Photochemistry of sulphur compounds related to the 1,2-dithiole system. Part IV. Photoisomerization of alpha;-7-(5-aryl or -alkyl-1,2-dithiol-3-ylidene)-4,5,6,7-tetrahydro-1,2-benzodithiol-3-ylidene ketones

摘要

2722 J.C.S. Perkin IPhotochemistry of Sulphur Compounds Related to the I ,2-DithioleSystem. Part IV. t Photoisomerization of a-7-(5-Aryl or -Alkyl-1,2-dithiol-3-yl idene) -4,5,6,7-tetrahydro-l,2- benzodit hiol-3-ylidene KetonesBy Carl Th. Pedersen and Christian Lohse," Department of Chemistry, Odense University, DK-5000 Odense,DenmarkMadeleine Stavaux, DBpartment de chimie, UniversitB de Caen, F-14032 Caen Cedex, FranceThe photochemistry of a series of higher poiysulphur analogues of 1.2-dithiol-3-ylidene ketones has been studied.The normal cis form is converted into the trans on irradiation of an ethanolic solution containing catalytic amountsof hydrogen chloride. The photoproduct reverts to starting material by a dark process, which obeys first-orderkinetics. The rate constants are only slightly affected by variation of substituents.IT has recently been shown that u-( 1 ,Z-dithiol-3-ylidene)ketones and aldehydes (I) upon irradiation are trans-formed into the trans-isomers (11), which revert tothe starting compound via a thermal proce~s.l-~The 1 ,Z-dithiol-3-ylidene ketone system can be ex-tended to include one more 1,2-dithiole nucleusin compounds of type (111).(The cyclohexane ring hast Refs. 1 and 2 are considered to be Parts I1 and 111, respec-C. Th. Pedersen and C. Lohse, J . C . S . Perkin I , 1973, 2837.C. Th. Pedersen and C. Lohse, J.C.S. Chem. Comm., 1973,tively, of this series.123.hitherto been an absolute prerequisite for the synthesisof this type of compound.)The geometry of the ketonic portion of compounds(111) has been shown to be closely related to that ofthe 1,2-dithiol-3-ylidene ketones (I).The geometryfor compound (IIIb) determined by X-ray crystallo-graphy is as shown.6Schmidt, and H. Behringer, Helv. Chem. Acta, 1973, 56, 597.Pinel, Tetrahedron Letters, 1973, 1257.3 G. Calzaferri, R. Gleiter, K.-H. Knauer, E. Rommel, E.4 R. Gleiter, K.-H. Knauer, E. Schmidt, Y. Mollier, and R.M. Stavaux, Bull. SOC. chim. France, 1971, 4429.6 J. Sletten and M. Velsvik, Acta Chem. Scand., 1973,27, 38811974 2723In the 1 ,2-dithiol-3-ylidene ketone series, S-0 distancesin the range 2-184-2.443 A and S(1')-S(2') distancess-s 05.*,L' 2hv eAin the range 2.101-2.137 A have been observed, tie-pending on the substitution ( c j .ref. 6). The sametype of cis-trans-photoisomerization as is observed fors-s s-s 02.061 2,856 2.110 2.327 s - - - 0mrl tmbl Distances in ,the 1 ,2-dithiol-3-ylidene ketones should therefore bepossible for compounds of type (111), giving rise to iso-mers such as (IV). However, isomerization around theother double bond or around both double bonds isalso possible and will lead to isomers such as (V) and(VI), respectively.s-s s-s .lWHa R 2R' w1RESULTS AND DISCUSSIONGleiter et aZ. originally claimed that the photoproductobtained from 1,2-dithio1-3-ylidene ketones was anO-S-bonded valence tautomer.' The formation ofsuch isomers from the irradiation of (111) can be excludedon the basis of the following observation.When acompound of type (111) was irradiated in a polymetha-crylate matrix it was converted into a photoproduct withan absorption spectrum slightly different from thatof the starting material (Figure). The photoproductonly reverted to starting material on heating at 120 "Cfor 24 h. If O-S-bonded tautomers were formed onlysmall geometrical changes in the molecule would benecessary during the reversion, whereas the changefrom trans to cis requires a large modification in geo-metry, which is only possible in the matrix at hightemperature.Flash-photolysis of (111) in neutral lO*M-ethanolicsolution converted it into the same photoproduct,which reverted so slowly to the starting material thatit was not possible to study accurately the kinetics ofthe back reaction owing to the long term instabilityof the equipment used, However, Calzaferri et aZ.3observed that the reversion of the photoproductsfrom 1 ,2-dithiol-3-ylidene ketones was catalysed by acids.We have observed that if the photolysis of (111) wascarried out in a ~ O + M solution containing 10-4~-hydro-gen chloride, reasonable rate constants for the backQ, .