Intramolecular [2+2+2] cycloaddition of bis(propargylphenyl)carbodiimides: Synthesis of L-shaped π-extended compounds with pyrrolo[1,2-a][1,8] naphthyridine corner units

Article abstract of DOI:10.1039/c3cc42792g

L-shaped π-extended penta-, hexa-, and heptacycles with a pyrrolo[1,2-a][1,8]naphthyridine junction were prepared from N,N′-bis[2-(2-alkyn-1-yl)phenyl]carbodiimides or their naphthyl analogs via Rh(i)-catalyzed intramolecular [2+2+2] cycloaddition and dehydrogenative aromatization. These L-shaped compounds emit sky-blue, yellow-green, or golden-orange fluorescence, with high quantum yields. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc42792g

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Intramolecular [2+2+2] cycloaddition of
bis(propargylphenyl)carbodiimides: synthesis of
L-shaped p-extended compounds with
Cite this: Chem. Commun., 2013,
49, 6206
pyrrolo[1,2-a][1,8]naphthyridine corner units†
Received 16th April 2013,
Accepted 22nd May 2013
Takashi Otani,ab Takao Saito,*a Ryota Sakamoto,a Hiroyuki Osada,a
Akihito Hirahara,a Naoki Furukawa,a Noriki Kutsumura,a Tsukasa Matsuob and
Kohei Tamaob
DOI: 10.1039/c3cc42792g
www.rsc.org/chemcomm
L-shaped p-extended penta-, hexa-, and heptacycles with a pyrrolo-
[1,2-a][1,8]naphthyridine junction were prepared from N,N0-bis-
[2-(2-alkyn-1-yl)phenyl]carbodiimides or their naphthyl analogs
via Rh(I)-catalyzed intramolecular [2+2+2] cycloaddition and dehydro-
genative aromatization. These L-shaped compounds emit sky-blue,
yellow-green, or golden-orange fluorescence, with high quantum
yields.
Transition-metal-catalyzed cycloadditions are powerful methods
for the synthesis of structurally complex and diverse carbocycles
and heterocycles.1–3 In particular, the [2+2+2] cycloaddition has
become an established tool for construction of a broad variety of
polycyclic aromatics and hetero-aromatics such as benzenes,
pyridines, and related systems.3 However, the majority of these
[2+2+2] cycloaddition reactions involve the use of (1) unsaturated
carbon components such as alkenes, alkynes, enynes, dienynes,
and endiynes, and (2) a limited number of unsaturated hetero-
components such as carbonyls, imines, nitriles, isocyanates, and
isothiocyanates.3,4 Furthermore, very few examples5,6 of utilizing
a fully intramolecular mode of the [2+2+2] cycloaddition with
hetero-components have been reported despite that three rings
can be constructed in a single reaction.4
Herein, we report the unprecedented, fully intramolecular
[2+2+2] cycloadditions of bis(propargylphenyl)carbodiimides,
and that the aromatized cycloaddition products, L-shaped
p-extended penta-, hexa-, and heptacycles with pyrrolo[1,2-a][1,8]-
naphthyridine corner units, show unique and interesting photonic
and electronic properties, such as high fluorescence quantum yield
in polar solvents, and spatially separated HOMOs and LUMOs.
a Department of Chemistry, Faculty of Science, Tokyo University of Science,
Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan.
E-mail: tsaito@rs.kagu.tus.ac.jp; Fax: +81 (0)3-5261-4631
b Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute,
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
† Electronic supplementary information (ESI) available: Experimental details,
characterization data, DFT calculations, and complete listing for ref. 11. CCDC
924311. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc42792g
Scheme 1 Synthesis of L-shaped (a) pentacycles 4, (b) hexacycles 5, 6, and
(c) heptacycles 7.
c
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We also note that L-shaped pentacyclic compounds with a
The L-shaped molecules can be regarded as p-extended
6–6–6–5–6 ring system have been investigated to a small extent,7 indolizines (pyrrolo[1,2-a]pyridine).9 Density functional theory
and no studies of hexacyclic and heptacyclic L-shaped compounds (DFT) calculations revealed that an extended HOMO resides on
have been reported.
