Iron asymmetric sulfoxidation

From a practical viewpoint the recently discovered vanadium-based and iron-based asymmetric sulfoxidation with hydrogen peroxide is worth mentioning [305, 306]. For vanadium, in principle as little as 0.01 mol% of catalyst can be employed (Fig. 4.111). With tridentate Schiff-bases as ligands, formed from readily available salicylaldehydes and (S)-tert-leucinol, ees of 59-70% were obtained for thioanisole [305], 85% ee for 2-phenyl-l,3-dithiane [305] and 82-91% ee for tert-butyl disulfide [307]. For iron, similar results were obtained using 4 mol% of an iron catalyst, synthesized in situ from Fe(acac)3 and the same type of Schiff base ligands as in Fig. 4.111 (see Ref. [306] for details). 

Figure 1.8 Conformation of iron porphyrins in asymmetric sulfoxidation.

Korte A, Legros J, Bolm C. Asymmetric synthesis of sulindac by iron-catalyzed sulfoxidation. Synlett 2004 2397-2399. 

The synthesis of sulfoximides and sulfimides has attracted considerable attention in recent years due to the potential utility of these compounds as efficient auxiliaries and chiral ligands in asymmetric synthesis (reviews [86-88]). Transition metal-catalyzed nitrene transfer to sulfoxides and sulfides is an efficient and straightforward way to synthesize sulfoximides and sulfimides, respectively. Bach and coworkers reported the first iron-catalyzed imination of sulfur compounds with FeCl2 as catalyst and B0CN3 as nitrene source. Various sulfoxides and sulfides were... 

Oxometalloporphyrins were taken as models of intermediates in the catalytic cycle of cytochrome P-450 and peroxidases. The oxygen transfer from iodosyl aromatics to sulfides with metalloporphyrins Fe(III) or Mn(III) as catalysts is very clean, giving sulfoxides, The first examples of asymmetric oxidation of sulfides to sulfoxides with significant enantioselectivity were published in 1990 by Naruta et al, who used chiral twin coronet iron porphyrin 27 as the catalyst (Figure 6C.2) [79], This C2 symmetric complex efficiently catalyzed the oxidation... 

Non-asymmetric Oxidations Suarez and coworkers described the oxidation of sulfides using FeBr3 or (FeBr3)2(DMSO)3 (10mol%) and nitric acid as oxidant (Table 3.6) [157]. Both iron catalysts are able to provide sulfoxides in good yields (65-99%). 

The major breakthrough in this field was achieved in 2003 by Legros and Bolm [164], who reported a highly enantioselective iron-catalyzed asymmetric sulfide oxidation. Optically active sulfoxides were obtained with up to 96% ee in good yields under very simple reaction conditions using Fe(acac)3 as precatalyst in combination with a Schiff base-type ligand (Table 3.9). Furthermore, inexpensive and safe 35% aqueous hydrogen peroxide served as terminal oxidant. 

Shortly thereafter, Bryliakov and Talsi described chiral [iron(salen)Cl] catalysts for the asymmetric sulfide oxidation using PhIO as terminal oxidant (Table 3.10) [166]. Whereas good selectivities in the formation of sulfoxides vs. sulfones were achieved, only poor to moderate enantiomeric excesses were obtained with this system (22-62% ee). 

The oxidative imination of sulfides and sulfoxides via nitrene transfer processes leads to N-substituted sulfilimines and sulfoximines. This reaction is interesting as chiral sulfoximines are efficient chiral auxiliaries in asymmetric synthesis, a promising class of chiral ligands for asymmetric catalysis and key intermediates in the synthesis of pseudopeptides [169]. However, very few examples of such iron-catalyzed transformations have been described. 

The selective oxidation of sulfides to sulfoxides is an important reaction in biological systems, ft is a well-studied model reaction for oxo atom transfer and it is of importance for applications in pharmaceutical and preparative organic chemistry [1, 2, 60]. Iron- and ruthenium-based sulfoxidation catalysts have also been explored in the area of asymmetric synthesis and enantioselective catalysis... 

Chiral iron acyl complexes have been applied to the asymmetric synthesis of cyclopropane carboxylic acids, sulfoxides and p-amino acids. Further details and applications may be found in the reviews given in the reference... 

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