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.

Stable and Persistent Acyclic Diaminocarbenes with Cycloalkyl Substituents and Their Transformation to β‐Lactams by Uncatalysed Carbonylation with CO

“…The C−Se bond lengths of 1 a Se and 1 b Se are ca. 1.85 Å, which compares well with values determined for closely related selenourea derivatives . The distances are in between the values typical of carbon–selenium single and double bonds, which has been rationalised in terms of a significant contribution of zwitterionic structures that feature single N 2 C + −Se − dative bonds…”

“…The structures of 1 a Se and 1 b Se are similar to those of other selenourea derivatives of ADACs containing very bulky amino groups such as, for example, ( i Pr 2 N)C(PipMe 2 ) . The most notable feature is the fact that one of the two amino groups is in an almost perpendicular orientation to the N 2 C plane (dihedral angle: 78.9 and 85.4° for 1 a Se and 1 b Se, respectively; see Table ).…”

“…The electrophilicity of ADACs 1 a – c and 2 was probed by a 77 Se NMR spectroscopic investigation of the corresponding easily available (see Experimental Section) selenourea derivatives of the type 1 Se. This well‐established method was recently applied by us for a broad range of ADAC‐derived selenoureas . The chemical shifts of the 77 Se NMR signals of these compounds in CDCl 3 lie in the range from 381–758 ppm.…”

“…This may be ascribed to their comparatively low electrophilicity, as indicated by the 77 Se NMR signal of the corresponding selenium adducts located at ca. 400 ppm . This value is already at the low‐field end of the region typical of selenium adducts of NHCs .…”

“…Selenium adducts of other acyclic diaminocarbenes exclusively exhibit signals at significantly lower field (≥ ca. 450 ppm) . According to the “selenium scale”, diamidocarbenes are considerably more electrophilic than ADACs, since their selenium adducts show signals at extremely low field (ca.…”