J . Am. Chem. SOC.1990, 112, 8941-8949 (4-[(p-nitrophenyl)azo]phenol); 0.0092 M, 0.092 M (phenol). The exceptions for the 'H N M R studies of 4 are listed with use of the same format: 0.015 M, 0.094 M (4-cyanophenol);0.0075 M, 0.075 M (4fluorophenol); 0.010 M, 0.050 M (6-nitro-2-naphthol);0.0075 M, 0.028 M (4-[(p-nitrophenyl)azo]phenol). For all studies, 400 p L of the host
solution was used. Acknowledgment. We gratefully acknowledge J . E. Cochran for his assistance with the 31P-iH correlation 2D N M R experiments. Acknowledgement is made for partial support of this work by the National Science Foundation and the Office of Naval
8941
Research Supplementary Material Available: Tables of X-ray data for 1, 2,4, and the 1:1 adduct of 4 with Ph2SnC12 including crystal
data, collection parameters, solution and refinement data, atomic coordinates and equivalent isotropic displacement parameters, bond lengths and angles, anisotropic displacement parameters, hydrogen atom coordinates, and isotropic displacement parameters (49 pages); tables of observed and calculated structure factors (121 pages). Ordering information is given on any current masthead page.
Reactivity of 8 , l l -Dihalo[ S]metacyclophanes' Leonardus W. Jenneskens,+ Henricus J. R. de Boer, Willem H. de Wolf, and Friedrich Bickelhaupt* Contribution from the Scheikundig Laboratorium, Vrije Uniuersiteit, De Boelelaan 1083, 1081 H V Amsterdam, The Netherlands. Received May 31, 1990
Abstract: The highly bent and strained 8.1 l-dihalo[5]metacyclophanes lb-d were subjected to a variety of reactions in order to compare their behavior with that of normal, planar aromatics. Indeed, a strongly deviating reactivity of 1 was observed. With acid, rapid Wagner-Meerwein rearrangement to the ortho isomers 3 occurred, with partial loss of the substituent at position I I (between the bridge) to furnish 4; theoretical calculations show that relief of strain on bridgehead protonation is the initiating step. Irradiation of 1 furnished mainly 3. This rearrangement was shown by '3C-labeling to proceed via a benzvalene intermediate; a minor side reaction was radical cleavage of the 1 I-halogen which leads to radical attack at the central methylene group of the bridge and finally to 15. The transformation 1 15 was more efficiently achieved with the complex reducing agent N ~ H / N ~ ( O A Cin) ~this ; case, however, removal of the second halogen to 18 and overreduction to 19 also occurred. Similarly, attempted catalytic reductive removal of the halogen was not successful, as the hydrogenation went on to the fully saturated bicyclo[5.3.l]undecane (16). The high reactivity of the aromatic nucleus was also apparent from the unusual ease of Diels-Alder reactions under rather mild conditions. Remarkable, too, is the reactivity of lb-d toward organolithium reagents. While n-butyllithium gave variable results depending on the halogen, tert-butyllithium showed either the unusual SN2-substitution of CI-1 1 in Ib (to give le), the expected double brominelithium exchange with IC (to give 25), or a single-electron-transfer reaction with Id (to give 1%). It is concluded that the enhanced and/or unusual reactivity of [5]metacyclophanesis mainly due to relief of strain in the initial stages of these reactions and not (primarily) to reduced aromatic character.
