摘要:
WZA drastic effect of halide anions on the nucleophilic substitution of mz1-phenylbenzo[61thiophenium salts xzxe-+ Tsugio Kitamura,**' Shin-ichi Soda," Kunihiko Morizane," Yuzo Fujiwara 'and2'ZEHiroshi Taniguchi a Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University 36, Hakozaki, Fukuoka 812-81, Japan Kurume National College of Technology, Komorino-cho, Kurume 830, Japan Nucleophilic substitution of 1,2,3-triarylbenzo[b] thiophenium salts with halide anions (Cl- and Br -) has been found to yield 2,3-diarylbenzo [b]thiophenes, halobenzenes and 1,2-diaryl-l-halo-2- [2-(phenylsulfanyl)phenyl] ethenes and especially the reaction with iodide anion gives only 2,3-diarylbenzo[b] thiophenes and iodobenzene, indicating that halide anions behave quite differently from the reaction with alkoxide anions reported previously.1-Phenylbenzo[b]thiophenium salts have unique structural features and are expected to show interesting chemical properties. The short, strong Ph-S+ bond resists nucleophilic cleavage in the reaction with nucleophiles and favours the ring- opening ofthe thiophene ring. Previously we have reported that the reaction of 1,2,3-triarylbenzo[b)thiophenium salts with alkoxide anion undergoes a nucleophilic ring-opening reaction and gives exclusively 1-alkoxy-l,2-diaryl-2-[2-(phenylsulfanyl)-phenyllethenes [reaction (1)].2 The stereochemistry of the ring-opened products, alkoxyethenes, is the 2 configuration which implies that the ring-opening reaction proceeds with the complete .retention of configuration.On the basis of the stereochemical outcome we have proposed two possible mechanisms: a mechanism involving nucleophilic substitution via o-sulfuranes and a rapid addition-elimination mechanism for the ring-opening reaction. In order to gain further information of the chemical behaviour toward nucleophiles, we have studied the nucleophilic reaction of 1,2,3-triarylbenzo[b)thiophenium salts with halide anions. Surprisingly, the reaction with the iodide anion caused exclusive bond breaking of the 1-phenylbenzo[b] thiophenium salts. Other halide anions also favoured the cleavage of the Ph-S+ bond rather than the ring-opening reaction. In this communication, we report the interesting drastic behaviour of halide anions in the nucleophilic reaction of 1,2,3-triaryl-benzo[b] thiophenium salts.Substituted 1-phenylbenzo[b]thiophenium salts 1 were prepared by halogenation of the corresponding [2-(phenylsul- fany1)phenyllalkenes."J The reactions of 1,2,3-triarylbenzo[b]- thiophenium salts with halide anions were conducted by the thermal decomposition of the 1,2,3-triarylbenzo[blthiophenium halides 1 (X = Br, Cl) and by the reaction of 1,2,3-triaryl-benzo[b]thiophenium perchlorates 1 (X = ClO,) with tetra- butylammonium iodide (Bu,NI). A typical reaction is as fol- lows. A suspension of 1,2,3-triphenylbenzo[blthiopheniumbro-mide la (X = Br) in toluene was refluxed for 24 h. After removal of the toluene, 2,3-diphenylbenzo[b]thiophene 2a (72%) and I -bromo-1,2-diphenyl-2-[2-(phenylsulfanyl)phenyl]ethene 3a (X = Br) (24%) [reaction (2)] were isolated by column chromatography on silica gel.The volatile product, bromo- benzene (56%), was analysed by GC before column chromato- graphy. Other 1,2,3-triarylbenzo[b]thiophenium halides 1 (X = Br and Cl) were decomposed by a similar manner. The reactions with the iodide anion were conducted by heating a mixture of triarylbenzo[b]thiophenium perchlorates and Bu,NI in propiononitrile. The conversion was low but no iodo- ethene 3 (X = I) was detected. The results are given in Table 1. PhI + -OR ,C'C, A? Ar' f Ar', A? = Ph, p-MeOC6H4;R = Me, Et, Pi, Me,CCH, p x-1 Reflux in a solvent h + PhX 2 3 a: Ar' = A? = Ph; b: Ar' =p-MeOC6H4,A? = Ph; c: Ar' = Ph, A? =p-MeOC6H4;d: Ar' = A? =p-MeOC,H4 The formation ofbenzo[b]thiophenes 2, haloethenes 3 (X = C1, Br, I) and halobenzene indicates that competition between the breaking of the Ph-S+ bond and the C-S+ bond of the thiophene ring takes place during the nucleophilic substitution.Of particular note is the reaction of 1,2,3-triarylbenzo[bl- thiophenium salts with Bu,NI in which only bond cleavage of the Ph-S+ bond occurs. This result is quite different from the reaction with alkoxide anions where exclusive ring-opening reactions are observed. The formation of haloethenes 3 is highly stereoselective. Namely, only one isomer of the two possible E-and Z-configurations was present.It was, however, difficult to determine the configuration of the haloethenes 3 by spectral means. Finally, the absolute structure was anaiysed by a single crystal X-ray diffraction of (Z)-1-bromo-I -(4-methoxyphenyl)-2-phenyl-2-[2-(phenylsulfanyl)phenyl]ethene 3b (X = Br).4 The reaction of 1-phenylbenzo[b]thiophenium salts 1 with halide anions involves bond cleavage of two types of Sf-C bonds: the phenyl-S+ bond and the S+-C bond of the thio- phene ring. From the crystal data la of 1,2,3-triphenylbenzo- J. Chem. SOC.,Perkin Trans. 