ipso Nitration. XXVIP.~ The crystal structure and stereochemistry of 3-bromo-6-methyl-6- nitrocyclohexa-2,4-dienyl acetate, 5-bromo-2-methyl-4-nitrocyclohexa-2,4-dieny acetate,

and 3-bromo-6-methyl-6-niltrocyclohexa-294-deny chloride

GORDON W. BUSIJNELL, ALFRED FISCIIER, GEORGE N. HENDERSON, A N D SUMIT RAY MAHASAY Department of Chemist?, Universih of Victoria, Victoria, B. C., Cnrzada V8W 2Y2

Received January 9, 1986

GORDON W. BUSHNELL, ALFRED FISCHER, GEORGE N. HENDERSON, and SUMIT RAY MAHASAY. Can. J . Chem. 64, 2382 (1986).

The adduct obtained on nitration of 4-bromotoluene in acetic anhydride is (Z)-3-bromo-6-n1ethyl-6-nitrocyclohexa-2,4-dienyl acetate. Its stereoselective rearrangement product, obtained on thermolysis in the presence of p-cresol, is (2)-5-bromo-2- methyl-6-nitrocyclohexa-2,4-dienyl acetate. Reaction with hydrogen chloride in ether is also stereospecific and gives (2)-3- bromo-6-methyl-6-nitrocyclohexa-2,4-dienyl chloride. The crystal structures of these compounds are reported.

GORDON W. BUSHNELL, ALFRED FISCHER, GEORGE N. HENDERSON et SUMIT RAY MAHASAY. Can. J. Chem. 64, 2382 (1986).

L'adduit obtenu par nitration du bromo-4 toluene dans I'anhydride acetique est l'acetate de bromo-3 methyl-6 nitro-6 cyclohexadii.ne-2,4 yle-(Z). Son produit de transposition stCreosClective, obtenu par thermolyse en presence de p-crCsol, est llacCtate de bromo-5 methyl-2 nitro-6 cyclohexadiPne-2,4 yle-(Z). Sa rkaction avec le chlorure d'hydrogene dans I'Cther est aussi stkreospbcifique et elle conduit au chlorure de bromo-3 methyl-6 nitro-6 cyclohexadiene-2,4 yle. On a determine les structures cristallines de ces composCs.

[Traduit par la revue]

Nitration in acetic anhydride of aromatic compounds in which a substituted position is of a comparable or greater degree of activation than an unsubstituted position often leads to the formation of nitronium acetate adducts (1, 2) in which the nitro group is attached to the activated substituted (ipso) position. Most commonly the 1,4 adduct is formed, normally as a pair of diastereomers. However, a 1,2 adduct is obtained when p-tert-butyltoluene (3), 2-methyl-2-(2'-methy1phenoxy)propa- noic acid (4), 4-methoxy- or a 4-halotoluene (5) is the substrate. Only one diastereomer of the 1,2 adduct is obtained. The 1,2 adducts undergo a 1,5 thermal rearrangement of the nitro group (6). Both stereospecific and stereorandom rearrangement processes have been observed (6). In the case of the diene from 4- bromotoluene, 3- bromo-6-methyl-6-nitrocyclohexa- 2,4-dienyl acetate (Pa), the stereorandom rearrangement is made stereospecific when p-cresol is added. The p-cresol is believed to suppress a radical rearrangement process (6: 7). Two stereochemical questions arise from these observations. (i) What is the stereochemistry (syn or anti) of the nitration addition reaction? (ii) What is the stereochemistry of the stereospecific rearrangement process, i .e . , is the nitro group rearrangement suprafacial or antarafacial? In the present work we report the results of single crystal X-ray diffraction structure determinations on l a and its stereospecific rearrangement product, 2a, a diastereomer of 5-bromo-2-methyl-6-nitrocyclo- hexa-2,4-dienyl acetate, which provide the answers to these questions. We have also determined the structure of the single diastereomer of 3 - bromo-6-methyl- 6-nitrocyclohexa- 2,4-dienyl chloride, 3a , obtained on reaction of l a with hydrogen chloride.

