Chemistry of the Sz.dbd;O bond. Part I. Nuclear magnetic resonance and infrared studies on trimethylene sulphite

摘要

458 J.C.S. Perkin I1Chemistry of the S=O Bond. Part I. Nuclear Magnetic Resonance andInfrared Studies on Trimethylene SulphiteBy C. H. Green and D. G. Hellier,' Department of Chemistry, Westfield College, London NW3 7STThe conformation of trimethylene sulphite has been studied by l H n.m.r. spectroscopy and by an i.r. investigationof the S=O bond stretching vibration in different solvents a t two concentrations. From analysis of the n.m.r.spectrum, cross-ring couplings are evident, although not previously rep0rted.t The i.r. studies are more comprehen-sive than those in previous reports and the interpretation of our data leads to different conclusions from those ofothers workers.THE conformation of the cyclic ester, trimethylenesulphite, has been studied by a variety of physical tech-n i q u e ~ .~ - ~ For example, Altona et aL4 obtained X-raydata on crystalline material at -100 "C and concludedthat the molecule is rigid and exists in a chair form withthe exocyclic oxygen atom in an axial position (I).Investigation of the conformation of trimethylene sul-t Note added in proof: Since submission of this paper, ananalysis of the 60 MHz 1H n.m.r. spectrum of neat trimethylenesulphite has been published (P. Albriktsen, Acta Chem. Scand.,1971, 25, 478). The results are similar to the data we obtainedfrom the analysis of the 100 MHz 1H n.m.r. spectrum of acarbon tetrachloride solution of trimethylene sulphite (1 0B. A. Arbuzov, Bull. SOG. chim. France, 1960, 27, 1311.D. G. Hellier, J. G. Tillett, H.F. Van Woerden, and R. F. ill.H. F. Van Woerden, Ph.D. Thesis, Leiden, 1964.wlv) *White, Chem. and I n d . , 1963, 1956.phite as a neat liquid or in solution has led to conflictingconclusions. Initially, Arbuzov suggested, from theresults of dipole moment measurements, that themolecule existed as an equilibrium mixture of the twoforms (I) and (11), with comparable amounts of both.But Van W~erden,~y~.* in a more detailed study of dipolemoments and i.r. spectra of trimethylene (and sub-stituted trimethylene) sulphites, concluded that theC. Altona, H. J . Geise, and C. Romers, Rec. Tvau. chim.,1966, 85, 1197.5 D. G. Hellier, Ph.D. Thesis, London, 1966.G. Wood and M. Miskow, Tetrahedron Letters, 1966, 4433.H. F. Van Woerden and E.Havinga, Rec. Trav. chinz., 1967,8 H. F. Van Woerden and E. Havinga, Rec. Trav. chin?., 1967,G. Eccleston, P. C. Hamblin, R. -4. Pethrick, R. P. amp;I.86, 342.86, 353.White, and E. Wyn-Jones, Trans. Furaday Soc., 1970, 66, 3101972 459molecule exists entirely in solution as the axial chair form(I). Recently, Wood and Miskow 6 adduced evidencefor flexible forms in solution by reinterpretation of the0IIdipole moment data together with some n.m.r. measure-ments. Wyn- Jones and his co-workers 9910 have usedtthe ultrasonic technique to study the conformation oftrimethylene sulphite (and other ring sulphites) . Theyconclude that the rigid chair form (I) is present in 90proportion but in equilibrium with various twist forms.Unfortunately, these authors did not realise that cyclicsulphites are highly associated in solution. Evidencefor association in cyclic sulphites has recently beenpublished by Wood and Miskow,ll and our own observa-tions confirm that this is an important characteristic ofthe solution chemistry of sulphites.Despite thisassociation the overall results of Wyn-Jones et al. are(probably fortuitously) not in great error. Since theultrasonic relaxation observed for trimethylene sulphiteoccurs by a single relaxation mechanism both for achloroform solution and for the neat liquid it seems mostprobable that their attribution of the relaxation to aconformational origin