CYCLIC OXONIUM IONS and RELATED STRUCTURES* Whose

CYCLIC OXONIUM IONS AND RELATED STRUCTURES* BY ROBERT J. GARGIULO AND D. STANLEY TARBELL

DEPARTMENT OF CHEMISTRY, VANDERBILT UNIVERSITY Communicated October 28, 1968 Abstract and Summary.-This paper describes the preparation by several methods of trialkyloxonium ions containing five-membered rings and the study of their structures by observing the nuclear magnetic resonance (NMR) spectra in several solvents. This provides independent evidence for these compounds, whose existence has been shown by kinetic, preparative, and stereochemical studies. The following SbCl6 oxonium salts have been prepared, isolated, and characterized as crystalline solids: the 1,2,5-trimethyltetrahydrofuranium (Ha), the 1,2,2,5-tetramethyl (JIb), the trans-l-methylperhydrobenzofuranium (IVa), the trans-1,2-dimethyl (IVb), and the trans-1,2,2-trimethyl (IVc) compounds. The trifluoroacetate salts IIc, IId, IVd, and LWe have been made in acid solution, and all the salts II and IV have been characterized by their NMR spectra as cyclic- oxonium salts, with the charged 1-CH3 group downfield from an uncharged -OCH3. Spectra of all the SbCl6 salts in nitrobenzene solution are reported. Further evidence for the cyclic structure is given by the identification of 2,2,5- trimethyltetrahydrofuran (VIII) from the action of pyridine and lithium chloride on the 1,2,2,5-tetramethyltetrahydrofuranium salt JIb. Solvolysis of this salt by ethanol gave 2-ethoxy-2-methyl-5-methoxyhexane (VII) by attack at the tertiary carbon of the oxonium ion. The structure of VII was established by its synthesis and that of the other possible isomer. The trifluoroacetate oxonium ion (IVe) gave, after three days in 5 per cent sulfuric-trifluoroacetic acid, trans-2,2- dimethylperhydrobenzofuran and methyl trifluoroacetate.

The occurrence of cyclic methoxonium ions derived from tetrahydrofurans has been indicated by observations in the fumagillin series' and in simpler compounds.2 The stereochemical implications of these intermediates have been explored, 4and an outstanding kinetic study by Winstein5 has given convincing support for the intermediacy of cyclic methoxonium ions in the solvolysis of compounds such as 4-methoxy-1-pentyl brosylate. The solvolysis of cis-5- methoxy-3-penten-1-yl tosylate involves a cyclic methoxonium ion and, to a smaller extent, an open-chain carbonium ion.6 The preparation of numerous types of onium ions in strong acid solution has been described,7 and several laboratories have recently reported studies on oxonium ions." In the present paper we describe the preparation of five 1-methyltetrahydro- furanium salts (II, IV) and their NMR spectra in several solvents; the ethanolysis of the 1,2,2,5-tetramethyltetrahydrofuranium ion (IIb) and the proof of structure of the product by synthesis are also presented. The salts JIb, IId, IVc, and lWe appear to be the first reported trialkyloxonium salts with a tertiary -carbon atom attached to oxygen. 52 Downloaded by guest on September 28, 2021 VOL. 62, 1969 CHEMISTRY: GARGIULO AND TARBELL 53

C1 OCH3 WI,. CH3-OCH2)2CHCH3 OH3>J7j

R CF3COOH/iCC I X_ Ia,R-H OHC3 IkR=CH3 aH2S04la,R=H,XSbC16 / b, R = CH3, X- SbC16 OCH3 R /c,RR=H,X=CF3COO I lHd,R = CH3,X CF3COO UH3OH(CHW),C=CH2 Va,R=H Vbk R = CH3 C1 J/R

HIa, R= R=H CH Mib,R-OH3,RI=H CF3COOH OH3X c,R= R'=CH3 2SO, Na, RR' H, X SbC16 NVb, R- CH3, R'= H, X - SbC16 X - CH2 lc, R- R=CH3, SbC16 ggCH2CICH2C=O-CHI IVd, R = CH3, R' =H, X =CF3COO R IVe, R CH3, R'- CH, X = CF3COO OCH3 VIa,R= H VIb, R= CH3 The chloromethoxy compounds Ia, Ib, IIIb, and IIMc were prepared by adding HC1 to the corresponding unsaturated methoxy compounds; all had satisfactory elemental analyses and spectral properties (infrared and NMR). The primary chloride MIIa was obtained from trans-2-(2-methoxycyclohexyl)-ethanol.9 Treatment8a 10 of the chlorides I or III with SbCh in CH2C12 at -78° for some and 0° for others gave rapid precipitation of the crystalline hexachloroantimonate salts, all of which were white, stable solids melting with decomposition, and all gave satisfactory carbon and hydrogen analyses. The formulation of compounds II and IV as cyclic oxonium salts (Table 1) is based on their analyses and on the NMR spectra in nitrobenzene, which show a marked shift to lower field of the 1-CH3 group in the salts, as compared to the uncharged OCH3 groups. The -0CH3 protons of compound IMIc absorb at 3.28 ppm in nitrobenzene. The cyclic oxonium structure is also supported by the nonequivalence of the geminal methyl groups in compounds IIb and JWc. If these compounds were not cyclic, the methyl groups would be equivalent and would appear as a singlet. Compounds Ha and IVb consist of mixtures of two diastereoisomers, because of the asymmetry at C-2; this leads to two 1-CH3 absorptions. The configuration of Ha is assigned on the grounds that if the methyl groups at C-2 and C-5 are trans, then the 1-CH3 will be cis to one; and hence will be more highly shielded.-" The 1-CH& group in the 2,5-cis compound will be less shielded because the 1-CH3 Downloaded by guest on September 28, 2021 54 CHEMISTRY: GARGIULO AND TARBELL PROC. N. A. S.

