Abstract
Small differences in the reactivity of weakly nucleophilic potassium aryltrifluoroborates are revealed in the silver-assisted Pd-catalyzed cross-coupling of K[4-RC6F4BF3] (R = H, Bu, MeO, EtO, PrO, iPrO, BuO, t-BuO, CH2=CHCH2O, PhCH2O, PhCH2CH2O, PhO, F, pyrazol-1-yl, pyrrol-1-yl, and indol-1-yl) with ArX (4-BrC6H4CH3, 4-IC6H4F and 3-IC6H4F). An assumed role of silver(I) compounds AgmY (Y = O, NO3, SO4, BF4, F) consists in polarization of the Pd–X bond in neutral complex ArPdLnX with the generation of the related transition state or formation of [ArPdLn][XAgmY] with a highly electrophilic cation and subsequent transmetallation with the weakly nucleophilic borate. Efficiency of AgmY as a polarizing agent decreases in order Ag2O > AgNO3 ≈ Ag2SO4 > Ag[BF4] > AgF. No clear correlation between the reactivity of K[4-RC6F4BF3] and substituent electron parameters, σI and σR°, of the aryl group 4-RC6F4 was found.
Highlights
The palladium-catalyzed reaction of organoboron compounds with C-electrophiles (Suzuki–Miyaura reaction) is one of the most intensively studied processes of the carbon–carbon bond formation
In continuation of systematic research of the Suzuki–Miyaura cross-coupling reaction of weakly nucleophilic organotrifluoroborates we report here the study of the relative reactivity of K[4-RC6F4BF3] in the Pd-catalyzed reactions with some aryl bromides and iodides in the presence of Ag2O
The general concept of the Pd-catalyzed Suzuki–Miyaura (SM) reaction applied to the cross-coupling of K[4-RC6F4BF3] with ArX is presented in Scheme 7
Summary
The palladium-catalyzed reaction of organoboron compounds with C-electrophiles (Suzuki–Miyaura reaction) is one of the most intensively studied processes of the carbon–carbon bond formation. Organoboronic acids, their esters and organotrifluoroborates are partners in these reactions and the choice of the desired reagent depends on the specific requirements in each particular case [1,2,3]. Organoboron reagents containing an electron-poor organic moiety exhibit a low reactivity under the usual cross-coupling conditions [3,4,5,6,7,8,9] and the target products are formed in low yield and/or are contaminated with byproducts.
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