Abstract
The reaction pathway for the formation of tetracenes from the diols 1,2-C6H4(CHOHCCAr)2, LiHDMS, CS2 and MeI has been modelled by computational methods at the CBS-QB3 level of theory. Comparison of PhCHOC(=S)YCCPh (Y=S− or SMe) indicates a slight kinetic advantage for the anionic system towards [3,3]-sigmatropic rearrangement [Eact(calcd) 19.7 vs 21.8 kcal mol−1]. Using anthracene-based models, 10-{SC(=O)Y}-4a,10-dihydroanthracene (Y=S− or SMe), allows direct comparison of both syn and anti-manifolds in the neutral versus anionic Chugaev elimination. syn-Elimination of [HSC(=O)S]- is distinctly favoured [Eact(calcd) 11.4 kcal mol−1] versus syn elimination of neutral methylated HSC(=O)SMe [Eact(calcd) 27.5 kcal mol−1]. The smaller barrier to syn elimination of the anionic leaving group is in accord with the low temperature conditions required for this Chugaev reaction (60 °C) and suggests a general advantage in carrying out Chugaev eliminations in anionic manifolds.
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