J . Chem. Soc., Faraday Trans. I , 1982, 78, 3153-3161 Electronic Interactions in Triatomic Cyclic Compounds Charge-transfer Complexes between Epoxide Donors and ICl as Acceptor BY SERGIO SANTINI" AND SALVATORE SORRISO Dipartimento di Chimica, Laboratorio di Chimica Fisica, Universita di Perugia, Via Elce di Sotto 10, 06100 Perugia, Italy Received 8th October, 198 1 Charge-transfer (c.t.) complexes between epoxide donors and ICI as acceptor have been studied spectrophotometrically. It was found that the basicity in the 3-, 4-, 5- and 6-membered rings depends on the size of the ring, following in the order 4 > 5 > 6 > 3 atoms. This is principally caused by the different LCOC bond angles and the consequent different types of hybridization of the oxygen orbitals in the various compounds.K,, data show that in the styrene oxides a small conjugative effect is present between the phenyl and oxiran rings. However, this classic conjugation is completely absent in both cis- and trans-stilbene oxides. The problem of conjugation between triatomic cycles acd the adjacent p or n systems has been confronted both theoretically1 and experimentally.2aa-c The majority of these studies refer to cyclopropane and not to its heteroatomic homologues. We have undertaken additional work directed towards highlighting the effects of resonance and of classical and non-classical electronic transmission across the triatomic ring, using theoretical3 and e~perimental*~-f approaches. Other authors have also debated the conjugative properties of the oxiran ring, obtaining contradictory results.5a In the present paper we have extended the investigations to the charge-transfer complexes between some oxirans (donors) and ICl (an acceptor) with the aim of obtaining further information about the presence of electronic effects between the heteroatomic ring and one or two adjacent aryl systems.We have furthermore extended the study to 4-, 5- and 6-membered cyclic systems which contain oxygen as a heteroatom. EXPERIMENTAL MATERIALS The propylene oxide, trimethylene oxide, tetrahydrofuran and tetrahydropyran used were commercial products and were purified as indicated in the literature.6* ' Styrene oxide (a commercial product) was purified by distillation. The styrene oxide derivatives p-chloro-, p-nitro- and rn-nitro- were available from a previous study3d and generously given by the authors; p-methyl- and rn-methyl-styrene oxides were prepared as described in ref.(8); p-fluoro- and rn-fluoro-styrene oxides were prepared as described in ref. (9)- The unsubstituted trans- and cis-stilbene oxides were prepared according to the method outlined in the 1iterature;lO thep-nitro- andpp'-dichloro-derivatives of the trans- and cis-stilbene oxides were available from a previous and given by the authors ; thepp'-dinitro-derivatives of the trans- and cis-stilbene oxides were prepared and purified as described in ref. (1 1). The carbon tetrachloride was a Carlo Erba RP product and was dried over P,O, and distilled. Iodine monochloride was prepared by a standard method (m.p. 27.2 O C ) .1 2 31533154 MOLECULAR COMPLEXES OF EPOXIDE DONORS MEASUREMENTS The spectra were recorded by an Optica CF4-DR double-beam spectrophotometer. Equilib- rium measurements at fixed wavelengths were taken using a single-beam Unicam SP 500 spectropho t ometer . The stability constants (&) were determined spectrophotometrically at 20 OC, both in the region of the halogen absorption band (500-600 nm) and in that of the blue-shifted perturbed halogen band (300-450 nm). A second new band (the c.t. band) appears on complexation at shorter wavelengths (< 300 nm); but this spectral region was not used for analysis owing to its overlapping with the donor absorption band. The stoichiometry of the complexes was assumed to be 1 : 1 as described later. Two different methods were used in determining Krt: (1) from the decrease in the free halogen absorption measured in the 500-600 nm region; (2) from the Benesi-Hildebrand procedure13 in the region of the perturbed halogen band (300-450 nm).The K,, values reported are averages of 4 or 5 values obtained in different experiments and have a maximum deviation of 4-5%. RESULTS AND DISCUSSION STOIC H I 0 MET RY AND S T R U C T URE 0 F CHAR G E-TR ANSFER COMPLEXES BETWEEN ETHERS, EPOXIDES AND THIOETHERS AND OACCEPTORS (I, AND Icl) o acceptors, such as iodine and iodine monochloride molecules, form c.t. complexes having a 1 : 1 stoichiometryl* when they react with oxygen-containing organic compounds, particularly with alcohols, ethers, cyclic ethers and heteroaromatic oxygen compounds.The iodine monochloride molecule forms stronger c. t. complexes than does the iodine molecule because of the greater basicity of the iodine atom in IC1 compared with the corresponding I, molecule, owing to the