Migration of aryl groups from silicon to carbon in α,β-epoxysilanes. A new model for hypervalent silicon study

Article abstract of DOI:10.1016/S0022-328X(97)00677-3

The reaction of (2R,3R)-3-(triphenylsilyl)glycidol (1) with n-Bu₄NF·3H₂O in THF, followed by treatment of the product with p-nitrobenzoyl chloride yields (2S)-glycidol p-nitrobenzoate (4) and trans-cinnamyl p-nitrobenzoate (6) in a ratio of 1:1.5. The reaction of (R)Si-(2R,3R)- or (R)Si-(2S,3S)-3-[(methyl)(phenyl)(1-naphthyl)silyl]-glycidols, 7 or 8 respectively, with n-Bu₄NF·3H₂O affords mixtures of the respective glycidol, trans-cinnamyl alcohol and 3-(1-naphthyl)allyl alcohol. No significant difference in the product distribution in reaction of these two diasteromers was observed.

Full text of DOI:10.1016/S0022-328X(97)00677-3

Journal of Organometallic Chemistry 558 (1998) 227–230
Priority communication
Migration of aryl groups from silicon to carbon in h,i-epoxysilanes. A
new model for hypervalent silicon study
Barbara Achmatowicz, Pawel Jankowski, Jerzy Wicha *, Andrzej Zarecki
Institute of Organic Chemistry, Polish Academy of Sciences, POB 58, 01-224 Warsaw 42, Poland
Received 16 September 1997
Abstract
The reaction of (2R,3R)-3-(triphenylsilyl)glycidol (1) with n-Bu4NF·3H2O in THF, followed by treatment of the product with
p-nitrobenzoyl chloride yields (2S)-glycidol p-nitrobenzoate (4) and trans-cinnamyl p-nitrobenzoate (6) in a ratio of 1:1.5. The
reaction of (R)Si-(2R,3R)- or (R)Si-(2S,3S)-3-[(methyl)(phenyl)(1-naphthyl)silyl]-glycidols,
7 or 8 respectively, with n-
Bu4NF·3H2O affords mixtures of the respective glycidol, trans-cinnamyl alcohol and 3-(1-naphthyl)allyl alcohol. No significant
difference in the product distribution in reaction of these two diasteromers was observed. © 1998 Elsevier Science S.A. All rights
reserved.
Keywords: h,i-Epoxysilanes; Aryl group migaration; Hypervalent Si species
1. Introduction
silicon atom [4]. However, all attempts including a very
recent one [5] failed to provide experimental evidence
for the formation of hypervalent silicon intermediates.
Now, we report on the first examples [6] of nucleophile-
induced aryl groups migration from silicon to the h-
carbon atom in h,i-epoxysilanes and on some
stereochemical implications of nucleophilic substitution
on a chiral silicon atom in h,i-epoxysilanes involving
the (methyl)(1-naphthyl)(phenyl)-silyl group.
There is continuous interest in hypervalent organosil-
icon species and the search for models that would make
possible a study of stereomutation in various reactions
involving transmission between four- and five- (or six-)
valent silicon atom [1]. The reaction of (chloromethyl)-
silanes with alkoxides or a fluoride anion is known to
involve an attack of the nucleophile on the silicon
atom, generating a hypervalent intermediate which sub-
sequently disproportionates with an alkyl group migra-
tion from silicon to carbon and expulsion of chlorine
[2]. Related h,i-epoxysilanes (which embrace a chiral
center in the h-position) react with a range of nucle-
ophiles regioselectively via an ‘h-opening pathway’ [3].
