Hadizamani, Katja

Langhals, Heinz

English

Open Access LMU

New Journal of Chemistry V O L U M E 14. N U M B E R 4 A P R I L 1990 I S S N 0398-9836 Code N J C H E 5 14 (4) 255-304 CONTENTS C H E M I C A L P A S T T I M E S C O M M U N I C A T I O N M . - C . M a s s e b i e a u , E . D u n a c h , M . T r o u p e l , J . P e r i c h o n (Thiais) 257 A follow-up on a recent disquisition about referees. 259 Efficient electrochemical reduction of carbon dioxide on freshly coated metal electrodes. A R T I C L E S W . J . G r i f f i t h s , F. M. H a r r i s (Swansea) A . S c l a f a n i , L. P a l m i s a n o , E . D a v i (Palermo) 261 Experimental determination of the single and double ionization energies of the halomethanes CBr2FCI and CBr2FH. 265 Photocatalytic degradation of phenol by T/O2 aqueous dispersions: rutile and anatase activity. Ε. E . B a t o v a , P. P. L e v i n , V . Y a . S h a f i ro -v i c h (Chernogo lovka) 269 Long-lived radical ion pair state of Zn-porphyrin-viologen-quinone triad. Μ . Z. Y a n g , M. G . D e B a c k e r , F. X . S a u v a g e (L i l le) 273 Electrochemical and photoelectrochemical characterizations of electrodes covered by zinc tetra 2,3 pyridinoporphyrazine layers. H. L a n g h a l s , Κ. H a d i z a m a n i ( M ü n c h e n ) 279 The relation between bridgehead reactivity and electronic effects. C . B e r r i e r , H. C a r r e y r e , J . - C . J a c q u e s y , M . - P . J o u a n n e t a u d (Poit iers) A . N. K i t a i g o r o d s k i i , Α . V . B u l a t o v (Chernogo lovka) M a n h T h o n g C u n g ( N a n c y ) , J . P. L a u s -s a c (Tou louse) M . M a u r i c e , J . S o u p p e (Ar t ix ) 283 Hydroxylation regioselective en meta de derives chlores de l'aniline et du phenol. 2 8 9 A 1Η NMR study of adducts formed between nitroaromatics and paramagnetic alizarine metal complexes in solution. 2 9 3 1D- and 2D-NMR investigation of the interaction of zinc (I I) and thymulin, an immunoregulatory nonapeptide from thymus, in aqueous solution. 301 Spectral, biochemical, and electrochemical properties of chemically modified nicotinamide adenine dinucleotides. A U T H O R I N D E X E . E . B a t o v a 269 C . B e r r i e r 283 A . V . B u l a t o v 289 H. C a r r e y r e 283 E . Dav i 265 M . G . D e B a c k e r 273 A . D u n a c h 259 W . J . G r i f f i t h s 261 K. H a d i z a m a n i 279 F. M . H a r r i s 261 J . C . J a c q u e s y 283 M . P. J o u a n n e t a u d 283 A . N. K i t a i g o r o d s k i i 289 H. L a n g h a l s 279 J . P. L a u s s a c 293 P. P. L e v i n 269 M a n h T h o n g C u n g 293 M . C . M a s s e b i e a u 259 M. M a u r i c e 301 L. P a l m i s a n o 265 J . P e r i c h o n 259 F. X . S a u v a g e 273 A . S c l a f a n i 265 V . Y a . S h a f i r o v i c h 269 J . S o u p p e 301 M . T r o u p e l 259 M . Z . Y a n g 273 New J. Chem., 1990, 14, 279-281 THE R E L A T I O N BETWEEN B R I D G E H E A D REACTIVITY AND E L E C T R O N I C E F F E C T S Heinz Langhals* and Katja Hadizamani Institut für Organische Chemie der Universität München Karlstraße 23, D-8000 München 2, Germany. Recerved August 16, accepted October 24, 1989. Dedicated to Prof. Dr. Ch. Rüchardt on the occasion of his 60th birthday. A B S T R A C T . — Electronic effects on bridgehead reactivity examined by means of UV /V IS and N M R spectroscopic methods lead to the conclusion that bridgehead reactivity is influenced both by angle strain and by electronic effects. In most cases both effects influence the reactivity in the same direction. Electronic effects are especially important when only a small change in bridgehead bond angles occurs during the reaction. This is given in highly strained bridgehead systems. Introduction Bridgehead reactivity ' is a convenient mechanistic t o o l 2 5 for the investigation of geometries of transition states6"9. For example, the main experimental