Diastereoselective addition of alcohol to diastereotopic silylenes

Article abstract of DOI:10.1021/ja028754w

The successful example of diastereoselective addition of alcohol to diastereotopic silylenes is reported. Copyright

Full text of DOI:10.1021/ja028754w

Published on Web 02/20/2003
Diastereoselective Addition of Alcohol to Diastereotopic Silylenes
Takanobu Sanji,*,† Hideyuki Fujiyama, Kaname Yoshida, and Hideki Sakurai*
Department of Pure and Applied Chemistry, Science UniVersity of Tokyo, 2641 Yamazaki, Noda,
Chiba 278-8510, Japan
Received September 30, 2002 ; E-mail: sanji@res.titech.ac.jp
Silylenes with two different substituents, R1R2Si:, are prochiral,
and addition of alcohol (or insertion of silylenes into an alcoholic
RO-H bond) should create chirality on the silicon.1 In particular,
if one of the two substituents is chiral, a diastereotopic face could
be defined, and diastereoselective addition of alcohol could be
expected. However, no such example has been reported either for
silylenes or for carbenes and germylenes.1,2 We report herein the
successful example of diastereoselective addition of alcohol to
diastereotopic silylenes.
To generate the requisite silylenes, trisilanes with a chiral
substituent, 1 and 3, were synthesized as the photochemical
precursors.3 A series of diastereotopic silylenes was generated by
photoirradiation of the trisilanes with 254 nm light in the presence
of t-BuOH or n-BuOH as a trapping reagent, as shown in eq 1.
Addition of alcohol to these silylenes gave the expected products.4
In this reaction, four diastereoisomers or two pairs of enantiomers
were obtained, because the silylene precursors were racemic.5 The
ratio of these two isomers was determined by GC with a capillary
column. For example, the photolysis of precursor 1 in hexane with
t-BuOH gave 2b with diastereoisomer excess (de) of 19% at 28.5%
conversion and 18% at 49.2% conversion at room temperature.6
This indicates that no secondary photochemical reaction occurred
at all under these conditions. The diastereoselectivity of alcohol
addition to the silylenes is also not dependent on the concentration
of alcohol (see Supporting Information). This suggests that the
stereochemistry of the products is determined by the primary
diastereofacial selection of the alcohol, because an intra- and
intermolecular transfer of the proton7 gave the same products. It is
rather surprising to find such a highly biased value for the
diastereoselectivity.
carbon, 5, which could give a cyclic trapping adduct 6, as shown
in eq 2. The de was found to be 24% at room temperature under
similar photochemical reaction conditions. Each cyclic trapping
product 6 could be separated, and the stereochemistry was analyzed
by NOE spectra (see Supporting Information). The stereochemistry
of the major isomer was found to be 6A, where the H- on the
chiral carbon and the t-BuO- group on the silicon atom are located
on the same side of the ring structure. This indicates that the alcohol
addition to the cyclic silylene occurred on the less hindered side to
produce the 6A isomer predominantly in the reaction.
The results are summarized in Table 1. Interestingly, alcohol
addition to the silylene proceeded in the diastereoselective manner
with a high value, where the ratio of diastereomers is found to
depend on the substituent groups on the silylene and/or alcohol.
For example, for n-BuOH addition to the i-Pr-substituted silylene
at room temperature, the de of 4a was estimated to be 9%, but the
de of 4b was 26% for t-BuOH. However, in the photoreaction of
1 with n-BuOH, almost no selectivity was observed. The temper-
ature dependence of the isomer ratio, as well as effects of the steric
demand of alcohol, were also examined. Indeed, fairly high
diastereoselection was observed, and the stereoselectivity gradually
increased with a decrease in temperature, especially for 4b, the
largest de being 52% at -76 °C.
Finally, the stereostructure of the trapping products could be
definitely determined by X-ray crystallographic analysis of 8A,
which was obtained by photolysis of 7 with 2,6-dimethylphenol as
a major product, as shown in eq 3. Figure 1 shows the ORTEP
drawing of 8A.9 Thus, the stereochemistry was determined to be
1R2R/1S2S.10
Table 1. Addition of Alcohol to Diastereotopic Silylenes
Generated in the Photolysis of 1 and 3
a,b
entry
R
R′OH
T/°C
de/%
1
2
3
4
5
6
7
8
9
Me
n-BuOH
t-BuOH
20
20
-75
20
0
19
33
9
26
32
35
39
46
52
i-Pr
n-BuOH
t-BuOH
To elucidate the reaction pathway, analysis of the configuration
of the trapping products was required. However, the stereochemical
structure for the diastereoisomers of 2 and 4 is difficult to determine
at the present stage because no crystals were obtained for these
isomers. Next, we designed a cyclic silylene with asymmetric chiral
20
-14
-30
-45
-62
-76
10
† Present address: Chemical Resource Laboratory, Tokyo Institute of Technol-
ogy, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan.
