New synthetic routes to poly (isothianaphthene) I. Reaction of phthalic anhydride and phthalide with phosphorus pentasulfide

ELSEVIER

Synthetic Metals 74 ( 1995) 65-70

New synthetic routes to poly (isothianaphthene) I. Reaction of phthalic anhydride and phthalide with phosphorus pentasulfide Rob van Asselt a, Ivan Hoogmartens a, Dirk Vanderzande a**,Jan Gelan a, Peter E. Froehling b, Marcel Aussems b, Olav Aagaard b, Ronald Schellekens b aInstitute for Material Research, Division of Chemistry, Limburg University, Universitaire Campus Gebouw D, 3590 Diepenbeek, Belgium b DSM Research, P.O. Box IS, 6160 MD Geleen, Netherlands

Received 2 March 1995; accepted 9 May 1995

Abstract A new route for the formation of the low bandgap polymer poly(isothianaphthene) is presented. The route comprises the reaction of phthalic anhydride or phthalide with phosphorus pentasulfide at elevated temperatures (T> 120 “C), and leads to the formation of poly(isothianaphthene) in one single step. The product obtained was analysed by elemental analysis, IR, Raman and solid state NMR spectroscopy. Chemical doping and dedoping of the material were investigated, and the maximum conductivity obtained was 10 S cm-’ by doping with NOSbF,. Both doped and undoped samples of poly( isothianaphthene) prepared by this new route were thermally very stable in air or in an inert helium atmosphere as shown by thermogravimetric analysis. Furthermore, the conductivity of a doped sample remained unchanged up to 250 “C in air. Preliminary results showed that the reaction with phosphorus pentasulfide can also be used to obtain nitrogen analogues of poly( isothianaphthene). Keywords: Synthesis; Poly(isothianaphthene);

Phthalic anhydride; Phthalide; Phosphorus pentasulfide

1. Introduction In the field of electrically conducting organic polymers considerable attention has been devoted to polymers with a low bandgap [ l-121. The interest in these low bandgap polymers stems from the fact that they may show intrinsic conductivity without the necessity of oxidation or reduction by a dopant, which could lead to better environmental stability. Furthermore, these polymers can become increasingly transparent upon doping due to a shift of the absorption maximum from the UV-Vis region to the near IR [ 3,4], whereas other conducting polymers remain dark coloured. Several systems have been reported, such as polymers containing alternating electron-donating and electron-accepting units, for which bandgaps of 0.5-l .O eV have been reported [ 11. Another promising strategy to obtain low bandgap polymers is to derivatize conjugated polymers that possess a modest bandgap and are easy to substitute. This approach was first employed to obtain poly (isothianaphthene) (I, PITN) by fusing a benzene ring on the /?,p positions of the thiophene * Corresponding author. Elsevier Science S.A. SSDIO379-6779(95)03353-L

rings of polythiophene [ 21. The optical absorption spectrum of PITN has an onset at 1 eV, to be compared to 2 eV for the parent polythiophene. Recent studies have shown that this reduction of the bandgap is also accompanied by an aromaticto-quinoid geometry transition [IO]. Other low bandgap derivatives of polythiophene which have recently been reported include poly (3,4_ethylenedioxythiophene) II [ 111 and poly (2,3-dihexyltbieno [ 3,4-b] pyrazine) III [ 12 1.

I

II

III

The application of PITN, with a narrow bandgap of about 1 eV, has been limited severely up to now because of its elaborate synthesis. Conventional routes ae based on the electrochemical or chemical polymerization of dihydroisothianaphthene or isothianaphthene (Scheme 1) , the synthesis of which is rather laborious and too expensive to perform on an

R. van Asselt et al. /Synthetic

66

(X = Cl. Br)

PITN

Scheme 1. Conventional pathways to PITN: (i) Na2S; (ii) H,Oz or NalO.,; (iii) AI,O,/ AT; (iv) Oz. FeC& or N-chlorosuccinimide (NCS) ; (v) H2S0, or NCS; (vi) H2S04 or electrochemical polymerization; (vii) CH,S03H or electrochemical polymerization; (viii) SO&&, NCS or electrochemical oxidation.

industrial scale. Furthermore, these monomers are rather unstable, which limits storage. Finally, synthesis of (substituted) PITN derivatives is hard to achieve via the routes indicated in Scheme 1. We have developed a new route to obtain PITN in one step by reaction of phthalic anhydride or phthalide with phosphorus pentasulfide ( P4Sro), which is described. In addition, we describe some results which show the applicability of this reaction to obtain nitrogen analogues of PITN.