support for the planarisation of the reaction center during S N I reactions derives mainly from this effect 9. The effect is well established for normal sized ring systems, although there are exceptions. For instance, the μ-triquinyl system is discussed to react in solvolysis reactions 10 9 times more slowly than expected from calculated bridgehead s t ra in 1 0 (but see for a further discussion ref . l 1 , , 2 ) . Moreover, with small ring systems — highly strained bridgeheads — solvolysis reactions proceed unexpectedly fas t 3 , 1 3 . Finally, the 4-homocubyl carbenium ion could be established as an intermediate in solvolysis reactions 1 4, but a classical ion is expected to be very highly strained and should resist the planarisation of the bridgehead. We think that the electronegativity of the bridgehead carbon atom — electron donating o f withdrawing properties in ground and transition state — plays an important part, especially in small ring systems, and that this effect must be added to the influence of bridgehead strain. This possible influence has been already mentioned by Bentley and Roberts 1 5. Thus, we sought a simple system which gave experimental information about bridgehead electronegativity. N C = N N £ i / \ C H 3 N 0 2 N 0 2 4^ 4^ X 8 X 9 ©* 9 & Results 14 15 16 A sensitive probe for electronic effects is the absorption wavelength o f donor-acceptor dyes 1 or 2, in which a donor D is conjugated to an acceptor A by a π-system X 1 6 (see also ref . , ? ) . D - X - A D - X - A - X - D 1 2 The absorption wavelength of these systems depends stron­gly upon the donor function of D and is thus sensitive to electronic properties of substituents on D . We used O-picrylethers as chromophors of type 1 with the oxygen as a donor and a nitro group as an acceptor. In order to minimize specific interaction the brideghead carbon under test has to be moved away from the oxygen, but should be linked to it by a group which transmits electronic effects. These conditions are fulfilled by the oximpicrates 3, the synthesis of which is well established 1 8" 2 3. For the UV/VIS measurements we used anti-methylketoximpicrates with brid­gehead substituents R 2 3 which can be prepared according to literature procedure 2 3 , 2 4 from the corresponding bridgehead carboxylic acid by the reaction with methyllithium to the methyl ketone and further with hydroxylamine to the anti-methylketoxime. The methylketoxime can be converted to the anti-methylketoximpicrate with picrylchloride and purified NEW JOURNAL OF CHEMISTRY, VOL. 14, N° 4-1990. — 0398-9836/90/4 279 03/$ 2.30/ © CNRS-Gauthier-Villars 280 Η. LAUGHALS A N D Κ. HAD1ZAMANI by low temperature crystallisation. The shift of these UV/VIS absorption spectra as a function of the bridgehead substituent, given in Table I , is small, but can be obtained quantitatively by a precise spectrometer. From Xmax of the absorption spectra the molar energy ET o f excitation can be calculated according to equation (1). ET = 28 590 [kcal x nm x m o l " 1 ] ^ ™ * (1) The ET values should be used in linear free energy relation­ships rather than Xmax. Another measure for electronic effects is the l H - N M R chemical shift o f the methyl singlet of 3. The methyl group is far enough away from the substituent R so that there cannot be a notable interference with anisotropic effects. These σ values are given also in Table I as well as the kinetic data of the Beckmann rearrangement for the mentioned groups 2 2 , 2 3 . These /revalues are a good measure of electronic effects, for the migrating group carries a large positive charge in the transition state, which is only slightly planarized 2 3. With bridgeheads the trend of the effect is similar, but not so pronounced as with the simple aliphatic groups. The l-bicyclo-[3.2.1]octyl bridgehead derivative (nr. 10) absorbs at short wavelengths (electron withdrawing) and