Diels, Otto Paul Hermann

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Diels, Otto Paul Hermann

(b. Hamburg, Germany, 23 January 1876; d. Kiel, Germany, 7 March 1954)

Diels’s father, Hermann, was professor of classical philology at the University of Berlin; his brother Paul, professor of Slavic philology at Breslau; and his brother Ludwig, professor of botany at Berlin. His mother, the former Bertha Dübell, was the daughter of a district judge.

As a student, Diels, with his brother Ludwig, eagerly conducted chemical experiments. He studied chemistry in Berlin from 1895 to 1899 and in 1899 obtained the Ph.D., magna cum laude, with a dissertation entitled “Zur Kenntnis der Cyanverbindungen.” From 1899 he studied under Emil Fischer and served as his assistant until he became a lecturer in 1904. He became department head in 1913 and in 1914 was appointed associate professor at the Chemical Institute of the Royal Friedrich Wilhelm (now Humboldt) University. In 1916 Diels accepted an invitation from Christian Albrecht University, Kiel, where he served as full professor and director of the Chemical Institute until his final retirement in October 1948.

Diels recorded his personal memories in a manuscript entitled “Werden und Wirken eines Chemieprofessors,” as well as in an illustrated diary (Diels was a weekend painter) which has been reviewed in detail by Sigurd Olsen. Diels was considered somewhat reserved, yet possessed of a good sense of humor. He was honest, sensitive, an outstanding educator, and a devoted family man. Diels was married in 1909 and had three sons, and two daughters. Toward the end of World War II two of his sons were killed at the eastern front, and air raids completely destroyed the Chemical Institute, the library, and his home. Since there was no possibility of carrying on his work and he was suffering from the general privations, Diels filed for retirement in September 1944, to be effective in March 1945. Nevertheless, in 1946 he agreed to resume the directorship of the Chemical Institute, and at the age of seventy he started anew, under the most primitive conditions in makeshift quarters.

Emil Fischer, Diels’s teacher, ended a period of chemistry that Willstätter called the age of simple methods and direct observation. During Diels’s time the importance of theoretical chemistry, physicochemical measuring methods, complicated experiments, and teamwork grew to such an extent that in some instances chemical research lost a little of its immediacy.

Diels’s work, which was in the field of pure organic chemistry with no significant digressions into biochemistry or into physical chemistry, reveals an outstanding experimenter with original and bold ideas. His Einführung in die organische Chemie (1907), which went through nineteen editions by 1962, has a clarity and a precision that have made it one of the most popular textbooks in the field. Diels’s lectures, accompanied by experiments, were outstanding and were enthusiastically received by his students.

In 1906 Diels obtained carbon suboxide by dehydrating malonic acid and investigated its properties. In the same year, with E. Abderhalden, he began his research on cholesterol, the structure of which had not yet been determined. He isolated pure cholesterol from gallstones and converted it into “Diels’s acid” through cleavage by oxidation. Meanwhile, Windaus had proposed a formula for cholesterol that did not agree with more recent observations. As a result, Diels decided first to establish the aromatic basic structure of cholesterol. Dehydration of cholesterol with sulfur was unsuccessful. Selenium was used next, and in 1927 it yielded a twofold success (Fig. 1): a new, milder, and very effective dehydrating agent had been

discovered; Diels had obtained the aromatic basic structure he had been looking for—3? methyl-l,2 cyclopentenophenanthrene, the structure of which was clarified by Robert Harper, Kon, and Leopold Ruzicka in 1934.

This “Diels hydrocarbon,” C₁₈H_l6, the identity of which with the corresponding synthetic product was demonstrated by Diels in 1935, proved to be the basic substance and structure of a number of very important natural products for the chemistry of natural substances. The investigation of these substances corresponded in importance to the discovery of the benzene ring for organic chemistry. The structure and behavior of the sex hormones, the saponins, the cardiac glycosides (digitoxin, strophanthin, etc.), the D vitamins, toad “venom,” bile pigments, adrenal-cortex hormones (cortisone), and similar substances could now be clarified.

