From homoconjugated push-pull chromophores to donor-acceptor-substituted spiro systems by thermal rearrangement

Article abstract of DOI:10.1002/chem.201303533

Series of homoconjugated push-pull chromophores and donor-acceptor (D-A)-functionalized spiro compounds were synthesized, in which the electron-donating strength of the anilino donor groups was systematically varied. The structural and optoelectronic properties of the compounds were investigated by X-ray analysis, UV/Vis spectroscopy, electrochemistry, and computational analysis. The homoconjugated push-pull chromophores with a central bicyclo[4.2.0]octane scaffold were obtained in high yield by [2+2] cycloaddition of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) to N,N-dialkylanilino-or N,N-diarylanilino-substituted activated alkynes. The spirocyclic compounds were formed by thermal rearrangement of the homoconjugated adducts. They also can be prepared in a one-pot reaction starting from DDQ and anilino-substituted alkynes. Spiro products with N,N-diphenylanilino and N,N-diisopropylanilino groups were isolated in high yields whereas compounds with pyrrolidino, didodecylamino, and dimethylamino substituents gave poor yields, with formation of insoluble side products. It was shown by in situ trapping experiments with TCNE that cycloreversion is possible during the thermal rearrangement, thereby liberating DDQ. In the low-yielding transformations, DDQ oxidizes the anilino species present, presumably via an intermediate iminium ion pathway. Such a pathway is not available for the N,N-diphenylanilino derivative and, in the case of the N,N-diisopropylanilino derivative, would generate a strained iminium ion (A1,3 strain). The mechanism of the thermal rearrangement was investigated by EPR spectroscopy, which provides good evidence for a proposed biradical pathway starting with the homolytic cleavage of the most strained (CN)C-C(CN) bond between the fused four-and six-membered rings in the homoconjugated adducts.

Full text of DOI:10.1002/chem.201303533

DOI: 10.1002/chem.201303533
Full Paper
&
Spiro Compounds
From Homoconjugated Push–Pull Chromophores to Donor–
Acceptor-Substituted Spiro Systems by Thermal Rearrangement
Cagatay Dengiz,[a] Oliver Dumele,[a] Shin-ichiro Kato,[a] Michal Zalibera,[b] Pawel Cias,[b]
W. Bernd Schweizer,[a] Corinne Boudon,[c] Jean-Paul Gisselbrecht,[c] Georg Gescheidt,[b] and
FranÅois Diederich*[a]
In memoriam Michael Bendikov
Abstract: Series of homoconjugated push–pull chromo-
phores and donor–acceptor (D–A)-functionalized spiro com-
pounds were synthesized, in which the electron-donating
strength of the anilino donor groups was systematically
varied. The structural and optoelectronic properties of the
compounds were investigated by X-ray analysis, UV/Vis spec-
troscopy, electrochemistry, and computational analysis. The
homoconjugated push–pull chromophores with a central
bicyclo[4.2.0]octane scaffold were obtained in high yield by
[2+2] cycloaddition of 2,3-dichloro-5,6-dicyano-p-benzoqui-
none (DDQ) to N,N-dialkylanilino- or N,N-diarylanilino-substi-
tuted activated alkynes. The spirocyclic compounds were
formed by thermal rearrangement of the homoconjugated
adducts. They also can be prepared in a one-pot reaction
starting from DDQ and anilino-substituted alkynes. Spiro
products with N,N-diphenylanilino and N,N-diisopropylanili-
no groups were isolated in high yields whereas compounds
with pyrrolidino, didodecylamino, and dimethylamino sub-
stituents gave poor yields, with formation of insoluble side
products. It was shown by in situ trapping experiments with
TCNE that cycloreversion is possible during the thermal rear-
rangement, thereby liberating DDQ. In the low-yielding
transformations, DDQ oxidizes the anilino species present,
presumably via an intermediate iminium ion pathway. Such
a pathway is not available for the N,N-diphenylanilino deriva-
tive and, in the case of the N,N-diisopropylanilino derivative,
would generate a strained iminium ion (A1,3 strain). The
mechanism of the thermal rearrangement was investigated
by EPR spectroscopy, which provides good evidence for
a proposed biradical pathway starting with the homolytic
cleavage of the most strained (CN)CÀC(CN) bond between
the fused four- and six-membered rings in the homoconju-
gated adducts.
