Synthesis and cyclic voltammetric behaviour of some 3-substituted thiophenes and pyrroles: Precursors for the preparation of conducting polymers

Synthetic Metals, 26 (1988) 153 - 168

153

SYNTHESIS AND CYCLIC VOLTAMMETRIC B E H A V I O U R OF SOME 3-SUBSTITUTED THIOPHENES AND P Y R R O L E S : P R E C U R S O R S F O R THE P R E P A R A T I O N OF CONDUCTING POLYMERS MARTIN R. BRYCE*, ANDRE D. CHISSEL, NIGEL R. M. SMITH and DAVID PARKER*

Department of Chemistry, University of Durham, Durham DH1 3LE (U.K.) POOPATHY KATHIRGAMANATHAN

Cookson Group PLC, 7 Wadsworth Road, Perivale, Greenford, Middlesex UB6 7JQ (U.K.) (Received May 3, 1988; accepted May 25, 1988)

Abstract The syntheses are reported of a series of 3-substituted thiophenes and pyrroles bearing long-chain amide, polyether and acyl functionality. The alkoxy-thiophenes and c~-ketopyrrole monomers are more easily oxidized than the alkyl-substituted thiophenes. Their anodic electrochemistry has been studied by cyclic voltammetry.

Introduction The development of conducting polymers as a new class of electronic materials has attracted considerable attention among chemists and physicists. The study of these materials had led to the postulation of new concepts in solid-state science. Furthermore, these conducting materials offer tremendous technological potential, e.g., in the fabrication of molecular electronic devices, solid-state batteries, chemically-modified electrodes and sensors [1 - 5]. Interest in the subject was stimulated a b o u t a decade ago with the discovery, b y Shirakawa and coworkers, that poly(acetylene) could be 'doped' with electron donors and acceptors to provide materials with conductivity levels (art ~ 10 3 S cm -~) approaching those of some metals [6]. It is now well established that the essential prerequisites for the attainment of high conductivity in an organic polymer are that the material must have extended 7r-conjugation and be partially oxidized or reduced ('p
© Elsevier Sequoia/Printed in The Netherlands

154

Derivatives of poly(pyrrole) and poly(thiophene) are currently attracting most study: the parent heterocycles electropolymerize readily and the doped polymers are highly conducting (amax typically 100 S cm-l). However, practical applications for these polymers are severely limited by the unsatisfactory mechanical and thermal properties of the doped (conducting} films. One approach, seeking to overcome these problems, has been to prepare copolymers or composites of the poly(heterocycle) with a traditional polymer, thus giving materials with enhanced processibility. Recently reported examples include the pyrrole-styrene graft copolymers [8] and composites of poly(3-methylthiophene) and poly(methylmethacrylate) [9]. The doping characteristics of poly(heterocycles) have also been exploited, and improved mechanical properties of poly(pyrrole} films have been obtained by the use of anionic polyelectrolytes [10] or covalently-bound dopant ions, i.e., (CH2)3803- groups attached to the nitrogen atom [11]. A new approach to improve processibility that we [12] and other groups [13] are developing is to substitute pyrrole and thiophene monomers at the 3-position with appropriate side-chains that will enhance the solubility and mechanical properties of the resulting polymer. While it is widely accepted that substitution of pyrrole at nitrogen leads to polymers with reduced conductivity [14], simple alkyl substitution at C-3 of pyrrole [14] and thiophene [15] is known not to have this adverse effect. Indeed, conductivity has been reported to increase in the order: poly(thiophene) poly(3-ethylthiophene) < poly(3-methylthiophene) [16]. At the outset of our work, no soluble, conducting poly(thiophene) or poly(pyrrole) derivatives had been described in the literature. Although poly(3-methoxythiophene} was reported to be soluble, no conductivity data had been presented [17]. Subsequently, poly(thiophene) derivatives (1} substituted at C-3 with long n-alkyl chains have been shown to be conducting (on = 11 - 9 5 S cm -1) [13a] and soluble (in the insulating state) in a range of organic solvents. More recently, water-soluble poly(thiophenes) have been prepared, where the C-3 substituent is an alkane-sulphonate group (2) [18]. In this paper we describe, in detail, the synthesis of a range of novel 3-substituted thiophene and pyrrole derivatives, some of which can be electropolymerized to yield soluble, conducting polymers (maximum an = 1050 S cm -l for compound 9} [12]. Further details of polymer characterization will be reported separately.

Results and discussion

Four classes of thiophene monomers with a long-chain substituent attached at C-3 are described, which are suitable for electropolymerization. These are: (a) ether and polyether (5 - 10} and (b) amide (13 - 15) derivatives of 3-methylthiophene; (c} alkoxy-substituted thiophenes (17- 21) and (d) alkoxy-substituted derivatives that also contain an amide linkage

155 TABLE 1 Structures of the compounds described in the text and Table 2. Typical reagents and reaction conditions are: (i) Na + or K + alkoxide, CC14, reflux. (ii) K+phthalimide -, 18crown-6, DMF, 80 °C. (iii) N2H4"H20, EtOH, reflux. (iv) RCOC1, Et3N, CH2C12, --5 °C. (v) Na + alkoxide, Cu20, KI, 110 °C. (vi) Tosyl chloride, pyridine, --20 °C. (vii) K + phthalimide-, 18-crown-6, DMF, 20 °C. (viii) RCOCI, A1C13, CH2C12, 20 °C. (ix) NaOH, dioxan, reflux

~

R

S

(1) (2) (3) < (4) > (5) (6) (7) (8) (9) (10)

(i)

(ii)

~

(CH2)nCH3 [n = 5, 7, 11, 17, 19] (CH2)nSO3H [n = 2, 4] CH3 CH2Br CH2OCH3 CH20--n-C4H9 CH20--n-C6Hll CH20(CH2)2OCH3 CH20(CH2)20(CH2)2OCH3 CH20(CH2CH20)4(CH2)3CH3 0

