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

Abstract

AbstractAzomethine 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.