Importance of Ligand Exchanges in Pd(II)-Brønsted Acid Cooperative Catalytic Approach to Spirocyclic Rings

Abstract

Increasing number of reports in the most recent literature convey the use of palladium and Brønsted acids as cooperative catalytic partners. However, the mechanistic understanding of several such cooperative catalytic reactions and the origin of cooperativity continue to remain limited. In transition metal catalysis, it is typically assumed that the native ligands, such as the acetates in palladium acetate, are retained throughout the catalytic cycle. Herein, we convey the significance of invoking ligand exchanges in transition metal catalysis by using the mechanism of a representative cooperative dual-catalytic reaction. Density functional theory (M06 and B3LYP) computations have been employed to decipher the mechanism of Pd(II)-Brønsted acid catalyzed migratory ring expansion reaction of an indenyl cyclobutanol to a spirocyclic indene bearing a quaternary carbon. The molecular role of water, benzoquinone and phosphoric acid has been probed by computing the energetics using several combinations of all these as ligands on palladium. Of the two key mechanistic possibilities examined, a Wacker-type pathway (involving a semipinacol ring expansion of cyclobutanol followed by a reductive elimination) is found to be energetically more preferred over an allylic pathway wherein the ring expansion in a Pd-π-allyl intermediate occurs subsequent to the initial allylic C-H activation. The Gibbs free energies of the transition states with the native palladium acetate are much higher than a Pd-bis-phosphate species generated through ligand exchanges.

Is silver a mere terminal oxidant in palladium catalyzed C–H bond activation reactions?

“…Under the reactions conditions, several ligand combinations as well as differences in the type of Pd-Ag heterobimetallic bridging ligand are likely. 36 In Fig. 1, we provide a subset of such ligand combinations around the metal, chosen from a larger set of such possibilities for the phosphorylation reaction (1) as an illustrative example.…”

Computational Studies of Carboxylate-Assisted C–H Activation and Functionalization at Group 8–10 Transition Metal Centers

“…In such processes the alkene moiety could be considered as a directing group, although the intrinsic reactivity of allylic C(sp 3 )-H bonds and the large geometric rearrangement required to form an η 3allyl ligand result in reported transition states being dominated by interaction with the carboxylate base and long PdC distances in excess of 3 Å that imply limited interactions. 131,132,133 While these processes literally involve a concerted metalation and deprotonation they do not feature the synergic involvement of the metal and carboxylate ligand that CMD might imply but that AMLA makes explicit.…”

Enantioselective cooperative catalysis

“…The enantioinduction was supposed to arise from the ion pairing/ligand association between the palladium center and the chiral phosphate. Very recently, Sunoj have studied mechanism of the reaction by DFT 19. This is the only example which belongs to the type of category explained inFig 2c.…”