Can Azomethine Ylides be Considered as Frustrated Lewis Pairs?

Azomethine ylides are typically transient synthons, heavily used in constructing N-heterocycles by dipolar cycloaddition reactions. We report here a pyridyl-tethered isolable azomethine ylide (AY) that unprecedentedly acts as a Frustrated Lewis Pair (FLP) in activating a series of H-E bonds (E = B, Si, Al, O). The reactions are thoroughly probed mechanistically by the aid of DFT calculations and each case appears to be distinct from the rest. While the HBpin activation follows a stepwise mechanism, the same of PhSiH3 has a concerted route. The AlH3 activation is also stepwise but takes place across the 1,5-(C+/N-) dipole involving the pyridyl-N. The H2O activation is better fitted with a ‘relay’ mechanism with two H2O molecules rather than one to interact with AY. The B-B bond of B2pin2 is also cleaved but in an intriguingly different way, by an oxidative addition at a carbene center formed in situ through a 1,3-(C+ to C-) H+ shift. Though the imperative H2 activation fails, a transfer hydrogenation by NH3•BH3 is achieved readily and mechanistically elucidated as a stepwise process. The AY also undergoes FLP-like cycloadditions with various dipolarophiles, among which the addition of CS2 but not of CO2 is alluring and counter-intuitive. DFT analysis again justifies this dichotomy by showing the addition of CS2 as thermodynamically favored but of CO2 as disfavored, mostly due to the larger ring strain in the cycloaddition product in the CO2 case.