Aust. J. Chem., 1977, 30, 1393-6

Halogenated Terpenoids. XV* An Unsaturated Tribromide from the Bromination of a-Pinene

Raymond M. Carman and Gary J. Walker

Department of Chemistry, University of Queensland, St. Lucia, Qld. 4067.

Abstract

Bromination of endo-2,endo-6-dibromobornane, obtained by bromination of a-pinene, yields unsaturated 2,3-endo-6-tribromoborn-2-ene (4).

The bromination of a-pinene (1) is of classical and historical interest.' Wallach2 in 1891 isolated bornyl bromide (2) together with a dibromide which he considered to be a true pinene derivative, but Semmler3a4 showed that the dibromide was a 2,6-dibromobornane. At the commencement of this work, the stereochemistry of the dibromide was unknown, although by analogy with the corresponding dichloride5 it probably had the endo,endo structure (3). Subsequently, the dibromide has been shown6 to give the endo derivative (2) on electrochemical reduction, proving that at least one of the bromines in the dibromide is endo, and this, together with the lack of optical activity and the p.m.r. spectrum which also shows a symmetrical system, proves the structure (3).

Compound (3) shows proton magnetic couplings of J2e,0,3ex, 10-5 Hz and J2exo,3endo 5.5 Hz. The vicinal exo,exo coupling is a little lower than those reported recently for n~rbornanes .~ There is however, which suggests that the presence of bromine may cause some reduction of vicinal exo,exo couplings.

Compounds (2) and (3) are obviously primary products from the addition of hydrogen bromide and bromine respectively to a-pinene (I). Prolonged bromination of bornyl bromide (2) gave a complex unstable mixture from which no pure com- pound could be isolated. However, bromination of dibromide (3) was cleaner and either in the light or in the dark yielded a tribromide, map. 116", in 30-60% yield.

* Part XIV, Aust. J. Chem., 1976, 29, 133.

Simonsen, J. L., and Owen, L. N., 'The Terpenes' Vol. 2, p. 168 (Cambridge University Press 194%.

Wallach, O., Justus Liebigs Ann. Chem., 1891, 264, 1. Semmler, F. W., Ber. Dtsch. Chem. Ges., 1900, 33, 3423. Semmler, I?. W., Ber. Dtsch. Chem. Ges., 1902, 35, 1019. Kwart, H., and Null, G., J. Am. Chem. Soc., 1956, 78, 594. Azizullah, and Grimshaw, J., J. Chem. Soc., Perkin Trans. I , 1973, 425. Marshall, J. L., Walter, S. R., Barfield, M., Marchand, A. P., and Marchand, N. W., Tetrahedron,

1976, 32, 537. Browne, K., and Kagi, R. I., Aust. J. Chem., 1973, 26, 1831.

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Mass spectral and elementary analysis (CloH13Br3) and a. strong infrared absorption at 1592 cm-I indicatedg a double bond, although the compound decolorized neither

bromine nor neutral permanganate, and was resistant to ozonolysis. The n.m.r. spectrum gave three methyl singlets, a one-proton octet at 6 1.70, a two-proton multiplet at 2 .6 and a one-proton quartet at 4.31, accounting for all the protons. No vinyl protons were apparent and so the double bond is tetrasubstituted and the tribromide is assigned structure (4). Decoupling experiments gave all coupling constants of expected magnitude, with J5,x0,6,x0 7.5 Hz (cf. 7.6 Hz in a similar system8), although J,,,,,,,, which is often1' c. 0 Hz in similar bicyclics, was 1.5 Hz.

A feature of the n.m.r. spectrum of (4) was the unsymmetrical quartet at 6 4.31 assigned to H 6. The first and third lines (reading from low field) in tetrachloromethane solution at 100 MHz were split by about 1 Hz, giving a total of six lines. In benzene solution the second and fourth lines were split, while in pyridine no extra splitting was observed and the normal symmetrical (c. 1 : 1 : 1 : 1) quartet was recorded. This extra splitting is ascribed to a 'poor man's double resonance' of the type de- scribed by Anderson, Mallory and Malloryll due to a fortuitous proximity of spin states of protons H 4 and H Sexo, which is manipulable to a degree through relative chemical shift movements upon change of solvent.

