Nucleophilic substitution induced by electron transfer at the bridgehead of polycyclic alkanes: Competition between polar and radical pathways

A series of 2,5(or 1,4)-dihaloadamantanes (4 and 5, X = Y = halogens) and 9,10-dihalotriptycenes (7, X = Y = halogens) as well as two 5-halo (X) adamantan-2-ones (6, Y = O, X = Br and I) have been treated with Me3SnLi in THF in the absence and presence of tert-butylamine (TBA) and dicyclohexylphosphine (DCHP). The product distributions of these reactions have been established by C-13 and Sn-119 NMR spectroscopy, vapor-phase chromatographic analyses, and GC/MS. The former compounds (4 and 5) appear to react exclusively by a free-radical chain process (S(RN)1 mechanism) to yield tin substitution products. By contrast, the triptycenes react predominantly by a polar mechanism initiated by the formation of a carbanion. In the case of the halo ketones (6, Y = O, X = Br and I), a mechanistic divergence of the reaction was unexpectedly encountered. Whereas the bromo ketone provides the substitution product (6, Y = O, X = SnMe3) in good yield (ca. 75%), apparently by a radical pathway, the iodo ketone yields a fragmentation product (ca. 95% yield) by a polar mechanism. This mechanistic switch highlights the importance of the electronegativity of the leaving group as well as substituent-induced electron delocalization as molecular factors governing the competition between radical and polar pathways.