2. Experimental All syntheses were performed in a nitrogen atmosphere, unless stated otherwise. Commercially available products were used without further purification. Solid state NMR spectra were recorded on a Bruker CXP 200 spectrometer at a spinning frequency of 2500 Hz. IR spectra were obtained on a Philips Pye Unicam SP-300 and on a Bruker IFS 48 spectrophotometer. Raman spectra were obtained on a Bruker IFS 66 FT-IR spectrophotometer equipped with a FRA 106 FI Raman module, by using a Nd:YAG laser ( 1064 nm) . Thermogravimetric analysis was performed on a Perkin-Elmer TGA 7 with a temperature increase rate of 20 “C min- ‘. Particle size distributions were determined on a Malvern Instruments M6.10 apparatus by using dispersions in ethanol/water after 2 min of ultrasonic vibration. The composition of samples was determined by elemental (combustion) analysis (C, H and N) , neutron activation (S, I and Cl) and X-ray fluorescence (P, Sb and Cl, relative to the amount of S) . Conductivity measurements were performed by the fourpoint method on pressed bars of cross-sectional area 1 X 2 mm and with a 9 mm distance between the inner electrodes. 2.1. Synthesis of PITN A mixture of 6.20 g phthalic anhydride (42 mmol) and 13.31 g P,Slo (30 mmol) in 50 ml xylene was heated to reflux. After 17 h the mixture was cooled to 20 “Cand filtered. The solid was dispersed in 50 ml methanol and heated to

Metals 74 (1995) 65-70

reflux for 1 h. The mixture was filtered, and the residue refluxed once more in 50 ml methanol for 1 h. After filtration, the black solid was transferred to a Soxhlet apparatus, and continuously extracted with successively THF (20 h), chloroform (8 h) and petroleum ether 40-60 (2 h) . The product was dried in vacua for 20 h to give 2.2’g of PITN as a black solid (40%). IR (KBr): 1382,1272,1215,1185,1148,1040, 970,740,44Ocm-‘. Raman (KBr): 1454,1443,1301,1192, 1167, 1058,986,886,466,452 cm-‘. Average particle size: 12 pm. The reaction of phthalide with P,Sr,,, performed in the same way, gave PITN in 80-90% yield after 20 h. IR (KBr) : 1379, 1261, 1210, 1189, 1140,1049,969,737,430 cm-‘. Raman (KBr): 1459,1448,1300,1196, 1165,1062,989,887,462, 445 cm-‘. Average particle size: 13 pm. 2.2. Dedoping of PITN 2.2.1. Method A

To 6.0 g PITN in 70 ml methanol was added 10 ml 85% hydrazine hydrate, and the mixture was refluxed for 1.5 h. Then the solution was cooled to 20 “C and filtered. The residue was washed with methanol (2 X 40 ml), followed by diethyl ether (2 X 30 ml) and dried in vacua to give 4.8 g of a black powder. 2.2.2. Method B PITN (2.5 g) was stirred in 50 ml of 32% aqueous ammonia at 20 “C for 16 h. The solid was filtered off and washed successively with water ( 15 ml), methanol (2 X 20 ml) and diethyl ether (2 X 20 ml). The product was dried in vacua to give 2.2 g of a black solid. IR (KBr): 1588, 1452, 1391, 1266, 1220, 1190, 1147, 1050,978, 875, 854,746,430 cm-‘. Raman (KBr): 1459, 1442, 1299, 1165, 1064,991,888,445 cm-‘. Solid state 13C CPMASNMR(t,,=5ms): 126,139ppm. 2.3. Doping of PITN with NOBF,, NOSbF,, Iz and Feel, A mixture of 1.0 g undoped PITN and 0.82 g NOBF, (7.0 mmol) in 10 ml acetonitrile was shaken ultrasonically at 30 “C for 1.5 h. The solid was filtered, washed with acetonitrile (3 X 10 ml) and diethyl ether ( 15 ml). Drying of the product in vacua yielded 0.89 g of a black powder. Doping with NOSbF6, I2 and FeC13was performed in the same way (the ratio of dopant to PITN monomer was 0.5-l in all cases) ; doping with iodine was also performed in hexane. IR (KBr): 1374,1262,1210,1185,1141,1049,967,741, 430 cm-‘. Additional frequencies of dopant species: 1075 cm-‘, V(BF,-);380cm-‘; y( Fe-Cl). Raman ( KBr) : 1442, 1421,1302, 1193,1166,1053,988,890,464,452 cm-‘. 2.4. Poly(thieno[jl,Cb]pyridine) This polymer was prepared by heating 10 g 2,3-pyridinedicarboxylic anhydride (67 mmol) and 20 g P4Sro (45