Results and discussi~n Diene l a was formed in 64% yield as a single diastereomer on

the nitration of 4-bromotoluene in acetic anhydride containing trifluoroacetic anhydride and was isolated by crystallization of the crude reaction product. Diene l a isomerized and partially aromatized when a solution in chloroform was allowed to stand at ambient temperature. The isomerization was carried out on a preparative scale by heating a solution of l a in benzene at 74°C for 1 h when a mixture of 2a (357~1, its diastereomer 2b (35%), unreacted l a (26%), and its diastereomer l b (4%), together with a trace of 5-bromo-2-methylphenyl acetate was obtained. The diastereomers 2a and 2b were isolated from the reaction mixture by fractional crystallization. Diene l b was obtained from the mother liquor by treatment with aqueous ammonia to selectively aromatize the rearranged dienes 2a and 2b, followed by chromatography. The secondary nitro group makes the elimination of acetic acid from the rearranged dienes extremely facile, whereas the tertiary nitrodienes are unreactive under the same conditions. Isomerization of diene l a in the presence of p-cresol gave only one product diene, 2u. The dienyl chloride 3a was obtained in 82% isolated yield when hydrogen chloride was bubbled through a solution of l a in ether at -78°C and the mixture then worked up.

The structures of l a , 2a, and 3a were determined by X-ray crystallography (Table 1). Dienes l b and 2b decomposed too rapidly on irradiation for structure elucidation. Table 2 contains fractional atomic coordinates and isotropic temperature para- meters for l a , 2a , and 3a. Table 3 contains bond lengths, which agree well with the literature values for similar bonds as compiled in the Chemical Society's special publications (8). Table 4 contains bond angles and shows sp3 and sp2 hybridised atoms subtending angles close to 109.5" and 12OU, respectively. Tables deposited2 contain anisotropic temperature parameters for l a and 3a (Tables S1, S2), intermolecular distances (Tables S3, S4, S5), mean planes, torsion angles, and Newman

l a 20

' For part XXVI, see ref. 2.

2Copies may be purchased from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont. , Canada KIA OS2.

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BUSHNELL ET AL.

TABLE 1. Crystal data for l a , 2a, and 3a

Crystal system Orthorhombic Monoclinic Orthorhombic Space group Pbca (No. 61) P21/n (No. 14) P c ~ 2 ~ (No. 29) 0 (A) 27.445(8) 7.386(3) 11.901(5) b 9.683(3) 7.618(2) 6.758(2) c 8.335(2) 19.498(6) 11.495(4) I3 ($g) - 92.91(4) -

v (A3) 2215(1) 1095.7(6) 924.5(6) Formula C9HI0NO4Br C9H 10N04Br C7H7NO2C1Br Mol. wt. 276.09 276.09 252.5 Z 8 4 4 Drneask cmP3) - 1.67 1.78 Dcalcd 1.656 1.674 1.814 Mounting axis c a a Standard reflections 18 00,080.006 060,006, 400 200, 006, 020 Measured range (20) 0-40" 0-30" 0-50" No. of steps 160 200 160 (0.01" in 20. 0.25 s) Background count (s) 40 50 40 Fraction of reflections

with F / u ( F ) > 3 0.82 0.93 0.89 p,(cm-') (A = 0.71069 A ) 39.25 39.67 49.40 Crystal shape. Perpendicular distance,

origin to faces: 0.081, 0.300, 0.231, 0.180, 0.237. 0.080, ?(loo), ?(010), ?(001) 0.473 mm 0.162 mm 0.189 mm Transmission 0.10-0.53 - 0.17-0.47 Convergence d/u(max) 0.11 0.007 0.004 R 0.0763 0.1033 0.0551 R ,< 0.0941 0.1029 0.0601 Difference map maximum (e AP3) 0.83 1.17 0.58 No. of observations 1033 597 862 No. of parameters 136 61 108

projections (Tables S6, S7, SS), and structure factors (Tables S9, S 10, S 1 1). The intermolecular distances are all greater than 3.1 P\, demonstrating the molecular nature of the crystals. When the three structures are compared there are many similarities. The nitro groups plus the adjacent C atom are planar as are the acetate groups. The h2xadienyl rings are non-planar, with the saturated atoms 0.24 A (ave.) from the weighted least squares (w.1.s.) planes. Each saturated ring atom is on the opposite side of the plane relative to the nearest ethylene moiety, and t h ~ unsaturated ring carbon atoms are generally close to 0.10 A from the w.1.s. plane. Birch, Hinde, and Radom attributed the non-planarity of the parent molecule, 1,3-cyclohexadiene, to angle strain at the saturated carbon atoms and steric interaction between eclipsed methylene hydrogen atoms outweighing the stabilizing effect of conjugation (9). Computed values for the C=C-C=C torsion angle in 1,3-cyclohexadiene range from 8 to 20°C (9-14) indicating a shallow minimum in the energy function (1 1). Experinlentally obtained values are consistently close to 17.5' (15-18). Our experimental values for the substituted cyclohexadienes are lower, 9.9(13)", 12(3)', and 13.2( 15)', respectively.

