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404 J.C.S. Perkin IMechanism and Stereochemistry of Nucleophilic Substitutions in Alkoxy-sulphonium Salts. Part 11.l Competitive Attack at Sulphur and Carbonin the Reaction of Alkoxydiarylsulphonium Salts with Halide IonsBy Rita Annunziata, Mauro Cinquini," and Stefan0 Colonna, Centro C.N.R. e lstituto di Chimica lndustrialedell'Universit8, Via C. Golgi 19, 201 33 Milano, ItalyThe stereochemical course of the reaction of optically active diarylethoxysulphonium salts with halide ions togive sulphoxides depends on the nature of the halide. For chloride, bromide, and iodide ions the reaction proceedswith retention of configuration at sulphur, whereas for fluoride ions net inversion i s observed ; this is tentativelyexplained on the basis of nucleophilic substitution involving apical attack by fluoride ion and equatorial departureof the leaving group, or of Berry pseudorotation.ALKOXYDIARYLSULPHONIUM salts (I) are ambident elec-trophiles and, depending on the nature of the nucleo-phile and of the substrate, attack can occur at sulphuror at carbon.1*2 In the reaction of optically activediarylethoxysulphonium salts with hydroxide ionsand with pyridine,l diary1 sulphoxides (11) are formedwith inversion and with retention of chirality at sulphur,respectively, by initial attack at the different reactioncentres.+We decided to establish the preferred pathway whena series of related nucleophiles is used, and treated(S)-ethoxy- (a-napht hyl) -+-tolylsulphonium fluorobor-ate (111) with tetraethylammonium iodide, bromide,chloride, and fluoride.From the results we coulddetermine a relative order of nucleophilicity and of thio-philicity. Reaction in anhydrous acetonitrile at roomtemperature gave the corresponding sulphoxide (IV),of variable optical activity, together with minor amountsof a-naphtyl fi-tolyl sulphide (V) (see the Table).Further information was obtained with the aid of a10% 180-enriched (I?)-(111).Part I, R. Annunziata, M. Cinquini, and S. Colonna, J.C.S.Per& I , 1973, 1231 ; see also references therein.a M. A. Khuddus and D. Swern, J . Amer. Chem. Sac., 1973,95,8393.DISCUSSIONThe stereochemical results, and the isotopic contentof the products, clearly indicate that reaction of (111)Reaction of the sulphonium salt (111) with halide ionsin acetonitrile a t 25"Sulphoxide Sul-* r- 7 phide1 8 0HalideI-Br-c1-F-I-R rc1-F-Mol .equiv.99II321111Yield9586747086806665(%)-410-410- 406 + 144- 380- 354- 156+ 129Retention Content Yield(%I (%) (%I100 100 3100 100 699 100 1832 c 1496 c 393 c 1069 44 3035 10 26a In acetone. b With respect to the starting material.c Not examined.with an excess of I-, Br-, or C1- occurs only via nucleo-philic attack at carbon by the halide ion, withoutaffecting the chirality at sulphur.Nucleophilic displacement at carbon is still largelyprefered in the reaction with equimolecular amountsof I- and Br-, but with 1 mol.equiv. of C1- the sulph-oxide (IV) obtained is substantially racemized.Inthe latter case, attack a t carbon, the extent of whichcan be measured from the isotopic content of (IV)(44%), must be accompanied by a competitive processinvolving achiral intermediates. Indeed the amountof retention based on the optical rotation of (IV) (69%)is in good agreement with the value (72%) obtained fromthe isotopic content, on the assumption that attackat sulphur leads to fully racemic material.It seems likely that racemization derives from sub-stitution at sulphur by C1- in the alkoxysulphoniumsalt (111), since both (111) and (IV) are optically stablein the reaction medium (see Experimental section).A possible mechanism is shown in Scheme 1.This mechanism involves the intervention of eithera tetraco-ordinate intermediate (VI) or a chlorosul-phonium salt (VII).Loss of chirality could be dueeither to formation of a diarylsulphur dichloride (VIII)or to halide exchange on (VII). Similar pathwayshave been proposed for the racemization of sulphoxidesC. R. Johnson and D. McCants, J . Anier. Chem. Soc., 1965,87, 64041.975 405by chloride i0ns.