With Kurt Alder, Diels developed over a period of twenty-two years the diene synthesis, which came to occupy a key position in the theory and practice of organic chemistry. It also yielded new facts concerning the three-dimensional isomerism of the carbon compounds. Starting with Thiele’s 1, 4-addition theorem and the knowledge of the additive power of azo esters, Diels and Alder attempted in 1928 to combine maleic anhydride with cyclopentadiene (Fig. 2). The dienes (compounds with conjugated carbon double bonds) united with philodienes (compounds with an ethylene radical flanked by carbonyl or carboxyl groups) to form ring-shaped structures. This type of synthesis is not only extraordinarily diverse but also occurs spontaneously

even at room temperature, and in general with good yields, without the use of condensing agents and catalysts. The diene system opens at positions 1 and 4, and the terminal carbons are located at the double bonds of the philodienes.

Diels published thirty-three papers on the practical applications of this new method of synthesis. Windaus, in the field of steroids, used it for the separation of ergosterol and its irradiation products. In a series of important terpenes, such as camphor, dl-santene, butadiene, and ?-phellandrene, the structure could be confirmed by synthesis, since these substances are composed of isoprene residues, the building blocks of the diene structure. Also, great progress could be made in the synthesis of heterocyclics.

In 1904 Diels participated in the Louisiana Purchase Exposition in St. Louis and received a gold medal for his exhibit. During the academic year 1925–1926 he served as rector magnificus of the University of Kiel. His inaugural address was entitled “Über die Bedeutung von Zufall und Instinkt bei grossen chemischen Entdeckungen.” In 1931 the Society of German Chemists awarded Diels the Adolf von Baeyer Memorial Medal, and in 1946 the Medical Faculty of Christian Albrecht University awarded him an honorary doctorate. Diels was a member of the academies of sciences of Göttingen and Halle (Leopoldina) as well as the Bavarian Academy of Sciences. In 1950 he and his pupil Kurt Alder shared the Nobel Prize in chemistry for the development of the diene synthesis. In his Nobel address, “Darstellung und Bedeutung des aromatischen Grundskeletts der Steroide,” Diels compared his research on cholesterol with his work in diene synthesis.

BIBLIOGRAPHY

Diels’s writings include “Über das Kohlensuboxyd I,” in Berichte der Deutschen chemischen Gesellschaft, 39 (1906), 689, written with B. Wolf; Einführung in die organische Chemie (Leipzig, 1907; 19th ed., Weinheim, 1962): “Über die Bildung von Chrysen bei der Dehydrierung des Cholesterins,” in Berichte der Deutschen chemischen Gesellschaft, 60 (1927), 140, written with W. Gädke; “Über Dehydrierungen mit Selen (II. Mitteil),” ibid., 2323, written with A. Karstens; “Synthesen in der hydroaromatischen Reihe, I. Mitteilung, Anlagerungen von ‘Dien’-kohlenwasserstoffen,” in Justus Liebigs Annalen der Chemie, no.460 (1928), 98, written with K. Alder; “Die Dien-Synthesen, ein ideales Aufbauprinzip organischer Stoffe. (Vortrag zum 100. Geburtstag von A. Kekulé),” in Zeitschrift für angewandte Chemie, 42 (1929). 911; “‘Dien-Synthesen’ als Aufbauprinzip und Hilfsmittel organisch-chemischer Forschung,” in Jaarboekie van de Natuur-Philosofische Facueitsvereniging (Groningen, 1934); “Über organischchemischer Entdeckungen und ihre Bedeutung für Gegenwart und Zukunft,” in Chemikerzeitung, 61 (1937), 7; “Bedeutung der Diensynthese für Bildung, Aufbau und Erforschung von Naturstoffen,” in Fortschritte der Chemie organischer Naturstoffe, 3 (1939), 1; “Mein Beitrag zur Aufklärung des Sterinproblems,” an address summarized in Angewandte Chemie, 60 (1948). 78; and “Darstellung und Bedeutung des aromatischen Grundskeletts der Steroide, Nobelvortrag,” in Les Prix Nobel en 1950 (Stockholm, 1950).