- Email: [email protected]
Geometrically constrained thiophene-pyrrole oligomers
Tetrahedron Letters, Vol. 36, No. 38, pp. 6835-6838, 1995 Elsevier Science Ltd Printed in Great Britain 0040-4039/95 $9.50+0.00
Pergamon 0040-4039(95)01441-1
Geometrically Constrained Thiophene-Pyrrole Oligomers Masatoshi Kozaki, James P. Parakka, and Michael P. Cava*
Departmentof Chemistry,The Universityof Alabama Box 870336, Tuscaloosa,Alabama 35487-0336, U.S.A.
Abstract: A series of novel mixed thiophene-pyrrole penta- and heptacyclic oligomers has been prepared in which two pyrrole nitrogens of each oligomer are connected by a polymethylene bridge. The stenc constraints produced in these bridged compounds are reflected in their spectroscopic and electrochemical properties.
OligothJophenes are currently attracting much attention as a result of their novel biological and physical properties. 1 Moreover, these oligomers have been recognized not only as model compounds for the corresponding polymer, but also promising candidates for electronic and optical devices.2 The insolubility and poor processability of higher thiophene oligomers have, however, limited both their detailed characterization as well as their application potentials. These problems have prompted us to study mixed thiophene-pyrrole containing oligomers in view of the ease of incorporation of solubilizing groups on the pyrrolic nitrogen as well as by the decrease in their oxidation potentials relative to the corresponding thiophene oligomers.3 In this paper, mixed thiophene-pyrrole, penta- and heptacyclic oligomers tethered by polymethylene chains were synthesized with the objective of constraining the conformation of the oligomer and observing the resulting effect on its spectroscopic and electrochemical properties. Pentacycle l a was obtained in 15% yield by refluxing tetraketone 24 and 3 equivalents of 1,6diaminohexane in the presence of propionic acid in toluene. In this reaction, diarnide 3a was also generated in 33% yield. The other pentacycles lb,e were also synthesized along with diamides 3b,¢ by a similar procedure using 4 equivalents of the corresponding diamine (Table 1). Double Knorr-Paal condensations of 4a, b 3a with
l, 12-diaminododecanegave beptacycles 5a, b in addition to diamides 6a,b. Table 1. Double Knorr-Paal Reactions of Bis-1,4-tetraketones and Diamines.
Entry
Tetraketone
Diamme
1
2
H2N(CH2)6NH2
la(15)
3a(33)
2
2
H2N(CH2)8NH2
lb(54)
3b(30)
3
2
H2N(CH2)I2NH2
le(45)
3e(27)
4
4a
H2N(CH2)12NH2
5a(27)
6a(ll)
5
4b
H2N(CH2)I2NH2
5b(28)
6b(23)
68 ~5
Yield (%)
6836
In the double Knorr-Paal condensation the starting diketones have a surprising tendency to undergo an intramolecular reaction in moderate yields, in spite of a considerable excess of diamine. In addition, in the case of l a , the C-6 chain provides a very short linkage of the two pyrrole rings so that the molecule l a is sterically very hindered. In the case of 5b, there are three heterocyclic rings between two linked pyrroles, and the central pyrrole ring already bears a bulky dodecyl chain which should impede the intramolecular bridging reaction.
o
o
2: R = tl~enyl 4a: R = bistl~enyl-l-methylpyrmle 4b: R = bisthienyl-l-dodecylpyrrole
3a: R 3b: R 3c: R 7: R
= = = =
1~,: R = C6H12 lb: R = CsHt6 lc: R = C12H24
C6H12NHCOEt CgHI6NHCOEt CI2H2,INHCOEt C12H25
5a: R = C H 3
5b: R =
C12H25
6a: R = CH3
R'= CI2H24NHCOEt 6b: R = C12H25
R'= CI2H2~/HCOEt 8: R = C12H25, R'= C18H37 Scheme
1
T a b l e 2. Absorption Maxima and Redox Values of Penta- and Heptacyclic Oligomers.
Ofigorner
kmax(nm) 1
Epa t / Epe l 2
Epa 2 / Epc 2 2
la
314
(0.70) 3 / 0.64
0.80 / (0.74) 3
lb
334
0.68 / 0.60
0.80 / 0.72
1c
349
0.66 / 0.60
0.78 / 0.72
7
349
0.67 / 0.58
0.83 / 0.75
5a
348
0.59 / 0.48
1.20 / 0.93
5b
349
0.60 / 0.51
1.26 / 1.08
8 363 0.64 / 0.51 1.26 / 1.19 (1) In CHCI3. (2) Measured in CICH2CH2CI containing 0.1 mol dm-3 tetrabutylammonium hexafluorophosphate using Pt disk as a working electrode at scan rate 100 mV s-I. V vs SCE. (3) Peak not sharply defined. The pentacyclic oligomers show UV-VIS absorption maxima in the range of 314-349 n m (Table 2). Notably, the m a x i m u m of l a appears at a significantly shorter wavelength than those of the other pentacycles.
