Sulfides, allyl selectivity

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

Whereas free singlet carbenes are rather unselective with respect to formation of cyclopropane 22 or ylide 23 and the cyclopropane is favored under conditions that populate the triplet state of a carbene (see Section I.2.I.2.4.2.6.2.), the metal carbenes generated with copper or rhodium catalysts display a selectivity for functional groups which are more nucleophilic than a double bond. Thus, no cyclopropanes are obtained from dialkylallylamines allyl sulfides -allyl dithioacetals , and allyl selenides under carbenoid conditions (copper or rhodium catalysts). 

These results can be explained by the relative rates of the formation of the sulfoxide (step a) and of the corresponding sulfone (step b) in the oxidation reaction of thioethers (reaction 1). It is known that, for dialkyl sulfides, such as Et2S, Pr2S and BU2S, sulfoxide formation proceeds much faster than sulfone formation [1], For the allyl sulfide the selectivity in sulfoxide is lower, because the difference in the rates of the two steps (a) and (b) of the oxidation reaction is less important, due to the conjugation of the lone electron pairs on the sulfur atom with the unsaturated system [1],... 

Sn2 displacements. The opening of allylic carbonates and displacement reactions of chlorides are regio- and stereoselective processes. The thio unit of functionalized allyl thiazolin-2-yl sulfides is selectively removed during the reaction. ... 

Gais H, Jagusch T, Spalthoff N, Gerhards F, Frank M, Raabe G. Highly selective palladium catalyzed kinetic resolution and enantioselective substitution of racemic allylic carbonates with sulfur nucleophiles asymmetric synthesis of allylic sulfides, allylic sulfones, and allylic alcohols. Chem. Eur. J. 2003 9 4202-A221. 

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

Allylic titanates having an electrofugal leaving group, e.g., trimethylsilyl68 75 - 77, at the 3-position are powerful reagents for the highly stereoselective synthesis of 1-hetero-substituted 3-alkadienes. For the carbonyl addition of the appropriate titanated allyl sulfides ( ) or carbamates ( and ), reliable y-selectivity and anti diastereoselectivity are reported. The... 

The Pacman catalyst selectively oxidized a broad range of organic substrates including sulfides to the corresponding sulfoxides and olefins to epoxides and ketones. However, cyclohexene gave a typical autoxidation product distribution yielding the allylic oxidation products 2-cyclohexene-l-ol (12%) and 2-cyclohexene-1-one (73%) and the epoxide with 15% yield [115]. 

Stereoselective preparation of ( )-allyl alcohols via radical elimination from anti-j-phenylthio-P-nitro alcohols has been reported.154 The requisite anti-P-nitro sulfides are prepared by protonation of nitronates at low temperature (see Chapter 4), and subsequent treatment with Bu3SnH induces anti elimination to give (E)-alkenes selectively (see Eq. 7.112). Unfortunately, it is difficult to get the pure yyw-P-nitro sulfides. Treatment of a mixture of syn- and anti-P-nitrosulfides with Bu3SnH results in formation of a mixture of (E)- and (Z)-alkenes. 

Hydrozirconation of allenic systems preferentially leads to allylic zirconocenes, which are highly reactive and thus very useful organometallic reagents. Allenic sulfides react in the expected fashion to give the (E)-y-thiophenylallylzirconocene chloride 20 (Scheme 4.18) [47]. These intermediates, upon introduction of an aldehyde or methyl ketone, give predominantly the anti isomer (ratios from 82 18 to > 97 3). Exclusive 1,2-addition was observed by Suzuki et al. in the case of an a,f5-unsaturated aldehyde. As long as the steric demands of the two substituents attached to the ketone carbonyl are significantly different, synthetically useful levels of selectivity can be achieved. 

Uemura and co-workers (91) demonstrated that copper catalysts effectively transfer nitrenoid groups to sulfides generating chiral sulfimides. A complex obtained from CuOTf and 55d catalyzes nitrenoid transfer to prochiral sulfides to afford products such as 139 in moderate to poor enantioselectivities (<71% ee, Eq. 78). Nitrenoid transfer occurs selectively to the sulfur atom of allylic sulfides generating allylic sulfenamide (140) in moderate selectivity, after [2,3] sigmatropic rearrangement of the initial sulfimide 141, Eq. 79. 

The first attempts to develop reactions offering control over the absolute stereochemistry of a chiral center, created by y-selective substitution of an achiral allylic alcohol-derived substrate, involved the use of chiral auxiliaries incorporated in the nucleofuge. The types of stereodirecting groups utilized vary, and have included sulfoximines [15], carbamates [16], and chiral heterocyclic sulfides [17-19]. 

Allylic and benzylic alcohols were oxidized to aldehydes or ketones with BnPhsPHSOs in refluxing CHsCN. The yield increased in the presence of bismuth chloride in a catalytic amount. Selective oxidation of various alcohols under solvent free conditions was also reported Interestingly, benzyl alcohols were oxidized selectively to benzaldehydes in very high yield (95-100%) when reacted with BnPhsPHSOs (1.2 eq.) and AICI3 (1 eq.) in the presence of an equimolar amount of 2-phenethyl alcohol, diphenyl carbinol or methyl phenyl sulfide (equation 72). 

Alkynylepoxy alcohols of high enantiomeric purity, obtained via Sharpless oxidation of allylic alcohols (see Section D.4.5) react smoothly with excess dialkylcuprate/magnesium bromide to give (/Vf.25)-3.4-alkadiene-1.2-diols in reasonable overall yield and with high anti selectivity when performed at low temperature and by using the dimethyl sulfide complex of copper(I) bromide to synthesize the cuprates42. 

AIkylthio)allylritanium reagentS, RSCH=CHCH2TiL (l).9 The reagents are prepared by deprotonation of allylic alkyl (aryl) sulfides with sec- or r-butyllithium followed by addition of Ti(0-/-Pr)4 at - 78°. They can react with carbonyl compounds at the a- or "/-position. a-Adducts predominate in reactions with a- and /1-mono- and disubstituted sulfides, whereas /-adducts predominate in reaction with /-substituted sulfides. The a-adducts show high eryr/iro-selectivity. The products are useful precursors to alkenyl oxiranes and to 2-(arylthio)-l,3-butadienes. 

A convenient method for the selective oxidation of oximes to carbonyl compounds using A-bromo-A-benzoyl-4-toluenesulfonamide is reported.172 A convenient method for the preparation of 2-phenylthio-3-bromopropene by bromination of allyl phenyl sulfide is shown to proceed via the formation of a l,3-dibromo-2-(phenylthio)propane intermediate.173... 

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