Studies of heterocyclic compounds. Part XVIII. Protonation of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes: formation of 1,2-oxathiolium and 1,2-oxaselenolium cations

摘要

1975Studies of Heterocyclic Compounds. Part XVIII.l Protonation of I ,6-Dioxa-Ga-thia- and 1,6-Dioxa-Ga-selena-pentalenes : Formation of I ,2-Oxathiolium and 1,2-Oxaselenolium CationsBy David H. Reid and Robert G. Webster, Department of Chemistry, The Purdie Building, The University,St. Andrews KYI 6 9ST, ScotlandA lH n.m.r. spectroscopic study of the protonation of 1.6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenesshows that 0- and C(3)-protonation occurs. Equilibrium mixtures are formed which contain the unprotonatedheterocycle and the 0- and the C(3)-protonated species in varying relative amounts, depending on the nature of theheterocycle and the acid medium. 1.6-Dioxa-6a-thia- and 1.6-dioxa-6a-selena-pentalene remained largelyunprotonated in trifluoroacetic acid, but the occurrence of H-D exchange a t C-3 and C-4 in trifluoroacetic 2Hacidrevealed the presence of the 3-formylmethyl-1.2-oxathiolium and -1.2-oxaselenolium cations.The majorspecies in a solution of 2.5-dimethyl-I ,6-dioxa-6a-thiapentalene in trifluoroacetic acid were the unprotonatedheterocycle and two isomeric 3-(2-hydroxypropenyl)-5-rnethyl-l,2-oxathiolium cations in a 4 : 6 : 1 ratio. 2.5-Dimethyl-I ,6-dioxa-6a-selenapentalene was largely unprotonated in trifluoroacetic acid. In trifluoroacetic acidcontaining 5 (v/v) perchloric acid 2,5-dimethyl-I ,6-dioxa-6a-thia- and 2,5-dirnethyl-l ,6-dioxa-6a-selena-pentalene gave the 3-acetonyl-5-methyl-1.2-oxathioliurn and -1.2-oxaselenolium cations, respectively, as theonly observable species. The 0- and C(3) -protonated 1,6-dioxa-6a-thia- and 1.6-dioxa-6a-selena-pentalenesare the first members of the 1.2-oxathiolium and the 1.2-oxaselenolium systems to be identified.The structureof the C(3)-protonated 1.6-dioxa-6a-thia- and 1.6-dioxa-6a-selena-pentalenes i s discussed in relation to themechanism of electrophilic substitution of 1,6-dioxa-6a-thiapentalenes and related hypervalent heterocycliccompounds.WE have recently described a convenient two-step syn-thesis of 1,6-dioxa-6a-thia- and 1,6-dioxa-6a-selena-pentalenes from 4-pyrones. We now report a study ofthe protonation of these compounds.Protonation of 1,6-Dioxa-6a-thiapentaZenes.-The lHn.m.r. spectrum of 1,6-dioxa-6a-thiapentalene (1) intrifluoroacetic acid showed two doublets ( J 2.7 Hz) at 87.02 and 8.66.These chemical shifts are similar inmagnitude to those of 3(4)-H (6 6.90) and 2(5)-H (6 8.64)(J 2.8 Hz) in 1,6-dioxa-6a-thiapentalene in CDCl,.lThe spectrum of 1,6-dioxa-6a-thiapentalene in trifluoro-acetic 2Hacid consisted simply of a singlet at 6 8.66. Ina preparative-scale experiment, a solution of compound(1) in trifluoroacetic 2Hacid was quenched with deuter-ium oxide containing an excess of sodium carbonateThis gave 3,4-dideut erio- 1,6-dioxa-6a-thiapent alene (2)quantitatively, the spectrum of which in CDCl, showeda singlet at 6 8.63 (2- and 5-H). These results indicatedthat the major species in trifluoroacetic acid is unpro-tonated 1,6-dioxa-6a-thiapentalene, but that a proton-ated species which we formulate as (5) is also present.This species is forrned rapidly and reversibly at concen-trations below the limit of detection by 1H n.m.r.spec-troscopy. The dideuterio-derivative (2) results from anH-D exchange involving C( 1rsquo;)-deuteriated derivativesof the intermediate (5).The lH n.m.r. spectrum of 2,5-dimethyl-l,6-dioxa-6a-thiapentalene (3) in trifluoroacetic acid revealed thepresence of unprotonated solute and two protonatedspecies in a 4 : 6 : 1 ratio Figure (a). The dimethyl-dioxathiapentalene (3) gives rise to two singlets at 6 2.43(2- and 5-Me) and 6.84 (3- and 4-H). (In CDCl, the cor-responding signals occur at 62.32 and 6.49.l) The spec-tral patterns of the protonation