Cyclophanes II

164 Pages·1983·3.371 MB·English

by  Edgar Heilbronner, Zhong-zhi Yang (auth.), F. Vögtle (eds.)

CyclophaneIIs
Editor: F. Vijgtle
With Contributions by
F. Gerson, E. Heilbronner, Y. Murakaxni,
F. VGgtle, Z. Yang
With 71 Figures and 39 Tables
Springer-Verlag
Berlin HeidelbergN ew York Tokyo
1983
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ISBN 3-540-12478-O Springer-Verlag Berlin Heidelberg New York Tokyo
ISBN 0-387-12478-O Springer-Verlag NewYork Heidelberg Berlin Tokyo
Library of Congress Cataloging in Publication Data. Main entry under title:
Cyclophanes II.
(Topics in current chemistry; 115)
Bibliography: p. Includes index.
1. Cyclophanes~~Addresses, essays, lectures.
I. VGgtle, F. (Fritz), 1939 -, II. Gerson, Fabian. III. Title: Cyclophanes 2. IV. Title:
Cyclophanes two. V. Series.
QDl.F58 vol. 115 540s [547’.59] 83-6860 [QD400]
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Editorial Board :
Prof. Dr. Michael J. S. Dewar Department of Chemistry, The University of Texas
Austin, TX 18712, USA
Prof. Dr. Jack D. Dmitz Laboratorium filr Organische Chemie der
EidgenGssischen Hochschule
UniversitatsstraBe 6/S, CH-8006 Zurich
Prof. Dr. Klaus Hafner Institut fib Organische Chemie der TH
PetersenstraBe 15. D-6100 Darmstadt
Prof. Dr. Edgar Heilbronner Physikalisch-Chemisches Institut der Universitgt
KlingelbergstraDe 80, CH-4000 Base1
Prof. Dr. Shd It8 Department of Chemistry, Tohoku University,
Sendai, Japan 980
Prof. Dr. Jean-Marie Lehn Institut de Chimie, Universite de Strasbourg, 1, rue
Blaise Pascal, B. P. Z 296/R8, F-67008 Strasbourg-Cedex
Prof. Dr. Kurt Niedenru University of Kentucky, College of Arts and Sciences
Department of Chemistry, Lexington, KY 40506, USA
Prof. Dr. Kenneth N. Raymond Department of Chemistry, University of California,
Berkeley, California 94720, USA
Prof. Dr. Charles W. Rees Hofmann Professor of Organic Chemistry, Department
of Chemistry, Imperial College of Science and Technology,
South Kensington, London SW7 2AY, England
Prof. Dr. Kiaus Schiifer lnstitut fiir Physikalische Chemie der Universitat
Im Neuenheimer Feld 253, D-6900 Heidelberg I
Prof. Dr. Fritz Viigrie Institut fiir Organ&he Chemie und Biochemie
der Universimt, Gerhard-Domagk-Str. 1,
D-5300 Bonn 1
Prof. Dr. Georg Wittig Institut fur Organ&he Chemie der Universitit
Im Neuenheimer Feld 270, D-6900 Heidelberg I
Table of Contents
The Electronic Structure of Cyclophanes as Suggested by their
Photoelectron Spectra
E. Heilbronner, Z. Yang . . . . . . . . . . . . . . . .
Radical Ions of Phanes as Studied by ESR and ENDOR
Spectroscopy
F. Gerson . . . . . . . . . . . . . . . . . . . . . . 57
dezifanoitcnuF Cyclophanes as Catalysts and Enzyme Models
Y. Murakami . . . . . . . . . . . . . . . . . . . . 107
gnidulcnoC Remarks
F. V6gtle . . . . . . . . . . . . . . . . . . . . . . 157
rohtuA Index Volumes 101-115 . . . . . . . . . . . . . 161
Table of Contents of Volume 113
Cyclophanes I
Synthesis of Medio- and Macrocyclic Compounds by High
Dilution Principle Technique
L. Rossa, F. V6gtle
Syntheses and Properties of the ]n2[ Cyclophanes
V. Boekelheide
Water Soluble Cyclophanes as Hosts and Catalysts
I. Tabushi, K. Yamamura
The Electronic Structure of Cyclophanes
as Suggested by their Photoelectron Spectra
"'Dedicated to Prof. J. D. Dunitz on the occasion of his 60th birthday"
Edgar Heilbronner and Zhong-zhi Yang 1
Physikalisch-Chemisches Institut der Universit~t Basel, KlingelbergstraBe ,08 CH-4056 Basel,
Switzerland
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Nomenclature and Experimental Details . . . . . . . . . . . . . . . . . 4
2.1 Designation of Structure . . . . . . . . . . . . . . . . . . . . . 4
