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.

The reactions of the acyclic diaminocarbenes (Me2N)2C and (Ph2N)(iPr2N)C with CO proceed in a 2 : 1 stoichiometric ratio, affording unprecedented betainic oxyallyl species of type [(R2N)2C]2CO.

The "Alder carbene" (iPr(2)N)(2)C undergoes a β-fragmentation in solution already at room temperature, affording propene and N,N,N'-triisopropylformamidine. This stands in sharp contrast to the indefinite stability previously claimed for this iconic compound.

This paper focuses on complexes of the N-heterocyclic carbene (NHC) 1-Np, which contains a 1,1Ј-ferrocenediyl backbone and neopentyl substituents at the N atoms flanking the divalent C atom. 1-Np is a ring-expanded NHC with a ring size of six atoms. The square-planar group 10 metal com-trans-[PdCl 2 (1-Np) 2 ] and trans-[NiCl 2 (1-Np)(PPh 3 )] have been prepared by the reaction of 1-Np with trans-[PdCl 2 (PhCN) 2 ], [PdCl 2 (PPh 3 ) 2 ], cis-[PdCl 2 (COD)] (COD = cycloocta-1,5diene) and [NiCl 2 (PPh 3 ) 2 ], respectively. In addition, the carbene-borane adduct 1-Np-BF 3 has been obtained by serendipity. All new compounds have been structurally characterised by single-crystal X-ray diffraction, which demonstrates the pronounced trans influence of 1-Np. 1-Np has been com- [a]

Diisopropylamino-cis-2,6-dimethylpiperidinocarbene reacts regio- and diastereoselectively with CO to afford a bicyclic β-lactam with 100% atom efficiency, whose spectrum of activity resembles that of penicillin G or amoxicillin.