- Email: [email protected]
Photo valence isomerization of sterically strained aromatic hydrocarbons: 8,9-dicarbethoxy[6]paracyclophane
0146-5724/88 $3.00+ 0.00 Copyright ~ 1988PergamonPress pk
Radlat. Phys. Chem. Vol. 32, No. 3, pp. 537-539, 1988 Int. J. R_~__o~.AppL lnstrum. Part C Printed in Great Britain.All fights reserved
PHOTO VALENCE ISOMERIZATION OF STERICALLY STRAINED AROMATIC HYDROCARBONS: 8,9-DICARBETHOXY[6]PARA CYCLOPHANE HERBERT DREESKAMP,1 PETER KAPAHNKE] and WERNER TOCHTERMANN 2 ~Institut ffir Physikalische und Theoretische Chemie der Technischen Universit/it, Hans-Sommer-Strasse 10, 3300 Braunschweig, F.R.G. 2Institut fur Organische Chemie der Universit~it,Olshansenstrasse 40, 2300 Kiel 1, F.R.G.
(Received 29 July 1987) Almtract--Irradiation of 8,9-diearbethoxy[6]paracyclophane (1) in the long-wavelength band with 2 ~ 310 nm in dilute fluid solutions gives the 1,4,Dewar isorner(2) with 2.4% quantum yield. The thermal back reaction 2~1 follows an Arrhen!us law (Ea = 88.3 + 3,6 kJ reel -I, log A = 9.3 + 0.6), By calorimetry an enthalpy of this reaction was determined as AH = - !9.8 + 3.8 kJ reel -t. This is significantly smaller than in unstrained systems with a planar aromatic ground state. N° other products were detected in this thermoreversible photoreaction: The photochemical rearomatization 2=-,1 occurs wfth a quantum yield of 0.12 =[:0.02 with light of 2 ~ 288 rim. Only by irradiation of 2 with 2 < 270 nm the prismane isomer (3) is formed.
INTRODUCTION Fluorescence opens an important way to investigate rapid processes in electronically excited molecules either in photo- or in radiation chemistry. ~) Substitution of aromatic hydrocarbons by t-butyl groups often drastically reduces the fluorescence quantum yields, sometimes termed the "loose bolt" effect. ~2'3)In the specific case of 9-t-butylanthracene it has been shown~4's~that the bulky snbstituent produces a deformation of the anthracene skeleton (see Fig. 1 for a definition of the angles of deformation). After irradiation into the longwave ~L,-band the molecule in fluid solution can pass over a small activation harrier to the potential surface of the S0-ground state forming in small yield (1.6%, ambient temperature) the 9-t-butyl-(9,10)-Dewar anthracene in which the anthracene skeleton is deformed much more strongly, (see Table 1). The low fluorescence quantum yield of 9-t-butylanthracene is thus caused only indirectly by the t-butyl group by a deformation of the aromatic system in the ground state and hence in the Franck-Condon state reached after photoexcitation. An alternative way of deforming a planar aromatic system is by bridging two opposite positions of a
benzene ring by a n-membered methylene chain --(CH2)~--. Synthesis, chemical reactivity and structure of these so called [n]paracyclophanes constitute a very active field of research. ~6) In particular it is known that the angles of deformation steadily increase with decreasing chain length (see Table 1). For [n]paracyclophanes with a long methylene chain (n/>7) the aromatic isomer is thermodynamically more stable, while for a short chain (n = 3) the equilibrium is shifted to the Dewar isomer. ~7) While for n = 4 and n = 5 a number of photoproducts a r e o b s e r v e d (s'9) the [7]paracyclophane exhibits no photoisomerization to the Dewar isomer. ~°) [5]paracyclophanes are obtained by irradiation of the corresponding Dewar isomers. ~lt-13~The borderline case of [ ~ a c y c l o p h a n e appears to be the most interesting one. Here Bickelhaupt, Jones Jr and co-workers. °°) have established a photoisomerization to the Dewar isomer and measured the activation parameters of the thermal rearomatization reaction. In this work a more complete investigation of the disubstituted [6]paracyclophane (I) was undertaken. Synthesis, chemical reactivity and some photo-
~2
Fig. 1. Definition of deformation angle. 537
1
538