has a low field shift of the methyl singlet in the ' H - N M R spectrum. On the other hand, the 1-homoadamantyl derivative 2 7 ' 2 9 (nr. 15) has a long wavelength absorption and an upfield shift for the methyl singlet. The migration tendencies of these groups (k in Table I) also show a corresponding behaviour. The other bridgehead derivatives give roughly similar results. An excep­tion is the l-bicyclo[2.2.1]heptyl derivative (nr. 8) which has a low reactivity in the Beckmann rearrangement and a downfield shift of the methyl singlet, but the absorption wavelength is unexpectedly high. A n explanation for this effect cannot be given so far. The spectroscopic data of the 1-cubyl and 4-homocubyl derivative (nr. 16 and 17) are especially interesting, for these systems are unexpectedly reactive in the Beckmann rearrangement 2 \ They give a long wavelength UV absorption and a highfield shift of the methyl singlet which suggests electron donating properties. Discussion Firstly, the sensitivity of the spectra of 3 to electronic effects of R was tested. To the end, oximpicrates with primary, secondary, tertiary and increasingly branched tertiary groups R were examined (see Table I , 1 ... 4 and 5 ... 7). The spectra changed in the expected manner from methyl to /m-buty l and to more branched tertiary substituents: the higher the electron donating ability o f R the longer the wavelength of absorption o f 3 (smaller ET values). Parallel to this long wavelength shift in the UV-spectra of 3 there is an upfield shift of the methyl singlet in the ' H - N M R spectrum. Both these effects are accompanied by a large rate enhancement for the Beckmann rearrangement. Table I. - Absorption spectra of bridgehead and other methylke-toximpicrats (3) — comparison with 'H-NMR and kinetic data from Beckmann rearrangement. Nr. Substituent R in 3 y α ^ m a x ETb 6 C [nm] [kcal/mol] [ppm] Is" 1] 1 CH 3 285.1 100.28 2.17 < 0.1 2 C 2 H 5 288.4 99.13 2.12 0.347 3 /-C 3 H 7 289.1 98.89 2.10 11.4 4 /-C4.H9 290.3 98.55 2.02 51.7 5 2,2-Dimethylpropyl 290.1 98.55 1.32 6 1,1,2,2-Tetramethyl- 290.6 98.38 888 propyl 7 1,1-Dipropylbutyl 291.1 98.21 909 8 l-Bicyclo[2.2.1]heptyl 292.6 97.71 2.15 4.72 9 l-Bicyclo[2.2.2]octyl 292.2 97.84 2.02 51.5 10 l-Bicyclo[3.2.1]octyl 291.9 97.84 2.10 87.5 11 l-Bicyclo[3.2.2]nonyl 292.1 97.88 2.02 239 12 1 -Bicyclo[3.3.1 ]nonyl 292.2 97.84 2.10 902 13 1-Twistyl 293.9 97.28 88.4 14 1-Adamantyl 292.3 97.81 2.04 520 15 1 -Homoadamantyl 293.4 97.44 2.04 1 550 16 1-Cubyl 300.4 95.17 2.03 ' 126 17 4-Homocubyl 302.8 94.42 2.02 192 β Absorption wavelength of 3 in cyclohexane; b Molar energy of excitation of 3 in cyclohexane;c *H-NMR chemical shift of the methyl singlet of 3 in CCU; d First order rate constant for the Beckmann rearrangement of 3 in 1,4-dichlorobutane at 80° C (see ref. 2 2" 2 3). 17 The spectral shifts in 3 give information about ground state properties of bridgehead electronic effects. For reaction kinetics the transition state must also be taken into account, but some of the electronic effects wil l have a similar influence there as in the ground state. Deviations in the comparison of spectral data with kinetic data can be understood this way. The unusual reactivity of 1-cubyl and 4-homocubyl bridge­head have been explained in terms of a vertical stabilisation 3 0 — it is interesting that this effect can also be found in the spectral shifts. The measurements indicate that electronic effects play an important part in bridgehead reactivity and must be taken into account as well as angle strain effects. This is all the more important when geometric changes during the reaction are small. Fortunately, in most cases both effects influence the reactivity in the same direction. Acknowledgements This work was supported by Deutsche Forschungsge­meinschaft und Fonds der Chemischen Industrie. Experimental section For the preparation and purification of the anti-methylketoximpi-crates see ref . 