60 J. Chem. SOC. (C), 1970 Thermal Decomposition of t-Butyl 2-Nitrobenzenesulphenate By D. R. Hogg * and P. W. Vipond, Department of Chemistry, University of Aberdeen, Aberdeen AB9 2UE t- Butyl 2-nitrobenzenesulphenate decomposes in refluxing anisole in the dark to give 2 - and 4-methoxyphenyl 2-nitrophenylsulphides, isobutene and di-2-nitrophenyl disulphide. Aniline and p-methoxybenzenesulphonic acid were also obtained and are considered to arise from an intramolecular oxidation-reduction followed by a trans-sulphonation reaction. t-Butyl-alcohol was identified in the refluxing liquid. The initial step in the d,ecornpo- sition is suggested to be the heterolysis of the sulphur-oxygen bond, and the isobutene is considered to be formed by the dehydration of t-butyl-alcohol rather than by a cyclic process involving fission of the carbon-oxygen bond as previously proposed.THE thermal decomposition of methyl Z-nitrobenzene sulphenate in aromatic solvents gives the appropriate aryl 2-nitrophenyl sulphides as the major isolable pr0ducts.l Evidence was adduced that these reactions and the related thermal rearrangements of the corre- sponding aryl sulphenate esters involved heterolytic fission of the sulphur-oxygen bond to give an ion-pair, or its precursor, e.g. ArS-OR =+= ArS+ -OR. The t-butyl esters of arylsulphenic acids are reported to give isobutene and sulphur dioxide on pyrolysis under nitrogen at 120-135", together with a nonvolatile tarry residue which could not be purified. No evidence could be obtained for a radical mechanism and the initial breakdown was suggested2 to occur via a cyclic transi- tion state. A concerted cyclic intramolecular process is also postulated for the isomerisation of allyl sulphenates to allyl sulphoxides.The results for the decomposition of the methyl and t-butyl esters indicated a possible change in inechan- ism as the methyl group in the ester is successively alkylated. In an attempt to obtain evidence for such a change we began by studying the thermal decom- position of t-butyl 2-nitrobenzenesulphenate under similar conditions to those previously used for the methyl analogue. D. R. Hogg, J. H. Smith, and P. W. Vipond, J . Chem. Soc. R. S. Irwin and N. Kharasch, J . Amer. Chem. SOC., 1960, (C), 1968, 2713. 82, 2502. The products from the thermal decomposition of t-butyl 2-nitrobenzenesulphenate in boiling anisole under nitrogen in the dark were found to vary slightly according to the experimental conditions.When a solution of the ester in anisole was slowly heated to boiling and nitrogen was passed through the solution, isobutene was evolved, and the residue contained di-2- nitrophenyl disulphide, aniline, P-methoxybenzene- sulphonic acid, and a mixture of 2- and 4-methoxy- phenyl 2-nitrophenyl sulphides in the ratio of ca. 1 : 4. Addition of the ester to boiling anisole led to a violent reaction and a greatly increased yield of 9-methoxy- benzenesulphonic acid. The nonvolatile products from this reaction also included %methyl-l-(2-nitrophenyl- thio)propene. A similar reaction occurred when 9- xylene was used as the solvent.In all of these reactions considerable charring occurred. These results agree with, but considerably extend, the previous observations on this reaction. The formation of the aryl 2-nitrophenyl sulphides, which are similarly formed in the thermal decomposition of the methyl and of the aryl esters, strongly suggests that the decomposition of the t-butyl ester also proceeds through a sulphenium ion, or its precursor, and that these products are formed by an electrophilic substi- tution reaction involving this intermediate. Such an intermediate could arise from an initial heterolysis of the sulphur-oxygen bond as previously proposed for the methyl ester or, alternatively, following the mechan- S. Braverman and Y . Stabinsky, Chew. Comm., 1967, 270; P.Bickart, F. W. Carson, J. Jacobus, E. G. Miller, and K. Mislow, J . Anzev. Chem. Soc., 1968, 90, 4869.Org. 61 ism proposed by Kharasch,2 from the initially formed sulphenic acid. e.g. ArSOCMe, -+ Me2C=CH, + Ar-S /* + iH+ 11 No ArS+ + H20 T- ArSOH,=+= Ar-S-OH The identification of t-butyl alcohol in the distillate, when the reaction mixture was not allowed to reflux, eliminates this reaction sequence and provides strong evidence that the initial bond fission occurs pre- dominantly between the sulphur and the oxygen atoms. The following scheme is therefore proposed for the re- action in anisole : t3s-oBut Q-s Me0 is reported to be 90% desulphonated after being heated a t 211" in aqueous acid for 4 hr., no reaction was ob- served in refluxing anisole in the presence of a trace of water.The considerable increase in the yield of sulphonic acid from the thermal decomposition when the reaction became more violent, leads us to assume that the model experiment was not sufficiently com- parable with the actual experiment. By analogy with the sulphonation of toluene,s the para-isomer should be the predominant product from the sulphonation of anisole. The apparent absence of other isomers in the decomposition products may be due to experimental difficulties in separating small quantities of isomeric sulphonic acids. It has been previously reported that, when alkyl substituted benzenesulphonic acids are \ Oxygen transfer from an o-nitro-group to sulphur , as postulated in reaction (l), or the alternative (lb), is comparatively well Zincke has shown that o-aminobenzenesulphonic acid is formed when %nitro- benzenesulphenyl chloride is heated in aqueous methanol, and it has recently been showng that in the base- catalysed rearrangement of 2-nitrobenzenesulphenanilide to azobenzene-2-sulphinic acid, the oxygens of the sulphinic acid group are derived from the nitro-group As desulphonation is accelerated by electron-releasing substituents, particularly when they are in the ortlzo- position ,' o-aminobenzenesulphonic acid should de- sulphonate comparatively readily.The formation of aniline and @-methoxybenzenesulphonic acid could therefore occur by a trans-sulphonation reaction involv- ing anisole and o-aminobenzenesulphonic acid [reactions (2) and (3)]. Although o-aminobenzenesulphonic acid NH, \ + H t Me2C:CH*S*C,H,*N0, Me0.C6HiS03H + Ht heated at 175-180" in toluene or xylene, trans-sulphon- ation occurs.The postulation of a similar reaction in this study at a slightly lower temperature is considered to be reasonable in view of the expected increased reactivity of both the acid arid the solvent. 4-Methoxy- phenyl 2-nitrophenyl sulphide was not appreciably affected by a prolonged period in refluxing anisole and, consequently, 4-methoxybenzenesulphonic acid is not formed by the thermal decomposition of this product. The isobutene formed in the decomposition is con- sidered to arise from the dehydration of t-butyl alcohol in the acidic medium [reactions (4) and (5)]. Isobutene is known 10 to undergo substitution reactions with electro- philic reagents, especially in solvents of low polarity, and the formation of 2-methyl-l-(2-nitrophenylthio)- propene in the reaction under more vigorous conditions is consequently not unexpected [reaction (6)j.In methylene chloride solution a t room temperature iso- butene forms a normal adduct with 2,kdinitrobenzene- sulphenyl chloride and the proposed structure for this adduct is in accord with previous work.ll 4 J. D. Loudon and G. Tennant, Quart. Rev., 1964, 18, 389. 7 E. E. Gilbert, ' Sulfonation and Related Reactions', Inter- 8 L. Vollbracht, H. Certfontain, and F. L. J. Sixma, Rec. Q 33. Drews, S. Meyerson, and E. K. Fields, J . Amer. Chem. Soc., 1961, 83, 3871. 10 M. L. Poutsma, J . Amer. Chem. Soc., 1965, 87, 4285; I. V. Bodrikov, 2. S. Smolyan, and G. A. Korchagina, Zhur.obshchei Khim., 1965, 35, 933. 