260060-85-3Relevant academic research and scientific papers
Atropisomerization Barriers of Configurationally Unstable Biaryl Compounds, Useful Substrates for Atroposelective Conversions to Axially Chiral Biaryls
Bringmann, Gerhard,Heubes, Markus,Breuning, Matthias,Goebel, Lothar,Ochse, Michael,Schoener, Bernd,Schupp, Olaf
, p. 722 - 728 (2000)
Configurationally unstable biaryl lactones of type (M)-1 ? (P)-1 and ring-opened 2-acyl-2′-hydroxy biaryl compounds of type (M)-4 ? (P)-4 are versatile precursors for the atroposelective preparation of axially chiral biaryls. The activation barriers of their atropisomerization process, which constitutes a fundamental precondition for the dynamic kinetic resolution, were determined by dynamic NMR spectroscopy for rapid processes and by HPLC-monitored racemization of enantiomerically enriched material for smaller interconversion rates. For the lactones, the free activation energies ΔG?298 increase with the steric demand of the substituent R ortho to the biaryl axis in the series H 1/2 ≈ ms) 1/2 ≈ s) 1/2 ≈ min) 1/2 ≈ d). The formally ring-opened 2-acyl-2′-hydroxy biaryls, which interconvert via the lactol isomers 5 as the cyclic (and thus configurationally less stable) intermediates, have a significantly slower atropisomerization rate as a result of the high loss in activation entropy ΔS? as a consequence of the required intermediate ring closure 4 → 5.
