(b. Montreal, Quebec, Canada, 8 October 1912; d. Los Angeles, California, 23 November 1969)
In 1923 Louis and Anne Dick Winstein moved to the United States. Their son, naturalized in 1929, attended Los Angeles public schools before enrolling in the University of California at Los Angeles (B.A. 1934; M.A., 1935) and the California Institute of Technology (Ph.D., 1938). On 3 September 1937 he married Sylvia V. Levin; they had a son and a daughter.
Winstein was a National Research Council fellow under Paul D. Bartlett at Harvard (1939–1940) and instructor at the Illinois Institute of Technology (1940–1941). In 1941 he became instructor of chemistry at UCLA, where he remained, becoming full professor in 1947. His many honors include election to the National Academy of Sciences (1955), UCLA Alumnus of the Year (1958), California Scientist of the Year (1962), and the National Medal of Science (posthumous). UCLA recognized his excellence with its Distinguished Teaching Award (1963). Winstein was active in the Academic Senate, the Alumni Association, and the UCLA Art Council and Friends of Music. He was at the peak of his career and influence at the time of his sudden death following a heart attack.
One of the leading figures in physical organic chemistry, Winstein started his career during the infancy of the field. In his undergraduate years E. D. Hughes and C. K. Ingold were making the first detailed kinetic studies of nucleophilic substitution reactions. At UCLA, William G. Young introduced him to the subject, an introduction that resulted in eight publications on allylic rearrangements and Grignard reagents by the time he had his M. A. Winstein’s love for the field deepened when he moved to Caltech to work with Howard Lucas, the American pioneer of physical organic chemistry. Lucas, at Caltech since 1913, was among the first organic chemists to explore electronic interpretations of organic reactions.
In 1939 Lucas and Winstein published an important paper on the stereochemical role of neighboring bromine in displacement reactions, providing an elegant proof that the bromine on the carbon atom adjacent to the reactive center formed a cyclic bromonium ion intermediate. Winstein was to be the foremost creator of these bridged ions. Except for research on antimalarials during World War II, most of Winstein’s work can be traced to the studies with Young and Lucas, the major interest always being the nature of the cationic intermediates in organic reactions.
A 1948 paper demonstrated that a great acceleration of the ionization process often accompanied neighboring group participation. Winstein originated the concept of driving force and developed means to determine its quantitative value. This concept first appeared in the context of the discovery of the homoallylic cholesteryl ion. Winstein studied the cholesteryl system as an extension of his work on allylic compounds, since the 5, 6-double bond in cholesteryl derivatives should act as a neighboring group. The solvolysis of cholesteryl-p-toluene sulfonate was 100 times faster than expected, which suggested the formation of an abnormally stable intermediate due to the driving force toward ionization in the displacement reaction. The product was a derivative of i-cholesterol, but an abnormally rapid conversion of this to the normal derivative meant that the intermediate was an electronically resonating ion. Winstein formulated it thus, as the common intermediate for the forward and reverse rearrangements:
In subsequent publications Winstein developed the subject of “nonclassical” ions, defined as carbonium ion intermediates with an electron pair delocalized and binding three centers. Such ions, formed by participation of the electron cloud associated with neighboring groups, were important in understanding the kinetics, stereochemistry, and mechanism of many substitution and rearrangement reactions. Winstein’s studies on bridged carbonium ions took continually new directions through the 1950’s and 1960’s though not without controversy. Herbert C. Brown, in particular, strongly opposed the postulation of such entities.
For the remainder of his life Winstein synthesized new substances, many of considerable structural beauty, to show the importance of neighboring groups. Whether they involved the pi participation of a double bond or the sigma participation of a saturated group, all were aspects of the principle of optimal charge delocalization through the most favorable overlap of atomic orbitals. Winstein justified his structural creations with molecular orbital calculations showing that the stabilization is due to electron delocalization.
Winstein’s studies in the 1950’s included the discerning of various kinds of ionic intermediates, from intimate ion pairs and solvent-separated ion pairs to dissociated ions. A reaction product may not derive from a carbonium ion but from ion pairs with varying degrees of tightness. He extended his kinetic analyses to the point where all rate constants were related to the various intermediates present.
During the 1960’s Winstein became fascinated with the similarities between a double bond and a three-membered ring. His inventiveness peaked with his creation of a series of “homoaromatic” compounds having all or part of the double bonds replaced by cyclopropane rings. Winstein prepared cations, radicals, and anions of enhanced stability owing to homoaromatic properties and developed this area into an active one taken up by many investigators. The unfolding of homoaromaticity provided a good example of his role as a trailblazer and innovator. The development of nuclear magnetic resonance in the last five years of his life enabled Winstein to directly observe a variety of such species, all of which existed because of cyclic electron delocalization and the induced ring current.
From Winstein’s creations and discoveries flowed new types of structures, syntheses, rearrangements, and concepts. His terms and concepts entered into the language and textbooks of organic chemistry, and were so common and familiar by the mid 1970’s that their recent origin was unknown to the current generation of students.
I. Original Works. Paul Bartlett collected Winstein’s most important papers in his Nonclassical lons (New York, 1965), which is a reprint of papers on the subject with commentary by Bartlett. Winstein himself provided two valuable reviews of his work, the first in George R. Roberson, ed., Modern Chemistry for the Engineer and Scientist (New York, 1957), 146–167; and the second as British Chemical Society centenary lecturer, “Nonclassical lons and Homoaromaticity,” in Quarterly Reviews, 23 (1969), 141–176.
II. Secondary Literature. Appreciations and analyses of Winstein’s work by his colleagues appeared shortly after his death. William G. Young and Donald J. Cram wrote the biography for Biographical Memoirs, National Academy of Sciences, 43 (1973), 321–353. Paul D. Bartlett’s study, “The Scientific Work of Saul Winstein,” is in Journal of the American Chemical Society, 94 (1972), 2161–2170; that of Andrew Streitwieser is in Progress in Physical Organic Chemistry, 9 (1972), 1–24. Each of these articles provides a bibliography of his publications.
Albert B. Costa