The compound (4) decomposed in the gas chromatograph (column 130°, injector 250") to give a peak with the same retention time as vinyl bromide. The base peak in the mass spectrum corresponded to loss of vinyl bromide. Finally, flash pyrolysis of the tribrornide in a closed system with trapping of the evolved gases and redistillation into a gas cell gave a product with an infrared spectrum identical with that from authentic vinyl bromide. The dibromocyclopentadiene (5) expected as the other product from the reversed Diels-Alder reaction of (4) polymerized and was not further studied.

The isolation of an unsaturated compound (4) after an extended time in an excess of brominating (and hydrobrominating) mixture is unusual, but literature precedents are available.12 The mechanism of the conversion of (3) into (4) is not entirely clear. Normally, a bromo compound brominates trans vicinal to the original halogen, either in the dark through elimination of HBr followed by bromination of the olefin, or in the light through hydrogen abstraction and stabilization through a three- membered cyclic bromo radical (Scheme l).13 The products (6) and/or (7) must then undergo a cis elimination of hydrogen bromide giving (8) and/or (9), which on bromination and subsequent dehydrobromination give the observed (4). Steric

Alford, J. R., Grant, D., and McKervey, M. A., J. Chem. Soc. C, 1971, 880. l o Meinwald, J., Meinwald, Y. C. , and Baker, T. N., J. Am. Chem. Soc., 1964, 86, 4074. I' Anderson, J. M., Mallory, F. B., and Mallory, C. W., J. Magn. Reson., 1975, 17, 340. l2 Le Quesne, P. W., Reynolds, M. A., and Beda, S. E., J. Org. Chem., 1975, 40, 142. l 3 Carman, R. M., and Venzke, B. N., Aust. J. Chem., 1973, 26, 571.

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crowding in the saturated tri- and tetra-bromo precursors of (4) appears to provide some driving force for the elimination reactions and precedents for cis elimination of hydrogen bromide in strained systems are available.14

- HBr _3

Scheme 1. All compounds shown are racemic.

Experimental P.m.r. spectra are in tetrachloromethane at 100 MHz. 1.r. spectra are in KBr disc.

Brorninatioit of a-Pinene (I)

Bromine (20 g) in CCI4 (50 ml) was added dropwise with stirring over 1 h to (-)-cc-pinene (1) (17 g) in CCI4 (100 ml) at 0". Decoloration of the bromine was accompanied by evolution of HBr and a rise in temperature. The CC14 was removed and the product distilled (2 mm) to give endo-2-bromobornane (2) (7 g), b.p. 80-95'12 mm, m.p. 90" (from MeOH) (lit.' 90") (Found: C , 55.4; H, 7.9; Br, 36.7. Calc. for CloH,,Br: C, 55.3; H, 7.9; Br, 36.8%). P.m.r. (6 ) : 0.86, 0.90,0.98, threeMe s; 2.52, degenerate 16-line system showing 10 discrete lines (1 : 2 : 1 : 1 : 3 : 3 : 1 : 1 : 2 : 1; Ll -LIO = 32 Hz), H3exo; 4.24, octet, H2, L1-L8 = 18 HZ; with J2,3exo 11 HZ, J z , ~ ~ ~ ~ ~ ~ ' ~ H z , J z , ~ ~ ~ ~ ~ . ~ H z , J3,,, 14Hz, J 3 e x o , 4 ~ . 4 H ~ , a n d J3e,0,5,x,c. 3Hz. 1.r.: 2940,2882, 1388, 1373, 902, 784, 660cm-I. m/e 216, 218 (5%, M); 215, 217 (35, M-H); 137 (100, M-Br); 136 (35, M-HBr), 121 (27), 95 (42), 93 (52), 91 (20), 81 (80). The compound was identical with endo-2-bromobornane synthesized by addition of HBr to a-pinene in CHCI, at 0".

