Computing handedness: Quantized and superposed switch and dynamic memory of helical polysilylene

Article abstract of DOI:10.1021/ja0026509

Two new conjugating helical polymers comprising a rodlike silicon backbone and enantiopure chiral pendants, poly{(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene} (PS-1) and its diastereomeric poly{(S)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene} (PS-2), were prepared. Molecular mechanics calculations of PS-1 and PS-2 model oligomers indicated a double well potential energy curve corresponding to almost enantiomeric helices with dihedral angles of 150-160° (P-motif, global minimum) and 200-210° (M-motif), regardless of their tacticity. Experimentally, it was found that PS-1 in dilute isooctane revealed switchable ambidextrous helicity on application of a thermal energy bias. Although PS-1 featured three distinct switching regions, viz. "region 1, between -80 and -10 °C", "region 2, between -10 and +10 °C", and "region 3, between +10 °C and +80 °C", the switching properties were interpreted as the result of superposed P- and M-helicities, undergoing dynamic pseudo-racemization or oscillation. Oscillating helicity in region 2 was roughly estimated to be about 13 cm^-1. The superposed helicity in region 2 was critical since it afforded molecular recognition ability with a dynamic memory function that was highly susceptible to solvent molecular topology and volume fraction. This could lead to potential as a molecular information processor to serve as a gauge of chemical properties. On the other hand, PS-2 could not switch its preferential screw-sense in the range of -80 to +80 °C. This may be related to greater differences the potential energy curve between P- and M-motifs.

Full text of DOI:10.1021/ja0026509

J. Am. Chem. Soc. 2001, 123, 6253-6261
6253
Computing Handedness: Quantized and Superposed Switch and
Dynamic Memory of Helical Polysilylene
Michiya Fujiki,*,†,§ Julian R. Koe,†,§ Masao Motonaga,§ Hiroshi Nakashima,†,§
Ken Terao,§ and Akio Teramoto‡,§
Contribution from the NTT Basic Research Laboratories, 3-1 Wakamiya, Morinosato, Atsugi,
Kanagawa 243-0198, Japan, Ritsumeikan UniVersity, Research Organization of Science and Engineering,
1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan, and CREST-JST (Japan Science and Technology
Corporation), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
ReceiVed July 18, 2000
Abstract: Two new conjugating helical polymers comprising a rodlike silicon backbone and enantiopure chiral
pendants, poly{(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene} (PS-1) and its diastereomeric poly{(S)-3,7-
dimethyloctyl-(S)-3-methylpentylsilylene} (PS-2), were prepared. Molecular mechanics calculations of PS-1
and PS-2 model oligomers indicated a double well potential energy curve corresponding to almost enantiomeric
helices with dihedral angles of 150-160° (P-motif, global minimum) and 200-210° (M-motif), regardless of
their tacticity. Experimentally, it was found that PS-1 in dilute isooctane revealed switchable ambidextrous
helicity on application of a thermal energy bias. Although PS-1 featured three distinct switching regions, viz.
“region 1, between -80 and -10 °C”, “region 2, between -10 and +10 °C”, and “region 3, between +10 °C
and +80 °C”, the switching properties were interpreted as the result of superposed P- and M-helicities,
undergoing dynamic pseudo-racemization or oscillation. Oscillating helicity in region 2 was roughly estimated
to be about 13 cm-1. The superposed helicity in region 2 was critical since it afforded molecular recognition
ability with a dynamic memory function that was highly susceptible to solvent molecular topology and volume
fraction. This could lead to potential as a molecular information processor to serve as a gauge of chemical
properties. On the other hand, PS-2 could not switch its preferential screw-sense in the range of -80 to +80
°C. This may be related to greater differences the potential energy curve between P- and M-motifs.
1. Introduction
molecular-based elements remain problematic, due to difficulties
in the wiring of these elements along with “memory” function,
defined threshold, processing, and error correction. However,
understanding and measuring the superposition of bi-stable
quantum states (“a quantum bit” or “qubit”) in nanosystems
based on semiconductors and superconductors are current
theoretical and experimental topics directed toward the develop-
ment of quantum computing.8 To overcome some problems in
conventional digital computing, “quantum computing” using the
controlled mixing of “on-off” states and “0-1” bits may
provide a method to save much processing time and energy.8
Designing and realizing miniature-scale computing devices
are now challenging issues in molecular science and nanotech-
nology.1 A chiral molecular switch and memory with a two-
state “on-off” function, linked with molecular motions such
as twist,2 rotate,3 fold,4 shrink,5 stack,6 and shuttle,7 may be a
candidate as basic elements in such devices. Nevertheless,
† NTT Basic Research Laboratories.
‡ Ritsumeikan University.
§ CREST-JST.
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Feringa, B. L. Nature 1999, 401, 152-154. (b) Feringa, B. L.; Jager, W.
F.; de Lange, B.; Meijer, E. W. J. Am. Chem. Soc. 1991, 113, 5468-5470
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Int. Ed. 1999, 38, 3528-3530. (b) Bedard, T. C.; Moore, J. S. J. Am. Chem.
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10.1021/ja0026509 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/09/2001

6254 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Fujiki et al.
Scheme 1. Chemical Structures of Poly[(R)-3,7-di-
methyloctyl-(S)-3-methylpentylsilylene] (PS-1), Poly-
[(S)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene] (PS-2),
Poly[(S)-3,7-dimethyloctyl-3-methylbutylsilylene] (PS-3),
and Poly[(S)-3,7-dimethyloctyl-2-methylpropylsilylene]
(PS-4)
Before these boom studies, although several quantum chemists9
had already discussed quantum tunneling and oscillation, and
the preparation and detection of superposed optical activity for
a hypothetical chiral molecule with a double-well potential, they
did not infer from their ideas the possibility of quantum
computing based on a chiral molecule.