HERBERTDREESKAMPet al. Table 1. Anglesof deformation of strained aromaticcompounds Substance n 8,9-Dicarbomethoxy[6]paracyclophane 6 19.4 19.5 8-Carboxy[6]paracyclophane 6 21.1 20.3 3-Carboxy[~a cyclophane 7 18.3 15.2 3,6-Heptanophthalide 7 14.9 13.8 4-Carboxy[8]paracyclophane 8 9.0 9.2 [IO]paracyclophane-4,6..diyne 10 1.8 1.8 9-t-Butylantbracene -16.1 7.0 9-t-Butyl-(9,10)-Dewaranthracene -83.1 81.8
~jo ~21 °
physical properties of I have been described in several publications since 1982.°4"1s) The structure of 8,9-dicarbomethoxy[6]paracyclophane is known. (~5) Here the angles of deformation were determined and may be assumed for the title compound 1 also (see Table 1). The photochemical formation of the Dewar isomer 2 and the prismane isomer 3 by irradiation of dilute solutions of 1 in cyclohexane with the unfiltered u.v.-light of a mercury lamp was established. (~5) In this work we found that irradiation of 1 with k ~ 310nm, i.e. in the long-wavelength absorption band, produces 2 with a quantum yield of 2.4% (in n-heptane at 300 K) exclusively. The identification of this product as 2 follows from the IH-NMR spectrum. (~s) The reaction is thermoreversible to better than 99%, i.e. even after 20 irradiation cycles no loss of 1 was obtained. Both by u.v.-absorption spectroscopy and ~H-NMR the thermal reaction 2-,1 was investigated. The reaction is first-order with the Arrhenius parameters listed in Table 2. The statement that bes~c[es 1 and 2 no other products are formed, is supported by the fact that perfect isosbestic points were observed in the spectra at k~ = 277 nm and k2 ffi 302,5 nm. Irradiation of 2 into the first absorption band (k~,, = 288 nm) produces only the starting material 1. The quantum yield of the photochemical rearomatization reaction 2-.1 by irradiation with light of k = 280 + l0 nm in n-heptane at 290 K was determined as 0.12 + 0.02. The enthalpy AH of the reaction 2-.1 was determined by calorimetry and is listed in Table 2, Samples of 2 (about I mg) in o-xyiene (0.02 mi) were measured in the temperature range of 3(X)-420 K with a calorimeter, type Heraeus TA 500. Table 2 summarizes the kinetic and thermo-
~(~2 E 1
)6
hv {-31Oh,> m) ( A,hv(-288nrn)
Ref. 15 16 17 18 19 20 21 21
dynamic parameters determined in this work together with some data of related compounds from the literature. Apparently the activation barrier for the thermal rearomatization reaction is not strongly affected by the presence of a methylene chain or the presence of a bulky substituent with the exception of the highly substituted hexamethylbenzene. On the other hand, the enthaipy of this reaction is strongly reduced by a steric strain which prevents the aromatic skeleton from attaining a planar geometry. This deviation from planarity in the ground state and hence in the Franck-Condon state after excitation seems to enhance the passage of the molecule on the surface of the excited state to the geometry of the Dewar isomer which deviates even more strongly from planarity. This process competes with a deactivation by light emission and thus reduces the fluorescence quantum yield. In rigid media or at reduced temperatures the fluorescence quantum yields of both the title compound 1 and 9-t-butylanthracenet4.~ increase as expected. Interestingly the quantum yield of the valence isomerization to the Dewar form is small while that of the photochemical rearomatization i s much larger in both cases (i.e. 0.024 vs 0.12 for 1~2 and 0.016 vs 0.4 for 9,t-butylanthracene~9-t-butyl-(9,10)-Dewar anthracene). Thus it may be assumed that the dependence of the energy surfaces on the angle of deformation is analogous in these two cases. The large quantum yield of the photochemical rearomatization reaction 2-,1 also agrees with the finding of Bickelhaupt eta/. H~mentioned above that the [5]paracyclophane can be obtained by photoisomerization of the corresponding Dewar isomer. Extrapolating the data of the reaction enthalpy AH presented in Table 2 we may assume that for the
~ 2 E
(hv(<_ 27Onto))> E ~ )S
2
- E=-CO2C2H5 Fig. 2. Reaction scheme.