11 N. Kharasch and C . M. Buess, J . Amer. Cheun. Soc., 1949, 71, 2724. T. Zincke and F. Farr, Annalen, 1912, 391, 57. C. Brown, Chenz. Comm., 1969, 100. science, New York, 1965, p. 429. Trav. chim., 1961, 80, 11. C62 J. Chem. SOC. (C), 1970 Di-2-nitrophenyl disulphide could be formed in this reaction by disproportionation 12 of 2-nitrobenzene- sulphenic acid which could arise from the hydrolysis of the sulphenium ion. Neither the other product in this disproportionation, 2-nitrophenyl 2-nitrobenzenethiol- sulphonate, nor any compound obviously derived from it, was, however, isolated from the reaction mixture. t-Butyl 2-nitrobenzenesulphenate decomposes much more rapidly than the methyl ester.l The lower rate of alkaline hydrolysis of the t-butyl ester compared with the methyl ester has been attributed l3 to the importance of steric effects in this bimolecular process.The re- lative rates of thermal decomposition suggest that bond fission must be virtually complete before bond formation commences at the sulphur and are consistent with the proposed mechanism. No evidence has been obtained for a change in the mechanism of the thermal decom- position of alkyl 2-nitrobenzenesulphenates with in- creasing a-substitution in the alkyl-group. EXPERIMENTAL Unless otherwise stated silica gel was used as the station- ary phase in chromatography and light petroleum refers to the fraction of b.p. 60-80". U.V. spectra were measured in ethanol solution and lH n.m.r. spectra in deuteriochloro- form .t-Butyl 2-nitrobenzenesulphenate,14 m.p. 53.5--54.5", and o-aminobenzenesulphonic acid 16 were prepared as previously described ; the former was purified by chromato- graphy with benzene-light petroleum (1 : 1) as eluant. 4-Methoxyphenyl 2-nitrophenyl sulphide, m.p. 92-94" (lit.,16 m.p. 95"), was obtained in 50% yield by treating 2-nitrobenzenesulphenyl chloride with anisole in the presence of aluminium ch1oride.l' No 2-methoxyphenyl 2-nitrophenyl sulphide was detected on examination of the reaction mixture by t .l.c. 2-Chloro-2-methylpropyl 2,4- dinitrophenyl sulphide, m.p. 83-86' (1it.,l1 m.p. 86-87O) was obtained by bubbling isobutene through a solution of 2,4-dinitrobenzenesulphenyl chloride in methylene chloride at room temperature for 2 hr. The solution was set aside for 24 hr., the solvent was removed under reduced pressure, and the product was crystallised from ethanol.Thermal Decomposition of t-Butyl 2-Nitrobenzenesulphen- ate. (a) A solution of t-butyl 2-nitrobenzenesulphenate (3.30 g.) in anisole (50 ml.) was slowly heated to boiling and then under reflux for 2 hr., when examination of the solution by t.1.c. indicated that all the ester had reacted. Light was excluded throughout the reaction and dry nitrogen was passed through the solution. The volatile products were collected in a trap cooled to -80". The small quantity of colourless, volatile product was treated with a solution of 2,4-dinitrobenzenesulphenyl chloride in methylene chloride together with a few drops of pyridine. After 24 hr. the solvent was evaporated off under reduced pressure and the residue was chromato- graphed with benzene-light petroleum (1 : 1) as eluant to give 2-chloro-2-methylpropyl 2,4-dinitrophenyl sulphide (0-20 g., 5.6%), i.r.spectrum identical with that of an authentic specimen. No t-butyl2,4-dinitrobenzenesulphen- ate could be detected in the residue. The black anisole solution was diluted with benzene and extracted with dilute hydrochloric acid. The acid ex- tracts were neutralised and extracted with ether. Re- moval of the solvent from the dried ethereal extracts yielded a pink oil which was treated with excess of Z-nitrobenzene- sulphenyl chloride (0.10 g.) in methylene chloride. Prepar- ative t.l.c., with benzene as irrigant, gave 2-nitrobenzene- sulphenanilide (0.03 g., 0.8%), m.p.93-95' (lit.,5 m.p. 94"), i.r. spectrum identical with that of an authentic specimen. The neutralised aqueous extracts were acidi- fied and evaporated to dryness to give a non-homogeneous red solid, the i.r. spectrum of which indicated the presence of a sulphonic acid. Examination by t.1.c. with ethanol as irrigant indicated the presence of several products, one of which had the same RF value as p-methoxybenzene- sulphonic acid. The dried benzene-anisole solution was chromatographed as previously described to give 2-methoxyphenyl 2-nitro- phenyl sulphide (0.16 g., 4.2%), 4-methoxyphenyl Z-nitro- phenyl sulphide (0.70 g., 18.5%) and di-2-nitrophenyl di- sulphide (0.26 g., 15%). A black residue remained a t the top of the column. (b) t-Butyl 2-nitrobenzenesulphenate (5.0 g.) was added to boiling anisole as rapidly as the vigorous reaction would allow and the dark solution was heated under reflux for 1 hr.The reaction mixture was chromatographed to give 2- methyl-l-( 2-nitrophenyZthio)propene (0.08 g., 6.7y0), an orange oil purified by low-temperature crystallisation, A,,, 248, 278sh, and 378 nm. (log E 4.2, 3.8, and 3.6); v,,,,. 