Affiliation: College of Arts and Sciences, Department of Chemistry
In situ generated [(PPP)Pt][BF4]2 (PPP = triphos) catalyzes the cycloisomerization of 1,6-enyne-ols by initiative pi-activation of the alkyne. This generates an isolable cationic Pt-alkenyl species which subsequently participates in turnover limiting protonolysis with in situ generated acid. This latter reactivity contrasts cationic Pt-alkyls which are more difficult to protonolyze. Mechanistic studies on isolated Pt-alkenyls, and deuterium labeling helped to elucidate the mechanistic details. A new class of tropos 3,3'-bis(diphenylphosphino)-2,2'-bipyridine ligated Pt(II) complexes was developed. Their ability to coordinate to chiral H-donor auxiliaries through the bipyridine moiety was examined. While d.r. was low for weak H-donors like BINOL and TADDOL, more acidic H-donors (L-DBT) afforded a 1.0:5.5 d.r. for bulkier bipyridine moieties. A single diastereomer of this complex crystallized from solution. These ligands could also be resolved across the metal by forming the (S)-BINOLate complex. However, upon cleavage of the chiral auxiliary, racemization was too rapid for the complex to be used for catalysis.