The (2) configuration of the a isomers is evident from the cis disposition of nitro and acetate in l a and 2u and the cis arrangement of nitro and chlorine in 3a (Figs. 1-3). The structures of l b and 2b follow from the fact that these are the diastereomers of the a isomers, as is indicated by the close similarity of the nmr spectra of l a with those of l b and of 2a with those of 2b. There are two possible conformations of each diastereomer and, in the cases of Pa and 30. that in which the adjacent methyl and acetate or chlorine groups are (pseudo) trans diaxial and the nitro group and neighbouring hydrogen are

therefore equatorial is preferred. This preference is reasonable in that the conformer with the smaller substituent (hydrogen) gauche to both methyl and nitro and the larger substituent (acetate or chlorine) gauche only to nitro, would exhibit less torsional strain than the other conformer in which the larger substituent is gauche to both neighbouring substituents. The preferred conformer of 2a is that which has axial nitro and equatorial acetate rather than vice versa. It is likely that this preference reflects a greater electronic repulsion between the neighbouring C-Br and C-NO2 dipoles than that between the C-OAc and C-CH3, the consequence being that the bromine and the nitro group are forced to be further apart (i.e. nitro axial) than the methyl and acetate or chlorine (acetate or chlorine equatorial). Interaction between the neighbouring dipoles at sp2 and sp3 carbons ~hou ld be minimized when the wbstituent at the tetrahedral carbon is axial.

On nitration of 4-bromotoluene formation of the cis 1,2 adduct l a is completely dominant, none of the trans diastereo- mer l b being detected in the product mixture. Generally the formation of adducts is under kinetic control (19) as is evident in the present instance from the fact that in the thermal rearrangement, when equilibrium conditions are approached, both diastereomers l a and l b are present together. Thus we must look to the transition state to account for the stereoselec- tivity. Three possible explanations come to mind. Ridd has suggested that in acetic anhydride the nitronium ion is com- plexed with acetic acid and that it only becomes free in the encounter pair (20). If the reaction with the substrate occurs immediately after the nitronium ion is liberated, and the further reaction of the carbocation occurs before the solvation shell has had time to relax, then the acetic acid molecule which was

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2384 C A N . J . C H E M . VOL. 64, 1986

TABLE 2. Fractional atomic coordinates and temperature parameters ( a ) l a U

Atom x / a ~ / b Z / c Uc,

- - ~-

"Coordinates X lo5 for Br and X 10' otherwise; temperature parameters X 104 for Br and X 10' otherwise.

( b ) 2aa

Atom

"Coordinates X lo4. temperature parameters X lo4 for Br and X 10' othem ise

( c ) 3aU -

Atom X / Q ~ / b z / L. Ue,

"Coordinates x104. The Br atom z coordinate was fixed: temperature parameters X lo4 for Br and C1. X 10' otherwise; estimated standard deviations are given in parentheses; U,, is the equivalent isotropic temperature parameter: U,, = (1/3)C,C,U,,n~a~(a, .a,) : T = exp -(8n2~, , ,s in ' @ / A 2 ) .

TABLE 3. Interatomic distances (A)"

Atoms Distance Atoms Distance

( c ) 3a C(3)-Br 1.947( 8 ) C(1)-CI 1.828( 9 ) 0 (1 ) -N( l ) 1.207(10) O(2)-N(1) 1.207(10) C(6)-N(l ) 1.540(11) C(7)-c(6) 1.530(13) C(3)-C(2) 1.325(13) C(5)-C(4) 1 .344(14) C( l ) - c (2 ) 1.517(12) C(6)-C(5) 1.480(13) C(4)-C(3) 1.457(13) c ( l ) -C(6) 1.527(!2)

"Estimated standard deviations are given in parentheses.

attached to the nitronium ion will be favourably placed to add to the 2-position in a cis manner. The second explanation proposes that the nitro group in the carbocation favours cis addition of the acetic acid molecule through hydrogen bonding of the latter to the nitro oxygen. This sets up the carbonyl group in a favourable position to add cis to the nitro group. The third explanation proposes that the carbocation is stabilized through carbon- carbon hyperconjugation involving the ring-methyl bond. This would result in the ring tending towards being planar with the carbon-nitrogen bond in the plane of the ring and the ring- methyl bond perpendicular to the ring. Approach of the acetic acid molecule would then be less hindered from the nitro face than from the methyl face.

