Mechanism and Selectivity Control in Ni- and Pd-Catalyzed Cross-Couplings Involving Carbon–Oxygen Bond Activation

Abstract

Conspectus Transition-metal-catalyzed C–O bond activation provides a useful strategy for utilizing alcohol- and phenol-derived electrophiles in cross-coupling reactions, which has become a research field of active and growing interest in organic chemistry. The synergy between computation and experiment elucidated the mechanistic model and controlling factors of selectivities in these transformations, leading to advances in innovative C–O bond activation and functionalization methods. Toward the rational design of C–O bond activation, our collaborations with the Jarvo group bridged the mechanistic models of C­(sp2)–O and C­(sp3)–O bond activations. We found that the nickel catalyst cleaves the benzylic and allylic C­(sp3)–O bonds via two general mechanisms: the stereoinvertive SN2 back-side attack model and the stereoretentive chelation-assisted model. These two models control the stereochemistry in a wide array of stereospecific Ni-catalyzed cross-coupling reactions with benzylic or allylic alcohol derivatives. Because of the catalyst distortion, the ligands can differentiate the competing stereospecific C­(sp3)–O bond activations. The PCy3 ligand interacts with nickel mainly through σ-donation, and the Ni­(PCy3) catalyst can undergo facile bending of the substrate–nickel–ligand angle, which favors the stereoretentive benzylic C–O bond activation. The N-heterocyclic carbene SIMes ligand has additional d­(metal)–p­(ligand) back-donation with nickel, which leads to an extra energy penalty for the same angle bending. This results in the preference of stereoinvertive benzylic C–O bond activation under Ni/SIMes catalysis. In addition to ligand control, a Lewis acid can increase the selectivity for stereoinvertive C­(sp3)–O activation by stabilizing the SN2 back-side attack transition state. The oxygen leaving group complexes with the MgI2 Lewis acid in the stereoinvertive activation, leading to the exclusive stereoinvertive Kumada coupling of benzylic ethers. We also identified that the competing C­(sp3)–O bond activation models have noticeable differences in charge separation. This leads to the solvent polarity control of the stereospecificity in C­(sp3)–O activations. Low-polarity solvents favor the neutral stereoretentive C–O bond activation, while high-polarity solvents favor the zwitterionic stereoinvertive cleavage. In sharp contrast to the nickel catalysts, the C­(sp2)–O bond activation under palladium catalysis mainly proceeds via the classic three-membered ring oxidative addition mechanism instead of the chelation-assisted mechanism. This is due to the lower oxophilicity of palladium, which disfavors the oxygen coordination in the chelation-assisted-type activation. The three-membered ring activation model selectively cleaves the weak C–O bond, resulting in the exclusive chemoselectivity of acyl C–O bond activation in Pd-catalyzed cross-coupling reactions with aryl carboxylic acid derivatives. This explains the overall acylation in the Pd-catalyzed Suzuki–Miyaura coupling with aryl esters. In collaboration wi...

From Esters to Ketones via a Photoredox‐Assisted Reductive Acyl Cross‐Coupling Strategy

“…Subsequent C(acyl)-O oxidative addition of 2-pyridyl ester to Ni I prefers to occur through a five-membered cyclic transition state TS-2 (DG °= 15.5 kcal mol À1 ). [18] The chelation of 2-pyridyl group with the metal center observed in TS-2…”

Ni(0)-promoted activation of Csp2–H and Csp2–O bonds

“…Furthermore, nickel is also known to effectively cleave C–O bonds. 3 In most cases, such reactions take place with C(sp 2 )–O bonds including aryl, 3,8 vinyl, 9 and acyl 10 C–O bonds. An important aspect of C(sp 2 )–O activation is the use of Lewis acidic boron, zinc or magnesium to help overcome the high-barrier of the oxidative addition of a C–O bond.…”

“…An important aspect of C(sp 2 )–O activation is the use of Lewis acidic boron, zinc or magnesium to help overcome the high-barrier of the oxidative addition of a C–O bond. 3,8–10 The Martin group has proposed that the Lewis acidity of the Grignard reagent is critical in the Ni-catalyzed Kumada–Corriu reaction of cyclic vinyl ethers. 9 b While the Agapie group has shown that Ni(0) reacts with an aryl ether to give a Ni( ii ) aryl alkoxide species, as the C–O oxidative addition product, 4 d , e similar studies for Ni(0) mediated C–O bond activation of vinyl ethers are lacking.…”

Palladium Catalyzed Regioselective Cyclization of Arylcarboxylic Acids via Radical Intermediates with Diaryliodonium Salts

“…In this view, multiple C–H activation reactions make use of cheap compounds, while alleviating the need for laborious synthetic manipulation. To efficiently produce high value-added chemicals in a minimum of operations, it should be one of the ultimate goals for new methodology development . To this end, direct functionalization of ArCO 2 H is of particular interest with controllable site specificity for diversification of the parent aromatic nucleus by π-extension .…”