A study on the relationship between the twisted π-conjugate system of 1,5-diaryl-1,5-diazapenta-1,3-dienes and their photophysical properties

Article abstract of DOI:10.1246/cl.2012.984

One-pot treatment of commercially available 1,3,3-tributoxy-2-cyanopropene with 2 equivalents of primary aromatic amines readily afforded 1,5-diaryl-1,5-diaza-3-cyanopenta-1,3-dienes. UV properties varied as steric size of the orthosubstituent in the N-aryl units increased. X-ray crystallographic analyses indicated preferable conformation of these compounds, and the twist angles between 1,5-diazapentadiene and aromatic units were measured. The relationship between the twisted angles and UV absorptions was rationalized by MO calculation, and the UV peak wavelengths are useful for the estimation of the twisted angles. A novel tetracoordinate diaryl-1,5- diazapentadiene was prepared. UV spectrum for its lithium complex in CHCl ₃ supported that the complex had a twisted conformation of the aryl units because the two etharial oxygen atoms coordinated to lithium cation.

Full text of DOI:10.1246/cl.2012.984

doi:10.1246/cl.2012.984
984
Published on the web September 8, 2012
A Study on the Relationship between the Twisted ³-Conjugate System
of 1,5-Diaryl-1,5-diazapenta-1,3-dienes and Their Photophysical Properties
Akio Kamimura,*1 Hideki Matsu,1 Hiroyuki Suzukawa,1 Eisuke Sato,1 Michinori Sumimoto,2 and Hidemitsu Uno3
1Department of Applied Molecular Bioscience, Graduate School of Medicine,
Yamaguchi University, Ube, Yamaguchi 755-8611
2Department of Material Chemistry, Graduate School of Science and Engineering,
Yamaguchi University, Ube, Yamaguchi 755-8611
3Department of Chemistry, Graduate School of Science and Engineering,
Ehime University, Matsuyama, Ehime 790-8577
(Received June 19, 2012; CL-120659; E-mail: ak10@yamaguchi-u.ac.jp)
One-pot treatment of commercially available 1,3,3-tribu-
CN
toxy-2-cyanopropene with 2 equivalents of primary aromatic
amines readily afforded 1,5-diaryl-1,5-diaza-3-cyanopenta-1,3-
dienes. UV properties varied as steric size of the ortho-
substituent in the N-aryl units increased. X-ray crystallographic
analyses indicated preferable conformation of these compounds,
and the twist angles between 1,5-diazapentadiene and aromatic
units were measured. The relationship between the twisted
angles and UV absorptions was rationalized by MO calculation,
and the UV peak wavelengths are useful for the estimation of the
twisted angles. A novel tetracoordinate diaryl-1,5-diazapenta-
diene was prepared. UV spectrum for its lithium complex in
CHCl3 supported that the complex had a twisted conformation of
the aryl units because the two etharial oxygen atoms coordinated
to lithium cation.
CN
CH(OBu)2
1) AcOH, H2O
2) ArNH2
BuO
N
N
Ar
H
1
Ar
Scheme 1.
Desired 1,5-diazapenta-1,3-dienes 1 were prepared by
treatment of 1,3,3-tributoxy-2-cyanopropene with catalytic
amounts of AcOH in wet CH3CN.7 ¢-Butoxy-¡-cyanoacrolein
was formed as an intermediate that was not very stable.
Additional amounts of aromatic amine finished the reaction to
give 1 in good yields (Scheme 1).8 The results are summarized
in Table 1.
Use of aniline gave compound 1a in 88% yield (Entry 1).
o-Substituents did not give significant steric hindrance of the
formation and compounds 1b to 1f were obtained in good yields
(Entries 2-6). Compounds 1a and 1b were isolated as a yellow
solid, but the yellow color became paler as the steric size of
the ortho-substituents became larger. Aliphatic amine such as
benzylamine also afforded the corresponding azadiene 1g in
moderate yields as a colorless solid (Entry 7). NMR analyses
revealed that all the compounds 1a to 1g should prefer U-shape
conformation in CDCl3 because broadened N-H protons were
observed at 12-13 ppm.
We next examined their UV-vis absorption spectra for
compounds 1. A spectral chart is depicted in Figure 1.
