At bridgehead carbons

Another kind of evidence for the Sn2 mechanism comes from compounds with potential leaving groups at bridgehead carbons. If the Sn2 mechanism is correct, these compounds should not be able to react by this mechanism, since the... 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

Many free-radical reactions have been observed at bridgehead carbons (e.g., see 14-37), ... 

For a review of bridgehead reactivity in nucleophilic substitution reactions, sec Muller Marcda. in Olah Cage Hydrocarbons Wiley New York. 1990, pp. 189-217. For a review of reactions at bridgehead carbons, see Fort Schleycr Adv. Alicyclic Chem. 1966, /. 283-370. 

Many free-radical reactions have been observed at bridgehead carbons, as in formation of bromide 18 (see 14-30), ° demonstrating that the free radical need not be planar. However, treatment of norbomane with sulfuryl chloride and benzoyl... 

Another approach to obtaining information about the geometric requirements of free radicals has been to examine bridgehead systems. It will be recalled that the small bicyclic rings strongly resist formation of carbonium ions at bridgehead carbon atoms because the skeletal geometry prevents attainment of the preferred planar... 

One chlorinated flame retardant that is useful for high processing polymers such as nylon is Dechlorane Plus . This has good thermal stability and is not prone to dehydrochlorination because the reaction would involve formation of double bonds at bridgehead carbons. 

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