PuPHOS: a synthetically useful chiral bidentate ligand for the intermolecular Pauson-Khand reaction.
Here we describe the synthesis and use of the Pulegone-derived bidentate P,S ligands PuPHOS and CyPuPHOS in the intermolecular Pauson-Khand reaction. Ligand exchange reaction of hexacarbonyldicobalt-alkyne complexes with PuPHOS provides a diasteromeric mixture of complexes (up to 4.5:1) from which the major isomers can be conveniently separated by simple crystallization. An isomerization-crystallization sequence of the original mixture results in a dynamic resolution that allows the preparation of the pure major Co(2)(mu-TMSC(2)H)(CO)(4)-PuPHOS (15a) in a multigram scale. Pauson-Khand reaction of 15a with norbornadiene provided, for the first time, the corresponding enone 18 with up to 93% yield and 97% ee. The use of (+)-18 as a surrogate of chiral cyclopentadienone is also demonstrated. Copper-catalyzed Michael addition of a Grignard reagent followed by removal of the TMS group with TBAF were the most reliable methods to transform (+)-18 into valuable starting materials 20a-e for the enantioselective synthesis of cyclopentenoid systems.
Verdaguer
Xavier
X
Unitat de Recerca en Síntesi Asimètrica (URSA), Parc Científic de Barcelona and Departament de Química Orgànica, Universitat de Barcelona, c/Josep Samitier, 1-5. 08028-Barcelona, Spain. xverdaguer@pcb.ub.es
Lledó
Agustí
A
López-Mosquera
Cristina
C
Maestro
Miguel Angel
MA
Pericàs
Miquel A
MA
Riera
Antoni
A