Sulfides methoxylation

Lomans BP, P Leijdekkers, J-P Wesselink, P Bakkes, A Pol, C van der Drift, HIP op den Camp (2001) Obligate sulfide-dependent degradation of methoxylated aromatic compounds and formation of methanethiol and dimethyl sulfide by a freshwater sediment isolate, Parasporobacterium paucivorans gen. nov., sp. nov. Appl Environ Microbiol 67 4017-4203. 

These facts indicate that an a-trifluoromethyl group remarkably promotes anodic substitutions with oxygen nucleophiles. Since these are very few successful examples of anodic methoxylation of phenylalkyl sulfides are known, it is notable that an a-trifluoromethyl group facilitates the anodic methoxylation. Therefore, such anodic substitutions have been systematically investigated from both mechanistic and synthetic aspects [42]. 

Table 7. Anodic methoxylation and acetoxylation of fluoroaikyl phenyi sulfides...

Since these methoxylated and acetoxylated sulfides have an acetal structure, it is expected that Lewis acid catalyzed demethoxylation should generate a carbocation intermediate which is stabilized by the neighboring sulfur atom. In fact, nucleophilic substitution with arenes has been successfully achieved as shown in Scheme 6.7 [43], This procedure is useful for the preparation of trifluoroethyl aromatics. As already mentioned, generation of carbocations bearing an a-trifluoromethyl group is difficult due to the strong electron-withdrawing effect. Therefore, this carbon-carbon bond formation reaction is remarkable from both mechanistic and synthetic aspects. 

When the anodic fluorination of 2-monofluoroethyl sulfide 5 is carried out in methanol containing Et3N-3HF instead of acetonitrile as a solvent, interestingly the a-methoxylated product 7 rather than the a-flourinated product 6 is obtained exclusively (Scheme 6.10) [51]. As described above, 7 is not obtained from 5 under conventional anodic methoxylation conditions. Therefore, that this novel fluoride ion promoted anodic methoxylation is remarkable. As shown... 

Table 9. Fluoride ion promoted anodic methoxylation of fluoroalkyl sulfides...

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

Kita has introduced a novel one-pot preparation of 5-methoxylated indoline 55 and indole 56 derivatives by intermolecular addition followed by cyclization between A-tosylaniline derivatives 53 and activated olefins 54 using phenyliodine(III) bis(trifluoroacetate) (FIFA) <99H511785>. In the reaction of 53 with phenyl vinyl sulfides, indoles were produced directly by the spontaneous elimination of thiophenol. 

Methoxyl Groups Lignin is partially demethylated by the action of hydrosulfide ions forming methyl mercaptan which is convertible to dimethyl sulfide by reaction with another methoxyl group. In the presence of oxygen, methyl mercaptan can be oxidized further to dimethyl disulfide (Fig. 7-28). Because the hydroxide ions are less strong nucleophiles than hydrosulfide ions, only small amounts of methanol are formed. Methyl mercaptan and... 

The eleclrolysis of sulfide 11 (1.5 mmol) was carried out at platinum electrodes (2x2 cm) at 17 C in 0.33 M Et,N 3HF in MeOH (15 mL). A constant current (25 mA) was passed until the starting material was consumed (monitored by GC) and then the electrolyte solution was neutralized with 12% aq NH,. The solution was extracted repeatedly with EtjO. The extracts were washed with brine, dried (MgSO ), and concentrated under reduced pressure. The residue was purified by TLC (silica gel. hcxane/F.tOAc 5 1) to provide x-methoxylated product 12 yield 98%. 

Bak F., Finster K., and Rothfuss F. (1992) Formation of dimethylsulfide and methanethiol from methoxylated aromatic compounds and inorganic sulfide by newly isolated anaerobic bacteria. Arch. Microbiol. 157, 529-534. 

Alkylthioacetonitriles when oxidized anodically in methanol and /7-toluenesulfonic acid as electrolyte produce a-methoxylated sulfides [104]. Their formation is tentatively explained by a Pummerer-type intermediate in which the presence of the cyano group appears to facilitate deprotonation at the a-position ... 

The anodic oxidation of a number of vinyl sulfides to produce fluorinated compounds was examined in MeCN-Et3N 3HF on Pt electrodes under controlled-potential conditions [130]. It became evident that the reactivity of the vinyl thioethers depends on, among others, their structure and the nucleophilicity of the medium. Two compounds, PhSCH = CH2 and PhSCH=CHPh, were studied in more detail to evaluate the influence of the different factors upon their electrochemical behavior, and comparisons with that in MeOH Et N 3HF and HOAc-KOAc, leading, respectively, to methoxylation and acet-oxylation were made (Table 3). 

Related to these reacations is the anodic methoxylation of sulfides carrying an electron-withdrawing substituent, such as cyano [105], fluoromethyl [106], difluoromethyl [106], or trifluoromethyl [106-109] in the a-position [Eq. (41)]. Yields are typically between 50 and 90%. An analogous reaction has been reported for selenides carrying electron-withdrawing perfluoroalkyl or cyano substituents [110]. 

Fuchigami and coworkers have proposed a Pummerer-type mechanism via the fluor-osulfonium cation for the anodic fluorination of sulfides [Eq. (14)], based on the comparative study of anodic methoxylation and fluorination of fluoroalkyl sulfides [43,45]. 

Anodic a-methoxylation of phenyl 2,2,2-trifluoroethyl sulfide was carried out using various solid-supported bases as shown in Equation 12.6. Polystyrene and silica -gel are suitable as the solid support for an organic base such as piperidine. It is noteworthy that anodic methoxylation was successfully carried out even after 10 recycles of the solid-supported base. The method has also been successfully applied to electrochemical acetoxylation in acetic add/acetonitrile. 

P2i2i2j Z = 4 D = 1.46 R = 0.105 for 961 intensities (film measurements). Compound obtained by glycol cleavage of methyl 6-O-trityl-a-D-glucopyranoside, followed by reduction, sulfonylation, and ring-closure by sulfide anion. The 1,4-oxathiane ring has a distorted chair conformation, with the sulfoxide bond equatorial and the bond to the methoxyl group axial. 

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