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. 2011 Apr 6;133(13):4710-3.
doi: 10.1021/ja111700e. Epub 2011 Mar 14.

Determining the enantioselectivity of chiral catalysts by mass spectrometric screening of their racemic forms

Christian Ebner  1 , Constanze A Müller, Christian Markert, Andreas Pfaltz
Affiliations
Free PMC article

Determining the enantioselectivity of chiral catalysts by mass spectrometric screening of their racemic forms

Christian Ebner et al. J Am Chem Soc. .
Free PMC article

Abstract

The enantioselectivity of a chiral catalyst can be determined from its racemic form by mass spectrometric screening of a nonequal mixture of two mass-labeled quasienantiomeric substrates. The presented method opens up new possibilities for evaluating catalyst structures that are not readily available in enantiomerically pure form.

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Figures

Scheme 1
Scheme 1. Screening Methodology
Scheme 2
Scheme 2. Synthesis of Phox Derivatives 4a and 4b
Conditions: (a) (CH3)2CHPPh3+I−, nBuLi, THF, 0 °C → RT, 4 h. (b) MCPBA, DCM, 0 °C, 2 h. (c) (1) PhCN, BF3·OEt2, 0 °C → RT, 3 h; (2) NaHCO3, RT, 2 h. (d) (1) sBuLi, TMEDA, THF, −78 °C; (2) Ph2PCl, −78 °C → RT, 18 h; (3) H2O2. (e) PhSiH3, 120 °C, 40 h.
Scheme 3
Scheme 3. Screening Conditions
Figure 1
Figure 1
ESI-MS spectrum from screening with phox ligand 4c in DCM. Substrate ratio: 2a/2b = 1:3.15. Observed ratio of allyl intermediates: 3a/3b = 30.5:69.5. Calculated selectivity: s = 3.5; ee = 56%.
Figure 2
Figure 2
Comparison of the screening methods.
Figure 3
Figure 3
Correlation of screening results and actual ee.
Scheme 4
Scheme 4. Synthesis of Phox Derivatives 4d−f
Conditions: (a) (1) nBuLi, THF, −78 °C, 20 min; (2) DMF, −78 °C → RT, 1 h; H2O. (b) Me3S+I−, H2O, KOH, MeCN, 60 °C, 4 h. (c) NaN3, AcOH, 1:1 acetone/H2O, 50 °C, 4 h. (d) H2, 5 mol % Pd/C, EtOH, RT, 4 h. (e) (1) 2-(diphenylphosphino)benzonitrile, ZnCl2, C6H5Cl, reflux, 7 days; (2) 2,2′-bipyridine, CHCl3, RT, 2 h.
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