Screening of chiral Diels-Alder catalysts by mass spectrometric monitoring of the retro reaction

A rapid screening procedure for the identification of chiral Diels-Alder catalysts by ESI MS
was developed. Screening of the retro reaction made it possible to indirectly determine the
catalyst’s enantioselectivity for the forward, product-forming reaction.
We were able to monitor positively charged catalytic intermediates for the retro-Diels-Alder
reaction. Use of mass-labelled quasienantiomers allowed the intermediates A and B to be
distinguished by mass spectrometry. The ratio of the mass peaks of A and B reflected the
catalyst’s intrinsic enantioselectivity. It was crucial to evaluate the influence of the mass
labels on the reactivity of the two quasienantiomers. Sufficiently strong binding between the
substrates and the metal catalyst was necessary to allow detection.
The method was used for the screening of bis(oxazoline), phosphinooxazoline and bis(imine)
ligands in the copper-catalysed retro-Diels-Alder reaction to identify highly selective ligands.
The selectivities were confirmed by product analysis (HPLC). In accordance to the principle
of microscopic reversibility the ratios observed for the retro-Diels-Alder reaction correlated
with those obtained for the preparative forward-Diels-Alder reaction.
The method was extended to organocatalytic retro-Diels-Alder reactions. Monitoring masslabelled
iminium ions C and D allowed direct determination of the catalyst’s intrinsic
enantioselectivity. This versatile method allowed not only rapid screening of imidazolidinone
and proline-based organocatalysts but also of oligopeptide-catalysts. The results obtained by
ESI MS were confirmed by preparative reactions.
After establishing a reliable protocol for the screening of organocatalysts, the procedure was
successfully applied to multi-catalyst screening. The catalytic intermediates originate from
different organocatalysts with different masses allowing facile assignment of the catalytic
intermediates. After optimising the conditions, multi-catalyst screening showed excellent
correlation with the single catalyst screening.