Diene 2 is thermodynamically more stable than diene f (in the rearrangement reaction, 1 forms 2 to an extent of more than 50%) yet no diene 2 is obtained from the nitration reaction. Thus, even though some (8%) attack of nitronium ion occurs at the position ortho to the bromine, none of the resulting nitrocyclohexadienyl cation is trapped by acetate, all of it undergoing deprotonation to the 4-bromo-3-nitrotoluene. Like- wise, none of the nitrocyclohexadienyl cation resulting from addition of nitronium ion ortho to methyl is trapped. Trapping of secondary nitrocyclohexadienyl cations is extremely rare (21, 22). The deprotonation reaction with its attendant gain in resonance energy is almost always greatly favoured over addition of a nucleophile. It is thus evident that the rearrange- ment cannot involve a cationic intermediate, rather stereo- random radical and stereospecific sigmatropic pathways are involved (4, 6, 7). This is of course also evident from the non-polar conditions under which the rearrangement is carried out. The addition of p-cresol suppresses the radical reaction and allows the observation of the sigmatropic process. Since (Z)-I

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BUSHNEL

TABLE 4. Bond anglesa (a) l a

Atoms Angle Atoms Angle

C(8)-O(3)-C(l) 116(2) C(7)-C(2)-C(l) 118(2) ( 2 ) - 1 - ( 1 ) 125(3) C(2)-C(1)-0(3) 108(2) 6 - 1 - 0 1 ) 123(2) C(6)-C(1)-O(3) 1 lO(2) C(6)-N(1)-O(2) 113(2) C(6)-C(1)-C(2) 115(2) C(4)-C(5)-Br 122(2) C(5)-C(6)-N(l) 108(2) C(6)-C(5)-Br 116(2) 1 - C 6 - 1 ) 1 lO(2) C(6)-C(5)-C(4) 122(2) C(1)-C(6)-C(5) 108(2) C(3)-C(4)-C(5) 121(2) O(4)-C(8)-0(3) 121 (2) C(2)-C(3)-C(4) 120(2) C(9)-C(8)-0(3) 11 l(2) C(1)-C(2)-C(3) 11 7(2) C(9)-C(8)-0(4) 128(3) C(7)-C(2)-C(3) 124(2)

( c ) 30

Atoms Angle Atoms Angle

(d) Endocyclic torsion angles (deg)

Torsion angle

Bonds l a 2a 3n

"Estimated standard deviations are given in parentheses.

gives (2)-2 this process is suprafacial as would be required if the new C-NO2 bond is formed synchronously with the fission of the original C-NO2 bond.

Reaction of acetate l a with hydrogen chloride gives the dienyl chloride with retention of configuration. This reaction would be expected to involve a carbocation intermediate or, at least, an incipient carbocation intermediate. A S N i type mechanism with a six-membered cyclic transition state can be

FIG. 1 . An ORTEP drawing of the molecular structure of l a showing the 25% probability ellipsoids.

FIG. 2. An ORTEP drawing of the molecular structure of 2a showing the 25% probability spheres.

envisaged in which hydrogen chloride, hydrogen bonded to carbonyl oxygen, is situated with the chlorine conveniently located to displace the acetate function. Alternatively, the chlorine could displace the acetate in an S N i f reaction by adding to the 5-position through a cyclic transition state. Both mechanisms would lead to the same diastereomer although, in principle, they can be distinguished since, if a single enantiomer of the reactant were used, different enantiomers of the product would be formed from the two pathways. If C-0 bond breaking runs slightly ahead of C-C 1 bond formation both the incipient carbocation intermediate and a cyclic transition state, accounting for retention of configuration, can be accommodated.

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CAN. J . CHEM. VOL. 64. 1986