~*6 Furthermore the occurrence ofintermediates such as (VI) and (VIII) has been provedin the reaction of sulphides with organic hypochlorites.6Isolation of bis-(p-chloropheny1)sulphur &chloride has/OEt >s=o\ + ,S-OEtSCHEME 1been reported7 and its crystal structure has been de-termined.8The presence of sulpliide (V) in the reaction mixturecould be accounted for on the basis of the equilibriumbetween (VIII) and the corresponding ~ulphide.~,~When the alkoxysulphonium salt (111) is treated withtetraethylammonium fluoride the stereochemical resultis different : the sulphoxide obtained (ca.30% opticallypure) is of opposite chirality to the starting material.Its isotopic content, 10% with respect to (111), indicatesthat the reaction proceeds only to a limited extent-Jia attack at carbon, i.e. with retention of configuration.Two competitive pathways, viz. racemization and in-version, must operate, both involving sulphur as thereaction centre.In blank experiments it was shown that under thereaction conditions (i) the sulphoxide (IV) is chemicallyand optically stable in the presence of fluoride ions;and (ii) the alkoxysulphonium salt (111) in solventalone is transformed, after the usual work-up, only to avery limited extent into (IV) which is almost racemic:assuming that the reaction of (111) with fluoride ionsproceeds through formation of a fluorosulphonium salt(X) followed by hydrolysis to sulphoxide (IV), oneof the two steps must occur with retention and the otherwith inversion of configuration.A possible explanation involves apical attack byfluoride ion on (111) to give the intermediate (IX) andequatorial displacement of the ethoxy-group to afford(X) with retention of configuration at sulphur (SchemeOther examples of nucleophilic substitutions a tsulphur which proceed with retention of chirality areknown.1° An alternative explanation is that the* Similar assumptions are required if conversion (111) +(X) occurs with inversion and hydrolysis of (X) occurs withretention of configuration.t In the reaction of 1-ethoxy-3-methylthietanium salts withhydroxide ion pseudorotation has been disproved.124 R.H. Rymbrandt, Tetrahedron Letters, 1971, 3653, andreferences therein.K. Mislow, Record Chem. Progr., 1967, 28, 217, and referen-ces therein.(a) C. R. Johnson and J. J. Rigau, J . Amer. Chem. SOL, 1969,91, 6398; (b) R. J. Arhart and J. C. Martin, ibid., 1972, 94, 4997,5003.2)-incoming fluoride ion approaches the tetrahedralsulphonium salt from the side opposite to the lone pair(Scheme 2) to give a trigonal bipyramidal intermediateOn energetic grounds (XI) could rearrange through aBerry pseudorotation,ll to the more stable form (XII),where the more electronegative groups (F, EtO) bothoccupy the f avourable axial positions.Assumingthat loss of ethoxide ion is slower than pseudorotation,the fluorosulphonium salt (X) is formed from (111)with net retention of configuration, and then collapsesto sulphoxide (IV) with inversion." It should be notedthat no example of pseudorotation in sulphur compoundshas yet been reported.?The racemization process as well as the formation ofsulphide (V) could be rationalized, as in the case ofchloride ions, by formation of difluorosulphuranes, whichhave recently been investigated.13Two features of the present work should be empha-sized: (i) under our experimental conditions, the re-lative order of nucleophilicity of the halide ions towards(XI).CA -c 10H7Ft F -a-C10H7 II S ) - ( m 1lF -CL-c'0H7<Tp - MeC Hl / T O E tF/ XI 1SCHEME 2alkoxysulphonium salts is I-, Br- > C1- >> F-, whilethe thiophilicity order is reversed; and (ii) reductionto sulphide occurs more readily with chloride and7 R.J. Maver, Diss. Abs., 1967, 29, 2B, S 48.8 N. C. Baenziger, R. E. Buckles, R. J. Maner, and T. D.Simpson, J . Amer. Chem. SOC., 1969, 93, 6749.9 G. Wilson and M. Chang, Tetrahedron Letters, 1971, 876.10 (a) S. Oae, M. Yokoyama, M. Kise, and N. Furukawa, Tetru-hedron Letters, 1968, 4131; (b) B. U. Christensen and A. Kjaer,Chem. Comm., 1969, 934.l1 (a) K. Mislow, Accounts Chem.lies., 1970,3, 321, and referen-ces therein; (b) E. L. Muetterties and R. A. Schunn, Quart. Rev.,1966, 20, 245.R. Tang and K. Mislow, J . Amer. Chew SOC., 1969,91, 6644.13 D. B. Denney, D. 2. Denney. and Y. F. Hsu, J . Amer. Chem.SOC., 1973, 95, 4064406fluoride ions and is clearly related to their higher thio-philicity with respect to bromide and iodide. In eachcase it is lowered by using an excess of reagent, whichleads to increased attack at carbon.As previously discussed for other nucleophiles,lpreferential attack by halide ions at sulphur or carbonin alkoxysulphonium salts can be explained by a posi-tively charged sulphur being a ' harder ' electrophiliccentre than carbon and fluoride being the ' hardest 'nucleophile am ang the halide ions.A comparison can be made between our results andthe acid-catalysed reaction of sulphoxide with halideions in protic so1venfs,l4 which has been shown to in-volve the protonated sulphoxide, which is similar toan alkoxysulphonium salt.In this case iodide ionleads to reduction whereas bromide and chloride iongive only racemization, both reactions proceeding viaattack at sulphur with formation of a halogenosul-phonium salt.l* In different conditions, i.e. in anhydrousmedium, reduction of sulphoxide is a side reaction,and may become dominant, depending on the mediumand the ~ubstrate.~It remains to be explained why the use of an excessof halide ions leads in the case of Br- and C1- to a dif-ferent ratio of attack at sulphur and carbon.Work isin progress to define the kinetics of the two processes.EXPERIMENTAL(R)-( +)-cr-Naphthyl p-Tolyl [lsO]Sulphoxzde (IV) .-Thiswas prepared, according to the procedure previouslydescribed,15 via hydrolysis of the corresponding ethoxy-1 4 D. Landini, G. Modena, F. Montanari, and G. Scorrano, J .Amer. Chew. SOC., 1970, 92, 7168, and references therein.J.C.S. Perkin Isulphonium salt with sodium[lsO] deuterioxide [fromD,lsO (10% lSO) and sodium], and had m.p. 136", [aJDz5+410" (G 1, acetone), 10% lSO (mass spectrum).(S) -Ethoxy - (a-naphthyl) -p- Tolylsulphoniunz Fhoroborate(111) .-This was prepared by the method previouslyreported from (S)-(IV), -410" (c 1, acetone), andwas isolated as an oil and used as such.Reactions of the Ethoxysulphortium Salt (111) with Tetra-ethylawtmonium Halides.-Anhydrous tetraethylammoniumhalide (1 or 2 mmol) was dissolved in anhydrous aceto-nitrile ( 1 0 ml) and added a t room temperature to a solutionof (111) (1 mmol) in the same solvent (10 ml).After asuitable reaction time (12 h for I- and Br-, 24 h for C1-,and 48 h for F-) the mixture was diluted with water andextracted several times with chloroform. After evapora-tion of the solvent the crude product was purified by columnchromatography (SiO, ; diethyl ether-light petroleum 3 : 7).Yields and optical rotations are reported in the Table.The result from the reaction with fluoride ion did notchange on working in a glass vessel or in a steel bomb.Blank Experiments.-(a) A solution of the sulphoxide(IV), -410" (G 1, acetone), and the tetraethylam-monium halide (2 equiv.) in acetonitrile was kept at roomtemperature for 48 h. Work-up as previously describedafforded the sulphoxide (IV) with unaltered optical rotation.(b) The (S)-ethoxysulphonium salt (111) was kept inacetonitrile for 48 11 at room temperature. Half thesolution after the usual work-up afforded the sulphoxide(IV) (10% yield), 4 1 4 " (c 3, acetone). The opticalstability of (111) was checked by hydrolysis of the remainingsolution with sodium hydroxide to afford a practicallyquantitative yield of the sulphoxide (IV), [aJD25 +410°(c 1, acetone).[4/1217 Received, 21st June, 19741l6 R. -4nnunziata, M. Cinquini, and S. Colonna, J.C.S. Pevkin I,1972, 2057

ISSN:1472-7781    

DOI:10.1039/P19750000404

出版商:RSC    

年代:1975

数据来源: RSC