remains valid, as the large associ-ation now known to exist is both solvent- and concentra-tion-dependent and any ultrasonic relaxation due tomolecular association presumably lies outside the experi-mental frequency range (25-105 MHz) used in theirwork.Anteunis l2 has introduced the term anancomeric todescribe structures having virtually a single conformationbecause of an extreme position of the conformationalequilibrium. This is the type of equilibrium suggestedfor trimethylene sulphite from results obtained by ultra-sonic and other techniques. We have analysed thelH n.ni.r.spectra of trimethylene sulphite to provideadditional evidence for the anancomeric conformation insolution.P. C. Hamblin, R. A. Pethrick, R. F. M. White, and E.G. Wood and M. Miskow, Tetvahedron Letters, 1970, 1775.Wyn-Jones, J . Mol.Structure, 1968, 1, 333.The 100 MHz proton spectrum of trimethylene sulphite(Figure 1) consists of four multiplets, (a), (b), (c), and (d),centred at 6 ca. 4.92, 3.79, 2-54, and 1.61 p.p.m. Fromthe magnitude of the couplings these may be assigned toFIGURE 1 100 MHz 1H N.m.r. spectrum of trimethylenesulphite lo in in CC1, (wlv); (a) X X spectrum, (b) AArsquo;spectrum, (c) (R __t Y ) spectrum, (d) M spectrumequivalent axial protons at C-4 and C-6, equivalentequatorial protons at C-4 and C-6, the axial proton atC-5 and the equatorial proton at C-5. The signals maybe analysed as an AArsquo;XXrsquo;M(R+ Y) spin system(terminology of Diehl and Lustig 13) ; this is similar to12 AI. Anteunis, D. Tavernier, and F. Borremans, Bull. SOG.l 3 P. Diehl and E. Lustig, J .Chem. Phys., 1966, 44, 2974.chim. belges., 1966, 75, 396460 J.C.S. Perkin I1the AA'XX'MR spin system except that the R part of thespectrum multiplet (c) shows additional lines due tosecond-order splittings."4 ( X ' IA )1 ( R lThe values of chemical shifts and coupling constantsgiven in Table 1 were obtained as ' best-fit ' parametersTABLE 1Calculated chemical shifts and coupling constants oftrimethylene sulphite *G/p.p.m.Nucleus (from ILie,Si) JIHzR S(1) 2-541 -14.18 (1,2) -0.27 (3,4)M 6(2) 1.618 12.77 (1,3) -0.55 (3,5)12.77 (1,4) -11.88 (3,6)4.85 (1,5) -11.68 (4,5)4.85 (1,6) -0.55 (4,6)XX2.64 (2,3) -1.60 (5,6)A4.9282.64 (2,4)1.89 (2,5)1.89 (2,6)3.794A W)* Chemical shifts and coupling constants were obtainedusing a modified version of LAOCOON I11 with an r.m.s.error of 0.055 from assigning 145 (90) of the 160 calculatedlines with relative intensities 0.02.Assignment of the re-maining 15 lines was ambiguous because of the complexity ofthe AA' spectrum (b). Chemical shifts are accurate to amp;-O-Olp.p.m. Coupling constants are considered accurate to h 0 . 1Hz except the cross-ring couplings J3,4, J3,5 = J4,s and J5,amp;,which are only accurate to f0.2 Hz.using the iterative program LAOCOON III.14 Althoughnot shown, there is good agreement between the calcu-lated and experimental spectra. The calculated valuesof the coupling constants are similar to those found forother carbocyclic system~.~J2J5~ The long-range Jcoupling between the equatorial protons on C-4 and C-6is the largest cross-ring coupling because these protonslie in a planar M arrangement.17The differences between the chemical shifts of protonsin cyclic sulphites have been interpreted as being largelydue to the electric field effect and magnetic anisotropyof the S=O b0nd.