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

Abstract

Both the 13 C chemical shift and the calculated anisotropy effect (spatial magnetic properties) of the electron-deficient centre of stable, crystalline, and structurally characterized carbenes have been employed to unequivocally characterize potential resonance contributors to the present mesomerism (carbene, ylide, betaine, and zwitter ion) and to determine quantitatively the electron deficiency of the corresponding carbene carbon atom. Prior to that, both structures and 13 C chemical shifts were calculated and compared with the experimental δ( 13 C)/ppm values and geometry parameters (as a quality criterion for obtained structures). K E Y W O R D S13 C chemical shift, carbenes/zwitterions, carbene electron deficiency, nucleus-independent chemical shifts (NICS), through space NMR shieldings (TSNMRS)

Quantification of σ-Acceptor and π-Donor Stabilization in O, S and Hal Analogues of N-Heterocyclic Carbenes (NHCs) on the Magnetic Criterion

“…However, the σ-acceptor/π-donor ylide stabilization of nitrogen remains extraordinary, dominating, and sufficient to stabilize the ylide mesomeric contributor of aminocarbenes. If the second substituent at the carbene center is diversified [R 2 N–C:–R 2 (R 2 = H ( 23a ), Me ( 23b ), CF 3 ( 23c ), OMe ( 18a ), SMe ( 22a ), F ( 23d ), Cl ( 23e ), Br ( 23f ), SiMe 3 ( 23g )], both the geometry and 13 C chemical shift variations of the carbene carbon atom [δ­( 13 C) = 284.1 ppm to 436.1 ppm] (Table ) are in between the 13 C chemical shift range of ylide-stabilized carbenes. ,, The spatial magnetic properties (TSNMRS) of 23 orchestrate this result. The anisotropic effect of the carbene carbon atom changes only insignificantly (Table ), and the amino substituent in all these ylides coins the spatial magnetic properties of 23 ; the additional substituent proves to be of minor influence.…”

“…To start with the geometry and carbene 13 C chemical shifts of hydroxy- and methoxycarbenes ( 12–14 ) in Table , the C, O bond lengths are clearly in between the ranges of C–O single and CO double bonds and the 13 C chemical shifts of the carbene electron-deficient center are, simultaneously, within the 13 C chemical shift range of ylide-stabilized carbenes . Both the geometry and magnetic parameters prove the ylide mesomeric contributor, for example, 12a of hydroxycarbene, to be the dominating electronic structure of oxycarbenes 12–14 (Scheme ).…”

“…This change in anisotropy effect of the electron-deficient center remains constant in 13­( tc ) and methoxycarbenes 14­( tt ) and 14­( tc ) (see Figure S1 in the Supporting Information). Actually, the pure carbene electronic structure of methylene carbene 11 is also distinctly indicated by the 13 C chemical shift of the carbene carbon atom in 11 [δ­( 13 C) = 1445.4 ppm], extremely outside the 13 C chemical shift range of ylide-stabilized carbenes …”

“…Sulfur, compared with oxygen and nitrogen, adjacent to the carbene function, still stabilizes the lone pair via π-donation, but the σ-acceptor property is lost and changed to only a minor σ-donor. The 13 C chemical shifts of the electron-deficient center in 15 and 16 are with 376.5 and 450.5 ppm, respectively, at the low field boarder of ylide-stabilized carbene 13 C chemical shifts; so low field shifts of stable carbenes have never been measured. Actually, the δ­( 13 C)/ppm values of dithiocarbenes cannot be compared directly with values in the former O -, O , O - ( vide supra ), N -, and N , N -carbenes due to the heavy atom effect on the 13 C chemical shift; however (the effect as a whole could not be that much), the δ­( 13 C)/ppm position of 15 and 16 remains extraordinary.…”

“…In the case of the flexible oxyamino- and thiaminocarbenes 18c and 22a , there is clear NMR-spectroscopic evidence of slow rotation about the partial C,N, C,O, and C,S double bonds . The 13 C chemical shifts of the electron-deficient carbon atom in 18c–22 are positioned at the low field end of the ylide-stabilized carbene carbon chemical shift range, and the spatial magnetic properties (Figure ) are comparable among themselves ( 18c , 19 , 21b , and 22a ). In the aromatic oxy- and thioaminocarbenes 20a and 21a (Supporting Information), the anisotropic effect of the carbene carbon atom is covered by the ring current effect of the conjugated 5-membered ring systems.…”