Small Molecule Activation with Intramolecular “Inverse” Frustrated Lewis Pairs

The intramolecular “inverse” frustrated Lewis pairs (FLPs) of general formula 1‐BR2‐2‐[(Me2N)2C=N]‐C6H4 (3–6) [BR2=BMes2 (3), BC12H8, (4), BBN (5), BBNO (6)] were synthesized and structurally characterized by multinuclear NMR spectroscopy and X‐ray analysis. These novel types of pre‐organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via an ortho‐phenylene linker, are capable of activating H−H, C−H, N−H, O−H, Si−H, B−H and C=O bonds. 4 and 5 deprotonated terminal alkynes and acetylene to form the zwitterionic borates 1‐(RC≡C‐BR2)‐2‐[(Me2N)2C=NH]‐C6H4 (R=Ph, H) and reacted with ammonia, BnNH2 and pyrrolidine, to generate the FLP adducts 1‐(R2HN→BR2)‐2‐[(Me2N)2C=NH]‐C6H4, where the N‐H functionality is activated by intramolecular H‐bond interactions. In addition, 5 was found to rapidly add across the double bond of H2CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise, 5 is capable of cleaving H2, HBPin and PhSiH3 to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules. Novel intramolecular “inverse” frustrated Lewis pairs have been prepared and their ability to activate small molecules investigated.