Anomeric effect in N-azidomethylpyrrolidine: gas-phase electron diffraction and theoretical study

Abstract

Gas-phase electron-diffraction data and high-level quantum chemical calculations have been used to study the conformational behaviour of N-azidomethylpyrrolidine. The two most stable conformers with a relative abundance of about 80% at 298 K possess gauche orientation of the azidomethyl group around the C-N(pyr) bond (C-N(azido)gauche with respect to the endocyclic N(pyr)-C bond). This orientation is a strong manifestation of an anomeric effect. The influence of the anomeric effect is also reflected in shortening of the C-N(pyr) bond and lengthening of the C-N(azido) bond as compared to such bonds in other compounds.

The structure of tris(chloromethyl)amine in the gas phase using quantum chemical calculations and gas electron diffraction and as a solid and melt using Raman spectroscopy

“…[ [18][19][20][21][22][23][24][25] made using GED regarding the anomeric effect in N(CH3)2(CH2F) and the theoretical work [26][27][28][29][30] in the literature regarding the anomeric effect in NH2(CH2F). A classic 'no-bond-double-bond' [18,19] resonance scheme, invoked to describe the geometric consequences of the anomeric effect in species such as N(CH3)2(CH2F) (measurably shortened and lengthened rN-C and rC-F internuclear distances, respectively, and an increase in the aN-C-F angle [18,19]) can be adapted to give the immonium-like resonance structures of Scheme 1 for A, similar to that given by Klapötke et al [1] give for the single crystal.…”

“…A classic 'no-bond-double-bond' [18,19] resonance scheme, invoked to describe the geometric consequences of the anomeric effect in species such as N(CH3)2(CH2F) (measurably shortened and lengthened rN-C and rC-F internuclear distances, respectively, and an increase in the aN-C-F angle [18,19]) can be adapted to give the immonium-like resonance structures of Scheme 1 for A, similar to that given by Klapötke et al [1] give for the single crystal. The anomeric effect in a range of fluorinated tertiary amines studied by Oberhammer et al [18][19][20][21][22][23][24][25] stabilizes the antiperiplanar configuration of the C-F bond with respect to the lone pair on the amine centre relative to the synperiplanar configuration, the strength of the stabilization conferred being both proportional to the orbital overlap between the C-F * orbital and the lone-pair-containing orbital on the amine centre and inversely proportional to the energy difference between the two orbitals [18]. Oberhammer et al consequently report observing only the antiperiplanar configuration for species such as N(CH3)2(CH2F) and N(CH3)2(CF3) [18,19] in the gas phase.…”

“…The characteristic geometric consequences of the anomeric effect are particularly obvious in the absence of intermolecular interaction in the gas phase. Oberhammer et al [18][19][20][21][22][23][24][25] have published prolifically on the subject and presented a number of theoretical calculations and high-quality structural solutions using GED, notably for N(CH3)2(CH2F) [19], NX2(CF3) (where X is F [20], Cl [21], and Br [22]), N(CH3)(CF3)2 [20], (N(CF3))2 [23,24], noting these geometric consequences in every case, and, more recently, for N-azidomethylpyrrolidine [25], although the strength of the anomeric effect, and consequently the extent to which they manifest, varies case-by-case. This work is summarized succinctly by Oberhammer in Ref.…”

“…One of the strongest anomeric effects has previously been predicted theoretically for NH2(CH2F) [26][27][28][29][30], although it has yet to be observed in the gas phase on account of the instability of the species in question [18]. On paper, TCMA shares structural similarities with tris(2-chloroethyl)amine, N(CH2CH2Cl)3, a powerful military-grade blister agent associated with acute cytotoxicity [31][32][33][34], but on the basis of the work by Oberhammer et al [18][19][20][21][22][23][24][25], the three-dimensional geometry around the amide centre should be anticipated to be quite different. Indeed, the structural solution in the gas phase from GED may be anticipated to be quite different to that obtained for the single crystal using XRD, as work by Mitzel et al [35][36][37][38][39] has demonstrated.…”