Dynamic NMR studies of a potential chiroptical switch based on dithiocarbamate-iminodithiolane interconversion

Article abstract of DOI:10.1021/ol016671z

Equation presented Variable temperature NMR spectra of the chiral spiro[(4-N,N-dimethyldithiocarbamato)-(2-N,N-dimethylimino)-1,3-dithiolane-5, 9′-xanthene] show complex dynamics including degenerate interconversion of the dithiocarbamate and iminodithiolane groups. The rate of this switching process can be controlled by chemical modification: the analogous spiro[dithiolane-fluorene] derivative shows no interconversion. These novel materials have potential application as molecular switching elements in information storage devices.

Full text of DOI:10.1021/ol016671z

ORGANIC
LETTERS
2
001
Vol. 3, No. 21
413-3416
Dynamic NMR Studies of a Potential
Chiroptical Switch Based on
Dithiocarbamate−Iminodithiolane
Interconversion
3
†
†
,†
‡
Lauren B. Aubin, Tracy M. Wagner, John D. Thoburn,* Brenda S. Kesler,
§
§
,§
Katherine A. Hutchison, Robert R. Schumaker, and James P. Parakka*
Departments of Chemistry, Santa Clara UniVersity, Santa Clara, California 95053 and
San Jose State UniVersity, San Jose, California 95192, and California Molecular
Electronics Corp., 50 Airport Parkway, San Jose, California 95110
jthoburn@scu.edu
Received August 30, 2001
ABSTRACT
Variable temperature NMR spectra of the chiral spiro[(4-N,N-dimethyldithiocarbamato)-(2-N,N-dimethylimino)-1,3-dithiolane-5,9′-xanthene] show
complex dynamics including degenerate interconversion of the dithiocarbamate and iminodithiolane groups. The rate of this switching process
can be controlled by chemical modification: the analogous spiro[dithiolane-fluorene] derivative shows no interconversion. These novel materials
have potential application as molecular switching elements in information storage devices.
Development of materials for high-density information
storage and for miniaturization of switching devices is one
of the major challenges in science and engineering. Organic
of nondestructive read-out, since the electronic absorption
band that is used for detection is often identical to that used
to promote isomerization. One approach to overcome this
difficulty uses chiral compounds whose configuration inverts
1
molecules are particularly attractive targets for these new
materials. A molecular switch must exist in two isomeric
forms corresponding to binary 0 and 1, show thermal
irreversibility so that stored information is not lost at room
temperature, have some nonthermal mechanism to drive the
interconversion, be fatigue-resistant, and have a mechanism
2
simultaneously with the photochromic isomerization. The
interconversion of isomeric states can be driven at one
wavelength, λwrite, and the chirality read at a different probe
wavelength, λread, using optical rotation, thereby avoiding
destructive read-out. The photochromic modulation of known
switches typically relies on cis-trans isomerization, photo-
cyclization, or helix reversal processes. We report here a new
molecular switch concept based on the degenerate valence
2
whereby the state can be read nondestructively. Among the
many organic compounds that possess photoreversible bi-
stability and high thermal barriers, only a few are capable
bond isomerization in the chiral 4-N,N-dimethyldithiocar-
†
bamato-2-N,N-dimethylimino-1,3-dithiolane cation (1):3
Santa Clara University.
San Jose State University.
California Molecular Electronics Corporation.
‡
§
(1) For a recent review, see special thematic issue: Photochromism:
Memories and Switches. Irie, M., Ed. Chem. ReV. 2000, 100, 1683-1890.
PhotoreactiVe Materials for Ultrahigh-Density Optical Memory; Irie, M.
Ed.; Elsevier: Amsterdam, 1994.
(2) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M.
Chem. ReV. 2000, 100, 1789.
1
0.1021/ol016671z CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/22/2001

