P,P,P′,P′-Tetraethynylated Bisphosphine and P-C-P Pincer Ligands with Bulky End Caps: Synthesis, Coordination Properties and Application to Platinum-catalyzed 1,8-Enyne Cycloisomerization

P,P,P′,P′-Tetraethynylated bisphosphine and P-C-P pincer ligands with tris[3,5-di(tert-butyl)-4-methoxyphenyl]methyl end caps were synthesized. The bulky end cap groups of these ligands form a nano-scale cavity, at the bottom of which the bisphosphine or P-C-P ligation motifs are located. Cationic Pt(II) complexes with these hollow-shaped chelating ligands catalyzed cycloisomerization of a 1,8-enyne to give seven-membered carbocyclic compounds.