Microwave Enhanced Formation of Electron Rich Arylboronates

Abstract

Microwave assisted synthesis of eight electron rich aryl boronates (2a-c, 4, 6a-d) via palladium catalyzed reactions of the corresponding aryl bromides with bis(pinacolato)diboron are described. Compared to conventional heating conditions, dramatic rate enhancements were found for reactions carried out under microwave irradiation, reducing reaction times from hours or days to only minutes.

Key words

palladium - bis(pinacolato)diboron - arylboronates - microwave irradiation - indoles

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15

Representative Experimental Procedure. Synthesis of tris{4-(4,4,5,5-tetra-methyl-[1,3,2]dioxaborolan-2-yl)-phenyl}amine 4: Tris(4-bromophenyl)amine 3 (48 mg, 0.1 mmol), bis(pinacolato)diboron (76 mg, 0.3 mmol), KOAc (88 mg, 0.9 mmol) and Pd(dppf)Cl₂ (8 mg, 3 mol%) were suspended in dry DME (2 mL) in a 10 mL glass tube which was tightly sealed with an aluminum/Teflon crimp. The sample was irradiated at 250 W, 150 °C for 17 min in a CEM-Discover mono-mode microwave apparatus. After completion of the reaction, the vessel was cooled down to 60 °C and the crude mixture was filtered through a thin plug of celite. The celite plug was washed with ether (3 × 5 mL), the organic fractions were combined and the solvent was removed under reduced pressure. The resultant crude product was then washed with methanol (× 3) and recrystallized from a diethyl ether-methanol mixture to furnish 56 mg (89%) of pure product 4 as light yellow crystals. m p > 300 °C. ¹H NMR (300 MHz, CDCl₃): δ = 1.36 (s, 36 H), 7.10 (d, 6 H), 7.70 (s, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 24.8, 83.6, 123.5, 135.90, 149.8. LR-MS (EI) [M⁺]: 624 (39), 623 (100), 622 (69), 497 (15), 496 (7). HR-MS (EI): C₃₆H₄₈B₃NO₆ Calcd 623.3760, found. 623.3755.