Stable Mesoionic N‐Heterocyclic Olefins (mNHOs)

Abstract

We report a new class of stable mesoionic N‐heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground‐state structure cannot be described through an uncharged mesomeric Lewis‐structure, thereby structurally distinguishing them from traditional N‐heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′‐diaryl‐1,2,3‐triazolium and N,N′‐diaryl‐imidazolium salts, respectively. In their reactivity, they represent strong σ‐donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.

Mesoionic N-heterocyclic olefin catalysed reductive functionalization of CO2 for consecutive N-methylation of amines

“…[32][33][34][35] Besides these NHOs, Hansmann and co-workers recently introduced a super nucleophilic mesoionic N-heterocyclic olefin (mNHO) 1 containing a polarised exocyclic double bond. 36,37 The Tolman electronic parameter (TEP) of 1 (2023 cm -1 )suggests that mNHO 1 has the strongest donor property compared to NHC (2051 cm -1 ), NHOs (2024-2031 cm -1 ) and aNHC (2039 cm -1 ), (Scheme 1a). 36 Moreover, the theoretical calculations predicted that the mNHO (1) possessing the highest proton affinity value of 293.3 kcal/mol is likely to be the strongest nucleophile among earlier reported N-heterocyclic olefins (NHOs) whose proton affinity values lie in the range of 262-282 kcal/mol (Scheme 1b).…”

Stable Mesoionic N‐Heterocyclic Olefins (mNHOs)

“…Slow addition of the colored mNHOs (2, 4,a nd 6)t o toluene solutions of [Rh(m-Cl)(CO) 2 ] 2 as well as B(C 6 F 5 ) 3 gave access to colorless or slightly yellow compounds 8-10 and 11-13,r espectively.T he solid-state structure of 9 (for 8 and 10 see the Supporting Information) [17] establishes the endon binding mode by the exocyclic a-carbon atom to square planar rhodium, with CO being trans coordinated ( Figure 5). TheC 1 ÀC2 bond distances are strongly elongated [8:1 .457-(2) ; 9:1 .458(3) ; 10:1 .461(5) ]a nd even longer in the solid-state structures of boron betains 12 [1.496(2) ]and 13 [1.482(4) ].…”

“…From a À35 8 8Cp entane solution we were able to obtain single crystals of 2 suitable for X-ray diffraction (Figure 1). [17] Most strikingly,t he CH 2 fragment is positioned coplanar to the five-membered ring. TheC1 À C2 distance [1.361 (1) ] is significantly shortened compared to the cationic starting material MICÀCH 3 (1) [ 1.478(5) ,s ee the Supporting Information] and slightly longer compared to Kuhnsreported tetramethyl substituted N-heterocyclic olefin [1.357-(3) ] [2a] (for ac omparison of bond distances,s ee the Supporting Information).…”

The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

“…Analogues of this type are known [14-17, 19, 20], and the exocyclic group C=C(CN) 2 is a good lone pair-free electronic analogue of C=O and C=NAr. Unsubstituted derivatives C=CH 2 are of some interest for two reasons: (i) mesoionic examples of this type were prepared and fully characterised (with X-ray structures) in 2020 [21]; (ii) the CH bonds may show some hyperconjugative electron donation, in a manner analogous to lone pairs. Table 4 compares calculated properties of C=C(CN) 2 , cis and trans C=CHCN and C=CH 2 mesoionic 1,3-diazoles 16.…”