Single versus Double Cu(I) Catalyzed [3 + 2] Azide/Platinum Diacetylide Cycloaddition Reactions

This report focuses on Cu­(I) catalyzed cycloaddition reactions between organic azides and the platinum diacetylide complexes trans-(PR3)2Pt­(C≡CR′)2 (where PR3 = P­(OEt)3, PEt3, PnBu3, PPhMe2, PPh3, and PBn3; and R′ = H, Ph, and p-PhNO2). Pt­(II)-Diacetylides supported by P­(OEt)3, PEt3, PnBu3, and PPhMe2 react with benzyl azide to provide syn/anti isomers of double cycloaddition products. In contrast, Pt­(II)-diacetylide complexes supported by PPh3 and PBn3 afford single cycloaddition products, exclusively. Steric congestion enforced by the larger phosphines PPh3 and PBn3 prevent the second cycloaddition. Ground state DFT computations provide some insight into the divergent reactivity and indicate that the π-acidity of the phosphine ligands is a variable in the single vs double cycloaddition outcome.