Expanding the chemical space for push-pull chromophores by non-concerted [2+2] and [4+2] cycloadditions: Access to a highly functionalised 6,6-dicyanopentafulvene with an intense, low-energy charge-transfer band

Article abstract of DOI:10.1039/c1cc10247h

Non-concerted [2+2] and [4+2] cycloadditions between N,N-dimethylanilino- substituted 1,1,2,4,4-pentacyanobuta-1,3-diene and 4-ethynyl-N,N-dimethylaniline are controlled by solvent polarity and provide access to a highly functionalised 6,6-dicyanopentaful

Full text of DOI:10.1039/c1cc10247h

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Expanding the chemical space for push-pull chromophores by
non-concerted [2+2] and [4+2] cycloadditions: access to a highly
functionalised 6,6-dicyanopentafulvene with an intense, low-energy
charge-transfer bandw
Govindasamy Jayamurugan,a Jean-Paul Gisselbrecht,b Corinne Boudon,b
Franziska Schoenebeck,*a W. Bernd Schweizer,a Bruno Berneta and Franc
¸
ois Diederich*a
Received 13th January 2011, Accepted 22nd February 2011
DOI: 10.1039/c1cc10247h
Non-concerted [2+2] and [4+2] cycloadditions between N,N-
dimethylanilino-substituted 1,1,2,4,4-pentacyanobuta-1,3-diene
and 4-ethynyl-N,N-dimethylaniline are controlled by solvent
polarity and provide access to a highly functionalised 6,6-
dicyanopentafulvene featuring an intense, low-energy charge-
transfer band and to an unusual spirocyclic zwitterion,
characterised by X-ray analysis.
Donor–acceptor (D–A) chromophores1 are of interest for a
range of potential technological applications, extending from
optoelectronic devices,2 such as waveguides,2d to near-infrared
dyes for use in optical recording and printing and as photo-
refractive materials.3 Much of our research over the past years
has focused on expanding the chemical space of push-pull
chromophores featuring intense, bathochromically shifted
intramolecular charge-transfer (CT) bands.4 Here we report
on the short, solvent-polarity-controlled synthesis of a highly
functionalised bis-N,N-dimethylanilino- (DMA-) donor-
substituted 6,6-dicyanopentafulvene featuring an intense
Scheme 1 Non-concerted [2+2] and [4+2] cycloadditions between 1
and 2: (i) solvent, 25 1C, 15–24 h (for details see Table S1, ESIw); (ii)
SiO2, 25 1C, CH2Cl2 4 h, 80%; or (NH4)2Ce(NO3)6, 12 h, CH3CN/
H2O 1 : 1, 90%; (iii) SiO2, 25 1C, 2 h, 95%.
1,1,3,6,6-pentacyanohexa-1,3,5-trienes E-3 and Z-3 (69% yield)
and 1,3,3,6,6-pentacyanocyclohexa-1,4-diene 4 (29% yield)
which were separated by reverse-phase HPLC (CH3CN/H2O
1 : 1) and characterised by 1H and 13C NMR spectroscopy.
Compounds E-3 and Z-3 are the products from a formal [2+2]
cycloaddition, followed by cycloreversion.6 Compound 4, on
the other hand, results from a previously not observed formal
[4+2] cycloaddition.
low-energy CT transition,
a highly unusual spirocyclic
compound and a bis-DMA-donor-substituted tricyano-
benzene. Their formation involves non-concerted formal [2+2]
and [4+2] cycloadditions as key steps.
DMA-substituted 1,1,2,4,4-pentacyanobuta-1,3-diene 1,5
a
highly nonplanar D–A chromophore with a torsional angle of
107.81 about the central C–C bond of the buta-1,3-diene
moiety (see Fig. S3, ESIw), was reacted with DMA-acetylene
2 in CH2Cl2 at 25 1C for 15 h (Scheme 1). LC-MS showed
complete conversion to a mixture of two diastereoisomeric
We subsequently analysed the solvent dependency of the
transformation of 1 on 2. In solvents of low polarity (toluene,
THF, 1,4-dioxane), compounds E/Z-3 and 4 were in each case
obtained as the major products in a ratio similar to the one
obtained in CH2Cl2 (see Table S1, ESIw). However in polar
solvents, such as CH3CN and DMF, the unusual spirocyclic
zwitterion 5 (see the X-ray crystal structure shown in Fig. 2)
was obtained in 47% and 46% yield, respectively, along with
minor amounts of E/Z-3 and 4. In the solid state, 5 can be
stored without decomposition for months in an ambient
laboratory atmosphere. The compound is well soluble in
CH3CN and DMF, scarcely soluble in CH2Cl2 and acetone
and insoluble in CHCl3, THF and Et2O.