cient linear D–A p-conjugation.[2] Organic materials in opto-
electronic devices are often used in the form of thin films, and
the morphology of the films must be carefully controlled.[3]
Non-planar, linearly conjugated push–pull systems have crucial
advantages in this respect as their structural features lead to
improved solubility and processability and ensure the desired
amorphous character of the resulting films. Therefore, they
represent attractive targets for application in advanced func-
tional materials.[4,5]
Introduction
Planar and non-planar push–pull chromophores (D-p-A) con-
sisting of strong electron donors and acceptors, have been ex-
tensively investigated due to their interesting optoelectronic
properties resulting from intramolecular charge transfer (CT).[1]
Most of the known push–pull systems are planar to retain effi-
[a] C. Dengiz, O. Dumele, Dr. S.-i. Kato, Dr. W. B. Schweizer,
Prof. Dr. F. Diederich
Intramolecular CT interactions have also been investigated
in chromophores, where appropriately aligned, rigid, non-p-
conjugating spacers separate the donor and acceptor p-sys-
tems. Examples are through-space CT interactions in donor–ac-
ceptor cyclophanes[6,7] and through-bond CT interactions via
rigid s-bond frameworks.[8] Only a few studies report on homo-
conjugated CT interactions in chromophores with a non-paral-
lel arrangement of donor and acceptor groups.[9]
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
Hçnggerberg, HCI, 8093 Zꢀrich (Switzerland)
Fax: (+41)44-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] Dr. M. Zalibera, Dr. P. Cias, Prof. Dr. G. Gescheidt
Institute of Physical and Theoretical Chemistry
Graz University of Technology
Stremayrgasse 9, 8010 Graz (Austria)
[c] Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
In our ongoing development of non-planar push–pull chro-
mophores available by a click-chemistry-type [2+2] cycloaddi-
tion–retroelectrocyclization (CA–RE) cascade between electron-
rich alkynes and electron-deficient alkenes, we became inter-
ested in expanding the variety of the olefinic electron accept-
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide
Institut de Chimie-UMR 7177, CNRS, Universitꢁ de Strasbourg
4, rue Blaise Pascal, 67000 Strasbourg (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303533.
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Results and Discussion
Syntheses of homoconjugated
adducts (Æ)-2a–h and D–A-p-
conjugated spiro compounds
(Æ)-3a–e
Target homoconjugated push–
pull chromophores (Æ)-2a–h
were prepared from various ani-
lino-substituted alkynes 1a–h by
the one-step procedure de-
scribed earlier.[13] Thermal [2+2]
cycloaddition of DDQ with the
alkynes proceeded smoothly in
1,2-dichloroethane at 258C, and
the desired novel DDQ adducts
(Æ)-2a–h were obtained in high
Figure 1. Series of homoconjugated push–pull chromophores (Æ)-2a–h obtained from alkynes 1a–h and D–A-p-
conjugated spiro compounds (Æ)-3a–e formed by thermal rearrangement.
isolated
yields
(76–94%)
(Table 1). All homoconjugated
ors beyond the commonly used cyanoolefins, such as tetracya-
Table 1. [2+2] Cycloaddition of alkynes with DDQ.
noethene (TCNE) and 7,7,8,8-tetracyano-p-quinodimethane
(TCNQ).[1c,d,10,11] Among others,[12] we explored the use of 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone (DDQ),[13] a well-known
oxidizing reagent for dehydrogenation reactions.[14] We ob-
served facile [2+2] cycloaddition to electron-rich alkynes under
formation of homoconjugated push–pull chromophores featur-
ing a bicyclo[4.2.0]octane scaffold, with interesting optoelec-
tronic properties including high third-order optical nonlineari-
ties.[13] While the cycloaddition was not followed by the usual
retroelectrocyclization, in one example, an unexpected thermal
rearrangement to the first member of a new class of p-conju-
gated D–A-functionalized spiro compounds was observed.