(iii) ) (12) CH2NH3+CI:> (13) CH2NHC(O)CH3

(iv)

(v)

(vi)

[

(14) (15) < (16) > (17)

CH2NHC(O)--n-CTH,5 CH2NHC(O)--n-C 11H23 Br O--n-C4H9 (18) O----n-C6H13 (19) O(CH2 )2OCH3 (20) O(CH2)20(CH2)2OCH3 < (21) O(CH2)2OH > (22) O(CH2)2OTosyl 0

(vii)

) (23)o(cH2)2~ (iii)

> (24) O(CH2)2NH3+CI-

(iv) > (25) O(CH2)2NHC(O)(CH2)6CH3 (26) O(CH2)2NHC(O)(CH2)IOCH3 R Z n - ~R"

(viii)

(ix)

R

~

< (27)Tosyl > (28) Tosyl

H CO(CH2)6CH3

(29) (30) > (31) (32) (33)

CO(CH2)10CH3 CO(CH2)16CH3 CO(CH2)6CH3 CO(CH2)loCH3 CO(CH2)16CH3

Tosyl Tosyl H H H

156 in the side-chain (25 - 26). One class of pyrrole m o n o m e r is also described, the keto-pyrroles (31 - 33). The methodologies employed for the synthesis of all the monomers have well-established precedents in the literature, and the reactions described are amenable to large-scale preparation.

Thiophene derivatives The ether (5 - 7), and di- and polyether derivatives (8 and 9 o 10, respectively), were prepared using a WiUiamson ether synthesis, by reaction of 3-(bromomethyl)thiophene (4) with the sodium or potassium alkoxide of the appropriate ether or polyether mono-alcohol. Amide-containing derivatives ( 1 3 - 15) were prepared in three steps from 4 via compounds 11 and 12, using the Ing-Manske modification of a Gabriel synthesis and coupling of the amine salt (12) with the appropriate acid chloride. Alkoxythiophenes ( 1 7 - 2 1 ) were prepared from 3-bromothiophene (16) and the appropriate sodium alkoxide in the presence of copper oxide and potassium iodide, using conditions developed by Gronowitz [19]. The alcohol (21) was converted to a tosylate (22), which served as a precursor for the alkoxyamides (25) and (26) [via compounds 2 3 - 24], using the methodology described above. The ketopyrroles (31 -33), our target monomers in the pyrrole series, were synthesized in four steps from pyrrole, using a modification of the N-tosylation m e t h o d of Papadopoulos and Haidar [20] and the procedures described by Anderson and Loader for 3-substitution of pyrroles [21]. This involved Friedel-Crafts acylation (to give compounds 2 8 - 30), followed by removal of the fl
Cyclic voltammetric study The behaviour of the monomers was examined systematically by cyclic v o l t a m m e t r y (CV) under standard conditions. For each monomer, a series of scans was made, varying the scan-rate from 50 to 400 mV s-1 and observing the initial scan only. In a second series o f experiments, consecutive scans were observed at a fixed scan-rate of 50 mV s-1 . The data and observations from these experiments are summarized in Table 2. Under the conditions examined, six monomers (compounds 5, 6, 20 and 31 - 33) were f o u n d to electropolymerize to give thin gold or brown polymer films on the working electrode. The primary wave {monomer oxidation potential) shown in Table 2 varies according to the nature of the ~-substituent. This oxidation corresponds to removal of an electron from the HOMO of the electron-rich ~r-system. Alkoxy groups (compounds 17 - 19 and 25 - 26) tend to stabilize the product radical cation and raise the m o n o m e r HOMO energy, so t h a t oxidation occurs at a lower voltage than with alkyl substituents (compounds 5 - 10). There was only a minor variation in oxidation potential with increasing length of the side chain. In most of the systems studied, a simple CV trace was observed (e.g., Fig. l(a)) where the anodic current associated with m o n o m e r oxidation exhibited a clear v i n dependence consistent with control by reactant diffusion to the electrode. A notable

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Fig. 1. C y c l i c v o l t a m m o g r a m s o f m o n o m e r s ( 0 . 0 2 M in a c e t o n i t r i l e in t h e p r e s e n c e o f 0.1 M E t 4 N P F s ; t~ = 5 0 m V s -~ ; c u r r e n t s c a l e - b a r is e q u i v a l e n t t o 50 /~A; (a) s c a n s 1, 3 and 5 with 3-(methoxyethoxymethyl)-thiophene (8); ( b ) s c a n s 2, 3 a n d 10 w i t h 3(methoxymethyl)-thiophene (5); (c) s c a n s 1, 2 a n d 4 w i t h 3 - ( d o d e c a n o y l ) - p y r r o l e (32). T h e s c a n s e q u e n c e f o l l o w s t h e o r d e r ( ....... ), ( . . . . ) a n d ( - - " --).

2500

2500

3000

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8

9

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13

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1.56

2.19

2.20

1.94

2.18

2.11

2.16

2.23

2.21

3000

3000

2.21

2500

0.159

0.048

0.075

0.040

0.110 (sh)

0.188

0.188

0.215

0.217

0.293

Prim. oxid. wave Eo (V) i o ( m A )

0.99

1.00

1.00

1.00

0.99

0.99

0.99

0.98

1.00

0.96

CLR b

Initial s c a n (P = 1 0 0 m V / s )

Anodic scan limit (mV)a

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2.86 0.127 C L R = 1.00

2.84 0.177 CLR = 0.99

2.64 0.088 C L R = 1.00

2.57 0.171 C L R = 0.97

broad wave at ~ 2 . 4 V

broad wave at ~ 2 . 5 V

2.54 0.046

Other peaks E (V);i(mA)

blue-green

none

none

none

none

none

blue-black

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blue-black

blue-black

Anode colour

Cyclic v o l t a m m e t r y s t u d y o f t h i o p h e n e a n d p y r r o l e m o n o m e r s