a Laboratorium fur Organische Chemie, ETH Zurich, Honggerberg,
HCI, 8093 Zurich, Switzerland. E-mail: diederich@org.chem.ethz.ch;
¨
¨
¨
¨
Fax: +41 44 632 1109; Tel: +41 44 632 2992
b 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, CS 90032, 67081 Strasbourg Cedex, France
w Electronic supplementary information (ESI) available: Synthesis
and characterisation of new compounds, details of the calculations,
spectroscopic and electrochemical data. Computational details,
energies, Cartesian coordinates and the full ref. 7 are provided. CCDC
801743–801746 and 802217. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c1cc10247h
We applied computational studies7,8 to gain insight on the
mechanisms of formation of 3, 4 and 5 (compare with Fig. 1).
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Fig. 2 ORTEP plots of 4–7 with atomic displacement parameter
Fig. 1 A non-concerted dipolar mechanism for the [2+2] and [4+2]
cycloadditions of 1 with 2 in formation of 3–5.
ellipsoids shown at the 50% probability level. T = 100 K. For selected
bond lengths (A) and angles (1) see Fig. S4–S7 (ESIw). Quinoid char-
acter: dr = {[(a + a0)/2 ꢀ (b + b0)/2] + [(c + c0)/2 ꢀ (b + b0)/2]}/2;13
for benzene: dr = 0, for fully quinoid rings, dr = 0.08–0.10 A. For
details of the X-ray analyses, including CCDC numbers, see the ESIw.
Different DMA residues in a molecule are marked I and II.
The electron-rich alkyne 2 attacks regioselectively at carbon
C1 in 1 under generation of a zwitterionic intermediate6b
featuring a 1,1,3-tricyanoallyl anion. The activation free
enthalpy barrier of this attack is DGz = 113.8 kJ molꢀ1 in
the gas-phase (104.6 kJ molꢀ1 in toluene and 94.1 kJ molꢀ1 in
MeCN), and is the rate-determining step. The regioselective
preference for attack at C1 relative to C4 is calculated to be
DDGz = 15.9 kJ molꢀ1 (in the gas-phase), which is in line with
C1 being more electrophilic than C4 (due to the lack of
conjugation with the DMA group) and more electrophilic
than C2 due to an additional cyano substituent. In solvents
of low polarity, in particular in toluene (see Table S1, ESIw),
provides 6 in 90% yield. The facile access to the highly
substituted 6,6-dicyanopentafulvene 6 is noteworthy, as such
chromophores show interesting optoelectronic properties.11
While theoretical calculations on donor-substituted 6,6-dicyano-
pentafulvenes have been reported,12 their synthesis so far has
not been explored.
The structures of 4–7 were confirmed by X-ray analysis
(see Fig. 2 and Fig. S4–S7, ESIw). The efficiency of the ground-
state CT from the donor to acceptor moieties was estimated by
the quinoid character dr (for definition, see caption to Fig. 2)
of the DMA rings.13 A large quinoid character value of
dr = 0.063 A corroborated the strong D–A conjugation
between DMA and adjacent dicyanovinyl moieties in 1 (for
bond lengths, see Fig. S3 ESIw). Due to the lack of efficient
acceptors and non-planarity (for dihedral angles (y), see
Fig. S4, S5 and S7 ESIw), small dr-values between 0.012 and
0.027 A were observed for the DMA rings of 4, 5 and 7. In
contrast, the DMA ring I in 6 in planar conjugation with the
tetracyanofulvene moiety (y = ꢀ13.61) displayed a very large
dr-value of 0.065 whereas the DMA ring II is turned out of the
plane of the fulvene moiety (y = 59.31) and only shows a low
dr-value of 0.03 (see Fig. S6, ESIw). Density functional theory
(B3LYP/6-31G*) calculations predict a high dipole moment of
m(G) = 9.5 D for 6.
pathway
a predominates leading to cyclobutene inter-
mediates which undergo cycloreversion to afford E/Z-3
(DGz = 49.0 kJ molꢀ1 in toluene). The initial zwitterionic
intermediate may also undergo E–Z isomerisation9 (the barrier
for isomerisation was calculated to be DGz = 33.9 kJ molꢀ1 in
the gas-phase, 46.4 kJ molꢀ1 and 68.6 kJ molꢀ1 in toluene
and MeCN, respectively). The resulting Z-isomer seems to
preferably undergo pathway a, leading to 3, or a formal [4+2]
cycloaddition (DGz = 31.8 kJ molꢀ1 in toluene) to give 4
(pathway b). In polar solvents, a formal [4+2] cycloaddition
(pathway c) generates the spirocyclic zwitterion 5. For MeCN,
a barrier of DGz = 49.4 kJ molꢀ1 was calculated for pathway c
and DGz = 44.4 kJ molꢀ1 for pathway a, while the
E/Z-isomerisation barrier (eventually leading to 4) is DGz =
68.6 kJ molꢀ1. Thus there is only a rough agreement with the
experimentally observed product mixture. However, all steps
following the initial, regioselective attack at C1 are overall of
relatively low barrier. Solvation has an effect on these barriers,
but the relatively flat potential energy landscape suggests that
the selectivity is likely effected also dynamically (such as
conformational distribution).
