Understanding Precatalyst Activation in Cross-Coupling
Reactions: Alcohol Facilitated Reduction from Pd(II) to Pd(0) in Precatalysts of the Type (η<sup>3</sup>‑allyl)Pd(L)(Cl) and (η<sup>3</sup>‑indenyl)Pd(L)(Cl)
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Abstract
Complexes of the type (η<sup>3</sup>-allyl)Pd(L)(Cl) (L =
PR<sub>3</sub> or NHC), have been used extensively as precatalysts
for cross-coupling and related reactions, with systems containing
substituents in the 1-position of the η<sup>3</sup>-allyl ligand,
such as (η<sup>3</sup>-cinnamyl)Pd(L)(Cl), giving the highest
activity. Recently, we reported a new precatalyst scaffold based on
an η<sup>3</sup>-indenyl ligand, (η<sup>3</sup>-indenyl)Pd(L)(Cl),
which typically provides higher activity than even η<sup>3</sup>-cinnamyl supported systems. In particular, precatalysts of the type
(η<sup>3</sup>-1-<sup>t</sup>Bu-indenyl)Pd(L)(Cl) give the highest
activity. In cross-coupling reactions using this type of Pd(II) precatalyst,
it is proposed that the active species is monoligated Pd(0), and the
rate of reduction to Pd(0) is crucial. Here, we describe detailed
experimental and computational studies which explore the pathway by
which the Pd(II) complexes (η<sup>3</sup>-allyl)Pd(IPr)(Cl)
(IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene),
(η<sup>3</sup>-cinnamyl)Pd(IPr)(Cl), (η<sup>3</sup>-indenyl)Pd(IPr)(Cl)
and (η<sup>3</sup>-1-<sup>t</sup>Bu-indenyl)Pd(IPr)(Cl) are
reduced to Pd(0) in alcoholic solvents, which are commonly used in
Suzuki–Miyaura and α-arylation reactions. The rates of
reduction for the different precatalysts are compared and we observe
significant variability based on the exact reaction conditions. However,
in general, η<sup>3</sup>-indenyl systems are reduced faster
than η<sup>3</sup>-allyl systems, and DFT calculations show
that this is in part due to the ability of the indenyl ligand to undergo
facile ring slippage. Our results are consistent with the η<sup>3</sup>-indenyl systems giving increased catalytic activity and provide
fundamental information about how to design systems that will rapidly
generate monoligated Pd(0) in the presence of alcohols