Other examples of thermal [2+2] cycloadditions of DDQ with
electron-rich alkynes have independently also been reported
recently.[15,16]
Substrate
R1
R2
H
Product, Yield [%]
1a
(Æ)-2a, 89
1b
1c
H
H
(Æ)-2b, 81
(Æ)-2c, 81
Starting from a series of differently N-substituted anilinoace-
tylenes 1a–h (Figure 1), we report here the high-yield prepara-
tion and optoelectronic properties of the homoconjugated D–
A-substituted bicyclo[4.2.0]octanes (Æ)-2a–h formed upon re-
action with DDQ. Subsequently, the scope of the thermal rear-
rangement to the linearly D–A-conjugated spirocyclic systems
(Æ)-3a–e is investigated as well as the structures and optoe-
lectronic properties of these novel chromophores. The yields
of this unusual transformation differ greatly, depending on the
N-substituents, and we describe a series of experimental stud-
ies to provide an interpretation of these findings. The novel re-
arrangement was also investigated by electron paramagnetic
resonance (EPR) studies, and a biradical reaction mechanism is
proposed.
1d
1e
1 f
H
(Æ)-2d, 76
(Æ)-2e, 94
(Æ)-2 f, 89
Me
H
1g
1h
H
H
(Æ)-2g, 77
(Æ)-2h, 85
adducts are deeply purple-colored in the solid state and stable
at room temperature.
Subsequently, five representative substrates of the DDQ ad-
ducts were chosen to investigate the thermal rearrangement
leading to the spiro compounds, for which we had reported
a single example in our earlier work.[13] DDQ adducts (Æ)-2a–
e were dissolved in dry toluene and stirred at reflux tempera-
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jugated compounds (Æ)-2a, (Æ)-2d, and (Æ)-2g (Figures 1SI–
3SI in the Supporting Information) confirmed the previously
observed[13] regioselective addition of the alkynes to the
double bond of DDQ bearing the two CN groups. The struc-
tures of the spiro compounds (Æ)-3a, (Æ)-3b, and (Æ)-3e were
also solved (Figure 2). Their N-substituted anilino rings are
roughly in plane with the adjacent dicyanocyclopentadiene
ring. Various intermolecular orthogonal dipolar interactions,
such as CN···CN, CO···CO, CO···CN, CN···CO, as well as p–p inter-
actions are observed in the crystal packings of both homocon-
jugated and spirocyclic systems (see Figures 8SI and 9SI in the
Supporting Information). The quinoid character values[17] (dr
between 0.028 and 0.046; in benzene, dr equals 0, whereas
fully quinoid rings show values between 0.08–0.1) were ob-
tained from the experimental bond lengths in the X-ray struc-
tures (Figure 7SI) and show high quinoid character for the DAA
rings in the ground states of both homoconjugated and p-
conjugated systems (Table 1SI).
Table 2. Synthesis of push–pull spiro compounds (Æ)-3a–e.
Substrate
R1
R2
H
Product, Yield [%]
(Æ)-2a
(Æ)-3a, 82
(Æ)-2b
(Æ)-2c
H
H
(Æ)-3b, 71
(Æ)-3c, 10
(Æ)-2d
(Æ)-2e
H
(Æ)-3d, 5
Me
(Æ)-3e, 32
UV/Vis spectroscopy
The homoconjugated D–A chromophores show intermolecular
CT bands (lmax values between 462 and 549 nm; 2.68–2.26 eV)
of moderate intensity (e values between 2000 and
3000mÀ1 cmÀ1, Figure 10SI). While the absorption maxima of
the linearly p-conjugated spirocyclic CT compounds are in
a comparable energy range (lmax values between 496 and
530 nm, 2.50–2.34 eV), the molar extinction coefficients are
higher by nearly up to one magnitude (e values between
12700 and 22400mÀ1 cmÀ1, Figure 3 and Figure 11SI in the
Supporting Information). The CT character of the longest-wave-
length absorption in both series of chromophores was proven
by protonation- (disappearance of the CT band) re-neutraliza-
tion (full recovery of the CT band) experiments for all com-
pounds except the diphenylanilino derivatives (Æ)-2b and (Æ)-
3b (see Figures 14SI–24SI). The basicity of the amino centers in
the latter is most probably not sufficient to enable protonation
by CF3COOH (TFA) to a sizeable extent, and the CT band does
therefore not disappear. Both homoconjugated and spirocyclic
chromophores feature positive solvatochromism (see Figur-
es 12SI and 13SI).[18]
Scheme 1. One-pot synthesis of spiro compounds (Æ)-3a,b.