a Determined by GLC. b Each isomer is not assigned for stereostructures.
9
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J. AM. CHEM. SOC. 2003, 125, 3216-3217
10.1021/ja028754w CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank Mr. T. Kondo of Tohoku Uni-
versity for the NOE measurement and Dr. M. Ichinohe and Prof.
A. Sekiguchi of the University of Tsukuba for the X-ray analysis.
We are also grateful to the Ministry of Education, Culture, Sports,
Science, and Technology of Japan for financial support.
Figure 1. ORTEP drawing of 8A.
Scheme 1
Supporting Information Available: Experimental details, spec-
troscopic and analytical data for 1-11, dependence of de on concentra-
tion of alcohol, Arrhenius plots of the diastereomers of 2b and 4b, 1H
NMR NOE difference spectra of 6, 1H-13C COSY spectra of 6, crystal
packing of 8A (PDF), and crystallographic file of 8A (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) For a recent review of silylenes, see: Gaspar, P. P.; West, R. In The
Chemistry of Organic Silicon Compounds, 2nd ed.; Rappoport, Z., Apeloig,
Y., Eds.; John Wiley & Sons: New York, 1999; Part 3, pp 2463-2568.
(2) Preliminary study of the stereochemistry of alcohol addition to silylenes,
see: (a) Ando, W.; Fujita, M.; Yoshida, H.; Sekiguchi, A. J. Am. Chem.
Soc. 1988, 110, 3310. (b) Zhang, S.; Wagenseller, P. E.; Conlin, R. T. J.
Am. Chem. Soc. 1991, 113, 4278. (c) Gaspar, P. P.; Beatty, A. M.; Chen,
T.; Haile, T.; Braddock, J.-W.; Rath, N. P.; Klooster, W. T.; Koetzle, T.
F.; Mason, S. A.; Albinati, A. Organometallics 1999, 18, 3921. (d)
Tortorelli, V. J.; Jones, M., Jr. J. Am. Chem. Soc. 1980, 102, 1425. (e)
Seyferth, D.; Annarelli, D. C.; Duncan, D. P. Organometallics 1982, 1,
1288. (f) Tortorelli, V. J.; Jones, M., Jr.; Wu, S.-H.; Li, Z.-H. Organo-
metallics 1983, 2, 759. (g) Ishikawa, M.; Nakazawa, K.-I.; Kumada, M.
J. Organomet. Chem. 1979, 178, 105. (h) Pae, D. H.; Xiao, M.; Chiang,
M. Y.; Gaspar, P. P. J. Am. Chem. Soc. 1991, 113, 1281.
The diastereoselectivities could be rationalized by reference to
the mechanism shown in Scheme 1. The selective addition of
alcohol to the silylene takes place from the less hindered face.
(3) Diastereoselective silacyclopropanations of chiral alkenes, see: (a) Cira-
kovic, J.; Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2002, 124,
9370-9371. (b) Driver, T. G.; Franz, A. K.; Woerpel, K. A. J. Am. Chem.
Soc. 2002, 124, 6524-6525.
(4) (a) Gaspar, P. P. In ReactiVe Intermediates; Jones, M., Jr., Moss, R. A.,
Eds.; Wiley: New York, 1978; Vol. 1, pp 229-277. (b) Gaspar, P. P. In
ReactiVe Intermediates; Jones, M., Jr., Moss, R. A., Eds.; Wiley: New
York, 1981; Vol. 2, pp 335-385.
Next, the diastereoselective addition of alcohol to a silylene with
an ethereal substituent was examined, where the oxygen atom can
coordinate intermolecularly to the silylene. The photoreaction of 9
with t-BuOH at -60 °C proceeded smoothly to give a mixture of
trapping products (eq 4). Interestingly, a slight predominance of
10B over 10A was observed; the ratio of A:B was 1:1.22. However,
the photoreaction of 11 with t-BuOH at -60 °C gave a 2.33:1
mixture of A and B isomers, which could be interpreted in the same
way as that described above. Most probably, the antistereoselectivity
in the reaction of 9 results when the oxygen atom on the ether
group coordinates to the silylene from the less hindered face, after
which the addition of alcohol to the silylene takes place.
Thus, we have demonstrated the clear example of diastereo-
selectivity in the reaction of group 14 divalent reactive intermedi-
ates. Details of the mechanism will be reported in due course.
Further work is in progress.
(5) See the Supporting Information for experimental details and spectral and
analytical data of the new compounds.
(6) Nomenclature of the relative configurations: (a) Masamune, S.; Kaiho,
T.; Garver, D. S. J. Am. Chem. Soc. 1982, 104, 5521. (b) Masamune, S.;
Ali, S. A.; Snitman, L.; Garvey, D. S. Angew. Chem., Int. Ed. Engl. 1980,
19, 557. (c) Prelog, V.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1982,
21, 567. (d) Seebach, D.; Prelog, V. Angew. Chem., Int. Ed. Engl. 1982,
21, 654.
(7) For experimental details, see the Supporting Information.
(8) The inversion process of the anionic silicon after the attack of alcohols
may be impossible, because the barrier to inversion of the silyl anions is
normally high, see: Lambert, J. B.; Urdaneta-Pe´rez, M. J. Am. Chem.
Soc. 1978, 100, 157.
(9) Crystal data for the major isomer at 120 K: MF ) C28H28OSi, MW )
408.59, monoclinic, P21/n, a ) 9.4120(6) Å, b ) 19.5450(12) Å, c )
12.8760(7) Å, â ) 105.840(4)°, V ) 2278.7(2) Å3, Z ) 4, Dcald ) 1.191
g/cm3. The final R factor was 0.0600 for 5455 reflections with I0 > 2σ(I0)
(Rw ) 0.0845 for all data).
(10) The stereochemistry of the major isomer of 2 and 4 is 1R2S/1S2R, because
the Me and i-Pr groups on the silylene have sterically lower priority than
the phenylethyl group.
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