separated and the residue was washed with ether (300 cm3). The combined ether solution was washed with cold water (4 X 300 cm3) and then dried. Removal of ether on the rotovapor at 15OC yielded a reddish-brown oil (25.5 g). The 'H nmr spectrum of the crude mixture gave the composition as (Z)-3- bromo-6-methyl-6-nitrocyclohexa- 2,4-dienyl acetate ( l a ) ( 6 4 8 ) , 4-bromo-3-nitrotoluene (8%) and 4-bromo-2-nitrotoluene (28% ) . Crystallization from ether-pentane mixture at -20°C afforded crude diene l a (9 g) as pale yellow crystals which, after further recrystallization, had mp 48-49°C: ir (KBr): 1745 and 1230 (OCOCH3), 1555 and 1365 (NO2) cmp' ; uv (CH2C12): 266.4 nm (350 m2 m o l l ) ; ' H nmr (250 MHz, CDCI,) 6: 1.77 (s. 3, CH3), 1.98 (s, 3, OCOCH?), 5.45 (dd, I . H(l)), 6.13 (dd, 1, H(4)), 6.37 (ddd, I , H(2)), 6.45 (ddd, 1, H(5)) ppm. J12 = 6.15, J l s = 1.76, Jz4 = 1.80. J 2 5 = 0.55. J45 = 10.23 Hz: 13C nmr (CDC13.

c2 62.9 MHz) 6,: 20.6 (OCOCH,), 23.1 (CH,), 70.4 (C(1)). 86.8 (C(6)). 122.0 (C(2)), 122.6 (C(3)). 127.8 (C(4)). 128.5 (C(5)), 169.0 (OCOCH3) ppm. Anal. calcd. for C9HI0NO4Br: C 39.15. H 3.65, N 5.07; found: C 39.36, H 3.52, N 5.04.

Isomerization of (Z) -3- bromo-6-methyl-6-nitrocyclohexn-2,4 -dien?;l acetate ( 1 a)

A solution of diene l a (10 g. 36 mmol) in benzene (20 cm3) was heated in a water bath at 74°C for 1 h. Removal of the benzene on the rotovapor at 25°C yielded a reddish-brown oil. The 'H nmr spectrum of

3. An ORTEP drawing of the molecular structure of 3a the oil indicated the presence of l a (26%), its diastereomer l b (4%), showing the 25% probability ellipsoids. (Z)- and (E)-5-bromo-2-methyl-6-nitrocyclohexa-2.4-dieny acetate

(2a) and (2b) (70%). with traces of 4-bromo-3-nitrotoluene and Ex~erimental 5-bromo-2-methylphenyl acetate. Fractional crystallization from 1: 1

Melting points are uncorrected and were determined on a Buchi SMP-20 melting point apparatus. Infrared spectra calibrated with polystyrene were recorded on a Perkin Elmer 283 spectrometer. Observations were made on potassium bromide discs for solids and on thin films between sodium chloride plates for liquids. Proton nuclear magnetic resonance spectra were recorded on Perkin Elmer R-32 (90 MHz) or Bruker WM 250 (250 MHz) spectrometers. The nmr spectra of the nitration reaction mixture was recorded with the acetic anhydride peak at 2.15 ppm as the lock signal. The substitution reaction with hydrogen chloride was followed using the ether peak at 1.85 ppm as the lock signal. For all other solutions, tetramethylsilane (90 MHz) or the solvent deuterium signal (250 MHz) was used as the lock signal. Carbon-13 nuclear magnetic resonance spectra were recorded on the Bruker WM 250 (62.9 MHz) spectrometer using solutions in CDC13 with TMS as the internal standard. Ultraviolet spectra were recorded on a Beckrnan DU-8 spectrophotometer. Elemental analyses were carried out by Canadian Microanalytical Service Ltd., Vancouver, B.C.

4-Bromotoluene was from J . T. Baker, acetic anhydride was certified ACS from Fisher, trifluoroacetic anhydride was Aldrich gold label. Fuming nitric acid (Fisher, 300 cm3) was purified by distilling from urea (10 g) and sulfuric acid (500 cm3) at < 35°C and was stored at -25°C.

Solvents for chromatography including pentane (reagent, Fisher). ether (Fisher), and petroleum ether (reagent, Fisher) were dried over sodium and distilled before use. Silica gel used for chromatography was 60-200 mesh, Davison Commercial grade H.

Nitration of 4-bromotoluerre A nitrating mixture was prepared by the careful addition of acetic

anhydride (51.1 g, 0 .5 mol) with stirring to freshly distilled nitric acid (12.6 g, 0.2 mol) at -78°C. After the completion of addition, the mixture was warmed to P C , stirred for 15 min at O°C and cooled to -40°C. Trifluoroacetic anhydride (21.05 g, 0.1 mol) was then slowly added to the mixture at -40°C. A solution of 4-bromotoluene ( 1 7.1 g. 0.1 mol) in acetic anhydride (10.2 g, 0.1 mol) was addeddropwise with stirring over 30 min to the nitrating mixture at -40°C. The reaction mixture was then stirred for an additional 90 min at -40°C and then cooled to -78'C and poured into ether (50 cm3) at -78°C. Ammonium hydroxide (450 cm3, 28%) was added in portions to the stirred mixture. After the addition was complete, stirring was continued for 1 h while the mixture warmed to room temperature. The ether layer was

ether-pentane at --~Pc gave diene 2a (2.56 g) in the first two crops. The third crop (0.96 g) contained a mixture of 2n and 2b (1 : 1). Further crystallization from 1:2 ether-pentane solution yielded a mixture of 2b and 2a (95 : 5 ) as the fourth crop (700 mg). Finally. from 1 : 3 ether - petroleum ether. diene l a was obtained as the fifth crop (600 mg).