the electron-rich pyrrole ring, whereas the LUMO mostly
Scheme 1 shows the synthesis of L-shaped pentacycles 4, resides on the electron-deficient pyridine ring.9b,10 We envi-
hexacycles 5, 6, and heptacycles 7. Carbodiimide 1a (R = Me) sioned that if such an electronic feature was preserved in the
was converted into 4a in an isolated yield of 90% by means of L-shaped molecules, their HOMOs and LUMOs would be
the intramolecular [2+2+2] cycloaddition using Wilkinson’s spatially separated, which would enable us to set the HOMO
catalyst, followed by dehydrogenation with MnO2 (Scheme 1). and LUMO energy levels independently. To determine the
Similarly, 4b (R = pent), 5a,b, 6a,b, and 7a,b were synthesized electronic structures of the L-shaped compounds, DFT compu-
from 1b, 2a,b, and 3a,b in 93%, 52%, 86%, 65%, and 79% tations at the B3LYP/6-31G(d,p) level were carried out for 4a, 5a,
yields, respectively (see ESI† for the details). In the [2+2+2] 6a, and 7a using the Gaussian 03 program package (Fig. 1).11 As
cycloaddition of unsymmetrical naphthyl phenyl carbodiimides expected, the HOMOs are primarily on the pyrrole-containing
2, two regioisomers of hexacycles 5a,b and 6a,b were obtained sides, while the LUMOs are on the pyridine-containing sides;
as a mixture in 52% yield (51 : 49) for a (R = Me) and 86% yield therefore, their HOMO and LUMO energy levels are signifi-
(53 : 37) for b (R = pent). The isomers 5a,b and 6a,b were then cantly influenced by the side on which their lobes reside. The
separated by fractional recrystallization. The structure of 5a was HOMO levels of molecules with the same pyrrole-containing
determined by the X-ray analysis (Fig. S1, ESI†).8
side were comparable (the 4a (ꢀ4.96 eV) and 5a (ꢀ5.00 eV) pair
compared with the 6a (ꢀ4.76 eV) and 7a (ꢀ4.82 eV) pair), as are
the LUMO levels of molecules with the same pyridine-containing
side (for example, the 4a (ꢀ1.81 eV) and 5a (ꢀ1.91 eV) pair
compared with the 6a (ꢀ2.13 eV) and 7a (ꢀ2.18 eV) pair).
Fig. 2 shows the UV-vis absorption, fluorescence spectra, and
photographs under irradiation with UV light of 4a, 5a, 6a, and 7a
in dichloromethane, and the spectral and the electronic data are
summarized in Table 1. Prominent features observed are as
follows. (i) Absorption and emission bands were red-shifted with
an increase in the p-conjugation length from 4 to 7, indicating
effective conjugation over the skeleton. The time-dependent
(TD)-DFT calculations were in good qualitative agreement with
lmax(abs). The lowest energy absorption bands are assignable to
the intramolecular charge-transfer transition from the HOMO to
the LUMO with an oscillator strength (f) of 0.29–0.36 (also see
ESI†); (ii) the compounds showed sky-blue to golden-orange
fluorescence with high quantum yields. The fluorescence wave-
lengths depended on the number of fused rings—4a (sky-blue,
FF = 0.83), 5a (yellow-green, FF = 0.76), 6a (yellow-green,
FF = 0.76), and 7a (golden-orange, FF = 0.67); (iii) even though
Fig. 1 Frontier MOs and energy levels of 4a, 5a, 6a, and 7a. The vacuum level: 0 eV. these compounds comprise a donor (pyrrole ring)–acceptor
Fig. 2 (a) UV-vis spectra, (b) fluorescence spectra (excited at lmax(abs)), and (c) photographs under irradiation with UV light (l = 365 nm) in CH2Cl2 of 4a, 5a, 6a, and
7a (from left to right).
c
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Table 1 Photophysical and electrochemical properties of L-shaped compounds
Calculation
max(abs)a (nm) [e (104 cmꢀ1
M
ꢀ1)]
l
max(em)a,b (nm)
FF
ts (ns)
lmax(abs)
f c
E
(V)
E
(V)
a,b
a
ox d,e, f
p,a
red a,d
1/2
Compound
l
4a
5a
6a
7a
442 (1.4), 421 (1.9), 403 (1.4, sh)
479 (1.6), 451 (2.1), 429 (2.0)
483 (1.6), 455 (2.3), 432 (1.6)
517 (1.6), 485 (2.1), 434 (2.6)
474, 508
523, 544
512, 544
556, 583
0.83
0.76
0.76
0.67
5.44
6.09
5.95
6.34
433
479
482
520
0.31
0.36
0.29
0.33
0.40
0.52
0.32
0.32
ꢀ2.40
ꢀ2.10
ꢀ2.30
ꢀ2.04
a
b
c
d
e
f
In CH2Cl2. Excitation at lmax(abs). Oscillator strength. Potential vs. ferrocenium/ferrocene. In THF. Irreversible.
Table 2 Photophysical properties of 4b in various solvents
max(abs) (nm) [e (10ꢀ4 cmꢀ1
M
ꢀ1)]
l
max(em)a (nm)
FF
ts (ns)
a
a
Stokes shift (cmꢀ1
)
Solvent
l
Methanol
Dichloromethane
Hexane
444 (1.1), 422 (1.8), 404 (1.4, sh)
448 (1.4), 425 (1.9), 406 (1.4, sh)
455 (1.5), 428 (2.0), 405 (1.4, sh)
473, 507
477, 512
462, 495, 531
0.83
0.82
0.80
6.90
5.78
4.26
1380
1360
330
a
Excitation at lmax(abs).
