Bridgehead halide, reactivity

Substitutions by the SRn 1 mechanism (substitution, radical-nucleophilic, unimolecular) are a well-studied group of reactions which involve SET steps and radical anion intermediates (see Scheme 10.4). They have been elucidated for a range of precursors which include aryl, vinyl and bridgehead halides (i.e. halides which cannot undergo SN1 or SN2 mechanisms), and substituted nitro compounds. Studies of aryl halide reactions are discussed in Chapter 2. The methods used to determine the mechanisms of these reactions include inhibition and trapping studies, ESR spectroscopy, variation of the functional group and nucleophile reactivity coupled with product analysis, and the effect of solvent. We exemplify SRN1 mechanistic studies with the reactions of o -substituted nitroalkanes (Scheme 10.29) [23,24]. 

Full details have now appeared concerning the photoisomerization of triquinacene its bridgehead substitution via photochlorination and the S l solvolytic reactivity of these halides The three deuterated, optically active 2,3-dihydrotri-quinacenes 463 of known absolute configuration have been prepared from +)-462 (Scheme XXXVII) The dextrorotatory monodeuterated triquinacene 464 was... 

Substituted bicycloalkyl halides are very unreactive toward nucleophilic substitution reactions. The low reactivity in S l reactions has been attributed to the fact that a planar configuration at the bridgehead carbon cannot be obtained without the introduction of considerable strain119. On the other hand, the S 2 reaction is precluded because a backside approach of the nucleophile cannot occur at a bridgehead position for a steric reason. The lack of reactivity of 1 -halobicycloalkanes toward nucleophiles by polar mechanisms makes them attractive substrates for the S l mechanism. 

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