Isolation of the key intermediates of base-promoted borylene–carbonyl coupling reactions

Abstract

The terminal alkylborylene complex [(η⁵–C₅H₅)(OC)₂MnBtBu] (1) reacts with trimethylphosphine and CO to form a borylene–carbonyl coupling product [(η⁵–C₅H₅)(OC)Mn{κ²–C,C′–C(O)B(tBu)(PMe₃)C(O)}] (3). This reactivity can be extended to nitrogen- and carbon-based ligands to form similar complexes [(η⁵–C₅H₅)(OC)Mn{κ²–C,C′–C(O)B(tBu)(L)C(O)}] (L = DMAP, 7, and IMe, 8). The key intermediates of these reactions were found to be neutral, Lewis base-stabilized borylenes of the form [(η⁵–C₅H₅)(OC)₂MnB(tBu)(L)] (L = 4-dimethylaminopyridine (DMAP), 4, 3,5-lutidine, 5 and :C[N(Me)CH]₂ (IMe), 6, which have been isolated and fully characterized. They can be synthesized from simple addition of the base to the parent borylene 1. The coordination of the base to the boron center seems to facilitate a rearrangement of the t-butyl borylene substituent.