6837
This can be attributed to a more non-planar structure forced by the shorter bridging C-6 chain. The resulting steric effect is also manifest in its 1H NMR, in which a large difference (0.12 ppm) was observed between two signals assigned to the protons of the pyrrole units of la. In contrast, the other pentacycles showed only a very small difference, less than 0.02 ppm, in these protons. On the other hand, while the C-12 bridged lc shows the same absorption maximum (349 run) as the linear pentacycle 7, 3b some conformational constraint exists in the C-8 bridged lb, the UV maximum of which is intermediate between the C-6 and C-12 bridged analogs. In the heptacyclic series, both the C-12 bridged 5a,b show a hypsochromic shift (around 15 nm) compared to the linear heptacycle 8, indicating an appreciable strain in the molecule. The anodic peak potentials (Epa) and cathodic peak potentials (Epe) of the bridged oligomers (la-e, 5a, b) and their non-bridged analogs 7 and g are listed in Table 2. The bridged pentacycles 1 reveal two quasireversible one-electron oxidation waves corresponding to the generation of radical cations and dications, respectively. The Epa values are all surprisingly close (within 0.03 V) to that of the linear oligomer 7, although the most strained member (la) has the highest oxidation potential. In the case of the linear heptacycle 8, the two observed oxidation waves are known to correspond to the formations of a dication and a trication radical, respectively.3d In this case, the C-12 bridged analogs 5a,b are more easily oxidized (by 0.05 and 0.04 V), implying at least a modest stabilization of the dication by the constraint of the bridge. AM1 calculations were performed for lb,c using two different starting structures, conformations A and B. For lb, both calculations gave the same optimized structure which is close to conformation B. Different optimized structures, closer to conformationa A and B, were obtained from calculations for le. While calculations for the bridged heptacycles 5a,b were not carried out, molecular models indicate a considerable curvature in these molecules, with the central N-alkyl group being out of the plane of the C-12 chain. The curvature of these oligomers make them attractive building units for the synthesis of large polycyclic rings, and of conjugated polymers containing curved moieties. Experiments aimed at these objectives are currently in progress.
ConformaaonA
ConformationB
ACKNOWLEDGEMENT The authors thank Dr. A. S. Jeevarajan and Dr. L. D. Kispert for assistance in performing the AM1 calculations. We also thank the National Science Foundation for a grant (CHE9224899) in support of this work.
6838
REFERENCES
1.
2.
3.
4.
(a) Nakayama, J.; Konishi, T.; Hoshino, M. Heterocycles 1988, 27, 1731. (b) Pham, C. V.; Burchardt. A.; Shabana, R.; Canningham, D. D.; Mark. Jr., B.; Zirnmer, H. Phosphorus, Sulfur, and Silicon 19119, 46, 153. (a) Akimichi, H.; Waragai, K.; Hotta, S.; Kano, H.; Sakaki, H. Appl. Phys. Lett. 1991, 58, 1500. (b) Geiger, F.; Stoldt, M.; Schweizer, H.; B~iuerle, P.; Umbaeh, E. Adv. Mater. 1993, 5, 922. (c) Gamier, F.; Deloffre, F.; Horowitz, G.; Hajlaoui, R. Synth. Met. 1993, 55-57, 4747. (d) Horowitz, G.; Delannoy, P.; Bouchriha, H.; Deloffre, F.; Fave, J.-L.; Gamier, F.; Hajlaoui, R.; Hryman, M.; Kouki, F.; Valat, P.; Wintgcns, V.; Yassar, A. Adv. Mater. 1994, 6, 752. (e) Gamier, F.; Hajlaoui, R.; Yassar, A.; Srivastave, P. Science 1994, 265, 1684. (a) Niziurski-Mann, R. E.; Cava, M. P. Adv. Mater. 1993, 5, 647. (b) Parakka, J. P.; Cava, M. P. Syntlt Met. 1995, 68, 275. (c) Liu, T.-L.; Parakka, J. P.; Cava, M. P.; Kim. Y.-T. Synth. Met. 1995, 71, 1989. (d) Parakka, J. P.; Jeevarajan, J. A.; Jeevarajan, A. S.; Kispert, L. D.; Cava, M. P., submitted for publication. Luo, T.-M. H.; LeGoff, E. J. Chin. Chem. Soc., 1992, 39, 325.