TABLE 2

const

dec

dec

dec

dec

dec

dec

dec

dec

dec

no 10 s c a n s

no 4 scans

no 4 scans

no 4 scans

no 10 s c a n s

no 10 s c a n s

no 10 s c a n s

no 5 scans

t h i n gold 5 scans

thin gold I0 scans

I0 ( m A ) c F i l m formation

M u l t i s c a n s at 50 m V / s

CV t r a c e i n v a r i a n t , s c a n s 1 - 10

P e a k s t r u c t u r e c o l l a p s e s a f t e r 3 scans. C r o s s o v e r s e e n o n reversal

P e a k s t r u c t u r e c o l l a p s e s a f t e r 3 scans. C r o s s o v e r s e e n o n reversal

P e a k s t r u c t u r e c o l l a p s e s a f t e r 4 scans. C r o s s o v e r s e e n o n reversal

2.57 V p e a k c o l l a p s e s ( s c a n 3). S h o u l d e r at 2 . 1 8 b e c o m e s b e t t e r d e f i n e d and shifts to E = 2.32 V

B r o a d w a v e at ~ 2 . 4 0 V d e c r e a s e s a n d disappears after 5 scans

B r o a d w a v e at ~ 2 . 5 0 V d e c r e a s e s a n d disappears after 5 scans

2.23 V peak s t r u c t u r e collapses after 6 scans

2 . 5 4 V p e a k d e c r e a s e s as s c a n r a t e increases. N o f i l m / a n o d e c o l o u r w i t h a s c a n limit o f 2 5 0 0 m V

N e w p e a k s at 2 . 3 3 V ( s c a n 2) a n d 1.97 ( s c a n 3). T h e f o r m e r h a d c o l l a p s e d b y scan 5 w h e n a n u c l e a t i o n loop was observed

General c o m m e n t s

O0

2500

2500

2500

2500

26

31

32

33

1.61

1.72

1.64

1.66

1.61

0.99

1.00

0.99

1.00

1.00

0.060 0.86 (prewave)

0.138

0.024

0.095

0.049

0.186

b r o a d waves 1.7 - 1.8 V < 100 m V / s

1.88 o n l y < 100 m V / s

2.14 only < 100 m V / s

1.84 (sh)

1.75 (sh)

1.75 0 . 0 0 9

none d

none d

none d

black

black

blue

dec

inc

inc

const

const

dec

const

const

const

dark-brown 4 scans

dark-brown 8 scans

gold-brown 5 scans

no 10 s c a n s

no 10 s c a n s

t h i n gold

no 5 scans

no 5 scans

no 8 scans

no 8 scans

T r a n s i e n t p e a k s s e e n at l o w s c a n s rate. Rapid nucleation/adsorption prewave observed

1 . 7 2 V p e a k s h i f t s to 1 . 4 8 ( s c a n 2) 1.88 V peak decrease at v > 100 m V / s

T r a n s i e n t at 1.87 V; 2 . 1 4 p e a k decreases as v is i n c r e a s e d

CV t r a c e ( s c a n s 1 - 1 0 ) a l m o s t invaria n t . S h o u l d e r at 1 . 8 4 V d e c r e a s e s as v is i n c r e a s e d

T r a n s i e n t at 2 . 2 1 V ( s c a n 5). O n e p e a k at 1 . 6 8 V b y s c a n 10. S h o u l d e r a t 1 . 7 5 V d e c r e a s e s as v i n c r e a s e d

Additional o x i d a t i o n peak at 2.21 V and nucleation loop seen on scan 6

C V t r a c e i n v a r i a n t , s c a n s 1 - 5. 1 . 7 5 V p e a k o n l y s e e n < 50 m V / s

CV t r a c e i n v a r i a n t , s c a n s 1 - 5 . 1 . 8 2 V p e a k o n l y s e e n < 50 m V / s . E x t r a p e a k at 2 . 3 2 V w h e n s c a n n e d t o 2 5 0 0 m V

E x t r a oxid. p e a k at 2 . 3 8 V. C V t r a c e invariant, scans 1 - 8

E x t r a o x i d . p e a k at 2.21 V. C r o s s o v e r s e e n o n reversal

a T h e s c a n range was f r o m + 1 2 0 0 m V t o t h e s p e c i f i e d limit. b C L R ( c o e f f i c i e n t o f linear r e g r e s s i o n ) w a s c a l c u l a t e d to s h o w t h e c o r r e l a t i o n b e t w e e n i0 a n d v 1/2 for v = 5 0 , 1 0 0 , 2 0 0 a n d 4 0 0 m V / s . ¢ C o n s t = c o n s t a n t ; inc = i n c r e a s e ; dec = d e c r e a s e . T h e s e t e r m s are u s e d to d e s c r i b e h o w t h e c u r r e n t a s s o c i a t e d w i t h t h e p r i m a r y o x i d a t i o n wave varies w i t h c o n t i n u o u s s c a n n i n g ; also s h = s h o u l d e r . d A t h i n gold film w a s o b s e r v e d o n t h e a n o d e a f t e r o n e scan.

2500

25

2500

1.61

1.82 weak

2000

1.00

20

0.149

blue-green

1.64

2000

1.61 0 . 1 0 4 (prewave)

19

0.96 blue-green

0.008

none

1.67

1800

2500

18

blue-green

2500

~D

160 exception was c o m p o u n d 33, where an adsorption prewave was observed (characterized by a high correlation for the dependence o f i0 with v). Compound 18 also showed a prewave separated by 60 mV from the m o n o m e r oxidation peak. Some systems displayed significant additional complexity, particularly where polymer film growth on the electrode occurred. First a 'nucleation loop' was often observed upon reversal o f the scan (e.g., Fig. l(b)). The crossover points observed m a y be related to the nucleation overpotential required to deposit the polymer phase on the foreign electrode. Similar phenomena have been observed previously during the anodic electrodeposition of pyrrole or thiophene [22] and, more recently, during the cathodic electrodeposition of a fluorocarbon m o n o m e r [23]. Secondly, the CV trace sometimes had oxidation postwaves (e.g., Fig. l(c)), which were usually weak and broad in nature. These were only observed at low scan rates (< 50 - 100 mV s-l). In some cases (e.g., with thiophenes 6, 8, 17 - 20, 25 and 26) a distinct blue or black colouration appeared to 'stream' from the anode surface during cycling. Such behaviour is considered to be due to the formation of soluble, low molecular weight oligomers [24], which do not adhere well to the electrode surface in the solvent used. Their formation was also sensitive to the anodic scan limit used: strongly coloured species were often only observed with a positive limit as much as 600 - 700 mV higher than the m o n o m e r oxidation potential {e.g., compounds 6 and 18). Such behaviour suggests that for the controlled electrodeposition of conducting polymeric films of certain of these monomers, careful choice of solvent and a large anodic overpotential are essential for sustained electropolymerization.