R. van Asselt et al. /Synthetic

Scheme 2. Synthesis of PITN by reaction of phthalic anhydride and phthalide with P,S,,,.

mmol) in 100 ml xylene at reflux temperature for 20 h. After workup and purification, as described above, 6 1% of a black powder was isolated. Analytical data: C7H4,6No.$1.4Po.3,10% unidentified. The product was dedoped by refluxing with hydrazine in methanol (method A). IR (KBr) : 3160, 3081, 1660, 1457, 1417,1400,1291,1271,1233,1168,1114,1071,1023,941, 896, 842,797,776 cm-‘. Conductivity: o= 10e9 S cm-‘. Oxidative doping by NOBF,, I2 and FeCl, was performed as described for PITN. IR, FeCl, doped ( KBr) : 1678, 1587, 1453,1402,1260,1230,1164,998,846,797,772,751 cm-‘. Conductivity (oxidant used in parentheses): cr = 2 X lo-’ S cm-’ (NOBF,), 3X 10e5 S cm-’ (I*), 6X lo-’ S cm-’ (FeCl,) .

2.5. Poly(thiem[3,4-blpyrazine) In a similar way as described above, this polymer was prepared from 2,3_pyrazinedicarboxylic anhydride and P&e in 59% yield, dedoped by hydrazine (method A), and doped by oxidation with NOBF,, I, and FeC&. Analytical data: CSHWN,.&PO.Z~ 10% unidentified. IR, after dedoping (KBr): 3170,1503,1458,1385,1309,1170,1060,993,918, 849 cm-‘. Conductivity: u= 10-6-10-7 S cm-’ (dedoped); (5-7) X 10-5Scm-’ (dopedbyNOBF,,I,orFeCls).

3. Results and discussion 3.1. Synthesis of PUN

Reaction of phthalic anhydride or phthalide with P,Si,, at elevated temperatures leads to the formation of PITN in one single step (Scheme 2). The advantage of this new method for the synthesis of PITN, as compared to conventional methods, is that readily available chemicals are used, which are very stable and can be stored for long times at room temperature without special precautions. The reaction gives PITN in one step in fair to good yields and no synthetic modifications of the monomers are required to obtain PITN. The reaction is best performed by refluxing in a high boiling aromatic solvent. In xylene ( 140-145 “C) a40-50% yield was obtained by starting from phthalic anhydride, and 8090% by starting from p&halide. For phthalide anhydride higher yields can be obtained by melting together equimolar amounts of phthalic anhydride and P,S,, and careful heating at 120 “C (80%)) but this reaction is difficult to control on larger scale. A high temperature is necessary to obtain good conversions, since reaction in refluxing toluene at 111 “C

