Facile and remarkably selective substitution reactions involving framework silicon atoms in silsesquioxane frameworks

Full text of DOI:10.1021/ja963904m

J. Am. Chem. Soc. 1997, 119, 3397-3398
Facile and Remarkably Selective Substitution
3397
Reactions Involving Framework Silicon Atoms in
Silsesquioxane Frameworks
Frank J. Feher,* Shawn H. Phillips, and Joseph W. Ziller
Department of Chemistry, UniVersity of California
IrVine, California 92697-2025
ReceiVed NoVember 12, 1996
Over the past several years, incompletely-condensed silses-
1-5
quioxane frameworks (e.g., 1-2) have attracted attention as
6
-9
models for silica,
aluminosilicates
as ligands in homogeneous models for
1
0-14
15-20
and silica-supported catalysts,
as
°
C. However, it is clear from experiments performed in CDCl3
comonomers for new families of silsesquioxane-based poly-
in NMR tubes that the reaction is complete within 20 min and
that only a slight excess (>5 equiv) of HBF4 is required.
The formation of a fluorine-containing framework was clearly
2
1,22
23
mers,
and as building blocks for network solids. For all
of these applications, any chemical modification of the silses-
quioxane has involved reactions which transform SiOH groups
into new siloxane (i.e., Si-O-Si) or heterosiloxane (i.e., Si-
O-M) linkages. In this paper we report several facile and
remarkably selective substitution reactions involving the frame-
work silicon atoms of 1. In addition to providing access to
several versatile new starting materials, these reactions provide
a powerful new methodology for functionalizing the rapidly
expanding pool of incompletely-condensed silsesquioxane frame-
works.
19
signaled by the appearance of F-coupled resonances at δ 22.27
13
(
(
d, J ) 24 Hz, 3 CH) in the C NMR spectrum and δ -65.43
29
d, J ) 24 Hz, 3 Si) in the Si NMR spectrum as well as a
19
prominent F resonance at δ -138.0. The ORTEP plot from
a preliminary X-ray crystal structure of 3 is shown in Figure 1.
The molecule crystallizes in the space group Pbca with the three
cyclohexyl groups adjacent to Si-F adopting mutually parallel
orientations with respect to their Si-C vectors. This arrange-
ment forces the silsesquioxane framework to adopt a substan-
tially more open structure compared to 1, but this structure can
be accommodated within the normal range of distances and
angles observed for cyclohexyl-substituted silsesquioxane frame-
The reaction of 1 with excess HBF4‚OMe2 occurs quickly
24
upon mixing in CH2Cl2/Et2O or CDCl3. Rather than effecting
cyclodehydration2 or producing a stable salt derived from
protonation of 1, this reaction affords a quantitative NMR yield
of a new Cs-symmetric Si/O framework, which was identified
,6
2
works.
Trifluoride 3 is surprisingly resistant to hydrolysis. It is
indefinitely stable in air, and it is uneffected by refluxing in
CDCl3 (65 °C, 4 h) with water/pyridine. However, net
hydrolysis can be accomplished in two steps by reacting 3 with
Me3SnOH (CDCl3, 65 °C, 12 h) to produce 4, which can be
subsequently hydrolyzed to 5 with aqueous HCl.
1
13
29
19
as 3 on the basis of multinuclear ( H, C, Si, F) NMR data,
25
mass spectral data, and a single-crystal X-ray diffraction study.
When performed on preparative scales in CH2Cl2/Et2O, the
reaction of 1 with excess HBF4‚OMe2 spontaneously produces
large, well-formed crystals of 3 in 96% after several days at 25
24
Both
(
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24) Detailed experimental procedures for the synthesis and characteriza-
tion of all new compounds are provided in the Supporting Information.
(
S0002-7863(96)03904-2 CCC: $14.00 © 1997 American Chemical Society

3398 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Communications to the Editor
philes. Both effects favor substitution with net inversion of
stereochemistry at silicon; protons from strongly acidic HBF4
2
7-29,32
should behave similarly.
It is tempting to propose that an excess of BF3 in commercial
(
Aldrich) HBF4 etherate might be the actual fluorinating agent,
1
9
but this suggestion is inconsistent with F NMR data as well
as the observation that the room temperature reaction of 1 with
BF3 (either alone or as an etherate) is slow and complicated by
the formation of numerous decomposition products. On the
basis of preliminary mechanistic studies, we suspect that HBF4
is indeed the fluorinating agent and that a slight excess of BF3
is required to prevent small amounts of HF from destroying
the silsesquioxane framework. Whatever the mechanism, it is
clear that the rapid, high yield formation of 3 requires both HBF4
and BF3.
The reactions of 3 with Me3SnOH and MeLi both proceed
with complete retention of stereochemistry at silicon. Although
the mechanistic details for nucleophilic substitution of these very
different reagents are not clear, both results are consistent with
the general observation that silyl fluorides react with hard
nucleophiles (e.g., alkyl lithium reagents) to give substitution
Figure 1. ORTEP plot of 3. For clarity, thermal ellipsoids are plotted
at 50% probability, and only C’s attached to Si’s are shown.
the highly stereoselective outcomes of many reactions.27-29
Numerous factors can influence the course of a reaction, and
the overall stereochemical outcome of substitution is usually
determined by the relative rates of competing pathways for
27-29,33
with net retention of stereochemistry at silicon.
The synthetic transformations reported in this paper provide
a potentially powerful new methodology for synthetically
manipulating silsesquioxane frameworks. Although the general-
ity of this methodology remains to be explored, both the
stereospecific nature of these transformations and the facility
with which they occur allow unprecedented synthetic manipula-
tion of incompletely-condensed silsesquioxane frameworks. Our
efforts to expand the scope of these reactions will be reported
in a future article.
28
inversion or retention of configuration. In general, stereo-
chemical inversion is favored with good leaving groups and/or
weakly nucleophilic (i.e., “soft”) reagents. However, cyclic
silicon compounds often show a greater preference for retention
of stereochemistry than open-chain analogs; for example, the
acetolysis of cyclic chlorosiloxanes in acetic acid/acetic anhy-
dride appears to proceed with a high degree (>92%) of
stereochemical retention.30 For the reaction of 1 with
HBF4‚OMe2, the formation of 3 with net inversion of stereo-
chemistry at Si parallels the known reactions of silanols (R3-
Acknowledgment. These studies were supported by the National
Science Foundation and Phillips Laboratory (Edwards AFB).
SiOH), silyl ethers (R3SiOR′), and silyl amines (R3SiNHR) with
BF3.31 In each of these cases, the presence of strongly Lewis
Supporting Information Available: A listing of experimental
procedures and supporting characterization data for the synthesis of
all new compounds and X-ray data for 3 and 6, including experimental
procedures, tables of crystal data, atomic coordinates, thermal param-
eters, bond lengths, angles, and ORTEP figures (24 pages). See any
current masthead page for ordering and Internet access instructions.
acidic boron centers facilitates loss of the leaving group via
coordination while preventing the formation of strong nucleo-
(
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JA963904M
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6.