Experimental Reactions were carried out under dry nitrogen, and solvents were distilled prior to use from an appropriate drying agent. Melting points were determined using a Reichert-Kofler block, and are uncorrected. Proton and C 13 n.m.r, spectra were recorded using a Bruker AC 250 instrument. Chemical shifts are given in ppm relative to SiMea (0 ppm). Infrared spectra were recorded as Nujol mulls, KBr discs or in the stated solvent, using a Perkin-Elmer 297 spectrophotometer. Mass spectra were recorded on a VG 7070E spectrometer and microanalyses were performed by Mrs. M. Cocks and Mr. R. Coult, University of Durham. Cyclic voltammetry was carried out under argon in a one-compartment cell with platinum working and counter-electrodes and a silver-silver chloride (SSC) reference electrode. Measurements were made with a BAS100 electrochemical analyser, scanning the potential range +1200 mV to + 1 8 0 0 - 3000 mV, as appropriate. For these electrochemical experiments, acetonitrile (Aldrich HPLC grade) was the solvent and was freshly distilled from Call 2. The supporting

161

electrolyte was tetraethylammonium hexafluorophosphate (0.1 M), which had been previously vacuum and oven dried (120 °C, 4 h, 10 -2 mm Hg). Monomer concentrations were typically 0.018 M. 3-(Bromomethyl)thiophene was prepared from 3-methylthiophene according to the method of Gronowitz [25], and was purified by distillation under reduced pressure (boiling point (b.p.) 97 - 98 °C, 13 mm Hg). Synthesis of ether- and polyether-substituted thiophene derivatives (5 - 10)

3-(Methoxymethyl)-thiophene (5) This substance is representative. A solution of 3-(bromomethyl)thiophene (4), (10.9 g, 0.062 mol) in carbon tetrachloride (50 cm 3) was added dropwise to a solution of sodium methoxide (6.6 g, 0.12 mol) in dry methanol (150 cm 3) and the mixture refluxed (15 min). After cooling to room temperature, the solvent mixture was removed under reduced pressure and the resultant oil dissolved in dichloromethane (100 cm3), extracted with distilled water (2 × 100 cm 3) and dried over potassium carbonate. After removal of the solvent, the crude oil was distilled under reduced pressure to yield compound 5 [2.8 g, 35% based on 4]. B.p. 5657 °C/10 mm Hg. Analysis found: C, 55.5; H, 6.0; S, 25.2. Calculated for C6HsOS: C, 56.2; H, 6.2; S, 25.0%; EI m/e (intensity): 128 (25, M+), 97 (100, C4H3S'CH2); 5H(CDC13): 3.37 (3H, s), 4.46 (2H, s), 7 . 0 9 - 7 . 2 8 (3H, m); 5C(CDC13): 57.2, 69.4, 1 2 1 . 9 , 1 2 5 . 2 , 1 2 6 . 6 and 139.3 ppm. Similar preparations were used for 6 - 10.

3-(n-Butoxymethyl)-thiophene (6) 49% yield. B.p. 93 S, 18.4. Calculated for 0.98 (3H, t), 1.46 (2H, (1H, m), 7.26 (1H, m), 70.2,122.5,125.7,127.4

- 94 °C/8 mm Hg. Analysis found: C, 64.2; H, 8.6; C9H~4OS: C, 63.5; H, 8.2; S, 18.9%; 5H(CDC13): m), 1.62 (2H, m), 3.54 (2H, t), 4.56 (2H, s), 7.13 7.33 (1H, m); 5C(CDC13): 14.0, 19.4, 21.4, 66.2, and 139.9 ppm.

3-(n-Hexyloxymethyl)-thiophene (7) 75% yield. B.p. 122- 123 °C/9 mm Hg. Analysis found: C, 66.1; H, 9.3; S, 15.8. Calculated for CllH18OS: C, 66.6; H, 9.1; S, 15.8%; 5H(CDC13): 0.88 (3H, t), 1.34 (6H, m), 1.59 (2H, m), 3.43 (2H, t), 4.48 (2H, s), 7.06 (1H, m), 7.17 (1H, m), 7.25 (1H, m); 8C(CDC13): 14.1, 22.7, 25.9, 29.8, 31.8, 68.2, 7 0 . 5 , 1 2 2 . 5 , 1 2 5 . 8 , 1 2 7 . 3 and 139.9 ppm.

3-(Methoxyethoxymethyl)-thiophene (8) 50% yield. B.p. 100- 101 °C/10 mm Hg. Analysis found: C, 55.7; H, 7.7; S, 18.3. Calculated for CsH~202S: C, 55.8; H, 7.0; S, 18.6%; EI m/e (intensity): 172 (5, M+), 97 (12, C4H3S'CH2); 5H(CDC13): 3.34 (3H, s), 3.54 (4H, m), 4.53 (2H, s), 7.05 -7.24 (3H, m); ~C(CDC13): 59.6, 67.0, 69.7, 7 2 . 5 , 1 2 3 . 4 , 1 2 6 . 4 , 127.9 and 140.0 ppm.

