Cyclopalladation and Reactivity of Amino Esters through CH Bond Activation: Experimental, Kinetic, and Density Functional Theory Mechanistic Studies

Kinetico-Mechanistic Studies on a Reactive Organocopper(II) Complex: Cu-C Bond Homolysis versus Heterolysis

Organocopper(II) reagents are an unexplored frontier of copper catalysis. Despite being proposed as reactive intermediates, an understanding of the stability and reactivity of the Cu^II-C bond has remained elusive. Two main pathways can be considered for the cleavage mode of a Cu^II-C bond: homolysis and heterolysis. We recently showed how organocopper(II) reagents can react with alkenes via radical addition, a homolytic pathway. In this work, the decomposition of the complex [Cu^IILR]⁺ [L = tris(2- dimethylaminoethyl)amine, Me₆tren, R = NCCH₂^-] in the absence and presence of an initiator (RX, X = Cl, Br) was evaluated. When no initiator was present, first-order Cu^II-C bond homolysis occurred producing [Cu^IL]⁺ and succinonitrile, via radical termination. When an excess of the initiator was present, a subsequent formation of [Cu^IILX]⁺ via a second-order reaction was found, which results from the reaction of [Cu^IL]⁺ with RX following homolysis. However, when Brønsted acids (R'-OH: R' = H, Me, Ph, PhCO) were present, heterolytic cleavage of the Cu^II-C bond produced [Cu^IIL(OR')]⁺ and MeCN. Kinetic studies were undertaken to obtain the thermal (ΔH^⧧, ΔS^⧧) and pressure (ΔV^⧧) activation parameters and deuterium kinetic isotopic effects, which provided an understanding of the strength of the Cu^II-C bond and the nature of the transition state for the reactions involved. These results reveal possible reaction pathways for organocopper(II) complexes relevant to their applications as catalysts in C-C bond forming reactions.

Amine-Directed Palladium-Catalyzed C-H Halogenation of Phenylalanine Derivatives

An efficient primary-amine-directed, palladium-catalyzed C-H halogenation (X=I, Br, Cl) of phenylalanine derivatives is reported on a range of quaternary amino acid (AA) derivatives thanks to suitable conditions employing trifluoroacetic acid as additive. The extension of this original native functionality-directed ortho-selective halogenation was even demonstrated with the more challenging native phenylalanine as tertiary AA.

Ligand Promoted, Palladium-Catalyzed C(sp2)-H Arylation of Free Primary 2-Phenylethylamines

A mono- N-protected amino acid (MPAA) ligand promoted, Pd(II)-catalyzed C(sp²)-H arylation of free primary 2-phenylethylamines using the native NH₂ as the directing group has been achieved. This method is compatible with challenging simple primary 2-phenylethylamines bearing α-hydrogen atoms. Application of this protocol in the direct structure modification of the drug molecule amphetamine is also demonstrated.

PdIV Species Mediation in PdII-Catalyzed Direct Alkylation of Arenes with Oxiranes: A DFT Study

The reaction mechanisms of Pd(OAc)₂-catalyzed dehydrogenative alkylation of 2-phenylpyridine with oxirane were investigated using DFT calculations. The most plausible reaction pathway was confirmed as a Pd^II/IV/II catalytic cycle consisting of four processes: C-H activation, ring-opening oxidative addition of oxirane, reductive elimination, and recovery of the catalyst. According to the B2PLYP/DGDZVP computational data, the oxidative addition of oxirane for converting Pd^II to Pd^IV was assigned to be the rate-determining step with a free-energy barrier of 28.1 kcal·mol^-1. For comparison, we also studied the alternative Pd^II-only pathway without a change of oxidation state and found that it was hindered kinetically by a high free-energy barrier of 75.1 kcal·mol^-1 occurring for the ring-opening migratory insertion of oxirane. In addition, the small-ring strain of oxirane should be responsible for the feasible C-O bond-cleavage and subsequent Pd^II → Pd^IV conversion, because the designed four-, five-, and six-membered-ring reagents did not display such an oxidative addition reactivity. Lastly, an extended reactivity order among oxirane, PhI, PhBr, and PhCl toward oxidative addition onto Pd^II to form Pd^IV was proposed in this article based on the computed kinetic parameters.

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

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Explicit roles of diverse directing groups in determining transition state energy and reaction exothermicity of C–H activation pathways

Effects of directing groups on the C–H activation transition state.

C-H Activation in Primary 3-Phenylpropylamines: Synthesis of Seven-Membered Palladacycles through Orthometalation. Stoichiometric Preparation of Benzazepinones and Catalytic Synthesis of Ureas

The dimeric cyclometalated complexes [Pd2{κ(2)C,N-C6H4CH2CH2C(R)(Me)NH2-2}2(μ-Cl)2] (R = Me (1a), H (1b)) are prepared by reacting 1,1-dimethyl-3-phenylpropylammonium or 1-methyl-3-phenylpropylammonium triflate with Pd(OAc)2 in a 1:1 molar ratio and subsequent treatment with excess NaCl. The mononuclear derivatives [Pd{κ(2)C,N-C6H4CH2CH2C(R)(Me)NH2-2}Cl(L)] (L = PPh3, R = Me (3a), H (3b); L = 4-picoline (4-pic), R = Me (4a), H (4b)) were prepared from 1a,b by splitting the chloro bridges with the neutral ligands L. A conformational analysis of the mononuclear palladacycles in solution has been carried out. Insertion of CO takes place into the Pd-C bond of complexes 1a,b, affording Pd(0) and the tetrahydro-benzazepinone 5a or 5b, which possesses potential pharmacological interest. Additionally, the triflate salt A or B undergoes catalytic carbonylation with CO to afford the corresponding N,N'-dialkylurea, using Pd(OAc)2/Cu(OAc)2, in boiling acetonitrile. The crystal structures of 3a·(1)/2H2O, 3b, 4b·CHCl3, and 5a were determined by X-ray diffraction studies.