Conjugated cyclic chlorocarbons: trichlorocyclopropenium ion, heptachlorotropenium ion, and octachlorofulvalene

The properlies of the aromatic trichlorocyclopropenium ion (C3Clj) are described. Evidence from vibrational spectroscopy indicates that the C-C bonds in C3Clj are unusually short and strong. C3Clj undergoes FriedetCrafts condensation reactions with aromatic hydrocarbons; with phenols, diarylquinocyclopropenes are obtained, which upon oxidation yield novel triquinocyclopropanes which show highly unusual electronic spectra The seven-ring analogue to C3Clj, heptachlorotropylium ion (C7Cl.j), is obtained from octachlorobicyclo[3,1,0]heptane and aluminium chloride at 150° to 175°. C7Clj undergoes chemical reactions similar to those of C3Clj. Water converts C7C; Al 2Cl7 to octachlorocycloheptatriene, which upon solvolysis with sulphuric acid is transformed to perchlorotropone. The compound octachlorofulvalene, C10Cl8, and its bromo analogue C10Br8 have been investigated. Both compounds show strong bathochromic shifts of electronic bands, due to twisting of the rings about the centrat double bond. C10Cl8 is a strongly electron-deficient molecule which serves as a powerful 1t -type charge-transfer acceptor toward aromatic hydrocarbons. The charge-transfer spectra are discussed. The chemistry of the chlorocarbons-fully or highly chlorinated organic compounds-is now undergoing a renaissance, in which many of the most interesting findings centre about the cyclic conjugated perchloropolyenes. Monocyclic members of this family are shown in Figure 1. Of these species only hexachlorobenzene was known before 1964. Within the last six years the stable species C3Clt, C7Clj and C8Cl 8 have been isolated (the cations as salts) and the transient existence of C4Cl4 , C5Clt and C5Cl5 has been convincingly demonstrated 1. The family of species in Figure 1, like the cyclic polyenes from which they are derived, can be classified as aromatic if they contain 4n + 2 and antiaromatic if they contain 4n pi-electrons. Thus C3Clj, C5Cl5, C6Cl6 and C7Clj can be considered aromatic and so are expected to be stabilized, but C4Cl4 , C5Clt and C8Cl8 are antiaromatic and should be destabilized, at least when planar. However, the chlorine substituents may perturb the carbocyclic pi system either by withdrawing electronic charge inductively 379 ROBERT WEST Cl ct( Cl ~ C*CI 0 Cl Cl Cl CI Cl Cl Cl Cl Cl Cl,)6z.P c~~( Cwl 0 Cl 8 Cl Cl Cl Cl Cl Cl Cl Cl ~ Cl Cl I Cl Cl Cl Cl Figure 1. The monocyclic conjugated chlorocarbons. through the C-Cl sigma bonds, or by direct participation of the nonbonding pairs on the chlorine in pi interaction with carbon. Fragmentary evidence suggests that both effects may be significant in certain delocalized chlorocarbons 1 . This paper will discuss the chemistry of two of these species, C3Clj(I) and C7Clt (II), both of which can be considered aromatic according to the usual rules. In ·addition, in the final section of the paper the chemistry of the interesting conjugated molecule octachlorofulvalene will be described. I. TRICHLOROCYCLOPROPEN~ION This simple aromatic species was first synthesized2 in 1964, by chloride abstraction from tetrachlorocyclopropene (nn with aluminium chloride or other powerful Lewis acid-halide acceptors : C3Cl4 + MC1 3 --+ C3Clj, MCl4 111 M = Al, Ga, Fe; also SbC15 The starting material !111) is synthesized by dehydrohalogenation of pentachlorocyclopropaneby warm concentrated aqueous potassium hydroxide solution in a two-phase system3 • Pentachlorocyclopropane is obtained by addition of dichlorocarbene to trichloroethylene; but the latter olefin is rather unreactive toward carbene addition, so most methods for carrying out this reaction give only very small yields. However, when sodium trichloroacetate is thermolysed in dimethoxyethane at 80° in the presence of trichloroethylene, pentachlorocyclopropane is produced in 25 per cent yield4 • H......._ .,......CI C=C ........ ......... Cl Cl Na+ccl 3COo- (CH3ocHzb 80° H Cl Cl Cl~ Cl Cl 380 KOH Cl Cl '?cl Cl 111 CONJUGATED CYCLIC CHLOROCARBONS Tetrachlorocyclopropene prepared by this route is now available commercially in the USA. lt shows promise as a vapour-phase fumigant as weil as being useful as a chemical intermediate. Substitution reactions on 111 normally take place with ring opening but with boron tribromide, III gives BC13 and tetrabromocyclopropene 3• This reaction probably takes pla:ce through the formation of I as an intermediate and successive Br-Cl exchange reactions: Cl l~ Cl Cl X Br ll •. ,, Br/ A Cl~r Br z 'sr Partially fluorinated cyclopropenes can also be prepared from lll, by treatm.ent with SbF 3 in the presence 3 of SbC15, or with KF in tetramethylene sulphone 5 : +Ä Cl CI Ä + Cl Ä Cl Cl Cl These halogenated cyclopropenes can all serve as precursors to trihalocyclopropenium ions, when treated with appropriate halide acceptors, I.e.: Br ~C Brj, AIBr4 I Ale!; c c1 + ci- c3c 3F--. 