Absorption peak -max in CHCl3 and some molar absorbance
coefficients are collected in Table 1. For example, compound 1a
had its absorption peak at 378 nm and the absorption edge of
the spectra remained over >400 nm region (Entry 1). Most of
compounds 1 that had aromatic rings at N-substituents had their
1,5-Diazapentadienes (¢-iminate) contain a unique conjugate
system because it is a merge of an enamine, an electron-donating
group, and an imine, an electron-withdrawing group, that
provides an electronic push-pull olefin unit.1 This unit has been
employed as an asymmetric catalyst for cyclopropanation.2
Recently, such conjugate systems have been extensively explored
as fluorescent material such as BODIPY.3 Due to an acidic
hydrogen atom at the enamine nitrogen, 1,5-diazapentadienes
prefer U-shaped conformation because the two nitrogens coor-
dinate to the hydrogen atom by hydrogen bonding.4 The
molecules serve as good bidentate ligands for various metallic
cations forming a metal complex.5 Use of the complex provides a
versatile synthesis of polycarbonate that was reported enthusias-
tically by Coats and his co-workers.6 For example they employed
an aluminum complex of 1,5-diaryl-1,5-diazapenta-1,3-diene
for the catalyst of the formation of polycarbonates. Sterically
hindered ortho-substituents for the aryl group were usually
employed and the two aromatic groups should prefer orthogonal
conformation to the plane of diazapentadiene ligands. We are
interested in the formation of the 1,5-diazapentadienes from
commercially available 1,3,3-tributoxy-2-cyanopropene and have
examined them to develop new functional materials. For the
development of their photophysical properties, we prepared a
variety of 1,5-di(o-substituted)aryl-1,5-penta-1,3-dienes. In this
paper we report the relationship between their UVabsorptions and
twist angles between the aryl group and 1,5-diazapentadienes.
MO calculation also supports the results. We also prepared the
ligands containing two additional oxygen ligand that are expected
to form tetradentate complexes with lithium cation.
-
max around 331 to 378 nm. It should be mentioned that apparent
blue shift was observed as the steric size of ortho-substituent
increased. For example, unsubstituted and o-methyl-substituted
diazapentadienes 1a and 1b showed -max around 378 and
371 nm (Entries 1 and 2), while absorptions for o-ethyl 1c,
o-isopropyl 1d, and o-tert-butyl 1e derivatives appeared -max at
367, 366, and 351 nm, respectively (Entries 3, 4, and 5). 2,6-
Dimethyl derivative 1f, that had congested substituents at ortho
positions, showed -max at 331 nm, and no shoulder peak was
observed in the visible region (Entry 6). N-Aliphatic-substituted
1,5-diazapentadiene 1g has a UV peak at 317 nm and all
absorbance appeared at the region below 350 nm (Entry 7).
Chem. Lett. 2012, 41, 984-986
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

985
Table 1. Preparation of 1,5-diaryl-1,5-diazapenta-1,3-diene 1
NH2
OH
NH2
t-BuOK/18-crown-6
MeI, THF
Yield Mp
/%a /°C
-
max
/nmb
Entry
1
Ar
O
–50 °C to r.t.
128.0-129.0 378 (2.34 © 104)
122.0-123.0 371 (2.28 © 104)
108.0-108.5 367 (2.05 © 104)
145.0-146.0 366 (2.11 © 104)
173.8-174.2 351 (1.94 © 104)
153.0-154.0 331 (1.66 © 104)
2; 55%
1
2
3
4
5
6
7
1a Ph
1b o-Tol
88
64
55
67
78
CN
1c o-EtC6H4
1d o-i-PrC6H4
1e o-t-BuC6H4
1f 2,6-Me2C6H3 58
1g Bn 45
O
BuOCH=C(CN)CH(OBu)2
CH3CN/AcOH/H2O/reflux
N
N
H
62.0-63.0
317 (1.67 © 104)
O
aIsolated yields. bMeasured in CHCl3. ¾ values were in
parentheses.
1h; 63%
Scheme 2.
To rationalize the relationship between the twisted angle and
steric hindrance of the ortho-substituents, we carried out MO
calculations for compounds 1a to 1f. Geometry optimizations
were performed using the DFT method, where the B3PW91
functional was used for the exchange-correlation term. Opti-
mized geometries were used for TD-B3PW91 calculations. To
investigate the solvent effect of CHCl3, we used the CPCM
method. The 6-311G(d,p) basis sets were employed in these
calculations. These calculations were performed with the
Gaussian 09 program.10
The calculation predicted the -max peaks for compounds 1
as follows: 1a for 380 nm (obs. 378 nm), 1b for 374 nm (obs.
