Facile synthesis of dicyanovinyl-di(meso-aryl)dipyrromethenes via a dipyrromethene-DDQ adduct

Article abstract of DOI:10.1039/b904446a

The dicyano-substituted vinyl dipyrromethene 4 and compound 5 are near-planar and were prepared from a simple meso-aryl dipyrromethane via the DDQ adduct 3; the unique structures of 3, 4 and 5 were confirmed by X-ray diffraction analysis.

Full text of DOI:10.1039/b904446a

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Facile synthesis of dicyanovinyl-di(meso-aryl)dipyrromethenes via a
dipyrromethene–DDQ adduct†
Ji-Young Shin,* Brian O. Patrick and David Dolphin*
Received 4th March 2009, Accepted 6th April 2009
First published as an Advance Article on the web 15th April 2009
DOI: 10.1039/b904446a
The dicyano-substituted vinyl dipyrromethene 4 and com-
pound 5 are near-planar and were prepared from a simple
meso-aryl dipyrromethane via the DDQ adduct 3; the unique
structures of 3, 4 and 5 were confirmed by X-ray diffraction
analysis.
2,3-Dichloro-5,6-dicyano-1,4-quinone (DDQ) has been used as a
powerful oxidant during the synthesis of polyaryl compounds,
and these oxidation processes have been intensively studied
over the past few decades.1 Initial radical formation to give
resonance-stabilizedintermediates is considered to be the principal
oxidation mechanism.2 In addition, 1,4-benzoquinones bear-
ing electron-withdrawing groups are strong dienophiles. Several
DDQ adducts have been prepared as precursors toward the
aromatization of saturated compounds.3 Such aromatization is
achieved by DDQ elimination via thermal processes in the
presence of a DDQ acceptor. Recently, we reported the isolation
The first, least polar, fraction (3) is red in organic solvents, and
of a C2-symmetric hexapyrrole by the oxidation of meso-2,6-
dichlorophenyl tripyrrane using DDQ under aerobic conditions.4
Here, a unique DDQ adduct, compound 3, in which the DDQ
moiety has linked two dipyrromethene units, has been isolated
from the simple DDQ oxidation of meso-2,6-dichlorophenyl-
dipyrromethane (1).‡ In addition to acting as a simple oxidant,
the current work shows that DDQ can form adducts with specific
substrates, which can the undergo subsequent transformations.
Other dipyrromethanes having different meso-substituents, such
as 4-nitrophenyl, 4-chlorophenyl and perfluorophenyl groups,
were also treated with DDQ under the same reaction conditions.
Only the perfluorophenyl-substituted dipyrromethane formed a
DDQ adduct in a reasonable yield (~20%) (see ESI†). The strong
electron-withdrawing groups seem to play a key role in this
reaction. Thus, treatment of adduct 3 with Et3N gave a new
compound 4 resulting from the partial elimination of the DDQ
moiety. Alternatively, refluxing a THF solution of 3 in the presence
of AlCl3 and MeOH formed 5. The structures of 3, 4 and 5 have
been confirmed by X-ray diffraction analysis.§
single crystals were obtained by the addition of hexane vapor
into a CHCl3 solution. The second (yellow) fraction was the
dipyrromethene 2. When pure compound 2 was treated with excess
DDQ, 3 was not obtained. Only dipyrromethane 1 produced 3 dur-
ing the oxidation step. As shown in Fig. 1, the structure of 3 showed
DDQ to be joined at the A-positions of the dipyrromethenes,
with a trans-configuration around the C–C bond of the two sp3
carbon atoms of the original DDQ. The amino-nitrogens share
a hydrogen atom with the imino-nitrogen and the oxo-oxygen by
hydrogen-bonding to create near-C2 symmetry. Consequently, the
DDQ ring is slightly tilted in order to make the oxygen atoms
point towards the amino-NH. Furthermore, the mean planes of
the two dipyrromethenes are nearly coplanar and perpendicular to
These two products,
2 and 3, were prepared by the
overnight DDQ oxidation reaction of meso-2,6-dichlorophenyl-
dipyrromethane (1) in CH2Cl2 and isolated using column chro-
matography on silica gel with CH2Cl2–hexane (3:2) as eluent.‡
Department of Chemistry, University of British Columbia, 2036
Main Mall, Vancouver, B.C V6T 1Z1, Canada. E-mail: jiyoung@
chem.ubc.ca, david.dolphin@ubc.ca; Fax: +1 604 822 9678; Tel: +1 604
822 4571
† Electronic supplementary information (ESI) available: Experimental and
spectral data for compounds, crystallographic data for compounds 3, 4 and
5. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b904446a
Fig. 1 Crystal structure of 3; (a) top and (b) side views; hydro-
gen-bond (dashed lines) lengths (A): 2.25(3) [N(1)–H ◊ ◊ ◊ N(2)], 2.55(3)
[N(1)–H ◊ ◊ ◊ O(1)], 2.19(3) [N(6)–H ◊ ◊ ◊ N(5)], 2.50(3) [N(6)–H ◊ ◊ ◊ O(2)].
