Table Of ContentREARRANGEMENTS
IN GROUND AND
EXCITED STATES
edited by
Paul de Mayo
Photochemistry Unit
Department of Chemistry
The University of Western Ontario
London, Ontario, Canada
1
1980
ACADEMIC PRESS
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Library of Congress Cataloging in Publication Data
Main entry under title:
Rearrangements in ground and excited states.
(Organic chemistry series ; )
Includes bibliographical references and index.
1. Rearrangements (Chemistry)—Addresses, essays,
lectures. I. Mayo, Paul de. II. Series: Organic
chemistry series (New York) ;
QD281.R35R42 547. Γ39 79-51675
ISBN 0-12-481301-1 (v. 1)
PRINTED IN THE UNITED STATES OF AMERICA
80 81 82 83 9 8 7 6 5 4 3 2 1
LIST OF CONTRIBUTORS
Numbers in parentheses indicate the pages on which the authors' contributions begin.
A. L. J. Beckwith (161) Department of Organic Chemistry, The Univer
sity of Adelaide, Adelaide, South Australia
Jerome A. Berson (311) Department of Chemistry, Yale University,
New Haven, Connecticut 06520
Richard D. Bowen (55) University Chemistry Laboratory, Cambridge,
CB2 1EW, England
Jayaraman Chandrasekhar (1) Institut für Organische Chemie, Universität
Erlangen-Nürnberg, 8520 Erlangen, Federal
Republic of Germany
D. H. Hunter (391) Department of Chemistry, University of West
ern Ontario, London, Ontario N6A 5B7, Canada
K. U. Ingold (161) Division of Chemistry, National Research
Council of Canada, Ottawa K1A OR6, Canada
W. M. Jones (95) Department of Chemistry, University of
Florida, Gainesville, Florida 32611
Martin Saunders (1) Department of Chemistry, Yale University,
New Haven, Connecticut 06520
J. B. Stothers (391) Department of Chemistry, University of West
ern Ontario, London, Ontario N6A 5B7, Canada
Paul von Ragué Schleyer (1) Institut für Organische Chemie, Universität
Erlangen-Nürnberg, 8520 Erlangen, Federal
Republic of Germany
E. W. Warnhoff (391) Department of Chemistry, University of West
ern Ontario, London, Ontario N6A 5B7, Canada
Dudley H. Williams (55) University Chemistry Laboratory, Cambridge
CB2 1EW, England
vii
FOREWORD
This volume had an elder sibling, but that was by another sire. Nonethe
less a family resemblance remains. In both cases the term "rearrange
ment" has been interpreted to mean what the editor wanted it to, at,
following the precedent set by Humpty Dumpty, whatever cost, provided
the interest justified it. The subjects of these separate essays, while not
covering the entire span of possibilities, do indeed enclose much territory.
When compared with the previously mentioned work on rearrangements
("Molecular Rearrangements," Part I, 1963) one may see, in part, the
remarkable and vigorous development of chemistry over the past fifteen
years.
One can discern at least three major surges of creative activity. First,
the flood from the lower reaches of the periodic table now makes it
impossible for the educated chemist to insist (while looking askance at
boron) that, with the exception of sulfur, the essentials of chemistry are
contained in the first row. Second, the period of the previous volume
coincided with, and so missed, the photochemical renascence, the latter
being such that virtually all laboratories are now equipped with the pale
blue lights which have provided rearrangements in abundance. The prin
ciples guiding these excited state transformations are beginning to
emerge, and chemistry is no longer a single surface subject. Indeed, even
the ubiquitous interest in energy hypersurfaces dates from the birth of the
earlier volumes. Further, some aspects of photochemistry were the stimu
lus for, perhaps, the most fruitful concept of this period: the emergence,
or, better, recognition of the view that the orbitals of reacting molecules
guided their transformation by placing barriers on certain pathways,
hitherto apparently as permitted as others, in order to maintain symmetry
or maximum overlap: the orbital correlation diagram has, in this period,
already found its way into the undergraduate text. The third supernova is,
therefore, the descent of orbital theory into the marketplace. In short, it
has become respectable, indeed barely highbrow, to be a theoretician; and
their mixing in polite society at meetings with experimentalists has be
come a commonplace and a stimulation. All these changes—imagine the
state of the art of the subjects of these essays fifteen years ago—in a
subject deemed, by some, dead.
