An N,N'-diamidocarbene: studies in C-H insertion, reversible carbonylation, and transition-metal coordination chemistry.
Treatment of dimethylmalonyl dichloride with N,N'-bis(2,6-diisopropylphenyl)-N-trimethylsilylformamidine followed by trimethylsilyl triflate (TMS.OTf) afforded the pyrimidinium salt [1H][OTf] in >99% yield. Subsequent deprotonation of this salt led to the formation of the corresponding free N-heterocyclic carbene (NHC) 1. Exhibiting a reactivity profile that is characteristic of traditional electrophilic carbenes, 1 was found to insert into a tertiary C-H bond and reversibly fix carbon monoxide (CO) under mild conditions to afford the first example of a diamidoketene. Remarkably, the aforementioned carbonylation reaction was found to be thermally reversible (K(eq) = 2.62 M(-1) at 30 degrees C; DeltaH degrees = -35.33 kJ mol(-1) and DeltaS degrees = -109.5 J mol(-1) K(-1)). NHC 1 also displayed nucleophilic characteristics. Treatment of 1 with [Ir(COD)Cl](2) (COD = 1,5-cyclooctadiene) afforded 1-[Ir(COD)Cl], a complex with bond lengths and angles that were in accord with known NHC analogues. Treatment of 1-[Ir(COD)Cl] with CO afforded the carbonyl complex 1-[Ir(CO)(2)Cl]. IR studies of this complex revealed a Tolman Electronic Parameter of 2057 cm(-1), a value similar to those for analogous metal complexes containing tricyclohexylphosphine.
Hudnall
Todd W
TW
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, USA.
Bielawski
Christopher W
CW