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Metal Salen Complexes in Catalysis

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Salen Ligand

Salen ligands are a type of Schiff base, which are formed by reacting two equivalents of salicylaldehyde with one equivalent of diamine.[1] Various metal centers in combination with diamine groups can bring distinct catalytic properties for the salen complexes.

Pinacol Coupling Reactions

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Ti(salen)Cl2 complex[2]
Ti(salen)Cl2 complex[2]
Enantioselectivity of reactions catalyzed by Ti(Salen) Complex[3]
Catalytic cycle of Ti(salen) complex[4]

Pinacol coupling can be catalyzed by titanium (IV) salen complexes enantioselectively.[1] Titanium (III) Salen complexes are capable of catalyzing redox reactions with chlorosilanes and silicon hydrides as additives.[1] Low valent titanium metal centers are stabilized by Salen ligands and are used for stereoselective organic synthesis. With DME (dimethylethane) as the solvent, the dll/meso ratio of the coupled pinacol is 84:16. As the reaction temperature drops from room temperature to -10˚C, the enantioselectivity increases from 65% to 95%.[5] As a reagent, aryl aldehydes afford the most gratifying results so far in enantiomeric control. Steric hindrance plays an essential role in lowering both yield and enantioselectivity as shown in the case of o-tolyl and 1-naphtyl.[5] Yet, yields and enantiomeric excess can be improved by increasing the concentration of the titanium Salen complex. The mechanism of these reactions is still under investigation, while some plausible pathways have been published. One of the most accepted pathway is the dimerization of the aldehyde radicals afforded by single electron transfer process.[6] Titanium (IV) is first reduced by zinc to Ti (II), followed by coordinating with the aryl aldehyde. The unpaired electron at the carbon forms the first carbon couples with another carbon radical, two aryl groups orient trans to each other and preferentially forms the product.

Catalyze Baeyer – Villiger Oxidation Reaction

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Chiral Zr(salen) complex[7]

Metal-salen complexes are also able to catalyze Baeyer – Villiger oxidation reaction with a high degree of enantioselectivity.[8] Zirconium Salen complexes are known to enantioselectively catalyze the Baeyer-Villiger reaction of 3-phenylcyclobutanone with urea-hydrogen peroxide adduct (UHP) as the oxidant.[8] The reaction runs at mild conditions: 5 mol% of Zr-Salen catalyst at room temperature, and can reach an enantiomeric excess of 87%.[8] Polar solvent, such as diethyl ether and alcohols, along with extreme temperatures decrease both yield and enantiomeric excess for the Baeyer – Villiger reaction. Racemic bicyclo[4.2.0]octan-7-one is used to determine the preference of the reaction in the presence of racemic reagents. Racemic products are obtained with a ratio of 1:6.6.

Baeyer – Villiger oxidation catalyzed by Zr(salen) complex[9]
Baeyer – Villiger oxidation catalyzed by Zr(salen) complex[9]

Catalyze Addition Reaction of HCN to Imines

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Al(salen) complex[10]
Al(salen) complex[10]
Effect of Aryl Substituents on Enantioselectivity[11]

Hydrogen cyanide (HCN) reacts with N-allyl benzaldimine rapidly at room temperature, while the reaction is completely repressed at -70˚C. Catalyzed by Al(salen) complex,[1][12] the reaction reaches completion within 15 h and attained an isolation of 91% and an enantiomeric excess of 95% at -70˚C.[1][12] In addition, aryl substituted imines are also great substrates for HCN additions, affording very high levels of enantioselectivity. However, alkyl substituted imines do not react with HCN enantioselectively, affording products with an enantiomeric excess less than 60%.  Recrystallization of the product can increase the enantiomeric purity up to 97.5%, while varying the imine nitrogen substituents, steric properties of Al(Salen) complex, and electronic properties of the catalyst do not have an observable impact on the enantioselectivity.[12]

Catalyze Asymmetric Epoxidation of cis-Olefins

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Mn(III)(salen) complex[13]
Mn(III)(salen) complex[13]

Chiral manganese Salen complexes, designed and described by Jacobson and Katsuki, are very efficient catalyst for asymmetric epoxidations of cis olefins. While Mn(II) Salen complex can be prepared by more accessible reagents, Mn(III) Salen complexes are much more stable and easier to obtain.[14][1] The asymmetric epoxidation, catalyzed by manganese Salen complexes, of olefins with aromatic substituents or triple bonds are usually used, since the orientation of the alkene facilitates the interaction with manganese Salen complexes sterically.[1] The details of the mechanism are under investigation, while a stepwise radical mechanism is suggested by computational methods, which explains, to some extend, the variable amount of trans epoxides formed during the reaction.[1]

Asymmetric Epoxidation catalyzed by Mn(salen) complex[15]
Asymmetric Epoxidation catalyzed by Mn(salen) complex[15]

