1
This structure has both an optically active center and a
substantial dipole moment. Substitution of suitable chromo-
phores at the 5 position of 1 is expected to yield light-
activated switching between R and S states via the prochiral
intermediate or transition state. The preferential existence
of one or the other enantiomeric states will be controlled by
the presence of an external electric field. Thus the influence
of both light and directional field on these molecules, which
we term Chiropticenes, will result in reversal of both the
At room temperature the 400 MHz H NMR spectrum of
1 in anisole-d
shows four unique methyl peaks (Figure 1).6
8
4
chirality as well as the dipole orientation. One of the key
requirements of molecular switches is a high thermal barrier
to interconversion between the two distinct states. This can
be investigated by means of variable temperature NMR
studies. We report in this paper the thermal stability,
stereoisomerization, and chemical control of switching for
model compound 1 and its xanthenyl and fluorenyl deriva-
tives, 2 and 3.
Compound 1 was synthesized in one step from 1,1,2-
tribromoethane and sodium dimethyldithiocarbamate fol-
3
lowed by anion exchange with KPF
6
. Compounds 2 and 3
Figure 1. 1H NMR (400 MHz) spectra for N-methyl region of 1
in anisole-d showing the exchange of the dithiocarbamate N-
8
methyls.
were synthesized by the general protocol outlined above. For
example, lithiation of the known methylene(bisdimethyl-
dithiocarbamate) 4 followed by quenching with xanthone
As temperature is raised the two dithiocarbamate methyl
peaks broaden, coalesce near 108 °C, and then begin to
resharpen. An Eyring plot of the rate constants derived from
5
yielded tertiary alcohol 5. Subsequent dehydration and
7
q
cyclization promoted by strong acids (HCl, HClO
or camphorsulfonic acid) followed by anion exchange using
KPF gave 2. Similarly, the fluorenyl analogue 3 was
prepared employing 9-fluorenone in the first step and HClO
in the cyclization sequence.
4
, pTSA,
line shape analysis gave a ∆G ) 17.7 ( 0.2 kcal/mol at
25 °C. This is slightly higher than 15 kcal/mol rotational
8
6
barriers seen in other alkylated dithiocarbamates. Signifi-
cantly, no other exchange is seen among the methyl groups,9
4
(
6) Peak assignments were made based on the relative rates of exchange.
(
3) Schumaker, R. R. Optoelectronic Tautomeric Compositions. U.S.
Patent 5,237,067, August 17, 1993.
4) (a) Hutchison, K. A.; Parakka, J. P.; Kesler, B. S.; Schumaker, R. R.
E & Z peak assignments alkyl dithiocarbamates are the reverse of those in
amides: Dahl, B. M.; Nielsen, P. H. Acta Chem. Scand., Ser. B 1974, B28,
1091.
(
Chiropticenes: Molecular Chiroptical Switches for Optical Data Storage.
In Micro- and Nano-photonic Materials and DeVices; Perry, J. W., Scherer,
A., Eds.; Proc. SPIE-Int. Soc. Opt. Eng. 3937, 2000, 64-71. (b) Parakka,
J. P.; Schumaker, R. R.; Kesler, B. S.; Thoburn, J. D. Molecular-Based
Chiroptical Dipole Switches. In Optical Engineering for Sensing and
Nanotechnology; Iwata, K., Ed.; Proc. SPIE-Int. Soc. Opt. Eng. 4416, 2001,
(7) Rate constants were obtained by piping calculated spectra from
DNMR5 and the experimental spectra into GNUPOT3.4 for interactive
iteration of parameters until the best fit was obtained. Stephenson, D. S.;
Binsch, G. DNMR5: IteratiVe Nuclear Magnetic Resonance Program for
Unsaturated Exchange-Broadened Band shape, QCPE. 569. Binsch, G. J.
Am. Chem. Soc. 1969, 91, 1304.
3
01-304.
(8) Holloway, C. E.; Gitlitz, M. H. Can. J. Chem. 1967, 45, 2659. Lemire,
A. E.; Thompson, J. C. Can. J. Chem. 1975, 53, 3732.
(
5) Nakai, T.; Okawara, M. Chem. Lett. 1974, 731.
3414
Org. Lett., Vol. 3, No. 21, 2001