Reaction of o- and p-alkyl substituted dithiobenzoic esters 5 with LDA and methyl iodide lead to the formation of the 1,2-bis(methylthio)stilbenes 9/10 and 16 by reductive dimerisation. The p-methyl dithiobenzoic ester 15b, however, suffers a Claisen condensation with lithium hexamethyldisalazane to yield the stilbene derivative 17. Even the o-methyldithiobenzoic acid 19 is reductively dimerized by LDA giving after methylation a mixture of the stilbenes 9a/10a and 20.
o-Alkyl substituted dithiobenzoates 3 are S-methylated with trimethyloxonium tetrafluoroborate to form the stable bis(a1-ky1thio)arylmethylium tetrafluoroborates 4. Their deprotonation with LiHMDS takes place either at the o-alkyl groups to form the postulated o-quinodimethanes 5 as intermediates or at the SMe groups to give the thiocarbonyl ylides 7. Both intermediates dimerize to furnish either the dibenzo[a,e]cyclooctenes 6 or the 1,4-dithianes 8/9. Their trapping with No-Quinodimethanes 1 are highly reactive intermediates in organic synthesis and have attracted considerable attention during the last forty yearsr21. They are generated by a variety of routes and normally trapped by [4 + 21 cycloaddition reactions. The high reactivity of o-quinodimethanes is attributed to a small HOMO-LUMO energy difference, allowing a rapid conversion of the singlet state l a to the triplet state lbC3]. The most convenient starting materials for the preparation of the hitherto unknown bis(alky1thio)-0-quinodimethanes 2 are supposed to be o-alkyl-substituted dithiobenzoic esters. Their S-alkylation at the thiocarbonyl group followed by deprotonation at the o-alkyl group should afford 2. phenylmaleimide (10) leads to the formation of the cycloadducts 11 or 12. The structure of 8e was confirmed by X-ray analysis. p-Substituted dithiobenzoates 13 show a similar reaction sequence. Their bis(methy1thio)arylmethylium salts, however, are only deprotonated at one of the SMe groups to give the 1,4-dithianes 15/16. Sterically crowded 1,4-dithianes (e.g. 8d,e/9d,e) rearrange with traces of acid to the ring opened stilbenes 18 and 19.Strong methylating agents such as trimethyloxonium tetrafluoroborate are capable of transforming aromatic dithioesters into stable salts"]. Thus, we obtained from 3 the bis(alky1thio)arylmethylium tetrafluoroborates 4 in high yield. Some of these salts were isolated (e.g. 4a, 4e) and characterized by elemental analysis and spectroscopic data.The experimental results summarized in Scheme 2 show that strong bases may abstract a proton from two different positions of 4 namely an cx proton from the o-alkyl group or a proton from the S-methyl group.The salt 4a is attacked by lithium hexamethyldisilazane (LiHMDS) almost exclusively at the o-methyl group, leading to the formation of o-quinodimethane 5a. The latter dimerizes in a [4 + 41 cycloaddition to the dibenzo[u,e]cyclooctene 6a. A few percent of 4a lose a proton from one of the SMe groups furnishing the thiocarbonyl ylide 7a, which dimerizes to the 1,4-dithiane 8. Crossing products arising from 5 and 7 were not found. From the two possible isomers 8 and 9, the (2) isomer 8a was formed exclusively (1.5% yield). A double or triple bond in the R3 group of the dithioester (e.g. 3b, c) is not sufficiently activated to trap the o-quinodimethane intermediates 5b or 5c by an intramolecular [4 + 21 cycloaddition reaction. Once again a dimerization to 6b or 6c was observed. In both cases the yield of the byproduct was increased to 15% for 8b and to 10% for 8c. Traces of t...
quinone derivativesquinone derivatives Q 0410
-092Dithio and Thiono Esters. Part 64. Attempted Synthesis of Bis( alkylthio)-orthoquinodimethanes from Dithiobenzoates.-Treatment of the salts (IIIa) and (IIIb) with LiNTms2 leads to the o-quinodimethanes (IV) which spontaneously undergo dimerization by (4 + 4) cycloaddition. Formation of the o-quinodimethanes (IV) is also confirmed by trapping with N-phenylmaleimide. In contrast, deprotonation of the salts (IIIc) and (IIId) does not yield o-quinodimethanes but the thiocarbonyl ylides (VI). The ylides dimerize to 1,4-dithianes by a (4 + 2) cycloaddition. In the presence of acid the 1,4-dithianes rearrange to stilbenes like (IX). In addition, it is shown that deprotonation of the p-substituted analogues (X) gives the dithianes (XI) and (XII) via formation of ylides. -(HARTKE, K.; BIEN, N.; MASSA, W.; WOCADLO, S.; Liebigs
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.