title: New Stable and Persistent Acyclic Diaminocarbenes.
abstract: The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [(iPr2N)CH(NRR')][PF6], for use as immediate carbene precursors. The corresponding ADACs (iPr2N)C(NRR') were sufficiently stable for isolation in the case of NRR' = 2-methylpiperidino (13), 3-methylpiperidino (14), 4-methylpiperidino (15), morpholino (17) and NiPrPh (20), but had to be trapped in situ in the case of NRR' = 2,2,6,6-tetramethylpiperidino (12) and NiPrMe (19). The tetraaryl-substituted ADACs (Ph2N)2C (22) and (Ph2N)C[N(C6F5)2] (24) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(μ-Cl)(cod)}2] did not work for 12 and 24. The (77)Se NMR chemical shifts, δ((77)Se), of the selenourea compounds derived from the new ADACs lie in the range 450-760 ppm, which indicates a much higher electrophilicity and π-accepting capability of ADACs in comparison with NHCs, which typically exhibit δ((77)Se)<200 ppm. The extreme low-field shift of 758 ppm observed for 12Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6-tetramethylpiperidino unit perpendicular to the N2C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Tim
authors: T
authors: Schulz
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Daniel
authors: D
authors: Weismann
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Lars
authors: L
authors: Wallbaum
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Robin
authors: R
authors: Guthardt
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Charlotte
authors: C
authors: Thie
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Michael
authors: M
authors: Leibold
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany).
authors: Clemens
authors: C
authors: Bruhn
authors: University of Kassel, Institute of Chemistry, Heinrich-Plett-Str. 40, 34132 Kassel (Germany). siemeling@uni-kassel.de.
authors: Ulrich
authors: U
authors: Siemeling
keywords: NMR spectroscopy; carbenes; electrophilicity; rhodium; selenium