Aug. 20, 1964
SYNTHESIS O F
8-VISYLHEPTAFULVENE
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[CONTRIBUTIOS FROM THE DEPARTMENT OF CHEMISTRY, THEUNIVERSITY OF CALIFORKIA, SANTA BARBARA, SANTA BARBARA, CALIF, A N D THEFRCITA N D VEGETABLECHEMISTRY LABORATORY, WESTERNUTILIZATION RESEARCH A N D DEVELOPMEKT DIVISION, AGRICULTURAL RESEARCH SERVICE, u. s. DEPARTMEKT OF AGRICULTURE, PASADENA, C A L I F ]
Synthesis and Study of Pseudoaromatic Compounds. I. The Synthesis of 8-Vinylheptafulvene BY DOMENICK J. BERTELLI,'CARLOGOLIKO,' AND DAVIDL. DREVER RECEIVED FEBRUARY 1, 1964 8-Yinylheptafulvene has been synthesized and by comparison of its n m . r and infrared spectra with those of heptafulvalene, 8,8-dicyanoheptafulvene, and tropone, it is concluded t h a t dipolar resonance interactions are not of major importance in the ground state
Heptafulvene ( l a ) has aroused considerable theoretical interest because of its close analogy to tropane* which suggests that dipolar resonance interaction in the ground state might lead to aromatic character. Molecular orbital and valence bond calculations based on a regular heptagonal ring agree qualitatively with this supposition, predicting a high delocalization energy
la, X=H lb, X=CN
for heptafulvene, 3-5 while recent calculations including bond alternation predict lower delocalization energies.6,7 Simple A 1 0 calculations predict a high dipole moment (4.6 D . ) 3with the ring positive and more refined calculations predict a smaller dipole moment (0.85 D . ) 4 in the same direction. However, SCF-MO calculations predict a small dipole moment (0.14 D . ) 5 in the opposite direction. The molecular orbital calculations allowing for bond alternation predict dipole moments of 0.2X6 or 0.63 D.'. Thus theoretical calculations are a t variance as t o the importance of dipolar resonance contributions, and to their predictions of delocalization energy for heptafulvene. The recent synthesis of heptafulvene by Doering and \Vileys and subsequent determination of the heat of hydrogenationg indicate that the molecule does not possess a high n-electron delocalization energy and that the formation of a double bond a t the 7-position of cycloheptatriene, to form a completely conjugated system, results in only a slight increase in n-electron delocalization Since the properties of heptafulvene deviate considerably from those based on theoretical prediction, it appeared worthwhile to study this ring system further in order to gain a more precise indication of its structure. The marked instability of heptafulvene severely limits an investigation of its physical properties and thus has precluded the determination of several 1 1 ) University of California S a n t a Barbara, Calif (2) H J Dauben, J r . , and H J . Ringold, J . A m Chem. S o c , 78, 876 (19;1), W von E Doering and F. L D e t e r t , ibid , 73, 876 (1951) (3) G Bet-thier and B. Pullman, T r a n s . F a r a d a y Soc., 46, 484 (1949) ( 4 ) E D Bergmann, E Fischer, D Ginsburg, Y. Hirshberg, D . Lauie, h1 h l a y o t . A Pullman. and B. Pullman, Bull soc chim. F r a n c e , 18, 684 (19.51) ( 5 , A Julg. J chiin p h y s . , 6 2 , 50 (1955) (6) A J u l g , ibzd , 5 9 , 757 ( 1 9 6 2 ) . ( 7 ) T . S a k a j i m a and S.Katagiri, Bull Chem. Soc. J a p a n , 86, 910 (1962). ( 8 ) U'. von E Doering and D W . Wiley, T e t r a h e d r o n , 11, 183 (1960). ( 9 ) R B. T u r n e r , U' R . Meador, W .von E. Doering, L H. Knox, J R . Xrayer. and D . W Wiley, J A m . Chem. Soc., 79, 4127 (1957).