-
One of the intriguing aspects in the chemistry of short-bridged [nlcyclophanes is the question how bending of the benzene ring influences the chemical and physical properties of these highly strained compounds. In contrast to the [n]paracyclophanes, which have been studied experimentally2s3and the~retically,~ the [n]metacyclophanes have received less attention. Although the synthesis of [n]metacyclophanes with n = IO, 7, 6,5 and S6 has been reported and the intermediacy of [4]metacyclophane' has been invoked, experimental data concerning their reactivity are scarce. Nevertheless, it could be deduced that [nlmetacyclophanes with n 5 7 possess extraordinary properties.2b~6b~8+9 This was initially rationalized by invoking the occurrence of bond fixation in the bent benzene ring toward a 1,3,5-cyclohexatriene-likestructure.9a However, when crystalline derivatives of the hitherto smallest isolable representative of [Slmetacyclophane (la), Le., 8 , l l -dihalo[5]metacyclophanes (lb-d) became available (Scheme 1),Io an X-ray structure determination of 8,1 I-dichloro[S]metacyclophane (lb) unambiguously showed that despite the large deviations from planarity of the bent benzene ring (e.g., at the bow of the boat, the bending angle is 26.8'), the aromatic carbon-carbon bond lengths were practically uniform and ty ical for a delocalized aromatic compound (1.393 f 0.007 ) . I i Obviously, bond fixation cannot be responsible for the extraordinary reactivity of [n]metacyclophanes with n I7. Consequently,
1
'Present address: AKZO Research Laboratories Arnhem. Corporate Research. P.O. Box 9300. 6800 SB Arnhem. The Netherlands.
0002-7863/90/ 15 12-8941$02.50/0
Scheme I'
18
"(a) 2a
2a : X = Y = C I
l b : X=Y=CI
2b : X=Y=Br
1c : X = Y = B r
2c : X-Br.Y=CI
l d : X=Br.Y=CI
+ 2Ph,SnH; (b) r-BuOK; (c) AgCIO,,
lutidine, T H F
an explanation was put forward based on a systematic theoretical investigation of [n]metacyclophanes with n = 7, 6 , 5, and 4.Iza ( I ) Taken in part from Jenneskens, L. W.Ph.D. Thesis, Vrije Universiteit, Amsterdam, The Netherlands, 1986. (2) For reviews see: (a) Keehn, P. M.; Rosenfeld, S. M. Cyclophanes; Academic Press: New York, 1983. (b) Bickelhaupt, F.; De Wolf, W. H. Red. Trau. Chim. Pays-Bas 1988,107.459. (c) Bickelhaupt, F.Pure Appl. Chem. 1990. 62. 373.
0 1990 American Chemical Society
8942 J . Am. Chem. SOC.,Vol. 112, No. 24, 1990
Jenneskens et al.
Scheme 11"
Scheme 111"
I
5'8-0
Y
Y
48-0
38-e
"a, X = Y = H; b, X = Y = CI; c, X = Y = Br; d, X = Br, Y = CI; e, X = t-Bu, Y = CI. * = I3C label in Ib and 3b. (a) H+, (b) -X+, (c) -H+.
The results suggested that the increase in strain with decreasing n is the dominating factor for the enhancement of reactivity. We (3) (a) Tobe, Y.; Kakiuchi, K.; Odaira, Y.; Hosaki, T.; Kai, Y.; Kasai, N. J . Am. Chem. SOC.1983, 105, 1376. (b) Tobe, Y.; Ueda, K.; Kakiuchi, K.; Odaira, Y.; Kai, Y.; Kasai, N. Tetrahedron 1986, 42, 1851. (c) Tobe, Y.; Ueda, K.; Kakiuchi, K.; Odaira, Y. Angew. Chem. 1986,98,364. (d) Liebe, J.; Wolff, C.; Krieger. C.; Weiss, J.; Tochtermann, W. Chem. Ber. 