Enhancement of Enantioselection in the Copper-Catalysed Intramolecular Büchner Reaction by Variation of the Counterion

Abstract

Variation of the enantioselection of the copper-catalysed intramolecular Büchner reaction of diazoketones with the nature of the counterion is reported; significantly the presence of the non­coordinating BARF anion leads to enhanced enantioselectivities when the aryl group bears electron-withdrawing halo substituents.

Key words

intramolecular Büchner reaction - carbenes - α-diazo­ketones - copper bis(oxazoline) catalyst - asymmetric catalysis

References and Notes

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15

BARF^- = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate.

22

General Experimental Procedure
The CuCl-NaBARF-1 catalyst was generated in situ from a mixture of CuCl (0.05 equiv), bis(oxazoline) ligand (0.06 equiv), and NaBARF (0.06 equiv) in CH₂Cl₂ (160 mL), which were stirred under nitrogen at 20 ˚C for 2 h. Diazoketone 2-5 (100 mg, 1 equiv) was then added dropwise over 1 h to the refluxing solution. The progress of the reaction was monitored by TLC and was found to be complete after a total of 3 h at reflux. Evaporation of the solvent at reduced pressure gave the crude product as green oil. A ^¹H NMR spectrum was recorded to determine the efficiency of the cyclisation. Purification by flash chromatography, using EtOAc-hexane as eluent, gave azulenones 6-9 as colourless oil. Spectral characteristics for the azulenone 6 were identical to those described earlier. [5]