- Email: [email protected]
Rolf Huisgen
TETRAHEDRON Pergamon
Tetrahedron 56 (2000) vii±viii
Preface
Rolf Huisgen
This issue of Tetrahedron is devoted to Professor Rolf Huisgen, University of Munich, one of the most prominent contemporary organic chemists, on the occasion of his 80th birthday. Rolf Huisgen's vita covers much of the 20th century. Being born in 1920 he experienced the turmoils of preworld-war II and was just ready to begin his university studies at the beginning of world-war II. After his start at the University of Bonn in 1939 he continued at the University of Munich where he obtained his diploma in 1940, having studied just six trimesters, the change from the semester to the trimester system being a consequence of the special war conditions. He was lucky to be able to engage immediately after the diploma examination in work for his doctoral degree. Under the guidance of Heinrich Wieland he completed his thesis in 1943. He continued earlier work of the Wielands group on the structure elucidation of the alkaloid vomicine without having modern instrumentation at his hands. Derivatization and degradation were the techniques available. Rolf Huisgen soon began independent research although activities were hampered by the unfavorable war and postwar conditions. He developed chemistry which did not follow the footsteps of his teacher Heinrich Wieland but which led him to study fundamental questions in mechanistic organic chemistry. In this way he founded and established physical organic chemistry in Germany. In 1949, at the age of 29, Rolf Huisgen was offered a position as Professor Extraordinarius at the University of TuÈbingen where he remained until 1952 when he returned to the University of Munich as successor of his teacher Heinrich Wieland.
The contributions of Rolf Huisgen to organic chemistry are numerous. In the early days it was the chemistry of diazo compounds which attracted his interest. He writes in his autobiography `The Adventure Playground of Mechanisms and Novel Reactions' (R. Huisgen, 1994): ªThe aromatic diazonium ion has fascinated generations of chemists: molecular nitrogen forced into the onium state, metastability despite a high energy level, a wealth of reactions equally signi®cant for laboratory and industryº. The Munich group was able to increase the synthetic possibilities of this chemistry and to broaden the mechanistic knowledge about the reactions of diazo- and diazonium compounds considerably. Thus, the reactions of N-nitrosoacylamides were studied in depth leading to the formulation of alkanediazonium ion pairs and the subsequent formation of carbocations in the various product forming steps. Huisgen's sharp analysis of experimental resultsÐa character feature which becomes apparent in all of his workÐled him to the postulation and the indirect identi®cation of diazoalkanes in the reactions of N-nitrosoacylamides. The intelligent design of experiments to prove a mechanistic consequence of experimental observations is another of Huisgen's attributes. Medium sized rings and benzyne chemistry were his main activities in the ®fties. Huisgen's contributions belong to the highlights in these areas. Chemistry involving one or several nitrogen atoms continued to be of high interest. The discovery of the existence of pentazoles, a long sought-after class of compounds, is certainly one of Huisgen's great scienti®c achievements. Proper analysis of experimental observations in reactions of diazonium ions with azide ion uncovered the existence of ®ve membered rings constructed only of nitrogen atoms. The combination of preparative and kinetic studies led to the development of 1,3-dipolar cycloaddition chemistry. Again, nitrogen-containing molecules, kinetic studies of the reactions of diazoalkanes with ole®ns, played a decisive role. These results induced the development of the general concept of 1,3-dipoles. Although scattered examples of reactions of alkenes with molecules, from hereon called 1,3-dipoles, could be found in the literature, it is Huisgen's great achievement and merit to have recognized the general principle and to have exploited the chemistry of 1,3-dipolar cycloadditions. Within a few years in the early sixties he discovered new 1,3-dipoles which followed from the formulation of the underlying principle. Unsophisticated
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0040-402 0(00)00332-X
Preface / Tetrahedron 56 (2000) vii±viii
theoretical models enabled the process, but the intelligent application of qualitative valence-bond theory established the generality of 1,3-dipolar cycloadditions. It is a characteristic label of Huisgen's work that after intellectual perception of a problem he masterly designs experiments to prove or disprove the validity of the presumption. Over the last 40 years Rolf Huisgen's group and many followers have developed 1,3-dipolar cycloadditions as the most powerful tool to synthesize ®ve-membered heterocyclic compounds. But Huisgen would not be himself if he had not had a basic interest to intellectually master the underlying principles. It is another of his features that he is able to recognize the importance of new developments immediately. Thus, when qualitative MO theory emerged as a guiding tool for new discoveries, he immediately seized the new possibilities.
