Jacobsen-Katsuki epoxidation

What is Jacobsen-Katsuki epoxidation?

In the 1980s, Kochi reported the epoxidation of olefins catalyzed by oxomanganese salen N,N-ethylenebis(salicylideneaminato) complex. However, it was in 1990 when Jacobsen and Katsuki concurrently reported the improvement of this reaction using a chiral salen ligand for manganese complexes, which resulted in high enantioselectivity.

Jacobsen-Katsuki epoxidation - general reaction scheme
Jacobsen-Katsuki epoxidation

This reaction is widely known as the Jacobsen-Katsuki epoxidation and is usually carried out in dichloromethane or acetonitrile. Jacobsen’s catalysts have two stereogenic centers, while Katsuki’s catalysts have four. The conformation of the chiral substituents attached to the C8- and C8-positions have a considerable influence on asymmetric induction. Additionally, the presence of a bulky group such as t-butyl at the C3- and C3-position of the salen ligand achieves high enantioselectivity. However, groups larger than t-butyl have not shown significant advantages for enhancing enantioselectivity. It has been reported that this reaction is strongly electrophilic and the presence and properties of substituents at the C5- and C5-position of salen ligand also affect the enantioselectivity, as well as the epoxidation rate.

Jacobsen catalyst

Jacobsen catalyst
Jacobsen-type catalyst
  • R1, R2 = H
  • R1,= t-Bu; R2 = H, Me, OMe
  • R1,= 9-methyl-9-fluorenyl; R2 = Me
Jacobsen catalyst
Jacobsen-type catalyst

R = Me, t-Bu, OSi(i-Pr)3

Katsuki catalyst

Katsuki catalyst
Katsuki-type catalyst
  • R1, R3 = Ph; R2 = H
  • R= Ph; R2 = Me; R3 = pt-Bu-Ph
  • R1R= (CH2)4R2 = Me; R3 = pt-Bu-Ph
Katsuki catalyst
Katsuki-type catalyst
  • R1 = Ph; R2 = Me
  • R= 3,5-Me2-Ph; R2 = Ph
  • R1R= (CH2)4R2 = Ph

The enantioselectivity of this reaction also depends on the structure of substrates. Jacobsen’s catalysts work better for cis-olefins, while Katsuki’s catalysts are better for trans-olefins. The counterion of Mn(salen)X complexes, the additive as an axial ligand, and the oxidant all have effects on the outcome of the asymmetric epoxidation.

Oxidants

The oxidants used for this epoxidation include iodosylbenzene (PhIO), sodium hypochlorite (NaOCl), oxygen/reductant, hydrogen peroxide (H2O2), alkylhydroperoxide (e.g., t-butylperoxide), peroxyacids (e.g., mCPBA, see list of acronyms), potassium monoperoxysulfate (KHSO5, oxone), dimethyldioxirane, and triphenyl phosphorus oxide (Ph3PO).

Reaction mechanism

The mechanism of this reaction is controversial; however, it is generally agreed that this reaction consists of a two-step catalytic cycle (the formation of Mn(salen)v complex and the transfer of activated oxygen to an olefinic double bond).

To complete the catalytic cycle, at least three mechanisms have been proposed, including the concerted, stepwise radical, and manganaoxetane pathways. A multichannel process including singlet, triplet, and quintet spin states, and proximal and distal approach (vectors) have also been proposed to explain the stereochemical outcome of the epoxidation. The transition state is held to be asynchronous; therefore, the order of mixing the reacting components is important for this reaction. This reaction has been extended to occur in the presence of ionic liquid (BMIMPF6), to resolve the racemic epoxides in a highly selective manner using a cobalt salen complex, and then oxidize the stilbene into diarylacetaldehyde under basic conditions.

References

  • Siddall, T. L., Miyaura, N., Huffman, J. C., and Kochi, J. K. (1983). Isolation and molecular structure of unusual oxochromium(V) cations for the catalytic epoxidation of alkenes. J. Chem. Soc., Chem. Commun., 21, 1185-1186. DOI: 10.1039/C39830001185
  • Srinivasan, K. and Kochi, J. K., Inorg. Chem., 1985, 24, 4671
  • Srinivasan, K.; Michaud, P. and Kochi, J. K., J. Am. Chem. Soc., 1986, 108, 2309
  • Zhang, W., Loebach, J. L., Wilson, S. R., and Jacobsen, E. N. (1990). Enantioselective epoxidation of unfunctionalized olefins catalyzed by salen manganese complexes. Journal of the American Chemical Society, 112(7), 2801-2803. doi: 10.1021/ja00163a052
  • Ryo Irie, Keiko Noda, Yoshio Ito, Naohide Matsumoto, Tsutomu Katsuki,
    Catalytic asymmetric epoxidation of unfunctionalized olefinsTetrahedron Letters, 31(50) 7345-7348, (1990) DOI: 10.1016/S0040-4039(00)88562-7
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  • Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N. and Katsuki, T., Tetrahedron: Asymmetry, 1991, 2, 481
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