Vanadium catalysts sulfoxidations

The first catalytic sulfoxidation of saturated hydrocarbons with S02/02 by vanadium catalysts to give alkanesulfonic acids has been reported.302 The best results were achieved with VO(acac)2 ... 

The Bolm protocol was recently used by Ellman et al. for the enantioselective oxidation of -butyl disulfide 22 [72], Excellent result was achieved in the formation of thiosulfinate 22 (91% ee, 93% yield) by using catalyst 20 (0.25 mol %) in a 0,5 mmol scale. In spite of extensive screening of chiral Schiff bases related to catalyst 20, better enantioselectivity was not realized. Chiral thiosulfinate 22 is a convenient starting material for the preparation of r-butyl sulfi-namides and t-butyl sulfoxides. Vetter and Berkessel modified the structure of the Schiff base moiety of catalyst 20 by replacing the aryl ring with a 1,l -binaphthyl system [73]. The corresponding vanadium catalyst realized 78% ee in the oxidation of thioanisol, which was better than that attained by the Bolm catalyst (59% ee). 

Bolm and Bienewald discovered in 1995 that some chiral vanadium (IV)-Schiff base complexes were efficient catalysts (1 mol %) for sulfoxidation [71a]. The catalyst 20 was prepared in situ by reacting VO(acac)2 with the Schiff base of a fJ-aminoalcohol (Scheme 6C.8). Reactions were conveniently performed in air at room temperature by slow addition of 1.1 mol equiv. of aqueous hydrogen peroxide (30%). Under these experimental conditions the reaction of methyl phenyl sulfide gave the corresponding sulfoxide in 94% yield and 70% ee. The best enantioselectivity was obtained in the formation of sulfoxide 21 (85% ee). Many structural analogues of catalyst 20 were screened for their efficacy, but none of... 

Andersson MA, Allenmark SG (1998) Asymmetric Sulfoxidation Catalyzed by a Vanadium Bromoperoxidase Substrate Requirements of the Catalyst. Tetrahedron 54 15293... 

Andersson M, Allenmark S (2000) The Potential of Vanadium Bromoperoxidase as a Catalyst in Preparative Asymmetric Sulfoxidation. Biocatal Biotransform 18 79... 

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). 

Organic hydroperoxides are generally used for the preparation of sulfoxides from sulfides, - while sulfones can be obtained in neutral organic solvents in the presence of metal catalysts such as V, Mo and Ti oxides at 50-70 C. Two polymer-supported reagents which involve peroxy acid groups and bound hypervalent vanadium(V) and molybdenum(VI) compounds have been developed for facile oxidation of sulfoxides to sulfones. 

A number of metal / -diketonates have been used to catalyze the oxidation of sulfides to sulfoxides, important synthetic intermediates for the construction of various biologically active molecules . For example, an elegant study by Ishii and coworkers demonstrated that VO(acac)2 (35) selectively catalyzed the sulfoxidation of adamantane (41) by SO2/O2 to give 1-adamantane sulfonic acid (42) (equation 10) . Although a number of metal acac complexes were examined as catalysts for this reaction, all but the vanadium compound failed to promote the sulfoxidation. The catalytic oxidation of triarylphosphines using the palladium complex Pd(acac)2 (29) has also been investigated . 

Picolinic acid also accelerates the H2O2 oxidations but less efficiently than pyrazine-2-carboxylic acid. It has been demonstrated recendy that the vanadium complex with picolinic acid, VO(PA)2 , encapsulated into the NaY zeolite retains solution-like activity in the liquid-phase oxidation of hydrocarbons [16a], It is noteworthy that pyrazine-2-carboxylic acid accelerates the hydrocarbon oxidation catalyzed by CH3Re03 [25 b]. Employing a (+)-camphor derived pyrazine-2-carboxylic acid as a potential co-catalyst in the CHsReOj-catalyzed oxidation of methyl phenyl sulfide with urea-H202 adduct, the corresponding sulfoxide was obtained with an e.e. of 15% [16b]. 

Asymmetric sulfoxidation of thioethers has been achieved enzymatically, mostly using heme- or vanadium-based haloperoxidases as the bio-catalysts... 

The vanadium oxide clusters in 5 showed catalytic activity in the sulfoxidation of thioethers or thiols (Scheme 6). When ferf-butylhydroperoxide (TBHP) was used as an oxidant, tetrahydrothiophene (THT) was oxidized much faster with 5 than in the control experiment without 5. More importantly, such sulfoxidation could be performed with 5 as catalyst using ambient air as oxidant. However, formation of the desired product dipropylsulfane (PrSSPr) was very slow, and a 41% yield was obtained after 30 days at 45°C. 

Enantioselective sulfoxidation has been also studied using polymeric aminoalcohol-derived Schiff bases. Polymers 354-357 have been prepared by copolymeristation of the corresponding monomer with either MMA and EGDMA or styrene and DVB (Scheme 150) [219], These four polymers were then stirred with VO(acac)2 for 4h at rt to lead to the vanadium complexes. For the enantioselective oxidation of thioanisole, although the yields were similar to those obtained with the homogeneous analogs, the enantioselectivity were lower. The best selectivity was obtained with the catalyst derived from 355. 

Oxidative desulfurizabon (ODS) of benzothiophenes and dibenzothiophenes has proved to be a promising alternative to hydrotreatment used to remove sulfur from oil heavy fractions, due to its moderate process conditions and lack of hydrogen consumption. In this process, organosulfur compounds in fuels are oxidised to form sulfoxides and sulfones. Alumina-supported vanadium-oxide catalysts have been... 

Asymmetric sulfoxidation with chiral vanadium complexes is much older than Ti and A1 because this metal provides more robust catalysts which are not deactivated by the presence of water. The first contribution to the field was made by Bolm and co-workers in the second half of 1990s. These authors developed chiral catalysts formed in situ by the reaction of VO(acac)2 with chiral enantiopure Schiff ligands bearing one stereocenter based on a t-leucinol scaffold." The maximum ee achieved was 85% using low catalyst loadings (<1% mol) and without any precautions to avoid moisture or oxygen, which was unusual at that time as hydroperoxides... 

Vanadium was also complexed in the cavity of a hemicryptophane providing the cage compound 61 (Fig. 21.23), which proved to be an efficient supramolecular catalyst for the oxidation of sulfide into sulfoxide with turnover number up to 180 [84], Moreover, an improvement of the reaction rate by a factor of 6 was observed when compared to the model molecule 70 that lacks a cavity. 

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