TheS,S-acetal to carbonyl transformation using the soft NO+or Cl+species

摘要

J. CHEM. SOC. PERKIN TRANS. I 1983 The S,S-Acetal to Carbonyl Transformation using the Soft NO+ or Cl+ Species M. T. M.El-Wassimy, K. A. Jargensen * and S.-0. Lawesson Department of Organic Chemistry, Chemical Institute, University of Aarhus, DK-8000 Aarhus C, Denmark A series of S,S-acetals (spiro-l,3-dithiolanes, 1,3-dithiolanest and 3,s-dibenzyt acetals) have been converted into the corresponding carbonyl compounds by sodium nitrite in aqueous solution (NO +, H20NO+, CINO) or with t-butyl hypochlorite (Cl") in anhydrous CCI4. Mechanisms for these trans- formations are suggested, based on the Hard and Soft Acids and Bases (HSAB) principle. The transformation of S,S-acetals into the corresponding carbonyl compounds is an important reaction.' For it to be successful it is important that a soft (borderline) acid (soft acids: low energy of the frontier unoccupied orbitals 2, attacks the soft base (the sulphur of the S,S-acetal; soft bases: high energy of the occupied frontier orbitals ".A variety of procedures for the title transformation are known, as for example use of transition-metal ions (Ti4+, Cu2+, Cd2+, Ag+, and Hg2+), oxidation of sulphur to a higher oxidation state, or S-a1kylation.l As a continuation of our work on reactions of soft (borderline) electrophiles with thiocarbonyl compounds we have studied the transform- ation of S,S-acetals into the corresponding carbonyl com- pounds (Scheme 1)by two different methods: sodium nitrite in aqueous acid solution (NO+) (Method A) and t-butyl hypo- chlorite (Cl+)in anhydrous Cell (Method B).Scheme 1. Results and Discussion A series of spiro-1,3-dithiolanes (l), (3), and (9,1,3-dithio-lanes (6),(8), and (lo), and S,S-dibenzyl acetals (2), (4), (7), (9), and (11) have been prepared by standard methods and treated with sodium nitrite in ~M-HCI solution (Method A) or t-butyl hypochlorite in anhydrous CCl, (Method B) at room temperature. When 3 mol equiv. of NaNOz (excess) are are allowed to react with S,S-acetals in ~M-HC~or 2 mol equiv. of Bu'OCl (excess) withS,S-acetalsin CCl, at room temperature the corresponding carbonyl compounds are formed after a relatively short time (see Table) and isolated in most cases in reasonable yield. Inspection of these results (see Table) reveals that aromatic S,S-acetals are transformed into the corresponding carbonyl compounds in reasonable yields by both Method A and Method B.For the alicyclic S,S-acetals the yields of the transformation are somewhat lower. Further, it should be noted that S,S-acetals of heptanal produce low yields of heptanal, besides a number of by-products when treated according to both methods. The advantages of using sodium nitrite in ~M-HC~or t-butyl hypochlorite in anhydrous CCI,, for the deprotection of the S,S-acetals are the mild reaction conditions and the simplicity of work-up together with the ready availability and cheapness of the compounds. Comparing the transformation of 0,O-acetals and S,S-acetals into their corresponding carbonyl compounds, it should be noted, that the choice of reagents is crucial.Hydrolysis of O,O-acetals and orthoesters is carried out by Table. Reaction of S,S-acetals with NaNOz in ~M-HC~(Method A) or Bu'OCI in anhydrous CCI, (Method B) Method A Method B Reaction Reaction Compd. time(h) 1 7 time (h) 1 88 1 94 1 100 4 43 20 24 20 42 20 55 0.25 88 1SO 65 1 67 2 94 5 100 0.25 90 1 100 1 80 1 100 1 70* 6 48 1 t 24 83 51 * Together with some unidentified products. t Complicated mixture of by-products (seems to contain carbonyl compound). mineral acids (H+),whereas S,S-acetals are transformed into the corresponding carbonyl compounds by transition-metal ions (Ti4+, Cuz+, Ag+, Cd2+, Hg2+), oxidation of sulphur, or S-alkylation.These differences can be accounted for by MO theory and the theory of Hard and Soft Acids and Bases (HSAB). Ward acids (electrophiles) are generally positively charged and have a relatively high LUMO energy (e.g.cations of the more positiveelementsofthePeriodicalTable:H+,Li+, Na+, K+)whereas soft acids (electrophiles) have low energy and do not necessarily have a positive charge. Hard nucleophiles (bases) are generally negatively charged and have a relatively low HOMO energy anions centred on the electronegative elements (e.g. OH-) and soft nucleophiles have a relatively high HOMO energy and do not necessarily have a negative charge (e.g. t hiocyanates). O,O-Acetals are hard, and are hydrolysed by the hard H+,5 due to the favourable hard-hard interaction, which gives a significant contribution to the Coulomb term in the Klopman equation equation (l) Coulonib term Frontier orbital term AE = qnucl'qetec 2(cnucl.Celec*P) +E I-EHOMO-~LUMO S,S-AcetaIs are soft, and are transformed into the corre- sponding carbonyl synthons by soft acids, and the orbital term in equation (1) is most important. Orbital-controlled reactions are soft-soft