Stable Carbenes or Betaines?

Abstract

The anisotropy effect in 1H NMR spectroscopy can be readily employed to indicate the position of carbene/betaine mesomeric equilibria. NR2 substituted carbene/betaines tend to adopt betaine structures, whereas in the absence of NR2 substituents, the betaine structures cannot stabilise the structure through both π‐donation effects of the NMe2 groups and the electronegativity of the nitrogen atoms, and the corresponding carbene‐like structures are preferred. These conclusions are supported by calculated bond orders and δ(13C)/ppm values. The spatial magnetic properties of isonitriles and carbon monoxide, which can be counted as stable carbenes or, at least, as carbene‐analogues, also exist as stable betaine structures, which is again supported by structural and magnetic properties.

The 13C chemical shift and the anisotropy effect of the carbene electron‐deficient centre: Simple means to characterize the electron distribution of carbenes

“…The simple reason therefore proves to be remarkable stabilization via resonance contributor 4 ′ in Scheme . Generally, π‐electron delocalization in ring moieties is most effective in stabilization of the studied ylide ( 1,2‐ and 1,3‐betaine ) structures (see previous studies and vide infra).…”

“…They are 5‐membered ring 1,3‐betaines (no uncharged resonance structure can be drawn) with one nitrogen atom aside the electron‐deficient carbon atom and, as in the case of 5 , enjoying stabilization via the participating resonance contributors 5 ′ (Scheme ) . Both −I and +M substituent effects of the adjacent N atom, obviously, are of dominating influence on the low field position of the δ( 13 C)/ppm values of 5 and 6 compared with 4 ; stabilization via π‐electron delocalization should be similar in 4 to 6 …”

“…Employing the ring current/anisotropy effect model (spatial magnetic properties—through space NMR shieldings [TSNMRS]), well known from proton NMR spectroscopy, the solely existence of NMe 2 ‐disubstituted allene carbene 9 as 1,4‐betaine structure 9 ′ could be proved unequicocally; in the absence of the dimethylamino groups ( 10 ), the carbene structure ( 10 ′) could be verified (Scheme ) …”

NMR spectroscopic conformational analysis of 4‐methylene‐cyclohexyl pivalate—The effect of sp2 hybridization

“…σ* C1−O7 ). [4] There is one earlier manuscript dealing with 1,4transannular effects in 4-substituted cyclohexanol on the present conformational equilibrium (determined by line width measurements of the R 2 C(OH)H proton signal in nuclear magnetic resonance (NMR) spectra at room temperature); [5] the axial conformation of the OH substituent increases with the electron attracting ability of C-4 substitution. [5] Steric destabilization of the axial conformer with increasing 1,3-diaxial interactions has been observed in various aliphatic cyclohexyl esters (−O−COAlk), [6] and the effect of hyperconjugation in 1,4-disubstituted cyclohexanes due to the polar effect of the second −OR substituent in 4position proved to be 0.5-0.7 kcal/mol (−OR= −OMe, −OEt, −Oiso-Pr, −Otert-Bu) [6] and 0.3-0.5kcal/mol (−OR =−OCOMe, −OCOEt, −OCOiso-Pr, −OCOtert-Bu), [3] respectively, preferring by this energy amount the axial conformation of the −OR substituent at cyclohexyl.…”

“…Figure 2). 4 Compared (cf. Scheme 2) with the corresponding cyclohexyl derivatives 2 (−ΔG°= 0.39 kcal/mol), the content of the axial conformer 1ax proves to be higher (−ΔG°= 0.02 kcal/ mol), however, compared with the corresponding cyclohexanone ester 4, on the other hand, 4ax is still more preferred (−ΔG°= −0.46 kcal/mol and this by about the same amount).…”