Column chromatography on SiO2 transformed the primary
products E/Z-3 and 4 in very high yield into 6,6-dicyano-
pentafulvene 6 (80%) and tricyanobenzene 7 (95%), respectively
(Scheme 1). The formation of 6 involves a formal 1,5-elimination
of HCN under cyclisation, while the formation of 7 proceeds
by 1,4-elimination of cyanogen ((CN)2)10 under aromatisation.
The elimination of HCN from E/Z-3 to 6 might involve a
radical mechanism as the one-electron oxidising agent ceric
ammonium nitrate (CAN, 50 mol% in CH3CN/H2O 1 : 1)
Fig. 3 UV/Vis spectra of 5 in MeCN, and of 6 and 7 in CH2Cl2,
T = 298 K.
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The crystal packings show a variety of short intermolecular
contacts involving the CN groups, such as dipolar, anti-
parallel and orthogonal CNꢁ ꢁ ꢁCN interactions14 and weak
CNꢁ ꢁ ꢁH–Carom hydrogen bonds (see Fig. S4–S7, ESIw).
In the solid state, spiro compound 5 is maroon-coloured,
fulvene 6 metallic maroon and tricyanobenzene 7 red. They
feature high melting points, and thermal gravimetric analysis
demonstrates high thermal stability (see Table S6, ESIw). The
UV/Vis spectrum of 5 in CH3CN shows an intramolecular CT
(ICT) band at lmax = 461 nm (2.69 eV) with weak intensity
(see Fig. 3), as previously observed for homoconjugated push-pull
chromophores.4b Time-dependent (TD) DFT (CAM-B3LYP/
6-31G* level) calculations suggest that the ICT band at 461 nm
predominantly arises from the homoconjugative D–A interaction
between the tricyanopropenide anion donor and the cyclo-
hexadienyl-imminium cation acceptor (see Fig. S16 and Table
S4, ESIw). This was experimentally confirmed by protonation–
re-neutralisation experiments in which protonation of the
DMA moiety with trifluoroacetic acid (TFA) in CH3CN
did not affect the ICT transition (see Fig. S8 and S27, ESIw).
Bis-DMA-substituted tricyanobenzene 7 displays a broad,
more intense ICT absorption in the range between 350–500 nm
(see Fig. 3). The CT band disappears completely upon proto-
nation (TFA) and is fully re-established by neutralisation
(Et3N) in CH2Cl2 (see Fig. S11, ESIw).
(see Table S3, ESIw). Based on the EA value and the HOMO
and LUMO levels, compounds related to 6 should be well
suitable for applications in organic bulk heterojunction solar
cells as recently proved for closely related 4-cyano-5-(dicyano-
methylene)-1H-pyrrol-2-ones.16
We are now exploring the full scope of the transformation, by
systematically changing the donor in 1 and 2 and by starting
from the TCNQ (tetracyanoquinodimethane) analogues of 1.
This work was supported by a grant from the ERC
Advanced Grant NO. 246637 (‘‘OPTELOMAC’’). We thank
Mr P. Rivera-Fuentes (ETHZ) for performing the TD-DFT
calculations.
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cyanopentafulvene 6 is quite remarkable (see Fig. 3). The
spectrum is dominated by a very intense, broad CT band
(lmax = 782 nm (1.59 eV), e/dm3 molꢀ1 cmꢀ1 = 27 500)
expanding from 600 nm into the NIR region with an end
absorption around 1100 nm, originating from ICT from the
conjugated DMA moiety I (see Fig. 2) to the tetracyano-
pentafulvene core. Protonation of 6 with TFA in CH2Cl2 leads
to a bathochromic shift (ca. 32 nm) of the band at lmax = 782 nm,
with the appearance of a new peak at lmax = 870 nm (see
Fig. S9, ESIw). This suggests that the acceptor strength of
the pentafulvene core is further enhanced by protonation of
the sterically deconjugated DMA moiety II (see Fig. 2) while
the conjugated DMA moiety I is untouched. Upon subsequent
re-neutralisation with Et3N (see Fig. S9, ESIw) or direct
treatment of 6 with a base (see Fig. S10, ESIw), the CT band
completely disappears, indicating substantial structural
changes presumably initiated by irreversible nucleophilic
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4522 Chem. Commun., 2011, 47, 4520–4522
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