Q: Why is it possible to transmetallate i-PrMgBr with PhBr to generate i-PrBr and PhMgBr, but not the other way around?
I suspect that it has something to do with the hybridization of i-Pr vs Ph (Ph would have a more stable anion, which pKa also shows) but I don't understand how it would effect the mechanism unless the intermediate is a phenylic anion?