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.
Nine partially halogenated benzylphosphanes Ar X CH 2 PR 2 (Ar X = 3,6-dichlorophenyl, 3,6-difluorophenyl and 3,4,5-trifluorophenyl; R = Ph, Cy, iPr) have been prepared and reacted with palladium acetate to obtain the cyclometallated dimers [Pd(μ-OAc)(κ 2-C,P-Ar X CH 2 PR 2)] 2. The acetate bridge has been exchanged by bromide using lithium bromide and the obtained dimers have been thoroughly characterised. The dimers with the non-halogenated phosphanes PhCH 2 PR 2 (R = Ph, iPr) have also been prepared.
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