Carbocation Chemistry / Edition 1 available in Hardcover
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About the Author
G. K. SURYA PRAKASH, the George A. and Judith A. Olah NobelLaureate Chair Professor and Scientific Co-director of the LokerHydrocarbon Research Institute at the University of SouthernCalifornia, has published 480 papers, coauthored six books, andholds more than a dozen patents.
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John Wiley & SonsCopyright © 2004 John Wiley & Sons, Inc.
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Chapter OneHISTORICAL PERSPECTIVE
Peter J. Stang Department of Chemistry University of Utah Salt Lake City, Utah
Carbocations are ubiquitous and arguably the most important reactive intermediates in all of organic chemistry. They are certainly the most important and occupy center stage at this special 2-day symposium on "100 years on carbocations." An indication of just how ubiquitous they are is the fact, as we learned from Professor Olah in the symposium with reference to Pierre Vogel's book (Carbocation Chemistry, Elsevier, New York, 1985), that carbocations are even present in all red wines. So, from now on, every time you enjoy a glass of red wine you may think of and remember carbocations.
We can divide a century of carbocation chemistry into approximately three equal periods: the beginning, from 1900 to the late 1930s, the golden age from the 1940s to the mid-1970s and the modern period since then. I will attempt to provide a brief overview of each period. In the process I have relied on several sources, among them Jack Roberts' wonderful scientific autobiography, The Right Place at the Right Time and others in the ACS Series on Profiles, Pathways and Dreams, edited by Jeff Seeman; Jerry Berson's recent little tome on "chemical creativity" and most of all the late CostinNevitzescu's informative chapter "Historical outlook," which constitutes Chapter 1 of the five-volume series "Carbonium Ions" edited by our host George Olah and my former postdoctoral mentor Paul Schleyer and published by Wiley between 1968 (Vol. I) and 1976 (Vol. V), which are still considered the definitive monographs on carbocations. I only regret that, at the time of the symposium, I did not have available George's new Autobiographical Reflections.
Briefly reviewing years of exciting chemistry, inevitably omissions of important contributions and/or individuals occur. My reflections are highly personal and certainly not comprehensive.
The independent observations and reports of J. F. Norris in the American Chemical Journal and F. Kehrmann in Chemische Berichte, both in 1901, on the heels of Moses Gomberg's discovery of the triphenylmethyl radical in 1900, are generally credited with the discovery of carbocations and hence our 100-year anniversary this year. They independently observed deep yellow solutions on the dissolution of the colorless triphenylcarbinol and triphenylmethyl chloride, respectively, in concentrated sulfuric acid. It was, however, Adolf Baeyer in Munich who in 1902 recognized and reported the saltlike character of compounds formed in solutions of triphenylcarbinol in sulfuric acid and the correlation between the formation of salt and the appearance of color.
There ensued a period of great activity in the field involving such luminaries as Baeyer, Gomberg, Walden, and many others, including a fairly acrimonious debate over nomenclature with Baeyer favoring the term carbonium salts while Gomberg suggested carbyl salts. Just as an example, Walden, of the Walden inversion fame, demonstrated as early as 1902, as reported in Chemische Berichte Vol. 35, p. 2018, the electrolyte character of triphenylmethyl chloride and bromide in sulfur dioxide solution by conductivity measurements. This was all the more remarkable for the early use of liquid S[O.sub.2], a nonaqueous solvent with a very high ionizing power. A. Hantzch, as early as 1907, established the ionic nature of the species formed by measuring the molal freezing point depression of a solution of triphenylcarbinol in concentrated sulfuric acid. These measurements were confirmed and refined by Louis Hammett in the early 1930s as reported in the Journal of the American Chemical Society (JACS) in 1933. Ziegler and coworkers measured the conductivity of a series of substituted triarylcarbonium perchlorates in S[O.sub.2] in the 1920s as reported in Annalen der Chemie during 1925-1930. These measurements were refined and extended by Lichtin and Paul Bartlett and their coworkers in the early 1950s and allowed them to measure the equilibrium constants between undissociated triarylchloride, their ion pairs, and the dissociated ions.
Meanwhile, two seemingly unrelated studies emerged to further develop and enhance the field. The pinacol rearrangement was first reported by R. Fitting in Annalen in 1860. Likewise, during the period 1896-1900, Georgh Wagner discovered the ring rearrangement reaction of certain terpenes, in particular, when borneol and isoborneol are dehydrated to camphene. It was, however, Hans Meerwein who recognized and demonstrated during 1910-1925 that cationic carbon intermediates were involved in these reactions and in particular in the camphene hydrochloride isobornyl chloride change, as first reported by Meerwein in Berichte in 1920. These rearrangements, proceeding through carbocations, have of course become known to every organic chemist as the Wagner-Meerwein rearrangement, a topic I shall return to later.