A stiff helical polysilylene, consisting of stretchable, rotatable
silicon-silicon single bonds in a helical backbone bearing
enantiopure chiral pendants10,11 may be a potential candidate
chiral molecule with which to test the idea of the superposition
of P (plus, right-handed) and M (minus, left-handed) states and
to construct a molecular processor susceptible to external
physical and chemical stimuli. This is because the helical
polysilylene has (a) essentially two conformational P- and
M-states, but may show marked cooperative conformational
change in response to external stimuli,11a,b similar to other stiff
helical polymers,13 (b) a unique, intense CD signal with
matching near-UV absorption with narrow bandwidth of 4-8
nm, characteristic of its “one-dimensional exciton with discrete
energy levels”,10 which allows the quantitative analysis of the
chiroptical switching states and population of P- and M-
states10,11 using the physically clearer Kuhn’s dissymmetry ratio
{gabs ) ∆ꢀ/ꢀ ) 2(ꢀL - ꢀR)/(ꢀL + ꢀR)}, rather than ∆ꢀ and
specific optical rotation ([R]), whose ∆ꢀ and ꢀ are the CD and
UV peak intensities, and ꢀL and ꢀR are the absorption intensities
for left and right circular polarized light,13 respectively, and (c)
an almost insulating semiconducting state which may be
rendered highly conducting by injection of electron or hole
carriers into the backbone.10e
ylpentylsilylene} (PS-2, Scheme 1), were prepared according
to the standard Wurtz procedure described in the literature.10d,11a
The former features opposite handedness in the γ-positions of
different enantiopure chiral pendants, while the latter contains
substituents with the same handedness.
Here we report that the choice of handedness in the two chiral
pendants definitively determines the inversion capability of the
chiroptical properties associated with a preferential screw-
sense: Only PS-1 in dilute solution revealed switchable
ambidextrous helicity on application of a thermal energy bias.
Although PS-1 in dilute isooctane features three distinct
switching regions, viz. “between -80 and -10 °C”, “between
-10 and +10 °C”, and “between +10 °C and +80 °C”, these
switching properties are interpreted as the result of the super-
position of P- and M-states. The superposed helicity between
-10 and +10 °C is of particular importance with respect to
molecular recognition ability and dynamic memory function,
since it is highly susceptible to solvent molecular topology and
volume fraction.
Previous stiff helical polysilylenes with enantiopure â-branched
chiral and long n-alkyl pendants, however, adopt only a single-
handed screw-sense helix (positive CD signal only) and are not
susceptible to any external physical or chemical stimuli, due to
severe restriction of twisting motions in their rigid structures.10
Also, the recently reported helical polysilylenes which undergo
inversion of preferential helicity are inadequate to test the
validity of these ideas, due to their lower inversion temperatures
of -20 °C.11a,b
With these problems in mind, two new stiff helical polysi-
lylenes, poly{(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene}
(PS-1, Scheme 1) and poly{(S)-3,7-dimethyloctyl-(S)-3-meth-
2. Results and Discussion
(9) (a) Cina, J. A.; Harris, R. A. Science 1995, 267, 832-833. (b) Cina,
J. A.; Harris, R. A., J. Chem. Phys. 1994, 100, 2531-2536. (c) Harris, R.
A., Chem. Phys. Lett. 1994, 223, 250-254. (d) Quack, M. Chem. Phys.
Lett. 1986, 132, 147-153. (e) Harris, R. A.; Stodolsky, L. Phys. Lett. 1978,
78B, 313-317. (f) Janoschek, R. In Chirality; Janoschek, R., Ed.;
Springer: Berlin, 1992; Chapter 2. (g) Barron, L. D. In New DeVelopments
in Molecular Chirality; Mezey, P. G., Ed.; Kluwer-Academic: Dordrecht,
The Netherlands, 1990; pp 34-37.
(10) (a) Fujiki, M. J. Am. Chem. Soc. 1994, 116, 6017-6018. (b) Fujiki,
M. Appl. Phys. Lett. 1994, 65, 3251-3253. (c) Ebihara, K.; Koshihara,
S-y.; Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Fujiki, M.
Jpn. J. Appl. Phys. 1997, 36, L1211-L1213. (d) Fujiki, M.; Toyoda, S.;
Yuan, C.-H.; Takigawa, H. Chirality 1998, 10, 667-675. (e) Ichikawa, T.;
Yamada, Y.; Kumagai, J.; Fujiki, M. Chem. Phys. Lett. 1999, 306, 275-
279. (f) Fujiki, M. J. Am. Chem. Soc. 1996, 118, 7424-7425. (g) Toyoda,
S.; Fujiki, M.; Suzuki, H.; Matsumoto, N. Solid State Commun. 1997, 103,
87-89.
2.1. Comparison of Polysilylenes with and without Helix-
Helix Transitions. As shown in Figure 1a, the positive-signed
CD spectrum of PS-1 for high-molecular weight polymer with
an extremum of 320 nm at -60 °C in isooctane is almost the
inverse of the negative-signed CD spectrum with an extremum
of 325 nm at +25 °C. This indicates that the helical conforma-
tions at the two temperatures are almost equivalent, except for
the direction of their preferential helical screw-senses. The
respective CD spectra, with matching UV absorption spectral
profiles in the range of -80 to +80 °C, are consistent with the
partial cancellation of positive and negative CD signals.14 These
are associated with an imbalance in the populations of pseudo-
enantiomeric P- and M-motifs in the same chain with the same
CD extrema.
(11) (a) Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336-3343. (b) Koe,
J. R.; Fujiki, M.; Motonaga, M.; Nakashima, H. Chem. Commun. 2000,
2000, 389-390. (c) Koe, J. R.; Fujiki, M.; Nakashima, H. J. Am. Chem.
Soc. 1999, 121, 9734-9735.
(12) Schilling, F. C.; Lovinger, A. J.; Zeigler, J. M.; Davis, D. D.; Bovey,
F. A. Macromolecules 1989, 22, 3055-3063.
Figure 1b shows that the ∆ꢀ value of PS-1 changes with
lowering temperature from ca. -4.3 at +80 °C, passes through
a broad minimum of -6.5 around +20 °C, increases abruptly
(13) (a) Green, M. M.; Park, J. W.; Sato, T.; Teramoto, A.; Lifson, S.;
Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138-
3154. (b) Teramoto, A. Rep. Prog. Polym. Phys. Jpn. 1998, 41, 25-65. (c)
Khatri, C. A.; Pavlova, Y.; Green, M. M.; Morawetz, H. J. Am. Chem.
Soc. 1997, 119, 6991-6995. (d) Green, M. M.; Khatri, C. A.; Peterson, N.