TABLE 1. NMR spectral data (in ppm) of oxonium ions in nitrobenzene.* Compound 1-CH. 2-OCH 5-OCH Ha, cis 4.38 1.78 (doublet, J = 6 cps) 1.78 (doublet J = 6 cps) trans 4.51 Same Same IIb 4.31 1.81 1.78 (doubletJ = 6 cps) 1.84 IVa 4.63 - MIb, cis 4.36 1.88 (doublet, J = 6 cps) trans 4.52 1.78 same W~c 4.20 1.82 2.00 * With TMS in CDC13 as standard: 0.6 ppm added to observed values to correct to TMS in nitrobenzene. Chemical shifts of the multiplets due to the methine and methylene protons were consistent with structure. will undoubtedly remain trans to them most of the time, even though it probably isomerizes rapidly.sb In nitromethane and sulfur dioxide, the geminal methyl groups of IIb appeared as a singlet, and the chemical shift difference between the two methyl groups of IVc decreased from 0.18 ppm in nitrobenzene to 0.13 ppm. One explanation of this is that there is less carbonium ion character in the ions JIb and IVc in nitrobenzene than in the other solvents. Further support for this can be found in the decrease of the 1-CH3 chemical shift with increasing substitution at C-2. Olah12 has observed a similar effect in trimethylethylenehalonium ions; the geminal methyl groups were equivalent in the bromonium and chloronium ions but not in the iodonium ion. The unsaturated compounds Vb and VIb were converted in trifluoroacetic acid to a mixture of the corresponding oxonium ions HId and lWe (as the trifluoroacetate salt) and the trifluoroacetate esters. Oxonium ions were not detected by NMR with the less substituted olefins Va and VIa, unless sulfuric acid was added to the solution. The ion lWe, after standing three days in 5 per cent sulfuric- trifluoroacetic acid solution, gave, by 1-CH8 cleavage, trans 2,2-dimethylperhydrobenzofuran and methyl trifluoroacetate. OCH3 OC H5 i CH. C,HOH I I CH Kg

The 1,2-dimethyltetrahydrofuranium ion underwent ethanolysis with attack predominantly at the less substituted carbon.5b As shown above, ethanol attacks Ilb exclusively at the most highly substituted carbon to give the tertiary ethoxy compound VII; LiCl in pyridine gives 2,2,5- trimethyltetrahydrofuran (VIII) by attack at the 1-CH3 group, as well as the unsaturated compound Va. The ethanolysis product VII was identified by synthesist of both the isomers VII and IX, which could be formed by attack of ethanol at C-2 or C-5 of 11M. * Aided by grants (2252-C) of the Petroleum Research Fund of the American Chemical Society and by a grant (AI-06328) from the National Institutes of Health. Grateful apprecia- tion is expressed for this assistance. lTarbell, D. S., et al., J. Am. Chem'. Soc., 83, 3096 (1961). 2Brust, D. P., D. S. Tarbell, S. M. Hecht, E. C. Hayward, and L. D. Colebrook, J. Org. Chem., 31, 2192 (1966) and references therein. 3 Novak, E. R., and D. S. Tarbell, J. Am. Chem. Soc., 89, 73 (1967). 4Friederang, A. W., and D. S. Tarbell, J. Org. Chem., 33, 3797 (1968). 6 (a) Winstein, S., E. L. Allred, R. Heck, and R. Glick, Tetrahedron, 3, 1 (1958); (b) Allred, E. L., and S. Winstein, J. Am. Chem. Soc., 89, 3991 (1967); (c) ibid., p. 3998; (d) ibid., p. 4008; (e) ibid., p. 4012. 6Hazen, J. R., and D. S. Tarbell, Tetrahedron Letters, in press. 7Olah, G. A., and P. E. Peterson, J. Am. Chem. Soc., 90, 4675 (1968) and many earlier papers. 8 (a) Kirrman, A., and L. Wartski, Bull. Soc. Chim. France (1966), p. 3826; (b) Lambert, J. B., and D. H. Johnson, J. Am. Chem. Soc., 90, 1349 (1968); (c) Klages, F., J. E. Gordon, and H. A. Jung, Chem. Ber., 98, 3748 (1965). 9 Cantor, S. E., and D. S. Tarbell, J. Am. Chem. Soc., 86, 2902 (1964). 10Meerwein, H., et al., J. Prakt. Chem., 154, 83 (1939). 11Mihailovic, M. L., et al., Tetrahedron, 23, 215 (1967); Olah, G. A., and J. Sommer, J. Am. Chess. Soc., 90, 927 (1968). 12Olah, G. A. and J. M. Bollinger, J. Am. Chem. Soc., 90, 947 (1968). 13Key steps in these syntheses were the addition of mercuric acetate in methanol or ethanol, followed by reductive removal of the mercuric group by sodium borohydride; cf. Bordwell, F. G., and M. L. Douglass, J. Am. Chem. Soc., 88, 993 (1966); Brown, H. C., and P. Geo- ghegan, Jr., J. Am. Chem. Soc., 89, 1522 (1967). Downloaded by guest on September 28, 2021