Frustrated Lewis Pair (FLP)-Like (Cyclo)Addition of Small Organic Molecules to a Stable Azomethine Ylide

Azomethine ylides are 1,3-dipolar zwitterions typically used for constructing N-heterocycles by 3+2-cycloaddition reactions. We report here a pyridyl-tethered isolable azomethine ylide (AY) that activates a series of H-E bonds (E═B, Si, Al, O) in FLP-like fashion. The reactions are probed mechanistically by DFT calculations and each case appears to be distinct from others. While the HBpin activation is stepwise, the same of PhSiH₃ is concerted. The AlH₃ activation is also stepwise but takes place across the 1,5-(C⁺/N^-) dipole. The hydrolysis of AY fits better with a "relay" mechanism with two H₂O molecules working in tandem. The B-B bond of B₂pin₂ is also cleaved but in an intriguingly different way, by delivering both the Bpin moieties at the carbenic site. Though the challenging H₂ activation fails, a transfer hydrogenation of AY by NH₃•BH₃ is readily achieved. AY also undergoes cycloadditions with various dipolarophiles including unactivated alkene and alkynes. In this regard, AY makes an interesting distinction between CO₂ and CS₂ by staying inert to the former but easily cycloadding the latter. DFT analysis justifies this dichotomy by showing the cycloaddition of CO₂ as thermodynamically disfavored.