1985, 118, 4144. (e) Noble, K. L.; Hopf, H.; Ernst, L. Ibid. 1984, 117, 455 and 474. (0 Tobe, Y.; Ueda, K.; Kakiuchi, K.; Odaira, Y. Chem. Lett. 1983, 1645. (8) Jenneskens, L. W.; De Wolf, W. H.; Bickelhaupt, F. Tetrahedron 1986, 42, 1571. (h) Kostermans, G. B. M.; De Wolf, W. H.; Bickelhaupt, F. Ibid. 1987, 43, 2955. (i) Kostermans, G.B. M.; Bobeldijk, M.; De Wolf, W. H.; Bickelhaupt, F. J . Am. Chem. Soc. 1987, 109,2471. 6)Tsuji, T.; Nishida, S.Ibid. 1988, 110. 2157. (k) Jenneskens, L. W.; De Kanter, F. J. J.; Kraakman, P. A.; Turkenburg, L. A. M.; Koolhaas, W. E.; De Wolf, W. H.; Bickelhaupt, F.; Tobe. Y.; Kakiuchi, K.; Odaira, Y. Ibid. 1985, 107, 3716. (I) Giinther, H.; Schmitt, P.; Fischer, H.; Tochtermann, W.; Liebe, J.; Wolff, C. Helv. Chem. Acta 1985,68,801. (m) Jennskens, L. W.; K d , A. H. P.; Kraakman, P. A.; Mcene, W.; De Wolf, W. H.; Bickelhaupt, F. J . Org. Chem. 1986,51, 2162. (n) Li, Z.-H.; Jones, M., Jr. Tetrahedron Lett. 1987,30,753. ( 0 ) Tobe, Y.;Ueda, K.; Kaneda, T.; Kakiuchi, K.; Odaira, Y.; Kai, Y.; Kasai, N. J . Am. Chem. Soc. 1987, 109, 1136. (p) Koningstein, V.; Tochtermann, W.; Peters, E.; Peters, K.; Von Schnering, G. Tetrahedron Lett. 1987,30,3483. (4) Hopf, H.; Haase, M.; Hunger, J.; Tochtermann, W.; Zander, M. Chem. Phys. Lett. 1986, 127, 145. (r) Kostermans, G. B. M.; Kwakman, P. J.; Pouwels, P. J. W.; Somsen. G.;De Wolf, W. H.; Bickelhaupt, F. J . Phys. Org. Chem. 1989,
2, 331. (4) (a) Allinger, N . L.; Sprague, J. T.; Liljefors, T. J. Am. Chem. SOC. 1974, 100, 5100. (b) Schmidt, H.; Schweig, A.; Thiel, W.; Jones, M., Jr. Chem. Ber. 1978, 111, 1958. (c) Carballeira, L.; Casado, J.; Gonzalez, E.; Rios, M. A. J . Chem. Phys. 1982, 77, 5655. (d) Spraque, J. T.; Tai, J. C.; Yuh, Y.; Allinger, N. L. J . Compur. Chem. 1987,8,581. (e) Lee, T. J.; Rice, J. E.; Allen, W. D.; Remington, R. B.; Schaefer, 111, H. F. Chem. Phys. 1988, 123, I . (fl Lee, T. J.; Rice, J. E.; Remington, R. B.; Schaefer, 111, H. F. Chem. Phys. Lett. 1988, 150,63. (g) Rice, J. E.; Lee, T. J.; Remington, R. B.; Allen, W. D.; Allen-Clabo, D., Jr.; Schaefer, 111, H. F. J . Am. Chem. Soc. 1987. 109, 2902. (h) Jenneskens, L. W.; Louwen, J. N.; de Wolf, W. H.; Bickelhaupt, F. J. Phys. Org. Chem. 1990, 3, 295. ( 5 ) Hirano. S.;Hara, H.; Hiyama, T.; Fujita, S . ; Nozaki, H. Tetrahedron 1975, 31, 2219. (6) (a) Van Straten, J. W.; De Wolf, W. H.; Bickelhaupt, F. Tetrahedron Lett. 1977,4667. (b) Turkenburg, L. A. M.; Blok, P. M. L.; De Wolf, W. H.; Bickelhaupt, F. Ibid. 1981, 22, 3317. (7) (a) Kostermans, G. B. M.; Van Dansik, P.; De Wolf, W. H.; Bickelhaupt, F. J . Am. Chem. SOC.1978, 109, 7887. (b) Kostermans, G. B. M.; Van Dansik, P.; De Wolf, W. H.; Bickelhaupt. F. J . Org. Chem. 1989, 53, 4531. (8) Noble, K. L.; Hopf, H.; Jones, M.. Jr.; Kammula, S.L. Angew. Chem. 1987, 90, 629. (9) (a) Turkenburg, L. A. M.; Blok, P. M. L.; De Wolf, W. H.; Bickelhaupt, F. Ibid. 1982, 94, 291. (b) Turkenburg, L. A. M.; De Wolf, W. H.; Bickelhaupt. F. Tetrahedron Lett. 1983, 24, 1817.
