166 J.C.S. Perkin IPhotoisomerisation of a-(Thiopyran-2-ylidene) Ketones ; SubstituentEffectsBy Carl Th. Pedersen and Christian Lohse,' Department of Chemistry, Odense University, DK-5000 Odense,Noel Lozac'h and Jean-Pierre SauvB, DBpartement de chimie de I'UniversitB de Caen, F-14032 CaenIn a series of ten substituted a- (thiopyran-2-ylidene) ketones, the stable Z-form i s transformed into the euro;-isomerupon irradiation. The photoproduct reverts to the starting material by a dark process, which obeys first-orderkinetics. The rate constants for the euro;Z-isomerization are controlled mainly by steric factors. The activation energyfor the thermal reversion is ca. 10 kcal mol-l.DenmarkCedex, France.COMPARISON of the carbonyl absorptions in the i.r.spectra of cc-thio?yranylidene ketones with those of180-enriched compounds shows that an interaction existsbetween sulphur and oxygen in these compounds.lY2This can be explained in terms of contributions by thecanonicd structures (1)-(3). Analogous resonanceforms have been assumed to describe the correspondinga-(l,2-dithiol-3-ylidene) ketones (4).394J.-P.Sauv6 and N. Lozac'h, Bull. SOC. claim. France, 1974,1196.R. Pinel, N. K. Son, and Y. Mollier, Compl. ./end., 1974,278. 729.N. Lozac'h, Adv. Heterocyclic Chcm., 1971, 13, 161.It has been shown ( c j . ref. 5 ) that a-(l,2-dithiol-3-ylidene) ketones and analogous multisulphur ketoness-s AVheatupon irradiation undergo geometrical isomerization.The thiopyranylidene ketones are structurally closelyrelated to these compounds (although no X-ray struc-ture determination of the former has yet been carried4 C.Th. Pedersen in ' Nonbenzenoid Aromatics,' vol. 3, ed.J , P. Snyder, Academic Press, in the press.C. Th. Pedersen and C . Lohse, J.C.S. Perkin I, 1973, 2837,and references therein.6 C. Th. Pedersen, C. Lohse, and M. Stavaux, J.C.S. Perkirt I,1974, 27221976 167out). We therefore expected to observe the same typeof isomerization as for the corresponding 1,2-dithiolederivatives, i.e. (6) (7).RL;13 k 2 k3 A 0' R'RESULTS AND DISCUSSIONIf an a-(thiopyran-2-ylidene) ketone is irradiated ina polymethacrylate matrix it is converted into a photo-product which is stable in the matrix and reverts to thestarting material only when heated at 110 "C for 24 h.The visible spectrum of the photoproduct is only slightlydifferent from that of the starting material (see Figure).0.60 4Visible spectra of (a) (IV) and (b) its photoproduct in apolymethacrylate matrixBy irradiation in ethanolic solution the same photo-product was formed, but the lifetime in this case variedfrom microseconds to minutes.These observations areconsistent with the assumption that the photoproduct isthe E-isomer of the thiopyranylidene ketone. Thethermal change from the E- t o the Z-form requires alarge modification in geometry, possible in the matrixonly at elevated temperature, whereas only small geo-metrical changes in the molecule would be necessary dur-ing the reversion if the photoproduct had an 0-S bondedstructure analogous to (2).The influence of substituents on the rate constantswas studied for the thermal reversal in 96 ethanol at25 "C.The results (Table 1) support the ZE-isomeriz-ation hypothesis.The compounds studied can be divided into threeclasses according to the magnitude of the rate constant forthe thermal back reaction: (a) compounds (1)-(IV), k1-6 x s-l, with no substituents in positions 2 and3'; (b) compounds (V)-(VW), k 30-110 sl, havinga cyano-group in the 2-position and a hydrogen atomin position 3'; and (c) compounds (IX) and (X), k 104-lo5 sL1, having a methyl group in the 3'-position.Electron-attracting groups in position 2 will lowerthe overlap population between C-2 and C-2' and thusaccelerate the thermal back reaction.This explains therise in rate in going from class (a) to class (6). Modelsindicate that the cyano-group introduces no stericstrain in the E-form.TABLE 1Rate constants for the dark reaction (7) --P (6)Compd. R1 RZ R3 R4 k * Et0Hjs-I k * Ch1s-l(I) Ph H H Ph 1.81 x($11) 4-MeC,H4 H H Ph 9.94 x(11) Ph H H 4-CIC6H, 6.53 x loe3(IV) pil H H H 9.52 x 10-3(VI) 4-C1C6H4 CN H Ph 110.0(V) Ph CN H Ph 74.0(VIII) Ph CN H 4-CIC6H, 32.2(VII) 4-MeC,H4 CN H Ph 43.0 1.62 x(IX) Ph H Me H 1.13 x lo4(X) Ph CN Me H 5 x 186.0a Estimated value.