371 nm), 1d for 373 nm (obs. 366 nm), 1e for 356 nm (obs.
351 nm), and 1f for 349 nm (obs. 331 nm). The calculation
indicated -max values in good agreement with observed data
except for 1f, the reason of which is not clear at the moment. The
optimized structure of 1d and 1e showed that the twist angles
between 1,5-diazapentadiene and aromatic ring were estimated
to be 30 and 48° for 1d, and 47 and 55° for 1e, respectively. The
fully optimized structures of 1 were usually not C2 symmetric
because the proton between the two nitrogen atoms preferred to
form enamine N-H bond in the optimized structures. Due to
non-C2 structures were obtained for the optimized structure for
these compounds, there was a pair of twist angels for each
compound. These values are close to the measured values for 1d
and 1e, and thus the present relationship between the twist
angles and UV absorption is well rationalized. We think that the
present relationship will be potentially useful for the UV on-off
functions using conformational change in the twist angles.
We next prepared tetracoordinative 1,5-diazapentadiene
ligand 1h (Scheme 2). Commercially available 2-aminobenzyl
alcohol was smoothly O-methylated to give 2 in 55% yield on
treatment with t-BuOK and MeI in the presence of 18-crown-6.
Conversion to 1h was achieved in 63% yield in a similar manner
for the preparation of other 1. Compound 1h showed 386 nm for
its -max in UV spectrum in CHCl3.
Figure 1. UV-vis absorption spectra of 1 in CHCl3 (concen-
tration of 1 was 2 © 10¹5 mol L¹1).
Figure 2. X-ray crystallographic chart of 1e.
Fortunately, compounds 1d and 1e afforded good crystals
that were useful for X-ray crystallographic analyses.9 ORTEP
chart for 1e is shown in Figure 2. As expected, 1,5-diazapenta-
dine took a planar U-shape conformation. The two aromatic
rings twisted in the same directions and at almost the same
angles; their structure was very symmetric and compounds 1d
and 1e showed C2 symmetric structure. The twisted angle was
measured to be 54° for 1e. The corresponding 1d structure
showed that the twist angle was 29°. Thus, the twist angle
increased as the steric size of the ortho-substituent became large.
Treatment of 1h with BuLi in CH2Cl2 or THF resulted in the
formation of corresponding lithium salt 1h-Li, which was
isolated by concentration under nitrogen atmosphere. Use of the
salt 1h-Li allowed us to investigate its UV and NMR properties.
For example, a singlet signal derived from benzyl protons and
O-methyl proton in 1h appeared in 4.46 and 3.22 ppm, which
moved to 4.41 and 3.30 ppm in 1h-Li, respectively. The -max
shift was also observed in UV spectra for 1h-Li in CHCl3 and its
-
appeared at 364 nm. It should be mentioned that this
max
Chem. Lett. 2012, 41, 984-986
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

986
1.5 Hz, 2H), 6.92 (dd, J = 7.7, 1.4 Hz, 2H), 1.30 (s, 18H);
13C NMR (126 MHz, CDCl3): ¤ 155.1, 145.4, 141.9, 127.4, 126.7,
126.1, 123.1, 121.1, 81.1, 35.0, 30.7; HRMS (ESI), [M ¹ H] :
complex showed remarkable stability toward the treatment with
MeOH. This was striking in contrast to a complex of lithium
cation with 1a, which decomposed immediately by addition of
MeOH and gave uncomplexed 1a quantitatively.
