO2; CH2Cl2) gave 3a (270 mg,
88%) as a purple solid. Rf = 0.24 (SiO2; CH2Cl2); mp 312−313 °C; 1H
NMR (400 MHz, CDCl3) δ = 3.12 (s, 12 H; NMe2), 6.76 (d, J = 9.1
Hz, 4 H; H−C(3′,5′)), 7.31 (s, 2 H; H−C(3)), 7.47 ppm (d, J = 9.1
Hz, 4 H; H−C(2′,6′)); 13C NMR (100 MHz, CDCl3) δ = 40.2
(NMe2), 80.1 (C(1)), 111.8 (C(3′,5′)), 113.8 (CN), 114.7 (CN),
118.6 (C(1′)), 132.3 (C(2′,6′)), 141.8 (C(3)), 153.7 (C(4′)), 167.2
̃
ppm (C(2)); IR (ATR) ν = 2957 (w), 2923 (w), 2851 (w), 2217 (m),
1598 (s), 1539 (w), 1500 (s), 1435 (w), 1379 (m), 1341 (m), 1285
(m), 1233 (w), 1208 (m), 1191 (w), 1174 (w), 1105 (w), 1063 (w),
985 (m), 943 (m), 825 (s), 802 (w), 751 (w), 739 (w), 693 cm−1 (w);
HR-MALDI-MS m/z (%) 441.1793 (75, [M + Na]+, calcd for
C26H22N6+Na+: 441.1804), 419.1976 (74, [M + H]+, calcd for
C26H23N6 : 419.1979), 235.0713 (100).
(E)-2-[4-(Dimethylamino)phenyl]-5-[4-(pyrrolidin-1-yl)-
phenyl]hexa-1,3,5-triene-1,1,6,6-tetracarbonitrile (3b). A solu-
tion of 2b (55 mg, 0.32 mmol) and 1a (97 mg, 0.35 mmol) in CHCl3
(30 mL) was stirred at 25 °C for 24 h and evaporated. FC (SiO2;
CH2Cl2) gave 3b (130 mg, 90%) as a black solid. Rf = 0.25 (SiO2;
EXPERIMENTAL SECTION
■
1
CH2Cl2); mp 280−281 °C; H NMR (400 MHz, CD2Cl2) δ = 2.06
General. Reagents and compounds 2a and 2e were purchased and
used as received. Flash column chromatography (FC) was carried out
with SiO2 60 (particle size 0.040−0.063 mm, 230−400 mesh) and
technical solvents. Thin-layer chromatography (TLC) was conducted
on aluminum sheets coated with SiO2 60 F254; visualization was with a
UV lamp (254 nm). The syntheses of 1a and 1b,6b 2b,16 2d,17 6,18 7,9
and 3-[4-(N,N-dimethylamino)phenyl]propynenitrile18 were per-
formed according to the literature. Melting points (m.p.) were
(m, 4 H; H2C(3,4) of pyrrolidin-1-yl), 3.11 (s, 6 H; NMe2), 3.42 (m, 4
H; H2C(2,5) of pyrrolidin-1-yl), 6.68 (d, J = 9.0 Hz, 2 H; H−
C(2″,6″)), 6.81 (d, J = 9.1 Hz, 2 H; H−C(2′,6′)), 7.25 (d, J = 15.2 Hz,
1 H; H−C(2)), 7.30 (d, J = 15.2 Hz, 1 H; H−C(3)), 7.48 and 7.49
ppm (2 d, J = 9.1 Hz, 4 H; H−C(2′,6′,2″,6″)); 13C NMR (100 MHz,
CD2Cl2) δ = 26.0 (C(3,4) of pyrrolidin-1-yl), 40.5 (NMe2), 48.4
(C(2,5) of pyrrolidin-1-yl), 79.7 and 80.8 (C(3,4)), 112.07 and 112.27
(C(3′,5′,3″,5″)), 114.24 and 114.48 (C(1)(CN)2), 115.18 and 115.43
(C(6)(CN)2), 118.70 and 119.18 (C(1′,1″)), 132.63 and 132.87
(C(2′,6′,2″,6″)), 141.93 and 142.43 (C(3,4)), 151.8 (C(4″)), 154.1
1
measured in open capillaries, and reported values are uncorrected. H
NMR and 13C NMR spectra were measured at 20 °C. Chemical shifts
are reported in parts per million (ppm) relative to the signal of
tetramethylsilane (δ = 0 ppm). Residual solvent signals in the H and
1
̃
(C(4′)), 167.6 and 167.8 ppm (C(1,6)); IR (ATR) ν = 2957 (w),
13C NMR spectra were used as an internal reference. Coupling
constants (J) are given in hertz. The apparent resonance multiplicity is
described as s (singlet), d (doublet), t (triplet), m (multiplet), and br.