~*l8 The shielding region of the S=Obond is shown in Figure 2; protons within the positivecone are shielded and those in the negative area are de-shielded.Assuming an axial chair form, it can be seen that both* In ref.13 Diehl and Lustig showed that the high and lowfield peaks of the central triplets are split by a factor p II JRx2/(vR - vX) Hz providing that vR - vx is much greater than othercoupling constants. The calculated value of p , 0.68 Hz, usingthe values given in Table 1, compare favourably with the experi-mentally observed value of 0.70 Hz.l4 S.Castellano and A. A. Bothner-By, J . Chem. Phys., 1964,41, 3863.l6 For references see J. E. Anderson, Quart. Rev., 1965, 19,434.axial and equatorial protons of C-4 and C-6 lie well withinthe deshielding region. In particular, the axial protonsof C-4 and C-6 lie in a syn-axial position with respect tothe exocyclic S=O bond and are subject to maximumdeshielding, confirming the assignment of these protonsto the lowest field multiplet (a). The assignment ofmultiplet (b) to the equatorial protons on C-4 and C-6 isalso in agreement with an axial chair f ~ r m . ~ . ~For trimethylene sulphite the axial proton on C-5 hasbeen shown to be less shielded than the equatorial proton.The C-5 methylene protons of (rigid) 1,3-dioxans havebeen given a similar assignment by Anteunis et aZ.12 eventhough the axial protons a t C-4 and C-6 absorb at higherfield than the equatorial protons, analogous to what isfound for cyclohexane.Anteunis attributes the ano-malous chemical shift difference of the C-5 methyleneprotons in 1,3-dioxans to a lone-pair interaction of thebsol; + / Ibsol;bsol; I ;*Ad + ' -FFIGURE 21,3-oxygen atoms with the equatorial proton on C-5.This type of interaction must be equally important in theorigin of the chemical shift differences between the C-5methylene protons for trimethylene sulphite (andderivatives). Anteunis has given a value of ca.0-8p.p.m. to the chemical shift difference between the C-5methylene protons of rigid 1,3-dioxan. The chemicalshift difference for C-5 methylene protons of trimethylenesulphite is ca. 0.9 p.p.m. (Table l), indicating that theeffect of the S=O bond on these chemical shifts is approxi-mately the same or small. The chemical shift differencebetween the axial and equatorial proton on C-4 and C-6must still be due principally to the electric field effectand magnetic anisotropy of the S=O bond,l*. l9 mainlybecause of their close proximity to this bond, particularlyfor the axial protons.In addition to the n.m.r. spectral analysis of tri-methylene sulphite we have studied the S=O stretchingvibration of this cyclic ester in different solvents and a ttwo concentrations in an attempt to obtain further in-113 For a review see H.Feltkamp and N. C. Franklin, Angew.Chem., 1965, 77, 804.17 J. Meinwald and A. Lewis, J . Amer. Chem. Soc., 1961, 83,2769; J. Meinwald and Y . Meinwald, ibid., 1963, $5, 2514.(a) B. A. Arbuzov and Yu. Yu. Samitov, Tetrahedron Letters,1963, 473; Yu. Yu. Samitov, Doklady Akad. Nauk S.S.S.R.,1965, 164, 347; J. G. Pritchard and P. C. Lauterbur, J . Amer.Chem. Soc., 1961, 83, 2105; (b) L. Cazaux and P. Maroni, Tetra-hedron Letters, 1969, 3667.19 Yu. Yu. Samitov and R. M. Aminova, Zhur. stvukt. Khim.,1964, 5, 5381972 461formation on conformation. The results are summarisedin Table 2.The presence of the band in the region of 1230 cm-l inthe i.r. spectrum of trimethylene sulphite and some5-substituted trimethylene sulphites 3 9 5 9 ' s 8 has beenassumed to indicate the presence of an equatorial S=Obond and/or ring conformation other than or in additionto a rigid chair conformation. Recently, investigationof 4,6-substituted trimethylene sulphites 18**20 estab-lished that for some isomers in such ring strained systemsthe higher i.r.stretching vibration of ca. 1230 cm-l wasassociated with an equatorial S=O bond and/or one inwhich the ring was so distorted that a true axial S=Obond was not possible.solvent shifts upon dilution differ though all shifts are' positive' (defined here as shifts to higher wave-numbers). ' Negative ' shifts upon dilution occur onlyfor polar solvents (CH,CN or CC1,-PhOH). Generally,for the solution