Thermodynamic vs. Kinetic Control in Synthesis of O-Donor 2,5-Substituted Furan and 3,5-Substituted Pyrazole from Heteropropargyl Precursor

Abstract

Elaboration of a convenient route towards donor-substituted pyrazoles from heteropropargyl precursors is challenging due to a number of thermodynamically favorable side reactions (e.g., acetylene–allene isomerization and Glaser homocoupling). In this work, Sonogashira cross-coupling conditions of 4-tert-butylphenyl propargyl ether with benzoyl chloride followed by tandem Michael addition/cyclocondensation with hydrazine into 3,5-disubstituted pyrazole (kinetic control), as well as cycloisomerization conditions of ketoacetylene intermediate into 2,5-disubstituted furan (thermodynamic control), were established through a variation of the catalyst loading, solvent polarity, excess of triethylamine, and time of reaction. During the optimization of process parameters, a number of by-products represented by a monophosphine binuclear complex (PPh3PdI2)2 with two bridging iodine atoms and diyne were identified and isolated in the pure form. The quantum-chemical calculations and solution-state 1H/13C NMR spectroscopy suggested that the 5(3)-(4-tert-butylphenyloxy)methoxy-3(5)-phenyl-1H-pyrazole exists in the tautomeric equilibrium in a polar methanol solvent and that individual tautomers could be characterized in case aprotic solvents employed. The pyrazole features a unique tetramer motif in the crystal phase formed by alternating 3(5)-phenyl-1H-pyrazole tautomers, which was stabilized by N–H···N bonds and stacking interactions of pyrazole rings, whereas pyrazole dimers were identified in the gas phase.