Organosulfur chemistry. II. Highly stereoselective reactions of 1,3-dithianes. Contrathermodynamic formation of unstable diastereoisomers

Treatment of the lithium derivative of cis-4,6-dimethyl-1,3dithiane (1) with DCl gives nearly exclusively (over 99%) the equatorial 2-deuterio compound (1a), treatment with methyl iodide gives exclusively the equatorial methyl compound (r-2,cis-6,cis-4trimethyl-1,3-dithiane, 2), and treatment with carbonyl compounds (formaldehyde, acetone, and pivalic aldehyde) gives the equatorial carbinols (4) in at least 99.8% isomeric purity. Treatment of the lithium derivative of the equatorial 2-methyl compound 2 with HCl yields exclusively the corresponding axial isomer 3 (r-2, trans-4,trans-G-trimethyl-1,3-dithiane); a similar contrathermodynamic'' equilibration occurs with the equatorial carbinols mentioned, above, with r-2-tert-butyl-cis-4,cis-6-dimethyl-1,3dithiane and with cis-2,4,4,6-tetramethyl-1,3-dithiane, suggesting a very large preference of the intermediate 2-dithianyl-lithium compound to have the metal in the equatorial position. The high degree of this preference (over 6 kcal/mol) suggests operation of a stereoelectronic rather than a simple steric effect. Carboxylation of the lithium derivative of 2 gives exclusively the equatorial carboxy derivative, r-2,cis-4,cis-6-dimethyl-1,3-dithianecarboxylic acid (6), whereas methylation of the lithium derivative of 2-carbinetgixy-cis-4,6-dimethyl- 1,3-dithiane with methyl iodide leads exclusively to the diastereoisomer 7. Lithiation followed by methylation of othe two diastereoisomeric 2-deuterio-cis- 4,8dimethyl-1,3-dithianes indicates that the kinetic stereoselectivity of the lithiation is only by a factor of 8.6 plus or minus 1.3 with an isotope effect of 2.5 plus or minus 0.1. The preference of the lithium derivative or carbanion to be equatorial is thus largely thermodynamically controlled. (auth)