¹
m/z 358.2276. Calcd for C24H28N3: 358.2283. Other compounds 1
were prepared in a similar manner.11 1a: Yellow solid; mp 128.0-
129.0 °C; 1H NMR (500 MHz, CDCl3): ¤ 13.20 (brs, 1H), 8.06
(s, 2H), 7.40 (dd, J = 8.3, 7.5 Hz, 4H), 7.22 (t, J = 7.4 Hz, 2H),
7.15 (d, J = 7.5 Hz, 4H); 13C NMR (126 MHz, CDCl3): ¤ 152.3,
144.4, 129.7, 125.8, 119.8, 119.0, 81.2; HRMS (ESI), [M + H]+:
m/z 248.1184. Calcd for C16H14N3: 248.1188; 1b: Yellow solid;
The present method provides a facile preparation of various
1,5-diaryl-1,5-diazapenta-1,3-dienes 1 which possess interesting
UV properties. The existence of the ortho substituents of
aromatic rings caused twist conformation between 1,5-diaza-
pentadiene and aromatic rings, and the twisting angles relate
to -max of their UV spectra. The twist was induced by the
coordination of the alkali metal complex for the ligand
containing two methoxy groups for the coordination. This novel
multidentate ligand for alkali metal effectively stabilized against
hydrolysis by MeOH. Further application of these 1,5-diaza-
pentadienes is now under investigation in our laboratory.
1
mp 122.0-123.0 °C; H NMR (500 MHz, CDCl3): ¤ 13.01-12.54
(br, 1H), 7.98 (s, 1H), 7.97 (s, 1H), 7.29-7.19 (m, 4H), 7.12
(t, J = 7.4 Hz, 2H), 7.03 (d, J = 7.9 Hz, 2H), 2.33 (s, 6H);
13C NMR (126 MHz, CDCl3): ¤ 153.2, 144.1, 130.9, 129.0, 127.4,
125.7, 121.0, 117.4, 81.7, 18.1; HRMS (ESI), [M + H]+: m/z
276.1470. Calcd for C18H18N3: 276.1501; 1c: Yellow solid; mp
108.0-108.5 °C; 1H NMR (500 MHz, CDCl3): ¤ 12.72 (t, J =
5.8 Hz, 1H), 7.97 (s, 1H), 7.96 (s, 1H), 7.29-7.22 (m, 4H), 7.18
(dd, J = 10.9, 4.3 Hz, 2H), 7.02 (d, J = 7.8 Hz, 2H), 2.70 (q,
J = 7.5 Hz, 4H), 1.17 (t, J = 7.4 Hz, 6H); 13C NMR (126 MHz,
CDCl3): ¤ 153.7, 143.8, 135.2, 129.1, 127.4, 126.0, 121.1, 118.2,
We are grateful to a financial aid from Yamaguchi
University based on The YU Strategic Program for Fostering
Research Activities (2010-2011).
¹
81.7, 24.5, 14.4; HRMS (ESI), [M ¹ H] : m/z 302.1662. Calcd
for C20H20N3: 302.1657; 1d: Yellow solid; mp 145.0-146.0 °C;
1H NMR (500 MHz, CDCl3): ¤ 12.71 (t, J = 5.8 Hz, 1H), 7.95 (s,
1H), 7.93 (s, 1H), 7.31 (dd, J = 7.3, 1.8 Hz, 2H), 7.27-7.19 (m,
4H), 7.00 (dd, J = 7.5, 1.6 Hz, 2H), 3.25 (dt, J = 13.7, 6.9 Hz,
2H), 1.20 (d, J = 6.9 Hz, 12H); 13C NMR (126 MHz, CDCl3): ¤
154.0, 143.3, 139.8, 127.2, 126.3, 125.9, 121.2, 118.7, 81.8, 28.1,
References and Notes
1
For review on 1,5-diazapenta-1,3-dienes and their cationic salts,
see: D. Lloyd, H. McNab, Angew. Chem., Int. Ed. Engl. 1976, 15,
459.
2
3
4
A. Pfaltz, J. Heterocycl. Chem. 1999, 36, 1437; A. Pfaltz, Synlett
1999, 835; A. Pfaltz, Chimia 1999, 53, 220.
Recent review on BODIPY: M. Benstead, G. H. Mehl, R. W.
Boyle, Tetrahedron 2011, 67, 3573.
¹
23.2; HRMS (ESI), [M ¹ H] : m/z 330.1968. Calcd for C22H24N3:
330.1970; 1f: Pale yellow solid; mp 153.0-154.0 °C; 1H NMR
(500 MHz, CDCl3): ¤ 12.40 (brs, 1H), 7.70 (s, 2H), 7.09 (d,
J = 7.5 Hz, 4H), 7.04 (dd, J = 8.5, 6.4 Hz, 2H), 2.24 (s, 12H);
13C NMR (126 MHz, CDCl3): ¤ 158.1, 143.8, 130.4, 128.7, 125.9,
a) L. C. Dorman, Tetrahedron Lett. 1966, 7, 459. b) E. Daltrozzo,
K. Feldmann, Angew. Chem. 1967, 79, 153. c) K. Feldmann, E.
Daltrozzo, G. Scheibe, Z. Naturforsch. 1967, 22b, 722. d) E.