Similarly, chromium-Salen complexes are also shown as an efficient catalyst for epoxidation reactions of trans-alkenes and display moderate enantioselectivity. Without substituents on the Salen ligand, the reaction proceeds via an electrophilic attack of chromium on the alkene.[16] YYet, with electron withdrawing substituents on the Salen ligand, the reaction proceeds at a faster rate and shows improved enantiomeric excess. Substrates and oxidants have an impact on the resulting enantiomeric excess as well. Normally double-bonded oxygen species increase the enantiomeric purity of the products up to 30%, while N-oxide has a detrimental effect for the ee, especially for beta-methylstyrene.[17] Unlike the radical process of manganese Salen complexes, chromium-Salen complexes proceed via a polar mechanism, with chromium metal center acting as an electrophile.[1] In addition, ruthenium Salen complexes are also efficient catalysts for asymmetric epoxidation of conjugated olefins.[14]

Catalyze Asymmetric Cyclopropanation

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Co(III)(salen) complex and effects of substituents on enantioselectivity[18]

Cobalt(II) Salen complexes are the first metallosalen complexes found to catalyze cyclopropanation enantioselectively.[19] After twenty years of studies, Cobalt (III) Salen complexes are commonly used as catalysts for asymmetric cyclopropanation. The catalytic reactivity and enantioselectivity of cobalt(III) Salen complexes can be controlled conveniently by introducing or removing substituents at C3 and C3’ positions.[14] With substituted C3 and C3’ positions, cobalt (III) Salen complexes are able to catalyze asymmetric cyclopropanation with high trans-selectivity and moderate enantioselectivity (5), while cobalt Salen complex without substituents at aforementioned positions shows no catalytic activity towards cyclopropanation.[14][20] Additionally, replacing the iodo group at axial position with an electron-donating group improves enantioselectivity of the cobalt Salen catalysts up to 93% without sacrificing the trans-selectivity.[14]

Asymmetric Cyclopropanation catalyzed by Co(III)(salen) complex[21]
Asymmetric Cyclopropanation catalyzed by Co(III)(salen) complex[21]

Catalyze hetero Diels-Alder reaction

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Hetero Diels-Alder reaction catalyzed by Cr(III)(salen) complex[22]
Hetero Diels-Alder reaction catalyzed by Cr(III)(salen) complex[22]
Effects of substituents on Yield and Enantioselectivity of D-A reaction catalyzed by Cr(III)(salen) complex[23]

Asymmetric hetero-Diels-Alder reactions can be catalyzes by cationic chromium Salen complexes in a highly stereocontrolled fashion.[24] Yield of these chromium Salen catalyzed reactions are usually quite high. The reaction mechanism is proposed to be a concerted [4+2] process.[25] Additionally, chromium Salen complex also simplify the preparation process for running Diels Alder reactions, since the active chromium catalysts are easier than the traditional Diels Alder catalyst to prepare.[14] 

