australian journal of chemistry (1975), 28(10), 2227-54
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Aust. J. Chem., 1975, 28, 2227-54
The Acetolysis of Some a-Cyclooctatetraenylalkyl p-Nitrobenzenesulphonates
George E. Gream and Michael MularOrganic Chemistry Department, University of Adelaide,
P.O.Box 498D, Adelaide, S.A. 5001.
AbstractThe synthesis and solvolytic behaviour (in buffered acetic acid) of the p-nitrobenzenesulphonate esters of 2-cyclooctatetraenylethanol, 3-cyclooctatetraenylpropan-1-01 and 4-cyclooctatetraenylbutan-1-01 are described. For comparison with the cyclooctatetraenylalkyl derivatives, kinetic data for the corresponding w-(cyclooct-1'-eny1)alkyl and w-cyclooctyl p-nitrobenzenesulphonates are reported. Kinetic and product studies have shown that rt-bond participation occurs in the acetolysis of 2-cyclooctatetraenylethyl p-nitrobenzenesulphonate. The nature of the cyclized products, which amount to 95 % when 1.1 equiv. of sodium acetate (the buffer) are used, depends markedly on the concentration of buffer. Acetolysis of 3-cyclooctatetraenylpropyl p-nitrobenzenesulphonates gives at least 98% noncyclized products. A probable cyclized product ( < 2 % ) is tentatively identified as bicyclo[6,3,0]undeca-1,3,5,7-tetraene. Cyclized products (44 %) are formed in the acetolysis of 4-cyclooctatetraenylbutyl p-nitrobenzenesulphonate.
Introduction As an extension of an interest in n-routes to carbonium and in the chemistry of cycl~octatetraene,~,~ it (CH&X was decided to investigate the acetolysis of some o-cyclooctatetraenylalkyl derivatives (1) (where n = 1-4, and X = good leaving group) Interest in a study of the capacity of the double bonds in a cyclooctatetraenyl moiety to behave as nucleophiles in the solvolysis of o-cyclooctatetraenylalkyl derivatives is enhanced by the following unique physical and chemical properties of cyclooctatetraene and some of its derivatives (for some reviews,
' Gream, G. E., Aust. J. Chem., 1972, 25, 1051.Gream, G. E., and Serelis, A. K., Aust. J. Chem., 1974, 27, 629. Gream, G. E., Serelis, A. K., and Stoneman, T. I., Aust. J. Chem., 1974, 27, 1711. Gream, G. E., Mular, M., and Wege, D., Aust. J. Chem., 1974, 27, 567. Huisgen, R., Konz, W. E., and Gream, G. E., J. Amer. Chem. Soc., 1970, 92, 4105. Gasteiger, J., Grearn, G. E., Huisgen, R., Konz, W. E., and Schnegg, U., Chem. Ber., 1971, 104, 2412. Schroder, G., 'Cyclooctatetraen' (Verlag Chemie: Weinheim 1965); Craig, L. E., Chem. Rev., 1951, 49, 103; Scott, L. T., and Jones, M., Chem. Rev., 1972, 72, 181. * Figes, H. P., in 'Topics in Carbocyclic Chemistry' (Ed. D. Lloyd) Vol. 1, p. 296 (Plenum: New York 1969).
