Jacobsen epoxidation
The Jacobsen epoxidation ( Jacobsen-Katsuki epoxidation ) is a name reaction in the field of organic chemistry. It was discovered independently by the chemists Eric N. Jacobsen and Tsutomu Katsuki.
Jacobsen epoxidation is an enantioselective method for the synthesis of asymmetric epoxides from the corresponding alkenes . A chiral manganese (III) - salen complex as a catalyst and stoichiometric amounts of an oxidizing agent (e.g. sodium hypochlorite ) are required for the reaction . In contrast to Sharpless epoxidation , it is not restricted to allyl alcohols , but can be used in general with multiply substituted, non-activated alkenes.
A special feature of Jacobsen epoxidation is its high enantioselectivity. The best results could be achieved with cis -alkenes as starting material. Depending on the reaction conditions, however, trans- alkenes and more highly substituted alkenes are also possible as starting materials. The addition of N -oxides is particularly recommended for the preparation of more highly substituted epoxides .
Structure of the catalyst
The core of the Jacobsen epoxidation is the manganese (III) salen catalyst. The catalysts developed by Jacobsen ( 1 and 2 ; t -Bu = tert - butyl , Me = methyl , Ph = phenyl ) and the catalyst proposed by Katsuki ( 3 ; Me = methyl, Ac = acetyl , Ph = phenyl) differ in the ligands on the salen skeleton and on the manganese:
The high enantioselectivity of Jacobsen epoxidation can be demonstrated using an example. The configuration of the catalyst is decisive for the configuration of the epoxide formed:
| Educt | Chiral catalyst | product | Enantioselectivity in ee |
|---|---|---|---|
|
( S, S ) catalyst ( 1 ) |
|
92% |
| ( R, R ) catalyst ( 2 ) |
|
84% |
Reaction mechanism
For the epoxidation, the catalyst must first be converted into the reactive manganese (V) form by the oxidizing agent - here sodium hypochlorite :
While there is consensus in the literature about the oxidation ( A ) of the catalyst, the further mechanistic details ( B ) of the Jacobsen epoxidation are still controversial:
Alkyl- substituted alkenes mainly form cis -epoxides, which is why a concerted mechanism is proposed for these .
However, conjugated alkenes mainly form trans epoxides, which is why a radical mechanism is assumed here . The free rotation of the radical intermediate ( 5 ) enables the formation of the trans product. A third mechanism via the intermediate of a mangano oxetane ( 6 ) could explain the formation of both isomers . This intermediate can react directly to form the cis epoxide, or it can react by homolysis to form the radical intermediate ( 5 ), which can also result in trans epoxides.
Individual evidence
- ^ W. Zhang, JL Loebach, SR Wilson, EN Jacobsen: Enantioselective epoxidation of unfunctionalized olefins catalyzed by salen manganese complexes . In: Journal of the American Chemical Society . 112, No. 7, March 1990, pp. 2801-2803. doi : 10.1021 / ja00163a052 .
- ↑ a b Eric N. Jacobsen, Wei Zhang, Alexander R. Muci, James R. Ecker, Li Deng: Highly enantioselective epoxidation catalysts derived from 1,2-diaminocyclohexane . In: Journal of the American Chemical Society . tape 113 , no. 18 , 1991, p. 7063-7064 , doi : 10.1021 / ja00018a068 .
- ↑ Ryo Irie, Keiko Noda, Yoshio Ito, Naohide Matsumoto, Tsutomu Katsuki: Catalytic asymmetric epoxidation of unfunctionalized olefins using chiral (salen) manganese (III) complexes . In: Tetrahedron: Asymmetry . tape 2 , no. 7 , 1991, pp. 481-494 , doi : 10.1016 / S0957-4166 (00) 86102-9 .
- ↑ Tsutomu Katsuki: Catalytic asymmetric oxidations using optically active (Salen) manganese (III) complexes as catalysts . In: Coordination Chemistry Reviews . tape 140 , February 1995, p. 189-214 , doi : 10.1016 / 0010-8545 (94) 01124-T .
- ↑ Christoph Elschenbroich: Organometallchemie , 5th edition, Wiesbaden 2005, p. 622.
- ↑ Bridget D. Brandes, Eric N. Jacobsen: Highly Enantioselective, Catalytic Epoxidation of Trisubstituted Olefins . In: The Journal of Organic Chemistry . tape 59 , no. 16 , 1994, pp. 4378-4380 , doi : 10.1021 / jo00095a009 .
- ↑ a b Torsten Linker: Jacobsen-Katsuki epoxidation and its controversial mechanism . In: Angew. Chem. Int. Ed. Engl. Volume 36 , no. 19 , 1997, pp. 2060-2062 , doi : 10.1002 / anie.199720601 .
- ^ László Kürti , Barbara Czakó: Strategic Applications of Named Reactions in Organic Synthesis . Elsevier Academic Press, Burlington / San Diego / London 2005, pp. 222-223, ISBN 978-0-12-429785-2 .
- ↑ Christian Linde, Moritz Arnold, Per-Ola Norrby, Björn Akermark: Is there a radical intermediate stage in the manganese (salen) -catalyzed epoxidation of alkenes? In: Angew. Chem. Band 106 , no. 16 , 1997, pp. 1802-1803 , doi : 10.1002 / anie.19971091615 .