Rearrangement of o-(chloromethyldimethylsilyl)phenylmethoxide: Evidence for an apical position of the migrating group in a trigonal bipyramid intermediate

Article abstract of DOI:10.1039/a801377b

Chloromethylsilane 5a undergoes a rearrangement reaction to give oxasilacyclopentane 9 rather than oxasilacyclohexane 10, indicating that the methyl group migrates in preference to the aryl group.

Full text of DOI:10.1039/a801377b

View Article Online / Journal Homepage / Table of Contents for this issue
Rearrangement of o-(chloromethyldimethylsilyl)phenylmethoxide: evidence for
an apical position of the migrating group in a trigonal bipyramid intermediate
Yousef M. Hijji,† Paul F. Hudrlik*‡ and Anne M. Hudrlik
Department of Chemistry, Howard University, Washington, DC 20059, USA
Chloromethylsilane 5a undergoes a rearrangement reaction
OH
OSiR3
OH
SiR3
to give oxasilacyclopentane 9 rather than oxasilacyclohex-
ane 10, indicating that the methyl group migrates in
preference to the aryl group.
Br
Br
5
6
Scheme 3
Rearrangements of a-substituted organosilanes, with migration
of an organic group from silicon to carbon, have been found to
take place under acidic,1 basic1–4 or thermal1 conditions. Little
is known about the stereochemistry of the migration reaction,
although migration from the apical position of a trigonal
bipyramid has been suggested.4 In contrast, the stereochemistry
of nucleophilic substitutions at silicon has been extensively
studied, and is generally interpreted as proceeding through
pentacoordinate trigonal bipyramid intermediates, usually with
entering and leaving groups in the apical positions.5 With the
increasing use of carbon-functional organosilicon compounds
in organic synthesis, it is important to establish the ster-
eochemistry of the fundamental reactions in organosilicon
chemistry.6 We report the first evidence for the stereochemistry
of the base-induced rearrangement.
We have previously found the g-oxidopropyl group on silicon
to be effective as an internal nucleophile, and reported the
cleavage reactions of g-hydroxysilanes 1 (for generation of
benzyl and allyl anion equivalents) (Scheme 1),7 and the
rearrangement reactions of g-hydroxysilanes 2 (forming car-
bon–carbon bonds via a silicon template) (Scheme 2).2
More rigid g-hydroxysilanes would be expected to be more
efficient for this purpose, and we have therefore been studying
the use of o-silylbenzyl alcohols 6 in the cleavage and
rearrangement reactions. In the course of this work, we have
found that o-silyl benzyl alcohols 6 can be easily prepared by
rearrangement of silyl ethers 5 of o-bromobenzyl alcohol with
Li or ButLi in Et2O (Scheme 3).8
Me2CH2Cl in Et2O–NEt3 for 1 h at room temperature. [An
impurity (ca. 6% by GC), believed to be (ClCH2Si)2O was
present: GC–MS m/z 181 (40%, M+ 2 CH2Cl), 153 (100);
1
additional peaks at d 2.76 (s) and 0.24 (s) in the H NMR
spectrum.] Treatment of 5a with ButLi in THF (30 min at
278 °C, then removal of the cold bath for 5 min, and workup by
addition of saturated NaHCO3) led to disappearance of 5a with
appearance of one product, assigned as oxasilacyclopentane 9.
[Some (ClCH2Si)2O (ca. 7% by GC) was also observed in the
GC and GC–MS.] Compound 9 is believed to arise via
rearrangement (see Scheme 3) to g-oxidosilane 7, followed by
methyl migration (presumably via trigonal bipyramid interme-
diate 8, see Scheme 4). Aryl migration would have given
oxasilacyclohexane 10.
The structure of 9 was supported by the spectra, in particular
the mass spectrum, which showed the base peak at 149 (M+ 2
Et), and the 1H NMR spectrum, which was different from that
reported for oxasilacyclohexane 10.9 The 1H NMR spectrum of
the crude product included d 7.2–7.6 (m, ArH) 5.16 (s, CH2O)
and 0.39 (s, MeSi), as well as peaks due to (ClCH2Si)2O
(singlets at d 2.76 and 0.24) and a small amount of THF.
[Comparison with the reported 1H NMR spectrum of compound
109 suggested that 10, if present, was less than 10%.] The 13C
NMR spectrum showed d 6.30 (SiCH2CH3), 8.83 (SiCH2CH3),
71.72 (CH2O), 121.55, 126.73, 129.49, 131.21, 134.14 (small),
150.07 (small), as well as 30.72 (small, SiCH2Cl), peaks for
THF, and three peaks in the SiMe region. The GC–MS showed
m/z 178 (15%, M+), 163 (8, M+ 2 Me), 149 (100, M+ 2 Et), 135
(8) and 105 (11). The structure of 9 was further confirmed by
Silyl ether 5a provides an opportunity to study the relative
importance of migratory aptitude (aryl vs. methyl) compared to
stereochemical preference of the migrating group (apical vs.
equatorial). Compound 5a§ was prepared in 78% yield by
treatment of o-bromobenzyl alcohol with 1.3 equiv. of ClSi-
Si
Cl
O
Z
(1) base
(2) E +
Si
O
Si
HO
+ E–Z
Br
5a
1 Z = Bn, allyl
–
Scheme 1
O
O
Cl
–
Si
Si
Cl
R2Li
R1
Si
HO
Cl
Si
O
R1
7
8
2
3
a R1 = Ph
b R1 = Me
c R1 = 2-furyl
d R1 = allyl
O
O
NOT
Si
R1
Si
HO
Si
R2
4
10
9
Scheme 2
Scheme 4
Chem. Commun., 1998
1213

