The Nonclassical Ion Problem

311 Pages·1977·5.897 MB·English

by  Herbert C. Brown (auth.)

mE
NONCLASSICAL ION
PROBLEM
mE
NONCLASSICAL ION
PROBLEM
Herbert C. Brown
Purdue University
West Lafayette, Indiana
With comments by
Paul von R. Schleyer
University of Erlangen-Nurnberg
Erlangen, West Germany
Plenum Press • New York and London
Library of Congress Cataloging in Publication Data
Brown, Herbert Charles, 1912-
The nonclassical ion problem.
Includes index.
1. Carbonium ions. I. Schleyer,PaulvonR.,1930- II. Title.
QD305.C3B76 547.7 76-45175
ISBN-13:978-1-4613-4120-8 e-ISBN- 13:978-1-4613-4118-5
DOl: 10_1007/978-1-4613-4118-5
©1977 Plenum Press, New York
Softcover reprint of the hardcover 1st edition 1977
A Division of Plenum PufJlishing Corporation
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All rights reserved
No part of this book may be reproduced, stored in a retrieval system, or transmitted,
in any form or by any means, electronic, mechanical, photocopying, microfilming,
recording, or otherwise, without written permission from the Publisher
To a fascinating, seductive intermediate,
the 2-norbornyl cation,
and to my wonderful wife,
who cheerfully coped.
Acknowledgments
The author is deeply indebted to Professor Paul von Rague Schleyer, with
whom a continuous discussion on a scientific level was maintained for many
years, even though each of us occasionally arrived at different conclusions;
to Dr. M. Ravindranathan, who gave exceptional assistance in preparing the
drawings, checking the references and manuscript, and in assisting with the
pUblication; and to Annette Wortman, who did her usual superlative job in
typing the manuscript.
H.CB.
The commentator has been debating the subject of nonclassical ions with
Professor Brown privately for more than a decade. Our letters fill a large
volume, and I am only one of the many persons with whom he has had an
extensive correspondence! I must confess that many of his writings, pub
lished and unpublished, have infuriated me and have on occasion elicited
intemperate responses. Nevertheless, communication has remained open
and I feel that real progress in understanding the nature of carbocations has
been made during this period. My own position has undergone considerable
change, as has that of Professor Brown. However, we still diverge in our
views of this topic. In our discussions, it is inevitable that areas of disagree
ment are emphasized more than areas of agreement, although the latter are
large. It is very much to Professor Brown's credit that he has repeatedly
attempted to arrange both open and published debates on the subject of
nonclassical ions. Professor Olah has presented his viewpoint, but I have
remained silent. I wish to thank Professor Brown, not only for the chance to
comment on the chapters of this book, but also for his hospitality during
visits to Purdue University, where most of these comments were written.
The manuscript was completed during the tenure of an Adjunct Professor
vii
viii Acknowledgments
appointment at Case Western Reserve University, and benefited from
helpful suggestions provided by Professor J. E. Nordlander and Professor G.
A. Olah.
P.v.R.S.
Preface
In the early 1950s it was proposed that the cationic centers of carboniun ions
in the usual solvolytic media could diminish their electron deficiency by
interacting with the <T-electrons in saturated carbon-carbon or carbon
hydrogen bonds within the structure to form <T-bridges, giving new struc
tures with markedly different properties, different symmetries, and
enhanced stabilities. Such <T-bridged "nonclassical" structures were pos
tulated to involve the formation of two electron-three center bonds of the
kind present in diborane and trimethylaluminum dimer.
It was further proposed that <T-participation was important in the
transition states leading to the formation of such stabilized <T-bridged
cations as intermediates. Consequently, many cases of enhanced rates of
solvolysis, previously attributed to relief of steric strain, were reinterpreted
as involving this new phenomenon.
This new proposal of nonclassical structures for carbonium ions proved
to be a popular one and was widely adopted. Indeed, in the 1950s nonclassi
cal structures appear to have been at least considered for every carbonium
ion known with the possible exception of methyl.
In science it is exceptionally rare to have a generally accepted concept
challenged. Scientists become highly conservative-once a concept has