2 2 2 3 . The UV/VIS-spectra were measured with 1 cm quartz cuvettes at 20° C in cyclohexane in order to prevent specific interactions with the solvent and to avoid Chapman rearrangement. The spectrometer — MIT 20/L from Bruins — had a resolution and an accuracy of 0.02 nm and was calibrated with holmiumoxide glass24 directly before and after each run. The extinctions of the samples at maximum wavelength were between 0.7 and 1.0 (4-homocubyl and 1-cubyl) derivative: 0.02 ... 0.1). The maximum wavelength was localized by the use of Mathias' rule 2 5 , 2 6 ' . REFERENCES 1 Bartlett P. D., Knox L. H., J. Amer. Chem. Soc, 1939, 61, 3184. 2 Schöllkopf U., Angew Chem., I960, 72, 147. NEW JOURNAL OF CHEMISTRY, VOL. 14, N° 4-1990 ELECTRONIC EFFECTS ON BRIDGEHEAD REACTIVITY 281 3 Fort R. C, v. R. Schleyer P., in Hart H M Karabatsos G. J., Advances in Alicyclic Chemistry, 1966, 1. 4 Fort R. C, in Olah G. Α., v. R. Schleyer P., "Carbonium Ions", J. Wiley-Interscience, New York, 1973, vol. IV, p. 1783. 5 Jefford C. W., McCreadie R., Mueller P., Siegfried B., J. Chem. Educ, 1971, 48, 708. 6 Golzke V., Groeger F., Oberlinner Α., Rüchardt C, Nouv. J. Chim.. 1978, 2, 169. 7 Breslow D. S., Edwards Ε. I . , Leone R., Schleyer P. v. R., J. Am. Chem. Soc, 1968, 90, 7097. 8 Firestone R. Α., J. Org. Chem., 1980, 45, 3604. 9 v. R. Schleyer P., Bingham R. C , J. Am. Chem. Soc, 1971, 93, 3189. 1 0 Bingham R. C, v. R. Schleyer P., Tetrahedron Lett., 1971, 23. 1 1 Müller P., Blanc J., Mareda J., Helv. Chim. Acta, 1986, 69, 635. 1 2 Müller P., Mareda J., Helv. Chim. Acta, 1987, 70, 1017. 1 3 Deila E. W., Pigou P. E., Tsanaktsidis J., J. Chem. Soc, Chem. Commun., 1987, 833. 1 4 Mergelsberg I . , Langhals H., Rüchardt C, Chem. Ber., 1983, 116, 360. 1 5 Bentley T. W., Roberts K., J. Org. Chem., 1985, 50, 5852. 1 6 Fritzsche K., Langhals H., Chem. Ber., 1984, 117, 2275 and ref. therein. 1 7 Reichardt C , "Solvents and Solvent Effects in Organic Chemistry", 2., Verlag Chemie, Wienheim 1988. 1 8 Huisgen R., Witte J., Ugi I . , Chem. Ber., 1957, 90, 1844. 1 9 Huisgen R., Witte J., Jira W., Chem. Ber., 1957, 90, 1850. 2 0 Huisgen R., Ugi I . , Assemi Μ. T., Witte J., Liebigs Ann. Chem., 1957, 602, 127. 2 1 Huisgen R., Witte J., Walz H., Jira W., Liebigs Ann. Chem., 1957, 604, 191. 2 2 Langhals H., Range G., Wistuba E., Rüchardt C, Chem. Ber., 1981, 114, 3813. 2 3 Langhals H., Rüchardt C , Chem. Ber., 1981, 114, 3831. 2 4 Vandenbelt J. M., J. Opt. Soc Am., 1961, 51, 802. 2 5 Brdicka R., "Grundlagen der Physikalischen Chemie", Deutscher Verlag der Wissenschaften, Berlin, 1972, 1 Ith ed., p. 252, 503. 2 6 Langhals H., Angew. Chem., 1982, 94, 739 ; Angew. Chem. int. Ed. Engl., 1982, 21, 724. 2 7 Langhals H., Rüchardt C , Chem. Ber., 1974, 107, 1245. 2 8 Winkler R., Langhals H., Chem. Ber., 1980, 113, 3381. 2 9 Langhals H., Mergelsberg I . , Rüchardt C , Burger U., Chem. Ber., 1982, 115, 1509. 3 0 Traylor T. G., Hanstein W., Berwin H. J., Clinton Ν. Α., Brown R. S., / . Am. Chem. Soc, 1971, 93, 5715. NEW JOURNAL OF CHEMISTRY, VOL. 14, Ν° 4-1990 

94, 739 ;

Advances in Alicyclic Chemistry,

Grundlagen der Physikalischen Chemie&quot;, Deutscher Verlag der Wissenschaften,

Soc Am.,

Solvents and Solvent Effects in Organic Chemistry&quot;,

The relation between bridgehead reactivity and electronic effects

Universität München: Elektronischen Publikationen

New Journal 
of Chemistry 
V O L U M E 14. N U M B E R 4 
A P R I L 1990 
I S S N 0398-9836 
Code N J C H E 5 14 (4) 255-304 
CONTENTS 
C H E M I C A L P A S T T I M E S 
C O M M U N I C A T I O N 
M . - C . M a s s e b i e a u , E . D u n a c h , 
M . T r o u p e l , J . P e r i c h o n (Thiais) 
257 A follow-up on a recent disquisition about referees. 
259 Efficient electrochemical reduction of carbon dioxide on freshly coated metal 
electrodes. 