Metals 74 (I 995) 65-70

67

after 20 h leads to only trace amounts of PITN from phthalic anhydride, and approximately 15% PITN from phthalide. When phthalide is reacted with P4Sn, in xylene while the temperature is kept at 120 “C, the yield drops to 48% after 20 h, as compared to the 80-90% yield at 140-145 “C. Refluxing phthalic anhydride and P,S,, in mesitylene ( 165 “C) or 1,2dichlorobenzene ( 181 “C) leads to quantitative yields of PITN after 16 h. In THF, diglyme or acetonitrile no PITN was formed under similar conditions, due to the reaction of P&, with these solvents upon heating. Purification of the product was achieved in two steps: In the first step phosphorus-containing products are removed by refluxing with methanol, and in the second step other contaminations such as soluble oligomers are removed by extraction with THF and chloroform. Dedoping of the products obtained in this way was achieved by compensation with hydrazine hydrate or ammonia. Subsequently, the product was p-doped by oxidation with nitrosonium salts, iodine or ferric chloride. 3.2. Characterization of the products Analytical data of the compounds indicate that PITN is formed by the reactions of phthalic anhydride and phthalide with PS i0 (Table 1) . Deviations from the expected formula CsH$ might arise from the presence of small amounts of impurities, possibly adsorbed water or oxygen. In some samples the amount of sulfur relative to the amount of carbon is higher than the expected molar ratio of 1:8, which might be due to either the presence of elementary sulfur formed during the reaction or to the presence of end groups (e.g. thiocarbonyl) in the polymer. The results of X-ray fluorescence indicate that in some samples a few mass percent of phosphorus is still present, but in most cases the phosphorus contaminations are satisfactorily removed by refluxing in methanol. The product synthesized from phthalic anhydride and P,_& in mesitylene shows considerable deviation from the expected C,HsS formula and indicates significant amounts of impurities or chain defects. These results show that, although the yield of product in mesitylene is much higher, at too high temperatures side reactions occur. Consequently, xylene is a better solvent for performing the reactionas in that case better-defined products are obtained. For this reason the product obtained from 1,Zdichlorobenzene solution was not investigated further. Analytical data of the doped samples show that after doping with NOSbF, the antimony content of the product is rather low (approximately 1 mol Sb per 2&30 monomer units for PITN obtained from phthalic anhydride and phthalide, respectively). Oxidation of PITN by iodine or ferric chloride leads to higher iodine and chlorine contents, respectively. The molar ratio S:I is 2-3, which indicates 1 mol dopant per 2- 9 monomer units, depending on whether the dopant is Ior I,-, whereas the ratio S:Cl of l-l.5 points to 1 mol dopant per l-6 monomer units (dopant is Cl- or FeCl,-).

R. van Asselt et al. /Synthetic Metals 74 (1995) 65-70

68

Table 1 Elemental composition and conductivity data of PITN a Compound PlTNfrom phthalic anhydride/PJ,, After synthesis Dedoped Doped/NOSbF, Doped/I, Doped/FeCl, Dedoped Doped/NOBF‘, Doped/I, Doped/FeCl, Dedoped PlTNfrom phthalide/P,S,,, After synthesis Dedoped Doped/NOBF4 Doped/NOSbF, Doped/I, Doped/FeCl,

Solvent b

Composition

xyl. xyl. xyl. xyl. xyl.

CsII&.2Po.os CsII4.sS~sNo.z CsIWi.3Sbo.~

IlIeS. IIES. IWS. IIMX.

dcb.

xyl. xyl. xyl. xyl. xyl. xyl.

PITNfrom dihydroisothianaphthene/NCS Dedoped tcm. Doped/NOBF., tcm.

Rest % ’

3 4 2

wL.3~0.9b.5

W-L.3S1.3Cb.8 C&,sSo.sPo.osNo.&b.i not determined not determined not determined not determined

CSII4.7~I.ZPO.04

C8&SS0_9N0.I not determined

9

9 5

u (Scm-‘)

0.05-O. 1 (24) x 1o-3 0.2-0.9 0.3-l .6 0.2-0.7 1 x 1o-3 3x 1o-3 7x10-3 3x 1o-3 4x 1o-4

1-2 (l-3)

x 10‘s

wb.3~l.oc~o.9

1-5 5-10 2-6 5-7

not determined not determined

5x 1o-3 0.5

CsH&o.9Sbo.o3 10

CSfwO.9b.3

a Brute formulae were calculated from analytical data (mass%) by setting the carbon content to be 8 C atoms. Conductivity data are an average of several independent measurements. b Solvent used for the synthesis of polymers; xyl. = xylene (mixture of isomers), n-es.= mesitylene, dcb. = 1,2-dichlorobenzene, tcm. = tetrachloromethane. ’ Deviation of the sum of the mass percentages of the determined elements from 100%.