As previously, the essays are intended to be critical, stimulating, per
sonal, and creative: they are not intended to be comprehensive, though
adequate referencing should make them a starting point for research.
ix
X FOREWORD
Some subjects will be found absent. In some of these too little change has
occurred in fifteen years to justify a new treatment. In other instances
adequate treatment elsewhere rendered an essay unnecessary. And it
must be admitted some lacunae are caused by the willingness of the spirit
of some potential authors, but, alas, their weakness of the flesh. . . .
The arrangement is roughly the following: first, ground state transfor
mations; a bridge passage of theory, with a codetta of those interesting
transformations that start on the ground state surface and finished in the
excited state; and, finally, transformations on upper surfaces. But the
separation is, in part, a formalism: silicon chemistry includes photo
chemistry, and cis-trans isomerism may be thermal—a sign that chemis
try, once a whole and then fragmented, is gradually becoming unified
again.
PAUL DE MAYO
"Contrariwise", continued Tweedledee, "if it was so, it might be; and if it
were so, it would be: but as it isn't, it ain't. That's logic."
L. Carroll
. . . Western culture has been profoundly affected by the findings, the
methodology, the attitudes, and the outlook of scientists. It is not wholly the
fault of scientists that what has passed into the general culture is grossly
distorted in two critical ways. One is the mistaken identification of science with
rationality. The other is the exaggerated dichotomy between science and
nonscience.*
G. Vickers
Reprinted with permission of MIT Press, Cambridge, Massachusetts.
CONTENTS OF OTHER VOLUMES
VOLUME 2
Rearrangements: A Theoretical Approach
Nicolaos D. Epiotis, Sason Shaik, and William Zandar
Rearrangements Involving Boron
Andrew Pelter
Molecular Rearrangements of Organosilicon Compounds
A. G. Brook and A. R. Bassindale
The Polytopal Rearrangement at Phosphorus
F. H. Westheimer
Rearrangements in Coordination Complexes
W. G. Jackson and A. M. Sarge son
Fluxional Molecules: Reversible Thermal Intramolecular Rearrangements of
Metal Carbonyls
F. A. Cotton and B. E. Hanson
Index
VOLUME 3
Chemical Generation of Excited States
N. J. Turro and V. Ramamurthy
Cis-Trans Isomerism of Olefins
J. Saltiel and J. L. Charit on
xiii
xiv CONTENTS OF OTHER VOLUMES
Photochemical Rearrangements in Trienes
W. G. Dauben, E. L. Mclnnis, and D. M. Michno
The Di-7r-Methane (Zimmerman) Rearrangement
Howard E. Zimmerman
Photochemical Rearrangements of Enones
David I. Schuster
Photochemical Rearrangements of Conjugated Cyclic Dienones
Kurt Schaffner and Martin Demuth
Rearrangements of the Benzene Ring
D. Bryce-Smith and A. Gilbert
Photorearrangements via Biradicals of Simple Carbonyl Compounds
Peter J. Wagner
Photochemical Rearrangements Involving Three-Membered Rings
Michel Nastasi and Jacques Streith
Photochemical Rearrangements of Five-Membered Heterocycles
Albert Padwa
Photochemical Rearrangements of Coordination Compounds
Franco Scandola
Index
ESSAYA I REARRANGEMENTS
OF CARBOCATIONS
MARTIN SAUNDERS,
JAYARAMAN CHANDRASEKHAR,
AND PAUL VON RAGUÉ SCHLEYER
I. INTRODUCTION 1
II. DIRECTLY OBSERVABLE CARBOCATIONS 3
A. Rapid Degenerate 1,2-Shifts 5
B. Deuterium Perturbation Method for the
Nonclassical Ion Problem 8
C. Hydride Shifts to More Distant Carbons . . .. 11
D. Solvents and Solvent Effects 14
III. MULTIPLE REARRANGEMENT REACTIONS 22
A. Monte-Carlo Methods of Analysis 23
B. Use of Graphs to Elucidate Rearrangement
Mechanisms 24
IV. THEORETICAL STUDIES 34
A. Migratory Aptitudes 34
B. Effect of Ring Size on the Ease of 1,2-Shifts . . 41
C. Rearrangements in Cycloalkenyl Cations . . .. 44
V. CONCLUSIONS 47
REFERENCES 48
I. INTRODUCTION
Rearrangements are characteristic of carbonium ion reactions. Carban-
