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Brown, Herbert Charles

BROWN, HERBERT CHARLES

(b. London, United Kingdom, 22 May 1912; d. Lafayette, Indiana, 19 December 2004)

organic chemistry, organoboron compounds.

Brown is best known for his work with boron compounds, for which he was awarded the Nobel Prize in Chemistry in 1979. He was also widely recognized in the chemical community for his work on steric strain, his work on aromatic substitution, and his long battle with much of the chemical establishment regarding the non-classical ion. In his extremely productive career, he published four books and more than one thousand scientific papers.

Early Life. Herbert Brown was born in England in 1912, the second of four children of Charles and Pearl (Gorinstein) Brovarnik. The family immigrated to the United States in 1914 and joined Herbert’s paternal grandparents in Chicago, where the family name had already been anglicized to Brown. Brown’s father was trained as a cabinetmaker, but there was little demand for his skills in Chicago, so he worked as a carpenter and eventually opened a small hardware store. After his death in 1926, Herbert was forced to drop out of high school and tend the store. He was adept at running the business but found it boring; he was much more interested in reading. He returned to high school in 1929 and graduated the next year.

The family having sold the store by this time, Brown went out and worked at several jobs. But it was the beginning of the Depression, and good work was hard to find. He went to Crane Junior College for one semester before Crane was forced to shut down. At Crane he became fascinated with chemistry and also met Sarah Baylen, whom he would later marry. Brown attended night courses at the Lewis Institute for a couple of semesters, took a correspondence course in qualitative analysis from the University of Chicago, and did some laboratory work at the home laboratory of Nicholas Cheronis, who had been one of the chemistry instructors at Crane. Brown and Sarah Baylen graduated from Wright Junior College in 1935, where Cheronis was in charge of the physical sciences, and with Cheronis’s help and encouragement, they entered the University of Chicago. Brown demonstrated what was to be his lifelong propensity for hard work and his inexhaustible energy by taking a large number of courses (up to ten per quarter) and completing his junior and senior course work in one year. He graduated with a Bachelor of Science degree in 1936. Julius Stieglitz, who had chaired the Chicago chemistry department for many years, convinced him to stay on and do graduate work.

Sarah had given Herbert a copy of Alfred Stock’s book The Hydrides of Boron and Silicon (1933) as a graduation present. He was sufficiently interested in the hydrides of boron that he took up graduate work under the guidance of Professor Hermann I. Schlesinger, who had an active program in boron chemistry. Sarah and Herbert were married in 1937, and he completed his graduate work in 1938, receiving a PhD.

At that time diborane, a gas, was made in only two laboratories in the world, those of Stock and Schlesinger, and in both laboratories, the process was difficult and the yields very low. Brown’s thesis work involved the reduction of aldehydes, ketones, and esters with diborane (B2 H6) and was certainly a precursor to the Brown-led revolution that eventually would take place in organic chemistry.

Early Research. After completing his graduate research, Brown could not find an industrial position, but Professor Morris Kharasch offered him an Eli Lilly postdoctoral fellowship. He was supposed to work on the isolation of the active principle from the pituitary gland, but the glands were slow in coming, so he began work on the possible chlorosulfonation of hydrocarbons with sulfuryl chloride, a reaction Kharasch hoped might be useful in the preparation of detergents. This alternate project turned out to be quite fruitful and resulted in ten papers from 1939 to 1942 on chlorination, chlorosulfonation, and chloroformylation of a variety of organic compounds, and some fundamental work on the stereochemistry and isomerization of free radicals. Brown later carried out additional free radical research at Wayne State and Purdue universities.

After his year with Kharasch, Brown was offered the position of research assistant to Professor Schlesinger, which carried the title of instructor. This post involved some teaching, working on Schlesinger’s research problems, directing Schlesinger’s research students, and if time and energy permitted, carrying out some independent research with master’s students. It was during this period, as a result of wartime research projects, that Brown and his coworkers first synthesized sodium borohydride (NaBH4). A few years later Schlesinger’s group synthesized lithium aluminum hydride (LiAlH4). These two compounds and the derivatives developed by Brown over the next thirty years had an enormous impact on organic synthesis. An additional result of the war research that was to prove extremely important was the discovery that diborane could be readily prepared in solution from boron trifluoride (BF3) and lithium hydride (LiH) or lithium borohydride (LiBH4).