3 2F , Al 4 At the time it was prepared C3Clj was the simplest aromatic species known, having just six atoms and D3h (triangular) symmetry6 • The bonding in I was investigated using vibrational spectroscopy and normal coordinate analysis. Observation of the infra-red spectrum presented no difficulty, but to determine the Raman spectrum measurement in solution in liquid sulphur dioxide was required. The resulting Raman spectrum is shown in Figure 2. Observed frequencies are given in Table 17• There are six constants in the U rey-Bradley force field used for the 381 ROBERT WEST Ra man spectru m of C3 Cl~ AlClL; in S02 1000 800 600 l.OO 200 SOz so2 Ct~ c3ctt «1320 0 C1l rT 1800 1600 1400 1200 1000 Frequency, cm-1 Figure 2. Raman spectr.um ofC3Clj A1Cl4 in liquid sulphur dioxide, showing polarization. -· calculation and only five fundamental frequencies were observed, so one constant must be assumed; but fortunately the chlorine atoms are so far apart that the nonbonded interaction constant can be taken as zero 7• The results of a Urey-Bradley calculation are shown in Table 2, together with those from a partial normal coordinate analysis for the corresponding brominated ion, C3Br j. In Table 3, the C-C and C-Cl Stretching force constants are compared with those for other known aromatic species. Note that both Kcc and Kcc1 are decidedly high er than for chlorobenzene; indeed Kcc for C3Clj is markedly higher than for any other known aromatic species. Consistent with this is the x-ray finding by Sundaralingam, that the C-C distance in triphenylcyclopropenium ion is shorter than that in benzene8 . cm- 1 200 459 735 1312 1348 1791 Table 1. Vibrational frequencies for C3Clj Infra-red Raman Assignment E s,pol Al vw E VS s, dep E s m,pol A 382 CONJUGATED CYCLIC CHLOROCARBONS Table 2. Force constants for C3Xj, millidynes/A 7 Species Kee Kex Heec Heeel Fee! CCICI * Values in parentheses are assumed. 6.31 2.99 -0.248 0.385 0.808 (0) 6.46 2.15 ( -0.248)* (0.275) (0.674) (0) Why should the C-C bond in cyclopropenium ions be so strong? Simple Hückel calculations predict a pi bond order of 0.667 for cyclopropenium, identical to that for benzene. The sigma bonding in cyclopropenium ions is probably external to the ring and 'bent' even more strongly than in cyclopropane or cyclopropene (Figure 3); if so the sigma bonds should be weaker Table 3. C--C and C-Cl stretching force constants for aromatic species Species Kcc Kcx p C6H6 5.59 4.67 0 5.15 4.79 0.35 CsH5 5.39 4.79 0.21 C6Cl6 4.81 2.30 0.37 C3Clj 6.32 2.99 0 p = Resonant constant. than in benzene. But bending of the sigma bonds may bring the carbon atoms closer together, allowing for much increased overlap of p orbitals on adjacent carbon atoms. According to this model the unprecedented bond strength in C3Clj arises from the increased pi-bonding in cyclopropenium ions, which more than makes up for the decreased sigma bond strength. In I, contributions from the chlorine atoms to the pi-bonding may also aid in raising both C--Cl and C--C bond strengths and force constants. Figure 3. Schematic drawing of orbitals forming C--C bonds in cyclopropenium ions, indicating overlap external to the three-membered ring. 383 ROBERT WEST Nuclear quadrupole resonance measurements of Lucken and Mazeline9 give a value of 0.35 for the asymmetry parameter 11 for the 35Cl nuclei iri I. This asymmetry parameter can be related directly to the partial double bond character, which is found to be 0.16. Both the asymmetry parameter and the bond order are higher than for any other known carbon-chlorine compound. The data suggest that about half of the positive charge on the ring (3 x 0.16) is delocalized through the pi system on to the chlorine atoms9 . The chemistry of I has been reviewed elsewhere 1 , so only those reactions which Iead to compounds of some theoretical importance will be treated here. Hydrolysis ofthe tetrachloroaluminate salt ofl, by flooding with water, Ieads to the regeneration ofiii in good yield7• Apparently water first attacks the AIC14 counterion rather than C3Clj, forming chloride which converts I to C3Cl4 . However, very ·slow solvolysis of I, preferably as the heptachlorodialuminate salt dissolved in dichloromethane, Ieads to the explosive liquid, dichlorocyclopropenone 10. The reaction is best carried out using alcohol, and the first product is the AIC13 complex of the cyclopropenone. Further alcoholysis of the complex converts it to an alkoxychlorocyclopropenone11, but the last chlorine atom is removed simultaneously with ring-opening. Upon cautious warming dichlorocyclopropenone is converted to a spirolactone : A C! ,AICI.~ AO:AIC13~ Ao -Li CIWO 0 CH2Cl 2 _h. -- Cl Cl Cl Cl C