ions, free radicals, and other intermediates are far less prone to such
transformations. Carbocations are electron-deficient species; they do not
possess enough electrons to satisfy the octet rule for each atom and
simultaneously to provide two electrons for each bond. Electron distribu
tions that differ significantly from classical bonding models result. Often a
repositioning of the nuclei occurs in order to increase the number of
bonding contacts. In effect, carbocations have a lower inherent structural
integrity than more electron-rich species; carbocation potential energy
surfaces are flatter, and the tendency to rearrange and to form bridged or
otherwise delocalized structures is great.
The first work in which carbonium ion rearrangements were observed
was carried out in nucleophilic solvents where cation lifetimes were
1
REARRANGEMENTS IN GROUND AND EXCITED STATES, VOL. I
Copyright © 1980 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-481301-1
2 MARTIN SAUNDERS ET AL
immeasurably short (7). Under these conditions, the rates of individual
rearrangement steps could not be determined. The structures of the car-
bocation intermediates themselves could be inferred only indirectly from,
for example, the rate- or product-determining transition states in a
solvolysis. Carbocation systems capable of undergoing degenerate rear
rangement to the same chemical structure are illustrative. If isotopie or
other labels in the products observed are not distributed statistically, it is
generally assumed that classical carbocation intermediates that do not
have sufficient time to equilibrate before quenching are involved. When
the statistical mixture of products is obtained, either fully equilibrated
sets of classical cations or symmetrically bridged species (the so-called
nonclassical ions) are considered to be responsible. Kinetic data have
often been used to attempt to differentiate between these possibilities.
However, solvolysis rates give information only about the transition
states between the starting materials and the ions. The transition states
are not always closely related in structure and energy to the ions them
selves. Quantitative prediction of solvolysis data is not yet possible. As a
result, the interpretation of solvolytic data in order to draw conclusions
about the structures of the ions often involves uncertainty and engenders
controversy.
Since the appearance of Part I of this volume's predecessor (2), several
major developments have occurred that have increased our understanding
of carbonium ions and their rearrangements. First, carbocations as stable
species in solution in nonbasic media can now be investigated directly (3).
Many new techniques have been developed for this purpose or have been
applied to carbocation problems.
Second, the chemistry of gas-phase ions has come of age (4). Although
not much structural information can be obtained directly, an increasing
body of invaluable quantitative data on the energies and reactions of ions
in the gas phase is being assembled, which, inter alia, facilitates the
interpretation of processes occurring in solution. We can now begin to
ascertain the degree to which solvent affects the properties of ions (5).
Third, theoretical calculations have become a valuable source of de
tailed information not available otherwise (6). The structures of only a few
stable carbocations and of no rearrangement transition states are known
experimentally. Ground and transition state structures can, however, be
calculated theoretically with a sufficiently high degree of accuracy, at
least for some interpretative purposes.
Fourth, it is now appreciated that typical solvolysis reactions, espe
cially of primary and secondary substrates, do not involve "free" carbo
cation intermediates but rather highly solvated "cationoid" species with
partially positive character (7). Nucleophilic solvent assistance can play a
