Oxidative Cross-Coupling of Substituted Phenols with Unactivated Aromatics

Visible-light-driven C(sp2)-H arylation of phenols with arylbromides enabled by electron donor-acceptor excitation

We have developed a catalyst-free visible-light-driven C(sp²)-H arylation of unprotected phenols with arylbromides to give 2-arylated phenols. This reaction proceeds through the excitation of an electron donor-acceptor complex between a phenolate and an arylbromide, electron transfer, and debrominative C(sp²)-C(sp²) coupling.

The recent advances in K2S2O8-mediated cyclization/coupling reactions via an oxidative transformation

Recently the K₂S₂O₈ mediated cyclization/coupling reactions to construct carbon–carbon/carbon–heteroatom bond via oxidative transformation is became much interesting in organic synthesis.

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.

Chromium-Salen Catalyzed Cross-Coupling of Phenols: Mechanism and Origin of the Selectivity

A highly chemoselective phenol cross-coupling reaction catalyzed by a Cr-salen catalyst was developed. Kinetic studies showed that the oxidation of Cr(III) to Cr(V) is the rate-determining step of the reaction. In addition, experimental stoichiometric analysis showed that a high valent Cr(V) species is the active catalyst for this process. The selectivity of the reaction was found to be determined by the cross-coupling carbon-carbon bond forming reaction, rather than any precoordination species. It appears that the lowest energy cross-coupling pathway requires a lesser degree of electronic reorganization in its transition state vs the lowest energy homocoupling pathway. This result was supported by stoichiometric Cr(V) kinetics, ¹³C kinetic isotope effects, and density functional theory (DFT) calculations. The understanding of the full landscape of this reaction allowed us to develop a general analysis to predict the regioselectivity of the cross-coupling reaction.

Structural Characterization of Phenoxy Radical with Mass-Correlated Broadband Microwave Spectroscopy

A combination of broadband microwave spectroscopy and VUV photoionization time-of-flight mass spectra has been used to record rotational spectra of the prototypical phenoxy radical, its per-deuterated isotopomers, and the full set of singly 13C-substituted analogues. Rotational parameters associated with the fits to the full set of isotopomers produce a highly accurate r0 structure for the phenoxy radical. High-level ab initio calculations accurately reproduce the rotational constants and spin-rotation parameters. The structure of the phenoxy radical is distinctly quinoidal, with delocalization of the unpaired electron spin density on the oxygen and phenyl ring. The fitted Fermi contact terms for the 13C atoms reflect a weighting of resonance structures that is 27% on the O atom, 21.5% on each of the two ortho C's, and 30% on the para C, providing a quantitative measure of its sites for subsequent reactions that will control its abundances in combustion and atmospheric environments.