1975 1557Formation of Meisenheimer-Jackson Complexes in a Non-polar SolventBy Anthony R. Butler, Department of Chemistry, The University, St. Andrews, FifeIn the presence of dicyclohexyl-18-crown-6 ether in benzene solution, trinitrobenzene forms Meisenheimer-Jackson complexes with a number of anions, some of which do not react in aqueous solution. Such complexesare also formed with dinitrobenzene under these conditions, and there is some evidence that nitrobenzene reactsin a similar way.ONE of the most important factors facilitating formationof Meisenheimer- Jackson or a-complexes from anionsand polynitro-compounds is delocalisation of the negativecharge by the inductive and mesomeric effects of thenitro-groups on the aromatic ring. Simple MO calcal-ations have shown that the electron density on the ben-zene ring in a a-complex is actually lower than in benzeneitself ,2 although more sophisticated Pariser-Parr-Popletype SCF-CI calculations do not confirm this r e ~ u l t .~The fact that a 2 : 1 complex is formed between sulphiteand picrate, with an overall negative charge of five: isfurther evidence of the remarkable effect of the nitro-groups. Any slight change in the geometry of the mole-cule which hinders the process of delocalisation has adramatic effect on its reactivity. For example, althoughsulphite a-complexes are normally formed readily, thereis no reaction between sulphite and 3-methyl-2,4,6-tri-nitr~phenol,~ presumably because the methyl groupprevents complete coplanarity.A possible consequenceof the ability of the nitro-groups to delocalise negativecharge is that solvation plays a smaller role in stabilisinga-complexes than it does with anionic species in which thecharge is more concentrated. Therefore, o-complexesshould be relatively more stable in non-polar solvents.It is possible to test this prediction by bringing thecationic counterion into the non-polar solvent by use of acrown ether. Such reagents are known to facilitatenucleophilic aliphatic substitution,6 and a a-complex hasbeen suggested, but not substantiated, as an intermediatein the reaction of potassium methoxide with dichloro-benzene in benzene solution.7RESULTS AND DISCUSSIONAddition of solid potassium hydroxide to a solution of1,3,5-trinitrobenzene (TNB) and dicyclohexyl-18-crown-6ether in benzene immediately produced an intense redcolour, the spectrum of which is shown in the Figure.This spectrum is so similar to that of the complex (I)obtained in aqueous solution by Gold and Rochester(maxima at 430 and 480 nm) that it may be identified asa a-complex without doubt.If sodium hydroxide re-places the potassium compound there is no immediatereaction, but over several hours the same red colour isJ. Meisenheimer, Annalen, 1902, 323, 205; C. L. JacksonP. Caveng, P. B. Fischer, E. Heilbronner, A. L. Miller, andH. Hosoya, S. Hosoya, and S. Nagakura, Theor. Chim. Acta,M. R. Crampton and M. El-Ghariani, J . Chem. SOC. (B),5 A. R. Butler and B. Medcalf, unpublished observation.C.L. Liotta and H. P. Harris, J . Amer. Chem. SOC., 1974,and F. H. Gazzolo, Amer. Chem. J., 1900, 23, 376.H. Zollinger, Helv. Chim. Acta, 1967, 50, 848.1968, 12, 117.1969, 330.96, 2250.formed. This reflects the greater ease with which thecrown ether complexes the potassium ion. If the a-complex in aqueous solution is prepared from potassiumhydroxide it is not extracted at all by benzene unless theorganic solvent contains crown ether; extraction is thenimmediate and complete. There is no immediate extrac-tion from aqueous solution if sodium hydroxide is used.~~ -400 500 600A InmSpectrum of the solution resulting from the reaction of a saturatedsolution of potassium hydroxide in benzene containing crownether (0.0006~) and TNB (0.0002~) (10 mm path length)NO A 2H OH t i \OH #4 ‘so;Sulphite anion readily forms a-complexes with TNB.gThis is surprising as in nucleophilic aliphatic substitutionsulphite is not a particularly strong nucleophile,1° and yetstronger nucleophiles, such as azide, do not form a-complexes with TNB.Addition of solid potassiumsulphite to a solution of TNB and crown ether in benzeneagain resulted in an intense red colour,ll whose spectrumis similar to that reported by Norris 11 for the 1 : 1 TNB-sulphite o-complex (11) in aqueous solution (A,, 460 and510-540sh nm). The same result was obtained withtoluene as solvent, so benzene does not play an intimateand unique role in the reaction. Most o-complexes inaqueous solution disappear during a few hours,12 but the7 D. J.Sam and H. E. Simmons, J . Amer. Chem. Soc., 1974,8 V. Gold and C. H. Rochester, J . Chem. SOC., 1964, 1710.9 F. Cuta and