(broad). NMR peaks were assigned by 2D COSY, NOESY, HSQC,
and HMBC experiments. Infrared spectra (IR) were recorded on an
FTIR spectrometer. UV/vis spectra were recorded in a quartz cuvette
with a 1 cm path length. The absorption wavelengths are reported in
nanometers with the molar extinction coefficient ε (M−1 cm−1) in
parentheses; shoulders are indicated as sh. HR-MALDI-MS spectra
were measured with 3-hydroxypicolinic acid (3-HPA) and 2-[(2E)-3-
(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile
(DCTB) as matrix. The signal of the molecular ion (M+) is reported in
m/z units.
2922 (w), 2864 (w), 2216 (m), 1602 (s), 1538 (w), 1504 (m), 1440
(w), 1401 (m), 1378 (m), 1341 (m), 1281 (w), 1206 (w), 1189 (m),
1105 (w), 1063 (w), 982 (w), 964 (w), 945 (w), 825 (m), 800 (w),
761 (m), 751 (m), 739 (w), 694 cm−1 (w); HR-ESI-MS m/z (%)
467.1957 (93, [M + Na]+, calcd for C28H24N6Na+: 467.1955),
+
445.2136 (100, [M + H]+, calcd for C28H25N6 : 445.2135).
(E)-2-[4-(Dimethylamino)phenyl]-5-(4-methoxyphenyl)hexa-
1,3,5-triene-1,1,6,6-tetracarbonitrile (3c). A solution of 2a (70
mg, 0.48 mmol) and 1b (138 mg, 0.53 mmol) in CHCl3 (30 mL) was
stirred at 25 °C for 24 h and evaporated. FC (SiO2; CH2Cl2) gave 3c
(147 mg, 75%) as a dark purple, metallic solid. Rf = 0.23 (SiO2,
CH2Cl2); mp 277−278 °C; 1H NMR (400 MHz, CD2Cl2) δ = 3.11 (s,
6 H; NMe2), 3.91 (s, 3 H; OMe), 6.80 (d, J = 9.1 Hz, 2 H; H−
C(2′,6′)), 7.11 (d, J = 8.9 Hz, 2 H; H−C(2″,6″)), 7.20 (d, J = 15.2 Hz,
1 H; H−C(4)), 7.35 (d, J = 15.2 Hz, 1 H; H−C(3)), 7.459 and 7.462
ppm (2d, J = 9.1 Hz, 4 H; H−C(3′,5′,3″,5″)); 13C NMR (100 MHz,
CD2Cl2) δ = 39.9 (NMe2), 55.6 (OMe), 80.7 (C(1)), 86.2 (C(6)),
111.5 (C(3′,5′)), 112.2 (CN), 113.1 (CN), 113.5 (CN), 114.5 (CN),
114.8 (C(3″,5″)), 118.3 (C(1′)), 123.8 (C(1″)), 131.4 and 132.0
(C(2′,6′,2″,6″)), 140.1 (C(4)), 142.7 (C(3)), 153.6 (C(4′)), 163.1
3[4-(Pyrrolidin-1-yl)phenyl]propiolonitrile (2c). A solution of
alkyne 2b16 (7.5 g, 43.8 mmol) in anhydrous THF (50 mL) was
cooled to −78 °C, treated dropwise with 1.6 M n-BuLi in hexane (4.2
mL, 65.7 mmol), stirred at −78 °C for 1 h, and treated portionwise
with a freshly prepared solution of PhOCN19 (8.97 g, 74.4 mmol) in
THF (15 mL).20 The mixture was stirred at −78 °C for 30 min and at
−40 °C for 15 min, warmed to r.t., and diluted with H2O. The
aqueous layer was extracted with EtOAc (3×). The combined organic
layers were washed with water and brine, dried over anhydrous
MgSO4, and evaporated. Column chromatography (SiO2; pentane/
EtOAc 9:1) gave 2c (6.0 g, 70%) as a yellowish brown solid. Rf = 0.60
̃
(C(4″)), 166.4 and 167.8 ppm (C(2,5)); IR (ATR) ν = 2953 (w),
2912 (w), 2839 (w), 2220 (m), 1601 (s), 1527 (w), 1506 (m), 1441
(w), 1379 (m), 1336 (m), 1306 (w), 1281 (m), 1262 (m), 1211 (w),
1180 (m), 1105 (w), 1029 (w), 982 (w), 951 (w), 840 (w), 828 (m),
805 (w), 740 cm−1 (w); HR-ESI-MS m/z (%) 428.1485 (100, [M +
Na]+, calcd for C25H19N5NaO+: 428.1482), 406.1667 (50, [M + H]+,
1
(SiO2; pentane/EtOAc 9:1); H NMR (400 MHz, CDCl3) δ = 2.04
(m, 4 H; H2C(3′,4′)), 3.33 (m, 4 H; H2C(2′,5′)), 6.47 (d, J = 8.9 Hz,
429
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The Journal of Organic Chemistry
Note
calcd for C25H20N5O+: 445.2135), 344.9848 (74), 304.2616 (99),
262.9819 (83).