G. E. Gream and M. Mular
Cyclooctatetraene is a tub-shaped molecule with alternating single and double A bonds of lengths 1 -476A and 1.340 A, respec~ively.~ recent analysisi0 of heats of hydrogenation and other data indicates that the strain and resonance energies in cyclooctatetraene are c. 50 and 60 kJ mol-', respectively. The compound is thus adequately described as 'a slightly strained, weakly conjugated cyclic tetra-olefin'.' An important property of cyclooctatetraene and its derivatives is the 'mobile' nature of the molecules at room temperature. Extensive n.m.r. studies have established that both bond-shifts and ring-inversions take p 1 a ~ e . l ' ~ ' ~ A kinetic examination of the reaction of cyclooctatetraene and some derivatives with dienophiles enabled Huisgen and coworkers13 to establish yet another important feature of these compounds, namely the existence of bicyclic valence tautomers (2) such as bicyclo[4,2,0]octa-2,4,7-triene for cyclooctatetraene itself. By the use of a range of dienophiles, and in particular the very reactive N-phenyltriazoledione, Huisgen5 and Paquettei4 have established that monosubstituted cyclooctatetraenyl derivatives (3) can exist in equilibrium with one or more of the four possible bicyclic valence tautomers (4)-(7). The nature of the substituent R in (3) is however very important in determining which of the valence tautomers (4)-(7) will be favoured.14
Traetteberg, M., Acta Chem. Scand., 1966, 20, 1724. Turner, R. B., Mallon, B. J., Tichy, M., Doering, W, von E., Roth, W. R., and Schroder, G., J. Amev. Chem Soc., 1973, 95, 8605. Anet, F. A. L., J. Amev. Chem. Soc., 1962,84,671; Anet, F. A. L., Bourn, A. J. R., and Lin, Y. S., J. Amer. Chem. Soc., 1964, 86, 3576; Gwynn, D. E., Whitesides, G. M., and Roberts, J. D., J. Amev. Chem. Soc., 1965, 87, 2862; Bryce-Smith, D., Gilbert, A., and Grzonka, J., Angew. Chem., Znt. Ed. Engl., 1971, 10, 746; Allinger, N. L., Sprague, J. T., and Finder, C. J., Tetrahedron, 1973, 29, 2519; Oth, J. F. M., Puve Appl. Chem., 1971,25, 582. Oth, J. F. M., Merknyi, R., Martini, T., and Shroder, G., Tetvahedvon Lett., 1966, 3087. l 3 Huisgen, R., and Mietzch, F., Angew. Chem., Int. Ed. Engl., 1964, 3, 83; Huisgen, R., Mietzch, F., Boche, G., and Seidl, H., 'Organic Reaction Mechanisms' Chem. Soc. Special Publ. No. 19, p. 3 (Chemical Society: London 1965). l4 Paquette, L. A., James, D. R., and Birnberg, G. H., J. Amev. Chem. Soc., 1974, 96, 7454.lo
An additional characteristic property of cyclooctatetraene is the ease with which it reacts with a proton and other electrophiles to form stabilized homotropylium cations1' [e.g. ion (8) from the action of strong acid on cyclooctatetraene itself]. The formation of homotropylium ions from substituted cyclooctatetraenyl derivatives has been reported.16
When the work described in this paper was commenced (1970), there were no reported examples of the cyclooctatetraene ring behaving as a nucleophile in the solvolytic reactions of cyclooctatetraenyl derivatives. In 1973 however, Paquette and H e n ~ e 1 ~ 'described the behaviour of cyclooctatetraenylmethyl chloride (9) in ~l~ aqueous ethanol and 2-cyclooctatetraenylethyl p-bromobenzenesulphonate (15)Vn acetic acid. A planned investigation of the solvolytic behaviour of cyclooctatetraenylmethyl bromide (10) (which had already been preparedlg) was therefore abandoned by us. A study of the behaviour of 2-cyclooctatetraenylethyl p-nitrobenzenesulphonate (14) in acetic acid was however well underway at the time and the results are reported here and compared with those of Paquette and Henzel. In this paper, the results of a study of the acetolysis of 3-cyclooctatetraenylpropyl p-nitrobenzenesulphonate (19) and 4-cyclooctatetraenylbutyl p-nitrobenzenesulphonate (25) are also reported. For comparative purposes, kinetic data for the w-(cyclooct-1'-eny1)alkyl (11, n = 2-4) and w-cyclooctylalkyl (12, n = 2-4) p-nitrobenzenesulphonates corresponding to (14), (19) and (25) have been determined.
