Sulphoxides, reduction

Van t Hoff t-factors 565 Vinylallenes rearrangement of 748 synthesis of 737 Vinyl carbonium ions 620 17a-Vinyl-17/f-hydroxysteroids, epimerization of 735 Vinyl sulphides, as alkyl sulphoxide reduction products 930, 932 Vinyl sulphones - see also Alkenyl... 

A number of other sulphoxide reduction reactions bear mentioning. The first, due to Marchelli and coworkers , is a very simple procedure whereby the sulphoxide is refluxed with t-butyl bromide and chloroform. A useful range of sulphoxides was studied and distillation of the reaction mixture (or percolation through a column of silica gel) gave pure sulphides in yields of > 90%. The procedure is appealing because of its experimental simplicity, and its use of a relatively inexpensive reagent. It may not be very successful with sterically hindered sulphoxides and the authors do not comment on this possibility. The mechanism of this reduction reaction is akin to that of BBrj (cf. Section II.A.3), except that the bromine trap is provided by a second mole of t-butyl bromide, as shown in equation (13) ... 

Kashiyama, E. Yokoi, T. Todaka, T. Odomi, M. Kamataki, T. Chiral inversion of drug role of intestinal bacteria in the stereoselective sulphoxide reduction of flosequinan. Biochem. Pharmacol. 1994, 48,... 

Lee, S.C. Renwick, A.G. (1995) Sulphoxide reduction by rat intestinal flora and by Escherichia coli m vitro. Biochem. Pharmacol. 49, 1567-1576. 

Reactions.—A variety of interesting and useful syntheses have been published involving the reaction of dimsyl anion [MeS(0)CHa ] with esters and lactones,with disulphides, with chlorosilanes, with sulphinate esters, with organoboranes, and with stilbenes. Simple and functionalized a-sulphinyl carbanions can be condensed with carbonyl compounds or alkylated, often in a stereocontrolled manner, as in a nicely conceived synthesis of biotin. Considerable attention has been given to methods for the removal of the sulphoxide function following carbon-carbon bond formation. Among the methods used are reduction by aluminium amalgam (with j3-keto-sulphoxides), reduction with Raney nickel, pyrolytic elimination of sulphenic acid, elimination of sulphur dioxide from sultines, e.g. (64), and sulphoxide-... 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Reaction of adsorbed inhibitors In some cases, the adsorbed corrosion inhibitor may react, usually by electro-chemical reduction, to form a product which may also be inhibitive. Inhibition due to the added substance has been termed primary inhibition and that due to the reaction product secondary inhibition " . In such cases, the inhibitive efficiency may increase or decrease with time according to whether the secondary inhibition is more or less effective than the primary inhibition. Some examples of inhibitors which react to give secondary inhibition are the following. Sulphoxides can be reduced to sulphides, which are more efficient inhibitorsQuaternary phosphonium and arsonium compounds can be reduced to the corresponding phosphine or arsine compounds, with little change in inhibitive efficiency . Acetylene compounds can undergo reduction followed by polymerisation to form a multimolecular protective film . Thioureas can be reduced to produce HS ions, which may act as stimulators of... 

The sulphoxide group is more amenable to chemical analytical methods than the sulphone group. Controlled oxidation and reduction are possible ... 

Drushel and Sommers96 also used zinc/hydrochloric add and Andreae97 chromium(II) chloride for reduction of sulphoxides. 

These results may easily be rationalized by assuming that the formation of hydroxy sulphoxides 91, 92 and 93 from hydroperoxysulphides 89 and 90 is an intramolecular oxidation-reduction reaction proceeding through a five-membered transition state 94. However, an alternative intermolecular mechanism in which the approach of the oxidant is directed by the hydroperoxy or the hydroxy function in the reductant cannot be excluded. 