become incorporated into accepted theory. This case proved to be no
exception. My call for a reexamination of the concept brought forth a
vigorous reaction. What became known as the "nonclassical ion con
troversy" has been raging for many years.
For reasons which are presented in this book, the 2-norbornyl cation
became the focus of the argument. Are the high exo : endo rate and product
ratios in this system the result of <T-participation in the exo, or are they the
result of steric factors? Is this cation a symmetrical <T-bridged species in
ix
x Preface
which the carbonium carbon has four nearest neighbors, with the bridging
carbon (C6) having five, or do we have a pair of rapidly equilibrating classical
cations with no unusual coordination numbers for the carbon atoms in the
structure?
One would have thought that the application of careful experiments
and intelligent thought to the problem would lead rapidly to a clear
conclusion. This has not been the case. Consequently, the nonclassical ion
problem provides an ideal area for examination of the actual operation of
the scientific method, with its strengths and weaknesses.
Perhaps the most unfortunate consequence of the difference of opinion
in this area has been the emotion it aroused. Indeed, since 1962 when I first
questioned the nonclassical proposal at a Reaction Mechanisms Conference
at Brookhaven, no open debate of the question was ever held. In fact,
even the attempts of the editors of Chemical and Engineering News and of
Accounts of Chemical Research to hold "symposia-in-print" on the subject
did not materialize.
In this book I have attempted to present all pertinent data, with frank
and open consideration of the pros and cons. It makes a truly fascinating
story. I hope that the reader will find it as fascinating an experience to read as
I encountered in pursuing this unusual hobby.
West Lafayette, Indiana Herbert C. Brown
March 1977
Contents
List of Figures xvii
Abbreviations xxi
1. That Fascinating Nonclassical Ion Problem
1.1. Introduction . . . . . 1
1.2. Origins ...... . 2
1.3. The Nonclassicallon Era 3
1.4. StericAssistance 4
1.5. An Alternative Interpretation 5
1.6. The Rococo Period of Carbonium Ion Structures 6
1.7. Difficulties in Challenging an Accepted Theory 8
1.8. Further Difficulties-A "Soft" Theory 9
1.9. Still Further Difficulties-Selective Reviews 11
1.10. Conclusion 12
Comments ........ . 13
2. Steric Assistance in Solvolytic Processes
2.1. Introduction . . . . . . . . . . . . . . . . . . 21
2.2. Steric Assistance in the Solvolysis of Highly Branched
Alkyl Derivatives . . . . . . . . . . . . . . . . 22
2.3. Steric Assistance in the Relative Effects of Methyl and
tert-Butyl Groups . . . . . . . . . 23
2.4. Steric Effects in Norbornyl Derivatives 26
2.5. Steric Effects in Ring Systems 28
2.6. Conclusion 30
Comments ....... . 32
3. u-Participation-A Factor in Fast Rates?
3.1. Introduction . . . . . . 35
3.2. n-, '1T-, and u-Participation 35
xi
xii Contents
3.3. u-Participation and Fast Rates 38
3.4. u-Participation vs. Steric Assistance 39
3.5. StericAssistance-Notu-Participation 40
3.6. The Fast Rates of Exo-Norbornyl and Cyclopropyl-
carbinyl 44
Comments . . . . . . . . . . . . . . . . . . . 45
4. Carbon-Bridged Cations
4.1. Introduction .. . . . . . . . 47
4.2. What Is a NonclassicalIon? ... 47
4.3. u-Bridgingvs. Hyperconjugation 50
4.4. Static Classical, Equilibrating Classical, or u-Bridged
Cations ............ . 54
4.5. Stereochemistry and u-Bridged Cations 56
4.6. '11"-and Aryl-Bridged Cations 59
4.7. Conclusion 66
Comments ....... . 67
5. The Cyclopropylcarbinyl Cation
5.1. Introduction . . . . . . . . . . . . . . . . . . 69
5.2. Exceptionally Fast Rates of Solvolysis ...... . 69
5.3. Rearrangements in the Solvolysis of Cyclopropylcar-
binyl Derivatives ....... . 72
5.4. u-Bridged Structures for the Cation 73
5.5. Stereochemical Characteristics 75
5.5. Conclusion 78
Comments ... 79
6. The 2-Norbomyl Cation
6.1. Introduction . . . . . . . . . . . . . . . . . . 83
6.2. Nonclassical "Structures" for the 2-Norbornyl Cat-
ion ............ 84
6.3. Basic Facts . . . . . . . . . 86
6.4. Transition State or Intermediate 88
6.5. Equilibrating Cations .... 89
6.6. Are Exo Rates Unusually Fast? 91
6.7. Do the High Exo: Endo Rate Ratios Require u-Bridged
Cations? . . . . . . . . . . . . . . . . . . . . 92
6.8. Do the High Exo: Endo Product Ratios Require u-
Bridged Cations? ................ 94