Iminoaziridine Synthesis, Configuration, and 15N NMR
induced shifts (ASISs):33 asymmetric solvation of the imine
moiety by benzene shifts resonances of protons that reside at
the side of the imino alkyl group stronger to high field than
protons at the side of the lone pair. Configurational assignments
for other iminoaziridines rest on analogy with 1a, 1g, 16, and
ASIS experiments, or lack any support.12 Homoallylic coupling
and ASISs are useful stereochemical indicators for imines,
imidates, and thioimidates,32,33 and their applicability to imi-
noaziridines seems reasonable, but cannot be taken for granted,
however. Obviously, these criteria cannot be applied when only
a single diastereomer is observable.
The 1H NMR signals were assigned with the help of 1H-1H
decoupling experiments and one-bond and long-range 13C-1H
shift correlations. 1D NOESY experiments were performed after
the relaxation times T1 had been determined. The 1D NOESY
experiment using 1g was carried out at a temperature of -60
°C, because, at room temperature, irradiation with the resonance
frequency of the N-methyl group enhanced the 3-H signal only
negligibly although the observed NOE effect for the tert-butyl
group at C-3 was indicative of the E-configuration. All 1D
NOESY results confirmed that the predominant diastereomers
do possess the E-configuration in accordance with the homoal-
lylic coupling and ASIS criteria. In addition, the signal enhance-
ments that were observed for the minor diastereomer of 4e
proved its Z-configuration. It is gratifying that these results lend
credence to the experimentally very simple relative ASIS
method. Therefore, only this method was used for the assign-
ment of E and Z to the diastereomers of exo- and endo-6.
A literature search uncovered an unexpected dearth of data
that parallel or may explain the unusual shift differences ∆sa in
Table 3.32a,34,37,39 An early 13C NMR and ASIS study of R/â-
unsaturated N,N-disubstituted N′-phenylamidines reported small
deshielding γ-effects (ca. 1 ppm) of the phenyl group.40 13C
NMR spectra of ethylidenecyclopropanes suggest that γ-effects
in these three-membered ring compounds may become deshield-
ing. Not only are the shielding γ-effects caused by the methyl
group of ethylidenecyclopropanes much smaller than in com-
parable five-membered ring compounds, they even approach
zero with increasing substitution of the ring carbons.41 Indeed,
the methyl groups of the (E)- and (Z)-ethylidenecyclopropanes
that bear a hydroxymethyl and a tert-butyl group in trans-
position at the ring fail to cause γ-effects; i.e., the value of ∆sa
is zero for both ring carbons.42 It is in line with this admittedly
scant data that the N-tert-butyl group in 1f and the N-phenyl
ring in 4n give rise to the by far strongest deshielding effects.
15N NMR Spectra of Iminoaziridines. So far, only 14N NMR
data of a single iminoaziridine12 but no 15N NMR spectra have
been reported. The results of the first 15N NMR study of
iminoaziridines are listed in Table 4. Amino nitrogens N(sp3)
resonate by 100 ppm at higher field than double-bonded nitrogen
atoms N(sp2).43 This allows the assignment of the two observed
signals to the exocyclic (dNR2) and ring nitrogens (>NR1).
The ranges of chemical shifts span more than 60 ppm and are
found where they are to be expected by comparison with the
few data reported for amidines.44,45
High-field shifts of 13C NMR signals by atoms in the
γ-position are extremely valuable and diagnostically the most
useful of all substituent effects in 13C NMR spectroscopy.34 In
13C NMR spectra recorded for N-alkylketimines, Fraser et al.
observed very pronounced upfield shifts of -6 to -12 ppm for
R-carbons syn to the N-alkyl groups in comparison with the
same carbon when anti.35 These differences of chemical shifts
are hereafter referred to as ∆sa ) δsyn - δanti. Jackman and Jen
reported upfield shifts of ∆sa ) -2.0 and -3.9 ppm for E/Z
diastereomeric amidines, viz., 2-[N-(2,6-dichlorophenyl)imino]-
pyrrolidine and -piperidine, respectively.36 Similar observations
in other classes of CdN compounds37 nurtured hope that shift
differences ∆sa in E/Z pairs of iminoaziridines would support
the stereochemical assignments based on homoallylic coupling
and ASISs. However, scrutiny of the 13C NMR spectra recorded
for such pairs frustrated this expectation as shown by the ∆sa
values in Table 3, which indicate downfield shifts for the syn-
carbons compared to the same carbons in anti-position to the
group at the imino nitrogen.
The values of ∆sa span a range of 8 ppm and show that the
γ-effects of the groups at the imino nitrogens are deshielding
in almost all cases provided that the configurations have been
assigned correctly. However, if this condition is not met, the
γ-effects would be normal and shielding as expected by analogy
with imines,35 amidines,36 and other CdN compounds.34,37 In
view of this disturbing situation and the recent revival of interest
in syn-anti isomerizations of imines,38 we embarked on an NOE
NMR study, which also allows unequivocal stereochemical
assignment when only a single diastereomer is observable. We
included the known iminoaziridines 1f, 1g,4 4e,11 4h,4 and 5.13
Resonance frequencies of 15N NMR signals are shifted
downfield by carbons in the â-position. These â-effects are
particularly large for both types of nitrogen when an N-methyl
group is exchanged for an N-isopropyl group; cf. (E)-1c vs (E)-
1f, ∆δ(NR1) ) 28, and (E)-4c vs (E)-4g, ∆δ(NR2) ) 49 ppm.
In contrast, replacement of an N-isopropyl with an N-tert-butyl
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