Why is 1-bromotriptycene inert to nucleophilic substitution?

According to my textbook, 1-bromotriptycene is inert to any nucleophilic substitutions.


[![1-bromotriptycene](https://i.stack.imgur.com/6FGWz.png)](https://i.stack.imgur.com/6FGWz.png)


Due to steric hinderance we can say SN2 reactions cannot occur, but I think SN1 reactions should be favoured as formed carbocation will be resonance stabilised.


Can someone explain why it doesn’t undergo SN1 reactions?




Sn1 reaction proceed through carbocation intermediate which has planar structure. The structure of 1-bromo triptycene is cage like structure so the bridgehead carbon cannot assume planarity , hence,the formation of a carbocation at the bridgehead position does not take place. Consequently 1-bromotriptycene is inert to sn1reaction.


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[![nucleophillic substitution](https://i.stack.imgur.com/XXtf7.png)](https://i.stack.imgur.com/XXtf7.png)


Bredt's rule is a consequence of the fact that having a double bond on a bridgehead or carbocation is not stable for small rings (fewer than eight atoms) due to a combination of ring strain, and angle strain (nonplanar alkene).Since SN1 reaction involves carbocation intermediate, and such intermediate is not possible here, hence SN1 IS RULED OUT.SN2, reaction involves a backside attack from the leaving group.Due to stearic hinderence to the backside attack, such a reaction is also not possible.Totally nucleophilic substitution is not possible.


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