Reactions between cinnamoyl(2‐hydroxybenzoyl)methanes and hydrazine hydrate in acetic acid gave 3‐(2‐hydroxyphenyl)‐5‐styrylpyrazoles, while the corresponding reactions with phenylhydrazine yielded 5‐(2‐hydroxyphenyl)‐1‐phenyl‐3‐styrylpyrazoles as the major products and 3‐(2‐hydroxyphenyl)‐1‐phenyl‐5‐styrylpyrazoles as by‐products. The reaction mechanism of this transformation is discussed. The first cycloaddition reactions between ortho‐benzoquinodimethane and either 3‐(2‐hydroxyphenyl)‐5‐styrylpyrazoles or 5‐(2‐hydroxyphenyl)‐1‐phenyl‐3‐styrylpyrazoles afforded 5‐[2‐(3‐aryl‐1,2,3,4‐tetrahydronaphthyl)]‐3‐(2‐hydroxyphenyl)pyrazoles or 3‐[2‐(3‐aryl‐1,2,3,4‐tetrahydronaphthyl)]‐1‐phenyl‐5‐(2‐hydroxyphenyl)pyrazoles, respectively. These cycloadducts were converted into the corresponding naphthylpyrazoles by oxidation with DDQ in dry 1,4‐dioxane. The structures of all new derivatives have been established by NMR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

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The outcome of the Suzuki–Miyaura cross‐coupling for the direct competition reaction between two boronic acids was evaluated under routine synthesis conditions. The reaction selectivity was found to depend on the amount of the base used, with fewer bases favoring the reactivity of the boronic acid with lower pKa (stronger acid). The dependence of the reaction selectivity on base stoichiometry was found to increase with the increase in the difference in the pKa values of the competing boronic acids. These results confirm a relationship between acid–base chemistry and the Suzuki–Miyaura reaction catalytic cycle. Moreover, the results indicate that under these specific conditions, the most reactive organoboron species toward transmetalation is the borate anion RB(OH)3− instead of the neutral boronic acid RB(OH)2. Hence, the main role of the base in the reaction mechanism is to increase the reactivity of the boronic acid toward the Pd–halide complex by converting it into the respective organoborate. In addition, boric acid, an important reaction byproduct, affects the selectivity in the Suzuki reaction because its gradual formation in the reaction medium disturbs the acid–base equilibrium.

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