(Received in USA 12 June 1995; accepted 26 July 1995)
Pergamon 0040-4039(95)01441-1
Geometrically Constrained Thiophene-Pyrrole Oligomers Masatoshi Kozaki, James P. Parakka, and Michael P. Cava*
Departmentof Chemistry,The Universityof Alabama Box 870336, Tuscaloosa,Alabama 35487-0336, U.S.A.
Abstract: A series of novel mixed thiophene-pyrrole penta- and heptacyclic oligomers has been prepared in which two pyrrole nitrogens of each oligomer are connected by a polymethylene bridge. The stenc constraints produced in these bridged compounds are reflected in their spectroscopic and electrochemical properties.
OligothJophenes are currently attracting much attention as a result of their novel biological and physical properties. 1 Moreover, these oligomers have been recognized not only as model compounds for the corresponding polymer, but also promising candidates for electronic and optical devices.2 The insolubility and poor processability of higher thiophene oligomers have, however, limited both their detailed characterization as well as their application potentials. These problems have prompted us to study mixed thiophene-pyrrole containing oligomers in view of the ease of incorporation of solubilizing groups on the pyrrolic nitrogen as well as by the decrease in their oxidation potentials relative to the corresponding thiophene oligomers.3 In this paper, mixed thiophene-pyrrole, penta- and heptacyclic oligomers tethered by polymethylene chains were synthesized with the objective of constraining the conformation of the oligomer and observing the resulting effect on its spectroscopic and electrochemical properties. Pentacycle l a was obtained in 15% yield by refluxing tetraketone 24 and 3 equivalents of 1,6diaminohexane in the presence of propionic acid in toluene. In this reaction, diarnide 3a was also generated in 33% yield. The other pentacycles lb,e were also synthesized along with diamides 3b,¢ by a similar procedure using 4 equivalents of the corresponding diamine (Table 1). Double Knorr-Paal condensations of 4a, b 3a with
l, 12-diaminododecanegave beptacycles 5a, b in addition to diamides 6a,b. Table 1. Double Knorr-Paal Reactions of Bis-1,4-tetraketones and Diamines.
Entry
Tetraketone
Diamme
1
2
H2N(CH2)6NH2
la(15)
3a(33)
2
2
H2N(CH2)8NH2
lb(54)
3b(30)
3
2
H2N(CH2)I2NH2
le(45)
3e(27)
4
4a
H2N(CH2)12NH2
5a(27)
6a(ll)
5
4b
H2N(CH2)I2NH2
5b(28)
6b(23)
68 ~5
Yield (%)
6836
In the double Knorr-Paal condensation the starting diketones have a surprising tendency to undergo an intramolecular reaction in moderate yields, in spite of a considerable excess of diamine. In addition, in the case of l a , the C-6 chain provides a very short linkage of the two pyrrole rings so that the molecule l a is sterically very hindered. In the case of 5b, there are three heterocyclic rings between two linked pyrroles, and the central pyrrole ring already bears a bulky dodecyl chain which should impede the intramolecular bridging reaction.
o
o
2: R = tl~enyl 4a: R = bistl~enyl-l-methylpyrmle 4b: R = bisthienyl-l-dodecylpyrrole
3a: R 3b: R 3c: R 7: R
= = = =
1~,: R = C6H12 lb: R = CsHt6 lc: R = C12H24
C6H12NHCOEt CgHI6NHCOEt CI2H2,INHCOEt C12H25
5a: R = C H 3
5b: R =
C12H25
6a: R = CH3
R'= CI2H24NHCOEt 6b: R = C12H25
R'= CI2H2~/HCOEt 8: R = C12H25, R'= C18H37 Scheme
1
T a b l e 2. Absorption Maxima and Redox Values of Penta- and Heptacyclic Oligomers.
Ofigorner
kmax(nm) 1
Epa t / Epe l 2
Epa 2 / Epc 2 2
la
314
(0.70) 3 / 0.64
0.80 / (0.74) 3
lb
334
0.68 / 0.60
0.80 / 0.72
1c
349
0.66 / 0.60
0.78 / 0.72
7
349
0.67 / 0.58
0.83 / 0.75
5a
348
0.59 / 0.48
1.20 / 0.93
5b
349
0.60 / 0.51
1.26 / 1.08
8 363 0.64 / 0.51 1.26 / 1.19 (1) In CHCI3. (2) Measured in CICH2CH2CI containing 0.1 mol dm-3 tetrabutylammonium hexafluorophosphate using Pt disk as a working electrode at scan rate 100 mV s-I. V vs SCE. (3) Peak not sharply defined. The pentacyclic oligomers show UV-VIS absorption maxima in the range of 314-349 n m (Table 2). Notably, the m a x i m u m of l a appears at a significantly shorter wavelength than those of the other pentacycles.