C. J. Am. Chem. Soc. 1993, 115, 4941-4942. (e) Cheon, K. S.; Selinger,
J. V.; Green, M. M. Angew. Chem., Int. Ed. 2000, 39, 1482-1485.
(14) Dekkers, H. P. J. M. In Circular Dichroism; Nakanishi, K., Berova,
N., Woody, R. W., Eds.; VCH: New York, 1994; Chapter 6. The gabs value
is in general defined as 4‚R/D, where R and D are the rotatory and dipole
strengths, respectively. A chiral polymer comprising a single helical state
at a given condition should have the maximum gabs value due to enantiopure
helicity in analogy to determining the enantio-purity of chiral small
molecules.

Computing Handedness
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6255
Figure 1. For PS-1 in isooctane, (a) CD and UV absorption spectra at -60 °C and +25 °C, (b) variable temperature UV (squares) and CD (circles)
intensities for cooling and heating runs, (c) variable temperature UV λext (squares) and CD λext (circles) wavelengths for cooling and heating runs.
through zero around 2 °C, and finally goes up to +11.8 at -80
°C. The compensation temperature (Tc), at which ∆ꢀ ) 0 (at 2
°C for PS-1 in isooctane), is a result of the mutual cancellation
of positive- and negative-CD signals due to the equal population
of P- and M-motifs with the same photoexcited energy. The
UV peak and CD extremum wavelengths of PS-1 against
temperature, however, are almost linearly blueshifted with
lowering temperatures, as plotted in Figure 1c. The blueshift
may be ascribed to a decrease in screw-pitch, deviating from
an ideal 73-helical structure.10d,11a
Thus, PS-1 can switch dynamically between P- and M-motifs
at these temperatures. The CD signal was found to follow
quickly the change in the solution temperature,15 even in a fast
heating run from -10 to 25 °C in less than 35 s, suggesting the
occurrence of rapid helical inversion near Tc, on a time scale
of less than 1 s.
Differing from PS-1, the positive-signed CD spectrum of
PS-2 with an extremum of 320 nm at -74 °C retains the positive
Cotton effect with an extremum of 328 nm at +80 °C, as shown
in Figure 2a. Also, from Figure 2b, the ∆ꢀ value monotonically
changes from +4.7 at +80 °C to +13.3 at -83 °C, indicating
no inversion of preferential screw-sense in this temperature
range, though the CD spectra are consistent with their corre-
sponding UV absorption profiles in the range of -83 to +80
°C.15 This conclusion is also supported by the UV peak and
CD extremum wavelength data shown in Figure 2c, which
shows monotonic blueshifts for UV and CD with lowering
temperature. However, it is not conclusive if PS-2 undergoes a
chiroptical inversion outside the temperature range investigated,
since a measurement was impossible due to the freezing (-100
°C) and boiling (+100 °C) for isooctane.
°C for PS-1, and from ca. 38 000 at +80 °C to ca. 75 000 at
-83 °C for PS-2. These high ꢀ values are due to their rodlike
conformations of PS-1 and PS-2 over this temperature range,
which have been established by viscometric and atomic force
microscopy measurements.10a-f Indeed an almost linear increase
in the ꢀ value with decrease in temperature is in parallel with a
progressive increase in the radius of gyration or end-to-end
distance of the polysilylene.10d
It is an open question as to why PS-1 undergoes such a steep
chiroptical switching response to change in solution temperature.
The answer may lie in the presence of quantized double well
local free-energy minimum potentials of the dihedral angle,9
leading to the coexistence of P- and M-motifs in the same silicon
chain at all temperatures, even very low and high.
Figure 3, a and b, shows the Si-Si-Si-Si dihedral angle
dependence of the potential energy of (R)-3,7-dimethyloctyl-
(S)-3-methylpentyloligosilylene with 31 repeating units termi-
nated with hydrogen (oligo-PS-1) and (S)-3,7-dimethyloctyl-
(S)-3-methylpentyloligosilylene 31 repeating units terminated
with hydrogen (oligo-PS-2) for their isotactic and syndiotactic
stereoisomers, respectively. For these calculations, standard
parameters with a Si-Si bond length of 2.34 Å and a Si-Si-
Si bond angle of 111°, and the pcff force field (Molecular
Simulation Inc., the Discover 3 module, Ver. 4.00) were used.
Details are given in the Experimental Section.
Both oligo-PS-1 and oligo-PS-2 clearly indicate double-well
potential curves which mean two local energy minima corre-
sponding to almost enantiomeric helices with dihedral angles
of about 150-160° (P-helix) and 200-210° (M-helix), regard-
less of their tacticity. The global minimum P-motif seems to
be more stable than the corresponding M-motif. Since both PS-1
and PS-2 exhibit positive CD signals at lower temperatures,
helical poly(dialkylsilylene)s adopting a preferential P-motif are
assumed to result in positive CD signal, or vice versa. The
relationship between helicity and the CD sign would be
As seen in Figures 1b and 2b, the UV peak intensities of
PS-1 and PS-2 almost linearly increase with lowering temper-
ature, ranging from ca. 40 000 at +80 °C to ca. 80 000 at -80
(15) See Supporting Information.

6256 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Fujiki et al.
Figure 2. For PS-2 in isooctane, (a) CD and UV absorption spectra at -74 °C and +80 °C, (b) variable temperature UV (squares) and CD (circles)
intensities for cooling and heating runs, (c) variable temperature UV λext (squares) and CD λext (circles) wavelengths for cooling and heating runs.
