Stable Di‐ and Tri‐coordinated Carbon(II) Supported by an Electron‐Rich β‐Diketiminate Ligand

Abstract

Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under http://dx.

Air‐Stable Oxyallyl Patterns and a Switchable N‐Heterocyclic Carbene

“…Theh igh reactivity of electron-rich 1,3-diaminooxyallyl moieties had to be tamed through the addition of electronwithdrawing groups.I nspired by our recent work on pyrimidine-1,3-diium salts, [21] we chose to introduce an N,N'amidinium bridge.Asprevious synthetic approaches towards stabilized oxyallyl species were irrelevant to such atarget, our novel strategy relied on the early introduction of a"masked" O-protected oxyallyl pattern. We reacted 1,3-dichlorovina- midinium salt 1, [22] featuring ab enzoyl O-protecting group, with one equivalent of N-(trimethylsilyl)amidine 2 [23] (Scheme 2).…”

Access to a Cationic, Electron-Poor N-Heterocyclic Carbene with a Quinazolinium Core by Postsynthetic Modification of Related Neutral Derivatives

“…40:60 which is due to the presence of the stereogenic center in the backbone in combination with the hindered rotation of the carbene ligand around the M− C(carbene) bond. While the signals for the olefinic protons of the COD ligand are rather broad and partially overlap, the 13 C spectra clearly reveal the presence of 2 × 4 well-resolved signals for the olefinic COD carbon atoms of the two diastereomers. The carbene carbon atoms resonate at 210.5 ( 1 J RhC = 44 Hz) and 210.4 ppm ( 1 J RhC = 47 Hz) for the rhodium complex and 203.7 and 203.5 ppm for the iridium analogue, respectively.…”

“…Interestingly, Martin and co-workers obtained a TEP of 2069 cm −1 by quantum chemical calculations for bis-(dimethylamino)-substituted cationic pyrimidiniumylidene F which is only 4 wavenumbers smaller than the value for G, despite the presence of the two strongly electron donating NMe 2 groups attached to the pyrimidine backbone. 13 We carried out DFT calculations for neutral and cationic carbenes 4 and G, respectively, in order to evaluate and compare the ligand properties of the two species (Chart 2). While the carbene lone pair of electrons is associated with the HOMO in both cases, the HOMO of cationic G (−9.9 eV) is much lower in energy than the HOMO of neutral 4 (−5.4 eV), indicating that the latter species should be a much stronger σ-donor.…”

“…After workup, the complex was isolated in 85% yield and characterized by standard analytical and spectroscopic methods. While no carbene resonance could be observed in its standard 13 C NMR spectrum, the signal was unambiguously identified at 196 ppm in the 1 H− 13 C HMBC spectrum due to a cross peak arising from the 3 J CH coupling to the N-methyl protons. As expected for a cationic species, the signal is shifted downfield in comparison to neutral complex 13.…”

“…12 While the work reported in this contribution was in progress, Martin and co-workers reported an intriguing way to tame the highly reactive cationic pyrimidinium-2-ylidene by attaching electron-releasing Me 2 N groups to the 4-and 6-positions of the ring (F), which even allowed for the isolation of the free carbene. 13 It should be noted that the sterically demanding di(isopropyl)phenyl groups attached to the N atoms also provide additional stability to the compound as they impede undesired reactions at the carbene center. In continuation of our previous studies on electron-poor NHCs, 14 we report here an alternative approach to the chemistry of a cationic benzo-anellated pyrimidiniumylidene (G) by postsynthetic modification of straightforwardly accessible neutral species.…”