Recrystallization of the first crop (300 mg) from 1:l ether - petroleum ether gave pure diene 2a as pale yellow crytals (240 mg), mp 93-95°C; ir (KBr): 1735 and 1230 (OCOCH,), 1560 and 1370 (NOz) cmp' ; uv (CH2C12): 279.75 nm (721 m2 mol-I): ' H nmr (250 MHz, CDCI3) 6: 1.82 (br dd, 3, CHI), 2.17 (s, 3. OCOCH,), 5.35 (d, 1, H(6)), 5.79 (ddq, 1, H(3)), 5.91 (ddq, 1. H(1)). 6.69 (d, 1, H(4)) ppm. J13 = 2.65, J lh = 8.70, JI.z.hfe = 1.00. Ji4 = 6.25, J3.2.Me = 1.63 Hz; I3c nmr (62.9 MHz, CDC13) 6,: 18.0 (CH3), 20.5 (OCOCH,), 70.9 (C(1)), 87.5 (C(6)), 109.5 (C(5)), 119.5 (C(3)), 134.1 (C(4)). 135.1 (C(2)). 169.8 (OCOCH,) ppm. Anal. calcd. for C9HION04Br: C 39.15, H 3.65. N 5.07; found: C 39.08, H 3.67. N 5.07.

Recr~;stallization of the fourth crop (500 mg) from 1:l ether - petroleun~ ether at -20°C gave pure diene 26 as pale yellow crystals (420 mg), mp 63-64°C; ir (KBr) 1745 and I210 (0COCH3), 1558 and 1360 (NO?) cm-I; uv (CH2C12): 279.75 nm (820 m2 mol-I): ' H nmr (250MHz. CDCI,) 6: 1.83 (d, 3, CH3). 2.15 (s, 3, OCOCH,), 5.24 (d. 1, H(6)). 5.86 (dq, 1 , H(3)). 5.93 (d. 1, H(1)). 6.66 (d. 1 , H(4)) ppm, Jlh = 3.47, Jj4 = 6.32, J3,2.hlr = 1.04 Hz; nmr (62.9 MHz, CDCI,) 8,: 19.9 (CH,), 20.7 (OCOCH,), 70.8 (C(1)). 90.1 (C(6)), 109.1 (US)), 122.4 (C(3)), 132.2 (C(4)), 132.5 (C(2)), 169.8 (OCOCH3)ppm. Anal. calcd. for C9HI0NO4Br: C 39.15, H3.65, N5.07;found: C39 .3 . H3 .54 , N5.18.

The 'H nmr spectrum of the mother liquor at this stage showed the presence of dienes 2a and 2b (37701, Pa (37%), 1 b (5%), 5-bromo- 2-methylphenyl acetate and 4-bromo-3-nitrotoluene (21 '3%). In order to isolate diene l b , the dienes 2 a and 2b were selectively aromatized. A solution of the reaction mixture (2 g) in ether (10 cm3) was stirred in an ice bath with ammonium hydroxide (5 cm3, 28%) for 15 min. The mixture was diluted with ether (80 cm3). washed with cold brine (4 x 20 cm3), and the solution dried over anhydrous magnesium sulfate. The 'H nmr of the residual oil, obtained after removal of solvent at 15"C, revealed the presence of 4-bromo-3-nitrotoluene (37%), 5-bromo-2-methylphenyl acetate (21 %), diene In (37%). and diene I b (5%). The mixture (1 .3 g) was separated by column chromatography on silica gel (180g) using a mixture of ether -