After five years as an instructor and as Schlesinger’s assistant, Brown was told there was no future for him at the University of Chicago, so he began looking for a new position. Neil Gordon, a former colleague of Morris Kharasch from the University of Maryland, was moving to Wayne University (as it was then called) in Detroit to chair the chemistry department and Kharasch recom

mended Brown for a position. Brown was on the faculty at Wayne from 1943 to 1947, during which time he published eighteen papers, established a reputation in the organic community for creativity and productivity, and had a son, Charles Alan, who became a chemist.

In 1947, Brown accepted a position as professor of chemistry at Purdue University. In 1959, he became R. B. Wetherill Professor and, in 1960, R. B. Wetherill Research Professor. He retired as professor emeritus in 1978 but continued to do research and publish until his death in 2004. (Although Brown spent the bulk of his academic career at Purdue, he was a visiting professor at UCLA [1951]; Ohio State [1952]; University of California at Berkeley [1957], Santa Barbara [1967], and San Diego [1979]; University of Colorado (1958); and a number of other major institutions. He also gave a large number of named lectures at most of the major universities in the country, including Harvard, Columbia, Cornell, Johns Hopkins, and the University of Pennsylvania.)

Steric Effects. During his years as Schlesinger’s assistant, Brown began a project in steric effects that would bring him considerable attention in the larger chemical community. This work was prompted in part by observations he

had made while a graduate student, and in part by what he considered a real neglect in the chemical literature of the part played by steric factors on chemical behavior. The work began with qualitative studies of the relative stabilities of a series of trialkylborane-alkylamine addition compounds.

Brown and his coworkers found that increasing the size of the groups attached to the nitrogen would cause instability in the addition compounds. For example, the trimethylamine-trimethylborane compound (A) shown in Figure 1 is more stable than the triethylamine-trimethylborane compound (B); that is, the equilibrium in the equation lies more to the left.

Brown then began a long-term quantitative study of the heats of formation of the amine-borane addition compounds and demonstrated the importance of steric effects in these systems and, by implication, in other areas of organic chemistry. He eventually identified three forms of steric strain, which are closely associated with his name: F(ront)-strain, B(ack)-strain, and I(nternal)-strain. Brown talked about these studies at the first Organic Reaction Mechanisms Conference at Notre Dame in 1946 and again at the third Organic Reaction Mechanisms Conference at Northwestern University in 1950. He summarized this work in a Centenary Lecture at Burlington House, London, before the Chemical Society (London) in 1955 and in a long article, “Chemical Effects of Steric Strains,” in the Journal of the Chemical Society the following year.

Brown, who seldom shied from an argument when he felt the facts were on his side, engaged in a minor skirmish in the early 1950s with Christopher Ingold, one of the founding fathers of physical organic chemistry, over the relative importance of steric effects in the formation of alkenes during the unimolecular hydrolysis of tertiary alkyl halides. In a series of papers in 1948, Ingold and his coworkers maintained that steric effects were seldom important in determining the products of unimolecular reactions and that the dominating factor involved electronic effects. In a 1950 paper with Roslyn Fletcher, Brown challenged this view and offered the results of a number of experiments in support of his position. Ingold took issue with Brown in a 1953 paper titled “The Comparative Unimportance of Steric Strain in Unimolecular Olefin Eliminations.” This eventually evoked a five-paper response from Brown in 1955, culminating with a paper titled, “The Importance of Steric Strain in the Extent and Direction of Unimolecular Elimination.” This argument with Ingold merely served as a preliminary bout for what was to become the most famous controversy of twentieth-century organic chemistry.

The Nonclassical Ion Problem. In the early 1930s, the concept of the carbocation, an organic ion with a positive charge on an electron-deficient carbon atom, was introduced. This new idea proved extremely useful in the development of organic theory, in particular in the detailed description of reaction pathways. Carbocations were proposed as intermediates in many reactions to explain rates, structural rearrangements, and steric consequences. In 1949, Winstein and Trifan suggested that some of their results could be better described by the introduction of a new reaction intermediate with unusual bonding. Intermediates of this type were eventually labeled “nonclassical ions.”

Once introduced, the nonclassical ion was used by many others to explain, what appeared to be, anomalous results. Brown, whose basic philosophy was “nature is simple,” challenged the concept of the nonclassical ion in a provocative paper titled “Strained Transition States” in 1962. He was following the principle of Occam’s razor that is applied to all scientific models: one should not increase, beyond what is necessary, the number of entities required to explain anything. He essentially argued that all of the results for which the nonclassical ion was invoked, could as easily be explained using classical ions and, of course, steric effects.

