Dithioesters thiophilic addition

The thiophilic addition of aliphatic Grignard reagents to dithioesters deserves some comment. Although they are less reactive than organolithium compounds, their reactions are selective. With enethiolizable dithioesters, enethiolization cannot be avoided with the organolithium species, and at low temperatures this can become exclusive [332]. Enethiolization does not interfere in the case of the less... 

A number of functionalized dithioesters have been utilized for carbophilic and/or thiophilic additions [120]. We shall mention only the potentialities offered by the readily available enolizable p-oxodithioesters (for a review, see [349]) R COCH(R2)CS2Me. 

A general way to achieve thiophilic addition is to use sulfines [149]. The sulfur atom of thiocarbonyl oxides is selectively attacked by carbanions. We have applied this property to aliphatic dithioester oxides and shown [150] that, despite the probably high acidity of the a-protons, thiophilic addition of alkyllithiums is observed, leading to stabilised carbanions. After treatment with electrophiles, dithioacetal oxides were obtained. From a synthetic point of view, it is worthy of note that these adducts are much more easily... 

Allyl silanes may also be employed for thiophilic addition to various sulfines, obtained by oxidation of thioketones, dithioesters [151] and trithio-carbonates [111]. It provides a new entry to allyl sulfoxides. 

Exceptions to the carbophilic addition of heteronucleophiles to thiocarbonyl compounds were demonstrated with thiols and thioaldehydes [160] or dithioesters [36] bearing an a-electron withdrawing group, which undergo selective thiophilic addition. 

Structural investigation of the intramolecular addition of aryl radicals to a dithioester group, in analogy to the carbanionic thiophilic addition reported above, has shown that 6-, 8- and 15-membered rings are accessible in this manner [142]. 

Reactions of organomagnesium compounds with a,(3-unsaturated dithioesters are somewhat unpredictable, 1,2-, 1,4-carbophilic and thiophilic addition having been observed [6], With a,(3-unsaturated thioamides [7] and imidothioesters [6], however, 1,4-carbophilic addition appears to predominate. 

When certain jS-keto-dithioesters (87) are treated with Grignard reagents, thiophilic addition occurs and 2-hydroxycyclopropanone dithioketals (88) are produced (Scheme 35) Only one isomer is obtained and a concerted cis-homo-l,4-addition has been proposed to account for the stereospecificity observed. Along somewhat similar lines, Giusti and coworkers have synthesized several cyclopropanone ketals by treating l,3-dibromo-2-propanone ketals with active metals (Table 20). Allenes are a byproduct in this reaction. Dihaloketals may also be cyclized electrolytically (Table 21). ... 

The thiophilic addition D is preferential when the temperature is less than — 17°C and when R is an ethyl or an isopropyl group similar results are observed with other dithioesters [33]. With CHaMgBr or CH3MgI, carbophilic addition is preferential if the reaction is performed at 20°C or at reflux, leading to the thiol E. 

Thiophilic addition of EtMgl to dithioesters leads to acyl anion equivalents that react with electrophilic reagents to yield to dithioacetals [34]. The latter compounds may be transformed into the corresponding carbonyl compounds on hydrolysis [Eq. (24)]. 

Related reactions involve the alkylation, allylation or tenzylation of (i) a,a-di(Aio)alkylmagnesium halides, resulting from the thiophilic addition of Grignard reagents on dithioesters (Scheme 55, entry 5).259.260 (li) 2-lithio-l,3-benzodithioles (Scheme 55, entry d) " (iii) 2-aryl-2-lithio-l,3-dithiolanes and 2-lithio-l,3-dithianes derived from die l,2-dimethyl-4,5-di(mercapto)methylbenzene (Scheme 55, entry c) ( 3,264... 

Thiophilic addition to dithioesters followed by capture of the thioacetal anion with an electrophile provides a useful approach to the formation of carbonyl compounds such as (42) and (43), and has been elegantly employed in the total synthesis of the macrocycles ( )-maysine and ( )-4,5-deoxymaysine. Trimethyl borate has been used to convert bis(phenylthio)methyl-lithium into the keten thioacetal (45) via the boron-containing carbanion (44), as shown in Scheme 4. Bis(phenylthio)methyl-lithium and related compounds are also the key reagents in the preparation of 2-(phenylthio)-enones, 2-(phenylthio)-butadienes, a-(phenyl-... 

From a synthetic point of view, both thiophilic and carbophilic additions on dithioesters afford pathways for carbon-carbon bond formation, and as the dithioacetals formed in these reactions are easily converted into carbonyl or methylene groups the dithioesters are equivalent to the four synthons... 

A new class of nucleophiles have been introduced for sulfur addition. Degl Innocenti and his group [145, 146] have shown that allyl or benzylsi-lanes, in the presence of tetra-n-butylammonium fluoride, reacted in a thiophilic fashion and led to allyl sulfides or dithioacetals. It is remarkable that this selective reaction is general for a large variety of thiocarbonyl compounds thioketones [145], dithioesters [146], and even with the normally sluggish trithiocarbonates [145]. With substituted allyl silanes retention of configuration of the allyl chain is observed. It is noteworthy that the possible [2,3] sigmatropic shift of the intermediate anionic species was not observed. 

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