C(3″,5″)), 7.14 (d, J = 9.1 Hz, 2 H; H−C(2″,6″)), 7.23 (d, J = 9.2 Hz,
2 H; H−C(2′,6′)), 7.87 ppm (s, 1 H; H−C(4)); 13C NMR (100 MHz,
CD2Cl2) δ = 25.3 (C(3,4) of pyrrolidin-1-yl), 40.0 (NMe2), 48.1
(C(2,5) of pyrrolidin-1-yl), 74.0 and 77.4 (C(1,6)), 111.4 and 112.2
(C(3′,5′,3″,5″)), 113.5 (CN), 113.6 (CN), 114.2 (CN), 114.7 (CN),
115.1 (CN), 119.3 and 119.5 (C(1′,1″)), 122.8 (C(3)), 131.8 and
132.3 (C(2′,6′,2″,6″)), 150.3 (C(4)), 152.2 (C(4′)), 154.1 (C(4″)),
(E)-2,5-Bis[4-(dibutylamino)phenyl]hexa-1,3,5-triene-
1,1,6,6-tetracarbonitrile (3d). A solution of 2d (466 mg, 0.2 mmol)
and TCNE (130 mg, 0.1 mmol) in CHCl3 (30 mL) was stirred at 50
°C for 36 h and evaporated. FC (SiO2; CH2Cl2) gave 3d (506 mg,
85%) as a purple solid. Rf = 0.49 (SiO2; CH2Cl2); mp 206−207 °C; 1H
NMR (400 MHz, CD2Cl2) δ = 0.98 (t, J = 7.3 Hz, 12 H; 4 Me), 1.35
− 1.42 (m, 8 H; 4 CH2Me), 1.60−1.67 (m, 8 H; 2 N(CH2CH2)2),
3.39 (br. t, J = 7.8 Hz, 8 H; 2 N(CH2)2), 6.75 (d, J = 9.2 Hz, 4 H; H−
C(3′,5′)), 7.27 (s, 2 H; H−C(3)), 7.49 ppm (d, J = 9.1 Hz, 4 H; H−
C(2′,6′)); 13C NMR (100 MHz, CD2Cl2) δ = 14.2 (2 Me), 20.8 (2
CH2Me), 29.8 (N(CH2CH2)2), 51.4 (N(CH2)2), 78.9 (C(1)), 111.8
(C(3′,5′)), 114.6 (CN), 115.6 (CN), 118.5 (C(1′)), 133.1 (C(2′,6′)),
̃
159.8 (C(5)), 162.6 ppm (C(2)); IR (ATR) ν = 3018 (w), 2981 (w),
2924 (w), 2860 (w), 2214 (m), 1602 (s), 1537 (w), 1491 (s), 1443
(m), 1407 (s), 1375 (m), 1345 (m), 1284 (w), 1209 (m), 1186 (m),
818 (m), 752 (m), 667 cm−1 (w); HR-ESI-MS m/z (%) 492.1906 (34,
[M + Na]+, calcd for C29H23N7Na+: 492.1907), 470.2090 (11, [M +
+
H]+, calcd for C29H24N7 : 470.2088), 420.9703 (33), 344.9844 (54),
262.9813 (61), 256.9645 (100).
