Palladium-catalyzed relay C–H functionalization to construct novel hybrid-arylcyclophosphorus ligand precursors

Copper-Catalyzed P-H Bond Functionalization to Construct Biaryl Phosphafluorene Oxides

In this study, in the presence of a certain amount of cuprous chloride catalyst and the synergistic action of ligand and base, the P-H bond activation of secondary biarylphosphine oxides and the attack on the β-site of orthoaryl groups were investigated. Phosphafluorene oxide was synthesized by C-H bond activation and an intramolecular dehydrogenation coupling reaction to construct a C-P bond. Subsequently, we conducted a control experiment and made reasonable speculations about its mechanism. In addition, the use of phosphafluorene as a ligand in some synthetic catalytic reactions has shown excellent results, demonstrating its excellent catalytic properties.

Eight-Membered Palladacycle Intermediate Enabled Synthesis of Cyclic Biarylphosphonates

Transition-metal-catalyzed coupling reactions that involve the direct functionalization of insert C-H bond represent one of the most efficient strategies for forming carbon-carbon bonds. Herein, a palladium-catalyzed intramolecular C-H bond arylation of triaryl phosphates is reported to access seven-membered cyclic biarylphosphonate targets. The reaction is achieved via a unique eight-membered palladacyclic intermediate and shows good functional group compatibility. Meanwhile, the product can be readily converted into other valuable phosphate compounds.

Organocatalytic Diastereoselective (4 + 1) Cycloaddition of o-Hydroxyphenyl-Substituted Secondary Phosphine Oxides

The first organocatalytic diastereoselective (4 + 1) cycloaddition of o-hydroxyphenyl-substituted secondary phosphine oxides (SPOs) has been established, which makes use of o-hydroxyphenyl substituted SPOs as suitable four-atom phosphorus-containing 1,4-dinucleophiles and 3-indolylformaldehydes as competent 1,1-dielectrophiles under Bro̷nsted acid catalysis. The reaction mechanism was suggested to involve the formation of 3-indolylmethanol intermediates and vinyliminium intermediates, which played an important role in controlling the reactivity and diastereoselectivity of the (4 + 1) cycloaddition under Bro̷nsted acid catalysis. By this approach, a series of benzo oxaphospholes bearing P- and C-stereocenters were synthesized in moderate to good yields (50%-95% yields) with excellent diastereoselectivities (all >95:5 dr). This reaction not only represents the first organocatalytic diastereoselective (4 + 1) cycloaddition of o-hydroxyphenyl-substituted SPOs but also provides an efficient and diastereoselective method for the construction of phosphorus-containing benzo five-membered heterocyclic skeletons bearing both P-stereocenter and C-stereocenter.

Synthesis of fused 3-trifluoromethyl-1,2,4-triazoles via base-promoted [3 + 2] cycloaddition of nitrile imines and 1H-benzo[d]imidazole-2-thiols

Here we report a strategy for the facile assembly of fused 3-trifluoromethyl-1,2,4-triazoles, which are difficult to synthesize using traditional strategies, in 50-96% yields through a triethylamine-promoted intermolecular [3 + 2] cycloaddition pathway. This protocol features high efficiency, good functional group tolerance, mild conditions, and easy operation. Furthermore, a gram-scale reaction and product derivatizations were carried out smoothly to illustrate the practicability of this method.

Ru(II)-Catalyzed Synthesis of Fused Imidazo[1,2-a]pyridine-chromenones and Methylene-Tethered Bis-imidazo[1,2-a]pyridines and Regioselective O-Acetoxylation of Imidazo[1,2-a]pyridines

Herein, we disclose an unprecedented protocol via ruthenium-catalyzed annulation for the synthesis of 6H-chromeno[4',3':4,5]imidazo[1,2-a]pyridin-6-one, and functionalized 2-(3-formylimidazo[1,2-a]pyridin-2-yl)phenyl acetate has been revealed by intramolecular chelation-assisted C-H activation. Additionally, a one-pot approach for creating bis(2-phenylimidazo[1,2-a]pyridin-3-yl)methane (BIP) has been realized through ruthenium catalysis with the use of formic acid. This method was used in gram-scale synthesis of BIP and step-economical late-stage functionalization of a marketed drug, zolimidine, in good yield.

Hydride Relay Exchange Mechanism for the Heterocyclic C-H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes

The mechanism and regioselectivity of the heterocyclic C-H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)₂ complexes were investigated using the density functional theory (DFT) method. The Pd(0)L₂(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (S_EAr) mechanism for the C-H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C-H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C-H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C-H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C-H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C-H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.