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

Synthesis of mono-, di- and tripalladated 1,3,5-benzenetristyryl complexes. CO insertion to give a dipalladated indenone

The tribrominated arenes 1,3,5-C₆(E-CHCHAr)₃Br₃ (Ar = Ph, (I), p-To (I')), add oxidatively to [Pd(dba)₂] ([Pd₂(dba)₃]·dba) in the presence of two equivalents of a phosphine (PPh₃ or PMe₂Ph) to form the monopalladated complexes trans-[Pd{C₆(E-CHCHAr)₃Br₂}Br(L)₂] (Ar = Ph, L = PPh₃ (1a), Ar = p-To, L = PPh₃ (1a'), Ar = Ph, L = PMe₂Ph (1b)), while the reaction in a 1 : 2 : 4 arene : Pd : PMe₂Ph molar ratio affords the dipalladated complex [{trans-PdBr(PMe₂Ph)₂}₂{μ₂-C₆(E-CHCHPh)₃Br}] (2b). Both I and I' add oxidatively to 3 equivalents of [Pd(dba)₂] in the presence of the chelating N-donor ligand tmeda (N,N,N',N'-tetramethylethylenediamine) to form the tripalladated complexes [{PdBr(tmeda)}₃{μ₃-C₆(E-CHCHAr)₃}] (Ar = Ph, (3c), p-To (3c')). Complex 3c reacts with PMe₃ to form [{trans-PdBr(PMe₃)₂}₃{μ₃-C₆(E-CHCHPh)₃}] (3d). Compound 3c also reacts with CO to give the novel dipalladated indenone [2-Ph-4,6-{PdBr(tmeda)}₂-5,7-(E-CHCHPh)₂-inden-1-one] (4). The crystal structures of 1a' and 1b were determined by X-ray diffraction studies.

Native Amino Group Directed Site-Selective ε-C(sp2)-H Iodination of Primary Amines

Selective remote C-H activating amines using unmodified NH₂ as a native directing group demonstrate compelling synthetic utilities. The 3-arylpropan-1-amine moiety is present in many drugs and candidates in clinical trials. Selective iodination of 3-arylpropan-1-amines on remote aryl rings gives valuable intermediates for modifying bioactive molecules and synthesizing quinolones. Here we report the first palladium-catalyzed selective ε-C(sp²)-H iodination of free 3-arylpropan-1-amines via a seven-membered palladacycle.

Sequential Insertion of Alkynes, Alkenes, and CO into the Pd–C Bond of ortho-Palladated Primary Phenethylamines: from η3-Allyl Complexes and Enlarged Palladacycles to Functionalized Arylalkylamines

The eight-membered metallacycles arising from the insertion of 1 equiv of alkyne into the Pd–C bond of ortho-metalated homoveratrylamine and phentermine can further react with alkenes to give two different types of mononuclear complexes depending on the nature of the olefin. When terminal alkenes (styrene and ethyl acrylate) are used, a mixture of the anti/syn η³-allyl Pd(II) complexes are isolated, which evolve slowly to the syn isomers by heating the mixtures appropriately. These η³-allyl Pd(II) complexes do not react with CO or weak bases, but when they are treated with a strong base, such as KO^tBu, they afford Pd(0) and the functionalized starting phenethylamines containing a 1,3-butadienyl substituent in an ortho position. When 2-norbornene was used instead of terminal alkenes, the strained olefin inserts into the alkenyl Pd(II) complex to afford a 10-membered norbornyl palladium(II) complex, in which the new C,N-chelate ligand is coordinated to the metal through an additional double bond, occupying three coordination positions. The reactivity of these norbornyl complexes depends on the substituents on the inserted alkenyl fragment, and thus they can further react with (1) KO^tBu, to give Pd(0) and a tetrahydroisoquinoline nucleus containing a tricyclo[3.2.1]octyl ring, or (2) CO and TlOTf, to afford Pd(0) and amino acid derivatives or the corresponding lactones arising from an intramolecular Michael addition of the CO₂H group to the α,β-unsaturated ester moiety. Crystal structures of every type of compound have been determined by X-ray diffraction studies.

Palladium-Catalyzed C(sp3 )-H Arylation of Primary Amines Using a Catalytic Alkyl Acetal to Form a Transient Directing Group

C-H Functionalization of amines is a prominent challenge due to the strong complexation of amines to transition metal catalysts, and therefore typically requires derivatization at nitrogen with a directing group. Transient directing groups (TDGs) permit C-H functionalization in a single operation, without needing these additional steps for directing group installation and removal. Here we report a palladium catalyzed γ-C-H arylation of amines using catalytic amounts of alkyl acetals as transient activators (e.g. commercially available (2,2-dimethoxyethoxy)benzene). This simple additive enables arylation of amines with a wide range of aryl iodides. Key structural features of the novel TDG are examined, demonstrating an important role for the masked carbonyl and ether functionalities. Detailed kinetic (RPKA) and mechanistic investigations determine the order in all reagents, and identify cyclopalladation as the turnover limiting step. Finally, the discovery of an unprecedented off-cycle free-amine directed ϵ-cyclopalladation of the arylation product is reported.

Carbon Dioxide-Mediated C(sp3)-H Arylation of Amine Substrates

Elaborating amines via C-H functionalization has been an important area of research over the past decade but has generally relied on an added directing group or sterically hindered amine approach. Since free-amine-directed C(sp³)-H activation is still primarily limited to cyclization reactions and to improve the sustainability and reaction scope of amine-based C-H activation, we present a strategy using CO₂ in the form of dry ice that facilitates intermolecular C-H arylation. This methodology has been used to enable an operationally simple procedure whereby 1° and 2° aliphatic amines can be arylated selectively at their γ-C-H positions. In addition to potentially serving as a directing group, CO₂ has also been demonstrated to curtail the oxidation of sensitive amine substrates.

Synthesis of 3-(2-Olefinbenzyl)-4H-chromen-4-one through Cyclobenzylation and Catalytic C-H Bond Functionalization Using Palladium(II)

An efficient strategy for synthesizing 3-(2-olefinbenzyl)-4H-chromen-4-one in two steps was developed. The first step is a cyclobenzylation reaction between (E)-3-(dimethylamino)-1-(2-hydroxyphenyl)prop-2-en-1-one and benzyl bromide to produce homoisoflavonoid. The second step involves intermolecular Pd-catalyzed π-chelating-assisted C-H bond olefination. Using the C-2/C-3 double bond of chromone, palladium-catalyzed aryl C-H bond activation can be functionalized to generate ortho-olefination derivatives in moderate to high yields.