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the DDQ-plane. The symmetry of 3 in solution was confirmed in
the 1H NMR spectrum, where only four b-protons were observed
(Fig. 3a). A broad NH peak was observed downfield at 11.18 ppm
due to the hydrogen-bonding. In addition, the sp3 nature of the
cyanide-bearing carbon atoms was confirmed at 30.28 ppm in
the 13C NMR spectrum (see ESI†). The optical spectrum of
3 (Fig. 4) showed a bathochromic shift (69 nm) for the two
dipyrromethene units compared to that of the dipyrromethene (2).
When compound 3 was treated with Et3N (Scheme 1), the
red color dramatically changed to green. X-Ray crystallographic
analysis revealed the structure 4, as shown in Fig. 2, in which
DDQ was partially eliminated and a double bond was newly
formed with concomitant extension of the p-conjugated network
along the main skeleton. The mean plane of the main skeleton
exhibited near-planarity (see the side-view in Fig. 2b). Due to the
extended p-conjugation, the sets of neighboring b-hydrogen peaks
Scheme 1 Formation of 4 and 5 from 3.
1
were widely separated in the H NMR spectrum (the red and
blue lines denote the peaks of b-hydrogens on the inner and outer
pyrroles in Fig. 3). The cyano-nitrogens also act as hydrogen-bond
acceptors and result in the NH peaks being extremely broadened
and highly downfield-shifted (12.94 ppm) in comparison to those
of compound 3 (Fig. 3b). The downfield shift of one of the
pyrrole b-hydrogens also results from intra-molecular hydrogen
bonding with the cyano-nitrogen. The extended p-conjugation
also generated a lower energy absorption band in its optical
spectrum (Fig. 4).
Fig. 4 Optical spectra of 2–5 in CH2Cl2.
The elimination to give 4 from 3 has aspects of a reverse
Diels–Alder reaction, and it is possible that, after attack of the
nucleophile, a concerted elimination does indeed occur to give a
“diketene”. However, considering the considerable resonance sta-
bilization of the carbanion, it is equally likely that the elimination
proceeds through this intermediate.
A dipyrromethene-eliminated compound 5 was obtained by
refluxing a THF solution of 3 in the presence of AlCl3 and
MeOH followed by purification using column chromatography.
A single crystal of 5 was grown by diffusion of hexane into a
CH2Cl2 solution. The structure (Fig. 5) exhibits an oxo-furan ring,
and Scheme 2 describes a mechanism for its formation. The oxo-
oxygen and imino-nitrogen atoms act as hydrogen-bond acceptors
Fig. 2 Crystal structure of 4; (a) top and (b) side views. The hydro-
gen-bond (dashed lines) lengths (A): 2.16(2) [N(1)–H ◊ ◊ ◊ N(2)], 2.31(2)
[N(1)–H ◊ ◊ ◊ N(4)], 2.52(2) [N(5)–H ◊ ◊ ◊ N(3)], 2.13(2) [N(5)–H ◊ ◊ ◊ N(6)].
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Fig. 3 1H NMR spectra of (a) 3 and (b) 4 in CD2Cl2.
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Scheme 2 A possible mechanism for the formation of 4 and 5 from 3.
Fig. 6 1H NMR spectra of 5 in CD2Cl2.
conjugation and near-planarity. Further research to address the
continual challenge of yield improvement is underway.
Acknowledgements
This work was supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada.
Fig. 5 Crystal structure of 5; (a) top and (b) side views; hydrogen-
˚
bond (dashed lines) lengths (A); 2.360(19) [N(2)–H ◊ ◊ ◊ O(1)], 2.140(17)
[N(2)–H ◊ ◊ ◊ N(1)].
Notes and references
‡ Selected data for 3: 8% yield (a small amount of compound 3 was lost
during the isolation due to its poor solubility), lmax (CH2Cl2, log e)/nm
500 (4.64); 1H NMR (400 MHz, CD2Cl2) d = 11.18 (bs, 2H, NH), 7.64 (m,
2H, AH), 7.52-7.40 (m, 6H, aryl-H), 6.79 (d, J = 4.3, 2H, b-CH of inner
pyrrole), 6.73 (d, J = 4.3, 2H, b-CH of inner pyrrole), 6.42 (m, 4H, b-CH
of terminal pyrrole);13C NMR (100 MHz, CD2Cl2) d = 176.67, 156.04,
146.84, 142.83, 141.78, 136.71 (bC), 135.96, 135.66, 134.93 (aC), 134.19,
131.56 (para-C), 131.50, 128.82 (meta-C), 128.68 (meta-C), 126.19 (bC),
121.95 (bC), 114.57 (bC), 114.27, 30.28 (sp3C); m/z HRESIMS found
802.9671, calcd. 802.9691 for C38H19N6O235Cl537Cl ([M + H]+). Selected
data for 4: 80% yield, lmax (CH2Cl2, log e)/nm 335 (4.18), 492 (3.59), 680
(4.31), 735 (4.50).1H NMR (400 MHz, CD2Cl2) d = 12.94 (bs, 2H, NH),
and result in the NH peaks being highly downfield-shifted
(12.94 ppm, Fig. 6).