Directing effect of the silyl group, in certain cases
overriding considerable steric shielding, has been ratio-
nalized by initial coordination of the nucleophile to the
2. Results and discussion
The reaction of (2R,3R)-epoxysilane [7] 1 (97:3 enan-
tiomer ratio by HPLC using a chiral column [8])
(Scheme 1) with n-Bu4NF·3H2O in THF at room
temperature for 6 h followed by in situ derivation of the
product with p-nitrobenzoyl chloride [9] (to avoid
losses of the volatile material) yielded two major prod-
ucts. After flash column chromatography (S)-glycidol
p-nitrobenzoate 4 [10] and trans-cinnamyl alcohol p-ni-
* Corresponding author.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00677-3

228
B. Achmatowicz et al. / Journal of Organometallic Chemistry 558 (1998) 227–230
Scheme 1.
trobenzoate 6 were isolated in 53 and 35% yield, respec-
tively. The esters were accompanied by small amounts
(2–4%) of the corresponding free alcohols, 3 and 5,
which reflects incompleteness of esterification.
While glycidol 3 was the expected product of the
epoxysilane 1 protio-desilylation which is well docu-
mented [11], cinnamyl alcohol (5) had to result from the
phenyl group migration from silicon to carbon. A
plausible mechanism for such transformation is de-
picted in Scheme 2. The addition of fluoride anion to
the silicon atom generates the hypervalent silicon
derivative (i) which rearranges with the phenyl group
migration to the h-carbon atom and simultaneous
epoxide ring opening to give i-hydroxysilane (ii). The
terminating stage of the rearrangement involves the
fluorodiphenylsilanolate elimination. It is noteworthy
that protio-desilylation reactions of (triphenylsi-
lyl)oxirane [7](a) and its deuterated analogue [11](a)
have been reported to yield oxirane and deuterated
oxirane (60% [7](a)), respectively; no other products
have been detected.
Scheme 3.
after derivation with p-nitrobenzoyl chloride. In a par-
allel series of experiments, the PNB derivative 2 was
treated with n-Bu4NF under similar conditions. The
results are summarized in Table 1.
As it can be seen from Table 1, in both examined
compounds in all solvents examined the product of
phenyl group migration was obtained. Free alcohol 1
appeared to be more prone to the rearrangement than
its PNB derivative 2. Taking separately epoxysilanes 1
or 2, there was virtually no difference between reactions
We examined first the effect of the solvent on the
reaction of 1 with a fluoride anion using additionally
DMSO, DMF and MeCN, and isolating the products
Scheme 2.
Table 1
Reaction of epoxysilanes with Bu4NF·3H2O in various solvents: yields and products ratio
Epoxysilane
1
The solvent
Products
4
Yield (%)a
4:6 ratiob
1
2
3
4
5
6
7
8
THF
6
6
88
81
74
58c
76
60
64
73d
1:1.5
1:1.8
1:0.4
1:1.7
1:0.25
1:0.25
1:0.12
1:0.2
DMSO
DMF
MeCN
THF
DMSO
DMF
MeCN
2
4
a The experiments were performed in the 0.1 mmol scale. Reaction of 1 with fluoride was followed by in situ derivation of the products with
p-nitrobenzoyl chloride. Yields refer to isolated product mixtures.
b The ratios were determined by HPLC analysis (uv detector 254 nm) of the crude mixtures.
c Allyl p-nitrobenzoate was isolated as an additional product (ca. 20% yield)(see the text).
d Allyl p-nitrobenzoate was the major product, isolated in ca. 60% yield.

B. Achmatowicz et al. / Journal of Organometallic Chemistry 558 (1998) 227–230
229
Scheme 4.
Scheme 5.
sults are presented in Scheme 3. As it can be seen from
the scheme, the products of phenyl as well as naphthyl
group migration were obtained (6 and 9, respectively)
in both cases. The product of phenyl group migration
clearly dominates. The ratio of phenyl- and naphthyl-
derivatives (6 and 9) is virtually the same regardless of
the epoxide configuration. The apparent lack of mem-
ory in the products with respect to the initial relative
configuration of epoxysilanes suggests that aryl group
migration is not simultaneous with the fluorine anion
addition. It is likely that the fate of the initially formed
five-coordinated silicon species may be followed by
NMR, for which our model is suitable.
in THF or DMSO (Table 1, entries 1 and 2, and
respectively, 5 and 6). In DMF a markedly smaller
amount of the allylic alcohol was generated (entries 3
and 7). When the reaction was carried out in acetoni-
trile [12] the product consisted of 4 and 6 and the third
component which was identified as allyl p-nitroben-
zoate [13] (ca. 20 and 60% yield from 1 or 2, respec-
tively). Proportions of 4 and 6 are shown in Table 1
(entries 4 and 8).
