A. Janiak et al. / Tetrahedron: Asymmetry 27 (2016) 811–814
813
Figure 4. Calculated structure of the lowest energy conformer of 2a and experimental ECD spectrum of 2a in acetonitrile.
In conformers 3.1–3.5 the trityl group forms a propeller,
attached to the rest of the molecule by an axis formed by the
para–N bond. Statistically, the P-type propeller conformers are
3. Conclusion
C
In conclusion, we have determined the structure of the tritylation
product of N-methyl- -methylbenzylamine. In this product the
slightly less populated (41%) compared to their M-type counter-
parts (58%). This is because the trityl central carbon atom is sepa-
rated by six bonds from the molecule’s stereogenic center and
there is weak stereodifferentiating steric crowding around the tri-
tyl group. The differences between conformers of 3, involving angle
u, are more significant. The absolute values of u are between 17°
and 73° and conformers with u < 0 dominate (96%) over conform-
ers having u > 0 (3%). Negative values of u in conformers 3.1 and
a
trityl group is N-substituted at the para rather than at the central
position, a situation not previously encountered in the reaction of
trityl chloride with aliphatic amines. Despite its extended structure,
the trityl derivative 3 shows a significantly strong ECD spectrum in
the region of trityl chromophore absorption spectrum. Although
the trityl group in 3 is dynamically nearly racemized, this compound
displays strong Cotton effects because of phenyl/p-phenylene
groups electronic excited state interaction in conformationally
biased structure. Such a situation is intuitively not obvious and the
result is a caution against the use of empirical models for CD spectra
interpretation, not based on a detailed computational analysis.
3
.4 are associated with the negative values of angles
formers (negative values of and u, population 58%) strongly
dominate over their diastereoisomers with positive values of both
and u angles (3%). Moreover, the structure of the conformer pre-
x. These con-
x
x
sent in the crystal matches very well with the structure of the low-
est energy conformer 3.1 [compare Figs. 1A and 2(3.1)]. ECD
spectra were simulated for the five lowest energy conformers
Supplementary data
3
.1–3.5 and the final spectrum presented in Figure 3 is an average
of all simulated spectra with population weighting.
For comparison, the structure of trityl derivative 2a is typical for
tritylated chiral amines. According to calculations, the dominating
conformer (92% population), shown in Figure 4, has a more com-
pact structure, the environment of the nitrogen atom is tetrahedral
References
1.
[
(S)-configuration] and the trityl group displays only very
approximate C symmetry of PPP helicity (angles are 19, 27,
7°). Other conformers of 2a are of MMP (6%) and MMM (2%)
2
3
.
.
3
x
Experimental data for 3: mp 99–102 °C; [
(CDCl ): d = 7.23 (m, 15H), 6.97 (d, J = 8.5 Hz, 2H), 6.75 (d, J = 8.8 Hz, 2H), 5.46 (s,
H), 5.08 (q, J = 6.9 Hz, 1H), 2.65 (s, 3H), 1.52 (d, J = 6.9 Hz, 3H); C NMR
CDCl ): d = 148.53, 144.67, 142.89, 131.84, 130.05, 129.38, 128.31, 128.14,
26.87, 126.75, 125.99, 112.76, 56.51, 55.92, 31.82, 16.28.
a] = +180 (c 0.15, CH Cl
23
); 1H NMR
D
2
2
7
3
13
1
helicity. These conformers are much less populated and do not
significantly contribute to the ECD spectrum of 2a.
(
3
1