Daltrozzo, K. Feldmann, Tetrahedron Lett. 1968, 9, 4983. e) H.-H.
Limbach, W. Seiffert, Tetrahedron Lett. 1972, 13, 371.
L. Bourget-Merle, M. F. Lappert, J. R. Severn, Chem. Rev. 2002,
102, 3031.
a) S. D. Allen, D. R. Moore, E. B. Lobkovsky, G. W. Coates,
J. Organomet. Chem. 2003, 683, 137. b) D. R. Moore, M. Cheng,
E. B. Lobkovsky, G. W. Coates, J. Am. Chem. Soc. 2003, 125,
11911. c) L. R. Rieth, D. R. Moore, E. B. Lobkovsky, G. W.
Coates, J. Am. Chem. Soc. 2002, 124, 15239. d) M. Cheng, D. R.
Moore, J. J. Reczek, B. M. Chamberlain, E. B. Lobkovsky, G. W.
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Preparation of 1e, typical procedure: 1,3,3-tributoxy-2-cyanopro-
pene (1.524 g, 5.39 mmol) was dissolved in CH3CN (10 mL) and
water (0.164 g, 9.11 mmol) and AcOH (0.117 g, 1.94 mmol) were
added. The resulting solution was stirred at room temperature for
27 h, when disappearing of 1,3,3-tributoxy-2-cyanopropene was
confirmed on TLC analysis. PPTS (0.561 g, 2.23 mmol) and o-tert-
butylaniline (2.1625 g, 14.49 mmol) were added to the solution
and the reaction mixture was heated to the refluxing temperature
for 47 h. Solvent was removed in vacuo and EtOAc (100 mL) was
added to the residue. The resulting solution was washed with sat.
NaHCO3(aq) (100 mL) and dried over Na2SO4. Filtration and
evaporation gave crude 1e, which was recrystallized from EtOAc
to give 1e in 78% yield (1.515 g, 4.21 mmol). Pale yellow solid;
mp 173.8-174.2 °C; 1H NMR (500 MHz, CDCl3): ¤ 12.40 (t,
J = 6.0 Hz, 1H), 7.77 (s, 1H), 7.75 (s, 1H), 7.38 (dd, J = 7.8,
1.4 Hz, 2H), 7.26 (td, J = 7.5, 1.5 Hz, 2H), 7.19 (td, J = 7.6,
¹
121.2, 78.9, 18.6; HRMS (ESI), [M ¹ H] : m/z 302.1644. Calcd
for C20H20N3: 302.1657; 1g: White solid; mp 62.0-63.0 °C;
1H NMR (500 MHz, CDCl3): ¤ 11.90-10.56 (brm, 1H), 7.66 (s,
2H), 7.35-7.24 (m, 6H), 7.18 (d, J = 7.6 Hz, 4H), 4.52 (s, 4H);
13C NMR (126 MHz, CDCl3): ¤ 157.7, 138.2, 128.8, 127.6, 127.5,
122.0, 76.8, 58.6; HRMS (ESI), [M + H]+: m/z 276.1452. Calcd
for C18H18N3: 276.1501; 1h; Pale yellow solid; mp 116.0-
5
6
1
117.0 °C; H NMR (500 MHz, CDCl3): ¤ 12.60 (brs, 1H), 7.94 (s,
1H), 7.93 (s, 1H), 7.37 (d, J = 6.3 Hz, 4H), 7.20 (t, J = 7.3 Hz,
2H), 7.08 (d, J = 8.0 Hz, 2H), 4.46 (s, 4H), 3.22 (s, 6H); 13C NMR
(126 MHz, CDCl3): ¤ 153.4, 144.7, 129.4, 129.4, 129.3, 125.5,
120.9, 118.4, 82.1, 71.5, 58.3; HRMS (ESI), [M + H]+: m/z
336.1680. Calcd for C20H22N3O2: 336.1712.
9
Crystallographic data for the structures 1d and 1e. CCDC 886786
and 886785, respectively.
7
8
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Chem. Lett. 2012, 41, 984-986
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/