Oxygen donor-mediated equilibration of diastereomeric alkene-palladium(II) intermediates in enantioselective desymmetrizing Heck cyclizations.
This investigation examines the origin of enantioselection in the desymmetrization of an acyclic prochiral Heck cyclization precursor. High asymmetric induction (97-98% ee) is attributed to a temporary interaction of a Lewis basic oxygen donor with weakly Lewis acidic palladium(II). A series of control experiments combined with quantum-chemical model calculations provided sound evidence for a mechanism involving oxygen donor-mediated, rapid equilibration of diastereomeric alkene-palladium(II) complexes prior to the selectivity-determining ring-closing event, a Curtin-Hammett scenario. Our study also highlights the importance of the cationic pathway (triflate counter anions versus halido ligand) and alkene stereochemistry (E versus Z) in asymmetric Heck reactions.
Machotta
Axel B
AB
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, Münster, Germany.
Straub
Bernd F
BF
Oestreich
Martin
M
D000475:Alkenes / Q000138:chemical synthesis* / Q000737:chemistry*; D003500:Cyclization; D008956:Models, Chemical; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism
D000475:Alkenes; D010165:Palladium; D010100:Oxygen