A R T I C L E S 
W . J . G r i f f i t h s , F. M. H a r r i s (Swansea) 
A . S c l a f a n i , L. P a l m i s a n o , E . D a v i 
(Palermo) 
261 Experimental determination of the single and double ionization energies of 
the halomethanes CBr2FCI and CBr2FH. 
265 Photocatalytic degradation of phenol by T/O2 aqueous dispersions: rutile and 
anatase activity. 
Ε. E . B a t o v a , P. P. L e v i n , V . Y a . S h a f i ro -
v i c h (Chernogo lovka) 
269 Long-lived radical ion pair state of Zn-porphyrin-viologen-quinone triad. 
Μ . Z. Y a n g , M. G . D e B a c k e r , 
F. X . S a u v a g e (L i l le) 
273 Electrochemical and photoelectrochemical characterizations of electrodes 
covered by zinc tetra 2,3 pyridinoporphyrazine layers. 
H. L a n g h a l s , Κ. H a d i z a m a n i ( M ü n c h e n ) 279 The relation between bridgehead reactivity and electronic effects. 
C . B e r r i e r , H. C a r r e y r e , J . - C . J a c q u e s y , 
M . - P . J o u a n n e t a u d (Poit iers) 
A . N. K i t a i g o r o d s k i i , Α . V . B u l a t o v 
(Chernogo lovka) 
M a n h T h o n g C u n g ( N a n c y ) , J . P. L a u s -
s a c (Tou louse) 
M . M a u r i c e , J . S o u p p e (Ar t ix ) 
283 Hydroxylation regioselective en meta de derives chlores de l'aniline et du 
phenol. 
2 8 9 A 1Η NMR study of adducts formed between nitroaromatics and paramagnetic 
alizarine metal complexes in solution. 
2 9 3 1D- and 2D-NMR investigation of the interaction of zinc (I I) and thymulin, 
an immunoregulatory nonapeptide from thymus, in aqueous solution. 
301 Spectral, biochemical, and electrochemical properties of chemically modified 
nicotinamide adenine dinucleotides. 
A U T H O R I N D E X 
E . E . B a t o v a 269 
C . B e r r i e r 283 
A . V . B u l a t o v 289 
H. C a r r e y r e 283 
E . Dav i 265 
M . G . D e B a c k e r 273 
A . D u n a c h 259 
W . J . G r i f f i t h s 261 
K. H a d i z a m a n i 279 
F. M . H a r r i s 261 
J . C . J a c q u e s y 283 
M . P. J o u a n n e t a u d 283 
A . N. K i t a i g o r o d s k i i 289 
H. L a n g h a l s 279 
J . P. L a u s s a c 293 
P. P. L e v i n 269 
M a n h T h o n g C u n g 293 
M . C . M a s s e b i e a u 259 
M. M a u r i c e 301 
L. P a l m i s a n o 265 
J . P e r i c h o n 259 
F. X . S a u v a g e 273 
A . S c l a f a n i 265 
V . Y a . S h a f i r o v i c h 269 
J . S o u p p e 301 
M . T r o u p e l 259 
M . Z . Y a n g 273 
New J. Chem., 1990, 14, 279-281 
THE R E L A T I O N BETWEEN B R I D G E H E A D REACTIVITY AND E L E C T R O N I C E F F E C T S 
Heinz Langhals* and Katja Hadizamani 
Institut für Organische Chemie der Universität München Karlstraße 23, D-8000 München 2, Germany. 
Recerved August 16, accepted October 24, 1989. 
Dedicated to Prof. Dr. Ch. Rüchardt on the occasion of his 60th birthday. 
A B S T R A C T . — Electronic effects on bridgehead reactivity examined by means of UV /V IS and N M R spectroscopic 
methods lead to the conclusion that bridgehead reactivity is influenced both by angle strain and by electronic effects. 
In most cases both effects influence the reactivity in the same direction. Electronic effects are especially important 
when only a small change in bridgehead bond angles occurs during the reaction. This is given in highly strained 
bridgehead systems. 
Introduction 
Bridgehead reactivity ' is a convenient mechanistic t o o l 2 5 for 
the investigation of geometries of transition states6"9. For 
example, the main experimental support for the planarisation 
of the reaction center during S N I reactions derives mainly 
from this effect 9. The effect is well established for normal 
sized ring systems, although there are exceptions. For instance, 
the μ-triquinyl system is discussed to react in solvolysis 
reactions 10 9 times more slowly than expected from calculated 
bridgehead s t ra in 1 0 (but see for a further discussion ref . l 1 , , 2 ) . 