1,4-dipolar cycloadditions. Important contributions were furnished in (212) cycloadditions. The mechanism of ketene cycloadditions was investigated thoroughly, in particular the threshold between concerted and stepwise formation of cyclobutanones was of interest. The experimental test of the predictions of the rules of conservation of orbital symmetry was the incentive for much of the work in this area. Elegant studies on (212) cycloadditions of donor and acceptor alkenes clari®ed many aspects of these reactions. It is not surprising that the scienti®c contributions of Rolf Huisgen to organic chemistry found wide recognition. Among the many honors which he received are several honorary doctoral degrees, honorary memberships of chemical societies worldwide, and membership of a number of academies. Many awards were attributed, among them the Liebig Medal of the German Chemical Society (1961), The Roger Adams Award of the American Chemical Society (1975), the Otto Hahn Prize in Chemistry and Physics, the highest Prize in Science in Germany (1979).
Sometimes lucky circumstances lead to synergetic success. Thus, when the Woodward±Hoffmann rules were ®rst published in the mid-sixties, Huisgen and his group recognized the power of this approach. He not only applied the principles to 1,3-dipolar cycloadditions, gaining important new insight into the mechanism of these reactions, but he also extended his studies to other types of pericyclic reactions. The combination of electrocyclic ring-opening of aziridines with the cycloaddition of the azomethine ylides formed is probably one of the most elegant demonstrations of the validity of the rules of conservation of orbital symmetry. Other investigations on electrocyclic reactions followed.
One of the most enjoyable experiences is that Rolf Huisgen, at the age of 80, is still present in Chemistry, publishes results of his many former students and associates, and loves scienti®c discussions, a feature which has been typical throughout his entire chemistry career. Reiner Sustmann UniversitaÈt Essen Institut fuÈr Organische Chemie 45117 Essen Germany E-mail address: [email protected]
One of Huisgen's main interests consisted of the elucidation of the mechanism of cycloadditions, not only 1,3- but also
viii
Tetrahedron 56 (2000) vii±viii
Preface
Rolf Huisgen
This issue of Tetrahedron is devoted to Professor Rolf Huisgen, University of Munich, one of the most prominent contemporary organic chemists, on the occasion of his 80th birthday. Rolf Huisgen's vita covers much of the 20th century. Being born in 1920 he experienced the turmoils of preworld-war II and was just ready to begin his university studies at the beginning of world-war II. After his start at the University of Bonn in 1939 he continued at the University of Munich where he obtained his diploma in 1940, having studied just six trimesters, the change from the semester to the trimester system being a consequence of the special war conditions. He was lucky to be able to engage immediately after the diploma examination in work for his doctoral degree. Under the guidance of Heinrich Wieland he completed his thesis in 1943. He continued earlier work of the Wielands group on the structure elucidation of the alkaloid vomicine without having modern instrumentation at his hands. Derivatization and degradation were the techniques available. Rolf Huisgen soon began independent research although activities were hampered by the unfavorable war and postwar conditions. He developed chemistry which did not follow the footsteps of his teacher Heinrich Wieland but which led him to study fundamental questions in mechanistic organic chemistry. In this way he founded and established physical organic chemistry in Germany. In 1949, at the age of 29, Rolf Huisgen was offered a position as Professor Extraordinarius at the University of TuÈbingen where he remained until 1952 when he returned to the University of Munich as successor of his teacher Heinrich Wieland.
