Vinyl sulfides oxidation

Modena and colleagues47 have developed use of some chiral, non-racemic terpene alcohols as directing groups for highly diastereoselective m-chloroperbenzoic oxidation of sulfides into sulfoxides. Specifically the isobornyl vinylic sulfides 8 undergo hydroxyl-directed oxidation to give a 9 1 ratio of diastereomeric sulfoxides (equation 11). 

The flotation of sulfidic, oxidic, and salt-type ores and, in special cases, silicate ores can be improved by the use of ether carboxylates as collectors [221,222]. In particular, the flotation of fluorite, barite, and scheelite is mentioned. Special synergistic combinations of ether carboxylates with fatty acids [223] and with vinyl- or alkylsulfonic acid polymer [224] are described. 

The preparative oxidation of vinyl sulfides is thought to proceed via an intermediate bridged sulfonium cation that can be attacked by an external nucleophile from the less encumbered side to give two isomers (Scheme 21) [87]. 

Finally, the intramolecular coupling reaction between an olefin and a pyrrole ring has been examined (Scheme 40). In this example, a 66% isolated yield of the six-membered ring product was obtained. A vinyl sulfide moiety was used as the olefin participant and the nitrogen protected as the pivaloyl amide in order to minimize the competition between substrate and product oxidation. Unlike the furan cyclizations, the anodic oxidation of the pyrrole-based substrate led mainly to the desired aromatic product without the need for subsequent treatment with acid. 

Aryl vinyl sulfides and sulfoxidesArylsulfenyl chlorides add to trimethyl vinyl-silane in CH2CI2 at -78 - 20° to form 2-chloro-l-(trimethylsilyl)ethyl aryl sulfides in 90-95% yield. These adducts are converted into aryl vinyl sulfides in high yield by KF-2H20 DMSO at 70 100°. The sulfides, as expected, can be oxidized to the corresponding sulfoxides by m-chlo roper benzoic acid in CH2C12 at 20". 

This early example is one of the numerous synthetic utilizations of the cycloadditions of olefins activated by sulfur atoms at various oxidation levels (vinyl sulfides, sulfoxides and sulfones = -S(0) R, n = 0,1, 2). Most of the work carried out in this field has been pertinently reviewed and discussed in a 1988 Tetrahedron Report with 204 references [485], Some specific aspects are underlined here and recent examples given. 

This chiral peracid oxidation was used to convert the vinyl sulfide 3, derived from (S)-( + )-camphor-10-sulfonyl chloride, to a chiral sulfoxide (96 4 selectivity),... 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

Alkyl enol ethers and vinyl sulfides may also be oxidatively annulated onto electron-rich aromatic rings (Scheme 71)168. 

Ethylenic cation radicals are also capable of rotating around the double bond. At the same time, the main specificity of ethylenic cation radical reduction consists of the high selectivity of the reaction. One-electron oxidation was developed as a strategy for selective and efficient reduction of relatively ionizable functionalities, including conjugated dienes, styrenes, and vinyl sulfides (Mirafzal et al. 1993). Reduction is highly sensitive to substrate ionizability and permits selective reduction of the more ionizable function in a difunctional compound. 

Thiopyran, derived from ethyl vinyl sulfide, is converted into thiopyrylium fluoroborate by reaction with triphenylcarbenium fluoroborate in 54% overall yield (Scheme 235) <2001EJ02477>. 3-Benzoylisothiochromene is oxidized to the 2-benzothiopyrylium salt in a similar manner <1994J(P1)3129>. 

Oxidation of the o/T/zo-substituted acetanilides 324 with DMP leads to the isolable o-iminobenzoquinones 325 (Scheme 94) (02JA2221). The imino-quinones undergo inverse electron demand Diels-Alder reactions with the vinyl ether and vinyl sulfide shown in Scheme 94, thus providing access to the benzoxazine derivatives 326 and 327. 

Annulation of furans via electrochemical oxidation at the anode has become an important process for the synthesis of complex polycycles, and was covered in a review <2000T9527>. Furans tethered at the 3-position to electron-rich alkenes, enol ethers, or vinyl sulfides were converted to [6,5] and [7,5]-fused ring systems <1996JOC1578, 2002OL3763, 2004JOG3726, 2005JA8034>, as illustrated in Scheme 20. Analysis of crude reaction mixtures and side... 

Problems can arise at the last stage due to difficulties in die isolation of die aldehyde and/or preferential vinyl sulfide formation. Nonetheless, the method has some potential Sulfoxides are prone to thermal elimination, and this has been used by Trost in his mediod, which can also be used for the oxidation of primary amines (Scheme 14). The procedure is limited to benzylic and allylic bromides. 

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