Moreover, with small ring systems — highly strained 
bridgeheads — solvolysis reactions proceed unexpectedly 
fas t 3 , 1 3 . Finally, the 4-homocubyl carbenium ion could be 
established as an intermediate in solvolysis reactions 1 4, but a 
classical ion is expected to be very highly strained and should 
resist the planarisation of the bridgehead. We think that the 
electronegativity of the bridgehead carbon atom — electron 
donating o f withdrawing properties in ground and transition 
state — plays an important part, especially in small ring 
systems, and that this effect must be added to the influence 
of bridgehead strain. This possible influence has been already 
mentioned by Bentley and Roberts 1 5. Thus, we sought a 
simple system which gave experimental information about 
bridgehead electronegativity. 
N C = N N £ i 
/ \ 
C H 3 N 0 2 
N 0 2 
4^ 4^ 
X 
8 
X 
9 
©* 9 & 
Results 14 15 16 
A sensitive probe for electronic effects is the absorption 
wavelength o f donor-acceptor dyes 1 or 2, in which a donor 
D is conjugated to an acceptor A by a π-system X 1 6 (see also 
ref . , ? ) . 
D - X - A D - X - A - X - D 
1 2 
The absorption wavelength of these systems depends stron­
gly upon the donor function of D and is thus sensitive to 
electronic properties of substituents on D . 
We used O-picrylethers as chromophors of type 1 with the 
oxygen as a donor and a nitro group as an acceptor. In order 
to minimize specific interaction the brideghead carbon under 
test has to be moved away from the oxygen, but should be 
linked to it by a group which transmits electronic effects. 
These conditions are fulfilled by the oximpicrates 3, the 
synthesis of which is well established 1 8" 2 3. For the UV/VIS 
measurements we used anti-methylketoximpicrates with brid­
gehead substituents R 2 3 which can be prepared according to 
literature procedure 2 3 , 2 4 from the corresponding bridgehead 
carboxylic acid by the reaction with methyllithium to the 
methyl ketone and further with hydroxylamine to the anti-
methylketoxime. The methylketoxime can be converted to the 
anti-methylketoximpicrate with picrylchloride and purified 
NEW JOURNAL OF CHEMISTRY, VOL. 14, N° 4-1990. — 0398-9836/90/4 279 03/$ 2.30/ © CNRS-Gauthier-Villars 
280 Η. LAUGHALS A N D Κ. HAD1ZAMANI 
by low temperature crystallisation. The shift of these UV/VIS 
absorption spectra as a function of the bridgehead substituent, 
given in Table I , is small, but can be obtained quantitatively 
by a precise spectrometer. From Xmax of the absorption spectra 
the molar energy ET o f excitation can be calculated according 
to equation (1). 
ET = 28 590 [kcal x nm x m o l " 1 ] ^ ™ * (1) 
The ET values should be used in linear free energy relation­
ships rather than Xmax. Another measure for electronic effects 
is the l H - N M R chemical shift o f the methyl singlet of 3. The 
methyl group is far enough away from the substituent R so 
that there cannot be a notable interference with anisotropic 
effects. These σ values are given also in Table I as well as 
the kinetic data of the Beckmann rearrangement for the 
mentioned groups 2 2 , 2 3 . These /revalues are a good measure 
of electronic effects, for the migrating group carries a large 
positive charge in the transition state, which is only slightly 
planarized 2 3. 
With bridgeheads the trend of the effect is similar, but not 
so pronounced as with the simple aliphatic groups. The 
l-bicyclo-[3.2.1]octyl bridgehead derivative (nr. 10) absorbs 
at short wavelengths (electron withdrawing) and has a low 
field shift of the methyl singlet in the ' H - N M R spectrum. On 
the other hand, the 1-homoadamantyl derivative 2 7 ' 2 9 (nr. 15) 
has a long wavelength absorption and an upfield shift for the 
methyl singlet. The migration tendencies of these groups (k 
in Table I) also show a corresponding behaviour. The other 
bridgehead derivatives give roughly similar results. An excep­
tion is the l-bicyclo[2.2.1]heptyl derivative (nr. 8) which has 
a low reactivity in the Beckmann rearrangement and a 
downfield shift of the methyl singlet, but the absorption 
wavelength is unexpectedly high. A n explanation for this 
effect cannot be given so far. The spectroscopic data of the 
1-cubyl and 4-homocubyl derivative (nr. 16 and 17) are 
especially interesting, for these systems are unexpectedly 
reactive in the Beckmann rearrangement 2 \ They give a long 
wavelength UV absorption and a highfield shift of the methyl 
singlet which suggests electron donating properties. 