2, 1996 473 Table 1 Reaction of 1,2,3-triarylbenzo[b]thiophenium salts 1with halide anions 1 Yields of products (%)" Product ratio Arl Ar2 X Reaction conditions 23 PhXb c21:c31 Ph C1 Ph Br Ph Br p-MeOC,H, Br p-MeOC,H, Br Ph CIO, Ph ClO, Ph Br Ph CIO, Toluene, 1 10 "C Toluene, 1 10"C Toluene, 110"C Toluene, 1 I0 "C Toluene, 1 10 "C Bu,NI, EtCN, 110'Cd Bu,NI, EtCN, 97"C" 48 47 (X = C1) 51 :49 L 72 24(X = Br) 56(X = Br) 75:25 55 34(X = Br) C 62:38 78 17(X = Br) C 82:18 69 19(X = Br) C 78:22 42 0 34(X = I) 1oo:o 25 0 33(X = I) 1oo:o NaOMe, MeOH, 64.5 "Ce 0 1OO(X = MeO) 0 0:100 NaOMe, MeOH, 64.5"C' 0 1OO(X = MeO) 0 0:100 a Isolated yields by column chromatography on silica gel.* Determined by GC. 'Not determined. Reactions were not completed. Ref. 2. IA2 4 5 6 [blthiophenium perchlorate la (X = C104), the Ph-S+ bond length is short (1.784 A) and comparable to the ring S+-C bond length (1.790 A).Accordingly, the ring S+-C bond is expected to be weak compared with the phenyl-S+ bonds and to undergo a nucleophilic bond fission. However, the choice of the bond fission is strongly dependent upon the nature of nucleophiles. Addition4imination is a generally accepted mechanism for nucleophilic substitution of unsaturated systems. The present reaction may proceed with the competing vinylic and aromatic addition-elimination processes yielding benzo[b]thiophenes 2 and ring-opened haloethenes 3. The stereochemical outcome of the ring-opened haloalkenes suggests a rapid addition- elimination process via a short-lived intermediate 4 as reviewed by Rapp~port,'~ where the stereochemistry of the product is retained and the 2 isomer is formed.On the other hand, the mechanisms involving a ligand-ligand coupling via o-sulfuranes have been proposed in the reactions of triarylsulfonium ions and diary1 and alkyl aryl sulfoxides with organo-lithium and -magnesium reagents. Recently Furukawa and co-workers have isolated a stable o-sulfurane and analysed it by X-ray diffraction.' Therefore, we postulate the formation of o-sulfuranes 5 and 6 as the intermediates in the thermal decomposition, although it is not certain at the present time whether such a ligand coupling process proceeds via a concerted process or a rapid addition4imination mechanism . In summary, we have found a drastic element effect on the nucleophilic substitution of 1,2,3-triarylbenzo[b]thiophenium salts. The nucleophilic reaction with halide anions indicates that the dephenylation and ring-opening reactions are in competition and the ring-opening reaction proceeds with the retention of the configuration.In particular, the reaction with the iodide anion is an extreme example in which the dephenylation reaction only takes place. Such competing processes do not take place in the reaction with alkoxide anions. The differences between these reactions may be attributed to the property of the counter anion (nucleophilic reagent). Detailed mechanistic studies of this interesting behaviour are now in progress. Acknowledgements This work was partly supported by the Ministry of Education, Science and Culture, Japan. References 1 (a)T. Kitamura, S. Soda, H.Kawasato, H. Taniguchi and M. Shiro, Tetrahedron, 1993,49,5055;(b) T. Kitamura, M. Yamane, R. Furuki, H. Taniguchi and M. Shiro, Chem. Lett., 1993, 1703. 2 T. Kitamura, M. Miyaji, S. Soda and H. Taniguchi, J. Chem. Soc., Chem. 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Miyagaki, Chem. Pharm. Bull., 1974, 22, 2020; S. Oae, T. Kawai, N. Furukawa and F. Iwasaki, J. Chem. SOC.,Perkin Trans. 2, 1987, 405; S. Ogawa, Y. Matsunaga, S. Sato, T. Erata and N. Furukawa, Tetrahedron Lett., 1992,33,93;S.Ogawa, S.Sat0 and N. Furukawa, Tetrahedron Lett., 1992, 33, 7925; S. Sat0 and N. Furukawa, Tetrahedron Lett., 1995,36,2803. 8 S. Ogawa, Y. Matsunaga, S. Sato, I. Iida and N. Furukawa, J. Chem. Soc., Chem. Commun., 1992,1141. Paper 5/08177G Received 18th December 1995 Accepted 19th January 1996 474 J. Chem. SOC.,Perkin Trans. 2,1996
ISSN:1472-779X
DOI:10.1039/P29960000473
出版商:RSC
年代:1996
数据来源: RSC