Figure 3. Dihedral angle dependence of the potential energy for PS-1 or PS-2 model molecules with 31 repeating units and hydrogen termini
(oligo-PS-1 or oligo-PS-2). For oligo-PS-1, (a) isotactic (it) sequences and (b) syndiotactic (st) sequences, and for oligo-PS-1, (c) it sequences and
(d) st sequences.
consistent with that of a recently reported poly{(S)-3,7-
dimethyloctyl-3-methyl-butylsilylene} (PS-3, Scheme 1) ex-
hibiting inversion of preferential helicity at -20 °C.11a
In it-oligo-PS-1, the P-motif is more stable than M-motif by
only 0.4 kcal‚(Si-repeat-unit)-1, while, in st-oligo-PS-1, M is
more stable than P by 1.8 kcal‚(Si-repeat-unit)-1 (Figure 3, a
and b). For it-oligo-PS-2, M is more stable than P by 1.5 kcal‚
(Si-repeat-unit)-1, while M is greatly more stable than P by 5.2
kcal‚(Si-repeat-unit)-1 for st-oligo-PS-2 (Figure 3, c and d).The
other important feature may be the ratio of energy barrier heights

Computing Handedness
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6257
4,11a ∆ν1/2 ) 65 Hz at -22.7 ppm. However, the chemical shift
of PS-1, PS-2, and PS-3 are upfield compared to that of PS-4
by ca. 2.5-3.0 ppm. The downfield chemical shift of PS-4 is
considered to originate from elongation of the Si-Si bond length
resulting from the steric demand by fixing hindered â-branched
alkyl side chains.10a This is because the chemical shifts of
flexible rodlike helical PS-1, PS-2, and PS-3 with less sterically
hindered γ-branched alkyl side chains10a,11 are very close to that
of greater flexibile poly(di-n-hexylsilane) in the disordered solid
phase.12 These observations infer that helical polysilylenes able
to switch the preferential screw sense may commonly have ∆ν1/2
of 30-50 Hz at around -25 ppm in 29Si NMR spectrum at
ambient temperature.
2.2. Quantized and Superposed Helicities. The variable
temperature gabs values of PS-1, PS-2, and PS-4 in isooctane
are plotted in the range from -80 to +80 °C (Figure 5a). The
slight temperature dependence of the gabs values even in PS-4
is due to a decrease of screw-pitch, as indicated by the
progressive blueshift of the UV λmax with decrease in
temperature.10d,11a,16 Here we assume that PS-4 includes purely
P-motif over the temperature range studied and use its gabs value,
gmaxP(T), to re-normalize the gabs values for PS-1 and PS-2 at
each temperature such that
Figure 4. 29Si NMR spectra of (bottom, a) PS-1 (low-molecular weight
fraction) and (top, b) PS-2 (low-molecular weight fraction) in CDCl3
at 20 °C.
between two minima at P or M and a hill existing at around
180° of dihedral angle.11a The role of conversion from one motif
to another will be determined by the forward and backward
barrier height energies. The barrier height ratios of it- and st-
oligo-PS-1s are only about 1.6 and 3.6, respectively, whereas
those of it- and st-oligo-PS-2s are fairly large, being about 4.1
and 3.4, respectively. This lower barrier height ratio of it-oligo-
PS-1 may lead to a rapid exchange between P- and M-twists,
and be responsible for switching of the preferential screw-sense
of actual PS-1 without thermal hysteresis.11a
To discuss the screw-sense switching, we try to simply
evaluate the energy parameters for the P- and M-motifs of oligo-
PS-1 and oligo-PS-2, in analogy with oligo-PS-3.11a Here, the
∆G is the difference in the Gibbs free energy between P and M
screw senses, and the enthalpy (∆H) and entropy (∆S) terms
are the differences in enthalpy and entropy between the P and
M turns, respectively. Here, our calculation results of the
oligomer models with sufficiently long repeating units would
be applicable to the real polysilylenes, since these parameters
are normalized as Si repeating unit. Thus we have
P(%) ) 100[1 + gabs(T)/gmaxP(Τ)]/2
M(%) ) 100 - P (%)
(3)
(4)
where P(%) and M(%) are the mol % of the P- and M-motifs,
respectively, and gabs(T) is the observed gabs at T. From the
second-order regression curve of gmaxP(T) for PS-4, we obtain
gmaxP (Τ) ) 2.29-3.22 × 10-3‚T -1.72 × 10-6‚T2 (5)
where T is in °C and gabs is in 10-4 unit.
It should be noted that PS-1 features three key, thermally
accessible regions, 1 (-80 to -10 °C), 2 (-10 to +10 °C),
and 3 (+10 to +80 °C), as indicated in Figure 5b. In region 1,
PS-1 contains a constant 20% P and 80% M (60% M excess),
but contrarily in region 3, it has 80% P and 20% M (60%
P-excess). However, PS-2 invariably contains 20% P and 80%
M (60% M-excess) over the entire temperature range (-80 to
+80 °C). This leads to the idea that the seesaw-like switching
of states between regions 1 and 3 may be quantized as the
stationary state of P- and M-superposition, but not due to 100%
pure P- and M-states. These estimates are derived from the
above assumption about P(%), whose accuracy is needed to
check by other means for a more quantitative discussion.
To explain this unique variable temperature helical state, it
might be necessary to assume that cooperativity in coupled
electronic and conformational transitions is related to the origin
of the steplike switching response to the thermal energy bias.
The effect of the electronic coupling has been theoretically
discussed for π-conjugating polyenes with conformational
defects,17 and the conformational effect has already been
established for electronically nonconjugating stiff helical poly-
isocyanate systems.13
∆G ) GP - GM ) ∆H - T∆S ) HP - HM - T (SP - SM)
(1)
At the inversion temperature Tc, the enthalpy and entropy terms
are just in balance to guide ∆G ) 0.11a,11b Therefore,
Tc ) ∆H/∆S
(2)
On the assumption that the value of ∆H is equal to the above
calculated energy difference between P- and M-motifs and Tc
) 275 K, we obtain ∆S ) -1.5 cal‚(Si-repeat-unit‚Κ)-1 and
∆H/RTc ) -0.74 at 275 K for it-oligo-PS-1 and ∆S ) -6.6
cal‚(Si-repeat-unit‚Κ)-1 and ∆H/RTc ) -3.3 at 275 K for st-
oligo-PS-1. The absolute magnitude of ∆H/RTc for the it is
almost identical to that of PS-3 with 1.0,10 whereas that for the
st is fairly large. Below Tc, ∆G is negative, and the P-motif is
more stable than the M-motif, which is reversed above Tc. The
inversion of ∆G term is caused by T∆S term, which is therefore
responsible for the switching of preferential helical screw-sense.
