Iron salen complexes

An alternative TS for asymmetric addition of cyanide to aldehydes catalysed by titanium-salen complexes has been proposed, based on a comparison with a related iron-salen complex for which a crystal stmcture is reported.252... 

An iron-salen complex can also catalyze the arylation of alkyl halides with organomagnesium compounds. Bedford and coworkers reported that the crosscoupling of primary and secondary alkyl halides with aryl Grignard reagents takes place in the presence of the iron complex 15 to give the corresponding alkyl-substituted arenes in good yields (Equation 5.28) [36]. 

Tonami, H., Uyama, H., Kobayashi, S., Higashimura, H., and Oguchi, T. (1999) Oxidative Polymerization of 2,6-disubstituted Phenols Catalyzed by Iron-Salen Complex. J. Macrom. ScL-Pure Appl Chem., A36,719— 730. 

Although salen complexes of chromium, nickel, iron, ruthenium, cobalt, and manganese ions are known to serve as catalysts for epoxidation of simple olefins, the cationic Mn-salen complex is the most efficient. 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

M. M. Bhadbhade, and D. Srinivas, Spin crossover in substituted N, N -ethylenebis (salicyli-denamine) iron(III) complexes variable temperature EPR and X-ray structures of [Fe(5-CH30-salen)(lm)2]Y, where Y = CIO and Cl , Polyhedron 17, 2699-2711 (1998). 

Ironically, oligomeric Co(III) salen complexes 5 (Figme 3) developed by Jacobsen et al. [57, 58] though highly active was non-recyclable. These complexes degraded during HKR of terminal/meso epoxides imder the reaction conditions used. 

Attempts to aziridinate alkenes with iron catalysts in an asymmetric manner have met with only limited success to date [101], In an early report on the use of various chiral metal salen complexes, it was found that only the Mn complex catalyzed the reaction whereas all other metals investigated (Cr, Fe, Co, Ni etc.) gave only unwanted hydrolysis of the iminoiodinane to the corresponding sulfonamide and iodoben-zene [102], Later, Jacobsen and coworkers and Evans et al. achieved good results with chiral copper complexes [103]. 

Finally, iron catalysts based on salen-type ligands have been used. These iron(III)-salen complexes were regarded as enzyme models, using PhIO as oxidant (Scheme 3.52) [162]. Initially, the corresponding active iron-oxo complexes were formed by reaction with PhIO and isolated before use. A stoichiometric amount of the iron-oxo complex allowed the efficient oxidation of a variety of aryl methyl sulfides in moderate to good yields. 

Hilt and coworkers developed radical addition/cyclization sequences of styrene oxides 42 and styrenes, dienes, 1,3-enynes, and acrylates 43 catalyzed by three catalyst systems (A) 20 mol% of the Fe(dppe)Cl2 complex [103], (B) FeCl2 in the presence of 10 mol% PPh3/10 mol% NHC 44 [103], or (C) 20 mol% of (salen)iron(II) complex 45 (Fig. 9) [104, 105]. In all three methods NEt3 and zinc were used as... 

The preparation and properties of some unusual organometallic Fe,n complexes of salen have been described by Floriani and Calderazzo.353 It was found that the iron(Il) complex (91) can be reduced to the corresponding monoanion (92) by sodium in THF. The extremely air-sensitive... 

Apart from the commonly used NaOCl, urea—H2O2 has been used/ With this reaction, simple alkenes can be epoxi-dized with high enantioselectivity. The mechanism of this reaction has been examined.Radical intermediates have been suggested for this reaction, polymer-bound Mn -salen complex, in conjunction with NaOCl, has been used for asymmetric epoxidation. Chromium-salen complexes and ruthenium-salen complexes have been used for epoxidation. Manganese porphyrin complexes have also been used. Cobalt complexes give similar results. A related epoxidation reaction used an iron complex with molecular oxygen and isopropanal. Nonracemic epoxides can be prepared from racemic epoxides with salen-cobalt(II) catalysts following a modified procedure for kinetic resolution. 

W. Adam, V. R. Stegmann, C. R. Saha-Moller, Regio- and Diastereoselective epoxidation of chiral allylic alcohols catalyzed by manganese(salen) and iron (porphyrin) complexes, J. Am. Chem. Soc. 121 (1999) 1879. 

Keywords C-H oxidation, Benzylic hydroxylation, AUylic oxidation, Desymmetrization of meso compound, Kharash-Sosnovsky reaction. Iron-porphyrin complex, (Salen)manganese(lll) complex... 

Sivasubramanian, K.V, M. Ganesan, S. Rajagopal, and R. Ramaraj (2002). Iron(III)-salen complexes as enzyme models Mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides. J Org. Chem. 67, 1506-1514. 

The need in new inexpensive, safe and effective processes for asymmetric sulfide oxidations is determined by pharmaceutical industry requirements [38], Recently, inexpensive and active. systems based on hydrogen peroxide as oxidant and non-toxic chiral iron(III) complexes as catalysts have been reported [39-41 ]. Different mctal-salen complexes have also been previously employed as catalysts for oxidation of sulfides with PhIO Mn "(salen) [42-44], salen) 45], salen) [46], The mechanism proposed in [46] involves intermediate formation of 0x0 iron(lV)-salcn cation radical, that seems doubtful based on the experimental results obtained. In this Chapter we present asymmcinc version of the latter system [(salen )Fc ClJ/PhlO (where salcn stands for the corresponding chiral Schiff base ligands. Scheme 5) and an NMR investigation of the active intermediates. 

---