The fraction of b.p. 110-125"/2 mm gave endo-2,endo-6-dibromobornane (3) (5 g), m.p. 171' (from EtOH) (lit.' 169-170") (Found: C, 40.5; H, 5.5; Br, 54.1. Cak. for C10H16Br2: C, 40.6; H, 5.5; Br, 54.0%). P.m.r. (6): 1.00, s, gem-Me2; 1.03, s, 1-Me; 1.85, 3H, m (distinguishable as a 2H q, H3endo and HSendo, overlapping a t, H4); 2.64, degenerate octet, H3exo and H Sexo; 4.35, q, H 2 and H 6 , with J2,3eso 5 J5exo,6 = 10.5 HZ, J ~ , j e n d o J~endo,~ = 5.5 HZ, J3,3r = J5,5p = 12 HZ, J3exo,4 = J4,5exo = 5 HZ and J3endo,4 C. 0 HZ. 1.r.: 2988, 2960, 1398, 1379, 817, 683, 633,590 cm-l. Optically inactive. mle 294 (9 %), 296 (18), 298 (9, M); 215,217 (16, M - Br); 214, 216 (5, M-HBr); 159, 161 (21); 136 (18, M-Br2), 135 (loo), 107 (14), 93 (88), 91 (19).

l4 Fujikura, Y., Inamoto, Y., Takaishi, N., Ikeda, H., and Aigami, K., Chem. Lett. Jpn, 1975, 1203.

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The dibromide (3) (5 g) in EtOH (50 ml) was refluxed with zinc powder (1.5 g) for 20 h to give t r icy~lene,~* '~ identical with an authentic sample synthesized from camphor hydrazone.16

Bromine (7 g) was added to dibromobornane (3) (10 g) in CC14 (60 ml) and the flask was stored in the dark. The stopper was removed occasionally to release HBr gas. After 3 weeks, the bromine colour had diminished and the solvent was removed to give a yellow oil. Chromatography over silica gel gave, in early hexane fractions, the vacemate (4) (5 g), map. 116" (from MeOH) (Found: C, 32.1 ; H, 3.5; Br, 63.9. C10H13Br3 requires C, 32.2; H, 3.5; Br, 64.3 %). P.1n.r. (6): 0.87, 1 .O3, 1 .l5, three Me s; 1.70, octet, H5endo; 2.60, m, H 4 and H5exo; 4.31, distorted q (see text), H 6. Decoupling gave J6,5,,o 7.5 Hz; J6 ,Sendo 3.5 Hz; J5,5, 13.5 Hz; J5CX0,4 C. 4 HZ and J5endo,4 1 .5 HZ. Irradiation at 6 2.6 removed the distortion of the H 6 quartet, giving a normal doublet with J 3 . 5 Hz. 1.r.: 2974, 2964, 2925, 1592, 1393, 1379, 1370, 1286, 1270, 1246, 1212, 1068, 1012, 970, 848, 752, 711, 650, 614 cm-'. mle 370 (4%), 372 (13), 374 (13), 376 (4, M); 264 (50), 266 (loo), 268 (50, M- CH2CHBr); 185, 187 (55, M-CH2CHBr2), 106 (32), 91 (13).

Gas chromatography of (4) (5 ft Porapak Q at 130' with injector at 250') gave a single peak with the retention time of vinyl bromide. Tribromide (4) was pyrolysed during passage in nitrogen gas down an evacuated glass tube held at 500°, and the condensate (- 195") was redistilled into an infrared gas cell to give material with an i.r. spectrum superimposable upon that of vinyl bromide.

Compound (4) in acetic acid at 17' was completely unchanged after a 2-h exposure to a stream of ozone.

Acknowledgments

The flash pyrolysis experiment was performed by Dr R. A. Marty. Lyn Lambert provided technical assistance with n.m.r. spectra.

Manuscript received 19 November 1976

Godlewski, J., and Wagner, G., Chem. Zentvalbl., 1897, 1, 1055. l 6 Meerwein, H., and Emster, K. van, Bev. Dtsch. Chem. Ges., 1920, 53, 1815.