The contributions of Rolf Huisgen to organic chemistry are numerous. In the early days it was the chemistry of diazo compounds which attracted his interest. He writes in his autobiography `The Adventure Playground of Mechanisms and Novel Reactions' (R. Huisgen, 1994): ªThe aromatic diazonium ion has fascinated generations of chemists: molecular nitrogen forced into the onium state, metastability despite a high energy level, a wealth of reactions equally signi®cant for laboratory and industryº. The Munich group was able to increase the synthetic possibilities of this chemistry and to broaden the mechanistic knowledge about the reactions of diazo- and diazonium compounds considerably. Thus, the reactions of N-nitrosoacylamides were studied in depth leading to the formulation of alkanediazonium ion pairs and the subsequent formation of carbocations in the various product forming steps. Huisgen's sharp analysis of experimental resultsÐa character feature which becomes apparent in all of his workÐled him to the postulation and the indirect identi®cation of diazoalkanes in the reactions of N-nitrosoacylamides. The intelligent design of experiments to prove a mechanistic consequence of experimental observations is another of Huisgen's attributes. Medium sized rings and benzyne chemistry were his main activities in the ®fties. Huisgen's contributions belong to the highlights in these areas. Chemistry involving one or several nitrogen atoms continued to be of high interest. The discovery of the existence of pentazoles, a long sought-after class of compounds, is certainly one of Huisgen's great scienti®c achievements. Proper analysis of experimental observations in reactions of diazonium ions with azide ion uncovered the existence of ®ve membered rings constructed only of nitrogen atoms. The combination of preparative and kinetic studies led to the development of 1,3-dipolar cycloaddition chemistry. Again, nitrogen-containing molecules, kinetic studies of the reactions of diazoalkanes with ole®ns, played a decisive role. These results induced the development of the general concept of 1,3-dipoles. Although scattered examples of reactions of alkenes with molecules, from hereon called 1,3-dipoles, could be found in the literature, it is Huisgen's great achievement and merit to have recognized the general principle and to have exploited the chemistry of 1,3-dipolar cycloadditions. Within a few years in the early sixties he discovered new 1,3-dipoles which followed from the formulation of the underlying principle. Unsophisticated
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0040-402 0(00)00332-X
Preface / Tetrahedron 56 (2000) vii±viii
theoretical models enabled the process, but the intelligent application of qualitative valence-bond theory established the generality of 1,3-dipolar cycloadditions. It is a characteristic label of Huisgen's work that after intellectual perception of a problem he masterly designs experiments to prove or disprove the validity of the presumption. Over the last 40 years Rolf Huisgen's group and many followers have developed 1,3-dipolar cycloadditions as the most powerful tool to synthesize ®ve-membered heterocyclic compounds. But Huisgen would not be himself if he had not had a basic interest to intellectually master the underlying principles. It is another of his features that he is able to recognize the importance of new developments immediately. Thus, when qualitative MO theory emerged as a guiding tool for new discoveries, he immediately seized the new possibilities.
1,4-dipolar cycloadditions. Important contributions were furnished in (212) cycloadditions. The mechanism of ketene cycloadditions was investigated thoroughly, in particular the threshold between concerted and stepwise formation of cyclobutanones was of interest. The experimental test of the predictions of the rules of conservation of orbital symmetry was the incentive for much of the work in this area. Elegant studies on (212) cycloadditions of donor and acceptor alkenes clari®ed many aspects of these reactions. It is not surprising that the scienti®c contributions of Rolf Huisgen to organic chemistry found wide recognition. Among the many honors which he received are several honorary doctoral degrees, honorary memberships of chemical societies worldwide, and membership of a number of academies. Many awards were attributed, among them the Liebig Medal of the German Chemical Society (1961), The Roger Adams Award of the American Chemical Society (1975), the Otto Hahn Prize in Chemistry and Physics, the highest Prize in Science in Germany (1979).
Sometimes lucky circumstances lead to synergetic success. Thus, when the Woodward±Hoffmann rules were ®rst published in the mid-sixties, Huisgen and his group recognized the power of this approach. He not only applied the principles to 1,3-dipolar cycloadditions, gaining important new insight into the mechanism of these reactions, but he also extended his studies to other types of pericyclic reactions. The combination of electrocyclic ring-opening of aziridines with the cycloaddition of the azomethine ylides formed is probably one of the most elegant demonstrations of the validity of the rules of conservation of orbital symmetry. Other investigations on electrocyclic reactions followed.
One of the most enjoyable experiences is that Rolf Huisgen, at the age of 80, is still present in Chemistry, publishes results of his many former students and associates, and loves scienti®c discussions, a feature which has been typical throughout his entire chemistry career. Reiner Sustmann UniversitaÈt Essen Institut fuÈr Organische Chemie 45117 Essen Germany E-mail address: [email protected]
One of Huisgen's main interests consisted of the elucidation of the mechanism of cycloadditions, not only 1,3- but also
viii