3 For recent reviews on [2+2+2] cycloadditions, see: (a) D. L. J. Broere
and E. Ruijter, Synthesis, 2012, 2639–2672; (b) Y. Shibata and
K. Tanaka, Synthesis, 2012, 323–350; (c) S. Okamoto, Heterocycles,
2012, 85, 1579–1602.
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see: (a) T. Shibata, T. Uchiyama, Y. Yoshinami, S. Takayasu,
K. Tsuchikama and K. Endo, Chem. Commun., 2012, 48, 1311–1313;
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G. Chouraqui, M. Malacria, C. Aubert and V. Gandon, Adv. Synth.
Catal., 2009, 351, 271–275. For fully intramolecular hetero-[2+2+2]
cycloaddition of nitrile–diynes, see: (d) H.-T. Chang, M. Jeganmohan
and C.-H. Cheng, Org. Lett., 2007, 9, 505–508.
5 Intermolecular [2+2+2] cycloadditions of carbodiimides with two
alkynes: (a) P. Hong and H. Yamazaki, Tetrahedron Lett., 1977, 18,
1333–1336; (b) H. Hoberg and G. Burkhart, Synthesis, 1979, 525–526;
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Soc., 2002, 124, 5059–5067.
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tions of polymer supported carbodiimide with diyne: (a) D. D.
Young and A. Deiters, Angew. Chem., Int. Ed., 2007, 46, 5187–5190;
(b) D. D. Young, J. A. Teske and A. Deiters, Synthesis, 2009,
3785–3790. Asymmetric rhodium-catalyzed [2+2+2] cycloaddition
of alkenyl carbodiimides with alkyne: (c) R. T. Yu and T. Rovis,
J. Am. Chem. Soc., 2008, 130, 3262–3263.
7 (a) S. V. Litvinenko, Y. M. Volovenko and F. S. Babichev, Chem.
Heterocycl. Compd., 1992, 1307–1311; (b) S. V. Litvinenko, Y. M.
Volovenko and F. S. Babichev, Chem. Heterocycl. Compd., 1993,
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H. Niwa and T. Hirano, Tetrahedron Lett., 2010, 51, 1401–1403.
8 CCDC 924311 contains the supplementary crystallographic data for
this communication†.
9 (a) A. Brandi and S. Cicchi, in Comprehensive Heterocyclic Chemistry III,
ed. A. R. Katritzky, C. A. Ramsden, E. F. V. Scriven and R. J. K. Taylor,
Elsevier Ltd., Oxford, 2008, vol. 11, pp. 367–408; (b) C.-H. Park,
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(pyridine ring) heterojunction, they exhibited weak solvato-
chromism (0–0) emission band of 4b at 473 nm in MeOH and
at 462 nm in hexane, and their fluorescence was hardly quenched
in polar solvents (FF: 0.83 in MeOH and 0.80 in hexane, Table 2);12
(iv) cyclic voltammetry studies in dichloromethane and THF
(summarized in Table 1) revealed that the first oxidation poten-
tials, Eopx,a, are affected mainly by the pyrrole-containing sides of
ox
p,a
the L-shaped molecules (note that E of 5a is similar to that of 4a).
On the other hand, the first reduction potentials, Er1e/2d, greatly
depend on the electronic structure of the pyridine-containing sides
red
1/2
(note that E of 5a is similar to that of 7a). These results are
consistent with the DFT calculations mentioned above.
In conclusion, we demonstrated a useful, one-flask synthesis
of penta-, hexa-, and heptacyclic L-shaped p-extended com-
pounds with a pyrrolo[1,2-a][1,8]naphthyridine junction, based
on the rhodium(I)-catalyzed [2+2+2] cycloaddition of N,N0-bis-
[2-(2-alkyn-1-yl)phenyl]carbodiimides or their naphthyl analogs.
The L-shaped compounds emit sky-blue to golden-orange
fluorescence in high quantum yields, even in polar solvents.
Cyclic voltammetry studies and DFT calculations indicate that
the HOMO and LUMO energy levels mostly depend on the
p-system of the pyrrole- and pyridine-containing sides, respec-
tively. p-Elongation from 4 to 5, and from 4 to 6, demonstrated
independent depression of the LUMO and elevation of the
HOMO levels, respectively, offering a new electronic tuning
method using these L-shaped structures.
We thank the Ministry of Education, Culture, Sports,
Science, and Technology of Japan for the Grant-in-Aid for
Specially Promoted Research (No. 19002008).
Notes and references
10 K. P. C. Vollhardt and N. E. Schore, Organic Chemistry: Structure and
Function, W. H. Freeman and Company, New York, 4th edn, 2003.
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3149–3159; (c) N. T. Patil and Y. Yamamoto, Chem. Rev., 2008, 108, 12 Many D–A molecules show intense solvatochromism in the emission
3395–3442.
and significant quenching of the fluorescence: (a) K. M. Omer, S.-Y. Ku,
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c
6208 Chem. Commun., 2013, 49, 6206--6208
This journal is The Royal Society of Chemistry 2013