It was thought of interest to determine the fate of the
aromatic substituents in (methyl)(1-naphthyl)(phenyl)-
silylglycidols 7 and 8 (Scheme 3). Two aryl groups with
a similar ability to stabilize the negative charge (which
is an important factor ([2]b,h) influencing the ‘migra-
tory aptitude’) but considerably differing in the steric
bulk are present in each of these compounds, 1-naph-
thyl and phenyl. On the other hand, two diastereomers
differing in relative configuration around the silicon
and the h-carbon atom lk (7) and ul (8) atom are
accessible for study (vide infra).
The diasteromeric epoxysilanes 7 and 8 were synthe-
sized using optically active allylic alcohol 15 as the
common
intermediate
(Scheme
4).
(1-Naph-
thyl)phenylsilane [15] 10 was treated with (1R,2S)-(−)-
ephedrine in the presence of the Wilkinson catalyst
according to Corriu and Moreau [16] to afford the
derivative 11 as a mixture of diastereomers. The crude
mixture was treated with an excess of methyl magne-
sium iodide to yield methylnaphthylphenylsilane 12
(90% yield, er (R):(S)=79:21 by HPLC). After several
crystallizations from pentane virtually optically pure
[17] (R)-(+)-12 was obtained ([h]2D0= +34.4, er 99:1,
Epoxysilanes 7 and 8 (for synthesis of these com-
pounds, [14] vide infra) were treated with n-
Bu4NF·H2O in THF at room temperature for 16 h and
the product was isolated by chromatography and the
components ratio was determined by HPLC. The re-
Table 2
Assymetric catalytic epoxidation of 15, (R):(S)=92:8 under Sharpless conditions using diisopropyltartrates
Catalyst
Product
7 Si (R); (1R,2R)
41.81 min
89%
41.33 min
2.5%
8 Si (R); (1S,2S)
44.85 min
3.5%
44.26 min
90.9%
Si (S); (1S,2S)
—
—
35.92 min
6.5%
Si (S); (1R,2R)
D-(−)-DIPT
L-(+)-DIPT
49.49 min
6.8%
—
—
Diasteromer content and retention times (chiral column with a precolumn [8], hexane-isopropanol (5%), flow 0.5 ml min−1, detection UV at 286
nm).

230
B. Achmatowicz et al. / Journal of Organometallic Chemistry 558 (1998) 227–230
Kreeger, P.R. Menard, E.A. Sans, H. Shechter, Tetrahedron
Lett. 26 (1985) 1115–1118; (h) E.A. Sans, H. Shechter, Tetrahe-
dron Lett. 26 (1985) 1119–1122; (i) A. Shirahata, Tetrahedron
Lett. 30 (1989) 6393–6394; (j) S.L Aprahamian, H. Shechter,
Tetrahedron Lett. 31 (1990) 1089–1092; (k) J.J. Eisch, C.S.
Chiu, J. Organomet. Chem. 358 (1988) C1–C5.
by HPLC). Pure silane 12 was chlorinated (retention of
configuration [15]) and the chloride was reacted with an
anion generated from the acetylene derivative 13 to
yield, after hydrolysis, 14 with the er of 96:4 (by
HPLC). Configuration around the silicon atom in the
prevailing enantiomer of 14 has been ascribed as (R)
since the intermediate silyl chloride is known to react
with lithioorganic reagents with inversion of the
configuration [18]. Reduction of the triple bond in 14
with lithium aluminum hydride afforded allylic alcohol
15 (79%, er 96:4).
