Digging into the mechanism of oxidative Pd(II)-catalyzed aminations
Although the Pd(II)-catalyzed alkene aminopalladation and allylic C-H activation have been much described in literature, the in-depth mechanism of such type of process is far from being a simple matter. This account focuses on the oxidative intramolecular Pd(II)-catalyzed amination of unsaturated N-sulfonyl carbamates and carboxamides, revealing that different mechanistic paths can be operative. In particular, after activation of the unsaturation by Pd(II) catalyst, aminopalladation can take place, affording the corresponding high-energy cyclic (5- or 6-membered) aminopalladated intermediate (AmPI). This latter can evolve along different pathways, such as: distocyclic β-H elimination, oxidation by a strong terminal oxidant, or carbopalladation. Otherwise, the cyclic AmPI can lay dormant, in equilibrium with the initial substrate. In this case, alternative reactivities may take place, such as allylic C-H activation of the olefinic substrate, [3,3]-sigmatropic rearrangement, or decomposition.
The two interlaced Pd-catalyzed allylation paths
Pd(0)-catalyzed allylation is a powerful transformation discovered in the sixties by Tsuji.(1) In 1970, the first catalytic version of this reactivity appeared in publications or patents (2, 3) and, a few years later, Trost reported the first asymmetric version(4), confirming the synthetic potential of this method. In this transformation, a Pd(0) complex undergoes oxidative addition by an allylic system, to generate an electrophilic η3-allyl-palladium intermediate I, which is finally trapped by a nucleophile to lead to the allylated product (Scheme 1, right). During the following decades, this Pd(0)-catalyzed allylation protocol gained a huge success. Many asymmetric variants,(5) as well as implementations as key steps in total and/or multistep syntheses of therapeutically relevant compounds have been reported. (6)
The same η3-allyl-palladium complex I can also be reached by the interaction between a Pd(II) complex and an alkene bearing an allylic hydrogen atom (Scheme 1, left). As above, trapping by a n ...