Reversibility Effects on the Stereoselectivity of Pt(II)-Mediated Cascade Poly-ene Cyclizations

Cyclization of 1,5-dienes bearing nucleophilic traps with electrophilic trisphosphine pincer ligated
Pt(II) complexes results in the formation of a polycyclic Pt-alkyl via a Pt(η2-alkene) intermediate. With electron-rich triphosphine ligands, an equilibrium between the Pt(η2-alkene) and Pt-alkyl was observed. The position
of the equilibrium was sensitive to ligand basicity, conjugate acid strength, solvent polarity, and ring size.
In cases where the ligand was electron poor and did not promote retrocyclization, the kinetic products
adhering to the Stork−Eschenmoser postulate were observed (E-alkenes give trans-ring junctions). When
retrocyclization was rapid, alternative thermodynamic products resulting from multistep rearrangements
were observed (cis-[6,5]-bicycles). Under both kinetic and thermodynamic conditions, remote methyl
substituents led to highly diastereoselective reactions. In the case of trienol substrates, long-range
asymmetric induction from a C-ring substituent was considerably attenuated and only modest diastereoselectivity was observed (∼2:1). The data suggest that for a tricyclization, the long-range stereocontrol
results from diastereo-selecting interactions that develop during the organization of the nascent rings. In
contrast, the bicyclization diastereoselectivities result from reversible cascade cyclization.