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

indicating that the parent compound 1 does not undergo
switching of the iminodithiolane and dithiocarbamate. Al-
though 1 appears to have the essential high thermal barrier,
it lacks the chromophore necessary for light-induced switch-
ing. We therefore undertook studies on model compounds 2
and 3.
The complex pathways by which the labeled nuclei can
exchange between the two enantiomers are shown in Figure
3. Line shape analysis indicates that the apparent line
The dynamics of Chiropticene 2 are significantly more
1
complex, as indicated by its variable temperature H NMR
6 5
spectra in C D Br (Figure 2). At room temperature four
Figure 3. Mechanistic pathways that result in scrambling of labels
Figure 2. 1H (400 MHz) variable temperature NMR spectra and
associated rate constants for 2 in C D Br. Asterisk (*) denotes
6 5
possible decomposition product.
(
R, â, γ, and δ) to all four chemical environments. kr1 ) rate
constant for rotation about dithiocarbamate C-N bond, kr2
rotation about iminodithiolane CdN bond, k ) switching.
)
s
inequivalent methyl groups are seen. The two upfield signals
broaden, coalesce at 97 °C, and then begin to resharpen,
corresponding to increased rate of rotation of the dithiocar-
bamate. Likewise, the two downfield peaks undergo a similar
transformation with coalescence at about 87 °C, and this
corresponds to exchange of the iminodithiolane methyls. At
temperatures above 97 °C these two peaks broaden, coalesce,
and begin to resharpen. This final process is the enantiomeric
switching that exchanges the iminodithiolane and dithiocar-
bamate methyls. Rate constant for dithiocarbamate C-N
rotation and enantiomeric switching are given in Figure 2,
broadening and coalescence seen for the iminium methyls
is not due to the direct CdN rotation but rather to an
indirect mechanism. Enantiomeric switching, followed by
s s
dithiocarbamate rotation, followed by reswitching (k , kr1, k
of Figure 3) results in an exchange that is identical to direct
rotation about the iminium CdN (kr2 of Figure 3). While
the coalescence of all four peaks to a single peak at high
temperature is clear evidence for enantiomeric switching, the
switching is also evident at much lower temperatures in the
coalescence of the iminium methyls. This complexity
provides a handle on the switching rate at lower temperatures.
1
1
q
corresponding to ∆G (25 °C) ) 17.3 ( 0.3 and 17.9 ( 0.3
q
kcal/mol, respectively.10
(10) Similar barriers were seen in anisole-d8 solvent (25 °C): ∆Gr1
)
q
1
7.4 ( 0.3, ∆Gs ) 18.4 ( 0.5.
(
11) Line shape analysis was done with kr2 as a varied parameter.
(
9) It was verified from other peaks that the symmetric and asymmetric
However, at low temperatures kr2 showed essentially no temperature
dependence, whereas at high temperature the line shape is virtually
unaffected by kr2 since the iminium line shape is dominated by the indirect
mechanism.
broadening seen at higher temperatures was not due to any exchange process
but rather field inhomogeneity resulting from difficulty in obtaining a good
shim at these temperatures.
Org. Lett., Vol. 3, No. 21, 2001
3415

carbamate functional groups can act as a switch. However,
the thermal barrier is too low. Even at room temperature
switching is fast enough to cause rapid racemization of
enantiomers. Given the resonance stabilization and the
N
tertiary nature of the inverting carbon, an S 1 mechanism
for the switching process is likely. This mechanism is
supported by the lack of switching in 1, where a carbocation
intermediate would be disfavored because it is primary and
lacks resonance stabilization. An S
for 1, but no switching is seen. If the mechanism is S
, then the rate of switching ought to be substantially slowed
N
2 mechanism is plausible
N
1 for
2
with a fluorenyl substituent (3) since the intermediate would
be an anti-aromatic 9-fluorenyl carbocation. The room
1
temperature H NMR of 3 in anisole-d
8
(Figure 4) again
shows four distinct peaks in the methyl region. As temper-
ature is increased the two upfield peaks from the N,N-
9
dimethylcarbamate broaden, coalesce, and resharpen. Sig-
nificantly, no other exchange broadening is seen, eVen in
the iminium methyls, which would show switching at lower
temperatures. Thus fluorenyl substitution stops the switching
process. Line shape analysis gave the rates listed in Figure
q
4
and ∆G ) 17.5 ( 0.1 kcal/mol at 25 °C for internal
rotation in the dithiocarbamate.
We have presented a general synthetic route for the
preparation of Chiropticenes, whose switching is based on
the degenerate intramolecular exchange of an iminodithiolane
and a dithiocarbamate. Using molecular engineering we have
shown that substituents at the 5 position influence the rate
of switching by stabilizing the incipient prochiral cation. The
electronic absorption maxima for 2 and 3 lie in the ultraviolet
(
λ
max(MeOH) ≈ 280 nm). Dithiocarbamates absorb in this
13
region and are known to undergo photofragmentation.
Figure 4. 1H (400 MHz) variable temperature NMR and rate
constants for 3 in anisole-d . Asterisk (*) denotes residual dioxane
from sample preparation that diminishes in intensity as sample is
warmed.
Consequently, for optical activation, a chromophore that
absorbs in the visible region will be required to avoid
degradation resulting from high energy ultraviolet radiation.
Modifications of these model systems employing extensively
conjugated chromophores are in progress and will be reported
in due course.
8
The 18.2 kcal/mol barrier for switching in 2 is slightly
higher than the 16.5 kcal/mol barrier in the previously
reported four-membered ring analogue, 4-dimethyldithio-
Acknowledgment. This work was funded in part by an
NSF REU grant (CHE98-20382) to Santa Clara University
and by NSF SBIR grant (9960048) to California Molecular
Electronics, Inc.
12
carbamato-2-N,N-dimethylimino-1,3-dithietane. The lower
barrier for the latter may be due to release of ring strain in
the dithietane intermediate or due to stabilization of a
carbocation by two adjacent sulfur atoms.
Supporting Information Available: Full characterization
of all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
These initial results were encouraging in that they dem-
onstrate that interconversion of the iminodithiolane-dithio-
(12) Schumaker, R. R.; Inoue, M.; Inoue, M. B.; Bruck, M. A.; Fernando,
OL016671Z
Q. J. Chem. Soc., Chem. Comm. 1991, 719. The 5 kcal/mol barrier reported
for the dithietane should be approximately 16.5 kcal/mol on the basis of
the reported spectra, which show a separation of about 40 Hz and a
coalescence temperature of about 60 °C (kC ≈ 90 s- ).
(13) Kennard, K. C.; Van Allen, J. A. J. Org. Chem. 1959, 24, 470.
Turner, S. R.; Blevins, R. W. Macromolecules 1990, 23, 1856.
1
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Org. Lett., Vol. 3, No. 21, 2001