of higher concentration, the greater thepolarity and solvation power of the solvent the lower thestretching frequency (in cm-l) of the main S=O band.This behaviour is similar to that found for sulphoxides.21(c) The v(S=O) band for the saturated vapour, and atlower pressures, gave a single sharp peak at 1212 cm-1with rotational P and Q branch maxima at 1203 and1217 cm-l, respectively.There was no observablevariation in these values with decrease of pressure.For trimethylene sulphite the observation of theSolventMeCNCC1, (+ 10 phenol)C6H12CCl,CS,C6H6PhNO,Neat (liquid)NH,*CH,,*NH,EtOH(vapour)S=O Frequencies (cm-1)Concentration A1194 12331194 12341198 12341190 12351190 12331189 12331181 12331183 123312121178 12321187 12331 11TABLE 2of trimethylene sulphite in various solventsConcentration B1187 1235 -7 + 2 21 14 71190 1233 -4 +I1200 1235 +2 + l 19 11 81196 1235 + 6 0 17 10 71192 1233 + 2 0 18 11 71194 1235 + 5 + 2 15 8 7i ii D1 D2 PA PB DAB28Concentration range A = 0*32-0-34~, D1 = i (B) - i (A).Concentration range B = 0-05-0-06~, D2 = ii (B) - ii (A).PB = Relative percentage of peak ii compared to PA = Relative percentage of peak ii compared to peak i, concentration A.peak i, concentration B.Although the change of relative intensities of the twoS=O bands from one solvent to another has been re-ported,6 there appear to have been no systematic solvent-solvent, solvent-concentration i.r.studies of trimethylenesulphite solutions. Also Wood and Miskow l1 have re-cently reported that several cyclic sulphites, in particulartrimethylene sulphite, are all highly associated in dilutesolutions, but previous i.r. studies of trimethylene sul-phite and substituted trimethylene sulphites have notconsidered or recognised the extensive molecular associ-ation now known to exist even at very low concentra-tions.From the i.r. data of the S=O stretching vibrations onecan note the following:(a) The position of the band at higher wavenumber(ca. 1234 cm-l) shows little or no variation withchange in solvent or solute concentration. However,the intensity of the band relative to that at thelower wavenumber (ca. 1195 cm-l) is dependent onboth solvent and solute concentration.The relativechange of intensity upon dilution is approximately thesame (ca. 30) for the five solvents used where sufficientlyaccurate data could be determined.(b) The position of the band at the lower wavenumbergives comparable values for carbon tetrachloride, carbondisulphide, and benzene at the higher concentration, but2O S. Sarel and V. Usieli Israel J . Chem., 1968, 6, 885; W.Wuchcrpfennig, Annalen, 1970, 787, 144.higher wavenumber band has been attributed to thepresence of species (11) in the equilibrium and/or flexibleforms species (111), e t ~ . ~Although intensity changes in such bands with changeof solvent and upon dilution have usually been inter-preted as changes in the position of the equilibrium, theamount of species (11)-(111) relative to species (I)deduced, on this assumption, from i.r.data for thehigher concentration, is too high to agree with the ultra-sonic relaxation results, which predict the presence of atleast 90 of species (I) in a yet more highly concentratedsolution and in the neat liquid. Another apparentinconsistency concerns the decreases in intensity of theband at 1234 cm-1 upon dilution. These decreases areapproximately the same (ca. 30) for solvents of areasonable range of polarity and solvation power.