H. Heaney
and B. A. Marples
Loughborough University of Technology, Loughborough, Leicestershire, England
I
I
Undersraduate Experiments with ~ e t r a t h l o r o b k ~ n e
Undergraduate exercises involving reactive int,ermedi;~teshave attracted attention recently. These include reactions with dichlorocarhene (1) and henzyne (2, 3). The coupling of st,udent preparations with spectroscopic investigations hss also increased in recent years ( 4 , 5 ) . Professor Fieser has always stressed the importance of ease of operation and accessibility of materials in his exercises, and it is with this in mind that we present some experiments which we have used in our laboratories. Tetrahalogenohenzynes are more electrophilic than benzyne (6'). Although henzyne reacts with benzene and naphthalene (7, 8) and presumably would do so with other aromatic compounds, the yields of the DielsAlder type adducts are such as to make the reactions unsuitable for use in undergraduate laboratories. An exception is the formation of triptycene from henzyne and anthracene (5). The tetrahalogenohenzynes (61, on the other hand, react with aromatic compounds to yield cycloadducts in good yield, and the reactions are easy to carry out using (in the case of tetrachlorobenzyne) relatively inexpensive chemicals. I n addition the pmr spectra of the products provide useful practice in interpretation. Tetrachlorobenzyne (I) may be conveniently generated from pentachlorophenyl lithium, prepared by the reaction of n-hutyl lithium with hexachlorobenzene in ether pentane (9). Addition of the co-reactant, followed by the removal of sufficient ether to enable a reflux temperature of about 50°C to he reached, forms the tetrachlorobenzyne and hence the adduct. Alternatively, tetrachlorohenzyne may be generated by the aprotic diazotization of tetrachloroanthranilic acid, as used in one method of generating benzyne (10). Tetrachloroanthranilic acid is easily prepared on a suitable scale from tetrachlorophthalic anhydride (11).
-
CI
c1
(-N1-CO,)
/
\
cl
I
I
II
-Me-
Me
CI
II ..
H~ .-n
H A n"
C1
0 V
CI
C1 VI
The pmr spectra of adducts (11), (111), and (IV) are shown in Figures 1 , 2 , and 3. In the spectrum of adduct (11), the singlet a t r 7.88 is assigned to the bridgehead methyl group, and the doublet ( J E 1.9 cps), centered a t r 8.08, to the vinyl methyl groups. The fine splitting of the high-field signal is due to long-range spin-spin coupling with the vinyl protons (allylic coupling), and the low-field position of the bridgehead methyl signal is due to the deshieldiug effect of the o-chlorine atom. The bridgehead proton signal, centered a t .r 5.24, occurs as a triplet ( J E 1.9 cps) due to allylic coupling with the vinyl protons. These latter give a signal centered a t r 4.0, which occurs as a quintet ( J = 1.8 cps) due to four equivalent allylic couplings to the vinyl methyl and bridgehead protons. I n the spectrum of adduct (111) the singlet a t .r 6.27 is assigned to the methoxyl group and the multiplets a t r 4.5-4.9 and r 2.8-3.4 are assigned to the hridgehead and vinyl protons. res~ectivelv. The low-field multiulet has kighdclearlyUdefined lines and is the A B portion of an ARX system in which the pro- ton a t the bridgehead is X (111). The signal due to X is a nine line multiplet due t,o its being coupled to two pairs of identical proI1 tons. The values J A X= 2.0 cps and J s x Me 5.8 cps determined from this multiplet are only ipproximate as a first-order analysis is inadequate. Similar approximate figures can
~:w Q
Me
Cl
CI I
\
I
c1
The products may he readily isolated in each of the react,ions by column chromatography on neutral alumina, part,icularly when using the tetrachloroant,hranilic acid route. When using pentachlorophenyl lithium, unchanged hexachlorobenzene makes the isolation of the pure, p-xylene adduct (IV) difficult except on a small scale. I t is therefore suggested that thc p xylene adduct is only prepared by the aprotic diazotization method. Octachloroacridone (V) is a by-product isolated in the reactions involving tetrachloroanthranilic acid. The ketone (VI) is a by-product in the reactions using anisole, hut since it is only obtained in very low yield it may be conveniently ignored. I t is adsorbed on alumina very strongly and is only elut,ed with chloroform.
\
I
C1
N
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2 and 4, and 6 and 8. The calculated chemical shift for protons A is i2.98 and that for protons B is r 3.16. The student may easily verify these figures and further, as a final check, compare calculated line frequency and intensity data with those observed. The spectrum of adduct (IV) shows a doublet (J E 1.7 cps) centered a t T S.03, which is assigned to the vinyl methyl groups. The fine splitting of this signal is due to allylic coupling to the vinyl protons. The bridgehead protons signal, centered a t 4.90-5.20, is a quartet due to allylic (J E 1.8 cps) and vicinal (J c x 5.9 cps) coupling with the vinyl protons. The vinyl protons signal a t T 3.5-3.9 is two overlapping quintets arising from vicinal coupling (J 1.5.8 cps) to the bridgehead protons and four equivalent allylic couplings (J = 1.8 cps) to the vinyl methyl and bridgehead protons. The following experimental procedures have been shown to he suitable for undergraduate use. Preparations
I
2
3
1
5
Figure 2.
P M R spectra of addvct (1111.
Figure 3.
P M R spectra of oddudIIV).
