Chiral cyclotrisiloxanes

Article abstract of DOI:10.1039/c0dt00135j

We have synthesized and separated the enantiomer pair of chiral cyclotrisiloxanes for the first time. Three-blade propeller-like alignment of three phenyl groups in tri(2-butyl)triphenylcyclotrisiloxane induced a positive Cotton effect with (R)-2-butyl group, and a negative effect with (S)-2-butyl substituent. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00135j

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www.rsc.org/dalton | Dalton Transactions
Chiral cyclotrisiloxanes†
Ryoji Tanaka,*a Susumu Kowasea and Masafumi Unno*a,b
Received 15th March 2010, Accepted 9th June 2010
DOI: 10.1039/c0dt00135j
We have synthesized and separated the enantiomer pair
of chiral cyclotrisiloxanes for the first time. Three-blade
propeller-like alignment of three phenyl groups in tri(2-
butyl)triphenylcyclotrisiloxane induced a positive Cotton
effect with (R)-2-butyl group, and a negative effect with (S)-
2-butyl substituent.
Scheme 1 Synthesis of (S)-2-butyldichlorophenylsilane 2.
Since the first isolation of chiral silicon compounds by Sommer,1
chiral silanes have been widely utilized as Lewis acids,2 chi-
ral auxiliaries,3 and chiral transfer units.4 Although silicon-
stereogenic silanes5 and polymeric compounds6 are relatively
well investigated, chiral silsesquioxanes or siloxanes are almost
unexplored.7,8 Cyclic siloxanes are easily accessible from chlorosi-
lanes, and are stable compounds. Therefore the preparation of
chiral siloxanes is promising in various applications like starting
compounds of chiral siloxanes or chiral templates. Here we will
report the first synthesis and isolation of an enantiomer pair of
chiral cyclotrisiloxanes.
In 2003, we reported the synthesis of hexakis(2,4,6-
triisopropylphenylsilsesquioxane) 1, and the X-ray crystal analysis
revealed that the enantiomer pair originating from the restricted
rotation of the bulky Tip (2,4,6-triisopropylphenyl) groups was
included in the single lattice.8 This result prompted us to prepare
a chiral cyclotrisiloxane, which is half part hexasilsesquioxane.
Regarding chiral cyclotrisiloxanes, Moller’s group reported the
synthesis of hexa((S)-2-methylbutyl)cyclotrisiloxane.9 Unfortu-
nately in their case, only slight optical rotation was observed
and no other information regarding stereostructure was obtained.
We devised to introduce chiral secondary alkyl groups and aryl
groups on the silicon atom in cyclotrisiloxane. In this compound,
controlling chirality by propeller-like orientation of aryl groups
can be expected.
both compounds to be 67% ((S)-2-butyldichlorophenylsilane) and
15% (butyltrichlorosilane). We then treated the mixture with Ph-
MgBr, and obtained products that could be successfully separated.
The yield of pure (S)-2-butyldichlorophenylsilane (S)-2 was 58%.
Because we could not determine the optical purity of chlorosilane
2, (S)-2 was transferred to 1,2-dibutyl-1,1,2,2-tetraphenyldisilane,
and optical purity was measured to be 94% ee after purification.
A similar experiment using (S)-2-diphenylphosphino-2-methoxy-
1,1¢-binaphtyl gave (R)-2-butyldichlorophenylsilane (R)-2.
Dichlorosilane (S)-2 was hydrolyzed under basic conditions,
and 2,4,6-tri((S)-2-butyl)-2,4,6-triphenylcyclotrisiloxane 3 was
obtained. The isomeric mixture was separated with recycle-type
HPLC, and (S)-cis,cis-3 and (S)-cis,trans-3 were obtained in 5%
and 28% yield, respectively (Scheme 2). The yields were further
improved by the reaction of (S)-2 with DMSO in toluene; (S)-
cis,cis-3 was obtained in 12% yield, and (S)-cis,trans-2 was given
in 61% yield (Scheme 2).11 A similar procedure could be applied
to (R)-2, and (R)-cis,cis-3 and (R)-cis,trans-3 were obtained. In all
cases, a single enantiomer was obtained by stereospecific manner.
Our synthesis began with hydrosilylation using a chiral template
reported by Uozumi and Hayashi.10 As shown in Scheme 1, 1-
butene was treated with trichlorosilane in the presence of (R)-2-
diphenylphosphino-2-methoxy-1,1¢-binaphtyl and [(C3H5)PdCl]2.
Hydrosilylation occurred at positions 1 and 2 of 1-butene, and
two trichlorosilanes were obtained. As the boiling points of the
two compounds are close, we could not separate by distillation. By
measuring NMR spectra of the mixture, we calculated the yields of
Scheme 2 Synthesis of tri((S)-2-butyl)triphenylcyclotrisiloxane 3.
The CD spectra of cis,cis-3 and cis,trans-3 showed Cotton band
of phenyl groups in the region of 250–280 nm, and (R)-cis,cis-
3 and (R)-cis,trans-3 indicated positive Cotton effect and (S)-
cis,cis-3 and (S)-cis,trans-3 showed negative Cotton effect (Fig. 1
and 2). These results clearly indicate that the phenyl groups
in 3 locate in chiral alignment, and that is the origin of the
Cotton effect. Noticeably, molar ellipticities of cis,cis-3 was more
than twice as large than that of cis,trans-3, indicating that the
diastereotopic structure of cis,cis-3 effectively leads to the three-
blade propeller form. Although cyclotrisiloxane rings are flexible,
aDepartment of Chemistry and Chemical Biology, Graduate School of Engi-
neering, Gunma University, Kiryu, Japan. E-mail: polysilane@gmail.com
bInternational Education and Research Center for Silicon Science, Graduate
School of Engineering, Gunma University, Kiryu, Japan. E-mail: unno@
chem-bio.gunma-u.ac.jp
† Electronic supplementary information (ESI) available: Detailed experi-
mental procedure of all compounds and X-ray crystal analysis report of
P-4 and M-4. CCDC reference numbers 769810 and 769811. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00135j
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9235–9237 | 9235
©

Fig. 1 Circular dichroism spectra of cis,cis-(S)-3 and cis,cis-(R)-3 in
hexane.
Fig. 4 Circular dichroism spectra of P-4 and M-4 (solid state).
Conclusions
We have prepared and separated the first enantiomer pair of
cyclotrisiloxanes by introducing chiral secondary alkyl groups
and phenyl groups. The three-blade propeller alignment of
phenyl groups was successfully induced by chiral alkyl groups,
and an enantiomer pair of cis,cis- and cis,trans-isomers were
separated.
Fig. 2 Circular dichroism spectra of cis,trans-(S)-3 and cis,trans-(R)-3 in
hexane.
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9236 | Dalton Trans., 2010, 39, 9235–9237
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