Checking for a Common Intermediate

Often similar reactions proceed via the same intermediate. For example, the SnI solvolysis of f-butyl bromide and f-butyl iodide in water would both be presumed to proceed via the f-butyl cation. We could easily verify this conclusion by performing a competition experiment where the addition of two nucleophilic traps would give two products resulting from the same intermediate in the same ratio. Any deviation in products and ratios would indicate different intermediates. This is exactly the same concept that was used in our example of competition experiments, except now we compare the product ratio from two different reactants. 

Checking for a Common Intermediate in Rhodium-Catalyzed Allylic Alkylations 

A very common structure in organometallic tran.sforma-tions is the ir-allyl complex (see Chapter 12). Such a structure is a resonance hybrid of two forms with a and tt bond character. These structures are formed from allylcarbo-nates and can undergo attack by various carbon-based nucleophiles to give extended allylic systems. 

A rhodium-based system that gives excellent control of stereochemistry has recently been reported, and it was shown by testing for a common intermediate that the putative allyl-Rh species is in fact unsymmetrical. The 

and Nelson, J. D. Conservation of Absolute Configuration in the Acyclic Rhodium-Catalyzed Allylic Alkylation Reaction Evidence for an Enyl (ir + rr) Organorhodium Intermediate. /. Am. Cliem. Soc., 120, 5581 (1998). 

---

Because eliminations are in competition with substitutions (see Sections 10.13.3 and 11.5.16), these two paths can be used to check for a common intermediate in the two different... 

---