(IY 1The band at 1234 cm-l cannot be attributed to non-monomeric species of the type (IV) since the value of nvaries disproportionately from solvent to solvent if the21 D. Barnard, J.M. Fabian, and H. P. Koch, J . Chem. SOG.,1949, 2442; T. Gramstad, SPectrochinz. Acta, 1963, 19, 829; T.Cairns, G. Eglinton, and D. T. Gibson, ibid., 1964, 20, 31J.C.S. Perkin I1band at ca. 1200 cm-l band is assumed to be that of thesolvated monomer. Combining our intensity measure-ments with Wood and Miskowrsquo;s data, this would predictrelative values of n of 40.5 (cyclohexane), 14-6 (carbontetrachloride) , and 2.5 (acetonitrile) for 1 solutions.The work of Gillespie and Robinson has shown thatthe S=O bond is very sensitive to its environment.22 Bystudying a large series of acyclic sulphites and sulphuryland sulphate compounds they showed that the length ofan S=O bond and the bond angle of an SO, group maybe correlated with the S=O bond stretching vibration,and that this correlation can be extended to includemany other compounds including neutral molecules andions.They derived the relationships (1) and (2),where v(S=O) = i.r. stretching frequency in cm-l,r(S=O) = bond length in A, and N(S=O) = bond order.v(s=o) ~(s=o) 3.7 = 4.79 x 103 (1)N(S=O) = 15-65/r(S=0)7*4 + 0.7Although equations (1) and (2) were originally appliedto acyclic molecules they appear to hold for the S=Obond in trimethylene sulphite. From the equations thedata in Table 3 can be calculated. The results of anTABLE 3v(S=O) (cm-1) Bond length (A) Bond order1180 1.460 1.6521190 1.456 1.6691200 1-453 1.6851210 1.450 1.7011220 1-447 1.7181230 1443 1.734X-ray analysis of crystalline trimethylene sulphitegave an estimated bond length of 1.45 amp; *011 A, equiva-lent to a calculated bond order of 1.70 amp; 0.04.The calculations show that only very small changes inthe S=O bond environment with resultant change inr(S=O) are required to produce changes in stretchingfrequencies of 0-50 cm-l.The solvent behaviour of the band at the lower wave-number follows, on the whole, a predictable pattern.The greater the polarity and solvation power of thesolvent, the greater the induced polarity of the S=Obond, the smaller the bond order, and the lower therelated stretching frequency.Thus for solvents of highpolarity, stretching frequencies of 1178 and 1181 cm-lare observed, but vibrations at higher wavenumber areobserved for solvents of lower polarity.For the vapourphase a highest value of 1212 cm-l is noted. This pre-sumably reflects the bond order of the molecule freefrom any solvent effects.Dilution in solvents of low polarity effectively decreasesthe dielectric constant of the medium (and molecularassociation) ; lsquo; positive rsquo; shifts are therefore observed.Dilution with polar solvents produces more effective22 R. J. Gillespie and E. A. Robinson, Canad. J . Chem., 1963,23 H. Fukushima, T. Pvliyazawa, T. Shimanouchi, and H.41, 3074.Takahashi, J . Mol. Spectroscopy, 1964, 13, 43.solvation and lsquo; negative rsquo; (lower wavenumber) shifts areobserved for these.In contrast, the band a t 1234 cm-l shows little changein frequency with changes in either solvent or soluteconcentration and is missing in the i.r.spectrum of thevapour. Thus, the behaviour of the band at 1234 cm-1for trimethylene sulphite does not appear to be entirelyconsistent with that expected for the S=O bond stretch-ing frequency for species (11) or (111). Since this bandoccurs in conjunction with that a t lower frequency fortrimethylene sulphite and in those isomers of the simple5-substituted trimethylene sulphites that have beensuspected of possessing flexible rings,3$ 7~186920 anotherpossible interpretation is that the flexibility is not sogreat as to cause ring inversion to any great extent, butsufficient to allow some ring distortion either by associ-ation with a polar solvent and/or by molecular associ-ation, which could give rise to additional -CH,- vibra-tions associated with the ring.Bands of varying in-tensity for other six-membered carbocyclic compoundssuch as cyclohexane 23 and 1,3-dioxan are also