(3E,5E)- and (3Z,5E)-2,4,7-Tris[4-(dimethylamino)phenyl]-
octa-1,3,5,7-tetraene-1,1,3,8,8-pentacarbonitrile ((3E,5E)/
(3Z,5E)-5). A solution of 3-[4-(dimethylamino)phenyl]propiolonitrile
(129 mg, 0.76 mmol) and TCNE (97 mg, 0.76 mmol) in toluene (30
mL) was stirred at 90 °C for 24 h, treated with 2a (220 mg, 1.52
mmol), and stirred at 90 °C for 48 h. Evaporation and FC (SiO2;
CH2Cl2) gave a 7:3 mixture of (3E,5E)/(3Z,5E)-5 (335 mg, 75%) as a
brownish-black solid. Rf = 0.64 (SiO2; CH2Cl2/EtOAc 98:2); 1H NMR
(400 MHz, CD2Cl2; (3E,5E)/(3Z,5E) 7:3) δ = 3.02 and 3.06 (2s, 1.8
and 4.2 H; NMe2), 3.10 (s, 6 H; NMe2), 3.11 (s, 6 H; NMe2), 6.64 (d,
J = 9.0 Hz, 0.6 H) and 6.77 (d, J = 9.1 Hz, 1.4 H), 6.71 (d, J = 9.3 Hz,
2 H), 6.86 (d, J = 9.0 Hz, 2 H) (3x H−C(3′,5′)), 6.80 (d, J = 15.2 Hz,
0.7 H), 7.19 (d, J = 15.2 Hz, 0.7 H), 7.24 (d, J = 15.2 Hz, 0.3 H), 7.50
(d, J = 15.1 Hz, 0.3 H) (H−C(5,6)), 7.13 (d, J = 9.0 Hz, 0.6 H), 7.36
(d, J = 9.1 Hz, 1.4 H), 7.50 (d, J = 9.0 Hz, 1.4 H), 7.52 (d, J ≈ 8.3 Hz,
0.6 H), 7.65 (d, J = 9.2 Hz, 1.4 H) 7.70 ppm (d, J = 9.2 Hz, 0.6 H) (3×
H−C(2′,6′); 13C NMR (100 MHz, CD2Cl2; (3E,5E)/(3Z,5E) 7:3)
signals of (3E,5E): δ = 40.4, 40.5, 40.5, 76.8, 79.7, 80.3, 109.2, 114.4,
115.3, 115.4, 115.7, 118.0, 119.0, 119.2, 121.7, 132.4, 132.5, 133.4,
139.8, 142.4, 153.3, 153.9, 155.0, 161.7, 163.1, 168.7, signals of (3Z,5E,
two signals are overlapping) δ = 40.44, 40.49, 78.1, 80.3, 108.6, 112.0,
114.3, 115.1, 115.4, 116.7, 119.4, 119.7, 120.4, 131.9, 132.6, 133.1,
̃
142.1 (C(3)), 152.6 (C(4′)), 167.2 ppm (C(2)); IR (ATR) ν = 2954
(m), 2928 (m), 2862 (w), 2213 (m), 1594 (s), 1533 (w), 1482 (s),
1435 (w), 1411 (m), 1366 (m), 1341 (s), 1289 (w), 1275 (m), 1230
(w), 1205 (m), 1187 (s), 1162 (m), 1105 (m), 991 (w), 976 (w), 925
(w), 820 (s), 798 (w), 763 (w), 738 (w), 692 cm−1 (w); HR-ESI-MS
+
m/z (%) 587.3846 (100, [M + H]+, calcd for C38H47N6 : 587.3857),
338.3414 (75).
(E)-2,5-Bis(4-methoxyphenyl)hexa-1,3,5-triene-1,1,6,6-tetra-
carbonitrile (3e). A solution of 2e (307 mg, 2.33 mmol) and TCNE
(149 mg, 1.16 mmol) in (CHCl2)2 (10 mL) was stirred at 120 °C for
60 h and evaporated. FC (SiO2; CH2Cl2) gave 3e (137 mg, 30%) as a
red solid and 1b6b (182 mg, 60%). Rf = 0.30 (SiO2; CH2Cl2); mp
271−272 °C; 1H NMR (400 MHz, CD2Cl2) δ = 3.91 (s, 6 H; OMe),
7.11 (d, J = 8.9 Hz, 4 H; H−C(2′,6′)), 7.26 (s, 2 H; H−C(3)), 7.44
ppm (d, J = 8.9 Hz, 4 H; H−C(3′,5′)); 13C NMR (100 MHz, CD2Cl2)
δ = 55.7 (OMe), 87.0 (C(1)), 112.0 (CN), 113.0 (CN), 114.8
(C(3′,5′)), 123.5 (C(1′)), 131.3 (C(2′,6′)), 141.2 (C(3)), 163.2
̃
(C(4′)), 167.3 ppm (C(2)); IR (ATR) ν = 3110 (w), 3075 (w), 3038
(w), 2958 (w), 2937 (w), 2913 (w), 2840 (w), 2225 (s), 1601 (s),
1575 (m), 1531 (m), 1507 (s), 1446 (w), 1442 (w), 1426 (w), 1335
(m), 1309 (m), 1285 (s), 1264 (s), 1182 (s), 1127 (w), 1108 (m),
1026 (s), 1011 (m), 992 (m), 972 (w), 959 (w), 843 (s), 830 (m), 793
(w), 741 (m), 697 (w), 632 cm−1 (w); HR-ESI-MS m/z (%) 415.1155
139.7, 144.7, 152.8, 154.0, 154.6, 161.4, 164.5, 168.7 ppm; IR (ATR) ν
̃
= 2911 (w), 2864 (w), 2811 (w), 2214 (m), 1601 (s), 1542 (w), 1489
(m), 1438 (w), 1377 (m), 1340 (m), 1209 (m), 1190 (m), 1167 (m),
944 (w), 820 cm−1 (w); HR-ESI/MALDI-MS (dual, DCTB as matrix)
+
(100, [M + Na]+, calcd for C24H16N4NaO2 : 415.1165), 344.9838
(52), 304.2606 (63), 268.9981 (53), 262.9811 (57), 186.9787 (41).