2.2 Characterization of the Radical Cation States
and Numbering of the PE-Bands . . . . . . . . . . . . . . . . . 5
2.3 Recording of the He(I0~) PE-Spectra . . . . . . . . . . . . . . . . 6
3 HeOa) PE-Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 [2.2] Paracyclophane and Derivatives . . . . . . . . . . . . . . . . 7
3.2 [2n] Cyclophanes . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 [2.2]Phanes Involving Higher Aromatic Systems . . . . . . . . . . . 13
3.4 Heterocyclic Phanes . . . . . . . . . . . . . . . . . . . . . . . 17
3.5 Miscellaneous Systems . . . . . . . . . . . . . . . . . . . . . . 19
4 Theoretical Model . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.1 Basis n-Orbitals . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2 Basis ~-Orbitals for the Bridging Alkyl Chains . . . . . . . . . . . . 25
4.3 "Through Space" n-n Interaction . . . . . . . . . . . . . . . . . 26
4.4 "Through Bond" Interaction . . . . . . . . . . . . . . . . . . . 29
5 Assignment and Discussion of the PE-Spectra . . . . . . . . . . . . . . 30
5.1 [2.] Cyclophanes . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2 Methyl-Substituted [2.2]Paracyclophanes . . . . . . . . . . . . . . 34
5.3 [2.2]Phanes Involving Higher Aromatic Systems . . . . . . . . . . . 36
5.4 [m.n] Paracyclophanes . . . . . . . . . . . . . . . . . . . . . . 38
1 Permanent address: Theoretical Chemistry Institute. Jilin University, Chang Chun, People's Repu-
blic of China.
Edgar Heilbronner and Zhongozhi Yang
5.5 Phanes with Unsaturated Bridges . . . . . . . . . . . . . . . . . 40
5.6 Heterocyclic Phanes . . . . . . . . . . . . . . . . . . . . . . . 43
5.7 Substituted [2.2]Paracyclophanes . . . . . . . . . . . . . . . . . 44
6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.1 The Influence of re-System Deformation on Ionization Energy ..... 47
6.2 Cyclophane-Type Interactions between Two-Centre ~-Systems ..... 48
6.3 Comment on the Inductive vs. Hyperconjugative Effect
of Alkyl Groups . . . . . . . . . . . . . . . . . . . . . . . . . 51
6.4 Breakdown of the Symmetry Restriction of Hyperconjugation ..... 52
7 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . 53
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Structure The Electronic of senahpolcyC
1 Introduction
The most intriguing question relating to cyclophanes is how the forced proximity
of two n-systems influences their chemical and physical properties, in particular
their photophysical behaviour. In this review we discuss the ionization energies (I j)
of cyclophanes, as determined by He(I~) photoelectron (PE) spectroscopy.
The X;ray analysis of the cyclophane prototype [2.2](1,4)cyclophane (1) (= para-
cyclophane) )1 reveals that the two benzene moieties are separated by a mean
interdeck distance D = 299 pm, much smaller than the usual n-system van derW aals
contact distance of ~ 340 pm between parallel aromatic hydrocarbons in crystals ,)2
or even the graphite layer distance of 335 pm. As shown, the bridged centres are
only 278 pm apart, the others 309 pm, which means that the two rings are bent out of
plane by a fold angle of °. 12.6
How are these rather unique structural features of I going to affect its ionization
energies IT, in particular the first one IT, relative to that of the parent compound
benzene? Rather naively one might have expected that the n -1 ionization energy
IT of 1 differs from the corresponding elg(n) -1 ionization energy of benzene
(IT = 9.25 eV) for at least three reasons:
)1 and/or Inductive evitagujnocrepyh effect of eht gnigdirb .spuorg It is known that
the n-1 ionization energies IT and '~I of 1,4-dialkylbenzenes are significantly smaller
than T = I IT = 9.25 eV of benzene, e.g.