The powder shows no change in transparency and remains dark after doping. For comparison, solid PITN obtained from dihydroisothianaphthene and N-chlorosuccinimide [ 81 was doped in similar ways as our PITN and this also remained black. The lack of transparency after doping might be due to (i) the morphology of the powder which prevents uniform distribution of dopant throughout the particles and leads to low local dopant contents, (ii) the presence of (dark coloured) impurities in chemically synthesized PITN or (iii) the presence of strongly absorbing end groups, such as thiocarbony1 functions, or coloured dopant molecules (e.g. after doping by IZ or FeCl,) . The suggestion that the morphology of the powder limits the accessibility for dopant molecules is supported by the low antimony content of SbF6--doped PITN and the relatively large average particle size of 12 and 13 pm for the samples synthesized from phthalic anhydride and phthalide, respectively. Low dopant content was also found for PITN synthesized by chemical polymerization of dihydroisothianaphthene using FeC&, giving PITN which contained only 1 mol dopant (FeCl,-) per 37 monomers 151. The conductivities of the PITN compounds (Table 1) obtained directly after synthesis, which are in the range 0.052 S cm-‘, indicate that doping occurs during synthesis. Dedoping by hydrazine or ammonia leads to an average conductivity decrease by two orders of magnitude (cr,,,=2~10-~-1~10-~ S cm-‘). Upon chemical doping with NOBF,, NOSbF,, I2 or FeCl,, the conductivity

typically is in the range 0.1-10 S cm-‘. The best results in terms of conductivity are obtained for PITN synthesized starting from phthalide (up to 10 S cm-’ after doping by NOSbF,). Remarkably, the conductivity of the product obtained from phthalic anhydride and P&n in mesitylene did not increase significantly upon doping with NOBF,, I2 or FeCl,. These results support the conclusion derived from analytical data that PlTN synthesized at too high temperatures contains chain defects or impurities, leading to a poorly conducting material. IR and Raman spectra of the products obtained by the reaction with P4Sio are comparable to those obtained for products synthesized by published procedures [ 2,9,13,14]. The absence of anhydride or lactone signals in the region 1700-1800 cm-’ in IR indicates that no starting materials are present in the product. The spectra furthermore show that the products are doped directly after synthesis, in agreement with the conductivity data. Dedoping by hydrazine hydrate or ammonia gives IR, Raman and solid state NMR spectra characteristic of undoped PITN. The solid state NMR spectra show two signals at 139 and 126 ppm and are qualitatively the same as the spectra from PITN obtained by polymerization of dihydroisothianaphthene with N-chlorosuccinimide [g-lo]. In the IR spectra the intensity of the signals has decreased, the signals are less broad and extra signals have appeared at 854,875, 1452 and 1588 cm-’ (Fig. 1). After dedoping, Raman spectra show increased intensity of the signals, less broad signals and disappearance of the signals

R. van Asselt et al. /Synthetic

Metals 74 (1995) 65-70

Raman

69

IR

\ 1800

ICQI

200

1800

mo

IIMI

1500

wavenumber (cm-‘)

wavenumber (cm-')

Fig. 1. Raman (left side) and IR (absorption as a function of wavenumber, right side) spectra of PITN initsundoped(top) and doped (bottom) state, recorded as KBr pellets.

at 466 and 1192 cm-’ (Fig. l), comparable to previously reported observations [ 131. IR and Raman spectra of p-doped products show the typical characteristics of doped PITN, independent of the type of dopant used. Furthermore, signals of the dopant are observed at 1075 cm-’ (BF,-) and 380 cm-’ (FeC&-) in the IR spectra of PITN doped by oxidation with NOBF, and FeCl,, respectively. 3.3. Thermal stability of PITN The thermal stability of PITN obtained from phthalic anhydride and phthalide in an inert helium atmosphere as determined by thermogravimetric analysis (TGA), shows qualitatively the same picture as PITN synthesized from dihydroisothianaphthene and N-chlorosuccinimide [ 81 and PITN synthesized by electrochemical oxidation of isothianaphthene [4]. Up to 300 “C less than 10% weight loss has occurred, whereas the observed weight loss at 900 “C was 35% for PITN synthesized from phthalide and 43% for PITN from phthalic anhydride. In agreement with earlier results PITN synthesized by our new route is less stable in a helium/ oxygen atmosphere than in an inert atmosphere. PITN doped with NOSbF, shows comparable stability to undoped PITN under helium as determined by TGA. In another experiment it was shown that the conductivity of samples doped with NOSbF,, IZor FeCl, remains unchanged when heated to 250 “C for 10 min in air.

