Ring Opening of Bridged Systems

Although the initial report included amine nucleophiles, the scope was limited to activated amines such as indole (which actually undergoes C-alkylation at the 3-position), phthalimide, and 7/-methylaniline. Furthermore, enantioselectivities were inferior to those observed with alcohols as nucleophiles. Lautens and Fagnou subsequently discovered a profound halide effect in these reactions. The exchange of the chloride for an iodide on the rhodium catalyst resulted in an increased enantioselectivity that is now comparable to levels achieved with alcoholic nucleophiles  

The halide exchange protocol also allows the use of other nucleophiles such as activated methylenes. The rhodium iodide complex was found to be the most 

It seems logical that these reactions are mechanistically related. In every case reported to date, the relative stereochemistry between the nucleophile and alcohol moiety is trans, indicating anti addition in the ring-opening event. Lautens and coworkers suggest a syn insertion of the rhodium to form the allyl rhodium alkoxide. Anti addition of the soft nucleophile, with possible initial protonation of the 

Enantioselective alkylative ring opening of these oxabicyclic alkenes has also been studied. Lautens and coworkers discovered that palladium complexes efficiently catalyze the addition of organozinc reagents to these activated alkenes with concomitant ring opening. In the presence of (Tol-BINAP)PdCl2, diethylzinc adds to oxabenzonor- 

The syn addition of the adducts suggests a mechanism different from that observed in rhodium-catalyzed alcoholysis and aminolysis reactions. Mechanistic investigations from the Tautens laboratory have revealed that the most likely mechanism involves an enantioselective carbopalladation followed by a (3-alkoxide elimination to afford the ring-opened product.  

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