The Impact of Palladium(II) Reduction Pathways on the Structure and Activity of Palladium(0) Catalysts

Abstract

Over the past decade, the scientific community has witnessed a dramatic increase in the number of catalytic transformations promoted by palladium complexes. [1] At the same time, continued improvements to both new and existing Pdcatalyzed reactions have resulted in milder conditions and greater substrate generality. These developments in Pd catalysis can be largely attributed to an increased understanding of the individual steps involved in catalytic reactions, particularly oxidative addition, [2] transmetalation, [3] and reductive elimination. [4] Because these elementary processes factor prominently in most catalytic cycles, improvements to palladium-catalyzed reactions have mainly focused on altering the electronic and steric properties of ligands coordinated to the Pd center to accelerate one or more of these steps. [5] However, a key step that remains poorly understood, yet directly impacts the overall rate and performance of a Pd 0catalyzed transformation, is the catalyst activation step. This step involves the reduction of a stable Pd II precursor to an active, zero-valent palladium catalyst and must occur prior to entering the catalytic cycle (Scheme 1). Despite the obvious implications of catalyst activation on a palladium-catalyzed reaction, there is a scarcity of detailed studies concerning the mechanism and efficiency of this reduction process. [6] Herein, we present studies that provide an in depth understanding of the in situ generation of {L n Pd 0 } (n = 1 or 2) catalysts under the standard conditions of a common Pd-catalyzed transformation, the Miyaura borylation [Eq. (1)]. Two pathways for catalyst activation were identified, which provide distinct {L n Pd 0 } complexes: (1) a bisphosphine Pd 0 species resulting from the diboron-mediated reduction of {L 2 Pd II } and (2) a monophosphine Pd 0 species resulting from the basepromoted reduction of {L 2 Pd II } by a ligated phosphine. Direct comparison of the catalytic activity of the resulting {L n Pd} species reveals the impact that catalyst activation has on both the identity and reactivity of a palladium catalyst.We began our studies by determining the reagent(s) responsible for the reduction of a Pd II precatalyst to an active Pd 0 species during the Miyaura borylation. The air-stable catalyst precursor [(Cy 3 P) 2 Pd(OAc) 2 ] (1), [7] which is readily formed from the combination of Pd(OAc) 2 and 2.0 equiv PCy 3 , was chosen for these investigations owing to its widespread application in the borylation of aryl halides. [8] A series of stoichiometric reactions were conducted between 1 and the typical reagents utilized in the borylation reaction to determine the effectiveness of each reagent towards the reduction of 1 (Table 1).The intramolecular reduction of the related complex [(Ph 3 P) 2 Pd(OAc) 2 ] to form an anionic Pd 0 species and triphenylphosphine oxide has been reported by Amatore [9] and others. [10] However, we found that prolonged heating of 1 at 70 8C in toluene, [11] either alone or with added PCy 3 , gave no observable formation o...

Nano-palladium is a cellular catalyst for in vivo chemistry

“…Owing to low s-donicity, TFP readily dissociates from Pd(II) species, as shown by direct equilibration experiments21. As one mechanism of in situ Pd(II) reduction, a phosphine ligand coordinated to the Pd(II) centre converts to its corresponding oxide, thereby generating the active Pd(0) species22, and we observe TFP-oxide generation from the precatalyst under physiological conditions (Supplementary Fig. 1e).…”

Mono-Oxidation of Bidentate Bis-phosphines in Catalyst Activation: Kinetic and Mechanistic Studies of a Pd/Xantphos-Catalyzed C–H Functionalization

“…11 Our studies showed that even simple changes to the order of addition of key reagents can fundamentally influence both the mechanism of Pd(II) reduction and the structure of the resulting catalytically active complex; for example, in a Miyaura borylation, changing the order of addition resulted in selective formation of either a mono-ligated or a bis-ligated Pd(0) complex from the same catalyst precursor. 11 Our studies showed that even simple changes to the order of addition of key reagents can fundamentally influence both the mechanism of Pd(II) reduction and the structure of the resulting catalytically active complex; for example, in a Miyaura borylation, changing the order of addition resulted in selective formation of either a mono-ligated or a bis-ligated Pd(0) complex from the same catalyst precursor.…”

Synthesis of axially chiral biaryl compounds by asymmetric catalytic reactions with transition metals

“…The starting point of both cycles is a Pd(0) species, suggesting that when a palladium(II) precatalyst is used, its reduction should occur first, as the oxidative addition of an aryl halide to a Pd(II) compound is not favored. Various palladium(II) reduction pathways using an excess of phosphine ligand [67] (oxidized to phosphine oxide) or alcohols (oxidized to aldehydes) [30] have been reported [22,68,69,70].…”