Pd Bites Back! Isopropyl Surprise

Although we don't always admit it, we chemists love a good surprise.

That's the feeling I got reading through a recent JACS ASAP, from the Chen group at PSU. They've been playing with an easy-to-remove C-H activation auxiliary (PA) first developed by Daugulis back in 2005. They've taught it some pretty neat tricks thus far: C-H amination, alkoxylation, and even alkylation reactions using simple primary iodides.

Their latest extends the chemistry to methyl iodide, which, when combined with some silver salts and a phosphate additive, usually plunks a new CH3 on an unhindered, kinetically-accessible gamma-methyl (i.e. the chain grows by one). However, in the case of norbornene, something wild happens: the Pd catalyst activates a secondary C-H bond, sticks a methyl on, but doesn't stop there; it "bites" into the newly formed methyl and adds on two more, to produce an isopropyl group. Sweet!

     
       Source: JACS ASAP 2013 | Chen group, PSU

Chen's group shows that the position of the new i-Pr depends upon exo ("up") or endo ("down") relationships of the initial amine, but doesn't speculate much on mechanism.

O Norbornene Tree, O Norbornene Tree;
I do not grok your sterics.

Well, allow me! I wonder what happens if you keep feeding iodomethane into the system. I'm sure there's a limit (likely steric) that prohibits additional branching past a certain point, but currently nothing completely rules out a dendrimeric "norbornene tree" compound, right?

Readers, can someone disabuse me of this crazy notion?