Enantioselective oxidation of sulfides

Encapsulation in Y zeohte was also the method chosen to immobihze Mn complexes of C2-symmetric tetradentate hgands (Fig. 24) [75]. These materials were used as catalysts for the enantioselective oxidation of sulfides to sulfoxides with NaOCl. The lack of activity when the larger io-dosylbenzene was used as an oxidant was interpreted as an indication that the reaction took place inside the zeolite microporous system. Both the chemo- and enantioselectivity were dependent on the structure of the sulfide. (2-Ethylbutyl)phenylsulfide led to better results than methylphenylsulfide, although in all cases the enantioselectivity was low (up to 21% ee). 

Colonna, S., Gaggero, N., Pasta, P., Ottolina G., Enantioselective oxidation of sulfides to sulfoxides catalysed by bacterial cyclohexanone monooxygenases. Chem. Commun., 1996, 2303. 

Reactions where NLE have been discovered include Sharpless asymmetric epoxi-dation of allylic alcohols, enantioselective oxidation of sulfides to sulfoxides, Diels-Alder and hetero-Diels-Alder reactions, carbonyl-ene reactions, addition of MesSiCN or organometallics on aldehydes, conjugated additions of organometal-lics on enones, enantioselective hydrogenations, copolymerization, and the Henry reaction. Because of the diversity of the reactions, it is more convenient to classify the examples according to the types of catalyst involved. 

Enantioselective Oxidation of Sulfides Synthesis of (-)-(S)-Methyl -Tolyl Sulfoxide... 

Enantioselective oxidation of sulfides. m-Chloroperbenzoic acid attacks the sulfide 1 preferentially from the less hindered direction to introduce an oxygen atom with high diastereoselectivity.3... 

The nature of the hydroperoxides is important in the enantioselective oxidation of sulfides [23-25], The best hydroperoxide is cumene hydroperoxide, a readily available compound. 

Whole-cell enantioselective oxidation of sulfides is in fast development and of preparative interest. Additional examples with Acinetobacter sp NCIMB 9871, Pseudomonas sp. NCIMB 9872, and Xanthobacter autotrophicus DSM 431 have been reported [113,114]. 

Uemura reported a highly enantioselective oxidation of sulfides to sulfoxides using a chiral titanium complex prepared from chiral BINOL and Ti(0-i-Pr)4, and this reaction exhibits a remarkable asymmetric amplification (Scheme 9.15) [33]. 

Colonna S, Gaggero N, Manfredi A, Casella L, Gullotti M, Carrea G, Pasta P (1990) Enantioselective Oxidations of Sulfides Catalyzed by Chloroperoxidase. Biochemistry 29 10465... 

Trevisan V, Signoretto M, Colonna S, Pironti V, Strukul G (2004) Microencapsulated Chloroperoxidase as a Recyclable Catalyst for the Enantioselective Oxidation of Sulfides with Hydrogen Peroxide. Angew Chem Int Ed 43 4097... 

Chloroperoxidase Enantioselective oxidation of sulfides Enantioselective oxidation of racemic epoxyalcohols Oxidation of benzyl alcohol Epoxidation of styrene Asymmetric oxidations Halogenation reactions [11, 15,77] [15, 48] [14] [78] [79] [80]... 

Asymmetric oxidation of sulfides to sulfoxides is undoubtedly the most straightforward route to optically pure (o.p.) sulfoxides. It is thus not surprising that various groups have been trying to develop efficient methods toward this end. Several o.p. sulfoxides have been obtained, by either diastereoselective or enantioselective oxidation of sulfides. 

The main methodologies developed until now for enantioselective oxidation of sulfides are effective only in the oxidation of alkyl aryl sulfoxides. Dialkyl sulfoxides on the other hand are generally oxidized with only poor selectivity. In an attempt to solve this problem, Schenk s group69 recently reported a stereoselective oxidation of metal-coordinated thioethers with DMD. The prochiral thioether is first coordinated to a chiral ruthenium complex by reaction with the chloride complexes [CpRu[(S,S)-chiraphos]Cl], 36. Diastereoselective oxygen transfer from DMD produces the corresponding sulfoxides in high yield and selectivity. The chiral sulfoxides 37 are liberated from the complexes by treatment with sodium iodide. Several o.p. aryl methyl sulfoxides have been obtained by this method in moderate to high ee (Scheme 12). 

Several approaches to the enantioselective oxidation of sulfides have been reported,61 including enzymatic approaches,62 use of optically pure oxidants,63 and several modifications of the Sharpless epoxidation procedure.63,64 The success of these procedures is somewhat substrate dependent for example, dialkyl sulfides and more complex substrates can give unpredictable results. 1,3-Dithiane itself is oxidized with only ca. 20% ee optically pure DiTOX has, however, been obtained by resolution.65... 

Several oxaziridines related to (14) (eq 8) have been used, most notably in the enantioselective oxidation of sulfides to sulfoxides, of selenides to selenoxides, and of alkenes to oxiranes, It is also the reagent of choice for the hydroxylation of lithium and Grignard reagents and for the asymmetric oxidation of enolates to give a-hydroxy carbonyl compounds, - A similar chiral fluorinating reagent has also been developed, ... 

Considerable advances have been made recently towards the enantioselective oxidation of sulfides to sulfoxides. The reactions can be technically challenging but high enantioselectivies can be achieved. 

Scheme 7.6 Enantioselective oxidation of sulfides using DET as ligand.

Scheme 7.8 Enantioselective oxidation of sulfide using titanium-salen catalyst 12.

Diethyl tartrate (compared to various tartaric acid derivatives) still remains the best ligand for the enantioselective oxidation of sulfides, as exemplified by the oxidation of methyl p-tolyl sulfide [49]. Yamamoto et al found that DET can be replaced by diols (30) or (31) in the Kagan reagent, giving sulfoxides of opposite... 

The development of other efficient methods for the enantioselective oxidation of sulfides to sulfoxides has been pursued for many years. Various chiral additives and metal complexes have been investigated. However, most of these systems have disadvantages such as formation of sulfone, long reaction times, unsatisfactory yields and poor enantiomeric excesses. Diethyl tartrate remains the additive (combined with titanium alcoholates) which gives the best results in terms of enantioselectivity. Since 1989, it has been found that other chiral ligands can be efficient in asymmetric induction. 

Chiral Modified Electrode Poly-L-valine coated anode promoted enantioselective oxidation of sulfide to sufoxide in 93 % ee [12]. 

More recently, Jansen and coworkers [90] used 4-hydroxyacetophenone monooxygenase (HAPMO) for aerobic oxidation of sulfides. Interestingly, both PhSMe and p-MeC6H4SMe gave the corresponding sulfoxides in >99% ee, which should be compared with the 99 and 37% ee, respectively, obtained with CHMO [86]. The flavoenzyme HAPMO, which has been cloned [90, 91], is a promising biocatalyst for enantioselective oxidation of sulfides to sulfoxides. 

Vanadium. and NMR spectroscopy has been used to study the peroxyvanadium intermediates in vanadium catalysed enantioselective oxidation of sulfides. The aerobic oxidation of Pr OH catalysed by per-oxovanadium complexes has been investigated using NMR spectroscopy. ... 

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