These early and remarkable results set the stage for the work of Hughes, Ingold, Hammett, and others and what might be called the "birth of physical organic chemistry" during the 1930s and 1940s that in turn led to the golden age of mechanistic organic chemistry in the 1950s through the mid-1970s. The players and contributors to this golden age of mechanistic, physical organic chemistry, centered around understanding reaction pathways and the identification and characterization of reactive intermediates by all available chemical and physical means and methods, read like a Who's Who in organic chemistry and they are among the titans of twentieth-century chemistry. Significant contributors to carbocation chemistry were Saul Winstein, Paul Bartlett, Bill Doering, Jack Roberts, H. C. Brown, N. C. Deno, Don Cram, Gardner Swain, George Olah, Jerry Berson, Ernie Grunwald, Andy Streitwieser, Paul Schleyer, Clair Collins, Chuck De Puy, Martin Saunders, William Saunders, Ned Arnett, Ron Breslow, Paul Gassman, Don Noyce, Jay Kochi, Ted Sorensen, Ken Wiberg, Fred Bordwell, Stan Cristoll, Ted Lewis, W. G. Young, Bob Taft, Frank Anet, Frank Westhheimer, Michael Dewar, Jack Shiner, Mike Karabatsos, Happy Hogoween, and Victor Gold. I can only hope that I did not miss any of the important contributors to the field.
Such topics as nonclassical ions, electrophilic aromatic substitutions, all manner of rearrangements, aromatic and homoaromatic cations, heteroatom-stabilized carbocations, addition-elimination reactions, solvolytic reactions, allylic and homoallylic ions, cyclic and bicyclic species, and so on ad infinitum were all extensively and rigorously investigated, and a treasure trove of knowledge, that greatly enriched organic chemistry, was acquired.
In fact, mechanistic studies and investigations of carbocations were carried out even by E. J. Corey, the paragon of synthetic organic chemistry, whom one would seldom associate with physical organic chemistry. In a series of studies with Joe Casanova. Jr., Corey reported on "the formation of carbocations by oxidative decarboxylation of carboxylic acids, with lead tetraacetate," in JACS in 1963. He even has a paper titled "On the norbornyl cation problem" in JACS, Vol. 85, pp. 169-173 (1963).
Obviously it would take a whole lot of time to discuss in only modest detail the elegant, seminal, and significant contributions of just the persons participating in this symposium: Jack Roberts, Ned Arnett, Paul Schleyer, Martin Saunders, Ted Sorensen, and of course Professor Olah, let alone all the people mentioned previously.
Hence, I will very briefly mention only the highlights: Jack Roberts' seminal studies on the cyclopropylcarbinyl-cyclobutyl systems. There is something special about history when narrated by an active participant. Thank you, Jack! Ned Arnett's very important calorimetric investigations; Paul Schleyer's careful solvolytic studies and investigations of bridgehead cations, not to mention his numerous, insightful computational contributions; Martin Saunders' ingenious isotope perturbation method that I'll return to again; Jerry Berson's incredibly erudite investigations of rearrangements; Ted Sorensen's novel NMR studies of transannually bridged systems; and, of course, the seminal contributions of our host Professor Olah, are all milestones in the rich history of carbocation chemistry.
Focusing more on George Olah's contributions, we now know of course that the Friedel-Crafts reaction, reported by the French chemist Charles Friedel, and the American chemist James Craft, in 1877 in Comptes Rendus and the Bulletin de Société Chimie de France, and related electrophilic aromatic substitutions, proceed via carbocations, involving [pi] and [sigma] complexes. Their formation was not recognized until well into the mid-1900s. Moreover, although various ionic arene complexes were extensively investigated over a period of more than half a century by numerous chemists including H. C. Brown, it was not until the late 1950s that Olah and coworkers were able to isolate several crystalline salts of various methylbenzenonium tetrafluoroborates, hexafluorophosphates, and hexafluoroantimonates and thereby unequivocally establish their nature. They also extensively and vigorously investigated Friedel-Crafts and other electrophilic aromatic substitution reactions. I can refer to a marvelous account of these reactions in the four-volume definitive monographs on this topic, Friedel-Crafts and Related Reactions, edited by George Olah and published by Wiley-Interscience in 1963. I particularly call attention to the wonderful historical chapter by George Olah and Robert Dear on Friedel and Craft that includes actual reproductions from the notebook entries of both Friedel and Craft.
George's knowledge and admiration of Hans Meerwein and his work, with whom he also carried out an extensive correspondence; his success in isolating and studying arenium salts; and his early realization that carbocations are very powerful Lewis acids that cannot long survive in the presence of any nucleophile or even very weakly basic solvents, all played a role in his subsequent seminal discoveries regarding carbocations. He hit upon the brilliant and ingenious idea of using powerful Lewis acids, not only to generate but also as the medium in which to observe carbocations. While still at Dow Chemical, he systematically screened nearly all high-valence Lewis acid fluorides and in the process came upon antimony pentafluoride, Sb[F.sub.5]. In the summer of 1962, at the Brookhaven Organic Reaction Mechanisms Conference, Olah literally shocked the organic chemistry community by his report that he was able to observe by NMR the tert-butyl cation as the Sb[F.sub.6] salt that was stable enough for both spectroscopic and chemical study and kept around at low temperature for many hours. This and Olah's and his coworkers' extensive subsequent NMR studies of carbocations were reported in a series of papers in JACS and elsewhere and served as a magnificent capstone on the numerous and extensive studies of carbocations that began at the dawn of the twentieth century. As the Nobel Foundation was to observe some 30 years later, "George Olah gave the cations of carbon longer life," longer by about a trillion-fold. These studies were definitely one of the major highlights of the golden age of mechanistic physical organic chemistry.