1,4-dimethylbenzene, IT = 8.60 eV, '~I = 9.1s eVa);
1,4-diethylbenzene, ~I n = 8.45 eV, IT = 9. lo eV;
1,4-dipropylbenzene, IT = 8.30 eV, IT = 9.0o eV .~4
This trend is the one expected, if we assume that the ionization energy reduction due
to an alkylsubstituent R is measured by its ~* value )5 (~ = 0.49, g~e = 0.00,
o~ t = --0.10, *ro r = --0.12) or, more precisely by the alkyl group parameters Rt~ H
deduced from the mean 5 p-1 ionization energies of alkyliodides RI )6 = (~t~ 0.00,
Z
mX
y I
and Heilbronner Edgar ihz-gnohZ Yang
ent~ = --0.89, tnrt~ = --1.08, ~npt~ = --1.18). Other things being equal, one would
therefore expect that the two bridging CH2CH2 groups shift IF of 1 to ,-~8.5 eV
by virtue of their inductive and/or hyperconjugative effect.
2) Steric interference of the n clouds. Obviously the two n clouds of the upper and
lower benzene rings in 1 seem to be pressed hard against each other as indicated
by the small mean distance D = 299 pm and the bent structure of the two rings.
This should lead to additional repulsion between the n-electrons crowded into the
space between the two rings. Intuitively one would predict that this destabilizing
interaction can be relieved by lowering the number of interacting n-electrons i.e.
by ionization. If this were true, we would be in the presence of a "toothpaste tube
effect" i.e. the harder you press the~electrons the easier they pop out. Needless to say
that this effect would tend to further reduce the ionization energy of I relative to
that of benzene. However, it is not obvious how big such an effect would be, except
that it should increase with decreasing distance D.
3) Bending of the n-systems. Curiously, but perhaps not unexpectedly, qualitative
reasoning along "resonance energy" lines is rather inadequate for dealing with the
question whether bending will increase or decrease the n-x ionization energies.
Indeed, lack of planarity of the benzene system could be construed to decrease its
"aromaticity" and hence its stability with the consequence that ionization should be
facilitated. On the other hand, bending a n-system will tend to localize its bonds i.e.
push it towards a more polyenic type of structure which should then be more difficult
to ionize.
Asw e shall see none of these explanations holds. In contrast to the usual conjugative
interaction of an alkyl group with a re-system, hyperconjugation in t is severely
limited by symmetry, so that a comparison e.g. with alkylsubstituted benzenes is
quite misleading. Furthermore, the postulated "toothpaste tube effect" is probably
very small if not negligible, and finally it is known that bending of tightly bound
n-systems such as double bonds ,)7 triple bonds ~8.7 or benzene rings ~* has no
significant influecne on their n-x ionization energies, as long as the deformations do
not exceed a critical limit .)9
In view of the failure of seemingly "obvious" explanations it is gratifying that the
body of experimental data accumulated to date can be rationalized in a straightforward
manner by using well-established model concepts borrowed from simple molecular
orbital theory, such as Hoffmann's "through-space" and "through-bond" orbital
interaction scheme .~01
2 Nomenclature and Experimental Details
2.1 Designation of erutcurtS
With few exceptions, the systems we are going to discuss consist of an upper and a
lower deck, u M and ~M shifted along the z-axis by a mean distance D with respect to
each other. They are linked by (CH2)n bridges (mainly n = 2) which connect e.g.
centres a, b of Mu to c, d of ~M as shown in the following diagram.
The Electronic Structure of Cyclophanes
H 2 C - - ~ HC 2
(1)
For simplicity we number the rc-centres of the upper and lower deck of 1 as
indicated in the left-hand formula, rather than according to the rules used normally
shown on the right. This has the advantage of assigning the same index to 2p atomic
orbitals facing each other.
6 5 7 8
(2)
~ 4 ' 01 1
*2 *3 21 31
Consequently, the linked centres in e.g. [24](1,2,4,5)cyclophane are given as 1--1',
2--2', 4--4', 5--5' rather than 3--12, 5--14, 6--15, 8--11, as required by the usual
convention depicted in the following formula.
81 91
71 , 02
To avoid any ambiguity in the symmetry designations of orbitals, configurations
and states of cyclophanes belonging to the point groups v2C and ,h2D the orientation
of the respective coordinate systems has been included in the structure formulae.
2.2 Characterization of the Radical Cation States
and Numbering of the PE-Bands
As is customary in PE spectroscopy the observed radical cation states jW2 of + M
are labeled R, ,~/ ~, C, etc. in ascending order of energy. If possible, they are
specified by including in parenthesis either the symmetry designation of the state
jW2 or, implying Koopmans' theorem, the vacated canonical molecular orbital %.
Thus the first four states of the paracyclophane radical cation ÷ 1 would be given
sa ,)s2B2(X ,)saB2(~, ,).3B2(~I ).2B2(C ro ,)lg2b(X ,)Xgfb(A ,)~.fb(~l .)~.;b(C