3.4. Scope of the reaction Except for the reaction of phthalic anhydride or phthalide with P&,,, PITN was also obtained by reaction of dimethyl-

phthalate or phthalic acid with P,S,o. In both cases 40-50% of PITN was obtained after 16 h refluxing in xylene and workup, as described for phthalic anhydride. The reaction of phthalic acid has to be carried out with an excess P&, ( 1.52 molar equivalents), as with 1 equivalent of P& a low yield of PITN was isolated ( 10%). Nitrogen-containing derivatives of PITN are readily accessible via our new route, as was shown by the reaction of 2,3-pyridinedicarboxylic anhydride with P&, to give poly ( thieno [ 3,4-b] pyridine) IV and the synthesis of poly (thieno[ 3,4-b] pyrazine) V from 2,3_pyrazinedicarboxylic anhydride and P4S10. N

IV

N

V

The products were characterized by elemental analysis and IR spectroscopy. Upon doping the signals in the IR spectra of poly (thieno [ 3,4-b] pyridine) are broader and less resolved and the spectra are independent of the type of dopant used. Furthermore, the signals at 3 160, 308 1, 107 1, 94 1 and 896 cm-’ have disappeared and the signals at 1280,998 and 75 1 cm-’ appear or are much more intense after doping of the polymer. The highest observed conductivity is in the order of low5 S cm-’ for both doped poly (thieno[ 3,4-b] pyridine) and poly( thieno[ 3,4_b]pyrazine), which is low as compared to PITN and the reported conductivity of 3.6X lo-* S cm-’ for poly (2,3_dihexylthieno [ 3,4-b] pyrazine) [ 121. The

70

R. van Asselt et al. /Synthetic Metals 74 (1995) 65-70

much lower conductivity of the nitrogen analogues as compared to PITN is surprising in view of the calculated bandgap which is lower than that of PITN [ 15,161. However, conductivities in this range have been reported previously for conjugated polymers containing alternating electron-donating and electron-accepting units, possessing a low bandgap of 0.5-l eV [ 11. Possibly, these are the value of truly undoped samples, whereas the values observed for ‘undoped’ PITN are in fact conductivities of oxygen-doped samples. Other explanations for the observed poor conductivity, which are also in agreement with the observed deviation of the analytical data from the expected values, are the formation of oligomers instead of polymers leading to a short conjugation length, the presence of impurities or defects, or the extreme sensitivity of the nitrogen-containing polymers towards air leading to degradation. Further investigations to study these aspects are currently being carried out. An interesting aspect of the presence of nitrogen atoms in the polymer is that the products might be soluble, e.g. by protonation. Indeed, poly (thieno [ 3,4-b] pyridine) is soluble up to approximately 35 mg ml-’ in acidic media such as concentrated sulfuric acid, formic acid and a 5% solution of camphorsulfonic acid in chloroform. In neutral solvents (acetonitrile, chloroform or dimethylsulfoxide) less than 4 mg ml-’ dissolves. Unfortunately, no ‘H NMR spectra of poly (thieno [ 3,4-b] pyridine) could be obtained in D2S04 due to extensive line broadening.

4. Conclusions A new route to obtain PITN from phthalic anhydride or phthalide by the reaction with P,Slo has been found. The advantage of this route is that commercially available starting materials can be used without synthetic modifications, giving PITN in one single step in fair to good yields. In view of the cost price of starting materials, phthalic anhydride is most interesting, but phthalide gives higher yields of PITN. The maximum conductivity obtained is 10 S cm-’ after chemical doping with nitrosonium salts, which is lower than the highest reported values of 50 S cm-’ for electrochemically prepared samples [ 41 and 30 S cm - ’ for PITN prepared by oxidation of poly( dihydroisothianaphthene) with sulfuric chloride [ 71. This is likely due to the morphology of the products, which limits uniform distribution of the dopant throughout the powder, and prevents high doping levels from being reached. It is also in agreement with the fact that no visible change in transparency occurs upon (de)doping. Experiments to investigate these aspects are currently in progress.

The reported procedure is not limited to phthalic anhydride and phthalide, but can be applied to obtain substituted derivatives and nitrogen analogues of PITN, as we have shown by some examples. This might lead to a wide variety of new (soluble) PITN derivatives as substituted anhydrides and phthalides are readily accessible, e.g. by chloromethylation of substituted benzoic acids with formaldehyde/HCl [ 171. Such derivatives were much more difficult to obtain by previously reported methods, because substituted derivatives of (dihydro) isothianaphthene require elaborate synthetic pathways [ 181.

Acknowledgements

The experimental assistance of Nicole Verhagen is gratefully acknowledged. The work at the Limburg University was financially supported by DSM.

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