Synthesis of Required Compounds w-Cyclooctatetraenylalkyl Derivatives Treatment of a solution of cyclooctatetraenyllithium in ether (prepared readily from bromocyclooctatetraene6 and butyllithium) with ethylene oxide a t low temperature gave a good yield (70%) of 2-cyclooctatetraenylethanol (13). The use of cyclo-
* Abbreviations used in the formulae: Bs, p-bromobenzenesulphonyl; Ns, p-nitrobenzenesulphonyl; Ts, p-toluenesulphonyl; Ac, acetyl.I5 Story, P. R., and Clark, B. C., in 'Carbonium Ions' (Ed. G . A. Olah and P. von R. Schleyer) VoI. 3, p. 1084 (Wiley-Interscience: New York 1972). I6 Hehre, W. J., J. Amer. Chem. Soc., 1973, 95, 5807, and references therein; Huisgen, R., and Gasteiger, J., Tetrahedron Lett., 1972, 3661, 3665; Paquette, L. A., Broadhurst, M. J., Warner, P., Olah, G. A., and Liang, G., J. Amer. Chem. Soc., 1973, 95, 3387; Ahlberg, P., Harris, D. L., Roberts, M., Warner, P., Seidl, P., Sakai, M., Cook, D., Diaz, A,, Dirlam, J. P., Hamberger, H., and Winstein, S., J. Amer. Chem. Soc., 1972, 94, 7063; Brookhart, M. S., and Atwater, M. A. M., Tetrahedron Lett., 1972, 4399. Paquette, L. A , and Henzel, K. A., J. Amer. Chem. Soc., 1973, 95, 2724. Paquette, L. A., and Henzel, K. A., J. Amer. Chem. Soc., 1973, 95, 2726. I9 Bowie, J. H., Gream, G. E., and Mular, M., Aust. J. Chem., 1972, 25, 1107.
G. E. Gream and M. Mular
octatetraenylmagnesium bromide, rather than the lithio reagent, gave a considerably reduced yield (21 %) of the required alcohol. Conversion of the alcohol into its p-nitrobenzenesulphonate ester (14) was achieved in good yield with p-nitrobenzenesulphonyl chloride in the presence of a slight excess of pyridine. When pyridine was used in large excess, water-soluble derivatives not extractable into ether were obtained. The preparation of 2-cyclooctatetraenylethyl acetate (16) was readily carried out by treating the alcohol (13) with acetic anhydride in pyridine.(13) X = O H (14) X = OKs (15) X = OBs (16) X = OXc (18) X = C0,Et (17) X = O H (19) X = OSs (20) X = 0 . 4 c (22) X = CN (23) X = C0,H (24) X = C0,Me
Initially, 3-cyclooctatetraenylpropan-1-01 (17) was prepared by reduction of ethyl 3-cyclooctatetraenylpro~ate(18) with lithium aluminium hydride. The required intermediate ester (18) was however not readily available. Treatment of cyclooctatetraenyllithium with ethyl acrylate in the presence of cuprous chloride gave a very low yield (5 %) of the ester. A similar reaction with cyclooctatetraenylmagnesium bromide gave an increased yield (20%) of (18). In later work, it was found that 3-cyclooctatetraenylpropan-1-01could be prepared more conveniently and in better yield (51 %) in a one-step reaction by heating a mixture of cyclooctatetraenylmagnesium bromide and two equivalents of oxetan. When an equivalent of oxetan was used, 3-bromopropan-1-01 (31 %) was the major product; 3-cyclooctatetraenylpropan-1-01 was formed in only 3 % yield. Bromide ion (from magnesium bromide present in the solution of the Grignard reagent) apparently reacts with oxetan to give the above bromo alcohol. When cyclooctatetraenyllithium was treated with oxetan at room temperature, no hydroxylic compounds were formed. At 80" (in boiling benzene), the major product was heptan-1-01 (30%); none of the desired alcohol (