(IO) Jenneskens, L. W.; De Kanter, F. J. J.; Turkenberg, L. A. M.; De Boer, H. J. R.; De Wolf, W. H.; Bickelhaupt, F. Tetrahedron 1984,40,4401. (11) Jenneskens, L. W.; Klamer, J. C.; De Boer, H. J. R.; De Wolf, W. H.; Bickelhaupt, F.; Stam. C. H. Angew. Chem. 1984, 96, 236. (12) (a) Jenneskens, L. W.;De Kanter, F. J. J.; De Wolf, W. H.; Bickelhaupt. F. J . Comput. Chem. 1987,8, 1154. (b) Jenneskens, L. W.; Louwen, J. N.; Bickelhaupt, F. J . Chem. SOC.,Perkin Trans. 2 1989, 1893.
+I
II
I
IL V I_I &I
1
/I
+
I/
CI
4e
"(a) E l , (b) E2, (c) -H+.
have now investigated the chemical behavior of the 8,l l-dihalo[5]metacyclophanes (lb-d) under a variety of experimental conditions. A number of unusual rearrangement, addition, and substitution reactions, which find no counterpart in ordinary aromatic chemistry, are reported. Since the 8,1l-dihalo[5]metacyclophanes(lb-d) are relatively easily accessible in contrast to the parent compound (la)? we have also investigated the possibility of opening a better preparative route from lb,c to l a as an ancillary goal. Results and Discussion Acid-Catalyzed Rearrangement of la-e. A characteristic reaction of short-bridged [n]cyclophanes is the acid-catalyzed rearrangement to their corresponding ortho isomers, the benzocycloalkenes. A tentative investigation of l a and Ib revealed that l a rearranged instantaneously and quantitatively to 3a at ambient temperature with catalytic amounts of acid.6a In contrast, the rearrangement of l b was considerably slower under similar conditions.gb Three hours were needed for complete reaction. Besides 3b, a side product, identified as 4b, was found in ca. 30% yield (Scheme 11). Similar results were obtained for IC and Id, which yielded 3c, 4c and 3d, 4d, respectively, in a ratio of 2:l. The formation of the rearrangement products was rationalized by the mechanism shown in Scheme 11. An analogous mechanism has been proposed for the acid-catalyzed rearrangement of 7,8benzo-1 l-chloro[5]metacyclophane.~3The key step is protonation at one of the bridgehead positions (C(6) or C( 10)) to give the primary cationic intermediate 5. The latter rearranges to 3 via a 1,2-alkyl shift to 6 followed by a 1,2-shift of a halonium ion to 7 and subsequent deprotonation. The formation of the side products 4 from lb-d can also be rationalized by the proposed mechanism. Formally, they can be derived by a cationic abstraction of the halogen substituent X from either the intermediate 6 or 7. In view of the much better nucleofugic properties of the proton compared to the halonium ion, it is likely that the latter is cleaved from 6. Apparently, this reaction competes efficiently with the 1,2-migration. Presumably, the anion of the trifluoroacetic acid used for the rearrangement acts as the nucleophile which attacks the halogen in an SN2-typesubstitution. A rearrangement with complete loss of the substituent X at C ( 1 1) was observed for 1 l-tert-butyl-8-chloro[5]metacyclophane (le), which was unexpectedly obtained from Ib with tert-butyllithium at -70 O C (vide infra).14 On treatment with catalytic amounts of acid, an instantaneous and quantitative rearrangement to 4e was observed; 3e could not be detected in the reaction mixture. This suggests that in the case of le, a 1,2-alkyl shift (6e 7e) of the tert-butyl group does not occur. This can be rationalized by the easy elimination of the tert-butyl group. It
-
(13) Grice, P.; Reese, C. B. J. Chem. SOC.,Chem. Commun. 1980, 424. (14) Preliminary communication: Jenneskens, L. W.; Klamer, J. C.; De Wolf, W. H.;Bickelhaupt, F. Ibid. 1984, 733.