ture. Large differences in the isolated yields of donor-substitut-
ed spiro compounds (Æ)-3a–e were reproducibly obtained:
only the N,N-diisopropyl ((Æ)-3a, 82%) and the N,N-diphenyl
((Æ)-3b, 71%) derivatives were formed in high yield, whereas
all other products were isolated in low yield (5–32%, Table 2).
The transformations of substrates (Æ)-2c, (Æ)-2d, and (Æ)-2e
afforded substantial amounts of insoluble, black decomposi-
tion products, and the origin of these side reactions was fur-
ther investigated (see below) after full characterization of the
desired products.
All spirocyclic products are highly colored solids and are
stable for prolonged periods of time at ambient temperature,
but decompose slowly in toluene at reflux. We also prepared
spiro compounds (Æ)-3a and (Æ)-3b, starting from alkynes
1a,b, in a one-pot reaction with DDQ in toluene at reflux
(Scheme 1). Yields were slightly lower ((Æ)-3a: 52%; (Æ)-3b:
48%) compared to the total yields in the two-step protocol
(82% and 71%, respectively), mainly due to purification diffi-
culties after the one-pot synthesis.
The increase in electron-donor strength among the various
N-substituted anilines follows the order established by Weringa
and Janssen.[19] With increasing donor strength, the CT bands
are bathochromically shifted. This shift is larger in the homo-
conjugated than in the linearly p-conjugated spirocyclic sys-
tems. CT bands in the former series shift in the following se-
quence: lmax =462 (2.68 eV, (Æ)-2g), 519 (2.39 eV, (Æ)-2b), 523
(2.37 eV, (Æ)-2h), 540 (2.30 eV, (Æ)-2c), 543 (2.29 eV, (Æ)-2e),
546 (2.27 eV, (Æ)-2a), and 549 nm (2.26 eV, (Æ)-2d and (Æ)-2 f).
In the spectra of the spirocyclic systems, the CT bands appear
at lmax =496 (2.50 eV, (Æ)-3e), 520 (2.38 eV, (Æ)-3b), 524
(2.37 eV, (Æ)-3d), 530 (2.34 eV, (Æ)-3a), and 530 nm (2.29 eV,
(Æ)-3c).[20]
X-ray crystal structure analysis
Crystals suitable for X-ray analysis were obtained by slow diffu-
sion of hexane into dichloromethane or ethyl acetate solutions
of the chromophores at 208C. The structures of the homocon-
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Electrochemistry
Electrochemical measurements were carried out in CH2Cl2 con-
taining 0.1m nBu4NPF6 in a classical three-electrode cell by
cyclic voltammetry (CV) and rotating-disk voltammetry (RDV).
All potentials are given versus Fc+/Fc (ferrocenium/ferrocene
couple), which was used as an internal reference. They are
summarized in Table 2SI and Figure 25SI in the Supporting In-
formation. For some species, due to electrode inhibition, repro-
ducible results could only be observed on freshly polished
working electrodes.
All N-substituted homoconjugated chromophores undergo
a reversible one-electron oxidation occurring at the anilino
moiety, whereas the reduction occurs in two steps on the di-
chloro-dicyano-substituted cyclohexenedione ring. The first re-
duction gives either a reversible one-electron transfer for (Æ)-
2b (À0.75 V, CV), (Æ)-2e (À0.78 V), and (Æ)-2g (À0.78 V), or
corresponds to an electrochemical–chemical reduction step. At
low scan rates, the peak current ratio Ipa/Ipc (peak anodic cur-
rent/peak cathodic current) was smaller than unity (at v=
0.1 VsÀ1 Ipa/Ipc =0.7). When the scan rate was increased, the
ratio reached unity at scan rates higher than 1 VsÀ1. This be-
havior is typical for an electrochemical–chemical mechanism
where the electrochemical step generates the radical anions,
which undergo a chemical reaction. This denotes that the gen-
erated radical is only stable at high scan rates and is rather re-
active. The second reduction (around À1.5 V) is always irrever-
sible at low scan rates and becomes reversible for (Æ)-2a, (Æ)-
2 f, (Æ)-2g, and (Æ)-2h at scan rates higher than 5 VsÀ1. EPR
results (see below) also show that the radical ions can be selec-
tively generated and that the spin distribution corresponds to
the spin residing in the “amino” and the “dichloro-dicyano-sub-
stituted cyclohexenedione” part of the molecule for the oxida-
tion and reduction, respectively.