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petroleum ether as eluent at -40°C. Initial fractions eluted with 3% ether gave 4-bromo-3-nitrotoluene. Mixtures of phenyl acetate, bromonitrotoluene, and diene l a were eluted next with 6% ether. Further elution with 10% ether gave mixtures containing diene l b (95%) and 4-bromo-3-nitrotoluene (5%). Diene l b did not crystallize from ether - petroleum ether and was characterized in solution. It had '~nmr(250MHz,CDCI , )6 : 1.74(s, 3, CH3), 2.13 (s, 3 ,0COCH3) , 5.94 (m. 1, H(2)), 6.23 (m, 3, H(1). H(4) and H(5)) ppm; nmr (62.9MHz, CDCl,, -15°C) 6,: 18.2 (CH3), 19.6 (OCOCH,), 70.8 (C(l)), 87.0 (C(6)), 117.1 (C(3)), 126.5 (C(2)), 127.2 (C(4)), 129.1 (C(5)), 169.5 (OCOCH3).

In some small scale reactions, diene l a was isomerized by heating a solution in CDC13 in an nrnr tube in a thermostated water bath. p-Cresol was added to some reactions as an inhibitor. Reactions were followed by 'H nmr at 90 MHz. Typical results were as follows: after 1 h at 60°C in the absence of inhibitor the composition was l a (72%). l b (trace), 2a (19'%), 2b (9%); after 15 h in the presence of p-cresol (0.25 mol proportion) the composition was l a (32%) and 2 a (68%) and no 2b was detected at any prior stage.

iZ)-3-Bromo-6-methyl-6-nitrocyclohe.xa-2,4-dienyl chloride (3a) Hydrogen chloride was bubbled through a solution of l a (275 mg,

1 mmol) in ether (5 cm3) at -78"C, for 15 min, and the mixture was then stirred for 45 min while the bath was allowed to warm to -40°C. The 'H nmr spectrum of the mixture at this stage showed the presence of a new diene as the only product. The solution was diluted with ether (20 cm3) at -78°C and neutralized by the addition of cxccss aqueous ammonia. The ether was separated and the aqueous layer extracted with ether. The combined ethereal extracts were dried (MgSO,) and the ether removed on the rotavapor at lS°C to give chloride 3 a (206 mg, 8 2 9 isolated yield). Crystallization from ether - petroleum ether at -20°C gave pale yellow crystals, mp 87°C: uv (CH,CI,): 273 nm (351 m' mol-I): ir: 1540 (NO2), 740 (C-CI) c m ' : ' H nmr (250 MHz, CDC13) 6 : 1.81 (s, 3, CH3), 4.89 (dd, 1, H(1)). 6.20 (dd, 1, H(4)). 6.36 (dd, 1, H(2)), 6.54 (dd, i . H(5)) ppm. J12 = 6.47, JIS = 1.72, J2, = 1.81, J45 = 10.30 Hz; ';c nmr (62.9 MHz. CDCI,) 6,: 24.2 (CH3), 58.6 (C(1)). 89.1 (C(6)). 121.2 (C(3)), 124.2 (C(2)), 128.1 (C(4)), 129.1 (C(5)) ppm. Anal. calcd. for C7H7N02CIBr: C 33.29, H 2.79, N 5.55; found: C 33.39, H 2.67, N5.51.

X-ray drffracrion studies Compounds l a , 2a , and 3 a were examined on Weissenberg and

precession cameras to determine the symmetry and approximate cell dimensions, and subsequently transferred to a Picker 4-circle diffrac- tometer automated with a PDP11/10 computer and using Zr filtered Mo radiation. The crystal data are given in Table 1. The symmetry positions for 2a were x, J, z ; -r , -y. - z ; (2 + x) , (4 - y) , (a + z); and (2 - x), (2 + y), (2 - z ) . The cell dimensions were refined by least squares using pairs (19, 13, and 8, respectively) of 28 measurements obtained by an automatic centering routine. The asymmetric unit was one molecule in each case. In measuring the intensities 8 / 2 8 scans were used. Three standard reflections preceded each batch of 50 measurements and the sum of their intensities was used to correct for crystal decomposition. The class of compounds under study was soft and low melting. l a , 2 a , and 3 a were selected as the best and most durable single crystals. but melting terminated the measurements on 2a. Lp corrections were done. Absorption corrections were applied by numerical integration for compounds l a and 3 a using Gaussian grids (4 x 10 X 12 for l a , 12 X 8 X 8 for 3 u ) . No reflections were omitted from the data sets.