Discussion 
Firstly, the sensitivity of the spectra of 3 to electronic effects 
of R was tested. To the end, oximpicrates with primary, 
secondary, tertiary and increasingly branched tertiary groups 
R were examined (see Table I , 1 ... 4 and 5 ... 7). The spectra 
changed in the expected manner from methyl to /m-buty l 
and to more branched tertiary substituents: the higher the 
electron donating ability o f R the longer the wavelength of 
absorption o f 3 (smaller ET values). Parallel to this long 
wavelength shift in the UV-spectra of 3 there is an upfield 
shift of the methyl singlet in the ' H - N M R spectrum. Both 
these effects are accompanied by a large rate enhancement 
for the Beckmann rearrangement. 
Table I. - Absorption spectra of bridgehead and other methylke-
toximpicrats (3) — comparison with 'H-NMR and kinetic data from 
Beckmann rearrangement. 
Nr. Substituent R in 3 y α 
^ m a x 
ETb 6 C 
[nm] [kcal/mol] [ppm] Is" 1] 
1 CH 3 285.1 100.28 2.17 < 0.1 
2 C 2 H 5 288.4 99.13 2.12 0.347 
3 /-C 3 H 7 289.1 98.89 2.10 11.4 
4 /-C4.H9 290.3 98.55 2.02 51.7 
5 2,2-Dimethylpropyl 290.1 98.55 1.32 
6 1,1,2,2-Tetramethyl- 290.6 98.38 888 
propyl 
7 1,1-Dipropylbutyl 291.1 98.21 909 
8 l-Bicyclo[2.2.1]heptyl 292.6 97.71 2.15 4.72 
9 l-Bicyclo[2.2.2]octyl 292.2 97.84 2.02 51.5 
10 l-Bicyclo[3.2.1]octyl 291.9 97.84 2.10 87.5 
11 l-Bicyclo[3.2.2]nonyl 292.1 97.88 2.02 239 
12 1 -Bicyclo[3.3.1 ]nonyl 292.2 97.84 2.10 902 
13 1-Twistyl 293.9 97.28 88.4 
14 1-Adamantyl 292.3 97.81 2.04 520 
15 1 -Homoadamantyl 293.4 97.44 2.04 1 550 
16 1-Cubyl 300.4 95.17 2.03 ' 126 
17 4-Homocubyl 302.8 94.42 2.02 192 
β Absorption wavelength of 3 in cyclohexane; b Molar energy of 
excitation of 3 in cyclohexane;c *H-NMR chemical shift of the methyl 
singlet of 3 in CCU; d First order rate constant for the Beckmann 
rearrangement of 3 in 1,4-dichlorobutane at 80° C (see ref. 2 2" 2 3). 
17 
The spectral shifts in 3 give information about ground 
state properties of bridgehead electronic effects. For reaction 
kinetics the transition state must also be taken into account, 
but some of the electronic effects wil l have a similar influence 
there as in the ground state. Deviations in the comparison of 
spectral data with kinetic data can be understood this way. 
The unusual reactivity of 1-cubyl and 4-homocubyl bridge­
head have been explained in terms of a vertical stabilisation 3 0 
— it is interesting that this effect can also be found in the 
spectral shifts. 
The measurements indicate that electronic effects play an 
important part in bridgehead reactivity and must be taken 
into account as well as angle strain effects. This is all the 
more important when geometric changes during the reaction 
are small. Fortunately, in most cases both effects influence 
the reactivity in the same direction. 
Acknowledgements 
This work was supported by Deutsche Forschungsge­
meinschaft und Fonds der Chemischen Industrie. 
Experimental section 
For the preparation and purification of the anti-methylketoximpi-
crates see ref . 2 2 2 3 . The UV/VIS-spectra were measured with 1 cm 
quartz cuvettes at 20° C in cyclohexane in order to prevent specific 
interactions with the solvent and to avoid Chapman rearrangement. 