The debate, with Brown on one side and the majority of the physical organic chemists on the other, continued for almost two decades and produced an enormous amount of research, much of it extremely inventive with respect to ideas and technology. Paul Bartlett, in his book, Nonclassical Ions (1965), wrote that the importance of the debate was: “(1) It has led to an extension of valence theory and has defined the meeting-ground of organic chemistry with the electron-deficient bonding principles as seen in boron compounds; (2) its study has provided new tools

and new insight relating to the ionization process in solution; (3) it has evolved some elegant methods of stereo-chemical study;” (p. v). Brown published a book, The Nonclassical Ion Problem (1977), and close to one hundred communications on the subject. Unfortunately, the dispute also produced some verbal abuse and considerable bitterness. The issue was never fully resolved to the satisfaction of all the parties, but the nonclassical ion remained a useful concept in the armament of the organic chemist.

Aromatic Substitution. As a further result of his interest in steric effects, Brown examined the electrophilic substitution of alkyl-substituted aromatic compounds (see Figure 2). Brown found that increasing the size of group R increased the ratio of I to II, demonstrating once more the profound effect that steric factors play in organic reactions.

Extending his interest to the general problem of electrophilic substitution, Brown and his coworkers developed a quantitative theory that could be used to predict the rates of a variety of reactions. In the process, he defined a new Hammett substituent constant, σ+, thereby extending considerably the applicability of the Hammett equation, which was such a powerful predictive tool for chemists. Brown’s work in this area was reviewed in detail in “A Quantitative Treatment of Directive Effects in Aromatic Substitution,” a 1963 paper with L. M. Stock.

Return to the Boranes. In 1953, Brown published eleven articles describing the research on boron compounds that had been done at Chicago during the war. The series started with a paper titled “New Developments in the Chemistry of Diborane and Borohydrides. I. General Summary.” He then initiated a research program in boron chemistry that eventually would lead to the Nobel Prize. It began with reductions of organic compounds using a variety of borohydrides, spread, as the result of an accidental observation, to the area of hydroboration, and finally led to the development of a large number of reactions using organoboranes.

Selective Reductions. In 1947, Schlesinger published “Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride and Some of Their Applications in Organic and Inorganic Chemistry,” which described the synthesis of lithium aluminum hydride, a compound that he and his coworkers had prepared during their war-related research. This solid reagent, which soon became commercially available, could easily reduce compounds containing carbonyl (C=O) groups to alcohols and compounds with carbon-nitrogen multiple bonds to amines. The procedure was relatively simple and the products could be isolated readily in very high yield. The major problem with the reagent was that it was highly reactive and thus unselective, and it reduced almost every compound that contained a carbon-oxygen or carbon-nitrogen multiple bond.

Weldon Brown at the University of Chicago worked with lithium aluminum hydride and collected data on its widespread use in organic synthesis. In 1951, he published “Reductions by Lithium Aluminum Hydride,” an extensive review citing hundreds of examples of the use of the compound in the few short years that it had been available.

Sodium borohydride, which had been synthesized in 1942 in Schlesinger’s laboratory but whose synthesis was not published until 1953, when Herb Brown finally found time to write up the war work, was a very mild and selective reducing agent and would only reduce aldehydes and ketones to the corresponding alcohols. After moving to Purdue University, Brown, who shared the patent with Schlesinger on the preparation of sodium borohydride, began a program designed to increase the reactivity of sodium borohydride and decrease the reactivity of lithium aluminum hydride. As in almost all his major research projects, Brown and his coworkers made systematic alterations to the two compounds, carefully observing and recording the effects of the changes. Over a period of thirty years, they developed a series of reducing agents that covered the entire spectrum of reactivity from the very selective sodium borohydride to the very reactive and unselective lithium aluminum hydride.

Hydroboration. While engaged in the reduction studies, one of Brown’s postdoctoral fellows, B. C. Subba Rao, observed an anomalous result. During the reduction of an ester containing a carbon-carbon double bond, more diborane was consumed than was required for the ester alone. It turned out that the diborane not only reduced the ester to an alcohol but also added to the carbon-carbon double bond. Further investigation showed that diborane and other so-called hydroborating agents could be added to a carbon-carbon double bond easily and rapidly and that the resulting product could be oxidized to an alcohol. The addition of diborane was regiospecfic; that is, hydrogen and boron added to the double bond in only one of two possible ways (the hydrogen always added to the more substituted carbon of the double bond), and the addition took place with no rearrangement. The addition was also stereospecific; that is, the elements of hydrogen and boron added on the same side of the double bond, which was generally the least hindered side.