E ~
CHz)s 3
539
Photo valence isomerization o f aromatic hydrocarbons Table 2. Thermodynamic and kinetic parameters of valence isomerizations Reaction
Solvent
2--,I [6]Dewar paracyclophane ---,[6]paracyelophane Hexamethyl Dewar benzene ~hexamethylbenzene (I,4)-Dewar naphthalene --*naphthalene 9-t-Butyl-(9,10)-Dewar anthracene --,9-t-butylanthracene (l,4)-Dewar anthracene --,anthracene (9, i 0)-Dewar anthracene --,anthracene
- AH(kJ tool- t)
Ea(kJ mol- 1)
log A
Ref.
o-Xylene' bn-Heptane Cyclohexane Cyclohexane --
19.8 _+3.8"
88.3 _+3.6b,c
9.3 _ 0.6b,c
This work
--249.1
83.3 _+3.8c 87.5 _+6.3d 155.7
9.3 _+0.6 9.8 _+0.9 15.03
10 10 23
Sqalene" n -Heptaneb Decaline" n -Heptaneb
248.5 + 8.0"
99.1 _+2.6b
12.5 _+0.4 b
24
172.3 + 5.7"
93.3 _ 2.5"
11.9 _ 0.8 b
5
n -Heptane
324 _ 10
110.9
--
25
90.4 + 2.5
12.22 _+0.41
26
Dodecane
--
• and b Specify the solvents used. CDetermined from u.v.-spectra, dDetermined from NMR-spectra.
[5]paracyclophane t h e D e w a r i s o m e r is t h e t h e r m o d y n a m i c a l l y m o r e stable f o r m . Acknowledgements--The
authors would like to thank H. Cammenga and S. Sarge for making the calorimetric measurements available to us and the " F o n d s der Chemie" for supporting this work.
REFERENCES I. See, for example: M. Burton and H. Dreeskamp, Faraday Discuss. Chem. Soc. 1959, 27, 64. 2. (3. N. Lewis and M. Calvin, Chem. Rev. 1939, 25, 273. 3. See, for example: N. J. Turro, Modern Molecular Photochemistry, p. 170. Benjamin/Cummings, New York, 1978. 4. H. Dreeskamp, B. Jahn and J. Pabst, Z. Naturforsch. 1981, 36a, 665. 5. B. Jahn and H. Drceskamp, Ber. Bunsenges. Phys. Chem. 1984, 88, 42. 6. P. M. Kcehn and S. M. Roscnfeld (Eds), Cyclophanes, Vol. I. Academic Press, New York, 1983. 7. I. J. Landheer, W. H. de Wolf and F. Bickelhaupt, Tetrahedron Lett. 1975, 349. 8. I. J. Landheer, W. H. de Wolf and F. Bickelhaupt, Tetrahedron Lett. 1974, 2813. 9. K. Weinges and K. Klcssing, Chem. Bet. 1976, 109, 793. 10. S. L. Kammula, L. D. Iroff, M. Jones Jr, J. W. van Straten, W. H. de Wolf and F. Bickelhaupt, J. Am. Chem. Soc. 1977, 99, 5815. 11. L. W. Jenneskens, F. J. J. de Kanter, P. A. Kraakman,
KP.C. 32/3--0
L. A. M. Turkenburg, W. E. Koolhaas, W. H. de Wolf, F. Bickelhaupt, Y. Tobe, K. Kakiuchi and Y. Odaira, J. Am. Chem. Soc. 1985, 107, 3716. 12. Y. Tobe, T. Kaneda, K. Kakiuchi and Y. Odaira, Chem. Lett. 1985, 1301. 13. G. B. M. Kostermans, W. H. de Wolf and F. Bickelhaupt, Tetrahedron Lett. 1986, 27, 1095. 14. J. Lieb¢, C. Wolff and W. Tochtermann Tetrahedron Lett. 1982, 23, 171.
