Search

A palladium‐catalyzed denitrogenative transannulation strategy to access various 3‐substituted isocoumarin‐1‐imine frameworks using 1,2,3‐benzotriazin‐4(3H)‐ones and terminal alkynes is described. The reaction is highly regioselective and tolerates a wide range of functional groups. The reaction is believed to proceed via a five‐membered palladacycle intermediate extruding environmentally benign molecular nitrogen as a by‐product. The utility of this method was showcased through the one‐pot synthesis of biologically relevant 3‐substituted isocoumarin scaffolds. [ABSTRACT FROM AUTHOR]

Abstract: The role of the reductant in the palladium N‐heterocyclic carbene (NHC) catalyzed reductive Heck reaction and its effect on the mechanism of the reaction is reported. For the first time in this type of transformation, the palladium‐NHC‐catalyzed reductive Heck reaction was shown to proceed in the presence of LiOMe and iPrOH even at 10 °C to give the products very efficiently in excellent yields and with exceptional chemoselectivities. This study shows that the reaction proceeds through two distinct mechanisms that depend on the nature of the reducing agent. In the presence of a protic solvent or acidic medium the reaction undergoes protonation to yield the reduced product, whereas in the absence of proton source, it proceeds through the insertion of the reductant followed by reductive elimination. The kinetic data reveal that the oxidative addition is the rate‐determining step in the reaction. The reaction profiles show first‐order kinetics in aryl iodide and Pd and zero‐order kinetics in LiOMe, benzylideneacetone, and the excess amount of NHC ligand. In addition, the reaction progress kinetic analysis shows that neither catalyst decomposition nor product inhibition occurs during the reaction. DFT calculations of the key steps confirm that the oxidative addition step is the rate‐determining step in the reaction. Deuterium‐labeling experiments indicate that the product is formed by the protonation of the Pd−Calkyl bond of the intermediate formed after enone insertion into the Pd−CAr bond. Application of chiral NHC ligands in the asymmetric reductive Heck reaction only results in poor enantioselectivities (enantiomeric excess up to 20 %) and is also substrate specific. DFT calculations suggest that the migration of the aryl group to the alkene of the substrate is the enantioselectivity‐determining step of the reaction. It is further shown that if the steric bulk at the enone is small (a methyl group), the two transition state barriers from [PdII(L2)(ArI)(enone)] species Cre and Csi, which have the re and si face of the enone substrate coordinated to Pd, are very similar, in line with the experimental results. With a slightly larger group (an isopropyl substituent) a significant difference in energy barriers is calculated (2.6 kcal mol−1), and in the experiment this product is formed with a modest enantiomeric excess (up to 20 %). [ABSTRACT FROM AUTHOR]

A palladium-catalyzed enantioselective reductive Heck reaction of enones using monodentate phosphoramidite ligands is described. α,α,α',α'-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL)-based phosphoramidites with palladium(II) acetate, and N-methyl dicyclohexylamine as reducing agent gives the reductive Heck product in high yields and enantioselectivities of up to 90 %. The solvent plays an important role and in diethyl carbonate, the chemo- and enantioselectivity appeared to be the highest. [ABSTRACT FROM AUTHOR]

We have studied the mechanism of the palladiumcatalyzed reductive Heck reaction of para-substituted enones with 4-iodoanisole by using N,N-diisopropylethylamine (DIPEA) as the reductant. Kinetic studies and in situ spectroscopic analysis have provided a detailed insight into the reaction. Progress kinetic analysis demonstrated that neither catalyst decomposition nor product inhibition occurred during the catalysis. The reaction is first order in the palladium and aryl iodide, and zero order in the activated alkene, N-heterocyclic carbene (NHC) ligand, and DIPEA. The experiments with deuterated solvent ([D7]DMF) and deuterated base ([D15]Et3N) supported the role of the amine as a reductant in the reaction. The palladium complex [Pd0(NHC)(1)] has been identified as the resting state. The kinetic experiments by stopped-flow UV/Vis also revealed that the presence of the second substrate, benzylideneacetone 1, slows down the oxidative addition of 4-iodoanisole through its competing coordination to the palladium center. The kinetic and mechanistic studies indicated that the oxidative addition of the aryl iodide is the rate-determining step. Various scenarios for the oxidative addition step have been analyzed by using DFT calculations (bp86/def2-TZVP) that supported the inhibiting effect of substrate 1 by formation of resting state [Pd0(NHC)(1)] species at the cost of further increase in the energy barrier of the oxidative addition step. [ABSTRACT FROM AUTHOR]

A surprisingly practical Pd(OAc)2 or Pd(TFA)2-catalyzed reductive Heck reaction between aryl iodides α and,β-unsaturated ketones is described using N,N-diisopropylethylamine (DIPEA, Hünigs base) as the reductant. In general, 1 mol% of Pd(OAc)2 is sufficient to afford good yields using electron-rich or halogen-substituted aryl iodides. Electron-deficient aryl iodides preferentially give homocoupling. Enones containing aryl or bulky substituents on the β-carbon react smoothly, producing mainly reductive Heck product, whereas enones with alkyl substituents on the β-carbon afford a mixture of reductive Heck and Heck product. Deuterium labeling experiments show that the reaction is a bona fide reductive Heck reaction and exclude a Heck reaction-conjugate reduction cascade. [ABSTRACT FROM AUTHOR]

A correction to the article "Palladium(II) Acetate Catalyzed Reductive Heck Reaction of Enones; A Practical Approach" that was published in the 2015 issue.