products indicate thatt Restricted rotation about the C-3,C-1lsquo; bond would double thenumber of possible isomeric 3-( 2-hydroxypropenyl)-l,2-oxa-thiolium cations.2 The assignments of structures (8a and b) to the major andminor protonated species, respectively, are tentative and mayrequire to be interchanged.they are the 3- (2-hydroxypropenyl) -1,2-oxat hioliumcations (8a and b)? resulting from O-protonation.Themajor species (8a) shows two methyl singlets at 6 1.71(2rsquo;-Me) and 2.69 (5-Me) and two one-proton singlets at64.43 (1lsquo;-euro;I) and 6.55 (4-H). The corresponding signalsof the minor species (8b) occur at 6 1.82, 2.75, 4.29, and6.64, respectively. f:In trifluoroacetic 2Hacid 2,5-dimethyl-l,6-dioxa 6a-thiapentalene underwent rapid H-D exchange at positions3 and 4, shown by the absence of all low-field signals fromits lH n.m.r.spectrum in this solvent. 3,4-Dideuterio-2,5-dimethyl-1,6-dioxa-6a-thiapentalene (4) was sub-sequently isolated; its spectrum in CDCl, consisted of asinglet at 6 2.31 (2- and 5-Me). These results showedthat a solution of compound (3) in trifluoroacetic acidcontains the C(3)-protonated species (6), in addition tothe unprotonated solute and the O-protonation products(8a and b). Unexpectedly, the cation (6) was the onlydirectly observable species in a solution of 2,5-dimethyl-1,6-dioxa-6a-t hiapentalene in trifluoroacetic acid con-taining 5(v/v) perchloric acid. The lH n.m.r. spec-trum of this solution Figure (b) showed two three-proton singlets at 6 2.68 (2rsquo;-Me) and 2.90 @-Me), a two-proton singlet at 6 5.08 C(lrsquo;)HJ, and a low-field singletat 6 7.64 (4-H).Spin-decoupling experiments showedthat the 2rsquo;-Me protons are weakly coupled to the methyl-ene protons. The signals of 5-Me and 4-H in the cation(6) occur much further downfield than the correspondingsignals of the cations (8a and b) (5-Me: A6 0.21 and 0.15;4-H : A6 1.09 and 1 .OO, respectively), because delocalisa-tion of the positive charge is confined to the ring in thecation (6) but extends into the 3-(2-hydroxypropenyl)substituent in the cations (8a and b). We conclude thatsolutions of the dioxathiapentalenes (1) and (3) intrifluoroacetic acid or in trifluoroacetic acid containingperchloric acid consist of equilibrium mixtures of theunprotonated bases (l) or (3), the O-protonated species1 Part XVII, D.H. Reid and R. G. Webster, (a) J.C.S.Chem. Comm., 972, 1283; (b) J.C.S. Perkin I , 1975, 7752098 J.C.S. Perkin I(7) or (S), and the C(3)-protonated species (5) or (6),whose relative amounts vary with the base and the nature0-s-06 6a 1Rrsquo; R2(1) H H(21 H D(31 Me H(4) Me DL 1lsquo;0 - S 2 OH+17a1 R-H(8a1 R-Me0- Se-06 60 1(91 R = H( 1 OIR: MeR W R0-s2 01 +(51 R=H(61 R=MeR ~ O H0-s R+17b) R = H(8b) R=Me0-Se2 o1 +111) R:H(12) R:Me0-2 0-s 0 x-Y-2+(13)Z-S (15) (1 6)(l41Z :Sex-Y AH 1 1 z Em x 1 1 Y-2+ +(171 (181of the acid medium. Interconversion of the protonatedspecies takes place through the unprotonated base,which acts as a lsquo; turntable rsquo;.1,6-Dioxa-Ga-thiapentalenes decompose upon pro-longed exposure to acids.The lH n.m.r. spectrum of 1,6-dioxa-6a-thiapentalene in trifluoroacetic acid showedadditional signals of progressively increasing intensityafter ca. 1 h. A solution in trifluoroacetic acid contain-ing 5 (v/v) perchloric acid showed numerous broadsignals immediately after preparation and rapidlybecame blue. 2 , 5-Dimet hyl- 1,6-dioxa-6a-t hiapent alene,although more stable to acid than compound (l), alsoshowed evidence of some decomposition in trifluoroaceticacid after several hours.Protonation of 1,6-Dioxa-6a-selenaj5entalelzes.