15. J. Liebe, C. Wolff, C. Krieger, J. Weiss and W. Tochtermann, Chem. Ber. 1985, 118, 4144 and earlier publications cited therein. 16. Y. Tobe, K. Kakiuchi, Y. Odaira, T. Hosaki, Y. Kal and N. Kasai, J. Am. Chem. Soc. 1983, 105, 1376. 17. N. L. Allinger, T. J. Waiter and M. G. Newton, J. Am. Chem. Soe. 1974, 96, 4588. 18. J. Hunger, C. Wolff, W. Tochtermann, E.-M. Peters, K. Peters and H. G. yon Schnering, Chem. Bet. 1986, 119, 2698. 19. M. G. Newton, T. J. Walter and N. L. Allinger, J. Am. Chem. Soe. 1973, 95, 5652. 20. T. Aono, K. Sakabe, N. Sakabe, C. Katayama and J. Tanaka, Acta Crystallogr. 1975, 1131, 2389. 21. K. P. Angermund, Dissertation, Bergische UniversitaetGesamthochschule Wuppertal, 1986. 22. K. Angermund, G. Goddard, C. Krueger Acta Crystallogr. Suppl. 1985, A40, 162. 23. J. F. M. Oth, Angew. Chem. Int. Ed. Engl. 1968, 7, 646. 24. W. Grimme and U. Heinzc, Chem. 8er. 1978, 111, 2563. 25. N. C. Yang, R. V. Carr, E. Li, J. K. McVey and S. A. Rice, J. Am. Chem. Soc. 1974, 96, 2297. 26. W. Pritschins, W. Grimme, Tetrahedron Lett. 1982, 23, ll51.
Radlat. Phys. Chem. Vol. 32, No. 3, pp. 537-539, 1988 Int. J. R_~__o~.AppL lnstrum. Part C Printed in Great Britain.All fights reserved
PHOTO VALENCE ISOMERIZATION OF STERICALLY STRAINED AROMATIC HYDROCARBONS: 8,9-DICARBETHOXY[6]PARA CYCLOPHANE HERBERT DREESKAMP,1 PETER KAPAHNKE] and WERNER TOCHTERMANN 2 ~Institut ffir Physikalische und Theoretische Chemie der Technischen Universit/it, Hans-Sommer-Strasse 10, 3300 Braunschweig, F.R.G. 2Institut fur Organische Chemie der Universit~it,Olshansenstrasse 40, 2300 Kiel 1, F.R.G.
(Received 29 July 1987) Almtract--Irradiation of 8,9-diearbethoxy[6]paracyclophane (1) in the long-wavelength band with 2 ~ 310 nm in dilute fluid solutions gives the 1,4,Dewar isorner(2) with 2.4% quantum yield. The thermal back reaction 2~1 follows an Arrhen!us law (Ea = 88.3 + 3,6 kJ reel -I, log A = 9.3 + 0.6), By calorimetry an enthalpy of this reaction was determined as AH = - !9.8 + 3.8 kJ reel -t. This is significantly smaller than in unstrained systems with a planar aromatic ground state. N° other products were detected in this thermoreversible photoreaction: The photochemical rearomatization 2=-,1 occurs wfth a quantum yield of 0.12 =[:0.02 with light of 2 ~ 288 rim. Only by irradiation of 2 with 2 < 270 nm the prismane isomer (3) is formed.
