Reactivity and Selectivity Controlling Factors in the Pd/Dialkylbiarylphosphine-Catalyzed C–C Cleavage/Cross-Coupling of an N-Fused Bicyclo α-Hydroxy-β-Lactam

High Selectivity in Csp2-Csp2 versus Csp3-O Reductive Elimination from Cycloplatinated(IV) Complexes

The cycloplatinated(IV) complexes trans-[Pt(p-MeC6H4)(C∧N)(OAc)2(H2O)] (C∧N = benzo[h]quinolate, bhq, 2a, and 2-phenylpyridinate, ppy, 2b) were prepared by reacting the corresponding [Pt(p-MeC6H4)(C∧N)(SMe2)] precursors with PhI(OAc)2 through an oxidative addition (OA) reaction. Thermolysis of 2a at 65 °C generates cis-[Pt(κ1N-10-(p-MeC6H4)-bhq)(OAc)2(H2O)], 3a, which is the product of a Csp2Ar-Csp2bhq reductive elimination (RE). The observed coupling reaction is significantly different from the previously reported analogous thermolysis of trans-[PtMe(C∧N)(OAc)2(H2O)] (C∧N = bhq, 2c, and ppy, 2d) that selectively releases Me-OAc (C-O RE). The density functional theory (DFT) calculations and experimental observations reveal that the Csp2Ar-Csp2bhq coupling reaction occurs through the dissociation of a coordinated water ligand. This in turn is followed by the concomitant bond forming and bond breaking process via a three-center ring transition state, in contrast to the Csp3Me-OAc coupling, which had taken place by an outer sphere SN2 type RE reaction in methyl complexes.

Mechanism and Selectivity of Copper-Catalyzed Bromination of Distal C(sp3)-H Bonds

Unactivated C(sp3)-H bonds are the most challenging substrate class for transition metal-catalyzed C-H halogenation. Recently, the Yu group [Liu, T.; Myers, M. C.; Yu, J. Q. Angew. Chem., Int. Ed.2017, 56 (1), 306-309] has demonstrated that a CuII/phenanthroline catalyst and BrN3, generated in situ from NBS and TMSN3 precursors, can achieve selective C-H bromination distal to a directing group. The current understanding of the mechanism of this reaction has left numerous questions unanswered. Here, we investigated the mechanism of Cu-catalyzed C(sp3)-H bromination with distal site selectivity using density functional theory calculations. We found that this reaction starts with the Br-atom transfer from BrN3 to the Cu center that occurs via a small energy barrier at the singlet-triplet state seam of crossing. In the course of this reaction, the presence of the N-H bond in the substrate is critically important and acts as a directing group for enhancing the stability of the catalyst-substrate interaction and for the recruitment of the substrate to the catalyst. The required C-centered radical substrate formation occurs via direct C-H dehydrogenation by the Cu-coordinated N3 radical, rather than via the previously proposed N-H bond dehydrogenation and then the 1,5-H transfer from the γ-(C-H) bond to the N-radical center pathway. The C-H bond activation by the azide radical is a regioselectivity-controlling step. The following bromination of the C-centered radical by the Cu-coordinated bromine completes the product formation. This reaction step is the rate-limiting step, occurs at the singlet-to-triplet state seam of the crossing point, and is exergonic.

Synthesis of Polycyclic Imidazoles via α-C-H/N-H Annulation of Alicyclic Amines

Secondary alicyclic amines are converted to their corresponding ring-fused imidazoles in a simple procedure consisting of oxidative imine formation followed by a van Leusen reaction. Amines with an existing α-substituent undergo regioselective ring-fusion at the α'-position. This method was utilized in a synthesis of fadrozole.

Formal γ-C-H Functionalization of Cyclobutyl Ketones: Synthesis of cis-1,3-Difunctionalized Cyclobutanes

1,3-Difunctionalized cyclobutanes are an emerging scaffold in medicinal chemistry that can confer beneficial pharmacological properties to small-molecule drug candidates. However, the diastereocontrolled synthesis of these compounds typically requires complicated synthetic routes, indicating a need for novel methods. Here, we report a sequential C-H/C-C functionalization strategy for the stereospecific synthesis of cis-γ-functionalized cyclobutyl ketones from readily available cyclobutyl aryl ketones. Specifically, a bicyclo[1.1.1]pentan-2-ol intermediate is generated from the parent cyclobutyl ketone via an optimized Norrish-Yang procedure. This intermediate then undergoes a ligand-enabled, palladium-catalyzed C-C cleavage/functionalization to produce valuable cis-γ-(hetero)arylated, alkenylated, and alkynylated cyclobutyl aryl ketones, the benzoyl moiety of which can subsequently be converted to a wide range of functional groups including amides and esters.

Regioselective α-Cyanation of Unprotected Alicyclic Amines

Secondary alicyclic amines are converted to α-aminonitriles via addition of TMSCN to their corresponding imines, intermediates that are produced in situ via the oxidation of amine-derived lithium amides with simple ketone oxidants. Amines with an existing α-substituent undergo regioselective α'-cyanation even if the C-H bonds at that site are less activated. Amine α-arylation can be combined with α'-cyanation to generate difunctionalized products in a single operation.