162

3-(Methoxyethoxyethoxymethyl)-thiophene (9) 46% yield. B.p. 136 - 138 °C/10 mm Hg. Analysis found: C, 56.0; H, 7.0; S, 15.1. Calculated for Cl0H1603S: C, 55.5; H, 7.4; S, 14.8%; EI m/e (intensity): 216 (12, M+), 97 (100, C4HaS-CH2); 5H(CDC13): 3.26 (3H, s), 3.40 (4H, m), 3.50 (4H, m), 4.45 (2H, s), 6.96-7.17 (3H, m); 5C(CDCla): 58.3, 66.1, 69.2, 70.2, 70.4, 71.7,121.7,125.2,126.7 and 139.6 ppm.

3-(Butoxyethoxyethoxyethoxyethoxymethyl)-thiophene (10) 57% yield. B.p. 162- 163 °C/0.15 mm Hg. Analysis found: C, 58.6; H, 8.7; S, 9.5. Calculated for C17H30OsS: C, 58.9; H, 8.7; S, 9.3%; EI m/e (intensity): 346 (9, M+), 97 (100, C4HaS.CH2) , 57 (75, C4H9); 5H(CDC13): 0.91 (3H, t), 1.34 (2H, m), 1.53 (2H, m), 3.45 (2H, t), 3.65 (16H, m), 4.56 (2H, s), 7.06 (1H, m), 7.20 (1H, m), 7.26 (1H, m); 5C(CDC13): 13.6, 18.9, 31.4, 68.1, 69.0, 69.8, 70.0, 70.3, 122.5, 125.5, 126.7 and 139.1 ppm.

3-(N-Phthalimidomethyl)-thiophene (11) Potassium phthalimide (5.7 g, 0.03 mol) and 18-crown-6-ether {0.05 g) were added to a stirred solution of 4 (5.5 g, 0.03 mol) dissolved in dry N,N-dimethylformamide (70 cm3), and the mixture heated (3 h, 80 - 90 °C), then cooled and filtered. The filtrate was poured on crushed ice (~ 100 g), stirred (5 h) and filtered to give solid product, which was dried and recrystallized from absolute ethanol to give 3-(N-phthalimidomethyl)-thiophene (5.2 g, 72% yield). M.p. 133 - 134 °C. Analysis found: C, 64.8; H, 3.8; N, 5.5; S, 12.7. Calculated for CI3H9NO2S: C, 64.2; H, 3.7; N, 5.8; S, 13.2%; EI m/e (intensity): 243 (100, M+), 97 (24, C~I-I3S.CH2+); i.r. (Nujol): 1760, 1700 cm-~; ~iH(CDC13): 4.84 (2H, s), 7.14-7.35 (3H, m), 7.65-7.85 (4H, m); 5C(CDC13): 36.2,123.3, 124.2,126.2,128.1, 132.1,134.0, 136.6 and 167.8 ppm.

3-(Aminomethyl)-thiophene hydrochloride (12) Hydrazine hydrate (1.4 cm 3, 0.03 mol) was added to a hot solution of 3-(N-phthalimidomethyl)-thiophene (11) (6.4 g, 0.026 mol) in absolute ethanol (200 cma), and the mixture refluxed (12 h). Hydrochloric acid was added (5 cm 3, conc.) and reflux continued for a further 30 min. After cooling (0 °C, 16 h) and filtration, the filtrate was evaporated to recover a white solid. This was dissolved in distilled water (50 cm3), the mixture filtered and the filtrate evaporated under reduced pressure to give a crude yellow solid, which was recrystaUized from absolute ethanol. Yield of 12 was 3.5 g (90%). M.p. 165 °C (dec). Analysis found: C, 40.4; H, 5.0; N, 9.6; S, 21.2. Calculated for CsHaNSCI: C, 40.1; H, 5.3; N, 9.4; S, 21.4%; 5H(D20): 3.80 (2H, s), 5.33 (br, s, exchanges in D20), 7.04 - 7.27 (3H, m); ~C(D20): 37.8,125.9,127.4, 127.6 and 132.9 ppm.

163

Synthesis of amido-substituted thiophenes (13 - 15)

N-( 3-Thienylmethyl)-acetamide (13) This substance is representative. Triethylamine (0.46 g, 4.5 mmol) was added dropwise to a suspension of 3-(aminomethyl)-thiophene hydrochloride (12) (0.6 g, 0.004 mol) in dry dichloromethane (20 cm 3) maintained at --5 °C under nitrogen, and the mixture stirred (15 min) before acetyl chloride (0.31 g, 0.004 mol) was added dropwise with continued stirring (3 h, --5 °C). The reaction mixture was then washed sequentially with hydrochloric acid (50 cm 3, 0.5 M) and sodium hydroxide solution (50 cm 3, 0.5 M). After separation, the organic phase was dried (magnesium sulphate) and the solvent removed under reduced pressure to give the crude product (13). Yield after recrystallization from hexane was 0.52 g (84%). M.p. 46 - 47 °C. Analysis found: C, 53.5; H, 6.2; N, 8.7; S, 20.4. Calculated for CTH9NOS: C, 54.2; H, 5.8; N, 9.0; S, 20.7%; i.r. (Nujol): 3250, 1650, 1550 cm-1; EI m/e (intensity): 155 (37, M+), 112 (55, C4H3S.CH2NH+), 97 (29, C4H3S-CH2+), 83 (12, C4H3S); ~H(CDCI3): 1.93 (3H, s), 4.41 (2H, d), 6.26 (1H, br, s), 7.00 -7.29 (3H, m); 5C(CDC13): 23.1, 38.7, 122.2, 126.3, 127.3, 139.8 and 170.0 ppm. Compounds 14 and 15 were similarly prepared.

