N-Geminal P/Al Lewis Pair-Alkyne Dipolar Cycloaddition to the Zwitterionic C₂PNAl-Heterocyclopentene

The N-geminal P/Al Lewis pair [Ph₂PN(2,6-iPr₂C₆H₃)AlEt₂]₂ (1) has been prepared and studied for reaction with a series of alkynes. The reaction of 1 with RC≡CR yielded zwitterionic C₂PNAl-heterocyclopentene [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CR=^⌝CR) (R = Me (2), Ph (3)); with PhC≡CEt produced two isomers, [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CPh=^⌝CEt) (4a) and [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CEt=^⌝CPh) (4b); and with other alkynes generated [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CR¹=^⌝CR²) (R¹, R² = CO₂Et, Ph (5); SiMe₃, Ph (6); PPh₂, Ph (7); SiMe₃,H (8); H, EtO (9)). Natural bond orbital analysis of the charge separation of the C≡C bond of alkynes was carried out, and then, the electronic matching interaction mode between the combined Lewis acid (AlEt₂) and base (PPh₂) groups of 1 and the C≡C bond of such alkynes was discussed. Reactions of 1 with alkene, nitrile, and carbodiimide molecules were also carried out, and cycloaddition compounds 10-12 were produced.