INTRODUCTION Fluorescence opens an important way to investigate rapid processes in electronically excited molecules either in photo- or in radiation chemistry. ~) Substitution of aromatic hydrocarbons by t-butyl groups often drastically reduces the fluorescence quantum yields, sometimes termed the "loose bolt" effect. ~2'3)In the specific case of 9-t-butylanthracene it has been shown~4's~that the bulky snbstituent produces a deformation of the anthracene skeleton (see Fig. 1 for a definition of the angles of deformation). After irradiation into the longwave ~L,-band the molecule in fluid solution can pass over a small activation harrier to the potential surface of the S0-ground state forming in small yield (1.6%, ambient temperature) the 9-t-butyl-(9,10)-Dewar anthracene in which the anthracene skeleton is deformed much more strongly, (see Table 1). The low fluorescence quantum yield of 9-t-butylanthracene is thus caused only indirectly by the t-butyl group by a deformation of the aromatic system in the ground state and hence in the Franck-Condon state reached after photoexcitation. An alternative way of deforming a planar aromatic system is by bridging two opposite positions of a
benzene ring by a n-membered methylene chain --(CH2)~--. Synthesis, chemical reactivity and structure of these so called [n]paracyclophanes constitute a very active field of research. ~6) In particular it is known that the angles of deformation steadily increase with decreasing chain length (see Table 1). For [n]paracyclophanes with a long methylene chain (n/>7) the aromatic isomer is thermodynamically more stable, while for a short chain (n = 3) the equilibrium is shifted to the Dewar isomer. ~7) While for n = 4 and n = 5 a number of photoproducts a r e o b s e r v e d (s'9) the [7]paracyclophane exhibits no photoisomerization to the Dewar isomer. ~°) [5]paracyclophanes are obtained by irradiation of the corresponding Dewar isomers. ~lt-13~The borderline case of [ ~ a c y c l o p h a n e appears to be the most interesting one. Here Bickelhaupt, Jones Jr and co-workers. °°) have established a photoisomerization to the Dewar isomer and measured the activation parameters of the thermal rearomatization reaction. In this work a more complete investigation of the disubstituted [6]paracyclophane (I) was undertaken. Synthesis, chemical reactivity and some photo-
~2
Fig. 1. Definition of deformation angle. 537
1
538
HERBERTDREESKAMPet al. Table 1. Anglesof deformation of strained aromaticcompounds Substance n 8,9-Dicarbomethoxy[6]paracyclophane 6 19.4 19.5 8-Carboxy[6]paracyclophane 6 21.1 20.3 3-Carboxy[~a cyclophane 7 18.3 15.2 3,6-Heptanophthalide 7 14.9 13.8 4-Carboxy[8]paracyclophane 8 9.0 9.2 [IO]paracyclophane-4,6..diyne 10 1.8 1.8 9-t-Butylantbracene -16.1 7.0 9-t-Butyl-(9,10)-Dewaranthracene -83.1 81.8
~jo ~21 °
physical properties of I have been described in several publications since 1982.°4"1s) The structure of 8,9-dicarbomethoxy[6]paracyclophane is known. (~5) Here the angles of deformation were determined and may be assumed for the title compound 1 also (see Table 1). The photochemical formation of the Dewar isomer 2 and the prismane isomer 3 by irradiation of dilute solutions of 1 in cyclohexane with the unfiltered u.v.-light of a mercury lamp was established. (~5) In this work we found that irradiation of 1 with k ~ 310nm, i.e. in the long-wavelength absorption band, produces 2 with a quantum yield of 2.4% (in n-heptane at 300 K) exclusively. The identification of this product as 2 follows from the IH-NMR spectrum. (~s) The reaction is thermoreversible to better than 99%, i.e. even after 20 irradiation cycles no loss of 1 was obtained. Both by u.v.