N-( 3-Thieny lmethyl)-octanamide (14) 89% yield. M.p. 72- 73 °C. Analysis found: C, 64.9; H, 8.9; N, 5.7; S, 13.3. Calculated for C13H:lNOS: C, 65.3; H, 8.8; N, 5.9; S, 13.4%; i.r. (Nujol): 3290, 1630, 1540 cm-1; EI m/e (intensity): 239 (19, M÷), 113 (35, C4H3S.CH2NH2) , 97 (63, C4H3S.CH2+); 5H(CDCI3): 0.87 (3H, t), 1.28 (8H, m), 1.61 (2H, m), 2.19 (2H, t), 4.44 (2H, d), 5.60 (1H, br, s), 7 . 0 3 - 7 . 3 3 (3H, m); 5C(CDC13): 14.2, 22.7, 25.8, 29.1, 29.3, 31.8, 36.9, 38.8,122.3, 126.5,127.1,140.0 and 173.1 ppm.

N-(3-Thienylmethyl)-dodecanamide (I5) 92% yield. M.p. 89 - 90 °C. Analysis found: C, 69.0; H, 10.2; N, 4.2; S, 11.1. Calculated for C17H29NOS: C, 69.1; H, 9.8; N, 4.7; S, 10.9%; i.r. (Nujol): 3300, 1630, 1545 cm-1; EI m/e (intensity): 295 (18, M+), 113 (100, C4H3S-CH2NH2), 97 (46, C4H3S.CH2+); ~iH(CDCla): 0.87 (3H, t), 1.25 (16H, m), 1.80 (2H, m), 2.20 (2H, t), 4.46 (2H, d), 5.80 (1H, br, s), 7.01 -7.30 (3H, m); 5C(CD2C12): 13.6, 22.4, 25.5, 29.1, 29.3, 31.7, 36.4, 38.2, 44.9, 53.6,121.6,125.7,127.0,140.1 and 174.1 ppm.

Synthesis of alkoxy-substituted thiophenes (17 - 21)

3-n-Butoxythiophene (17) This substance is representative. Sodium metal (2.0 g, 0.087 mol) was completely reacted with n-butanol (125 cm 3) under an atmosphere of argon. Copper(II) oxide (1.25 g, 0.016 mol) and potassium iodide (0.05 g,

164 0.3 mmol) were then added, followed by 3-bromothiophene (16) (5.0 g, 0.031 mol). The mixture was stirred (100 °C, 3 days), more potassium iodide was added (0.05 g, 0.3 mmol), and reaction continued (100 °C, 2 days). After filtration, the butanol solution was poured into distilled water (150 cm a) and extracted with ether (2 X 100 cm3). The combined extracts were dried over magnesium sulphate, ether removed under reduced pressure and the residue chromatographed on a silica column (15 X 2.5 cm o.d.) eluted with petroleum ether (b.p. 40 - 60 °C). Compound 17 was obtained as a colourless oil (2.0 g, 42%). High-resolution MS: 156.00676 theoretical: 156.00689); EI m/e (intensity): 156 (30, M+), 100 (100, C4H4OS), 57 (20, C4H9), 43 (21, C3H~); 5H(CDC13): 0.93 (3H, t), 1.45 (2H, m), 1.71 (2H, dt), 3.93 (2H, t), 6.20-7.17 (3H, m); 5C(CDC13): 13.8, 19.2, 31.3, 69.8, 98.6,119.5,124.4 and 158.0 ppm. Derivatives 18 - 21 were similarly prepared.

3-n-Hexyloxythiophene (18) 43% yield. (117 °C/10 mm Hg). Analysis found: C, 64.9; H, 9.2; S, 17.2. Calculated for C10H16OS: C, 65.2; H, 8.7; S, 17.4. High-resolution MS: 184.09354 (theoretical: 184.09219); EI m/e (intensity): 184 (64, M+), 100 (100, C4H4OS), 43 (50, C3H7); 5H(CDC13): 0.71 (3H, t), 1.39 (6H, m), 1.70 (2H, d), 3.67 (2H, t), 6.15- 7.10 (3H, m); 5C(CDC13): 14.0, 22.5, 25.7, 29.2, 31.5, 70.0, 96.7,119.4, 124.3 and 158.0 ppm.

3-(Methoxyethoxy)-thiophene (19) 68% yield. Analysis found: C, 53.1; H, 6.6; S, 20.2. Calculated for CTHmO2S: C, 53.1; H, 6.3; S, 20.3%; EI m/e (intensity): 158 (25, M÷), 100 (26, C4H4OS), 59 (100, C3H70), 45 (50, C2HsO); ~H(CDCI3): 3.42 (3H, s), 3.70 (2H, t), 4.07 (2H, t), 6.23-7.16 (3H, m); 5C(CDC13): 58.9, 69.2, 70.8, 97.2,119.4,124.5 and 157.6 ppm.

3-(Methoxyethoxyethoxy)-thiophene (20) 34% yield. Analysis found: C, 53.3; H, 7.2; S, 16.0. Calculated for C9H1403S: C, 53.4; H, 6.9; S, 15.9%; EI m/e (intensity): 202 (3, M+), 100 (9, C4H4OS), 59 (53, C3H70}, 45 (12, C2HsO); ~H(CDC13): 3.34 (3H, s), 3.53 (2H, dd), 3.65 (2H, dd), 3.76 (2H, dd), 4.05 (2H, dd), 6.22-7.15 (3H, m); 6C(CDC13): 58.6, 69.2, 69.4, 70.3, 71.6, 97.2, 119.2, 124.4 and 157.3 ppm.

3-(Hydroxyethoxy)-thiophene (21) 43% yield. M.p. 38- 39 °C. Analysis found: C, 50.2; H, 5.8; S, 22.5. Calculated for C6HsO2S: C, 50.0; H, 5.6; S, 22.3%; i.r. (Nujol): 3300, 1535, 750 cm-~; EI m/e (intensity): 144 (100, M÷), 100 (47, C4I-I4OS), 45 (17, C2HsO); 5H(CDC13): 4.00 (4H, m), 6.19- 7.10 (3H, m); 5C(CDC13): 60.8, 71.4, 97.7,119.4, 124.8 and 157.4 ppm.