interactions and are exothermic; some of the reactions studied in this investigation are, in fact, exothermic. The electrophiles used here for the S,S-acetal-+ carbonyl cp aR2 J.CHEM. SOC. PERKIN TRANS. I 1983 isobutylene, the thiol, and the corresponding carbonyl com- pound.S-Benzyl thionitrite' (12) has been isolated in low yield and was characterized on the basis of i.r. and mass R1 spectral evidence see equation (2)J. Compound (12) is also R' R2 (1) R1 ,R2 = -S-CH,-CH~-S-(3) R',R*=-S-CH,-CH~-S-2 PhCH2-S-NO ---3~-PhCH2-S-S-CH2Ph + 2 NO (2) (2) R'= R2=-S-CH2Ph (4)R', R2=-S-CH2 Ph (12) (1 3) known to form dibenzyl disulphide (13) and nitrogen oxide,' which accounted for the isolation of (13) from all the reactions /R' with S,S-dibenzyl acetals. It is also envisaged, that the soft sulphenyl chloride (RSCI), formed by Method B can also fJCYR' attack the soft sulphur of (I) or (11) to give (13). It is well known from nitrosation reactions that anions such as C1- and SCN- increase the reaction rate and yields.' (5) (6) R' ,R2 =--S-CHZ-CHZ-S Preliminary investigations seem to indicate that when Method f 7) R' ,R2 =-S-CHzPh A is used for the transformation of (1) and (3) into the corresponding carbonyl compounds by the presence of SCN-in the reaction mixture, the yields of the carbonyl compounds increase by 14 and 63, respectively (the same reaction time). Further work is in progress to elucidate the effect.C5H11-C-Me s' 's U Experimental 'H N.m.r. spectra were recorded at 60 MHz on a Varian (8) R',R2=-S-CH,-CHz-S-(10) EM 360 spectrometer, SiMe, being used as an internal (9)R',R2=-S-CH2Ph standard. 1.r. spectra were recorded on a Beckman IR 18 spectrometer. Mass spectra were recorded on a Micromass 7070F spectrometer operating at 70 eV using a direct inlet.Microanalyses were carried out by L~vens Kemiske Fabrik, DK-2750 Ballerup (Microanalytical Lab.). The starting compounds were prepared according to known method^.^ (11 1 General Procedure.-Sodium nitrite in aqueous acid solution. Method A. The S,S-acetal (5 mmol) was mixed with ~M-HC~ (15-20 ml) (CH2Ci2 was also used if the S,S-acetal was 4-transformation are NO+ (H,ONO and ClNO of which the insoluble in the acidic medium) and NaN02 (15 mmol) in last one is assumed to be present when HCI is used as acid) or water (5-10 ml) was added dropwise with vigorous stirring Cl+. Both NO+ (borderline) and C1+ are soft.2 The first step at room temperature; the reaction times are given in Table 1. in the reaction is the attack of X+ (X = NO or C1) at the The reaction mixture was extracted with CHzClz and the soft sulphur of the S,S-acetal (I) giving (11): combined organic layers were washed with water and dried (MgSO,).The organic phase was evaporated and then subjected to column chromatography on silica gel with CH2C12 as eluant. The products were identical ('H n.m.r., i.r. and mass spectra and m.p.) with authentic samples of the carbonyl compounds. t-Butyl hypochlorite in anhydrous CCl,. Method B. The S,S-acetal (5 mmol) was mixed with anhydrous CC14 (15-20 ml) and Bu'OCI (10 mmol) in anhydrous CCl, (5 ml) was added dropwise with vigorous stirring at room temperature; the reaction mixture was worked up as above. Acknowledgements Thanks are expressed to DANIDA for a post-doctoral fellowship to one of us (M.T. M. E.-W.). Met hod A )=O Method B References 1 (a) B.-T. Grobel and D. Seebach, Synthesis, 1977, 6, 357 and Scheme 2. references therein: (6) N. J. Cussans, S. V. Ley, and D. H, R. Barton, J. Chern. Soc., Perkin Trans. I, 1980, 1654 and references therein; (c) G. A. Olah, S. C. Narang, G. F. Salem, and B. G. B. Gupta, Synthesis, 1979, 4, 274; (d) H. Ikehira, S. Tanimoto, T.By Method A RSNO is substituted by water, and subsequent Oida, and M. Okano, ibid., 1982, 12, 1087.decomposition produces the carbonyl compound and the 2 (a) R. G. Pearson, 'Hard and Soft Acids and Bases,' Dowden, thiol. Similarly, t-butoxide and (11) give the sulphenyl chloride Hutchinson, and Ross, Inc., 1973, and references therein; and an 0,s-acetal, of which the last one decomposes to give (b) T.-L. Ho, 'Hard and Soft Acids and Bases Principle in J.CHEM. SOC. PERKIN TRANS. I 1983 Organic Chemistry,’ Academic Press, London and New York, 1977. 3 (a) K. A. Jsrgensen, A. B. A.-G. Ghattas, and S.-0. Lawesson, Tetrahedron, 1982, 38, 1163; (b) K. A. Jsrgensen, M. T. M. El-Wassimy, and S.-0. Lawesson, ibid., 1983, 39, 469; (c) K. A. Jsrgensen and S.-0. Lawesson, Chemica Scripta, 1982, 20, 227; (d) M. T. M. El-Wassimy, K. A. Jprrgensen, and S.-0. Lawesson, Tetrahedron, 1983, 39, 1729. 4 (a) A. Schoenberg and 0. Schutz, Liebigs Ann. Chem., 1927, 454, 47; (b) H. J. Bacher and A. F. Tainoma, Reel. Trav. Chim. Pays-Bas, 1938,57, 1183; (c)H. Hauptmann and M. M. Campos,J. Am. Chem. Soc., 1950, 72, 1405; (d) T. Posner, Chem. Ber., 1902,35, 2343; (e) H. Fasbender, ibid., 1888, 21, 1473; (f)E. E. Reid and A. 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