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Notes and references
‡ Aziridine 1 has been reported previously by Hallas and Choi who
indicated that it is best prepared by base induced ring closure to form
the aziridine as the final step.16 This approach was adopted herein
although considerable refinement of the synthetic route was required to
obtain 1 in a satisfactory state of purity. Full details are provided in ESI.†
§ Lowering of the photoirradiation temperature to ꢀ20 1C led to no
improvement in the ratio of 1 : 2. Below 10 1C, the cis isomer was
observed to be thermally stable, so that during the VT-NMR experi-
ments (see Fig. 1), the ratio of 1 to 2 did not change.
¶ Aziridine 1 is stable provided it was kept away from sources of acid.
8 In aziridines 1 and 2, a complex AA0BB0 spin system is predicted.17
Fortuitously, the aziridine signals were observed as pairs of singlets at low
temperature, coalescing to one singlet on warming. As such they could be
analysed as A2B2 spin systems, simplifying the line shape analysis.
Fig. 2 Ground state and transition state structures for 1 and 2 optimised at the
1 For reviews, see: (a) J. J. Zhang, Q. Zou and H. Tian, Adv. Mater.,
2013, 25, 378; (b) A. A. Beharry and G. A. Woolley, Chem. Soc. Rev.,
2011, 40, 4422; (c) S. Silvi, M. Venturi and A. Credi, Chem. Commun.,
MP2/aug-cc-pVDZ level.
´
2011, 47, 2483; (d) D. Bleger, Z. Yu and S. Hecht, Chem. Commun.,
2011, 47, 12260; (e) E. R Kay, D. A. Leigh and F. Zerbetto, Angew.
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2 For a review, see E. Merino and M. Ribagorda, Beilstein J. Org. Chem.,
2012, 8, 1071.
without implicit solvent. The optimised structures are pre-
sented in Fig. 2, with the numerical values given in Table 1.
Although the absolute values derived from these MP2/aug-cc-
pVDZ level calculations using toluene as implicit solvent over-
estimate the barriers for both 1 and 2, they successfully predict
a higher barrier for 2, and compute a difference (DDG‡) of
3.6 kJ molꢀ1 between the cis and trans isomers that agrees
closely with the experimental values (vide supra). Significant
distortion away from planarity as evidenced in the minima and
TS for 2 disrupts conjugation of the aziridine lone pair into the
extended p-system, leading to a higher barrier. This can be
quantified using a natural bond orbital analysis which revealed
not only the greater extent of delocalisation from the nitrogen
into the ring for 1 compared with 2, but also a greater change
in this delocalisation in moving from the GS - TS for 1
(46.3 kJ molꢀ1) compared with 2 (44.5 kJ molꢀ1) (see ESI†).
These studies demonstrate how it is possible to modify the
atomic pyramidal inversion rates in an aziridine using only the
physical inputs of light and heat. Switching 1 - 2 is success-
fully achieved photochemically, with the reverse reaction facili-
tated with simple heating. Careful choice of solvent and
conditions are required to achieve acceptable conversion during
the photoreaction. Experimental and computed differences in
the activation barrier (DDG‡) for the pyramidal motion for 1 and
2 are of the order of 3 kJ molꢀ1, with cis-2 inverting more slowly
due to a twist in the azo system which reduces the extent of
conjugation of the aziridine lone pair into the p-system. The
process proceeds without degradation and can be cycled several
times. As such, it represents the first example of the use of light
to control the rate of pyramidal atomic inversion and it also
illustrates how dynamic molecular motion may be principally
controlled by changes in bond conjugation rather than sterics.5
Future studies will focus on increasing the magnitude of the
rates differences between the two states, and the development of
molecular devices that exploit this phenomena.
3 For recent examples of photochemical control over binding or
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5 (a) A. S. Lubbe, N. Ruangsupapichat, G. Caroli and B. L. Feringa,
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6 W. B. Jennings and D. R. Boyd, in Cyclic Organonitrogen Stereodynamics,
ed. J. B. Lambert and Y. Takechi, VCH, Cambridge, UK, 1992, pp. 105–158.
7 The data analysis is simplified by the fact that 1a = 1b and 2a = 2b.
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9 M. W. Davies, M. Shipman, J. H. R. Tucker and T. R. Walsh, J. Am.
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10 J. B. Lambert, Top. Stereochem., 1971, 6, 19.
11 H. M. D. Bandara and S. C. Burdette, Chem. Soc. Rev., 2012, 41, 1809.
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15 H. J. Reich, J. Chem. Educ.: Software, Ser. D, 1996, 3, 2.
16 G. Hallas and J.-H. Choi, Dyes Pigm., 1999, 40, 99.
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We acknowledge EPSRC (EP/F021054/1 and EP/F021275/1)
and the University of Warwick for financial support. The
computing facilities of VPAC, Australia, are gratefully acknowl-
edged. TRW thanks VESKI for an Innovation Fellowship.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.