165

3-(Toluenesulphonylethoxy)-thiophene (22) Solid p-toluenesulphonyl chloride (0.88 g, 4.6 mmol) was added in small amounts to a cooled (--5 °C) solution of 21 (0.6 g, 4.2 mmol) in dry pyridine (50 cmS), the mixture was stirred for 5 min, then maintained at --20 °C {48 h) and then stirred with crushed ice (5 h). After filtration, the solid was recrystallized twice from hexane to give a white crystalline product (22) (1.0 g, 83%). M.p. 76 - 77 °C. Analysis found: C, 52.1; H, 4.5; S, 21.8. Calculated for C13H1404S2: C, 52.4; H, 4.7; S, 21.5%; i.r. (Nujol): 3090, 1590, 1545, 1380, 1180 cm-l; EI role (intensity): 298 (0.5, M÷), 199 (57, tosyl-OCH2CH2), 155 (8, tosyl), 91 (18, CH3"C6H4); ~H(CDC13): 2.4 (3H, s), 4.0 (2H, t), 4.2 (2H, t), 6.1 (1H, m), 6.5 (1H, m), 7.0 (1H, m), 7.37.7 (4H, m); 6C(CDCla): 21.3, 67.1, 67.9, 97.7,118.9,124.6, 127.6, 129.6, 132.3,144.7 and 156.3 ppm.

3-N-(Phthalimidoethoxy)-thiophene (23) 3-(Toluenesulphonylethoxy)-thiophene (22) (0.4 g, 1.4 mmol) and potassium phthalimide (0.3 g, 1.6 mmol) were stirred together (85 °C, 12 h) with 18-crown-6 ether (50 mg) in dimethylformamide (60 cmS). The mixture was cooled to room temperature and stirred into crushed ice (12 h), and the solid filtered and recrystallized from absolute ethanol to give a white crystalline product (23) (0.26 g, 72%). M.p. 1 4 6 - 1 4 7 °C. Analysis found: C, 62.2; H, 4.1; N, 4.9; S, 11.8. Calculated for C14HllNOaS: C, 61.5; H, 4.0; N, 5.1; S, 11.7%; i.r. (Nujol): 1760, 1710, 1550 cm-X; EI m/e {intensity): 273 (4, M÷), 174 (100, phthalNCH2CH2); 8H(CDCla): 4.0 (2H, t), 4.1 (2H, t), 6.2 (1H, m), 6.6 (IH, m), 7.0 (1H, m), 7 . 6 - 7 . 9 (4H, m); 5C(CDC13): 37.2, 66.8, 97.9, 119.5, 123.3, 124.7, 132.0, 134.0, 157.0 and 168.1 ppm.

3-(Aminoethoxy)-thiophene hydrochloride (24) Hydrazine hydrate (0.7 cm a, 0.015 mmol) was added slowly to a solution of compound 23 (2.6 g, 9.5 mmol) in hot absolute ethanol (200 cm 3) and the mixture stirred at reflux (12 h) under nitrogen. Hydrochloric acid (5 cm a, conc.) was carefully added to the system and reflux continued for a further 30 min. After cooling and filtration, the filtrate was evaporated to dryness (10 -a mm Hg, 15 °C) to leave a white crystalline solid, which was extracted with water. Filtration and removal of the water (10 -3 mm Hg, 15 °C) gave the product 24 (1.5 g, 88%). M.p. 135- 145 °C (dec). Analysis found: C, 39.8; H, 5.8; N, 8.1. Calculated for C6H10NOSCI: C, 40.1; H, 5.6; N, 7.8%; ~H(CDC13): 3.00 (2H, t), 3.90 (2H, t), 6.20 (1H, m), 6.70 (1H, m), 7.10 (1H, m); 5C(CDCla): 40.8, 71.5, 96.7,118.7 and 124.1 ppm. Synthesis of alkoxy-amido-substituted derivatives (25 - 26)

No(3-Thienyloxyethyl)-octanamide (25) This substance is representative. Triethylamine (1.3 cm 3, 9.2 mmol) was added (10 min) to a stirred suspension of 3-(aminoethoxy)-thiophene

166 hydrochloride (24) (1.5 g, 8.4 mmol) in dry dichloromethane (60 cm 3) cooled to --5 °C. Octanoyl chloride (1.37 g, 8.4 mmol) in dichloromethane (20 cm 3) was then added dropwise and stirring continued (4 h, --5 °C). The mixture was washed sequentially with hydrochloric acid (2 × 50 cm 3, 0.1 M) and sodium hydroxide (2 × 50 cm 3, 0.1 M) and dried with magnesium sulphate. Filtration and solvent evaporation resulted in a white solid, which was recrystallized from hexane to give 25 (1.4 g, 62%). M.p. 7879 °C. Analysis found: C, 61.9; H, 8.6; N, 4.9. Calculated for C1~I23NO:S: C, 62.4; H, 8.5; N, 5.2%; CI m/e (intensity%): 270 (0.5, M+), 170 (100, C4H3SOCH2CH2NHCO); i.r. (Nujol): 3300, 1635, 1545 cm-t; 5H(CDC13): 0.88 (3H, t), 1.27 (8H, m), 1.63 (2H, t), 2.19 (2H, t), 3.64 (2H, m), 4.02 (2H, t), 5.93 (1H, br, s), 6.27 (1H, m), 6.75 (1H, m), 7.19 (1H, m); ~C(CDC13): 13.8, 22.3, 25.4, 28.8, 29.0, 31.4, 36.4, 38.6, 68.7, 97.5, 116.9, 124.7,157.0 and 173.2 ppm.