electron affinity of the iodine atom, which is higher in ICl than in I,.15 Oxygen has two hybridized electron lone-pairs, one of which is engaged in the formation of one c.t. bond.16 The halogen molecule is directly linked to the oxygen atom in a geometry that affords maximum overlap between the lone-pair donor orbital and the antibonding acceptor molecular orbital. As regards the ether-halogen complexes, structural data17 have shown the presence of a linear arrangement : In the c.t. complex of 1,4-dioxan with IC1 an arrangement in which two molecules of iodine monochloride are attached to the oxygen atoms of 1,4-dioxan in an equatorial position has been proposed :I8 n In view of the absence of experimental evidence to the contrary or clearly contradictory theoretical reasons, it seems fairly probable that the same geometry is characteristic of these complexes in solution.X-ray studies on thioether-halogen complexes show a structural situation analogous to that found for ether complexes, i.e. 1 : 1 stoichiometry and a linear arrangementS. SANTINI AND S. SORRISO 3155 of the complex 's- --x-x. / For our c.t. complexes of cyclic ether and thioether donors with ICl we can suppose, on the grounds of the previous considerations, that complexes of a 1 : 1 stoichiometry are formed in which the iodine atom is directly linked to the donor atom in a linear arrangement : O(S)---I-Cl.\ / The hybridization of the oxygen and sulphur atoms in these complexes is an important factor in c.t. complexation.16 OXYGEN BASICITY I N THE 3-, 4-, 5- AND 6-ATOM RINGS K,, data for the cyclic ether series show that the electron-donating ability of the oxygen atom is dependent on the size of the ring, the basicity being in the order 4 > 5 > 6 > 3 atoms. An order of this type has previously been reported in studies of c.t. complexes with iodine' as well as in other studies.lg TABLE 1 .-STABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN IC1 AND VARIOUS CYCLIC OXIDES IN cc1, AT 20 OC TOGETHER WITH ABSORPTION MAXIMA OF THE PERTURBED HALOGEN BANDS Kc t compound i/nm /dm3 mol-' propylene oxide 388 30.0 trimethylene oxide 398 84.0 tetrahydrofuran 402 56.5 tetrahydropyran 405 48.0 The steric and inductive effects of the methyl groups do not appear to be important in determining this order of basicity for the oxygen atom in different rings.It thus seems reasonable to assume that the availability of the lone-pair oxygen electrons in the various cyclic ethers differs with the size of the ring, and in particular with the LCOC angle. The change in this angle must result in an altered hybridization of the orbitals, which in turn affects the electron distribution on the oxygen atom. This explanation may be valid for the 3-membered ring, in which the hybridization of the oxygen is fundamentally different from that in other rings.2o According to theoretical calculations, however, the difference in hybridization in 4-, 5- and 6-membered rings is very Undoubtedly other factors are involved.Arnett and Wu22 have explained the difference in electron-donating abilities of tetrahydrofuran and tetra- hydropyran on the basis of interactions between the electrons in non-bonded oxygen orbitals and in the adjacent C-H bonds, these being greater when the ring is planar than when it is puckered. Since the tetrahydrofuran ring is, in turn, probably slightly less nearly planar than the trimethylene oxide ring, the explanation may be extended in some degree to the latter. The basicity of the cyclic sulphides towards the iodine molecule in c.t. complexes varies with ring size in the order 5 > 6 > 4 > 3 members.z3? 23 This further establishes that the ring-size effects differ for two heteroatoms even if they both belong to the same family in the periodic table.Steric factors alone do not account adequately for31 56 MOLECULAR COMPLEXES OF EPOXIDE DONORS the results of the interaction of cyclic sulphides with molecular iodine. It has been suggested that the basicity differences were due rather to differences in electron availability caused by different ring sizes; i.e. the electron distribution is altered by ring size. It is also interesting to note how the basicity of the oxygen atom varies in the series of epoxides shown in table 2. (The parent compound, ethylene oxide, was not TABLE 2.-sTABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN Icl AND VARIOUS EPOXIDES IN eel, AT 20 O C TOGETHER WITH ABSORPTION MAXIMA OF THE PERTURBED HALOGEN BANDS compound Kct l/nm /dm3 mol-l propylene oxide 388 30.0 styrene oxide 390 25.4 cis-stilbene oxide 410 16.2 trans-s tilbene oxide 420 10.2 included because its boiling point is below the temperature at which spectroscopic measurements were made.) The data show how the introduction of a methyl or phenyl group in place of a hydrogen atom in ethylene oxide produces a small variation in the K,, value, indicating that the methyl and phenyl groups have an almost similar effect on the basicity of the oxygen atom.A greater decrease in the value of K,, is, instead, observed in the cis- and trans-stilbene oxides. Before giving an explanation of this behaviour on the basis of steric and electronic effects, we must consider the molecular and electronic structure of the ethylene and styrene oxide molecules.Ethylene oxide is a molecule having the following structure (a = 61' 24'):25 The H-C-H plane that of the ring. formed by the carbon and hydrogen atoms is perpendicular The crystal structure of p-nitrostyrene oxide has been determined by Williams and is depicted below (where = nitrogen, @ = oxygen and a = 60" 7'): to et The dihedral angle between the plane of the phenyl ring and the plane of the oxiran ring is 80'2'. The relative arrangement of the two rings should essentially beS. SANTINI AND S. SORRISO 31 57 determined by the interaction between the two sp5 hybridized orbitals of the oxiran ring and a p orbital on an adjacent carbon atom, in accordance with the bent-bond model developed for cyclopropane.26 The optimum geometry for this interaction has been shown experimentally to be that where the plane of the oxiran ring and the axis of the p orbital are parallel, as shown below: In the case of p-nitrostyrene oxide the configuration of the molecule does not coincide exactly with that necessary for maximum interaction.This is caused by non-bonding interactions of the ortho-hydrogens of the phenyl ring with the hydrogens of the oxiran ring. One may consider that the small difference in basicity between propylene and stryene oxides, as illustrated by their K,,, values, is caused by the presence in the latter (in addition to the small electron-withdrawing inductive effect of the phenyl ring) of a small electron-donating conjugative effect between the phenyl and oxiran rings, as expected from the bent-bond theory described above.This is confirmed by the values of ionization potential (i.p.) assigned to the oxiran ring in ethylene and styrene oxides, which are slightly lower in styrene oxide,*' thus indicating that the phenyl group has a greater electron-donating capacity than hydrogen, evidently caused by the prevalence in this compound of the conjugative electron-donating effect over the inductive electron-withdrawing effect. In the cis- and trans-stilbene oxides the lower electron- donating capacity of the oxygen may be explained not only on the basis of a probable loss of conjugation between the phenyl groups and the oxiran ring, for which only the inductive electron-withdrawing effect of the two phenyls would remain operative, but also on that of the presence of steric hindrance of the relatively big IC1 molecule.EFFECT OF SUBSTITUENT ON THE COMPLEXATION REACTION OF STYRENE AND STILBENE OXIDES WITH I c l The data in table 3 show the existence of a small effect produced by the substituents on the complexation centre formed by the oxygen atom. The small variations obtained in K,,, are insufficient to allow any appropriate considerations to be made as to which type of electronic effect (inductive and/or resonance) operates in these systems. One can, in any case, note that the substituents produce little effect on the K,, values. An attempt was made to correlate the values of K,, with the op and om values obtained by McDaniel and Brown2X using the Hammett method.A value of p = - 0.47 (r = 0.991 ; s = 0.02) was obtained (see fig. 1). The Hammett plot has been used in previous studies for donor substrates of various types whose complexation centre was conjugated to an adjacent 7c system, in particular for substituted thiophenols complexed with I, ( p = - 1.1 l),2s for mono- and di-substituted diphenyl sulphides complexed with I, (p,,,, = -0.64; pdi = -0.43)30 and for aryl diphenylmethyl sulphide complexes with I, ( p = - 1 .04).31 In our case, the small value of p is a clear indication of the overall small size of the electronic (inductive and/or resonance) transmission effect between the two rings. This result is confirmed by p.e.s. measurements on the aryloxiran molecule, where a similarity between the3158 MOLECULAR COMPLEXES OF EPOXIDE DONORS 1.5 1.4 h I .- - 0 E 1.3 m E -0 .1.2 2 00 - 1.1 1.0 TABLE 3.-sTABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN Icl AND SUBSTITUTED STYRENE OXIDES IN cc1, AT 20 " c TOGETHER WITH ABSORPTlON MAXIMA OF THE PERTURBED HALOGEN BANDS - - - - - - Kr t subs ti tuen t A/nm /dm3 mol-l H m-C1 p-Me m-Me m-NO, m-F p-c1 P-NO, P-F 390 395 410 385 390 420 420 395 412 25.4 20.2 16.2 27.2 26.7 10.2 11.2 22.0 17.2 0.4 -0.2 0.0 0.2 0.4 0.6 0.8 FIG. 1 .