Phosphonylpyrazoles from Bestmann-Ohira Reagent and Nitroalkenes: Synthesis and Dynamic NMR Studies

Application of diethyl 1-diazo-2-oxopropylphosphonate (Bestmann-Ohira reagent) as a cycloaddition partner with nitroalkenes has been extensively investigated. Base-mediated reaction of the Bestmann-Ohira reagent with various nitroalkenes such as beta-substituted alpha,beta-disubstituted, and nitroethylene that are part of a carbocyclic or heterocyclic ring provided functionalized phosphonylpyrazoles through a one-pot regioselective reaction at room temperature in high yield. The substituted nitroalkenes employed in these reactions also included Morita-Baylis-Hillman adducts of conjugated nitroalkenes with various electrophiles. Detailed dynamic NMR Studies were performed on the prototropic tautomerism exhibited by the phosphonylpyrazoles using CDCl3 and DMSO-d(6) as solvents and H-1 and P-31 as the probe nuclei. These studies unraveled the existence of two tautomers in solution with a small energy difference but considerable barrier to interconversion.