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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General Experimental
Procedure
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 CH2 Cl2 (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 ¹ H
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
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