Catalytic asymmetric epoxidation [9] of 15 (Scheme
5) using diisopropyl D-(−)-tartrate (DIPT)(99% ee,
Aldrich®) followed by flash chromatography afforded
the product in 74% yield, consisting of three compo-
nents in a ratio shown in Table 2 (determined by HPLC
on chiral column [8], the mixture was inseparable by
HPLC on a Nucleosil column). Epoxidation of 15 with
L-(+)-DIPT (99% ee) as the catalyst yielded a mixture
of epoxides in 89% yield (Table 2). Structure assign-
ments were made on the grounds of the rules governing
the Katsuki–Sharpless epoxidation [19] and were confi-
rmed by rotation of glycidol ent-4 obtained after pro-
tio-desilylation (see above).
[3] For reviews, see: (a) P.F. Hudrlik, A.H. Hudrlik, in: G.L.
Larson (Ed.), Advances in Silicon Chemistry, vol. 2, Jai Press,
Greenwich, 1993, p. 1; (b) A.R. Bassindale, P.G. Taylor, in: S.
Patai, Z. Rappoport (Eds.), The Chemistry of Organosilicon
Compounds, John Wiley, Chichester, 1989, p. 893–964. P.
Jankowski, S. Marczak, M. Masnyk, J. Wicha, J. Organomet.
Chem. 403 (1991) 49–62.
[4] (a) J.J. Eisch, Z.-R. Liu, M. Singh, J. Org. Chem. 57 (1992)
1618–1621; (b) J.J. Eisch, C.C. Chiu, J. Organomet. Chem. 358
(1988) C1–C5 and references quoted therein.
[5] (a) P.F. Hudrlik, D. Ma, R.S. Bhamidipati, A.M. Hudrlik, J.
Org. Chem. 61 (1996) 8655–8658; (b) P.F. Hudrlik, Y.M. Abdal-
lah, A.K. Kulkarni, A.H. Hudrlik, J. Org. Chem. 57 (1992)
6552–6556.
[6] (a) An alkyl group migration has been noted in a silacyclobu-
tane, K. Matsamuto, Y. Takeyama, K. Oshima, K. Utimoto,
Tetrahedron Lett. 32 (1991) 4545–4548; (b) For an example of
acid-catalysed migration of the phenyl group from silicon to
carbon in h,i-epoxysilanes, see: I. Fleming, T.W. Newton, J.
Chem Soc. Perkin Trans. 1 (1984) 119–123.
[7] (a) J.M. Schwab, T. Ray, C.-K. Ho, J. Am. Chem. Soc. 111
(1989) 1057–1063; (b) P. Raubo, J. Wicha, Tetrahedron: Asym-
metry, 7 (1996) 763–770.
[8] A Chiracel® OD-H analytical column, 0.46(I.D.)×25 cm with a
precolumn 0.46×5 cm of Daicel Chemical Industries, Tokyo,
Japan, has been used.
3. Conclusion
[9] (a) Y. Gao, R.M. Hanson, J.M. Klunder, S.Y. Ko, H. Masa-
mune, K.B. Sharpless, J. Am. Chem. Soc. 109 (1987) 5765–5780;
(b) For a review on non-racemic glycidols, see: R.M. Hanson,
Chem. Rev. 91 (1991) 437–475.
In conclusion, h,i-epoxysilanes bearing an aromatic
group on the silicon atom, on treatment with n-Bu4NF
undergo a rearrangement with the aryl group migration
from silicon to carbon. Epoxysilanes with (methyl)(1-
naphthyl)(phenyl)silyl group in this reaction afford
products of phenyl- and naphthyl-group migration, the
former prevailing. No differences could be noted in the
reactions course of two diastereomeric epoxysilanes
differing in the relative configuration around the silicon
and the neighboring carbon atom.