- c. 30fx20lo 10 I I 7 -X /nm400 500 600 70 0Visible spectrum of (IIIa) in a polymethacrylate matrix: fullline before irradiation, dotted line after irradiationreaction were obtained at 25 "C, and the process wasfound to obey first-order kinetics. The rate constantsfor various substituted compounds are given in theTable.Rate constants for thermal back reaction of photoproductsfrom (111) at 25 "C obtained from 1 O d 6 ~ solutions inabsolute ethanol containing 10-4~-hydrogen chlorideCompound R1(IIIa) But(IIIb) But(IIIC) But(IIId) Ph(IIIe) Ph(IIIf) Ph(IIIh)4-MeOC6H,4-MeOC6H,(IIIg)R24-MeOC6H,PhRut4-MeOC,H4PhButPhButk1s-l0.650.900.430.260.280.191-830-88Both compounds of type (111) and dithiolylideneketones can be regarded as resonance hybrids of formssuch as (111) and (VIII) and (I) and (VII), respectively.YIII CYnI)' R.Gleiter, D. Werthemann, and H. Behringer, J . Amer.Chern. SOC., 1972, 94, 6512724 J.C.S. Perltin I, 1974It has been shown for the dithiolylidene ketonesby comparison of i.r. spectra with those of lSO enrichedcompounds that the contribution of the ketonic formsin some cases is as low as 27.8 As O-S-bondedstructures can be excluded by means of ESCA spectra 9,this means that it is the polar forms which dominate.These polar forms which lower the double-bondcharacter between C(2) and C(3rsquo;) in the dithiolylideneketones and that of the corresponding bond in com-pounds (111) will probably give rise to a shortening ofthe 0-S distance.The relatively long 0-S distancein (111) may thus indicate a strong contribution of theketonic form, which means that the bond to the dithiolenucleus has high double-bond character. Catalysisis therefore necessary in these compounds. Protonationof the oxygen will give rise to structures such as (IX)which lower the double-bond character, and thereforegive a faster back reaction.S-S S-S OH Rrsquo;L+yy+R2U(Dc)Rate measurements at several concentrations ofhydrogen chloride were performed.I t is to be notedthat the isomerization reactions are very stronglycatalysed by acid, and moreover, a plot of log kobsvs. -HH, is linear with a slope of 1-00. The same slopewas found for 1,2-dithio1-3-ylidene ketones. Thusa similar process seems to be operative in both isomeriz-ations. In the U.V. spectrum a shift of the carbonylabsorption towards longer wavelengths was observedon addition of acid.The results discussed hitherto are consistent withthe presence of isomer (IV) but do not exclude isomers(V) and (VI). The 90 MHz IH n.rn.r. spectrum of (IIIh)shows two singlets at 6 6-88 and 6.52 in carbon tetra-chloride, consistent with the previously reported valuesin carbon disulphide by S t a ~ a u x , ~ who assigned thesignal at 6 6-88 to Hb and that a t 6.52 to Ha.Thesesignals were not shifted on addition of 1 0 - 5 ~ trifluoro-acetic acid.When the photolysis experiments were carried outin 10-5~-trifluoroacetic acid in carbon tetrachloridethe photoproduct had a lifetime of ca. 20 min, whichwas long enough to obtain its n.m.r. spectrum. Owingto the low solubility of the product and its relativelyshort lifetime, lH Fourier transform spectra were foundmost suitable for the study.After irradiation of a 0.2 solution with a mercurylamp without a filter in a Pyrex n.m.r. tube for 60 s,the signal at 6 6.52 had disappeared and a new signalhad appeared at 6.25, whereas the signal at 6.88 was notshifted.Thus the signal corresponding to Ha wasshifted to higher field whereas that corresponding toHb was not shifted, which is consistent only withstructure (IV) for the photoproduct. If isomer (V)were formed, the Hb signal should be shifted and thatof H, not, and if (VI) were formed both signals shouldbe shifted. The shift observed is in accordance withthe observations of Calzaferri et al. of the analogousshift in the case of 1,2-dithiolylidene ketones.If the concentration of trifluoroacetic acid was in-creased to 10-lhI. the compounds were photostable andthe colour was shifted from red or red-violet to blue.At this concentration of acid the compounds are pro-bably entirely in the salt form and there is free rotationaround the former double bond, as is observed for( l,dit hiol-3-ylio) phenolat es .4As shown in the Table, the influence of changing thesubstituents on the rate constants is only slight, inaccord with the observations for the 1,2-dithiol-3-ylid-ene ketones2 It was found for these compounds thatthe rate constants for the thermal back reaction areonly weakly dependent on the substituents in positions1 and 5rsquo;, but strongly dependent on those in position4rsquo;.All the compounds studied here have the samesubstituent, viz. part of the cyclohexane ring, in theposition corresponding to the 4rsquo; position of the 1,2-di-thiolylidene ketones.EXPERIMENTALThe rate constants for the trans-cis-isomerization wereobtained by using the flash photolytic equipment de-scribed previously.rsquo; The kinetics were extremely sensitiveto the presence of water.The lH n.m.r. spectra were recorded on a Bruker WH 90Fourier transform n.m.r. spectrometer.Compounds (IIIa-h) were prepared as previously de-scribed.6We thank Dr. Klaus Bock, Department of Organic Chem-istry, The Technical University, Lyngby, for recording theFourier transform n.m.r. spectra and the Danish Re-search Council for use of the Fourier transform spectro-meter.411617 Received, 2nd AzLgust, 197418 D. Festal and Y. Mollier, Tetrahedron Letters, 1970, 1259.0 B. J. Lindberg, S. Hogberg, G. Malmsten, J.-E. Bergmark,0. Nilsson, S.-E. Karlsson, A. Fahlman, U. Gelius, R. Pinel, M.Stavaux, Y . Mollier, and N. Lozacrsquo;h, Chimica Scripta, 1971,1, 183.0 Copyright 1974 by The Chemical Societ