The positions of equilibrium, established by acid catalysis, between diastereomeric cis- and trans-5-(tert-butylthio)-(1), 5-(tert-butylsulfiny1)- (2), and 5-(tert-butylsulfonyl)-2-isopropyl-l,3-dioxanes (3) are reported and compared with previously published data for the 5-methylthio (4), 5-methylsulfinyl (5), and 5-methylsulfonyl (6) analogues. Although AG° values for the sulfides 1 and 4 are very similar, the difference in conformational behavior for sulfoxides 2 and 5 is significant, and the effect of changing from methyl to tert-butyl in the sulfones (6 — 3) is quite dramatic: the large preference of the methyl analogue for the axial position (1.19 kcal/mol) is reversed in 3 where the equatorial isomer is more stable by 1.14 kcal/mol. The conformational behavior in 1–6 is discussed in terms of the rotamer population of the axial isomer, in which steric and electrostatic effects are dominant. X-ray crystallographic data on cis-2-tert-butyl-5-(tert-butylsulfonyl)-l,3-dioxane (cis-9 axial sulfonyl) show that the SS-£er£-butyl group is outside the ring, with both sulfonyl oxygens above the dioxane ring and eclipsing the endocyclic C-C bonds. The electrochemical behavior of cis-1 and trans-1 supports the idea that lone-pair/lone-pair electron repulsion is responsible for the large predominance of the equatorial isomer.
ASJC Scopus subject areas
- Organic Chemistry