[10] [h]2D0= +38.2 (CHCl3), reported in ref. 9: [h]2D0= +38.7
[11] Relevant work includes: (a) T.H. Chan, P.W.K. Lau, M.P. Li,
Tetrahedron Lett. (1976) 2667–2670; (b) I. Hasan, Y. Kishi,
Tetrahedron Lett. 21 (1980) 4229–4232; (c) T. Katsuki, Tetrahe-
dron Lett. 25 (1984) 2821–2822; (d) Y. Takeda, T. Matsumoto,
F. Sato, J. Org. Chem. 51 (1986) 4728–4731; (e) Y. Kobayashi,
T. Ito, I. Yamakawa, H. Urabe, F. Sato, Synlett (1991) 811–813.
[12] For examples of desilylation in acetonitrile, see: D.R. Boyd,
G.A. Berchtold, J. Org. Chem. 44 (1979) 468–470 and ref. 11a.
[13] Origin of allyl alcohol is not quite clear at the present. Supple-
mentary experiment has shown that k-(triphenylsily)allyl alcohol
undergoes partial desilylation under the applied conditions (in
acetonitrile) although, in principle, vinylsilanes are desilylated
much slower then epoxysilenes, see ref. 14.
References
[14] All new compounds were fully characterized by 1H and 13C
NMR spectra, while elemental composition was confirmed by
combustion analysis.
[15] L.H. Sommer, C.L. Frye, G.A. Parker, K.W. Michael, J. Am.
Chem. Soc. 86 (1964) 3271–3276.
[1] (a) C. Chuit, R.J.P. Corriu, C. Reye, J.C. Young, Chem. Rev. 93
(1993) 1371–1448; (b) R.J.P. Corriu, C. Gue´rin, J.J.E. Moreau,
in: E.L. Eliel, S.H. Wielen, N.L. Allinger (Eds.) Topics in
Stereochemistry, vol. 15, 1984, pp 43–198; (c) R.R. Holmes,
Chem. Rev. 96 (1996) 927–950; (d) E.F. Perozzi, R.S. Michalak,
G.D. Figuly, W.H. Stevenson, D.B. Dess, M.R. Ross, J.C.
Martin, J. Org. Chem. 46 (1981) 1049–1053.
[2] (a) C. Eaborn, J.C. Jeffrey, J. Chem. Soc. (1957) 137–144; (b) C.
Eaborn, Organosilicon Compounds, Academic Press, Butter-
worths, New York, 1960; (c) A.G. Brook, Acc. Chem. Res. 7
(1974) 77–84; (d) R.J.P. Corriu, C. Gue´rin, J. Organomet.
Chem. 198 (1980) 231–320; (e) R. Damrauer, S.E. Danahey,
V.E. Yost, J. Am. Chem. Soc. 106 (1984) 7633–7634; (f) J.Y.
Corey, E.R. Corey, V.H.T. Chang, M.A. Hauser, M.A. Leiber,
T.E. Reinsel, M.E. Riva, Organometallics 3 (1984) 1051; (g) R.L.
[16] R.J.P. Corriu, J.J.E. Moreau, Bull. Soc. Chim. Fr. (1975) 901–
902.
[17] Reported [16] specific rotation for enantiomer prepared with the
use of (1S,2R)−(+)−ephedrine: [h]2D0= −34.4.
[18] (a) L.H. Sommer, W.D. Korte, P.G. Rodewald, J. Am. Chem.
Soc. 89 (1967) 862–868. (b) R.J.P. Corriu, G. Royo, Bull. Soc.
Chim. Fr. (1972) 1497–1510. Please note that in transforming of
12 (R) into 14 (R) inversion of configuration around the silicon
atom has occurred.
[19] B.E. Rossiter, in: J.D. Morrison (Ed.), Asymmetric Synthesis,
vol. 5, Academic Press, Orlando, 1985, Chapter 8.
.