Sulfoxides deoxygenations

BBr is a very useful reagent for cleaving ethers, esters, lactones, acetals, and glycosidic bonds it is used to deoxygenate sulfoxides and in the preparation of image-providing materials for photography (5). 

In addition to deoxygenation sulfoxides undergo reductive cleavage at the carbon-sulfur bond when heated in tetrahydrofuran with aluminum amalgam. Keto sulfoxides were thus converted to ketones in usually quantitative yields (the keto group remained intact) [141]. 

In general, the chemistry of fluoro-l,3,5-triazines resembles that of the chloro derivatives, and therefore will not be discussed in detail. There is evidence that cyanuric fluoride is less reactive than cyanuric chloride in reaction with aniline (76CCC3378). Chambers et al. have reported the isolation of a stable anion cr- complex from the reaction of cyanuric fluoride and cesium fluoride (equation 32) (77JCS(Pl)l605). Such species are believed to be intermediates in the formation of (68) from cyanuric fluoride and perfluoropropene (equation 33). Olah et al. have shown that cyanuric fluoride deoxygenates sulfoxides efficiently (equation 34) (80S221). 

Miscellaneous Reactions. Bromodimethylborane can also be used to convert dialkyl, aryl alkyl, and diaryl sulfoxides to the corresponding sulfides (eq 15). Typically, the sulfoxides are treated with 2.5 equiv of Me2BBr in dichloromethane at —23 °C for 30 min and at 0°C for 10 min. Bromine is produced in the reaction and must be removed in order to avoid possible side reactions. This is accomplished by saturating the solution with propene prior to introducing the reagent or by adding cyclohexene. Phosphine oxides and sulfones failed to react under the conditions used to deoxygenate sulfoxides. 

A direct attack of nucleophiles on the sulfur atom of the sulfone or sulfoxide group in acyclic or large-ring sulfones and sulfoxides is rather rare, or unknown, excluding metal hydride reductions and/or reductive deoxygenations. The situation is completely different in the three-membered ring systems. 

Quebrachitol was converted into iL-c/j/roinositol (105). Exhaustive O-isopropylidenation of 105 with 2,2-dimethoxypropane, selective removal of the 3,4-0-protective group, and preferential 3-0-benzylation gave compound 106. Oxidation of 106 with dimethyl sulfoxide-oxalyl chloride provided the inosose 107. Wittig reaction of 107 with methyl(triphenyl)phos-phonium bromide and butyllithium, and subsequent hydroboration and oxidation furnished compound 108. A series of reactions, namely, protection of the primary hydroxyl group, 0-debenzylation, formation of A-methyl dithiocarbonate, deoxygenation with tributyltin hydride, and removal of the protective groups, converted 108 into 7. 

The successful deoxygenation of the sulfoxide 18a by either hexachlorodisilane as the reducing agent, or diiron nonacarbonyl according to the deoxygenation-complexation route can also be rationalized in terms of electrophilic attack of the reagents used on the nucleophilic sulfoxy oxygen. 

The reaction appears to be facilitated by a y-carbonyl group. In the absence of this activation, sulfoxide deoxygenation appears to be the favored reaction pathway (equation 130). 

The Hurd-Mori reaction,where a tosylhydrazone is converted by thionyl chloride to the corresponding thiadiazole, involves the formation of a 1,2,3-thiadiazole-3,3-dioxide. In one example, this type of compound was isolated and subsequently deoxygenated with thiourea <1991PS175>. There have been no further reports of S-linked sulfoxide or sulfone derivatives of 1,2,3-thiadiazoles since the publication of CHEC-II(1996). 

Complex compounds of sulfoxides Deoxygenation reactions of sulfoxides... 

Care must also be taken to obtain the correct reaction conditions deoxygenation reactions of sulfoxides have been known to occur during relatively simple preparative procedures (see Section IV,C). In addition, displacement of other ligands can easily occur, as, for example, in Eqs. (12) and (13). 

Problems of dilution and pH sensitivity have also been encountered in the synthesis of cts-[Pt(S-R2SO)(olefin)Cl2] complexes (85), where deoxygenation of the sulfoxide with concomitant oxidation of the metal center occurs at low pH. The reactions of [M(Ph2PCH2CH2PPh2)Cl2] (M = Pd, Pt) with one equivalent of silver perchlorate in the presence of Me2SO yield either the O-Me O complex or its deoxygenation product, depending upon reaction conditions. A sequence of reactions, Eq. (27), has been proposed (143). 

