Mechanistic Definition of Trimethylstannylation of 1,4-Dihalobicycloheptanes: Bridgehead Nucleophilic Substitution Mediated by the Intermediacy of Radicals, Radical Anions, Carbanions, and Propellane

Article abstract of DOI:10.1021/jo00287a014

A series of 1,4-dihalobicyclo<2.2.1>heptanes (2; X=Y=halogens) have been synthesized, characterized, and treated with (trimethylstannyl)lithium (Me3SnLi) in the absence and presence of tert-butylamine (TBA).The product distributions of these reactions have been established by 13C and 119Sn NMR spectroscopy and vapor-phase chromatographic analyses.The results clearly indicate that a polar mechanism involving the formation of a carbanion can compete effectively in these systems against a free-radical chain process (S_RN1).The latter mechanism was previously shown to dominate the stannlyation reactions of 1,4-dihalobicyclo<2.2.2>octanes (1; X = Y = halogens).Most of the initially formed (4-halo(X)bicyclo<2.2.1>hept-1-yl)lithium derivatives (2; X = halogen, Y = Li) collapse partially (X = Cl) or completely (X = Br and I) to yield the highly reactive <2.2.1>propellane, which serves as a transient intermediate in the stannylation process.Most importantly, the competition between the radical and polar mechanisms in 2 is shown to be dependent not only on the nature of the leaving group (Y = Br or I) but also on the substituent (X = H, F, Cl, Br, or I).Factors governing the partitioning between the two mechanisms are considered.