Noticeable differences among PS-1, PS-2, PS-3, and PS-4
are their signal line widths (∆ν1/2) and chemical shifts in 29Si
NMR in CDCl3 at 20 °C, as displayed in Figure 4. The line
width and chemical shifts are closely related to the degrees of
main chain rigidity and mobility of polysilylenes.10a,11a The
greater degree of flexibility of PS-2 compared to PS-1, PS-3,
and PS-4 is suggested by the narrower ∆ν1/2: for PS-2, ∆ν1/2
) ca. 14 Hz at -25.7 ppm; for PS-1, ∆ν1/2 ) 46 Hz at -25.3
ppm; for PS-3,11a ∆ν1/2 ) 29 Hz at -25.3 ppm; and for PS-
Since the values of ꢀ and λmax around Tc change almost
linearly with the variation of the solution temperature (Figure
1, b and c), it is possible that, in an electronically conjugating,
stiff helical polysilylene with discrete energy levels10g in a
double-well potential, the wave function of the lowest ground-
(16) Teramae, H.; Takeda, K. J. Am. Chem. Soc. 1989, 111, 1281-1285.
(17) (a) Stratt, R. M.; Smithline, S. J. J. Chem. Phys. 1983,79, 3928-
3937. (b) Schweizer, K. S. J. Chem. Phys. 1986, 85, 4181-4193.

6258 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Fujiki et al.
Figure 5. (a) Variable temperature gabs values of PS-1 (high-molecular weight fraction, filled circles), PS-2 (high-molecular weight fraction, open
circles), and PS-4 (filled squares) in isooctane in the range of -80 and +80 °C. (b) P-M fraction of PS-1 (filled circles) and PS-2 (open circles)
as a function of temperature.
state at an M-state electronically communicates with that of a
P-state through quantum mechanical tunneling between the P-
and M-electronic states. Although the situation has already been
discussed theoretically for wave functions of hypothetical chiral
molecules,9 it is necessary to invoke the presence of helix
reversals in the case of helical polymers.13,17 The helix reversal,
probably consisting of several silicon-silicon single bonds
(0.2-1.0 nm in length) is the section in which the screw-sense
changes direction, and might act as a moderately small tunneling
barrier. The reversal may play a crucial role in correcting errors
from thermal noise through dynamic winding and rewinding
motions from one screw-sense to the other. The helix reversal
may exist as very short all-anti, transoid, or TG+TG- sequence.18
For the purpose of presentation, the idea was schematically
illustrated in Supporting Information,15 where a helix reversal
was expressed by a triad “bit -1 bit +1 bit -1” or “bit +1 bit
Figure 6. Potential energy as a function of main chain dihedral angles
-1 bit +1” and a helical sequence by a consecutive bit -1’s
with three switching regions. Below Tc (bottom trace with circles
or bit +1’s. Since the chain conformation is expected to change
calculated at zero Kelvin), at Tc (middle trace, hypothetical curve), and
above Tc (upper trace, hypothetical curve). Dotted arrows indicate
tunneling processes between wave functions of P- and M-motifs existing
in the same backbone.
as such a triad moves along the chain, the direct molecular
imaging and spectroscopic identification of the helix reversal
as well as structural analysis of the dynamic motion are currently
ongoing.
Apparent time-averaged optical inactivity at Tc may result
from a rapid interconversion between P and M (oscillating
helicity or dynamic pseudoracemization). In this quasi-stationary
state, the wave functions of the superposed P- and M-states (Ps-
and Ms-states) may produce two splitting sublevels, Ψ ) 1/x2-
[ΨPs ( iΨMs].9 This is a result of electronic coupling between
the lowest energy levels of P- and M-helical states, which are
almost degenerate, confined in the same one-dimensional helical
silicon semiconducting wire.10g,11a These helical exciton absorp-
tions are unique because of negligible weak electron-phonon
coupling,10g a feature not seen in π-conjugating polymers or
small molecules. Actually, vibronic phonon-side bands in the
high energy region of the near-UV band at 320 nm are just
discernible in Figures 1 and 2. Two hypothetical energy models
with two splitting sublevels are illustrated in Figure 6.
In region 1 below Tc, the higher level in the P-state mainly
interacts with the lower level in the M-state, whereas in region
3 above Tc, the lower level in the P-state interacts with the higher
level in the M-state. If these interactions are reasonably strong,
both the P- and M-state populations may be significant and
rather insensitive to temperature change. On the other hand, in
region 2 encompassing Tc, higher and lower levels in the P-state
interact with the higher and lower levels in the M-state each
other. Assuming that ∆EPM ≈ 20 K or 1.1 × 10-3 eV (transi-
tion width in region 2 for superposed P and M), one may
suppose a racemization time (Trac) of 2.7 × 10-12 sec,
corresponding to oscillating helicity (δ) of ca. 13 cm-1 from
the time-energy uncertainty principle (∆E∆T g h).9 The direct
detection of the tiny splitting levels or oscillation frequency may
be in the realm of high-resolution vibration in far-infrared region,
fluorescence, and UV spectroscopy,9,19 although Si-Si vibra-
tions are observed in the range of 450-550 cm-1. The expected
oscillation frequency may be the almost similar magnitude of
the proton tunneling splitting energy for tropolone derivatives
in a supersonic free jet, determined by fluorescence spectros-
copy.19
More detailed dimensional characterization including chi-
roptical analysis as functions of molecular weight and temper-
ature will provide more realistic molecular and energetic
parameters of PS-1 and PS-2, such as the energy costs of the
helix reversal (∆Gr) and preferential helix induction (∆Gh),
persistence length (q), and lower-molecular weight limit of
switching properties.13 Preliminary investigation of low-molec-
ular weight PS-1 (Si repeating number is approximately 290)
revealed that the P- and M-ratios in regions 1 and 3 and the Tc
(18) (a) Zink, R.; Magnera, T. F.; Michl J. J. Phys. Chem. A 2000, 104,
3829-3841. (b) Ottosson C-H.; Michl J. J. Phys. Chem. A 2000, 104,
3367-3380.
(19) Sekiya, H.; Tsuji, T.; Ito, S.; Mori, A.; Takeshita, H.; Nishimura,
Y. J. Chem. Phys. 1994, 101, 3464-3471.

Computing Handedness
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6259
value are almost identical to those of the high-molecular weight
fraction, except that the transition width in region 2 is slightly
broadened.15 This suggests that even such a lower Mw PS-1
can effectively switch a preferential screw-sense in response to
change in temperature.
