STEREOCHEMISTRY OF CARBANIONS DERIVED FROM 1,3-DIOXANES

摘要

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 C-13 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 carbanion(13) 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. [References: 41]