(Z)-5-[4-(Dimethylamino)phenyl]-2-[4-(pyrrolidin-1-yl)-
phenyl]hexa-1,3,5-triene-1,1,3,6,6-pentacarbonitrile (Z-4a)
and (E)-5-[4-(Dimethylamino)phenyl]-2-[4-(pyrrolidin-1-yl)-
phenyl]hexa-1,3,5-triene-1,1,3,6,6-pentacarbonitrile (E-4b). In
a sealed tube, a solution of 2c (213 mg, 1.1 mmol) and 1a (270 mg, 1
mmol) in (CHCl2)2 (10 mL) was stirred at 120 °C for 24 h.
Evaporation and FC (SiO2; CH2Cl2 → CH2Cl2/EtOAc 98:2) gave Z-
4a (172 mg, 37%) as a bluish-maroon solid and E-4b (23 mg, 5%) as a
brownish-black solid.
+
m/z (%) 588.2750 (100, [M]+, calcd for C37H32N8 : 588.2746).
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra, quinoidal character, and electro-
chemistry of all new compounds; and crystal structure data for
compounds 3a, 3c, 3d, 3e, Z-4a, E-4b, and (3E,5E)-5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Z-4a: Rf = 0.48 (SiO2; CH2Cl2/EtOAc 98:2); mp 227.5−228.5 °C;
1H NMR (400 MHz, CD2Cl2) δ = 2.06 (m, 4 H; H2C(3,4) of
pyrrolidin-1-yl), 3.14 (s, 6 H; NMe2), 3.46 (m, 4 H; H2C(2,5) of
pyrrolidin-1-yl), 6.67 (d, J = 9.2 Hz, 2 H; H−C(3′,5′)), 6.80 (d, J = 9.3
Hz, 2 H; H−C(3″,5″)), 7.69 (s, 1 H; H−C(4)), 7.75 (d, J = 9.2 Hz, 2
H; H−C(2′,6′)), 7.80 ppm (d, J = 9.3 Hz, 2 H; H−C(2″,6″)); 13C
NMR (100 MHz, CD2Cl2) δ = 25.8 (C(3,4) of pyrrolidin-1-yl), 40.5
(NMe2), 48.6 (C(2,5) of pyrrolidin-1-yl), 74.5 and 77.1 (C(1,6)),
112.3 and 112.9 (C(3′,5′,3″,5″)), 113.3 (NC−C(3)), 114.9 (CN),
115.0 (CN), 115.4 (CN), 115.5 (CN), 118.0 and 119.1 (C(1′,1″)),
122.0 (C(3)), 133.1 and 133.6 (C(2′,6′,2″,6″)), 150.1 (C(4)), 152.9
AUTHOR INFORMATION
Corresponding Author
*E-mail: diederich@org.chem.ethz.ch.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
̃
(C(4′)), 154.9 (C(4″)), 162.3 and 162.4 ppm (C(2,5)); IR (ATR) ν =
This work was supported by the ERC Advanced grant no.
246637 (“OPTELOMAC”). A.D.F. acknowledges the NSF-
IRFP (USA) for a postdoctoral fellowship.
3018 (w), 2981 (w), 2953 (w), 2924 (w), 2865 (w), 2214 (m), 1603
(s), 1495 (m), 1443 (m), 1409 (m), 1383 (m), 1340 (m), 1209 (m),
1186 (m), 820 (w), 750 cm−1 (w); HR-ESI-MS m/z (%) 492.1902
(100, [M + Na]+, calcd for C29H23N7Na+: 492.1907), 470.2086 (40,
+
[M + H]+, calcd for C29H24N7 : 470.2088), 344.9843 (93), 262.9815
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