-The lHn.m.r. spectrum of 1,6-dioxa-6a-selenapentalene (9) intrifluoroacetic acid showed two broad signals at 6 7.21(3- and 4-H) and 8.91 (2- and 5-H) (J ca.2.7 Hz) arisingfrom the unprotonated base. The spectrum of 2,5-di-methyl-l,6-dioxa-6a-selenapentalene (10) also showedexchange-broadened signals at 6 2.51 (2- and 5-Me) and6.92 (W, ca. 60 Hz) (3- and 4-H) arising from the un-protonated heterocycle. The signals from 3- and 4-Hin the spectra of compounds (9) and (10) in trifluoroaceticacid were absent from the spectra of these compoundsin trifluoroacetic 2Hacid, and in the case of com-pound (9) the signal from 2- and 5-H had collapsedto a singlet. The occurrence of H-D exchange atpositions 3 and 4 indicated that in trifluoroaceticacid compounds (9) and (10) are in equilibrium withsmall amounts of the C(3)-protonated species (11)and (12), respectively. In trifluoroacetic acid con-taining 5 (v/v) perchloric acid, compound (9) wasrapidly destroyed, but 2,5-dimethyl-l,6-dioxa-6a-selena-pentalene (10) underwent C(3)-protonation cleanly toI I I I 1 I7 6 5 4 3 2s1L8 7 6 5 1 3 2 s100 MHz 1H N.m.r.spectrum of 2,6-dimethyl-l,6-dioxa-6a-thiapentalene; (a) in trifluoroacetic acid, (b) in trifluoroaceticacid containing 5 (vlv) perchloric acidgive the cation (12) as the only observables species, thespectrum of which showed two methyl signals at 6 2.772rsquo;-Me, weakly coupled to C(1rsquo;)HJ and 2.81 (amp;Me), a tw1975proton singlet at 6 5.10 C(l')Hd, and a singlet at 6 7.81(4-H) . Although O-protonated dioxaselenapentaleneswere not detected in the acid solutions, it is likely thatthey too exist in equilibrium with the correspondingunprotonated bases and the C(3)-protonated species, inview of the behaviour of 2,5-dimethyl-1,6-dioxa-6a-thiapentalene (3) in trifluoroacetic acid.Disc.ussion.-The cations (6), (Sa and b), and (12) arethe first reported derivatives of the hitherto unknown1,2-oxathiolium (13) and 1,2-oxaselenolium (14) systems.We have previously suggested that the derivative (15)of the oxathiolium cation is an intermediate in the nitro-sation of 1,6-dioxa-6a-thiapentalene (1).The spectro-scopic identification of the cations (6) and (12) lendsexperimental support to this suggestion and to the moregeneral proposal that the electrophilic substitution ofhypervalent heterocyclic systems of the type (l6), whereX, Y, and 2 are heteroatoms of Groups V and VI and Y isa second- or lower-row element, proceeds by way of in-termediate 6x-electron monocyclic cations (17) and/or(18) -EXPERIMENTALM.p.s.were determined with a Kofler hot-stage apparatusFor 3H n.m.r. measurements tetramethylsilane was used asinternal reference. Mass spectra were obtained with ana R. M. Christie, A. S. Ingram, D. H. Reid, and R. G. Webster,J.C.S. Perkin I, 1974, 722.2099A.E.I. MS902 spectrometer. Petroleum was of boilingrange 40-50". Perchloric acid refers to 70-72 (w/w)perchloric acid.Preparation of Deuteriated 1,6-Dioxa-6a-thiapentalenes.-3,4-Dideuterio- 1,6-dioxa-6a-thiapentalene( 2). 1,6-Dioxa-Ga-thiapentalene (1) (256 mg, 2 mmol) was dissolved in tri-fluoroacetic ,Hacid (7.7 ml, 100 mmol ; isotopic purity99.8). After 3 min sodium carbonate (6 g, 56.6 mmol) indeuterium oxide (20 ml; isotopic purity 99.7) was addedand the resulting mixture was extracted with dichloro-methane ( x 3). The extracts were dried (Na,SO,) andevaporated, and the residue (255 mg, 98) was recrystallisedfrom petroleum to give needles, m.p. 61-63', 6 (CDCI,; 100MHz) 8.63 (2- and 5-H), Mf 130.0048 (Found: C, 46.2.C5H,D,0S requires C, 46.1 ).3,4-Dideuterio-2, 5-dimethyt- 1,6-dioxa-6a-thiapentaZene(4).The procedure was identical with that of the precedingexperiment, with 2, B-dimethyl- 1,6-dioxa-Ga-thiapentalene(3) (3 12 mg, 2 mmol) in place of 1,6-dioxa- 6a-thiapentalene .The residue from the extracts was chromatographedalumina(5 x 1.9 cm) with benzene. The eluates affordedspars (119 mg, 38y0), m.p. 7 0 - 7 1 O (from petroleum), 6(CDC1,; 100 MHz) 2.31 (2- and amp;Me), M+ 158.0380 (Found:C, 53.2. C,H,D,O,S requires C, 53.1 ).The isotopic purity of compounds (2) and (4) was 95.We thank the Carnegie Trust for the Universities ofScotland for financial assistance.6/910 Received, 13th May, 1976