Brown pursued this new discovery very systematically, with energy and with considerable resources. This new reaction permitted Brown and his coworkers to synthesize a plethora of new organoboranes. The products were usually trialkylboranes (R3B), but by careful selection of the starting alkene, they were able to synthesize a variety of monoalkylboranes (RBH2) and dialkylboranes (R2BH). The monoalkylboranes, RBH2, and dialkylboranes, R2 BH, can also add to alkenes to produce mixed trialkyl boranes, B and Once Brown and his colleagues had perfected the synthesis of the organoboranes, they turned their attention to the use of these compounds.

Reactions of the Organoboranes. . In addition to oxidizing the organoboranes to the corresponding alcohols, and in certain instances to the corresponding ketones, they found that the boron could be replaced by halogen, by an amino group, by hydrogen or deuterium, and by mercury. Furthermore, the organoboranes could be used to build more complex organic molecules by making new carbon-carbon bonds. The boranes can undergo coupling to form dimers, they can be utilized in the formation of cyclopropanes, and they can react with carbon monoxide and be converted to tertiary alcohols, secondary alcohols, ketones, and aldehydes. Organoboranes can also be used to extend carbon chains by two, three, or more carbon atoms by reactions with α,B-unsaturated aldehydes and ketones and α-halo esters and ketones. Utilizing well-chosen cyclic organic compounds containing multiple double bonds, Brown was able to “stitch” with boron and “rivet” with carbon monoxide to make complex polycyclic systems.

The uses of the organoboranes in organic synthesis seem almost limitless, constrained only by the imagination and the technical skill of the researcher. In 1978, James Brewster, who has written several sketches of Brown and his research, wrote, “Organoborane chemistry was not just discovered, it was also the product of a high order of creative imagination—an artistic triumph as well as a scientific one” (p. 331).

Awards. Brown was the Centenary Lecturer of the Chemical Society of London in 1955 and the George Fisher Baker Non-Resident Lecturer at Cornell University in 1969. The Baker lectures formed the basis of his 1972 book, Boranes in Organic Chemistry. He was elected to the National Academy of Sciences in 1957 and the American Academy of Arts and Sciences in 1966. Brown was the recipient of the William H. Nichols Medal in 1959; the American Chemical Society Award for Creative Research in Synthetic Organic Chemistry in 1960; the Linus Pauling Medal in 1968; the National Medal of Science in 1969; the Roger Adams Medal in 1971; the Charles Frederick Chandler Medal in 1973; the Madison Marshall Award in 1975; the City College of New York Scientific Achievement Award Medal in 1976; the Ingold Memorial Lecturer and Medal and the Elliott Cresson Award, both in 1978; the Nobel Prize in 1979; the Priestly Medal in 1981; the Perkin Medal in 1982, the Gold Medal of the American Institute of Chemists in 1987; and the Emperor’s Decoration (Japan): Order of the Rising Sun, Gold and Silver Star, in 1989.

BIBLIOGRAPHY

The Chemistry Department at Purdue University in West Lafayette, Indiana, is in possession of “Remembering HCB: Memoirs of Colleagues and Students of Herbert C. Brown.” In addition to the memoirs, it also contains an essay by James Brewster on Brown’s researches and Brown’s bibliography through 1977. Purdue’s Chemistry Department also holds “Herbert C. Brown: A Life in Chemistry,” which honors Brown’s Nobel Prize. It contains reprints of three articles covering his work with boranes, his Nobel lecture, an autobiographical extract from his book, Boranes in Organic Chemistry (1972), and a biographical note by James Brewster. The Chemical Heritage Foundation in Philadelphia holds “Oral History with H. C. Brown,” the transcript of an interview of Brown by Leon Gortler in 1982.

WORKS BY BROWN

With Roslyn Silber Fletcher. “Chemical Effects of Steric Strain. II. The Effect of Structure on Olefin Formation in the Hydrolysis of Tertiary Aliphatic Chlorides.” Journal of the American Chemical Society72 (1950): 1223–1226.

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With Hermann I. Schlesinger et al. “New Developments in the Chemistry of Diborane and the Borohydrides. I. General Summary.” Journal of the American Chemical Society 75 (1953): 186–190.

With Hermann I. Schlesinger and Albert E. Finholt. “The Preparation of Sodium Borohydride by the High Temperature Reaction of Sodium Hydride with Borate Esters.”Journal of the American Chemical Society 75 (1953): 205–209.