Synthesis of Polycyclic Isoindolines via α-C-H/N-H Annulation of Alicyclic Amines

Relatively unstable cyclic imines, generated in situ from their corresponding alicyclic amines via oxidation of their lithium amides with simple ketone oxidants, engage aryllithium compounds containing a leaving group on an ortho-methylene functionality to provide polycyclic isoindolines in a single operation. The scope of this transformation includes pyrrolidine, piperidine, azepane, azocane, and piperazines.

Computational Study of Key Mechanistic Details for a Proposed Copper (I)-Mediated Deconstructive Fluorination of N-Protected Cyclic Amines

Using calculations, we show that a proposed Cu(I)-mediated deconstructive fluorination of N-benzoylated cyclic amines with Selectfluor^® is feasible and may proceed through: (a) substrate coordination to a Cu(I) salt, (b) iminium ion formation followed by conversion to a hemiaminal, and (c) fluorination involving C-C cleavage of the hemiaminal. The iminium ion formation is calculated to proceed via a F-atom coupled electron transfer (FCET) mechanism to form, formally, a product arising from oxidative addition coupled with electron transfer (OA + ET). The subsequent β-C-C cleavage/fluorination of the hemiaminal intermediate may proceed via either ring-opening or deformylative fluorination pathways. The latter pathway is initiated by opening of the hemiaminal to give an aldehyde, followed by formyl H-atom abstraction by a TEDA²⁺ radical dication, decarbonylation, and fluorination of the C3-radical center by another equivalent of Selectfluor^®. In general, the mechanism for the proposed Cu(I)- mediated deconstructive C-H fluorination of N-benzoylated cyclic amines (LH) by Selectfluor^® was calculated to proceed analogously to our previously reported Ag(I)-mediated reaction. In comparison to the Ag(I)-mediated process, in the Cu(I)-mediated reaction the iminium ion formation and hemiaminal fluorination have lower associated energy barriers, whereas the product release and catalyst re-generation steps have higher barriers.

Mechanism, reactivity, and selectivity in a palladium-catalyzed organosilicon-based cross coupling reaction

A thermodynamic control mechanism unravels the unusual chemical selectivity in Pd-catalyzed cross-coupling reaction of silacycles.

Regio- and Stereoselective Alkylboration of Endocyclic Olefins Enabled by Nickel Catalysis

Whereas there is a significant interest in the rapid construction of diversely substituted saturated heterocycles, direct and modular access is currently limited to the mono-, 2,3-, or 3,4-substitution pattern. This Communication describes the straightforward and modular construction of 2,4-substituted saturated heterocycles from readily available materials in a highly stereo- and regioselective manner, which sets the stage for numerous readily accessible drug motifs. The strategy relies on chain walking catalysis.

Sequential Norrish-Yang Cyclization and C-C Cleavage/Cross-Coupling of a [4.1.0] Fused Saturated Azacycle

Methods that functionalize the periphery of azacylic scaffolds have garnered increasing interest in recent years. Herein, we investigate the selectivity of a solid-state Norrish-Yang cyclization (NYC) and subsequent C-C cleavage/cross-coupling reaction of a strained cyclopropane-fused azacyclic system. Surprisingly, the NYC primarily furnished a single lactam constitutional and diastereo-isomer. The regioselectivity of the C-C cleavage of the α-hydroxy-β-lactam moiety could be varied by altering the ligand set used in the coupling chemistry. Experimental and computational observations are discussed.

α,α'-C-H Bond Difunctionalization of Unprotected Alicyclic Amines

A simple one-pot procedure enables the sequential, regioselective, and diastereoselective introduction of the same or two different substituents to the α- and α'-positions of unprotected azacycles. Aryl, alkyl, and alkenyl substituents are introduced via their corresponding organolithium compounds. The scope of this transformation includes pyrrolidines, piperidines, azepanes, and piperazines.

Bioinspired Diversification Approach Toward the Total Synthesis of Lycodine-Type Alkaloids

Nitrogen heterocycles (azacycles) are common structural motifs in numerous pharmaceuticals, agrochemicals, and natural products. Many powerful methods have been developed and continue to be advanced for the selective installation and modification of nitrogen heterocycles through C-H functionalization and C-C cleavage approaches, revealing new strategies for the synthesis of targets containing these structural entities. Here, we report the first total syntheses of the lycodine-type Lycopodium alkaloids casuarinine H, lycoplatyrine B, lycoplatyrine A, and lycopladine F as well as the total synthesis of 8,15-dihydrohuperzine A through bioinspired late-stage diversification of a readily accessible common precursor, N-desmethyl-β-obscurine. Key steps in the syntheses include oxidative C-C bond cleavage of a piperidine ring in the core structure of the obscurine intermediate and site-selective C-H borylation of a pyridine nucleus to enable cross-coupling reactions.