-Hammett plot for the stability constants of charge-transfer complexes between IC1 and substituted styrene oxides in CCI, at 20 OC. U n systems in the aryloxiran and benzene is indicating a small interaction within the aryloxiran molecule between the n system of the aromatic hydrocarbon part and the lone-pair orbitals of the ethoxy group.In the trans-stilbene oxides the substituent still has a weak effect on the K,, value (table 4), but the size of this effect is smaller than that for the corresponding styrene oxide derivatives. One might think that in this case only the inductive effect would remain operative. Structural data on trans-stilbene oxide are not available in the literature, and therefore the hypothesis suggested here cannot be confirmed, but it is supported by the results obtained for the cis-derivatives. In the cis-stilbene oxides the effect of the substituent is nearly analogous to that observed in the trans-derivatives, and is therefore of a smaller size with respect to the effect observed in the styrene oxides.It thus seems that in these derivatives also, onlyS. SANTINI A N D S. SORRISO 3159 TABLE 4.-sTABILITY CONSTANTS OF MOLECULAR CHARGE-TRANSFER COMPLEXES BETWEEN Icl AND SUBSTITUTED Cis- AND lranS-STILBENE OXIDES IN cc1, AT 20 “c TOGETHER WITH ABSORPTION MAXIMA OF THE PERTURBED HALOGEN BANDS compound cis-stilbene oxide P-NO, PP’-(NO,)2 PP’-Cl, P-NO, PP’-(NO2) 2 PP’-CL trans-stilbene oxide 410 41 8 432 422 420 428 43 5 425 16.2 12.2 9.4 11.4 10.2 8.4 6 . 6 8.0 the inductive effect is operative. A similar result has been obtained in a previous study for the cis-phenylpyridylcyclopropanes X For these compounds the hypothesis of a direct electronic interaction between the two rings was suggested. Supposing that an interaction of this type is also present in the cis-stilbene oxides, one might think that the conjugative effect of the substituents (and of the phenyl groups linked to them) would be transmitted from one ring to the other, bypassing oxiran ring so that it would experience only weak inductive effects. In regard to this, identical behaviour with respect to the transmission of electronic effects is found between the cyclopropane ring and the oxiran ring when they are linked to two aromatic systems.In the trans-derivative of both types of compound an absence of conjugative effects between the phenyl groups and the triatomic cycle is noted; in the cis-derivatives of both types of substrates one may hypothesize the presence of direct electronic interactions between the two aromatic systems, with no conjugation between these systems and the triatomic ring.ABSORPTION I N THE VISIBLE REGION When a halogen molecule is complexed with a donor molecule to give a c.t. complex, the absorption band of the halogen in the visible region is shifted towards the blue. For complexes of the same acceptor with analogous donors, the size of the blue-shift is a measure of the strength of the complex. Fig. 2 shows a plot of log,, K,, against the difference in energy of electron promotion between the free and perturbed halogen [A,,,(free) = 459 nm in CCl,]. The plot shows a good correlation for all the oxiran compounds, indicating a structural analogy among the relative complexes for which the shift of the ICl band is a rough measure of the strength of the c.t.interaction. The deviation observed for 4-, 5- and 6-membered oxides is then due to different steric and electronic requirements for the complexation reaction.3160 MOLECULAR COMPLEXES OF EPOXIDE DONORS 3000 4ooo! - - I E 2 2000- 2 1000- 0 ’ 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 See e.g. R. Hoffmann and R. B. Davidson, J. Am. Chem. Soc., 1971,93, 5699 and references therein; W. J. E. Parr and T. Schaefer, J. Am. Chem. SOC., 1977,99, 1033 and references therein; R. C. Hahn, P. H. Howard and G. A. Lorenzo, J. Am. Chem. Soc., 1971, 93, 5816. (a) R. S . Brown and T. G. Taylor, J . Am. Chem. SOC., 1973, 95, 8025 and references therein; (b) Y. Kuyusama and Y. Ikeda, Bull. Chem. SOC. Jpn, 1973,46,204; (c) D. J. Williams, P. Crotti, B. Macchia and F.Macchia, Tetrahedron, 1975, 31, 993. S. Sorriso, F. Stefani, E. Semprini and A. Flamini, J. Chem. SOC., Perkin Trans. 2, 1976, 374. (a) V. Mancini, P. Passini and S . Santini, J . Chem. SOC., Chem. Commun., 1978, 100; (6) V. Mancini, P. Passini and S . Santini, J. Chem. SOC., Perkin Trans. 2, 1980, 463; ( c ) V. Mancini, G. Morelli and L. Standoli, Gazz. Chim. Ital., 1977,47, 107; ( d ) S . Sorriso, C. Battistini, B. Macchia and F. Macchia, Z . Naturforsch., Teil B, 1977, 32, 1467; (e) R. S . Cataliotti, G. Paliani, S. Sorriso, B. Macchia and F. Macchia, Z . Phys. Chem., N.F., 1977, 105, 1; ( f ) M. T. Foffani, G. Innorta, S. Sorriso and S . Torroni, J. Org. Mass Spectrom., in press. See e.g. (a) L. A. Strait, R. Ketcham, D. Jambotkar and V. P. Shoh, J. Am. Chem.SOC., 1964, 86, 4628; (b) L. A. Strait, D. Jambotkar, R. 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