Functionalized Pyrazoles as Agents in C–C Cross-Coupling Reactions
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Marijana Pejic
The 1-tetrahydropyranyl-(THP-)protected pyrazoles 4-R-pz(THP) (R=pinacolatoboryl=Bpin (3a(THP)), Me3Si (4a(THP)), Me3Sn (5a(THP)), and 4-R-3,5-Ph2pz (R=Bpin (3b(THP)), Me3Si (4b(THP)), Me3Sn (5b(THP)) were obtained by the following syntheses: i) In a first step, 4-X-pz (X=Br (1a), I (2a)) and 4-X-3,5-Ph2pz (X=Br (1b), I (2b)) were reacted with 3,4-dihydro-2Hpyran (DHP) to give the related THP-protected bromo- or iodopyrazole derivatives. ii) In a second step these THP derivatives were metalated by treatment with nBuLi or iPrMgCl. Subsequent reactions of the THP-protected metallopyrazoles 4-M-pz(THP) and 4-M-3,5-Ph2pz(THP) (M=Li, MgBr) with Bpin(OiPr), Me3SiCl, and Me3SnCl yielded the pyrazole derivatives 3a(THP), 3b(THP), 4a(THP), 4b(THP), 5a(THP), and 5b(THP). In contrast to the stannylated pyrazoles 5a(THP) and 5b(THP), the corresponding borylated and silylated derivatives could be easily deprotected: treatment of 3a(THP), 3b(THP), and 4a(THP) with HCl yielded the parent pyrazoles 3a, 3b and 4a. The microwave-assisted C-C cross-coupling reactions of these pyrazoles with aryl halides were examined, e. g. Suzuki reactions of 3a with p-pentylphenylbromide, p-hexylphenylbromide, and p- (2-ethylhexyl)phenylbromide. Similar reactions were also performed with 1a, 1b, 2a, and 2b and aryl-substituted pinacolatoboranes or boronic acids. Crystals of 5b(THP) suitable for X-ray diffraction were grown (monoclinic P21/c) and their structure determined. The crystal structures of 1a·HBr (monoclinic P21/n), 1b (triclinic P̄1̄), (1c)2·HBr (monoclinic P2/c), 1c·HBr·(Br2)0.5 (triclinic P̄1̄), (2a)3·H2SO4 (triclinic P̄1̄), 3a (orthorhombic P212121), (3a)3·H2O (trigonal R3c), 3b (orthorhombic Pna21), and 4a (monoclinic Pc) reveal interesting hydrogen bonding networks.
Graphical Abstract
Functionalized Pyrazoles as Agents in C–C Cross-Coupling Reactions
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