Spiro species (Æ)-3a–e undergo a reversible one-electron ox-
idation, whereas the reduction occurs as an irreversible two-
electron transfer at around À1.1 V (Table 2SI, Figure 25SI). For
these compounds, the generated radical anion is much more
reactive than in the homoconjugated series. The estimations of
electrochemical gap (EoxÀEred) are rather approximate due to
the irreversible nature of the reduction step, nevertheless the
trends are significant and can be compared with the optical
gaps determined from lmax or lend in the UV/Vis spectra (Ta-
ble 3SI).
Computational study of the electronic transitions
On the optimized molecular structures of (Æ)-2a–h and
(Æ)-3a–e (except (Æ)-2c and (Æ)-3c), the vertical optical tran-
sitions were calculated by time-dependent density functional
theory (TD-DFT) using the software package Gaussian 09 (for
further details on the computational methods see the Support-
ing Information).[21] In all cases, the computed transition ener-
gies are slightly larger than the experimental values (Tables 4
SI–14SI). Differences between computed excitation energies
and experimental absorption maxima are in the range of 0.03–
0.23 eV [root-mean-square deviation (RMSD)=0.18 eV] for
Figure 2. ORTEP representations of spirocyclic (Æ)-3a, (Æ)-3b (one of the
two symmetry independent molecules shown) and (Æ)-3e with vibrational
ellipsoids shown at the 50% probability level, T=100 K. Arbitrary number-
ing. H-atoms and solvent molecules are omitted for clarity.
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is no correlation between the
electron-donor strength of the
anilino substituents and the
yields of the spirocyclic com-
pounds.
DDQ is well known to oxidize
N-alkylated anilines, leading to
iminium ions via a-Csp3–H ab-
straction,[26] while anilines are
also polymerized to polyani-
lines.[27] In fact, reacting N,N-di-
methylaniline with DDQ in dry
toluene at reflux under N2 pro-
duced an insoluble black-green
solid within one hour. The reac-
tion of N,N-diisopropylaniline
was much slower and afforded
an insoluble black film after
6 h.[28] However, there was no re-
action under the same condi-
Figure 3. UV/Vis absorption spectra of D–A-functionalized spiro compounds (Æ)-3a–e.
(Æ)-2a–h and in the range of 0.33–0.41 eV (RMSD=0.366 eV)
for (Æ)-3a–e, well within the expected error for anilino-cyano
type push–pull chromophores.[22–25]
tions when N,N-diphenylaniline was used as a substrate. We
subsequently explored whether the homoconjugated [2+2] cy-
cloadducts would undergo cycloreversion, thereby liberating
DDQ which could then oxidize the aniline species present
under formation of the undesired side products. In the litera-
ture, [2+2] cycloadducts obtained by via polar intermedi-
ates[29,30] have in fact been observed to undergo cyclorever-
sion.[31] We reacted the homoconjugated chromophores (Æ
)-2a and (Æ)-4[13] with TCNE in 1,2-dichloroethane at reflux
(848C) for 6 h and obtained by the thermal cycloreversion, fol-
lowed by a [2+2] CA–RE reaction the anilino-substituted
1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) 5 and 6, respectively,
in 41–42% yield, thereby confirming that cycloreversion
indeed takes place (Scheme 2). No transformation was ob-
served for the diphenylaniline derivative (Æ)-2b; in this case,
only the spiro compound (Æ)-3b formed in a very slow reac-
tion. Cycloreversion also occurs in toluene at reflux (1118C);
here the formation of the spiro compounds is more competi-
tive and starting from (Æ)-2a, TCBD 5 is obtained in 16% yield
besides (Æ)-3a (40%). Starting from (Æ)-4, TCBD 6 forms in
12% yield besides the corresponding known[13] spiro chromo-
phore (18%).
