Abstract
The synthesis and kinetic resolution of quaternary oxindoles through copper catalysed azide-alkyne cycloadditions is presented. Selectivity factors (s) up to 22.1 ± 0.5 are reported. Enantioenriched alkynes and triazoles were obtained in ≥80% enantiomeric excess (e.e.).
Highlights
Click chemistry was first outlined in 2001 by K
Sharpless and co-workers, the copper catalysed azide alkyne cycloaddition (CuAAC) has lived up to Sharpless’ accolade as ‘‘the cream of the crop’’ in this area.[1]
To the best of our knowledge there are no reports of successful Kinetic resolution (KR) of alkynes via the CuAAC reaction
Summary
Resolution of racemic azides via the CuAAC reaction has been achieved by Meng et al.[7] using a Cu–PyBox catalyst,[8] selectivity factors (s) up to s = 8 where obtained.[9] Meng et al were unable to kinetically resolve racemic terminal alkynes (s = 1).[7] A range of five different terminal alkynes were tested and no enantiomeric discrimination was observed whatsoever with any compounds tried. They accounted for this observation using a mononuclear transition state model. A series of readily available copper sources were probed, including Cu(I) Cu(II) and Cu(0) species, under standard conditions of 0.6 equivalents of benzyl azide (3b), 12.5 mol% of copper source and 15 mol% of PhPyBox ligand (L1) at 0 1C for Scheme 1 (a) General scheme for the CuAAC KR strategy; (b) application of KR to alkynyl C-3 quaternary oxindoles
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