Catalyst-Controlled Site-Selectivity Switching in Pd-Catalyzed Cross-Coupling of Dihaloarenes

The Road Less Traveled: Unconventional Site Selectivity in Palladium-Catalyzed Cross-Couplings of Dihalogenated N-Heteroarenes

The vast majority (≥90%) of literature reports agree on the regiochemical outcomes of Pd-catalyzed cross-coupling reactions for most classes of dihalogenated N-heteroarenes. Despite a well-established mechanistic rationale for typical selectivity, several examples reveal that changes to the catalyst can switch site selectivity, leading to the unconventional product. In this Perspective, we survey these unusual cases in which divergent selectivity is controlled by ligands or catalyst speciation. In some cases, the mechanistic origin of inverted selectivity has been established, but in others the mechanism remains unknown. This Perspective concludes with a discussion of remaining challenges and opportunities for the field of site-selective cross-coupling. These include developing a better understanding of oxidative addition mechanisms, understanding the role of catalyst speciation on selectivity, establishing an explanation for the influence of ring substituents on regiochemical outcome, inverting selectivity for some "stubborn" classes of substrates, and minimizing unwanted over-reaction of di- and polyhalogenated substrates.

Different Oxidative Addition Mechanisms for 12- and 14-Electron Palladium(0) Explain Ligand-Controlled Divergent Site Selectivity

In cross-coupling reactions, dihaloheteroarenes are usually most reactive at C─halide bonds adjacent to a heteroatom. This selectivity has been previously rationalized. However, no mechanistic explanation exists for anomalous reports in which specific ligands effect inverted selectivity with dihalopyridines and -pyridazines. Here we provide evidence that these ligands uniquely promote oxidative addition at 12e- Pd(0). Computations indicate that 12e- and 14e- Pd(0) can favor different mechanisms for oxidative addition due to differences in their HOMO symmetries. These mechanisms are shown to lead to different site preferences, where 12e- Pd(0) can favor oxidative addition at an atypical site distal to nitrogen.

Unconventional Site Selectivity in Palladium-Catalyzed Cross-Couplings of Dichloroheteroarenes under Ligand-Controlled and Ligand-Free Systems

Halides adjacent to nitrogen are conventionally more reactive in Pd-catalyzed cross-couplings of dihalogenated N-heteroarenes. However, a very sterically hindered N-heterocyclic carbene ligand is shown to promote room-temperature cross-coupling at C4 of 2,4-dichloropyridines with high selectivity (∼10:1). This work represents the first highly selective method with a broad scope for C4-coupling of these substrates where selectivity is clearly under ligand control. Under the optimized conditions, diverse substituted 2,4-dichloropyridines and related compounds undergo cross-coupling to form C4-C_(sp2) and C4-C_(sp3) bonds using organoboron, -zinc, and -magnesium reagents. The synthetic utility of this method is highlighted in multistep syntheses that combine C4-selective cross-coupling with subsequent nucleophilic aromatic substitution reactions. The majority of the products herein (71%) have not been previously reported, emphasizing the ability of this methodology to open up underexplored chemical space. Remarkably, we find that ligand-free "Jeffery" conditions enhance the C4 selectivity of Suzuki coupling by an order of magnitude (>99:1). These ligand-free conditions enable the first C5-selective cross-couplings of 2,5-dichloropyridine and 2,5-dichloropyrimidine.

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.

Selective ortho-Functionalization of Adamantylarenes Enabled by Dispersion and an Air-Stable Palladium(I) Dimer

Contrary to the general belief that Pd-catalyzed cross-coupling at sites of severe steric hindrance are disfavored, we herein show that the oxidative addition to C-Br ortho to an adamantyl group is as favored as the corresponding adamantyl-free system due to attractive dispersion forces. This enabled the development of a fully selective arylation and alkylation of C-Br ortho to an adamantyl group, even if challenged with competing non-hindered C-OTf or C-Cl sites. The method makes use of an air-stable PdI dimer and enables straightforward access to diversely substituted therapeutically important adamantylarenes in 5-30 min.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

Haloselective Cross-Coupling via Ni/Photoredox Dual Catalysis

The chemoselective functionalization of polyfunctional aryl linchpins is crucial for rapid diversification. Although well-explored for C_sp² and C_sp nucleophiles, the chemoselective introduction of C_sp³ groups remains notoriously difficult and is virtually undocumented using Ni catalysts. To fill this methodological gap, a “haloselective” cross-coupling process of arenes bearing two halogens, I and Br, using ammonium alkylbis(catecholato)silicates, has been developed. Utilizing Ni/photoredox dual catalysis, C_sp³–C_sp² bonds can be forged selectively at the iodine-bearing carbon of bromo(iodo)arenes. The described high-yielding, base-free strategy accommodates various protic functional groups. Selective electrophile activation enables installation of a second C_sp³ center and can be done without the need for purification of the intermediate monoalkylated product.

Governing effects in the mechanism of the gold-catalyzed cycloisomerization of allenic hydroxylamine derivatives

The formation of chiral heterocycles via cycloisomerization reactions of allene derivatives has gained relevance due to their associated efficiency and atom-economy. The only drawback that keeps these reactions away from being routine synthetic strategies is the control in the regioselectivity (most often 5-endo vs. 6-endo). In this work, we computationally explore the experimental chemistry reported by Krause using N-hydroxy-α-aminoallenes and hydroxylamine ethers as substrates and provide a rationale for the different reactivity observed. The drastic effects observed experimentally when changing the nature of the gold catalyst have also been studied mechanistically. These results are expected to help in the design of improved regioselective protocols for the formation of medium sized chiral heterocycles from allene substrates.

Sequential and iterative Pd-catalyzed cross-coupling reactions in organic synthesis

ABSTRACT

Sequential and iterative Pd-catalyzed cross-coupling reactions can be performed in which the order of C-C bond formations can be controlled either by the attenuated leaving groups of the multireactive substrate or by specific catalyst/ligand combinations. This tutorial review gives an overview about recent developments in this field and the various strategies used for the assembly of oligoarenes and -alkenes.

GRAPHICAL ABSTRACT

Regioselective Suzuki couplings of non-symmetric dibromobenzenes: alkenes as regiochemical control elements

The regioselective Suzuki coupling of dibromobenzenes has been investigated. The regiochemistry is high when one bromine atom is proximal to an alkene.