Compounds 3, 4 and 5 are unique and their formation is facile.
The considerable electron deficiency of 3 results in the stabilization
of carbanion intermediates (Scheme 2, before protonation) which
allows for their formation. The chemical properties of dicyano-
substituted vinyl dipyrromethene compounds render them good
candidates as useful metal-chelating ligands due to the extended
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7.59 (m, 2H, A-CH), 7.54–7.42 (m, 6H, aryl-H), 7.16 (d, J = 4.5, 2H,
b-CH of inner pyrrole), 6.72 (d, J = 4.3, 2H, b-CH of inner pyrrole), 6.47
(dd, J = 4.1 and 1.2, 2H, b-CH of terminal pyrrole), 6.42 (dd, J = 4.3
and 2.2, 2H, b-CH of terminal pyrrole); 13C NMR (100 MHz, CD2Cl2)
d = 140.04, 136.82 (aC), 135.95, 134.58, 133.75, 133.57 (bC), 131.47,
128.77 (aryl-C), 126.19 (bC), 125.77 (bC), 119.57, 115.38 (bC), 114.09; m/z
HREIMS found 650.03271 (100%), calcd. 650.03471 for C34H18N635Cl4
(M+). Selected data for 5: 30% yield, lmax (CH2Cl2, log e)/nm 332 (4.51),
420 (3.94), 542 (4.48), 735 (4.50); 1H NMR (400 MHz, CD2Cl2) d = 12.43
(bs, 2H, NH), 7.54–7.35 (m, 6H, aryl-H), 6.99 (dd, J = 4.0 and 2.4, 2H,
b-CH), 6.56 (d, J = 4.8, 2H, b-CH), 6.33 (d, J = 4.8, 2H, b-CH), 6.05 (dd,
J = 4.4 and 1.6, 2H, b-CH), 4.29 (s, 6H, OCH3); 13C NMR (100 MHz,
CD2Cl2) d = 178.39, 160.22, 149.84, 144.99, 140.59, 138.79, 137.14
(bC), 136.90, 134.20, 131.24 (aryl-C), 129.45, 128.79 (aryl-C), 124.18,
122.67 (bC), 122.00, 120.70 (bC), 118.04 (bC), 113.99, 91.93, 58.36; m/z
HRESIMS found 505.9636 (100%), calcd. 505.9633 for C22H12N335Cl4O3
[(M + H)+].
R1 = 0.030 [I > 2.00s(I)], wR2 = 0.079 (all data), GOF = 1.03, CCDC
716868.
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§ Crystal data for 3: C38H18N6O2Cl6·CH2Cl2, T = 173 K, M = 888.21,
¯
˚
˚
˚
triclinic, P1 (no. 2), a = 11.2796(18) A, b = 12.066(2) A, c = 14.734(2) A,
◦
◦
◦
3
˚
a = 83.100(8) , b = 79.807(8) , g = 89.782(8) , V = 1959.0(6) A ,
Dc = 1.506 g cm-3, Z = 2, R1 = 0.053 [I > 2.00s(I)], wR2 = 0.174
(all data), GOF = 1.17, CCDC 708612. Crystal data for 4: C34H18N6Cl4,
˚
T = 173 K, M = 652.34, monoclinic, P21/c (no.◦14), a = 14.1530(16) A,
◦
˚
˚
b = 10.3574(11) A, c = 20.116(2) A, a = 90.0 , b = 92.535(5) , g =
◦
90.0 , V = 2945.8(6) A , Dc = 1.471 g cm-3, Z = 4, R1 = 0.033 [I >
3
˚
2.00s(I)], wR2 = 0.082 (all data), GOF = 1.02, CCDC 710025. Crystal
¯
data of 5: C22H11N3Cl2O3, T = 173 K, M = 507.14, triclinic, P1 (no. 2),
◦
˚
˚
˚
a = 7.8137(8) A, b = 9.9057(11) A, c = 14.0837(16) A, a = 84.895(5) , b =
4 J.-Y. Shin, S. S. Hepperle, B. O. Patrick and D. Dolphin, Chem. Commun.,
2009, DOI: 10.1039/b901171b.
◦
◦
3
-3
˚
77.368(5) , g = 81.828(5) , V = 1051.0(2) A , Dc = 1.603 g cm , Z = 2,
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