  1. ^ a b c d e f g h i Cozzi, Pier Giorgio. "Metal–Salen Schiff base complexes in catalysis: practical aspects". Chem. Soc. Rev. 33 (7): 410–421. doi:10.1039/b307853c.
  2. ^ Cozzi, Pier Giorgio. "Metal–Salen Schiff base complexes in catalysis: practical aspects". Chem. Soc. Rev. 33 (7): 410–421. doi:10.1039/b307853c.
  3. ^ Chatterjee, A.; Bennur, T. H.; Joshi, N. N. (2003-06-12). "Truly Catalytic and Enantioselective Pinacol Coupling of Aryl Aldehydes Mediated by Chiral Ti(III) Complexes †". The Journal of Organic Chemistry. 68 (14): 5668–5671. doi:10.1021/jo0342875.
  4. ^ Chatterjee, A.; Bennur, T. H.; Joshi, N. N. (2003-06-12). "Truly Catalytic and Enantioselective Pinacol Coupling of Aryl Aldehydes Mediated by Chiral Ti(III) Complexes †". The Journal of Organic Chemistry. 68 (14): 5668–5671. doi:10.1021/jo0342875.
  5. ^ a b Chatterjee, A.; Bennur, T. H.; Joshi, N. N. (2003-06-12). "Truly Catalytic and Enantioselective Pinacol Coupling of Aryl Aldehydes Mediated by Chiral Ti(III) Complexes †". The Journal of Organic Chemistry. 68 (14): 5668–5671. doi:10.1021/jo0342875.
  6. ^ Wirth, Thomas (1996-01-19). ""New" Reagents for the "Old" Pinacol Coupling Reaction". Angewandte Chemie International Edition in English. 35 (1): 61–63. doi:10.1002/anie.199600611. ISSN 1521-3773.
  7. ^ Chatterjee, A.; Bennur, T. H.; Joshi, N. N. (2003-06-12). "Truly Catalytic and Enantioselective Pinacol Coupling of Aryl Aldehydes Mediated by Chiral Ti(III) Complexes †". The Journal of Organic Chemistry. 68 (14): 5668–5671. doi:10.1021/jo0342875.
  8. ^ a b c Wirth, Thomas (1996-01-19). ""New" Reagents for the "Old" Pinacol Coupling Reaction". Angewandte Chemie International Edition in English. 35 (1): 61–63. doi:10.1002/anie.199600611. ISSN 1521-3773.
  9. ^ Chatterjee, A.; Bennur, T. H.; Joshi, N. N. (2003-06-12). "Truly Catalytic and Enantioselective Pinacol Coupling of Aryl Aldehydes Mediated by Chiral Ti(III) Complexes †". The Journal of Organic Chemistry. 68 (14): 5668–5671. doi:10.1021/jo0342875.
  10. ^ Cozzi, Pier Giorgio. "Metal–Salen Schiff base complexes in catalysis: practical aspects". Chem. Soc. Rev. 33 (7): 410–421. doi:10.1039/b307853c.
  11. ^ Sigman, Matthew S.; Jacobsen, Eric N. (1998-05-13). "Enantioselective Addition of Hydrogen Cyanide to Imines Catalyzed by a Chiral (Salen)Al(III) Complex". Journal of the American Chemical Society. 120 (21): 5315–5316. doi:10.1021/ja980299+.
  12. ^ a b c Sigman, Matthew S.; Jacobsen, Eric N. (1998-05-13). "Enantioselective Addition of Hydrogen Cyanide to Imines Catalyzed by a Chiral (Salen)Al(III) Complex". Journal of the American Chemical Society. 120 (21): 5315–5316. doi:10.1021/ja980299+.
  13. ^ Cozzi, Pier Giorgio. "Metal–Salen Schiff base complexes in catalysis: practical aspects". Chem. Soc. Rev. 33 (7): 410–421. doi:10.1039/b307853c.
  14. ^ a b c d e f Katsuki, Tsutomu (2003-02-01). "Some Recent Advances in Metallosalen Chemistry". Synlett (3): 0281–0297. doi:10.1055/s-2003-37101.
  15. ^ Cozzi, Pier Giorgio. "Metal–Salen Schiff base complexes in catalysis: practical aspects". Chem. Soc. Rev. 33 (7): 410–421. doi:10.1039/b307853c.
  16. ^ Samsel, E. G.; Srinivasan, K.; Kochi, Jay K. (2002-05-01). "Mechanism of the chromium-catalyzed epoxidation of olefins. Role of oxochromium(V) cations". Journal of the American Chemical Society. 107 (25): 7606–7617. doi:10.1021/ja00311a064.
  17. ^ Bousquet, Claudine; Gilheany, Declan G. (1995-10-16). "Chromium catalysed asymmetric alkene epoxidation. greater selectivity for an E-alkene versus its Z-isomer". Tetrahedron Letters. 36 (42): 7739–7742. doi:10.1016/0040-4039(95)01577-5.
  18. ^ Katsuki, Tsutomu (2003-02-01). "Some Recent Advances in Metallosalen Chemistry". Synlett (3): 0281–0297. doi:10.1055/s-2003-37101.
  19. ^ Nakamura, Akira; Konishi, Akira; Tatsuno, Yoshitaka; Otsuka, Sei (2002-05-01). "A highly enantioselective synthesis of cyclopropane derivatives through chiral cobalt(II) complex catalyzed carbenoid reaction. General scope and factors determining the enantioselectivity". Journal of the American Chemical Society. 100 (11): 3443–3448. doi:10.1021/ja00479a028.
  20. ^ Nakamura, Akira; Konishi, Akira; Tatsuno, Yoshitaka; Otsuka, Sei (2002-05-01). "A highly enantioselective synthesis of cyclopropane derivatives through chiral cobalt(II) complex catalyzed carbenoid reaction. General scope and factors determining the enantioselectivity". Journal of the American Chemical Society. 100 (11): 3443–3448. doi:10.1021/ja00479a028.
  21. ^ Katsuki, Tsutomu (2003-02-01). "Some Recent Advances in Metallosalen Chemistry". Synlett (3): 0281–0297. doi:10.1055/s-2003-37101.
  22. ^ Bandini, Marco; Cozzi, Pier Giorgio; Umani-Ronchi, Achille. "[Cr(Salen)] as a 'bridge' between asymmetric catalysis, Lewis acids and redox processes". Chemical Communications (9): 919–927. doi:10.1039/b109945k.
  23. ^ Bandini, Marco; Cozzi, Pier Giorgio; Umani-Ronchi, Achille. "[Cr(Salen)] as a 'bridge' between asymmetric catalysis, Lewis acids and redox processes". Chemical Communications (9): 919–927. doi:10.1039/b109945k.
  24. ^ Bandini, Marco; Cozzi, Pier Giorgio; Umani-Ronchi, Achille. "[Cr(Salen)] as a 'bridge' between asymmetric catalysis, Lewis acids and redox processes". Chemical Communications (9): 919–927. doi:10.1039/b109945k.
  25. ^ Schaus, Scott E.; Brånalt, Jonas; Jacobsen, Eric N. (1998-01-07). "Asymmetric Hetero-Diels−Alder Reactions Catalyzed by Chiral (Salen)Chromium(III) Complexes". The Journal of Organic Chemistry. 63 (2): 403–405. doi:10.1021/jo971758c.
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