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(eta2-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.