Electrochemical synthesis of biaryls by reductive extrusion from N,N'-diarylureas

The synthesis of biaryl compounds by the transition-metal free coupling of arenes is an important contemporary challenge, aiming to avoid the toxicity and cost profiles associated with the metal catalysts commonly used in the synthesis of these pharmaceutically relevant motifs. In this paper, we describe an electrochemical approach to the synthesis of biaryls in which aniline derivatives are coupled through the formation and reduction of a temporary urea linkage. The conformational alignment of the arenes in the N,N'-diaryl urea intermediates promotes C-C bond formation following single-electron reduction. Our optimized conditions are suitable for the synthesis of a variety of biaryls, including sterically hindered examples carrying ortho-substituents, representing complementary reactivity to most metal catalysed methods.

Sustainable and Highly Controlled Aryl Couplings Revealed by Systematic Assessment of Photoactivatable Linkers

The controlled synthesis of biphenyls, which play a prominent role in pharmaceuticals, agrochemicals, and liquid crystals typically requires hazardous organometallic reagents, aryl halides, and heavy metal catalysts. We recently reported...

Radical 1,4-Aryl Migration Enabled Remote Cross-Electrophile Coupling of α-Amino-β-Bromo Acid Esters with Aryl Bromides

We report an unprecedented, efficient nickel-catalysed radical relay for the remote cross-electrophile coupling of β-bromo-α-benzylamino acid esters with aryl bromides via 1,4-aryl migration/arylation cascades. β-Bromo-α-benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce-Smiles rearrangement. This reaction enables the formation of two new C(sp³ )-C(sp² ) bonds using a bench-stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox-neutral C(sp³ )-C(sp² ) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross-electrophile coupling and radical 1,4-aryl migration through the generation of C(sp³ )-centred radical intermediates from the homolysis of C(sp³ )-Br bonds and the switching of the transient alkyl radical into a robust α-aminoalkyl radical.

Metal-Free β-Amino Alcohol Synthesis: A Two-step Smiles Rearrangement

A novel method for the synthesis of β-amino alcohols has been demonstrated under mild reaction conditions with a broad scope via a two-step Smiles rearrangement. What is more, theoretical calculations have been performed to confirm the rationality of the mechanism. The method has been proved to be notably effective for N-arylated amino alcohols, which are difficult to synthesize by traditional methods.

Uncovering the Neglected Similarities of Arynes and Donor-Acceptor Cyclopropanes

Arynes and donor-acceptor (D-A) cyclopropanes are two classes of strained systems having the potential for numerous applications in organic synthesis. The last two decades have witnessed a renaissance of interest in the chemistry of these species primarily because of the mild and robust methods for their generation or activation. Commonly, arynes as easily polarizable systems result in 1,2-disubstitution, whereas D-A cyclopropanes as polarized systems lead to 1,3-bisfunctionalization thereby showing striking similarities. Transformations with 1,2- and 1,3-dipoles afford cyclic structures. With arynes, emerging four-membered rings as intermediates might react further, whereas the analogous five-membered rings obtained from D-A cyclopropanes are most often the final products. However, there are a few cases where these intermediates behave surprisingly differently. This Minireview highlights the parallels in reactivity between arynes and D-A cyclopropanes thereby shedding light on the neglected similarities of these two reactive species.

Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers

The metal-free, highly selective synthesis of biaryls poses a major challenge in organic synthesis. The scope and mechanism of a promising new approach to (hetero)biaryls by the photochemical fusion of aryl substituents tethered to a traceless sulfonamide linker (photosplicing) are reported. Interrogating photosplicing with varying reaction conditions and comparison of diverse synthetic probes (40 examples, including a suite of heterocycles) showed that the reaction has a surprisingly broad scope and involves neither metals nor radicals. Quantum chemical calculations revealed that the C-C bond is formed by an intramolecular photochemical process that involves an excited singlet state and traversal of a five-membered transition state, and thus consistent ipso-ipso coupling results. These results demonstrate that photosplicing is a unique aryl cross-coupling method in the excited state that can be applied to synthesize a broad range of biaryls.

Transition-Metal-Free Aryl-Aryl Cross-Coupling: C-H Arylation of 2-Naphthols with Diaryliodonium Salts

Transition-metal-free regioselecitive C-H arylation of 2-naphthols with diaryliodonium salts has been developed. The reaction proceeds under very simple experimental conditions and affords a range of products with various substitution patterns. The method allows for the incorporation of electron-deficient aryls, which complements well currently existing metal-free aryl-aryl cross-couplings of phenols that have been so far restricted to the introduction of electron-rich aryl moieties. The mechanism of the reaction was studied by means of DFT calculations, demonstrating that the C-C bond formation occurs via a dearomatization of 2-naphthol substrate, followed by a subsequent rearomatization by tautomerization. The computations show that the use of a low polarity solvent and an insoluble inorganic base is key to securing the high selectivity of the C-C coupling over a competing C-O arylation pathway, by preventing the incipient deprotonation of 2-naphthol.

Construction of Condensed Polycyclic Aromatic Frameworks through Intramolecular Cycloaddition Reactions Involving Arynes Bearing an Internal Alkyne Moiety

Facile synthetic methods for condensed polycyclic aromatic compounds via aryne intermediates are reported. The generation of arynes bearing a (3-arylpropargyl)oxy group from the corresponding o-iodoaryl triflate-type precursors efficiently afforded arene-fused oxaacenaphthene derivatives, which were formed through intramolecular [2+4] cycloaddition. Extending the method to the generation of arynes bearing a 1,3-diyne moiety led to a continuous generation of naphthalyne intermediate through the hexadehydro Diels-Alder reaction involving the aryne triple bond. This novel type of aryne-relay chemistry enabled the synthesis of a unique aminoarylated oxaacenaphthene derivative and highly ring-fused anthracene derivatives.