Taming the 1,5-sigmatropic shift across protonated spirocyclic 4H-pyrazoles

Area of Science:

• Organic Chemistry
• Synthetic Chemistry
• Heterocyclic Chemistry

Background:

• The synthesis of 4H-pyrazoles from 1,3-diketones and hydrazine is a known method.
• This reaction proceeds via a 4H-pyrazol-1-ium intermediate.
• Spirocyclic pyrazoles are valuable building blocks in organic synthesis.

Purpose of the Study:

• To investigate the formation of spirocyclic 4H-pyrazoles.
• To understand the influence of substituents on the reaction pathway and intermediate stability.
• To enable the isolation of novel spirocyclic 4H-pyrazoles for further applications like click chemistry.

Main Methods:

• Density functional theory (DFT) calculations were employed to predict reaction rates.
• Experimental synthesis was performed to validate computational predictions.
• Spectroscopic and analytical techniques were used to characterize the products.

Main Results:

• DFT calculations and experiments confirmed a rapid 1,5-sigmatropic shift in the protonated intermediate for diphenyl-substituted pyrazoles, leading to tetrahydrocyclopenta[c]pyrazole.
• Replacing phenyl groups with 2-furanyl groups drastically reduced the rate of the 1,5-sigmatropic shift (by 6.2 × 10^5-fold).
• This significant rate reduction allowed for the isolation of previously inaccessible spirocyclic 4H-pyrazoles.

Conclusions:

• The electronic nature of substituents on the pyrazole ring critically affects the stability of the 4H-pyrazol-1-ium intermediate.
• Strategic modification of substituents can control reaction pathways, enabling the synthesis of specific heterocyclic scaffolds.
• The newly isolated spirocyclic 4H-pyrazoles are promising candidates for use in click chemistry.