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Book Review Stable Carbocation Chemistry. Edited by G. K. S. Prakash and P. von R. Schleyer

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~~
general chemical training. Again, we find
that although a “rotary evaporator with
vacuum pump” (p. 97, paragraph 6) is
used in nearly every case, it appears repeatedly in the list of apparatus for several
of the procedures. It would have been better to include a short chapter collecting
together general information about laboratory practice, which could have been
placed immediately before the list of suppliers at the end of the book. The space
thereby released would perhaps make it
possible to list the most important spectroscopic data for the compounds described, or at least to give the relevant literature references, as is provided, for
example, in Organic Syntheses.
However, despite these minor shortcomings, this is a very useful compilation
of important synthetic methods for
macrocycles, which is clearly set out and
convenient to use. The detailed laboratory
recipes will be helpful for organic chemists
and for those working in the area of coordination chemistry, and they will undoubtedly give inspiration for the synthesis of new and interesting compounds.
Helmut Sonnenschein
Institut fur Angewandte Chemie
Berlin-Adlershof e. V. (Germany)
Stable Carbocation Chemistry. Edited
by G. K. S. Prakash and P. von
R. Schleyer. Wiley, New York, 1997.
587 pp., hardcover E 65.00.-ISBN
0-471-59462-8
Carbocations persist under conditions
where the absence of nucleophiles ensures
that no intermolecular reactions can occur, and the energy minimum is deep
enough to prevent isomerizations. This
book on such “stable” cations is based on
a symposium held to mark the 65th birthday of G. A. Olah, and contains 17 articles by leading researchers in the field.
I n Chapter 1 George Olah describes his
pioneering work on carbocations, the importance of which was recognized by the
award of a Nobel Prize in 1994. Some
more recent results from Olah’s laborato-
Angew Chem Inr Ed Engi 1997.36, No 16
ry, including the latest work, are reported
in Chapter 4 by G. K. Surya Prakash.
These include studies of carbocations with
silyl groups or very bulky substituents,
and of cations and dications derived from
pagodane. Olah’s recipe for success can be
summarized very briefly as “superacids
and NMR”. The superacids concerned
have remained virtually unchanged since
the beginning (usually SbF, and/or
FSO,H,
sometimes
diluted
with
SO,ClF), whereas the range of NMR
instruments and techniques has greatly
expanded. The isotopic-perturbation
method enables one to distinguish between static and equilibrating carbocations (M. Saunders et al., Ch. 9). Similar
information is obtainable by CPMAS
solid-state NMR spectroscopy in amorphous SbF,, which allows measurements
down to temperatures as low as 5 K (P. C.
Myrhe and C. S. Yannoni, Ch. 10). Measurements of intrinsic isotope shifts can be
used for structural and conformational
analysis (D. Forsyth, Ch. 9). Theoretical
calculations of NMR chemical shifts (using IGLO and GIAO methods) can be
used to verify proposed structural models.
Theoretically calculated vibration frequencies were found to be essential for
interpreting infrared spectra of carbocations in SbF, matrices (D. E. Sunko, Ch.
11). Such calculations have been made
possible through the development of better and faster quantum-chemical methods, which yield exact structures even
for quite large carbocations (P. von
R. Schleyer et al., Ch. 2). The only experimental method that can give information
of comparable quality is X-ray crystallography (T. Laube, Ch. 14), but unfortunately its scope is limited by the extreme
difficulty of growing single crystals of carbocation salts. Calorimetric and electrochemical measurements have yielded important thermodynamic data for carbocations (E. M. Arnett and R. A. Flowers,
Ch. 8). An interesting approach is the use
of X-ray photoelectron spectroscopy to
study carbocations at interfaces, but the
account of this work is somewhat disappointing as it gives little definite information (D. T. Clark, Ch. 13).
0 WILEY-VCH
Verlag GmbH, D-69451 Wemhelm, 1997
In addition to these chapters that concentrate on methods, there are others that
deal with special types of carbocations.
Chapter 3 (T. S. Sorensen) is devoted to
carbocations with p-hydride bridges,
Chapter 5 (H.-U. Siehl) to vinyl cations,
and Chapter 6 (P. Ahlberg) to 9-barbaralyl cations (C,H;) and isomers thereof.
Some important reactions of stable
cations are also treated. These include rearrangements, especially of arenonium
ions (V. G. Shubin and G. I. Borodkin,
Ch. 7), alkane carbonylation reactions
(J. Sommer, Ch. IS), and electrophilic
substitutions of arenes (K. Shudo and T.
Ohwada, Ch. 16). A topic that deserves
particular attention is the enormous variety of highly selective reactions that can
be performed on natural products (especially terpenes and steroids) in superacids
(J.-C. Jacquesy, Ch. 17). Most of the reactions described in Chapters 15-17 do not
directly involve the carbocations as such
but their protonated forms or Lewis
acid adducts (“superelectrophiles”) . Thus
even the chemistry in superacids involves
reactive intermediates that can only be detected indirectly; here dications have a
role similar to that of simple carbocations
in nucleophilic media.
The editors have managed to avoid significant overlapping of subject matter between the contributions, and the work of
the authors (and of the publishers) is of a
high quality. The presentation is excellent,
and there are very few misleading errors.
(Examples: 26b/p. 148, 9/p. 167, 15/p.
204, 2/p. 5.50; in Fig. 11.7 the experimental data are missing, and in Fig. 15.5 the
numbering of the curves should be interchanged.) The book gives the reader “at
first hand” a comprehensive and up-todate survey of this fascinating and highly
productive area of physical-organic chemistry; the literature coverage extends up to
199.5 in most cases. The ideas and methods described here also have the potential
to contribute to advances in other fields.
The book should be of interest to a wide
readership.
Wo(fgung Kirmse
Lehrstuhl fur Organische Chemie
der Universitat Bochum (Germany)
0570-083319713616-1771 S 17 50+ 50,O
1771
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