Stereochemistry of carbanions derived from 1,3-dioxanes

Article abstract of DOI:10.1021/ja00143a004

Proton abstraction from 4-methyl-1,3-dioxane (1), 2-phenyl-1,3-dioxane (4), 4-phenyl-1,3-dioxane (5), trans-4-methyl-2-phenyl-1,3-dioxane (7), and cis- (12) and trans-2-methyl-4-phenyl-1,3-dioxane (6) by means of BuLi/KO-t-Bu ("Lochmann-Schlosser base") gave the corresponding carbanions as shown by subsequent deuteronation with EtOD. In contrast, attempted proton abstraction from cis-2-phenyl-4-methyl-1,3-dioxane (3) under the same conditions was unsuccessful, as was abstraction from phenylcyclohexane. Competitive experiments showed that abstraction of the equatorial hydrogen from 7 was about 4 times faster than corresponding abstraction from 6 and that abstraction of the equatorial hydrogen in 6 was well over 25 times faster than abstraction of the axial hydrogen in 12. Deuteronation of the 2-phenyl carbanion from 7 gave the trans isomer 8 in a 70:1 or greater ratio. Deuteronation of the carbanion derived from 6 or 12, in contrast, produced the axial and equatorial 4-phenyl compounds 10 and 11 in a ratio of only about 2:1. The preliminary conclusion that the carbanion derived from 2-phenyl-1,3-dioxane is largely pyramidal, similar to that from 2-phenyl-1,3-dithiane,⁸ whereas the 4-phenyl-1,3-dioxanyl carbanion is planar, similar to the phenylcyclohexyl (benzylic) carbanion,¹³ was confirmed by ¹³C NMR study of these carbanions. The ion derived from 4-phenyl-1,3-dioxane is red and shows the large upfield shifts of the ortho and para ring carbons also seen in the phenylcyclohexyl carbanion¹³ and characteristic of planar benzylic carbanions.¹² In contrast, the orange carbanion derived from 2-phenyl-1,3-dioxane (which was very unstable and easily oxidized), to the extent that measurement was possible, showed the smaller upfield para carbon shift characteristic of pyramidal benzylic carbanions.^8b,12 It is concluded that, while two adjacent oxygen atoms mildly stabilize equatorial carbanions (presumably inductively), the destabilizing effect of antiperiplanar lone pairs prevents the 2-phenyl carbanion from becoming planar and inhibits abstraction of the axial hydrogen in compound 3. Reprotonation of the pyramidal ion is stereoselective from the equatorial side. In contrast, in the 4-phenyl carbanion, benzylic resonance stabilization overrides the antiperiplanar effect of one neighboring oxygen atom: Both axial and equatorial hydrogen atoms at C(4) can be abstracted and the carbanion is planar; accordingly reprotonation is essentially nonstereoselective.