| People
- Faculty
- Professor:
Bruce Rickborn |
| Field(s): |
Organic Chemistry |
 |
| Email: |
rickborn@chem.ucsb.edu |
| Phone: |
(805)
893-
2818 |
Fax:
(805)
893-
4120 |
| Office: |
2148 Chem |
 |
Selected
Publications |
|
Go
to Research Group website |
| Bio: |
Professor Rickborn received the BA from UC Riverside (1956), and the Ph.D. degree from UCLA (1960). After two years on the Berkeley faculty, he transferred to Santa Barbara in 1962. He has been an A.P. Sloan Fellow, an NSF Senior Postdoctoral Fellow (MPI, Mulheim/Ruhr), and a Fulbright Senior Lecturer (U. Nacional, Bogota). Dr. Rickborn retired in 2001 after 39 years of service to the department of Chemistry. |
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Current
Research
The Diels-Alder reaction is indisputably among the most important processes in organic chemistry. In spite of the many thousands of papers dealing with this cycloaddition, novel features of this classical reaction continue to appear. Catalyis by acids has been an area of intense study for over 30 years, but base catalysis, in spite of earlier efforts to prove its existence, was first demonstrated only recently in our laboratory. We found that anthrone and some analogs in the presence of weak bases (amines) give room temperature cycloadditions. Dienophiles that span a reactivity range ³ 105 gave cycloadducts, but only substrates that contain at least one electron-withdrawing-group exhibit this reactivity.
The cycloadducts subsequently undergo, more or less readily, an intramolecular cleavage reaction to form the analogous Michael adduct isomer; these are the products, found in the earlier literature, of attempts to demonstrate base-catalyzed Diels-Alder reactions of anthrone.
The base-catalyzed Diels-Alder reaction embodies a mechanistically intriguing "catch 22". For second-order reactions, an increase in the concentration of a reactive species will normally lead to faster reaction. However,the use of an equivalent of stronger base(e.g. alkoxides) with anthrone effectively causes cycloaddition to shut down, since the reactive educt phenoxide becomes the thermodynamically favored species under these conditions. The system thus presents the unusual feature of both weak and strong bases delivering enhanced rates by accessing the anionic reaction energy surface, but only catalytic conditions allow cycloadduct isolation.
Current efforts are aimed at increasing the reactivity range of dienophiles that can be employed, and testing the stability of the cycloadducts towards typical reagents in order to demonstrate synthetic utility. To be most useful, the cycloadduct must withstand commonly employed acids, bases, oxidants, and other reagents. The ultimate goal is to add to the value of the already powerful (Diels-Alder - cycloadduct manipulation - retro-Diels-Alder) sequence.
Uncharged anthracenes have some potential advantages for use in Diels-Alder chemistry, although all neutral derivatives examined to date exhibit reactivity much lower than the anion described above. Exploration of substituted anthracenes and naphthalenes may provide better dienes, and a better understanding of how substituents affect the reactions.
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| Selected
Research Publications |
| Unusual Stability of N-Methylmaleimide Cycloadducts; Characterization of Isobenzofuran Retro-Diels-Alder Reactions. D. Tobia, R. Harrison, B.Phillips, T.L. White, M. DiMare and B.F. Rickborn, Org. Chem. 58, 6701 (1993). | | Anthracenediols as Reactive Dienes in Base Catalyzed Cycloadditions: Reduction-Cycloaddion Reactions of Anthraquinones. M. Koerner and B.F. Rickborn, Org. Chem. 56, 1373 (1991). | | Base Catalyzed Reactions of Anthrones with Dienophiles. M. Koerner and B.F. Rickborn, Org. Chem. 55, 2662 (1990). |
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