In addition to the catalysis of sulfoxide deoxygenation (Section IV,C) and sulfoxide oxygenation (Section IV,D), other interesting reactions have been shown to be catalyzed by sulfoxide complexes, and these are detailed below. 

The ability of vanadium(II) chloride to facilitate sulfoxide deoxygenation has been discussed (Section IV,C), and it appears that vana-dium(III) sulfoxide complexes may be prepared by air oxidation of van-adium(II) salts in the presence of the sulfoxide. In this manner, [V(Me2S0)6][C104]3 was prepared from vanadium(II) perchlorate (119) and the kinetics of substitution with thiocyanate ion detailed. Care is necessary in handling the pure compound, as it is reported to be sensitive to detonation. A large number of oxovanadium(IV) species have... 

Electron-withdrawing groups decrease the rate of the fluoro Pummerer reaction, which, in certain cases,7 allows a DAST-mediated deoxygenation to compete with the introduction of fluorine alpha to sulfur. The reaction is compatible with a number of functional groups and can readily be carried out with nucleosides. Robins and coworkers4 reported the synthesis of a 5 -fluoro-5 -S-phenyladenosine analog using antimony trichloride as catalyst at room temperature. It should be noted that a-fluoro sulfoxides provide a convenient entry to terminal fluoroalkenes.3 8 9 10... 

An unsuccessful attempt to repeat this reaction was made by Bird however, he was able to show that dibenzothiophene 5-oxide did, in fact, react with either thionyl chloride or phosphorus oxychloride to yield 2-chlorodibenzothiophene in good yield. Since all methods of nitration of dibenzothiophene yield a mixture of 2-nitrodibenzothio-phene and dibenzothiophene 5-oxide, which have identical melting points, it was concluded that the earlier workers had in fact been working with the sulfoxide and not the nitro compound. The reaction was rationalized as being a deoxygenative halogenation of a heterocyclic 5-oxide akin to the Meisenheimer reaction of A-oxides, which already had precedents in the sulfoxide field. Unfortunately the 2-chlorodibenzothiophene prepared by this route is contaminated with 2,8-dichlorodibenzothiophene which cannot be removed by crystallization. The best method of preparation of this compound is therefore via a Sandmeyer reaction on 2-aminodibenzothiophene. ... 

Stannous chloride is used most frequently for the reduction of nitro compounds [177, 178, 179] and of quinones [180, 181], It is also suitable for conversion of imidoyl chlorides [182] and of nitriles [183] to aldehydes, for transformations of diazonium salts to hydrazines [184], for reduction of oximes [f[Pg.30]

Titanous chloride (titanium trichloride) is applied in aqueous solutions, sometimes in the presence of solvents increasing the miscibility of organic compounds with the aqueous phase [199, 200]. Its applications are reduction of nitro compounds [201] and cleavage of nitrogen-nitrogen bonds [202] but it is also an excellent reagent for deoxygenation of sulfoxides [203] and amine oxides [199] (Procedure 38, p. 214). 

Dialkyl and diaryl sulfoxides were reduced (deoxygenated) to sulfides by several methods. As a little surprise sulfides were obtained from sulfoxides by catalytic hydrogenation over 5% palladium on charcoal in ethanol at 80-90° and 65-90 atm in yields of 59-99% [684. In the case of p-tolyl -styryl sulfoxide deoxygenation was achieved even without reduction of the double bond giving 66% of p-tolyl -styryl sulfide and only 16% of p-tolyl -phenyl-ethyl sulfide [684],... 

Chemical deoxygenation of sulfoxides to sulfides was carried out by refluxing in aqueous-alcoholic solutions with stannous chloride (yields 62-93%) [186 Procedure 36, p. 214), with titanium trichloride (yields 68-91%) [203], by treatment at room temperature with molybdenum trichloride (prepared by reduction of molybdenyl chloride M0OCI3 with zinc dust in tetrahydrofuran) (yields 78-91%) [216], by heating with vanadium dichloride in aqueous tetrahydrofuran at 100° (yields 74-88%) [216], and by refluxing in aqueous methanol with chromium dichloride (yield 24%) [190], A very impressive method is the conversion of dialkyl and diaryl sulfoxides to sulfides by treatment in acetone solutions for a few minutes with 2.4 equivalents of sodium iodide and 1.2-2.6 equivalents of trifluoroacetic anhydride (isolated yields 90-98%) [655]. 

REDUCTION WITH STANNOUS CHLORIDE Deoxygenation of Sulfoxides [186]... 

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