Highly Selective N-Alkylation of Pyrazoles: Crystal Structure Evidence for Attractive Interactions

N1-Selective Methylation of Pyrazoles via α-Halomethylsilanes as Masked Methylating Reagents

The development of a highly selective N-methylation of pyrazole heterocycles using commercially available, bench-stable α-halomethylsilanes as masked methylating reagents is described. Sterically bulky α-halomethylsilanes significantly improve the selectivity of N-alkylation over traditional methylating reagents and readily undergo protodesilylation in the presence of a fluoride source and water to give N-methyl pyrazoles. Selectivities of 92:8 to >99:1 N1/N2 regioisomeric ratios were achieved with a range of pyrazole substrates in good yields.

Synergistic Organoboron/Palladium Catalysis for Regioselective N-Allylations of Azoles with Allylic Alcohols

A method for regioselective palladium-catalyzed allylic alkylation of ambident nitrogen heterocycles, employing simple allylic alcohols as electrophile precursors, is described. An organoboron co-catalyst serves both to activate the azole-type nucleophile toward selective N-functionalization and to accelerate the formation of a π-allylpalladium complex from the allylic alcohol. The method can be applied to various heterocycle types, including 1,2,3- and 1,2,4-triazoles, tetrazoles, pyrazoles, and purines, and can be extended to substituted allylic alcohol partners.