View Article Online
treatment (in THF) with excess MeLi (278 °C, 1 h) which gave
a mixture of o-(ethyldimethylsilyl)benzyl alcohol8 (major) and
o-(trimethylsilyl)benzyl alcohol8 (minor).10
Notes and References
† Present address: Morgan State University, Baltimore, MD 21251, USA.
‡ E-mail: phudrlik@fac.howard.edu
§ The IR, 1H NMR and mass spectra were in agreement with the
structure.
Migratory aptitudes in rearrangements of halomethylsilanes
under basic conditions have been correlated with the ability of
the migrating group to stabilize a negative charge.3,4 In our
studies of the rearrangement reactions of g-hydroxysilanes 2,
we found that the phenyl group had a greater migratory aptitude
than the methyl group, as expected.2 Treatment of
g-hydroxysilane 2a with several bases resulted in oxasila-
cyclopentane 3a, the product of phenyl migration; in some
cases, a small amount ( < 5%) of 2-phenyl- 2-ethyl-1-oxa-
2-silacyclopentane, the product of methyl migration, was also
observed. (Treatment of g-hydroxysilane 2a with 2 equiv. of
MeLi in Et2O gave product 4a, presumably by trapping of 3a
with MeLi.)2
The comparison of the migratory aptitudes in the acyclic
system 2a (phenyl migration) with those in the cyclic system 5a
(methyl migration) is very interesting. Perhaps the cyclic
system 8a has geometrical constraints which disfavor aryl
migration. To the extent oxygen would prefer the apical position
in a trigonal bipyramid intermediate (apical entry, electro-
negativity, see 8),11 the Si–Ar bond in the o-benzyl alcohol
system would have to be equatorial. The preference for methyl
migration in 5a suggests that migration is favored by an apical
position of the migrating group in a trigonal bipyramid
intermediate. This work also suggests that o-silylbenzyl alcohol
substrates should be useful for carbon–carbon bond forming
reactions via rearrangements without interference from the aryl
group.
1 For leading references, see P. F. Hudrlik, Y. M. Abdallah, A. K.
Kulkarni and A. M. Hudrlik, J. Org. Chem., 1992, 57, 6552.
2 P. F. Hudrlik, Y. M. Abdallah and A. M. Hudrlik, Tetrahedron Lett.,
1992, 33, 6743 and references cited therein.
3 R. Damrauer, V. E. Yost, S. E. Danahey and B. K. OAConnell,
Organometallics, 1985, 4, 1779.
4 S. L. Aprahamian and H. Shechter, Tetrahedron Lett., 1990, 31,
1089.
5 Although not simultaneously for reactions proceeding with retention of
stereochemistry: R. R. Holmes, Chem. Rev., 1990, 90, 17; A. R.
Bassindale and P. G. Taylor, in The Chemistry of Organic Silicon
Compounds, ed. S. Patai and Z. Rappoport, Wiley, New York, 1989,
part 1, pp. 839–892.
6 I. Fleming, A. Barbero and D. Walter, Chem. Rev., 1997, 97, 2063.
7 P. F. Hudrlik, Y. M. Abdallah and A. M. Hudrlik, Tetrahedron Lett.,
1992, 33, 6747.
8 Y. M. Hijji, P. F. Hudrlik, C. O. Okoro and A. M. Hudrlik, Synth.
Commun., 1997, 27, 4297.
9 E. Baciocchi, R. Bernini and O. Lanzalunga, J. Chem. Soc., Chem.
Commun., 1993, 1691.
10 For other examples of cleavage at silicon by intramolecular alkoxide,
see: C. Ru¨cker, Tetrahedron Lett., 1984, 25, 4349; W. Kirmse and
F. So¨llenbo¨hmer, J. Chem. Soc., Chem. Commun., 1989, 774.
11 Nucleophilic substitution reactions at silicon are felt to occur via apical
entry, and although pseudorotation of the pentacoordinate intermediate
is possible, oxygen is more apicophilic than carbon. See ref. 5 and
C. Chuit, R. J. P. Corriu, C. Reye and J. C. Young, Chem. Rev., 1993,
93, 1371; R. R. Holmes, Chem. Rev., 1996, 96, 927.
Financial support from the National Science Foundation
(CHE-9505465 and CHE9007879) is gratefully acknow-
ledged.
Received in Corvallis, OR, USA, 16th February 1998; 8/01377B
1214
Chem. Commun., 1998