With Ichiro Moritani. “Steric Effects in Elimination Reactions. V. The Importance of Steric Strain in the Extent and Direction of Unimolecular Elimination. The Role of Steric Strains in the Reactions of Highly Branched Carbonium Ions.” Journal of the American Chemical Society77 (1955): 3623–3628.

“Chemical Effects of Steric Strains.” Journal of the Chemical Society (1956): 1248–1268. A review of his work on steric effects.

Hydroboration. New York: W. A. Benjamin, 1962. Detailed description of the early work in hydroboration and the reactions of the organoboranes.

“Strained Transition States.” In The Transition State. London: The Chemical Society, Special Publication No. 16, 1962: 140–158. First presented at a symposium in Sheffield, England, in April 1962.

With L. M. Stock. “A Quantitative Treatment of Directive Effects in Aromatic Substitution.” In Advances in Physical Organic Chemistry. Vol. 1, edited by Victor Gold. London: Academic Press, 1963.

Boranes in Organic Chemistry. Ithaca, NY: Cornell University Press, 1972. A distillation of his Baker Lectures at Cornell University in 1969. Contains an autobiographical introduction and organized treatments of almost all of his research prior to 1970.

Organic Syntheses via Boranes. New York: John Wiley, 1975. Essentially a “how to” book for chemists, covering the formation and utilization of organoboranes. Contains short overviews of the synthesis and reactions of organoboranes and specific procedures for many syntheses.

The Nonclassical Ion Problem. New York: Plenum Press, 1977. Brown’s view of the problem, with commentary and rebuttal by Paul von R. Schleyer.

“From Little Acorns to Tall Oaks—From Boranes through Organoboranes.” Nobel Lecture, 8 December 1979. Available from http://nobelprize.org/chemistry/laureates/1979/brown-lecture.pdf “The Nonclassical Ion Problem: Twenty Years Later.” Pure and Applied Chemistry54 (1982): 1783–1796.

OTHER SOURCES

Bartlett, Paul D., ed. Nonclassical Ions: Reprints and Commentary. New York: W. A. Benjamin, 1965. An overview of the nonclassical ion problem through a collection of literature reprints and commentary by Bartlett, one of the most important and most respected physical organic chemists. Brewster, James H. “Herbert C. Brown—A Biographical Note.” In Aspects of Mechanism and Organometallic Chemistry: A

Volume in Honor of Professor Herbert C. Brown, edited by James H. Brewster. New York: Plenum Press, 1978. Brown, Weldon G. “Reductions by Lithium Aluminum Hydride.” In Organic Reactions, edited by Roger Adams. Vol. 6. New York: John Wiley, 1951.

Cooper, K. A., E. D. Hughes, C. K. Ingold, and B. J. McNulty.“Mechanism of Elimination Reactions. Part VI. Introduction to a Group of Papers. Unimolecular Olefin Formation from tert-Butyl and tert-Amyl-sulfonium Salts.” Journal of the Chemical Society (1948): 2038–2042. First of ten papers in this issue of the journal. Paper XVI, the final paper in this series, discusses the mechanisms of elimination reactions in great detail.

Finholt, Albert E., A. C. Bond Jr., and Hermann I. Schlesinger. “Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride and Some of Their Applications in Organic and Inorganic Chemistry.” Journal of the American Chemical Society 69 (1947): 1199–1203.

Hughes, Edward D., Christopher Kelk Ingold, and V. J. Shiner Jr. “Mechanism of Elimination Reactions. Part XVII. The Comparative Unimportance of Steric Strain in Unimolecular Olefin Eliminations.” Journal of the Chemical Society(1953): 3827–3832.

Winstein, S., and Daniel S. Trifan. “The Structure of the Bicyclo[2,2,1]2-heptyl (Norbornyl) Carbonium Ion.” Journal of the American Chemical Society 71 (1949): 2953.

Leon B. Gortler

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Brown, Herbert Charles

Herbert Charles Brown, 1912–2004, American chemist, b. London, Ph.D. Univ. of Chicago, 1938. A professor at Wayne State Univ. (1943–47) and Purdue Univ. (1947–78), he shared the 1979 Nobel Prize in Chemistry with Georg Wittig. Brown developed boron-containing compounds as important reagents in organic synthesis. These organoborones provided an inexpensive means of making organic chemicals used in agricultural, pharmaceutical, and other industries.

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