B
4
ld
be obtained from the AB octet. ;\lore accurate values,
Adduet (II). A '/J three-necked flask is fitted with a small pressure equalized dropping funnel, a low temperature thermometer, a gas bubbler containing liquid paraffin, and a magnetic stirrer bar. Hexachlorobenzenel (1.575 g, 0.005 M ) and dry ether (100 ml) are placed in the flask and stirred while the temperature is reduced to -25 to -35'C. The apparatus is then flushed with a slow stream of dry nitrogen through a T-piece adapter attached to the dropping funnel and n-butyl-lithium solution (1 e q u i ~ a l e n t ) ~ is added to the dropping funnel by means of a hypodermic syringe through a serum cap. The n-butyllithium solution is then added to the hexachlorobenzene suspension over about 15 min; care should be taken that the temperature does not rise above -25'. The mixture is then stirred a t this temperature for 1 hr prior to adding dry mesitylene (100 ml). The apparatus is then removed from the cooling bath and converted to a distillation set-up and ether distilled until the vapor temperature reaches -50°C. At this stage a brief reflux period (about 1 hr) is sufficient to complete the reaction. After the reaction mixture is cooled, it is poured into hydrochloric acid (100 ml, 2 N ) , and the organic layer separated, dried, and the ether and excess mesitylene removed a t a rotary evaporator under reduced pressure. The residue is placed on neutral alumina (20 g) and eluted with petroleum ether (bp GO-80°C) until no further weight increase is obtained. Colored impurities are held on the alumina. This material is then placed on neutral alumina (1.50 g Activity I) and eluted with petroleum ether (bp GOSO°C). After a first fraction of unchanged hexachlorobenzene, the adduct (11) (-GOYo) is obtained, mp 110°C after recrystallizat,ion from ethanol. Adduel (III) may be obt,ained by essentially the same method using anisole in place of mesitylene. I t may be prepared on a 2-4 times scale sincc it is more strongly held on alumina and hence more easily separated from unchanged hexachlorobenzene. Elution from alumina with petro1:benzene (9: 1) or with petrol:ether (9: 1)
JAx E 1.6 cps and Jax"6.2 cps, can be obtained by
carrying out an analysis of the AB octet by the method outlined by Banwell (18). The lines are numbered 1-S from high to low field and JiB= 7.4 cps is determined directly from the separation of lines 1 and 3, 5 and 7, 802
/
Journal of Chemical Education
1
Obtainable from East,man Organic Chemicals.
' Obtainable from the Foot,e Chemical Co.
affords pure (111) (- G5%), mp 1 2 2 T after recrystallization from cyclohexane. Adduct (IV) is best prepared by the following method. Tetrachloroanthranilic acid (IS) (1.38 g, 0.005 M ) in dry ether is added over 20 min to a solution of isoamyl nitrite (,5 ml) in dry p-xylene (200 ml) kept a t 45'C. The mixture is warmed a t 45°C for a further '/z hr, and the solvents and excess iso-amyl nitrite removed under reduced pressure to yield a yellow product which is placed on alumina (.50 g). Elution with petroleum ether yields (IV) (-G5%), mp 128-130°C after recrystallization from ethanol. Literature Cited
(1) Fmssn, L. F., A N D Sacns, D. H., J. Org. Chem., 29, 1113 (1964). M. J., Canad. J. Chem., 439 (2) FIESER,L. F., A N D HADD.~DIN, 1599 (1965). M. J., T3elected Experiments (3) CAVA,M. P., A N D MITCHELL, in Organic Chemistry," W. A. Benjamin, New York, 1966. M. C., "Experimental Orgmic Chemistry," W. A. (4) CAS~RIO, Benjamin, New Yark, 1967. (5) BALDWLN, J., "Experimental Organic Chemistry," MeGrilwHill Book Co., New York, 1965.
J. P. N., AND HEANEY, H.,Tct~ahedronLetters, (6) (a) BREWER, 4709 (196.5). (b) HEANEY, H., AND JARLONSKI, J. M., J . P. N., Tetrahedm Letters, 4529 (1966). (c) BREWER, HEANEY,H., A N D MARPLES,B. A., Chem. Comm., 27 J . M., T ~ t r a (1967). (d) HEANEY, H., AND JARLONSKI, hpdron Letters, 2733 (1967). (e) BREWF.R,J. P. N., I. F., HEANEY,H., AND MARPLES,B. A,, ECKHARD, J. Chem. Soc. (C),664 (1968). if) HEANEY,H., A N D JABWNSKI, J.M., J. Chem. Soc. (C),1895 (1968). (7) MILLER,R. G., A N D STILES,M., J. Amw. Chem. Soc., 85, 1798 (1963). L.,, J. Amrr. Chem. Soc., 89,3071 (1967). (8) F R I ~ M . \ N (9) RAUSCH, M. D., TIRRETTS, F. E., AND GORDON, H. B., J. Oroanomclal. Chem.., 5.493 (1966). . . . (10) FRIEDMAN, L., A N D L O G U I . ~F., M., J. Amcr. Chtm. Soe., 85,1540 (1963). W. R., .AND NICHOLS,E. H., Amm. C h m . J., (11) ORNDORFF, 48,473 (1012). (12) BANWELL, C. N., in "N~mlertr Magnetic Resonance for D. R.), Organic Chemists," (Editor: MATHIK~ON, Academic Preas,London, 1967, p $5. (13) Easily prepared on a small scale from tetrachlorophthalic anhydride (obtainable from Aldrich Chemical Co., or Eastman Organic Chemicals) by the method given by W. R.., A N D NICHOLS, E. H., Amer. Chem. J., ORNDORFF, 48,473 (1912).
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