Reactioily of 8,I I - Dihalo/.5/metacyclophanes
J . Am. Chem. SOC.,Vol. 112, No. 24, 1990 8943
Table I. M N D O Heats of Formation (AH;) and Proton Affinities
Table It. Heats (AH) of Homodesmotic Reactions"
(PAV
comDd
la
lb Sa Sb Sa
AH,'
PAb
45.6' 34.7c 218.2 222.7 230.3 232.1 5.9 (4.12)"
194.6 179.2 182.5 169.2
AH
readion
entry
-46.0 CI
-46.9 CI
@
-27.9
-5.9
196.6
176.5
189.0
184.1
lb
'o=.
+
H
C2H6
-19.7' (-20.2)' -24.9' (-24.8)"
CtH, "In kcalmol-I. b P A derived from the equation PA = (AHfo(B, MNDO) AHf'(H+)) - AHfo(BH+, MNDO): AHfo(H+) = 367.2 kcalmol-'.i6b 'Cf. ref 12a. 'Experimental AHfo.'6E'Cf. ref 16d.
Jb+
+
Sb
can proceed via an El-type mechanism giving 4e and the relatively stable tert-butyl cation, which subsequently deprotonates to 2methylpropene. The latter was detected in the IH-NMR spectrum in equimolar amounts with respect to 4e. However, an E2-type mechanism seems equally feasible (Scheme 111). The results for l e support the assumption that the side products 4 observed on rearrangement of Ib-d are derived from the intermediate 6 (Scheme 11). Although all the experimental results can be rationalized conveniently by the proposed mechanism, none of the cationic intermediates proposed in Scheme I1 could be characterized directly. Therefore, further support was obtained from the acid-catalyzed rearrangement of [ 1 l-13C]lb(13C enrichment ca. 10%). The latter was prepared from the appropriately I3C labeled precursor 2a. It should be noted that the I3C-NMR spectra of [5]metacyclophanes are highly character is ti^;'^ in comparison with the m-xylenes having the same substitution pattern, a downfield shift of approximately 8 ppm was observed for the aromatic carbon atom C( 11). In agreement with the proposed mechanism, the acid-catalyzed rearrangement of [ 1 l-I3C]l b yielded 3b, the I3C-NMR spectrum of which revealed an enhanced signal at 6 = 139.3 ppm for C(9a) (Scheme 11). We have investigated the key step, i.e., protonation of l a and Ib, theoretically. Proton affinities (PA's) for the carbon atoms C(6) and C(7) were calculated by the MNDO method.'6a For comparison, the corresponding PA's of the unstrained reference compound m-xylene were calculated. In the case of l a and lb, protonation at the bridgehead carbon atom C(6) or C(10) is favored thermodynamically by about 10-1 2 kcalmol-I; expectedly, the reverse order is calculated for protonation of m-xylene (Table 11.17'
Also informative is an analysis of the homodesmotic'* reactions presented in Table 11. The calculated heats of reaction ( A H ) are either a measure for the strain energy (SE = - A H ) or a ( I S ) Jenneskens, L. W.; De Kanter, F. J . J.; De Wolf, W. H.; Bickelhaupt, F. Magn. Reson. Chem. 1986, 24, 308. (16) (a) For the calculation of PA, the experimental AHfo(H+) = 367.2 kcal-mol-' was used: Olivella, S.; Urpi, F.; Vilarrasa, J. J . Comput. Chem. 1984.5.230. (b) Bowers, M. T. Gas Phase fon Chemistry; Academic Press: New York, 1979; Vol. 2. (c) Cox, J . D.; Pilcher, G. Thermochemisfry of Organic and Organometallic Compounds; Academic Press: New York, 1970. (d) Dewar, M. J. S.;Thiel, W. J . Am. Chem. Soc. 1977, 99, 4907. (17) Cf.: Devlin, 111, J . 1.; Wolf, F.; Taft, R. W.; Hehre. W. J . fbid. 1976, 98. 1991. (18) George, P.; Trachtman, M.; Bock, C. W.; Breh, A. M. J . Chem. Soc., Perkin Trans. 2 1976. 1222.
