Bridgehead carbons, and

Although the bonding is unusual, the central bond in A [l.l.l]propellane has 0.8 times the electron density at the center of the C—C bond in butane. There is a significant amount of electron density near the bridgehead carbons, and this presumably is responsible for the high reactivity of the compound toward electrophiles... 

It is quite possible to form a radical at a bridgehead carbon, and sometimes this is the only way to perform any chemistry at such a position, for we have already seen that a bridgehead carbon is normally inert to attack by both the SN1 and the SN2 pathways. 

In all of these studies with polycyclic cyclopropane systems, the proton underwent addition to the methylene group. However, there are reports on the addition reaction of hydrogen bromide in which the proton attacked the bridgehead carbon and the bromide finally became the substituent at the methyl group, e.g. formation of 10 and 11. °... 

Although Br attacks simple alkylcyclopropanes at the least substituted carbon atom, with bicyclo[n. 1.0]alkanes attack can be at the more substituted bridgehead carbon atom [Scheme 15, paths (a) and (b)] ". In the case of bicyclo[ 1.1.0]butane or bicyclo[2.1.0]pentane, attack at the bridgehead carbon and opening of the internal cyclopropane bond relieves considerable strain. However, with bicyclo[3.1.0]hexane attack of Br still seems to predominate at the bridgehead position ". ... 

From the structural point of view, the substrates reported in Table 6 have halogens which are bonded to bridgehead carbons and are equatorial or pseudoequatorial. This stereochemistry allows us to compare the results obtained with these derivatives with those reported in Table 3 for cyclohexanes which also have equatorial halogens. Contrary to the cyclohexane derivatives, the strained a-haloketones give exclusively ring-contracted products regioselec-tively. This regioselectively, as well as the absence of secondary substitution... 

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