The spectrometer — MIT 20/L from Bruins — had a resolution and 
an accuracy of 0.02 nm and was calibrated with holmiumoxide glass24 
directly before and after each run. The extinctions of the samples at 
maximum wavelength were between 0.7 and 1.0 (4-homocubyl and 
1-cubyl) derivative: 0.02 ... 0.1). The maximum wavelength was 
localized by the use of Mathias' rule 2 5 , 2 6 ' . 
REFERENCES 
1 Bartlett P. D., Knox L. H., J. Amer. Chem. Soc, 1939, 61, 3184. 
2 Schöllkopf U., Angew Chem., I960, 72, 147. 
NEW JOURNAL OF CHEMISTRY, VOL. 14, N° 4-1990 
ELECTRONIC EFFECTS ON BRIDGEHEAD REACTIVITY 281 
3 Fort R. C, v. R. Schleyer P., in Hart H M Karabatsos G. J., 
Advances in Alicyclic Chemistry, 1966, 1. 
4 Fort R. C, in Olah G. Α., v. R. Schleyer P., "Carbonium Ions", J. 
Wiley-Interscience, New York, 1973, vol. IV, p. 1783. 
5 Jefford C. W., McCreadie R., Mueller P., Siegfried B., J. Chem. 
Educ, 1971, 48, 708. 
6 Golzke V., Groeger F., Oberlinner Α., Rüchardt C, Nouv. J. 
Chim.. 1978, 2, 169. 
7 Breslow D. S., Edwards Ε. I . , Leone R., Schleyer P. v. R., J. Am. 
Chem. Soc, 1968, 90, 7097. 
8 Firestone R. Α., J. Org. Chem., 1980, 45, 3604. 
9 v. R. Schleyer P., Bingham R. C , J. Am. Chem. Soc, 1971, 93, 
3189. 
1 0 Bingham R. C, v. R. Schleyer P., Tetrahedron Lett., 1971, 23. 
1 1 Müller P., Blanc J., Mareda J., Helv. Chim. Acta, 1986, 69, 635. 
1 2 Müller P., Mareda J., Helv. Chim. Acta, 1987, 70, 1017. 
1 3 Deila E. W., Pigou P. E., Tsanaktsidis J., J. Chem. Soc, Chem. 
Commun., 1987, 833. 
1 4 Mergelsberg I . , Langhals H., Rüchardt C, Chem. Ber., 1983, 116, 
360. 
1 5 Bentley T. W., Roberts K., J. Org. Chem., 1985, 50, 5852. 
1 6 Fritzsche K., Langhals H., Chem. Ber., 1984, 117, 2275 and ref. 
therein. 
1 7 Reichardt C , "Solvents and Solvent Effects in Organic Chemistry", 
2., Verlag Chemie, Wienheim 1988. 
1 8 Huisgen R., Witte J., Ugi I . , Chem. Ber., 1957, 90, 1844. 
1 9 Huisgen R., Witte J., Jira W., Chem. Ber., 1957, 90, 1850. 
2 0 Huisgen R., Ugi I . , Assemi Μ. T., Witte J., Liebigs Ann. Chem., 
1957, 602, 127. 
2 1 Huisgen R., Witte J., Walz H., Jira W., Liebigs Ann. Chem., 1957, 
604, 191. 
2 2 Langhals H., Range G., Wistuba E., Rüchardt C, Chem. Ber., 
1981, 114, 3813. 
2 3 Langhals H., Rüchardt C , Chem. Ber., 1981, 114, 3831. 
2 4 Vandenbelt J. M., J. Opt. Soc Am., 1961, 51, 802. 
2 5 Brdicka R., "Grundlagen der Physikalischen Chemie", Deutscher 
Verlag der Wissenschaften, Berlin, 1972, 1 Ith ed., p. 252, 503. 
2 6 Langhals H., Angew. Chem., 1982, 94, 739 ; Angew. Chem. int. Ed. 
Engl., 1982, 21, 724. 
2 7 Langhals H., Rüchardt C , Chem. Ber., 1974, 107, 1245. 
2 8 Winkler R., Langhals H., Chem. Ber., 1980, 113, 3381. 
2 9 Langhals H., Mergelsberg I . , Rüchardt C , Burger U., Chem. Ber., 
1982, 115, 1509. 
3 0 Traylor T. G., Hanstein W., Berwin H. J., Clinton Ν. Α., Brown 
R. S., / . Am. Chem. Soc, 1971, 93, 5715. 
NEW JOURNAL OF CHEMISTRY, VOL. 14, Ν° 4-1990 


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The relation between bridgehead reactivity and electronic effects

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