*
Q? +
CI
+
'a
w+ w+
+
-47.6
C3HS CI
-0.9
CI CI
-16.3
Cl -1 4.2 CI
"In kcalmol; AHl' ( M N D 0 ) ' s were used in the calculation (see Table I )
measure for the effect of chlorine substitution. Entries 1 and 2 yield the S E of l a and Ib, respectively, which are in excellent agreement with those calculated with Bensonlg group increments.'2a Of special interest are the calculated A R s of entries 3 and 4. The calculations clearly predict that protonation at C(6) or C(10) (5a) leads to a substantial reduction of the strain (ASE = 46.0-27.9 = 18.1 kcal-mol-'). In contrast, protonation at C(7) or C(9) (5'a) yields SE = 47.6 kcal which is essentially the same as that of unprotonated l a . Thus, the driving force for the acid-catalyzed rearrangement is the release of strain realized in protonation, Le., in the very first step of the reaction sequence. A similar conclusion has been reached by calculations in the [nlparacyclophane series3' Next we will turn to the influence of chlorine substitution. The results of entries 1,2, and 5 reveal that chlorine substitution does not exert much influence on the SE. Therefore, electronic factors must be responsible for the experimentally observed lower rate of rearrangement of Ib. A comparison of the two PA's of la and l b (Table I) shows that for the latter they are 15.4 kcalmol-I (PA(Sa)-PA(Sb)) and 13.3 kcalmol-I (PA(5'a)-PA(%)) smaller. As a n t i ~ i p a t e d this , ~ ~ is a consequence of the destabilization of the cationic intermediate 5(') by the electron-withdrawing halogen substituents. Diels-Alder Reactions of [5]Metacyclophanes (Ib-e). In general, benzene and its simple derivatives do not engage in DielsAlder reactions with dienophiles under mild conditions.20 Obviously, the low reactivity is a consequence of the high activation energy needed to break up the resonance of the aromatic ring. However, this barrier can be lowered by either a decrease of the resonance energy or by an increase in ground-state energy of the aromatic "diene" due to strain or imposed steric factors. In the (19) Benson, S. W. Thermochemical Kinefics; Wiley: New York, 1968. (20) (a) Ciganek, E. Tetrahedron Lett. 1967, 3321. (b) Krespan, C. G.; McKusick, B. C.; Cairns, T. L.J . Am. Chem. SOC.1%1,83, 3428. (c) Bryce Smith, D.; Gilbert, A,; McColl, 1. S.; Yianni, P. J . Chem. Soc., Chem. Commun. 1984, 951. (d) Arriau, J.; Fernandez, J.; Yianni, P. J . Chem. Soc., Perkin Tram. 2 1986, 2013. ( e ) Bryce Smith, D.; Gilbert, A,; McColl, I. S.; Drew, M. G. B.; Yianni, P. J . Chem. Soc., Perkin Trans. I 1987, 1147. (f) Cookson, T. L.: Dance, J . Tetrahedron Lett. 1962, 879.
Jenneskens et ai.
8944 J. Am, Chem. SOC..Vol. 112, No. 24, 1990 Table IV. ' H N M R Data of 18
Scheme IV' V
U 18-d
0
U
-15.5 11.5 6.1 1.5 0.5 7.8
TCE : 81-d MA : 91-d DMAD: 100-d
"TCE, tetracyanoethene; MA, maleic anhydride; DMAD, dimethyl acety lenedicarboxyla te. Table 111. Conditions and Yields of Diels-Alder Reactions of la-e' dienophile cyclodimethyl ohan& tetracvanoethene maleic anhvdride acetvlenedicarboxvlate lac 8r 100% 9n, 100% loa, 100% 25 OC, 5 h 25 OC. < I 5 min 25 OC,