A recent theory and analysis for “temperature effect” of
optically active polyisocyanate terpolymers able to alter a
preferential screw-sense13e could be applicable to an advanced
discussion for PS-1. Detailed characterization of those ∆Gr,
∆Gh, and q parameters based on quantitative analysis on
molecular weight and temperature dependence of chiroptical,
light scattering, and viscometric data is now ongoing using the
current theories for wormlike chain.20
Interest in the superposition of bi-stable quantum states at
low temperature is now growing in the area of the quantum
computing.8 The apparent superposition of PS-1 results from
time averaged M- and P-states at almost ambient temperature.
This may lead to the idea that the helical superposition could
be achievable even in a single molecule. If we assume for a
chainlike 70-mer that the 10 73 helical unit cell corresponds to
a bi-stable bit responsive to thermal and chemical sources, a
single helical polysilylene molecule with 70 Si-Si bonds,
corresponding to 10 qubits, may be able in principle, therefore,
to process huge amounts of chemical information, reaching 210.
Alternatively, if fine temperature resolution of 0.1 K is
practicably attainable, a transition temperature of 20 K in region
2 might possibly permit the recognition of ca. 200 different
molecular shapes. Although direct detection of oscillating
helicity might be the next challenge, our understanding and
insight of the quantized and superposed helicities masking the
switchable optically activity of conjugating polymers may lead
to the more realistic design of a molecular information processor
linked to the dynamic twisting molecular motion. As a pre-
liminary test of this idea, we investigated the solvent molecule
recognition ability of PS-1, as described in section 2.3.
2.3. Molecular Shape Recognition by Superposed Switch-
ing and Dynamic Memory. The most important feature of PS-1
exists in region 2, because the superposed state linearly varies
with thermal energy bias, ranging from 60% M to 60% P ex-
cess. It is noted that PS-1 sensitively recognizes the topology
of small molecules21 due to the three-branched structures of the
two chiral pendants. In other words, the superposed states linked
by dynamic twisting motions may also be considered to
comprise a “dynamic memory” function, since if the solvent
molecules (external chemical bias) are taken away, the super-
posed state may result in modifying the P-M population of
PS-1. This may be proved by a change in Tc value with a range
of nonpolar hydrocarbon solvent molecules with different
degrees of branching. This may be called as “solvent effect” of
optically active polymers able to alter a preferential screw-
sense, as demonstrated in optical activity of poly(n-hexyliso-
cyanate) induced by enantiopure chiral, racemic, and achiral
solvents.13c The energetic analysis for this may be applicable
to the present PS-1 systems, similar to the temperature effect
mentioned above.
Figure 7. Variable temperature P-M fraction of PS-1 (low-molecular
weight fraction) in 2,2,4,4,6,8,8-heptamethylnonane (filled circles),
methylcyclohexane (filled squares), and n-heptane (open circles) in the
range of -80 and +80 °C.
Table 1. Solvent Molecular Topology Effect of the Tc Value in
PS-1
The population was evaluated from the variable temperature
gabs values of PS-1 with the regression gabs curve of PS-4. The
quantized helicity below and above Tc, and the superposed
helicity around Tc are preserved even in these solvents. Although
the value of Tc greatly varies with the degree of branching of
the solvent, the P-M population below Tc seems to be
insensitive to the degree of branching but above Tc, to be very
sensitive.
The effects of solvent topology on Tc for various solvents
are summarized in Table 1, where the degree of branching is
defined as the Balaban index number21 and numbers of
branching carbons per linear five-carbon sequence. Considering
in intrusion of branched solvent molecules into the branched
side chains in the ordered state of PS-1 in an ordered state below
Tc, which may lead to an unfavorable packing of pendants,11a
this results in an increase of Sp but no change of SM in eq 1.
This means a decrease in the ∆S term and an increase in Tc,
since the entropy term is responsible for determining Tc.
The degree of the interaction might be greatly affected by
the degree of solvent branching. As seen in Table 1, straight
chain alkyl molecules give a lower Tc, whereas branched chain
alkyl molecules tend to afford a higher Tc and branched cyclic
solvents give an intermediate value. Quantitatively, it is shown
Figure 7 compares the variable temperature P-M population
of PS-1 (low-molecular weight fraction) in three typical solvents
with different degree of branching, including n-heptane (linear
hydrocarbon), methylcyclohexane (cyclic hydrocarbon), and
2,2,4,4,6,8,8-heptamethylnonane (highly branched hydrocarbon).
(20) (a) Yamakawa, H. Helical Wormlike Chains in Polymer Solutions;
Springer: Berlin, 1997. (b) Kratky, O.; Porod, G. Recl. TraV. Chim. 1949,
68, 1106. (c) Norisuye, T.; Tsuboi, A.; Teramoto, A. Polym. J. 1996, 28,
357-361.
(21) Rouvray, D. H. Sci. Am. 1986, 255(3), 36-43.

6260 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Fujiki et al.
Figure 8. Change in the Tc value of PS-1 (low-molecular weight) as a function of degree of branching in a range of hydrocarbon solvent molecules.
The degree of branching is evaluated by (a) Balaban index number,21 and by (b) number of branching points, compared to five linear carbon
sequence since (S)-3-methylpentyl pendant is regarded to contain five carbon atoms and one branching carbon atom. In the case of cyclic hydrocarbons,
we are regarded that methylcyclopentane consists of five carbon atoms of ring and one branched carbon and methylcyclohexane has five carbon
atoms of ring and two branched carbons.
Figure 9. (a) Value of gabs and (b) P-M fraction of PS-1 (low-molecular weight fraction) at -10 °C as a function of volume fraction of isooctane
and n-octane.
in Figure 8, a and b, that Tc increases nonlinearly with increasing
the Balaban index number, whereas Tc increases almost linearly
with increasing the degree of branching of the solvent molecule
defined here. The correlation of Tc to the Balaban index number
is not as good as the degree of branching.
It was also found that the superposition states derived from
the gabs value and P-M population of PS-1 in the region 2 are
very susceptible to the volume fraction of mixed isooctane and
n-octane solvents at -10 °C (Figure 9, a and b). The superposed
states give a slightly nonlinear response to the volume fraction
of isooctane, indicating a weak cooperative interaction between
the pendants and the solvent molecules.
potential curves, showing two local steric energy minima
corresponding to almost enantiomeric helices with dihedral
angles of about 150-160° (P-helix, global minimum) and 200-
210° (M-helix), regardless of their tacticity.