That brings us to the "modern" period of carbocation investigations, approximately since the mid-1970s. Here, I would like to but briefly mention half a dozen recent (as of 2004) studies that illustrate the continued interest and vitality of carbocations. A major forward step in this field and in particular the NMR observation of carbocations was, as I already mentioned, Martin Saunders' development and application of the method of isotopic perturbation of equilibrium, observable in the carbon-13 ([sup.13]C) NMR spectrum. The application of this ingenious technique in the 1980s provided compelling evidence that the parent 2-norbornyl cation, under strongly acidic conditions, indeed has a bridged nonclassical structure and is not a rapidly equilibrating mixture of two classical ions. This was elegantly confirmed by Yanoni's solid-state NMR studies at 5 Kelvin, reported in JACS in 1985. Likewise, one must mention the very nice X-ray structure determinations of several carbocations, including some substituted norbornyl ions and the tert-butyl cation as reported by Thomas Laube in a series of papers in Angewandte and JACS from the late 1980s through the mid-1990s.
Similarly, Jay Kochi's marvelous comprehensive study, by a judicious combination of X-ray crystallography, NMR, time-resolved UV-vis spectroscopy, and kinetics, elucidates in exquisite details the precise structure of the aforementioned [pi] and [sigma] complexes in electrophilic aromatic substitutions. As we now know, these intermediates can have lifetimes from femtoseconds to hours, and hence one needs all available techniques and methods to thoroughly investigate them in order to understand them even better. A "perspective" on Jay Kochi's elegant studies since 1990 or so was featured in the October 2002 issue of the Journal of Organic Chemistry (JOC). Most important, of course, is the cover of issue 9 of JOC in 2001 honoring 100 years of carbocations.
Among the important recent developments in the carbocation field of course are George Olah's studies on C-H and C-C bond activation; in other words, protonation under strong, superacid conditions of saturated hydrocarbons, the parent ion being, of course, C[H.sup.+.sub.5].
Likewise, there are, of course, Professors Olah and Prakash's elegant studies on superelectrophiles that include the protonitronium dication, protohydronium dication, protosulfonium dication, trialkyloxonium dications, protoacetylium dication, and others. These are fascinating, new species whose structure, reactivity, and exact role we are just beginning to understand; likewise, of course, Helmut Schwartz' elegant gas-phase studies and Herbert Mayer's nucleophilicity scale.
Finally, I would like to conclude by briefly discussing the importance of carbocations in bioorganic and natural product chemistry. As mentioned earlier, Wagner discovered, and Meerwein recognized, the role of carbocations in the acidcatalyzed rearrangements of the components of turpentine, molecules we now know as terpenes or isoprenoids. At that early point of development, monumental work and insight was required to merely deduce the structures of the products of these complex reactions.
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Table of ContentsPreface.
1. Historical Perspective (Peter J. Stang).
2. 100 Years of Carbocations and Their Significance in Chemistry(George A. Olah).
3. Zwitterionic ‘‘Neutral’’ and‘‘Anionic’’ Carbocation Analogs (ChaitanyaS. Wannere, Zhongfang Chen, and Paul von Rague´ Schleyer).
4. Recent Studies of Long-Lived Carbocations and Carbodications(G. K. Surya Prakash and V. Prakash Reddy).
5. Antiaromaticity Effects in Cyclopentadienyl Carbocations andFree Radicals (Annette D. Allen and Thomas T. Tidwell).
6. Long-Lived Carbocations in Cold Siberia (Vyacheslav G. Shubinand Gennady I. Borodkin).
7. Polyfluorinated Carbocations (Vitalij D. Shteingarts).
8. Carbocations, Fast Rearrangement Reactions, and the IsotopicPerturbation Method (Martin Saunders and Olga Kronja).
9. Stable Ion Chemistry of Polycyclic Aromatic Hydrocarbons(PAHs); Modeling Electrophiles from Carcinogens (Kenneth K.Laali).
10. Chromium Tricarbonyl–Coordinated Carbocations (BruceN. Hietbrink, Dean J. Tantillo, Kendall N. Houk, and Craig A.Merlic).
11. Carbocations in Gold Chemistry (Hubert Schmidbaur and KeithA. Porter).
12. Proton Exchange between Strong Acids and Alkanes (JeanSommer and Alain Goeppert).
13. Electrophilicity Scales for Carbocations (Herbert Mayr andArmin R. Ofial).
14. Organic Synthesis in Superacids (Jean-Claude Jacquesy).