6837
This can be attributed to a more non-planar structure forced by the shorter bridging C-6 chain. The resulting steric effect is also manifest in its 1H NMR, in which a large difference (0.12 ppm) was observed between two signals assigned to the protons of the pyrrole units of la. In contrast, the other pentacycles showed only a very small difference, less than 0.02 ppm, in these protons. On the other hand, while the C-12 bridged lc shows the same absorption maximum (349 run) as the linear pentacycle 7, 3b some conformational constraint exists in the C-8 bridged lb, the UV maximum of which is intermediate between the C-6 and C-12 bridged analogs. In the heptacyclic series, both the C-12 bridged 5a,b show a hypsochromic shift (around 15 nm) compared to the linear heptacycle 8, indicating an appreciable strain in the molecule. The anodic peak potentials (Epa) and cathodic peak potentials (Epe) of the bridged oligomers (la-e, 5a, b) and their non-bridged analogs 7 and g are listed in Table 2. The bridged pentacycles 1 reveal two quasireversible one-electron oxidation waves corresponding to the generation of radical cations and dications, respectively. The Epa values are all surprisingly close (within 0.03 V) to that of the linear oligomer 7, although the most strained member (la) has the highest oxidation potential. In the case of the linear heptacycle 8, the two observed oxidation waves are known to correspond to the formations of a dication and a trication radical, respectively.3d In this case, the C-12 bridged analogs 5a,b are more easily oxidized (by 0.05 and 0.04 V), implying at least a modest stabilization of the dication by the constraint of the bridge. AM1 calculations were performed for lb,c using two different starting structures, conformations A and B. For lb, both calculations gave the same optimized structure which is close to conformation B. Different optimized structures, closer to conformationa A and B, were obtained from calculations for le. While calculations for the bridged heptacycles 5a,b were not carried out, molecular models indicate a considerable curvature in these molecules, with the central N-alkyl group being out of the plane of the C-12 chain. The curvature of these oligomers make them attractive building units for the synthesis of large polycyclic rings, and of conjugated polymers containing curved moieties. Experiments aimed at these objectives are currently in progress.
ConformaaonA
ConformationB
ACKNOWLEDGEMENT The authors thank Dr. A. S. Jeevarajan and Dr. L. D. Kispert for assistance in performing the AM1 calculations. We also thank the National Science Foundation for a grant (CHE9224899) in support of this work.
6838
REFERENCES
1.
2.
3.
4.
(a) Nakayama, J.; Konishi, T.; Hoshino, M. Heterocycles 1988, 27, 1731. (b) Pham, C. V.; Burchardt. A.; Shabana, R.; Canningham, D. D.; Mark. Jr., B.; Zirnmer, H. Phosphorus, Sulfur, and Silicon 19119, 46, 153. (a) Akimichi, H.; Waragai, K.; Hotta, S.; Kano, H.; Sakaki, H. Appl. Phys. Lett. 1991, 58, 1500. (b) Geiger, F.; Stoldt, M.; Schweizer, H.; B~iuerle, P.; Umbaeh, E. Adv. Mater. 1993, 5, 922. (c) Gamier, F.; Deloffre, F.; Horowitz, G.; Hajlaoui, R. Synth. Met. 1993, 55-57, 4747. (d) Horowitz, G.; Delannoy, P.; Bouchriha, H.; Deloffre, F.; Fave, J.-L.; Gamier, F.; Hajlaoui, R.; Hryman, M.; Kouki, F.; Valat, P.; Wintgcns, V.; Yassar, A. Adv. Mater. 1994, 6, 752. (e) Gamier, F.; Hajlaoui, R.; Yassar, A.; Srivastave, P. Science 1994, 265, 1684. (a) Niziurski-Mann, R. E.; Cava, M. P. Adv. Mater. 1993, 5, 647. (b) Parakka, J. P.; Cava, M. P. Syntlt Met. 1995, 68, 275. (c) Liu, T.-L.; Parakka, J. P.; Cava, M. P.; Kim. Y.-T. Synth. Met. 1995, 71, 1989. (d) Parakka, J. P.; Jeevarajan, J. A.; Jeevarajan, A. S.; Kispert, L. D.; Cava, M. P., submitted for publication. Luo, T.-M. H.; LeGoff, E. J. Chin. Chem. Soc., 1992, 39, 325.
(Received in USA 12 June 1995; accepted 26 July 1995)