PS-1 in solution was found to exhibit switchable helicity with
a “dynamic memory”, that is susceptible to thermal energy bias
and chemical environment. Although PS-1 in isooctane features
three distinct switching regions, “1, between -80 and -10 °C”,
“2, between -10 and +10 °C”, and “3, between +10 °C and
+80 °C”, these switching properties were understood as a result
of the superposition of P- and M-helicities, due to dynamic
pseudoracemization or oscillating helicity.
These change in Tc with solvent topology and composition
allows one to determine topology and composition of a wide
range of molecules. This may lead to new method for tackling
the current issue of predicting the chemical and biological
properties of unknown chemicals from the molecular-connectiv-
ity index, which could help in modeling the toxicity and spread
of pollutants, in developing anesthetics and psychoactive drugs,
in predicting the degree of pollutants in the environments, in
estimating the cancer-causing potential of chemicals, and in
predicting the taste and smell of new substances.21
Electronic coupling between the almost degenerate, lowest-
energy levels of P- and M-helical motifs in the same main chain
may give wave functions of the superposed P- and M-states
with two splitting sublevels. In region 2, higher and lower levels
in the P-state mutually interact with those in the M-state. We
derived an oscillating helicity of about 13 cm-1, corresponding
to a racemization timeof 2.7 × 10-12 sec, from the time-energy
uncertainty principle (∆E∆T g h) and ∆EPM ≈ 20 K (1.1 ×
10-3 eV). The superposed helicity in region 2 is critical, since
it affords molecular recognition ability with a dynamic memory
function which is highly susceptible to solvent molecular
topology and mixed solvent volume fraction.
3. Conclusions
Electronically conjugating helical polymer, poly{(R)-3,7-
dimethyloctyl-(S)-3-methylpentylsilylene} (PS-1) consisting of
a rodlike silicon backbone and enantiopure chiral pendants, and
its diastereomeric poly{(S)-3,7-dimethyloctyl-(S)-3-methylpen-
tyl-silylene} (PS-2), were prepared. Molecular mechanics
calculations of these oligomer models revealed double-well
PS-2, however, did not switch preferential screw-sense in the
range from -83 to +80 °C. This may be related to the fact that
the backbone mobility of PS-2 may be greater, as suggested
from the narrower 29Si NMR line widths, and greater differences
in relative energy stability between P and M, and energy barrier
heights in the potential energy.

Computing Handedness
J. Am. Chem. Soc., Vol. 123, No. 26, 2001 6261
and Br2 in CCl4, followed by hydrogenation of (R)-(+)-â-citronellol
4. Experimental Section
(7) (Fluka, [R]24 ) +4.46° (neat), 97.4% ee).
D
4.1. Measurements. CD and UV absorption spectra were recorded
simultaneously on a JASCO J-720 spectropolarimeter equipped with a
liquid nitrogen-controlled quartz cell with path length of 5 mm in a
cryostat, ranging from 23 to -90 °C, and a Peltier-controlled quartz
cell with path length of 10 mm, ranging from 80 to -10 °C. Detailed
measurement conditions have already described in the literature.11 The
sample concentration was 2 × 10-5 (Si-repeat unit)-1 dm-3 for UV
and CD measurements. 13C (75.43 MHz) and 29Si (59.59 MHz) NMR
spectra were taken in CDCl3 at 30 °C with a Varian Unity 300 NMR
spectrometer using tetramethylsilane as an internal standard. Optical
rotation at the Na-D line was measured with a JASCO DIP-370
polarimeter using a quartz cell with path length of 10 mm at room
temperature (24 °C). The weight-average molecular weight (Mw) and
number-average molecular weight (Mn) of the polymers were evaluated
using gel permeation chromatography (Shimadzu A10 instruments,
Shodex KF806M as a column, and HPLC-grade tetrahydrofuran as
eluent at 30 °C), based on a calibration with polystyrene standards.
The enantiopurity of starting materials and intermediates were
determined at Toray Research Center (TRC, Shiga, Japan) using the
chiral gas chromatography technique (Spelco, â-DEX-325 and â-DEX-
225, 30 m × 0.25 mm ID, column oven temperature 70-95 °C, He
carrier with 1.2 mL/min). Detailed analytical conditions were â-DEX-
325 at 70 °C for 4 and 9, â-DEX-225 at 82 °C for 6 and 10, and â-DEX-
225 at 90 °C for 7 and 11 (see in section 4.3.). We concluded that the
enantiopurities of 4, 6, 7, 9, 10, 11 used in this work were sufficiently
high with almost identical ee’s. However, the chiral GC analysis at
the TRC indicated that the enantiopurity of (S)-(-)-â-3,7-citronellol
Filtration of the reaction mixture and vacuum distillation of the
filtrate afforded pure 1. Colorless liquid. Yield 9.8 g (42%); bp 108-
113°/0.20 mmHg, [R]24 ) +3.18° (neat). 29Si NMR (CDCl3, 30 °C,
D
ppm) 34.73, 13C NMR (CDCl3, 30 °C, ppm) 11.33, 17.38, 18.64, 22.63,
22.72, 24.76, 27.99, 28.70, 28.73, 29.14, 34.81, 36.44, 36.48, 39.32.
(S)-(+)-3,7-Dimethyloctyl-(S)-3-methylpentyldichlorosilane (8) was
synthesized in a similar way using (S)-3,7-dimethyloctylbromide (9,
[R]24 ) +6.05° (neat), 95.0% ee) (Chemical Soft) from (S)-(-)-3,7-
D
dimethyloctanol (10, [R]24 ) -4.22° (neat), 95.9% ee) and (S)-(-)-
D
â-citronellol (11, Fluka, [R]24D ) -4.55° (neat), 97.4% ee) as starting
material. Colorless liquid; bp 109-113°/0.20 mmHg, [R]24D ) +5.60°
(neat). 29Si NMR (CDCl3, 30 °C, ppm) 34.75, 13C NMR (CDCl3, 30
°C, ppm) 11.32, 17.37, 18.64, 19.11, 22.63, 22.71, 24.75, 27.98, 28.70,
29.14, 34.80, 36.44, 36.47, 39.31.