* 10-5-~f-So1utions at 25 "C; Ch = cyclohexane.Introduction of a substituent into the 3'-position givesrise to strong steric strain in the E-form. If such a sub-stituent is present, the carbonyl group and the phenylsubstituent in position 1 cannot be coplanar with thethiopyran part of the molecule in the E-form.Thisaccounts for the great increase in rate constant in goingfrom class (71) to class (c). If a cyano-group is furtherintroduced into position 2, its electron-attracting effectand the steric hindrance of the methyl group destabilizethe E-form to such an extent that it is not possible toobserve the decay in ethanol. In cyclohexane, where thestability of the E-form is higher (see later), it is possibleto observe the decay of the E- to the Z-form. From therate constant obtained in this solvent it can be estimatedthat the rate constant must be ca. 5 x lo5 in 96ethanol.9-Chloro-substitution in the phenyl group cf.com-pounds (11), (VI), and (VIII) results in only a smallchange (an increase) in the rate constant. The samewas observed in the case of 1,2-dithio1-3-ylidene ketone^,^whereas introduction of a $-methyl group gives rise to asmall decrease in both series.The observation that the rate constants are muchlower in cyclohexane is in accord with the postulate thatnon-polar solvents will favour true ketonic structuressuch as (1) which have a high degree of 2,2'-double-bondcharacter.A. 0' R(81 ( 9 ) 110)The thermal back reaction is strongly catalysed byacid. For example in ethanolic 10-3~-hydrogen chlor-ide, k p ) is 1580 s-l (cf. 9.52 x loe3 in 96 ethanol).This is probably due to protonation of the carbonylgroup, which gives rise to forms such as (8) where there islittle 2,2'-double-bond character168 J.C.S.Perkin ITwo different conformations are possible for the E-forms (9) and (10). For steric reasons we will assumethat (9) is preferred except where substituents arepresent in the 3'-position, in which case the carbonyl andthe phenyl group in position 1 are twisted out of plane.From the Arrhenius plots for a compound from group(a) and one from group (6) the energy of activation E,and the entropy of activation AS* have been calculated.The values (Table 2) fit well with those reported byTABLE 2s-sCNkEtoH 4.68 x 10-3s-1 kEt0R 5.79 s-1E a 1 1.6 kcal mol-I E a 12.3 kcal ~ n o l - ~AS* -32.3 cal mol-l I P AS* - 15.9 cal mol-I 1C-lGleiter and his co-workers for the corresponding dithiol-ylidene ketones.' The correlation strongly supports theG.Calzaferri, R. Gleiter, K.-H. Knauer, E. Rommel, E.Schmidt, and H. Behringer, Helv. Chiwc. A d a , 1973, 56, 697.hypothesis that the same mechanism is operative in thetwo cases. The highly negative AS* values furthersupport a polar transition state with a high degree oforientation of the solvent molecules. The high AS*values also rule out the possibility that the photoproductis an 0-S bonded isomer, since the reaction (3) + (1)will probably have only a small entropy of activation.The observed energies of activation are also in agree-ment with CND0/2 calculations of the energy differencebetween theE- and the Z-form of ct-( 1,2-dithiol-3-ylidene)-acetaldehyde (a value of 20 kcal mo1-l has been foundby inclusion of d orbitals on sulphur * a 9 ) .EXPERIMENTALThe compounds used were prepared as described in ref.1 : (a) compounds (1)-(IV) and (IX) by rearrangement,with loss of sulphur, of 2-(phenacylmethylthio) thiopyryl-iuni bromides; (b) compounds (V)-(VIII) and (X) bycondensing 2-cyanoacetophenone with a 2-(methylthio)-thiopyrylium iodide.The rate constants for the EZ-isoinerization were ob-tained by using the flash photolytic equipment describedprevi~uslv.~5/807 Rcceived, 29th April, 19751R. Piiicl, 1'. Rfollier, J.-P. de Barbeyrac, and G. Phster-J.-P. de Barbeyrac, D. Conbeau, and C;. Pfister-Guillouzo,Guillouzo, Cowzpt. rend., 1972, 275, 909.J . MoE. Structure, 1973, 16, 103
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