-absorption spectroscopy and ~H-NMR the thermal reaction 2-,1 was investigated. The reaction is first-order with the Arrhenius parameters listed in Table 2. The statement that bes~c[es 1 and 2 no other products are formed, is supported by the fact that perfect isosbestic points were observed in the spectra at k~ = 277 nm and k2 ffi 302,5 nm. Irradiation of 2 into the first absorption band (k~,, = 288 nm) produces only the starting material 1. The quantum yield of the photochemical rearomatization reaction 2-.1 by irradiation with light of k = 280 + l0 nm in n-heptane at 290 K was determined as 0.12 + 0.02. The enthalpy AH of the reaction 2-.1 was determined by calorimetry and is listed in Table 2, Samples of 2 (about I mg) in o-xyiene (0.02 mi) were measured in the temperature range of 3(X)-420 K with a calorimeter, type Heraeus TA 500. Table 2 summarizes the kinetic and thermo-
~(~2 E 1
)6
hv {-31Oh,> m) ( A,hv(-288nrn)
Ref. 15 16 17 18 19 20 21 21
dynamic parameters determined in this work together with some data of related compounds from the literature. Apparently the activation barrier for the thermal rearomatization reaction is not strongly affected by the presence of a methylene chain or the presence of a bulky substituent with the exception of the highly substituted hexamethylbenzene. On the other hand, the enthaipy of this reaction is strongly reduced by a steric strain which prevents the aromatic skeleton from attaining a planar geometry. This deviation from planarity in the ground state and hence in the Franck-Condon state after excitation seems to enhance the passage of the molecule on the surface of the excited state to the geometry of the Dewar isomer which deviates even more strongly from planarity. This process competes with a deactivation by light emission and thus reduces the fluorescence quantum yield. In rigid media or at reduced temperatures the fluorescence quantum yields of both the title compound 1 and 9-t-butylanthracenet4.~ increase as expected. Interestingly the quantum yield of the valence isomerization to the Dewar form is small while that of the photochemical rearomatization i s much larger in both cases (i.e. 0.024 vs 0.12 for 1~2 and 0.016 vs 0.4 for 9,t-butylanthracene~9-t-butyl-(9,10)-Dewar anthracene). Thus it may be assumed that the dependence of the energy surfaces on the angle of deformation is analogous in these two cases. The large quantum yield of the photochemical rearomatization reaction 2-,1 also agrees with the finding of Bickelhaupt eta/. H~mentioned above that the [5]paracyclophane can be obtained by photoisomerization of the corresponding Dewar isomer. Extrapolating the data of the reaction enthalpy AH presented in Table 2 we may assume that for the
~ 2 E
(hv(<_ 27Onto))> E ~ )S
2
- E=-CO2C2H5 Fig. 2. Reaction scheme.
E ~
CHz)s 3
539
Photo valence isomerization o f aromatic hydrocarbons Table 2. Thermodynamic and kinetic parameters of valence isomerizations Reaction
Solvent
2--,I [6]Dewar paracyclophane ---,[6]paracyelophane Hexamethyl Dewar benzene ~hexamethylbenzene (I,4)-Dewar naphthalene --*naphthalene 9-t-Butyl-(9,10)-Dewar anthracene --,9-t-butylanthracene (l,4)-Dewar anthracene --,anthracene (9, i 0)-Dewar anthracene --,anthracene
- AH(kJ tool- t)
Ea(kJ mol- 1)
log A
Ref.
o-Xylene' bn-Heptane Cyclohexane Cyclohexane --
19.8 _+3.8"
88.3 _+3.6b,c
9.3 _ 0.6b,c
This work
--249.1
83.3 _+3.8c 87.5 _+6.3d 155.7
9.3 _+0.6 9.8 _+0.9 15.03
10 10 23
Sqalene" n -Heptaneb Decaline" n -Heptaneb
248.5 + 8.0"
99.1 _+2.6b
12.5 _+0.4 b
24
172.3 + 5.7"
93.3 _ 2.5"
11.9 _ 0.8 b
5
n -Heptane
324 _ 10
110.9
--
25
90.4 + 2.5
12.22 _+0.41
26
Dodecane
--
• and b Specify the solvents used. CDetermined from u.v.-spectra, dDetermined from NMR-spectra.