The lowest-energy transitions are composed mostly of
HOMO-to-LUMO excitations and thus can be best described as
intramolecular charge-transfer processes involving the transfer
of electron density from the anilino donor to the dichloro-di-
cyano-substituted cyclohexenedione acceptor moiety in com-
pounds (Æ)-2a–h and from the anilino donor to the adjacent
dicyanocyclopentadiene acceptor in spiro compounds (Æ)-3a–
e, as depicted in the molecular orbital representations of these
levels (Tables 4SI and 11SI). Small but distinctive orbital over-
laps in the cyclobutene moiety suggest that through-bond in-
teractions are responsible for the intramolecular CT transition
in cycloadducts (Æ)-2a–h.
The differences in intensity of the low-energy transitions be-
tween the series (Æ)-2a–h and (Æ)-3a–e are theoretically well
predicted by the computed oscillator strengths f. While values
of f range in the series of the non-p-conjugated chromophores
(Æ)-2a–h from 0.053 to 0.109 (average 0.085), larger numbers
in the range of 0.401–0.673 (average 0.564) are obtained in the
conjugated push–pull chromophores (Æ)-3a–e. This can be ex-
plained by the enhanced electronic communication between
the anilino donor and the dicyanocyclopentadiene moiety
through Csp2-p-conjugation in (Æ)-3a–e compared to the Csp3
-
homoconjugated chromophores (Æ)-2a–h.
Origin of the large difference in yields obtained in the ther-
mal rearrangement
While spiro compounds (Æ)-3a and (Æ)-3b were obtained in
high yield, derivatives (Æ)-3c–e were only isolated in poor
yield (Table 2) and their formation, starting from the pure ho-
moconjugated compounds (Æ)-2c–e, was accompanied by
substantial amounts of insoluble, black decomposition prod-
ucts. Slow product decomposition in toluene at reflux is not re-
sponsible of the low yields obtained for (Æ)-3c–e. Also, there
Scheme 2. In situ trapping of the anilinoalkyne, formed by cycloreversion,
with TCNE.
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Based on all these experimental findings, we propose that
aniline species in the transformation to spiro derivatives (Æ)-
3c–e are oxidized by DDQ formed by the cycloreversion of the
homoconjugated starting materials (Æ)-2c–e. In contrast, such
oxidation side reaction does not compete in the formation of
the two spiro compounds (Æ)-3a,b, that are formed in high
yield (Table 2). Diphenylanilino species lack an a-Csp3–H group
and are not oxidized to iminium ions, whereas such oxidation
is disfavored for diisopropylanilino species due to severe A1-3
strain in the iminium ions. Other, steric and thermodynamic
stability arguments also cannot be dismissed. At this point, we
cannot propose, which aniline species is actually oxidized by
the DDQ[28] liberated in the cycloreversion but the analysis
above presumably holds for all of them.
ed upon heating compounds (Æ)-2a, (Æ)-2b, and (Æ)-4 in tolu-
ene under the exclusion of oxygen (Figure 4).
It was even possible to obtain a well distinguishable ENDOR
1
spectrum revealing H isotropic hyperfine coupling constants
(hfcs, Figure 4d). The spectra displayed in Figure 4 cannot be
obtained, when DMSO solutions of (Æ)-2a, (Æ)-2b, and (Æ)-4
are treated in an analogous way. The analysis of the reaction in
DMSO indicates cycloreversion of (Æ)-2a and (Æ)-4 yielding
DDQ and DAA-substituted acetylenes. The simulation of the
EPR spectra shown in Figure 4 reveals two rather small 14N hfcs
attributable to the amino nitrogen and one CN nitrogen atom
1
and additional H hfcs (Table 15SI). To test if these data corre-
spond to electron-transfer-generated species, we selectively
generated the radical anions and cations of (Æ)-2b and (Æ)-4.