1619, 1592, 1568, and 742 cm.-l; lH n.m.r. bands at T 8*01-8.05 (3H, m), 4.05 (lH, m), 1-70-3-04 (4H, m) (Found: C , 57-6; H, 5.4; N, 6.5; C,,H,,NO,S requires C, 57.4; H, 5.3; N, 6.7y0), identical with the compound obtained by treating the yellow oil from the reaction of isobutene and 2-nitrobenzenesulphenyl chloride with potassium t-butoxide in t-butyl alcohol at room temperature for 16 hr.2- and 4-Methoxyphenyl Z-nitrophenyl sulphides and di-2-nitrophenyl disulphide were also obtained by elution with benzene-light petroleum. Elution with methanol gave, after removal of the solvent, a black solid which was dissolved in methylene chloride and extracted with water. The red aqueous extracts were evaporated to give a red oil from which p-methoxybenzenesulphonic acid (0.21 g., 5.10/,), colourless crysyals, vmL 3400, 1190, 830 cm.-l; lH n.m.r. bands a t T 6.15 (3H, s), 2.11-3*03 (4H, q, J 9 Hz), was obtained by careful addition of ethanol. This product was obtained in 11.3% yield from a similar reaction by extracting the reaction mixture with water. (G) t-Butyl 2-nitrobenzenesulphenate (3- 10 g.) dissolved in anisole (15 ml.) was slowly heated to the boiling point in the dark and the pale yellow distillate was collected in two fractions (9 ml.and 4 ml.). These fractions and the reaction mixture were examined by g.1.c. on a column con- taining ethylene glycol succinate (15% by weight) on g-cel. By comparison with standard solutions the first fraction was shown to contain t-butyl alcohol (>0.05 g., l2 E. Vinkler and F. KlivCnyi, Acta Chim. Acad. SC~. Hung., 15 E. Wertheim, Org. Synth., Coll. Vol. 11, 1943, 471. 1957, 11, 15. 16 R. Passerini, Boll. sci. Fac. Chim. ind. Bologna, 1950, 8, l7 C. M. Buess and N. Kharasch, J . Amer. Chem. SOC., 1950, l3 C. Brown and D. R. Hogg, Chem. Comrn., 1967, 38. 122. 72, 3529. l4 C. Brown, D. R. Hogg, and D. C. McKean, submitted for publication in Spectrochim. A cta.Org.5%) together with a very volatile component, presumably isobutene. Water was also present. No t-butyl alcohol was detected in the reaction mixture, and only a trace in the second fraction. A similar decomposition of ester (3.5 g.) gave a colourless distillate (8 ml.). Treatment of the first fraction with 2-nitrobenzenesulphenyl chloride (0.5 9.) and a few drops of pyridine gave an oil which was purified by chromato- graphy to give t-butyl 2-nitrobenzenesulphenate (0- 1 g., 3%), m.p. 51-53', i.r. spectrum identical with that of an authentic specimen. Very little decomposition occurred when the ester was heated under reflux for 1 week in ethylene dichloride solution in the presence of cyclohexene. ( d ) Similar products were obtained when the ester was decomposed in p-xylene. Attempted Thermal Decomposition of 4-Methoxyphenyl 2-Nitrophenyl SuZphide.--4-Methoxyphenyl 2-nitrophenyl sulphide (0.70 g.), dissolved in boiling anisole, was heated under reflux for 48 hr. in the dark. The solution darkened slightly but no 9-methoxybenzenesulphonic acid could be detected. The starting material (0.59 g., Say0), m.p. 88- 91 O, was recovered by chromatography followed by crystallisation. Attempted trans-Sulphonation Reaction with o-Amino- benxenesulphonic Acid. A stream of nitrogen was passed through a solution of o-aminobenzenesulphonic acid (1.00 g.) in refluxing anisole (20 ml.) for 16 hr. Extraction with water gave material (0.90 g.) identical (i.r.) with the starting material. No p-methoxybenzenesulphonic acid could be detected by t.1.c. Similar results were obtained when water was added to the initial solution. One of us (P. W. V.) thanks the S.R.C. for the award of a maintenance grant. [9/1003 Received, June 13th, 19691