data which might be of use. Several derivatives of heptafulvene, substituted a t the 8-position, are known which exhibit enhanced stability over the parent compound. lo,llHowever, i t cannot be ascertained t o what extent derivatives containing cyano or carboalkoxy substituents change the ground state properties of the molecule. Thus, the relatively high dipole moment ( 3 D . ) calculated12 for heptafulvene on the basis of the known dipole moment of 8.8-dicyanoheptafulvene might be substantially in error owing to the enhancement of dipolar resonance contributions ( l b ) by the highly stabilizing influence of cyano groups toward a negative charge.l3 For these reasons it appeared desirable to investigate the possibility of synthesizing a derivative of heptafulvene which would introduce a minimum perturbation on the parent system while conferring sufficient stability to enable the determination of certain properties. It is assumed that a vinyl group a t the 8-position does not constitute a severe perturbation on the ground state of heptafulvene owing to the stabilization of dipolar resonance forms. Furthermore, from the n.m.r. spectrum i t might be possible to ascertain any tendency for a displacement of T-electron density from the seven-membered ring to the side chain by- the presence of an additional conjugated double bond. 7-Allylcycloheptatriene ( 3 ) can be readily prepared by the addition of cycloheptatrienyl 7-methyl etherL4 (2) to allylmagnesium chloride. The resulting hydrocarbon reacts rapidly with either triphenylmethylcarbonium fluoroborate or hexachloroantimonate to give the corresponding allyltropenium compound and triphenylmethane.'j Although the hexachloroantimonate salt (-1) could be isolated in pure form, the fluoroborate salt was obtained only as an oil a t room temperature. The n.m.r. spectrum was completely consistent with the structure of an allyltropenium ion and showed that no rearrangement to the vinyl tropenium system had occurred. Because of the insolubility of the allyltropenium hexachloroantimonate salt in both dichloromethane and chloroform, it was found more convenient to use the fluoroborate compound for all subsequent reactions, (10) H J . Dauben. J r . , and R . B Medz have reported t h e synthesis of 8,8-diphenylheptafulvene. Abstracts, 14.5th National Meeting of t h e American Chemical Society. S e w York, X Y , Sept . 1963, p 7.5 (11) T Nozoe, T . Makai, A S a t o , and I . Osaka, Bull Chem Soc J a p a n , 84, 1384 (1961) (12) M. Yamakawa, H Watanabe, T Mukai, T Kozoe, and M Kuho, J . A m Chem. Soc , 82, 5665 (1960). (13) A-akajima has indicated t h a t t h e experimental dipole moment may he in error, see ref. 7. (14) A. G Harrison, L . R Honnem. H J Dauben, J r . , and F P Lossing. J A m Chem Soc., 82, 3593 (1960). (15) H J Dauben, J r , F A. Gadecki, K. M. H a r m o n , and D L Pearson, i b i d . , 7 9 , 4557 (19.57).
D. J. BERTELLI,C. GOLIXO,A N D D. L. DREYER
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I
I
I
I
5
4
Fig. 1.-The n.1n.r. spectrum of 8-vinylheptafulvene in carbon tetrachloride with tetrarnethylsilane as internal reference at 60 Mc. ( T M S equal t o 10 7 ) .
even though it could not be isolated in pure form. Addition of a solution of allyltropenium fluoroborate in dichloromethane to a solution of trimethylamine16 in the same solvent produced an immediate and intense red solution and rapid precipitation of trimethylammonium fluoroborate. Since the allyltropenium fluoroborate could not be isolated in pure form, the compound, which was only slightly soluble in chloroe / H c H 3
+
CH,=CCHCHzMgCl
--L
+
"((2%)~ BF d -
form, was prepared in chloroform and washed several times with the same solvent before dissolving in dichloromethane and treatment with trimethylamine. For this reason, the yield of the deprotonation step could not be determined. The solutions thus obtained were stable for several days a t room temperature with no apparent change. Concentration of these solutions to ca. 0.05 -11 with a stream of deoxidized tiitrogen, addition of carbon tetrachloride, concentrating again, and finally chromatography over alumina gave solutions free of amine which showed no apparent change upon standing for a few days a t room temperature. Further concentration of these solutions by a deoxidized nitrogen stream gave solutions suitable for the determination of the n . m . r . and infrared spectra of the S-vinylheptafulvene. These more concentrated solutions would slowly polymerize, but were stable for scl-era1 hours if kept under nitrogen. I 1 J I>aul,en, J r , ~1 n l , J A m Chiiii .Soc.. 8 3 , 4657 (1961). a n d in rd 10. have shown this method t o be a general a n d useful method of del,! rit
