Exploring the reactivity of dimethylzinc with fluorine substituted 1-phenyl-4,5-dihydro-1H-pyrazol-5-ols

• Fluorine substituted 1-phenyl-4,5-dihydro-1H-pyrazol-5-ols have been synthesized and characterized.
• The cyclic hemiaminal form has been observed as major isomer in the solid state.
• The 1-phenyl-4,5-dihydro-1H-pyrazol-5-ols were reacted with dimethylzinc and N,N,Ń,Ń-tetramethylethylenediamine (tmeda).
• In contrast to earlier reports complexes with the RO–Zn(tmeda)Me motif were formed, probably due to the fluorine substitution.

In the present study, different fluorine substituted 1-phenyl-4,5-dihydro-1H-pyrazol-5-ols have been synthesized in moderate yields and characterized by X-ray crystallography, revealing a high stability of the cyclic hemiaminal form in the solid state. Furthermore, NMR investigations confirmed this structural motif in solution. Interestingly, the formation of the corresponding pyrazoles by elimination of water was not observed under described reaction conditions, probably due to stabilization effects of the fluorine substitution. The ligands were treated with dimethylzinc and N,N,Ń,Ń-tetramethylethylenediamine (tmeda) in a 1:1:1 molar ratio to form tetrahedral monomeric complexes with a RO–Zn–Me motif. The additional coordination sites at the zinc are occupied by the tmeda ligand (η2-coordination). On the other hand, increasing the amount of ligands revealed the formation of complexes with a (RO)2–Zn–(tmeda) motif. Interestingly, with the highly fluorinated 1-phenyl-4,5-dihydro-1H-pyrazol-5-ols a new coordination mode (monodentate O-coordination) of pyrazoline ligands was observed.

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