N-(3-Thienyloxyethyl)-dodecanamide (26) 48% yield. M.p. 97- 98 °C. Analysis found: C, 66.6; H, 9.9; N, 3.9; S, 9.7. Calculated for ClsH31NO2S: C, 66.5; H, 9.5; N, 4.3; S, 9.9%; i.r. (Nujol): 3295, 1632, 1545 cm-1; CI m/e (intensity%): 326 (7, M+); EI m/e (intensity%): 226 (100, CH3(CH2)10C(O)N(H)CH2CH2), 126 (5, C4H3S.OCH=CH2), 98 (11, C~I30); 5H(CDC13): 0.88 (3H, t), 1.24 (16H, br), 1.62 (2H, t, br), 2.19 (2H, t), 3.64 (2H, m), 4.02 (2H, t), 5.98 (1H, br), 6.27 (1H, m), 6.74 (1H, dd), 7.19 (1H, m); 5C(CDC13): 13.7, 22.3, 25.3, 28.9, 29.2, 31.5, 68.6, 97.4,118.8,124.5, 156.8 and 173.1 ppm. Synthesis of pyrrole derivatives (28 - 33)

3-Octanoy lpyrrole (31) This substance is representative. Octanoyl chloride (2.18 g, 13.4 mmol) was added dropwise to a suspension of aluminium trichloride (1.95 g, 14.6 mmol) in methylene chloride (25 cm3), the mixture stirred (10 rain, 20 °C) and N-tosylpyrrole (27) (2.7 g, 12.2 tool) in methylene chloride (5 cm 3) added slowly to the cooled (5 °C) reaction mixture. After further stirring (2 h, 20 °C) the reaction was quenched by pouring the mixture into iced water. The organic layer was separated, dried with magnesium sulphate and evaporated under reduced pressure (40 °C/10-2 mm Hg) to give N-tosyl-3octanoylpyrrole (28) as an oil that crystallized on standing (3.3 g, 78%). M.p. 47- 48 °C. Analysis found: C, 66.2; H, 6.6; N, 3.9. Calculated for C1~-I25NO3S: C, 65.8; H, 7.2; N, 4.0%; EI m/e (intensity%): 347 (3, M+), 263 (70, C13H13NO3S), 192 (13, C12HIsNO), 155 (59, C~H7SO2); ~H(CDC13): 0.76 (3H, t), 1.18 (8H, m), 1.56 (2H, t), 2.30 (3H, s), 2.62 (2H, t), 6.57, 6.58 (1H, dd), 7.03, 7.04 (1H, dd), 7.22 (2H, d), 7.65, 7.66 (1H, dd), 7.70 (2H, d); 5C(CDC13): 13.9, 21.5, 22.5, 24.2, 28.9, 29.1, 31.5, 39.6, 112.2, 121.3, 124.0, 127.1, 129.0, 130.2, 135.0, 145.6 and 195.5 ppm. Used without further purification, compound 28 (1.5 g, 4.32 retool) was dissolved

167 in dioxan (25 cm 3) and refluxed (17 h) with sodium hydroxide (25 cm 3, 5 M). After cooling to room temperature, the aqueous phase was separated and extracted with ethyl acetate (3 × 50 cm3). The combined organic phases were shaken with concentrated brine solution, separated, dried with sodium sulphate and the solvent evaporated under reduced pressure (40 °C, 10 -2 mm Hg) to give an oil that slowly crystallized. Recrystallization from hexane gave 31 [0.59 g, 66% based on 28]. M.p. 71-72 °C. Analysis found: C, 74.9; H, 10.3; N, 7.5. Calculated for C12H19NO: C, 74.6; H, 9.8; N, 7.3%; EI role (intensity%): 193 (6, M÷), 122 (8, CTHsNO), 109 (74, C6HTNO), 94 (100, CsH~NO); 8H(CDC13): 0.87 (3H, t), 1.29 (8H, m), 1.71 (2H, t), 2.76 (2H, t), 6.65 (1H, dd), 6.78 (1H, dd), 7.44 (1H, dd), 9.43 (1H, br, s); 5C(CDC13): 14.0, 22.6, 25.2, 29.1, 29.5, 31.7, 39.8, 108.7, 119.5, 123.2, 126.0 and 196.2 ppm.

3-Dodecanoylpyrrole (32) N-Tosyl-3-dodecanoylpyrrole (29) (m.p. 55- 56 °C) was detosylated to give 32, 94% yield. Mpt. 62- 63 °C. Analysis found: C, 77.3; H, 11.0; N, 5.3. Calculated for C16H27NO: C, 77.1; H, 10.8; N, 5.6%; EI m/e (intensity%): 249 (6, M+), 122 (11, C~HsNO), 109 (90, C6HTNO), 94 (100, CsH~NO); 5H(CDC13): 0.87 (3H, t), 1.30 (16H, m), 1.71 (2H, t), 2.76 (2H, t), 6.65 (1H, dd), 6.78 (1H, dd), 7.44 (1H, dd), 9.48 (1H, br, s); 5C(CDC13): 14.0, 22.7, 25.2, 29.3, 29.5, 29.6, 31.6, 39.8,108.6, 119.5,123.3, 126.1 and 197.6 ppm.

3-Octadecanoy lpyrrole (33) N-Tosyl-3-octadecanoylpyrrole (30) (m.p. 64 - 65 °C) was detosylated to give 33 (74% yield). Mpt. 84 - 85 °C. Analysis found: C, 79.3; H, 11.8; N, 4.4. Calculated for C22H3~NO: C, 79.3; H, 11.7; N, 4.2%; EI m/e (intensity%): 333 (10, M+), 122 (12, CTHsNO), 109 (100, C6HTNO), 94 (67, CsH4NO); 5H(CDC13): 0.88 (3H, t), 1.25 (28H, m), 1.71 (2H, t), 2.76 (2H, t), 6.64 (1H, m), 6.77 (1H, d), 7.44 (1H, m), 9.88 (1H, br, s); 5C(CDCI3): 14.1, 22.7, 25.3, 29.5, 29.7, 31.7, 39.8, 108.5, 119.7, 123.6, 125.6 and 197.7 ppm. Acknowledgements

We thank Cookson Group plc for financial support (to A.D.C. and N.R.M.S.), and the SERC and Research Corp. for assistance in the purchase of the electrochemical analyser.

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