Figure 5 displays the EPR spectra obtained upon electrolytic
reduction of (Æ)-2b and (Æ)-4 in THF. The spectral widths are
Mechanistic studies by EPR spectroscopy
small (ca. 0.5 mT) and the g factor of 2.0053 rather high (and
[33]
CÀ
In our first report on the thermal rearrangement of homocon-
jugated (Æ)-4 to the corresponding spiro compound, we pro-
posed as the first step a conrotatory retroelectrocyclization to
an eight-membered ring intermediate, which would subse-
quently have to be followed by a cis/trans isomerization to fur-
ther proceed with the rearrange-
similar to DDQ ), indicating that the spin and the charge
are residing in the acceptor moiety of the molecule, as also re-
flected by the shape of the LUMO (Table 4SI).
Oxidation of (Æ)-2b and (Æ)-4 with (bis(trifluoroacetoxy)io-
do)benzene (PIFA) leads to EPR spectra clearly indicating an
ment.[13] While a corresponding
free enthalpy reaction profile
could be computed at the
B3LYP/6-31 G(d) level with PCM
solvation in toluene (see Fig-
ure 28SI), all attempts to model
further reaction steps leading to
the final spiro product were un-
successful.
We believe that no ionic
mechanism including any kind
of transannular reactivity be-
tween the two carbonyl groups
leads to the product. Modeling
of many different possible trans-
annular attacks, even stepwise,
did not lead to any reasonable
result. We concluded that a neu-
tral biradical mechanism, which
Figure 4. EPR spectra (a–c) obtained upon heating of (Æ)-2a, (Æ)-2b, and (Æ)-4 in toluene and the corresponding
simulations. ENDOR spectrum (d) obtained upon heating of (Æ)-2a in diphenyl ether.
had actually been proposed by
one of the reviewers of the earli-
er communication,[32] is more
likely and continued performing
EPR studies.
The thermal rearrangement of
the homoconjugated push–pull
chromophores (Æ)-2a, (Æ)-2b,
and (Æ)-4 was investigated by
EPR spectroscopy to inspect
whether the reaction proceeds
via open shell stages, that is,
(bi)radicals or radical ions. Well-
resolved EPR spectra with re-
CÀ
CÀ
Figure 5. EPR spectra of a) (Æ)-4 , g=2.0053(1); b) (Æ)-2b , g=2.0053(1), generated by electrochemical reduc-
markable intensity were detect- tion in Ar-saturated THF solutions (0.1m nBu4NPF6) at 250 K.
Chem. Eur. J. 2014, 20, 1279 – 1286
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amino-type radical cation, in which the predominate portion of
the spin is concentrated at the amino nitrogen atoms and the
directly adjacent substituents, corresponding to the HOMO of
the parent molecules (Table 4SI). This is mirrored by the hyper-
ment of the rather closely related homoconjugated systems
(Æ)-2a–e differed rather strikingly. Whereas the bulkier N,N-di-
isopropyl ((Æ)-3a) and N,N-diphenyl ((Æ)-3b) derivatives were
obtained in high yield, the three other compounds with pyrro-
lidino, didodecylamino, and dimethylamino residues were only
formed in poor yield. It was then found by in situ trapping ex-
periments with TCNE, that the homoconjugated adducts un-
dergo cycloreversion under liberation of DDQ, which is known
to oxidize N-alkylated anilines via Csp3–H abstraction and for-
mation of iminium ions. The low yields of (Æ)-3c–e are most
probably due to competing oxidation of aniline species by the
DDQ formed in the cycloreversion. The high yields of (Æ)-3a,b
were explained by the absence of oxidation to an iminium ion
in the case of the diphenylanilino derivative and by strain in
the iminium ion that would be formed by the diisopropyl de-
rivative. A mechanism for the thermal rearrangement initiated
by retroelectrocyclization to an eight-membered ring and fol-
lowing cis/trans isomerization, which had been proposed in
the earlier communication[13] could not be further supported in
DFT calculations of the free enthalpy reaction pathway. On the
other hand, EPR investigations of the thermal rearrangement
yielded well-resolved EPR spectra with remarkable intensity,
suggesting that the reaction proceeds by open-shell biradical
or radical ion species. Based on this finding, a biradical mecha-
nism was proposed, as had already been suggested by one of
the reviewers of the earlier communication.[32]
+
+
C
C
fine data of (Æ)-2b and (Æ)-4 presented in Table 16SI. The
14
+
+
C
C
size of the N hfcs in (Æ)-2b and (Æ)-4 of approximately
0.9–1.0 mT are significant for triphenylamine and dimethylami-
nobenzene radical cations, respectively.[34] This bears out that
the EPR data representing the spectra obtained upon convert-
ing (Æ)-2a, (Æ)-2b, and (Æ)-4 to the corresponding spiro iso-
mers do not stem from one-electron reduced or oxidized spe-
1
cies. Remarkably, the multiplicities of the H hfcs in Table 15SI
are in line with species connected to (Æ)-2a, (Æ)-2b, and (Æ)-
4. They can be tentatively related to a biradical species formed
by homolytic cleavage of the most strained (CN)CÀC(CN) bond
between the fused four- and six-membered rings in the homo-
conjugated adducts. We therefore now tentatively propose the
mechanism for the thermal rearrangement shown in Scheme 3.
