Bridgehead positions free radicals

More recently, other approaches to this interesting ring system have also been developed. These are illustrated in Eqs. (41), 29< 13°) and (42) 131 As indicated, photochlorination of the parent hydrocarbon occurs only at the methylene positions 13°). The correspondence between free radical and car-bonium ion reactivities at the bridgehead positions of polycyclic hydrocarbons suggests that the bridgehead position of bisnoradamantane should also be highly unreactive in carbonium ion processes 1321. 

The remarkable activation of sec-alkyl of B-sec-alkyl-9-BBN is attributed to the stabilization of the free radical, owing to the interaction of the odd electron with the vacant p orbital of boron atom [8]. Attack on the two a-bridgehead positions is less facile because in these positions the odd electron would necessarily occupy an orbital that is orthogonal to the vacant p orbital on boron. 

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