Exploring the reactivity of electrophilic trisphosphine platinum(II) complexes in the cycloisomerization of dienes

The cycloisomerization of 1,5-dienes bearing nucleophilic traps with electrophilic trisphosphine Pt(II) complexes generates a cationic Pt-alkyl species which is stable to protonolysis by bulky diaryl ammonium acids. An investigation of tridentate pincer ligand effects in a model system where the alkyl group was -Me revealed that small electron donating substituents at phosphorus enhanced the rate of protonolysis by almost two orders of magnitude. Mechanistic experiments suggested that protonation at Pt generated a 5-coordinate intermediate which eliminated methane by reductive coupling and rapid associative ligand substitution. The large difference in protonolysis rates between pincer and non-pincer systems was attributed to torsional strain inherent to square planar pincer systems. Polyene cyclizations with dicationic Pt complexes typically resulted in a large forward rate constant for cyclization with diastereoselectivity of the polycyclic products governed by the Stork-Eschenmoser postulate. Ligand effects, more specifically electronics, were observed to affect the mode of cyclization (concerted or stepwise). The first direct observation of the equilibrating species in a polycyclization reaction (Pt( 2-alkene) and Pt-alkyl) was made using the electron donating bis(2- diethylphosphinoethyl)ethylphosphine (EtPPPEt) ligand and a 1,5-dienyl sulfonamide. Cyclization was determined to be stepwise in nature, generating the more thermodynamically favored cis ring junction in the 6,5-bicyclic Pt-alkyl product. The variables which affect the cyclization equilibrium were investigated and included: solvent polarity, metal electrophilicity, acid/base strength, and ring strain. These factors were used as a guideline to control stereoselectivity in polyene cascade cyclizations. Medium range stereocontrol was observed using a 1,5-dienol substrate but such control was not present in the cyclization of trienol substrates. The effects of ligand design on Pt(II) catalyzed cyclopropanation reactions was also investigated. Deconstructing the PPP ligand framework into a combination of mono- and bidentate phosphine ligands allowed for a modular approach to catalyst optimization. The optimal achiral catalyst for the cyclopropanation of 1,6- and 1,7-dienes was found to be (dppm)(PMe3)Pt2+. This catalyst was extremely electrophilic and carbophilic; increasing rates by a factor of 20 and allowing for more functional group tolerance. An asymmetric ligand with a similar bite angle to dppm was also synthesized and tested for enantioselective catalysis

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 an 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 retro-cyclization, then kinetic products adhering to the Stork-Eschenmoser postulate were observed (E-alkenes give trans-ring junctions). When retrocyclization was rapid, then alternative thermodynamic products resulting from multi-step 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 suggests 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