摘要:
C hem i cal Corn m u n i cat ions NUMBER 18/1965 22 SEPTEMBER Asymmetric Synthesis and Racemization of Disulphide Monoxides By W. E. SAVIGE and A. FAVA (Istituto di Chimica Generale, Universitli di Pisa, Italy) IT is known that oxidation of an unsymmetrically substituted thioether by an optically active peracid can lead to asymmetric synthesis of the corre- sponding sulphoxide (RmSO-R’) .1 S-Oxides of symmetrically or unsymmetrically substituted disulphides (R.SO6.R or R-SO-S-R’) should in theory be resolvable into stereoisomers, and this has been shown to be possible by the recent isola- tion of the S-monoxides of L-cystine.* We have found that asymmetric synthesis of disulphide monoxides (thiolsulphinates) can be effected by oxidation of the disulphide with percamphoric acid in chloroform at 0-10”.Under these conditions, the diaryl disulphide monoxides obtained (Ar.SO.S-Ar, Ar = C,H,, P-Cl-C,H,, p-M&,H, or /3-naphthyl) showed some optical activity ([or12 ca. loo, c 1 in chloroform), while the corresponding alkyl (dodecyl) or aralkyl (benzyl) compounds were optically inactive. The purity of the products, after successive recrystallizations from cold chloroform-light petroleum and aqueous acetone, was established by thin-layer chromato- graphy (silica gel, benzene ; location by iodine vapour3 or i~doplatinate~) , which gave satisfactory separation of the disulphide monoxides (R, ca. 0.5) from initial impurities such as the disulphide (R, ca. 0.9), the disulphide dioxide (RSO,SR, R, ca. 0.7) and camphoric acid (R, 0). Apart from optical properties, the physical properties (appear- ance, melting point, and U.V.absorption spectrum) of the diaryl disulphide monoxides agreed well with those of the inactive monoxides prepared by oxidation of the disulphides with perbenzoic acid (cf. ref. 6 ) or by other methods described in the literat~re.6,~1* Diary1 disulphide monoxides are reported to undergo rather ready disproportionation to the disulphide and disulphide dioxide, either in the solid state on rigorous drying7 or in hexane solution.6 2 R-SO-S-R __+ RSSR + RS0,SR K. BalenoviC, N. Bregant, and D. FrancetiC, Tetrahedron Letters, 1960, No. 6, 20; K. BalenoviC, I. Bregovec, D. FrancetiC, I. MonkoviC, and V. Tomasid, Chem. and Ind., 1961, 469; A. Mayr, F. Montanari, and M. Tramontini, Gazzetta, 1960, 90, 739; A.Maccioni, F. Montanari, M. Secci, and M. Tramontini, Tetrahedron Letters. 1961, 607; G. Barbieri, U. Folli, F. Montanari, and G. Torre, Boll. Sci. Fac. Chim. Ind. Bologrta, 1965, 23, 47. 4 W. E. Savige, J. Eager, J. A. Maclaren, and C. M. Roxburgh, Tetrahedron Letters, 1964, 3289. a “Anfarbereagentien fur Dunnschicht- and Papier Chromatographie,” E. Merck Co., Darmstadt, Germany, 1961. ti H. J. Backer and H. Kloosterziel, Rec. Trav. chim., 1954, 73, 129. ? D . Barnard, J . Chem. Soc., 1957, 4675. 8 G. Modena and P. E. Todesco, Ricerca Sci., 1960, 30, 1788. H. M. Winegard, G. Toennies, and R. J. Block, Science, 1948, 108, 506. E. Vinkler and F. Klivdnge, Acta Chim. Acad. Sci. Hung., 1957, 11, 15.418 CHEMICAL COMMUNICATIONS We find that solutions of a monoxide in the com- mon solvents remain stable for at least several days in the dark or in dim light, but rapidly dispropor- tionate in strong light (Pyrex vessels) .D These monoxides of symmetrical disulphides show considerable variation of optical stability according to the solvent medium.They appear to be comparatively very stable in certain solvents such as acetonitrile, dioxan, butanone, or acetic acid, where the first-order specific rate for racemi- zation is less than 2 x 10-5 sec.-l at 50". In other solvents such as benzene, cyclohexane, or ethanol they appear to racemize fairly easily. In dry benzene at 50", di-p-chlorophenyl disulphide mon- oxide (all kinetic measurements were made on this compound) racemizes with a half-life of about 1 hr.: krac = 1.9 x lo-* sec.-l The first-order specific rate appears to be independent of the concentration of the monoxide (in the range 0-07-0.2~).The addition of small amounts of water or ethanol to the benzene strongly decreases the racemization rate. In benzene saturated with water a t room temperature (ca. 0 . 0 3 ~ ) the rate is decreased by a factor of about 20. However, the rate is then dependent upon the concentration of monoxide as well, and it increases with increas- ing [monoxide] / [water] or [monoxide] / [ethanol] ratio. It is remarkable that although small amounts of ethanol in benzene (up to 0 . 5 ~ ) decrease the racemization rate, pure ethanol (Krac = 1.1 x sec.-l a t 50") causes racemization faster than pure benzene. Pyridine is outstanding in that racem- ization occurs immeasurably fast at room tem- perature.The exceptional behaviour of pyridine suggests that racemization may occur in this solvent by way of a special mechanism made possible by its nucleophilic character. In fact, racemization could arise by way of a nucleophilic attack on one of the sulphur atoms, such as Ar.SO.S.Ar + Py ---+ Ar-SO- S-Ar I +Py In such a reaction the sulphinyl and pyridinium ions need not become separated, but racemization could occur on collapse of the ion-pair.ll To test the hypothesis of a nucleophilic attack, the effect was determined of a nucleophile such as ionic chloride. It was found that chloride ion has an enormous effect in that in acetonitrile a t 25" in the presence of 10-4~-tetrabuty~ammonium chloride the first-order specific rate is krac = 6.9 x sec.-1 Tetrabutylammonium perchlorate was found to have no appreciable effect up to a concentration 10-fold higher. It is likely that this "nucleophilic mechanism" may be operative in ethanol as well. However, some other mechanism must operate in the other solvents, particularly in benzene and in cyclohexane. This matter is currently under investigation. (Received, August 2nd, 1965; Corn. 479.) @ Kice (ref. 10) cites unpublished work by Venier according to which aryl thiolsulphinates in the absence of strong 10 J. L. Kice and N. E. Pawlowsky, J . Amer. Chem. Soc., 1964, 86, 4898. 11 A. Fava, U. Tonnellato, and L. Congiu, Tetrahedron Letters, 1965, 1657. acid decompose only very slowly in solution even at 65".
ISSN:0009-241X
DOI:10.1039/C19650000417
出版商:RSC
年代:1965
数据来源: RSC