Formyl Substituent at C-4 of Pyrazoles: A Temporary Protecting Group for Regioselective Palladium-Catalyzed Direct Arylation at C-5

Construction of multi-substituted pyrazoles via potassium carbonate-mediated [3 + 2] cycloaddition of in situ generated nitrile imines with cinnamic aldehydes

A highly efficient potassium carbonate-mediated [3 + 2] cycloaddition reaction of hydrazonoyl chlorides with cinnamic aldehydes to furnish multi-substituted pyrazoles under nontoxic and mild conditions has been developed. A plausible stepwise cycloaddition reaction mechanism is proposed. This protocol featured broad substrate coverage, good functional group tolerance, wide scalability, and operational simplicity, as well as conveniently constructed pyrazole scaffolds.

Effective Tools for the Metal-Catalyzed Regiodivergent Direct Arylations of (Hetero)arenes

The direct functionalization of two different C-H bonds of the same organic molecule using different procedures - also called regiodivergent C-H bond functionalization - currently represents an important research topic in organic chemistry, as it demonstrates the versatility of C-H bond functionalization methodology. Over the last decade, the number of tools to control such regiodivergent C-H bond functionalizations has increased significantly. In this account, we will present the various tools that allowed us to arylate different positions of various (hetero)arenes, via a C-H bond functionalization, using palladium or ruthenium catalysis.

Transition-metal-catalyzed C–H functionalization of pyrazoles

This review describes recent advances in transition-metal-catalyzed C–H functionalization reactions of pyrazoles to form new C–C and C–heteroatom bonds on the pyrazole ring.

Polysubstituted 3-trifluoromethylpyrazoles: regioselective (3 + 2)-cycloaddition of trifluoroacetonitrile imines with enol ethers and functional group transformations

Non-catalysed addition of trifluoroacetonitrile imines to enol ethers provided fully regioselectively (3 + 2)-cycloadducts, which either spontaneously or via Brønsted acid-induced elimination of ROH molecules led to the formation of 3-trifluoromethylated pyrazoles. In the case of 2,3-dihydrofuran, the respective bicyclic intermediate was isolated and its structure was confirmed by X-ray analysis. Using the developed protocol the synthesis of a known antitumor compound SC-560 was performed in 45% yield. Subsequent functionalisations of selected 4-(ω-hydroxyalkyl)pyrazoles at C(5) through lithiation/addition, cross-coupling reactions or via intramolecular Pd-catalysed C-H arylations opened up an access to polysubstituted pyrazoles including unusual tricyclic systems comprising 7-membered rings (oxepane, thiepane and azepane) as the central unit.

Regioselectivity in palladium-catalysed direct arylation of 5-membered ring heteroaromatics

In the past few years, several results using modified catalysts and new reaction conditions have been reported permitting better control of the regioselectivity of Pd-catalysed arylations.

Conditions for palladium-catalyzed direct arylations of 4-bromo and 4-iodo N-substituted pyrazoles without C–Br or C–I bond cleavage

The Pd-catalyzed arylation at the C5 position ofN-protected pyrazole derivatives bearing bromo or iodo substituents at the C4 position is described.