Strut ture s o l ~ ~ r ~ o n and refinemer~t SHELX (23). MULTAN (24), and ORTEP (25) and local programs

were used The structures were solved using direct methods and refined uslng electron-density maps and the method of least squares mininxsing 2 15 I IF, I - F, I I ' The atomlc qcatterlng factor5 u ere those included in the SHELX progrdm (23. 26) The we~ght~ng schemesemployed were 11 = I / (u ' (F ) + 0 301 F') for l a a n d 3 a . and unit weights for 2a No speclal posltlons were occupied For 3a. the z

coordinate of the Br atom was fixed, since there was no reference point in the z direction provided by the symmetry elements. For compounds l a and 3 a anisotropic thermal parameters were used for all atoms, but for compound 2a isotropic temperature factors were employed. Hydrogen atoms were not found. Final difference maps were calculated as a further check and the maxima, given in Table 1. indicate no chemical mistakes.

Acknowledgements We thank Mrs. Katherine A. Beveridge for technical assis-

tance, the Natural Sciences and Engineering Research Council of Canada for financial support, and the University of Victoria for the award of a University of Victoria Fellowship (to S.R.M.) .

1. D. J . BLACKSTOCK, A. FISCHER, K. E. RICHARDS, J. VAUGHAN. and G. J . WRIGHT. Chem. Commun. 641 (1970).

2. A. FISCHER. G. N. HENDERSON, and L. M. IYER. Can. J . Chem. 63, 2390 (1985).

3. A. FISCHER and R. RODERER. Can. J . Chem. 54, 3978 (1976). 4. G. S . BAPAT. Ph.D. Dissertation, University of Victoria, Vic-

toria, B.C. 1983. 5. A. FISCHER, D. L. FYLES, and G. N. HENDERSON. J . Chcm. Soc.

Chem. Commun. 513 (1980). 6. G. S . BAPAT, A. FISCHER, G. N . HENDERSON, and S . RAY

MAHASAY. J . Chem. Soc. Chem. Commun. 1 19 (1983). 7. S. RAY MAHASAY. Ph.D. Dissertation, University of Victoria,

Victoria, B.C. 1984. 8. L. E. SUTTON (Editor). Tables of interatomic distances and

configurations in molecules and ions. The Chemical Society, London, 1958; Tables of interatomic distances in molecules and ions supplement 1956- 1959. The Chemical Society, London. 1965.

9 . A. J . BIRCH. A. L. HINDE, and L. RADOM. J. Am. Chem. Soc. 103, 284 (1983).

10. N. L. ALLINGER and J . C. TAI. J. Am. Chem. Soc. 99, 4256 (1977).

11. A. WARSHEL and M. KARPLUS. J. Am. Chem. Soc. 94, 5612 (1972).

12. G. FAVINI, F. ZUCCARELLO, andG. BUEMI. J. Mol. Struct. 3,385 (1963).

13. A. KOMORNICKI and J. W. MCIVER. J . Am. Chem. Soc. 96,5798 (1974).

14. N. L. ALLINGER and J . T. SPRAGUE. J. Am. Chem. Soc. 95,3893 (1973).

15. S. S. BUTCHER. J . Chem. Phys. 42, 1830 (1965). 16. G. DALLINGA and L. H. TONEMAN. J . Mol. Struct. 1 , 11 (1967). 17. M. TRAETTEBERG. Acta Chem. Scand. 22, 2305 (1968). 18. H. OBERHAMMER and S. BAUER. J. Am. Chem. Soc. 91, 10

(1969). 19. A. FISCHER and G. N. HENDERSON. Can. J . Chem. 59, 2314

(1981). 20. J . H. RIDD. Adv. Phys. Org. Chem. 16. 1 (1978). 21. 6. G. CROSS, A. FISCHER, G. N. HENDERSON, and T. A. SMYTH.

Can. J. Chem. 62, 1446 (1984). 22. R. G. CLEWLEY. Ph.D. Dissertation, University of Victoria,

Victoria, B.C., 1985. 23. G. M. SHELDRICK. SHELX-76. a program for crystal structure

determination. Anorganisch-Chemisches Institut der Universitat Gottingen. 1976.

24. P. MAIN. MULTAN-78, a system of computer programs for the automatic solution of crystal structures from X-ray diffraction data. Department of Physics, University of York, England. 1978.

25. C. K . JOHNSON. ORTEP: A Fortran thermal ellipsoid plot program for crystal structure illustrations. ORNL-3794, Revised. Oak Ridge National Laboratory, Oak Ridge, Tennessee. 1965.

26. D. T. CROMER and J . T. WABER. International tables for X-ray crystallography. Vol. 4 . Kynoch Press, Birmingham, England. 1974. pp. 99 and 149.

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