foundin the region 1220-1260 cm-l, probably due to -CH2-twist vibrations. We have found from an extensivestudy of the i.r. spectrum of five-membered ring cyclicsulphites, such as ethylene ~ulphite,,~ that these onlypossess one S=O band, at ca. 1190 cm-l, and the band at1230 cm-l is absent.Our conclusions from the i.r. studies are as follows.1. Ultrasonic relaxation techniques indicate that somerelaxation process occurs. This can be due to (a) theequilibrium suggested or (b) the molecular associationnow known to exist. Since the relaxation occurs via asingle (solvent-independent) mechanism , either process(a) or (b) is involved but not both. If the process is (a),then implicit in this statement is that the equilibriummust lie at least 90 in favour of the conformation withan axial S=O bond species (I).However, if the band at1234 cm-l were due only to an S=O stretching vibration,the i.r. spectrum of the neat liquid would suggest thatthis percentage must not be greater than 72.2. Frequency changes with change in solvent andsolute concentrations are observed for the band at ca.1196 but not for that at 1234 cm-l. However, theintensity of the latter is dependent upon solvent andsolute concentration though dilution effects for differentsolvents are approximately the same.3. Methylene vibrations for other carbocyclic corn-pounds are also observed in the 1230 cm-l region andthese vibrations are probably present in trimethylenesulphite (and other six-membered ring sulphites).It therefore seems likely that an exclusive and directassignment of the band at 1234 cm-l, for trimethylenesulphite and some of its substituted derivatives to anequatorial S=O bond or to an S=O bond in flexible con-formers is an oversimplification and that another factor,24 R.Mecke and F. Langenbucher, lsquo; Infra-red Spectra ofSelected Chemical Compounds,rsquo; Heyden, vol. l71I, London, 1970 ;H. Wunderlich, Z. analyt. Chein., 1968, 241, 234.25 Unpublished observations1972 463or combination of factors, is also probably involved. Asimilar i.r. investigation of a large number of 5-substi-tuted trimethylene sulphite derivatives confirms ourconclusions and we are also investigating the i.r.be-haviour of the S=O bond of some 4,6-substituted deriva-tives, where ring distortion and flexible conformationsare known to exist.Summary.-The n.m.r. and i.r. data confirm thegeneral conclusion that the conformation of trimethylenesulphite in solution is similar to that in the solid stateand that despite an earlier claim; it exists predomin-antly, if not exclusively, in a chair conformation with anaxial S=O bond.EXPERIMENTALThe lH n.m.r. spectra were run on a Varian HA100 instru-ment for 10 (wlv) trimethylene sulphite solutions incarbon tetrachloride. Peak positions were calibrated bydirect frequency count. At least three spectra were run(forward and reverse sweep) and the reproducibility of theline positions was f0.05 Hz.All i.r. measurements were obtained using the Perkin-Elmer 521 spectrophotometer. The trimethylene sulphitesamples and all solvents were purified by standard tech-niques to a spectroscopically pure level. For solutions oftrimethylene sulphite in six different solvents a t the twoconcentration 0-33 and 0 . 0 5 ~ (giving ca. 8 and ca. 1solutions) the i.r. spectra were recorded on a linearlyexpanded calibrated scale and are accurate to 4 1 cm-1.For the solvents ethanol, methylenediamine, and nitro-benzene, accurate measurements of absorption frequenciesin the required region could only be made for solutions atthe higher concentration, so the relative intensities of the1234 and the 1200 cm-l bands could not be measured withany certainty.One of us (C. H. G.) thanks to the S.R.C. for a researchstudentship.1/263 Received, 12th iWavck, 1971