Experimental Section
Only the general procedures for the synthesis of the target chro-
mophores (Æ)-2a–h and (Æ)-3a–e are reported here, where as all
other details and protocols can be found in the Supporting Infor-
mation.
General procedure A (thermal [2+2] cycloaddition)
DDQ (1 equiv) was added under N2 to a solution of electron rich
alkyne (1 equiv) in 1,2-dichloroethane (20 mL). The mixture was
stirred at 258C for 30 min and the solvent removed in vacuo. The
residue was purified by FCC to give homoconjugated chromo-
phore.
Scheme 3. Proposed biradical mechanism for the thermal rearrangement of
(Æ)-2a–e to (Æ)-3a–e. B and B’: biradical intermediates.
General procedure B (thermal reaarangement)
Conclusion
A solution of homoconjugated chromophore (100 mg) in dry tolu-
ene (250 mL) was heated to reflux for 5–24 h. The solvent was re-
moved in vacuo. Dichloromethane was added to the residue and
the mixture stirred vigorously for 10 min. Insoluble material was re-
moved by filtration, and the filtrate was evaporated in vacuo. The
residue was purified by FCC to give spiro compound.
A series of novel homoconjugated push–pull chromophores,
(Æ)-2a–h with a central bicyclo[4.2.0]octane scaffold was ob-
tained in high yield by [2+2] cycloaddition of DDQ to anilino-
activated alkynes. In this series, the N-substituents on the ani-
line ring were systematically varied according to their electron-
donation strength. Starting from (Æ)-2a–e, the thermal rear-
rangement to the p-conjugated spirocyclic push–pull systems Acknowledgements
(Æ)-3a–e was investigated, for which only a single example
had been described in a previous communication.[13] The struc-
tural and optoelectronic properties of the two new series of
push–pull chromophores were investigated by comprehensive
study, including X-ray analysis, UV/Vis spectroscopy, electro-
chemistry (CV and RDV), and computational analysis. The yields
of the spirocyclic products (Æ)-3a–e in the thermal rearrange-
This work was supported by the ERC Advanced Grant No.
246637 (“OPTELOMAC”). Oliver Dumele acknowledges the re-
ceipt of a Kekulꢁ fellowship from the Fonds der Chemischen
Industrie (Germany). Dr. Adam R. Lacy, Dr. Tristan Reekie (ETH
Zꢂrich), and Murat K. Deliomeroglu (University of Texas, Austin)
are acknowledged for reviewing the manuscript.
Chem. Eur. J. 2014, 20, 1279 – 1286
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O. A. Dyachenko, Tetrahedron Lett. 2010, 51, 5028–5031; c) L. N. Sobeni-
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Kazheva, G. G. Alexandrov, O. A. Dyachenko, B. A. Trofimov, Tetrahedron
2011, 67, 4832–4837.
Keywords: charge
homoconjugation
transfer
·
EPR
spectroscopy
·
·
push–pull chromophores
·
spiro
compounds · thermal rearrangement mechanism
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Received: September 6, 2013
Published online on January 2, 2014
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