Functionalized Pyrazoles as Agents in C–C Cross-Coupling Reactions
2014
Pejic, Marijana | Popp, Sebastian | Bolte, Michael | Wagner, Matthias | Lerner, Hans-Wolfram
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.
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