E. BerAnek, Coll. Czech. Chem. Comm., 1988,23, 1501 : C. F. Bernasconi and R. G. Bergstrom, J . Amer. Chem.SOC., 1973, 95, 3603; M. R. Crampton, J . Chem. SOC. (B), 1967,1341.l o W. P. Jencks and J. Carriuolo, J . Amer. Chem. SOC., 1960,82, 1778.l1 A. R. Norris, Canad. J . Chem., 1967, 45, 174.12 R. A. Henry, J . Org. Chem., 1962, 27, 2637.96, 2252J.C.S. Perkin ITNB-sulphite complex in benzene appears to be in-definitely stable. If any one of the reactants is omittedno colour develops and there is no obvious reactionbetween TNB and the crown ether. Thus formation of0-complexes in benzene occurs readily.A more surprising observation is that some anionswhich do not form a-complexes in aqueous solution reactreadily with TNB in benzene solution.Intense colourswere obtained with the following potassium salts :carbonate, hydrogen carbonate, nitrite, fluoride, andacetate. At the crown ether concentration employed(0. l ~ ) , chloride, bromide, nitrate, and perchlorate did notreact. There was no obvious reaction with sodium salts.The spectra of all the coloured species obtained weresimilar to that shown in the Figure and are certainlythose of 0-complexes. Absorption maxima are listed inthe Table. The observation that a number of anions,Spectra of a-complexes of TNB in benzene solutionAnion A,,./nm A,,./nmHydroxideCarbonateHydrogen carbonateFluorideAcetateNitriteSulphite* Extinction coefficients are notstudy, it is not known whethera-complex is complete or partial.430 495426 500432 500450452 520infl435 510458 500-540shgiven as, from the presentconversion of TNB intowhen not perturbed by solvation, react with TNB to forma-complexes is further evidence that reactivity in aqueoussolution is controlled as much by solvation as by elec-tronic influences.Reactionwith potassium iodide produced an orange solution witha single absorption maximum at 444 nm. The shape ofthe band was different from those of the a-complexes ob-tained previously and a different reaction may occur inthis instance.Further study is in progress. Thesolubility of potassium permanganate in benzene in thepresence of crown ether is well known.Addition ofTNB resulted in the ubiquitous red colour and it appearsthat even the permanganate anion can form a o-complex.The species is not stable and rapidly deposits manganesedioxide.l3 C. J. Pedersen and K. H. Frensdorff, Angew. Chem. Internat.Edn., 1972, 11, 16.Two salts gave results of special interest.The other products were not determined.Previous work, and the present study, show thatanions have enhanced nucleophilicity in benzene solutionand may, therefore, form 0-complexes with substratesless reactive than TNB. Potassium hydroxide, carbon-ate, and acetate in benzene all react with 1,3-dinitro-benzene (DNB) to give coloured solutions.The spectrasuggest the products are all a-complexes. There is noreaction with DNB in aqueous solution although DNBdoes form a coloured species with potassium hydroxide inaqueous dimethyl sulphoxide. The structure of the C-complexes of DNB is almost certainly (111) ; attack a t the4-position is more likely because of the electronic in-fluence of a nitro-group in the para-position of the result-ing adduct. With nitrobenzene the results are lesscertain. Addition of potassium hydroxide to nitro-benzene and crown ether in benzene gave no obviouscolour change. However, a broad, low shoulder (430-650 nm) appears on the intense absorption band of nitro-benzene, which commences at 430 nm. A similar, butsmaller effect occurs with potassium carbonate. Theevidence is too slight to lead to any firm conclusion.As o-complexes of TNB are highly coloured and readilyextracted from aqueous solution by crown ethers, theymay have some use as indicators of the complexing abilityof the array of coronate, catapinate, and cryptate13ligands and ' octupus ' molecules l4 which have beenreported recently.EXPERIMENTALAll inorganic reagents were AnalaR grade and dried in anoven before use. Potassium sulphite was prepared bydigesting barium sulphite with an equimolar amount ofpotassium sulphate. TNB was recrystallised from methan-ol. Dicyclohexyl- 18-crown-6 ether (Emanuel) was dried ina desiccator before use. Crown ether and TNB were dis-solved in reagent grade benzene to give 0 . 1 ~ - and 0 . 0 4 ~ -solutions, respectively.The spectra were recorded on a Unicam SP 800 spectro-photometer. In some cases considerable dilution withbenzene was necessary.Mr. A. Duffin is thanked for preliminary experiments.[5/273 Received, 10th February, 1976114 F. Vogtle and E. Weber, Angew. Chem. Internat. Edn., 1974,13, 814
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