[5]paracyclophane t h e D e w a r i s o m e r is t h e t h e r m o d y n a m i c a l l y m o r e stable f o r m . Acknowledgements--The
authors would like to thank H. Cammenga and S. Sarge for making the calorimetric measurements available to us and the " F o n d s der Chemie" for supporting this work.
REFERENCES I. See, for example: M. Burton and H. Dreeskamp, Faraday Discuss. Chem. Soc. 1959, 27, 64. 2. (3. N. Lewis and M. Calvin, Chem. Rev. 1939, 25, 273. 3. See, for example: N. J. Turro, Modern Molecular Photochemistry, p. 170. Benjamin/Cummings, New York, 1978. 4. H. Dreeskamp, B. Jahn and J. Pabst, Z. Naturforsch. 1981, 36a, 665. 5. B. Jahn and H. Drceskamp, Ber. Bunsenges. Phys. Chem. 1984, 88, 42. 6. P. M. Kcehn and S. M. Roscnfeld (Eds), Cyclophanes, Vol. I. Academic Press, New York, 1983. 7. I. J. Landheer, W. H. de Wolf and F. Bickelhaupt, Tetrahedron Lett. 1975, 349. 8. I. J. Landheer, W. H. de Wolf and F. Bickelhaupt, Tetrahedron Lett. 1974, 2813. 9. K. Weinges and K. Klcssing, Chem. Bet. 1976, 109, 793. 10. S. L. Kammula, L. D. Iroff, M. Jones Jr, J. W. van Straten, W. H. de Wolf and F. Bickelhaupt, J. Am. Chem. Soc. 1977, 99, 5815. 11. L. W. Jenneskens, F. J. J. de Kanter, P. A. Kraakman,
KP.C. 32/3--0
L. A. M. Turkenburg, W. E. Koolhaas, W. H. de Wolf, F. Bickelhaupt, Y. Tobe, K. Kakiuchi and Y. Odaira, J. Am. Chem. Soc. 1985, 107, 3716. 12. Y. Tobe, T. Kaneda, K. Kakiuchi and Y. Odaira, Chem. Lett. 1985, 1301. 13. G. B. M. Kostermans, W. H. de Wolf and F. Bickelhaupt, Tetrahedron Lett. 1986, 27, 1095. 14. J. Lieb¢, C. Wolff and W. Tochtermann Tetrahedron Lett. 1982, 23, 171.
15. J. Liebe, C. Wolff, C. Krieger, J. Weiss and W. Tochtermann, Chem. Ber. 1985, 118, 4144 and earlier publications cited therein. 16. Y. Tobe, K. Kakiuchi, Y. Odaira, T. Hosaki, Y. Kal and N. Kasai, J. Am. Chem. Soc. 1983, 105, 1376. 17. N. L. Allinger, T. J. Waiter and M. G. Newton, J. Am. Chem. Soe. 1974, 96, 4588. 18. J. Hunger, C. Wolff, W. Tochtermann, E.-M. Peters, K. Peters and H. G. yon Schnering, Chem. Bet. 1986, 119, 2698. 19. M. G. Newton, T. J. Walter and N. L. Allinger, J. Am. Chem. Soe. 1973, 95, 5652. 20. T. Aono, K. Sakabe, N. Sakabe, C. Katayama and J. Tanaka, Acta Crystallogr. 1975, 1131, 2389. 21. K. P. Angermund, Dissertation, Bergische UniversitaetGesamthochschule Wuppertal, 1986. 22. K. Angermund, G. Goddard, C. Krueger Acta Crystallogr. Suppl. 1985, A40, 162. 23. J. F. M. Oth, Angew. Chem. Int. Ed. Engl. 1968, 7, 646. 24. W. Grimme and U. Heinzc, Chem. 8er. 1978, 111, 2563. 25. N. C. Yang, R. V. Carr, E. Li, J. K. McVey and S. A. Rice, J. Am. Chem. Soc. 1974, 96, 2297. 26. W. Pritschins, W. Grimme, Tetrahedron Lett. 1982, 23, ll51.