4.4. Polymer Preparation. A typical synthetic procedure is described
as follows for PS-1. To a mixture of 12 mL of dry toluene (Kanto),
0.50 g (22 mmol) of sodium (Wako), and 0.06 g (0.23 mmol) of 18-
crown-6 (Wako), 1.6 g (4.9 mmol) of 1 was added dropwise in an
argon atmosphere. The mixture was stirred slowly at 110 °C. After 2
h, 150 mL of dry toluene was added to reduce solution viscosity and
stirring was continued for a further 30 min. The hot reaction mixture
slurry was passed immediately through a 5-µm PTFE filter under argon
gas pressure. To the clear filtrate, 2-propanol and ethanol as precipitating
solvents were added carefully. Several portions of white precipitates
were collected by centrifugation and dried at 120 °C under vacuum
overnight.
For PS-1, high-molecular weight (HMW) fraction: Mw ) 5.83 ×
106 (DPw ) ca. 23 000) and Mn ) 3.44 × 106; low-molecular weight
(LMW) fraction: Mw ) 7.36 × 104 (DPw ) ca. 290) and Mn ) 2.47
× 104; for PS-2, HMW fraction, Mw ) 4.67 × 106 (DPw ) ca. 18 400)
and Mn ) 2.09 × 106, LMW fraction, Mw ) 6.37 × 104 (DPw ) ca.
250) and Mn ) 23400. 29Si NMR (CDCl3, 20 °C, ppm); for PS-1 (LMW
fraction): -25.32; for PS-2 (LMW fraction): -25.69, 13C NMR (CDCl3,
20 °C, ppm, major peaks); for PS-1 (LMW fraction):11.95, 17.38,
18.77, 22.57, 22.75, 25.58, 28.11, 28.70, 28.73, 29.14, 34.81, 36.86,
36.48, 39.39; for PS-2 (LMW fraction, major peaks): 11.91,19.00,
22.56, 22.74, 25.58, 28.13, 29.06, 35.34, 37.18, 38.51, 39.36.
(Merck, [R]24 ) -3.11° (neat), corresponding to 68.4% ee based on
D
[R]24D of purer 11 by Fluka) was only 64.8% ee, and also (S)-(-)-3,7-
dimethyloctanol (Chemical Soft (Kyoto, Japan), based on the Merck
product) was 66.4% ee. On the basis of the analysis, we used 7(Fluka)
and 11 (Fluka) only as starting materials.
4.2. Molecular Mechanics Calculations. Molecular mechanics
calculation was performed using the Molecular Simulation Inc., the
Discover 3 module, Ver. 4.00 on Silicon Graphics Indigo II XZ based
on standard default parameters with a Si-Si bond length of 2.34 Å
and a Si-Si-Si bond angle of 111° using the MSI pcff force field
suitable for polymers. For this calculation, the MSI built-in function
of simple-minimize (default: dihedral angle restraints, the steepest
descents with the first derivative for 1.0, iteration limit 1000, movement
limit 0.2, followed the conjugate gradients) and simple-dynamics
(default: constant volume and constant temperature, initial temperature
300 K, time step 1 fs, velocity Verlet integration method, initial velocity
is random from Boltzman distribution) were used.
Initially, after an oligomer model (31 repeating units terminated with
hydrogen) with dihedral angle of 180° was generated, two oligomer
models with dihedral angle of 150° and 210° were preliminarily
generated. Next, calculation of P-screw-sense oligomer model setup
with desired dihedral angle was carried out based on the 150°-model,
and M-models were based on the 210°-model, respectively.
Acknowledgment. We thank Hiromi Tamoto-Takigawa for
preliminary CD and UV measurements. Drs. Masao Morita, Kei-
ichi Torimitsu, and Hideaki Takayanagi are gratefully acknowl-
edged for their continuing support. Professors Takahiro Sato,
Junji Watanabe, Masashi Kunitake, Shoji Furukawa, and Eiji
Yashima are acknowledged for fruitful comments and discus-
sion. Drs. Zhong-Biao Zhang and Hongzhi Tang are acknowl-
edged for discussion. Correspondence and requests for material
and Supporting Information should be addressed to M.F.
(e-mail: fujiki@will.brl.ntt.co.jp).
4.3. Monomer Preparation. The synthetic procedure for (R)-(+)-
3,7-dimethyloctyl-(S)-3-methylpentyldichlorosilane (1) is described. (S)-
3-Methylpentyltrichlorosilane (2) was obtained by coupling 68 g (0.40
mol) of tetrachlorosilane (Shin-Etsu) with the Grignard reagent formed
from 29.5 g (0.25 mol) of (S)-(+)-3-methylpentyl chloride (3) (Chemical
Soft). Colorless liquid. Yield 33.3 g (76%); bp 70-73°/6.5 mmHg,
Supporting Information Available: Variable temperature
UV and CD spectra of PS-1 (HMW fraction) in isooctane at
-80, -40, -20, -10, -8, -5, 0, 5, 10, 25, 55, and 80 °C,
variable temperature UV and CD spectra of PS-2 (HMW
fraction) in isooctane at -83, -61, -40, -19, -5, 0, 25, 55,
and 80 °C, variable temperature gabs values of PS-1 (HMW and
LMW fractions) and PS-2 (HMW fraction), which are normal-
ized by the regression gabs curve of PS-4, and schematic
illustration of twisting and translational motions of helical motifs
and helix reversals (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
[R]24 ) +2.25° (neat). 29Si NMR (CDCl3, 30 °C) 13.66 ppm, 13C
D
NMR (CDCl3, 30 °C, ppm) 19.00, 21.66, 22.61, 22.70, 24.67, 27.95,
28.93, 34.38, 36.39, 39.24. Dichlorosilane 1 was obtained by slowly
adding Grignard reagent (5) obtained from 33 g (0.15 mol) of (R)-(-
)-3,7-dimethyloctylbromide (4, [R]24 ) -5.96° (neat), 96.6% ee) to
D
17.4 g (79 mmol) of 2 in dry diethyl ether at room temperature. The
bromide 4 was prepared at Chemical Soft by bromination of (R)-(+)-
3,7-dimethyloctanol (6, [R]24 ) +3.90° (neat), 95.7% ee) with PPh3
JA0026509
D