The addition of sulphinyl chlorides to trimethylsilyl enol ether 138 affording a-ketosulphoxides 139 (equation 76) represents an extension of the reaction of sulphinyl chlorides with ketones. This reaction has attracted attention only recently. Sergeev and coworkers192 reported that treatment of sulphinyl chlorides with acyclic enol ethers afforded a-ketosulphoxides 139 in good to excellent yields. Meanwell and Johnson193 observed that in the case of cyclic enol ethers the corresponding sulphoxides were formed only in very low yields. They found, however, that the introduction of an equivalent amount of a Lewis acid into the reaction mixture markedly promotes the desired reaction, whereas the use of catalytic amounts of a Lewis acid led to a substantial reduction in the yield. This is most probably due to the formation of a complex, between the a-ketosulphoxide and the Lewis acid. 

The well-known fact that enantiomers exhibit different reactivity towards chiral reagents has been used to obtain optically active sulphoxides in a process which is called kinetic resolution. Kinetic resolution of sulphoxides usually involves either oxidation to the corresponding sulphones or reduction to sulphides by means of proper chiral oxidizing or reducing agents. 

A very interesting approach to optically active sulphoxides, based on a kinetic resolution in a Pummerer-type reaction with optically active a-phenylbutyric acid chloride 269 in the presence of /V,A -dimethyIaniline, was reported by Juge and Kagan332 (equation 149). In contrast to the asymmetric reductions discussed above, this procedure afforded the recovered sulphoxides in optical yields up to 70%. Chiral a, /1-unsaturated sulphoxides 270 were prepared via a kinetic resolution elaborated by Marchese and coworkers333. They found that elimination of HX from racemic /i-halogenosulphoxides 271 in the presence of chiral tertiary amines takes place in an asymmetric way leading to both sulphoxides 270 and 271, which are optically active (optical yields up to 20%) with opposite configurations at sulphur (equation 150). 

A kinetic resolution of racemic sulphoxides was observed in the reduction by chiral polyiminoalanes. The efficiency of this process depends on the molecular structure of the polyiminoalane. With open pseudo-cubic tetra [JV-(l-phenylethyl)]imidoalane, unreacted sulphoxides were isolated in enantiomeric enrichment up to 75%. Optical purity was shown to increase with increasing the reaction temperature, a maximum enrichment being observed between 55 and 70 °C336. 

Reaction of benzylideneaniline with optically active methyl p-tolyl sulphoxide 449 in the presence of lithium diethylamide produces the corresponding jS-anilinosulphoxide 450 with 100% asymmetric induction. Its reductive desulphurization with Raney nickel leads to the enantiomerically pure amine 451524 (equation 270). When the same optically active... 

In the reduction of racemic /i-ketosulphoxides (e.g. 464a) with actively fermenting yeast (Saccharomyces cerevisiae) the enantiomers are reduced at sufficiently different rates to allow isolation of optically active /1-hydroxy sulphoxide 524 and unreacted optically active /1-ketosulphoxide with at least 95% optical purity617,618 (equation 323). 

This chapter deals with the reverse of these reactions. Since this is not a simple process, a brief discussion of the nature of the S—O bonds in sulphoxides and sulphones will be of assistance in understanding the requirements for reduction of these functional groups. 

This area of organosulphur chemistry has been fortunate to have been well and regularly reviewed in recent years12, the reviews by a team of Polish and Japanese workers in 197713 and 198414 meriting special mention. This review will concentrate on recent developments in the reduction of sulphoxides. 

Although dichloroborane has been successfully used to reduce dialkyl sulphoxides, it was less useful for the reduction of diaryl sulphoxides and recent work26 has found that fully substituted bromoboranes can fill this gap. Three reagents were introduced, viz. bromodimethylborane, 9-bromo-9-borabicyclo[3.3.1]nonane (9-BBN-Br) and tribromo-borane, the reactions being shown in equation (5) ... 

Sulphoxides may be reduced by heating in weakly basic ethanol with one of a range of hydrazine or hydroxylamine derivatives40. The process has been patented both for simple sulphoxides and for more complex molecules such as a-methylsulphinylacetophenone, PhCOCH2SOMe, although the yield for this reduction was only 48%. 

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