Alkyl halides 1,3-dithiane lithio derivatives

Sulfur compounds are useful as nucleophilic acyl equivalents. The most common reagents of this type are 1,3-dithianes, which on lithiation provide a nucleophilic acyl equivalent. In dithianes an umpolung is achieved on the basis of the carbanion-stabilizing ability of the sulfur substituents. The lithio derivative is a reactive nucleophile toward alkyl halides and carbonyl compounds. 11... 

The acidity difference of hydrogen atoms adjacent to divalent sulfur compared to oxygen stems from the greater polarizability of sulfur and the longer C-S-bond length d-orbitals are not involved. In most cases treatment of dithianes with w-BuLi at temperatures of -30 °C is sufficient for the preparation of the lithio-derivatives. With pKA values of approximately 30, lithiated dithianes can react with aldehydes or ketones, epoxides and acid derivatives, but also with alkyl halides without competing elimination reactions. 

All types of electrophiles have been used with 2-lithio-l,3-dithiane derivatives, including alkyl halides, sulfonates, sulfates, allylic alcohols, arene-metal complexes, epoxides, aziridines, carbonyl compounds, imines, Michael-acceptors, carbon dioxide, acyl chlorides, esters and lactones, amides, nitriles, isocyanates, disulfides and chlorotrialkylsilanes or stannanes. The final deprotection of the dithioacetal moiety can be carried out by means of different types of reagents in order to regenerate the carbonyl group by heavy metal coordination, alkylation and oxidation184 or it can be reduced to a methylene group with Raney-nickel, sodium or LiAIII4. 

The lithio derivatives of 1,3-dithiane react readily with alkyl halides to afford 2-alkylated dithianes in excellent yields. Dithiane anions are useful for the formation of small- and medium-sized rings via alkylation. ... 

The proof given in support of dianion formation does not however seem to be sufficients Nevertheless the single step process is reasonably efficient when primary alkyl halides are used but none of the dialkylated compound is produced with isopropyl iodide and benzyl bromide. 2-(o>-Haloalkyl)-l,3-di-thianes have proved to be valuable precursors of 1,3-dithianes derived from cyclic ketoncs. Haloge-nated 1,3-dithianes have been in turn prepared from 2-lithio-l,3-dithiane and stoichiometric amounts of u-chloro- or bromo-alkyl iodides or with an excess of the corresponding dichloride (Scheme 64, entry c). Use of u-bromoalkyldithianes is often impractical because of the ease with which these compounds are transformed into cyclic sulfonium salts. ... 

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... 

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