Reactivity control of an allylsilane bearing a 2-(phenylazo)phenyl group by photoswitching of the coordination number of silicon

Article abstract of DOI:10.1021/ja037404m

Photoswitching of the coordination number of silicon between four and five in allyldifluoro[2-(phenylazo)phenyl]silane, which was confirmed by X-ray analysis and multinuclear NMR spectroscopy, caused multistep reactions to proceed or stop, yielding tetraf

Full text of DOI:10.1021/ja037404m

Published on Web 05/04/2004
Reactivity Control of an Allylsilane Bearing a 2-(Phenylazo)phenyl Group by
Photoswitching of the Coordination Number of Silicon
Naokazu Kano, Masaki Yamamura, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
Received July 19, 2003; E-mail: takayuki@chem.s.u-tokyo.ac.jp
Highly coordinated organosilicon compounds are known to
exhibit reactivities that are different from those of ordinary
organosilicon compounds with a tetracoordinate silicon atom.1 If
the coordination number of the silicon can be reversibly controlled
by external stimuli such as magnetism, light, etc., without influenc-
ing the reaction conditions or adding external reagents to the
reaction system, the reactivities can be altered by such stimuli.
Although a change in the coordination number of silicon between
five and six in response to heat and light was recently reported in
some organosilicon compounds, this change was not applied to
switching of the reactivities.2,3 In this paper, we report the reactivity
Figure 1. ORTEP drawing of (E)-1 (left) and 2 (right) with thermal
ellipsoid plots (50% probability). Selected bond lengths (Å) and angles (deg).
(E)-1: Si1-F1, 1.6228(12); Si1-F2, 1.5955(13); Si1-C1, 1.8640(18); Si1-
C4, 1.8578(18); N1-N2, 1.2595(18); Si1-N2, 2.389(2); F1-Si1-F2,
97.28(6); F1-Si1-C1, 102.89(7); F1-Si1-C4, 99.81(7). 2: Si1-F1,
1.6487(13); Si1-F2, 1.6865(12); Si1-F3, 1.6790(13); Si1-F4, 1.7062-
(12); Si1-C4, 1.9003(18); Si1-N2, 2.1664(17); N1-N2, 1.442(2); F1-
Si1-F2, 88.36(6); F1-Si1-F3, 94.48(6); F1-Si1-F4, 94.64(7); F1-Si1-
C4, 100.15(7).
control of an allyldifluorosilane bearing a 2-(phenylazo)phenyl
group by switching of the coordination number of silicon between
four and five induced by photoirradiation.
Allyldifluorosilane (E)-1 bearing a 2-(phenylazo)phenyl group
was synthesized from 2-iodoazobenzene2,4 via allyldiethoxy[2-
(phenylazo)phenyl]silane. In the 29Si NMR spectrum of (E)-1 in
CDCl3 at room temperature (rt), a triplet coupled with two fluorine
nuclei (1JSiF ) 278 Hz) was observed at δSi -38.5, which shifted
upfield compared with allyldifluorophenylsilane (δSi -20.4). In the
19F NMR spectrum, a broad singlet at δF -140.4 with satellite peaks
(1JSiF ) 278 Hz) at rt was split into two broad peaks at δF -141.27
(1F) and -127.17 (1F) at -90 °C. Such coupling patterns and
chemical shifts, which are similar to those of pentacoordinate silanes
bearing a 2-(dimethylaminomethyl)phenyl group,5 indicate non-
equivalency of fluorine nuclei due to a trigonal bipyramidal (TBP)
geometry around silicon constructed by coordination of a lone pair
of nitrogen of the azo group in the solution state. The pentacoor-
dinate state of the silicon atom in (E)-1 in the crystalline state was
determined by X-ray crystallographic analysis (Figure 1).6 The N2
atom of the azo moiety is directed to the Si1 atom despite the steric
repulsion of a bulky silyl group.4a The intramolecular N2‚‚‚Si1
distance (2.389(2) Å), which is almost similar to that of difluoro-
[2-(dimethylaminomethyl)phenyl](methyl)silane (2.346(3) Å), is
much shorter than the sum of the corresponding van der Waals
radii (3.65 Å).7 Therefore, the crystal structure of (E)-1 is most
likely interpreted as a distorted TBP structure with N2 and F1 atoms
at apical positions and C1, C4, and F2 atoms at equatorial positions.
In the UV/vis spectra of (E)-1, an absorption maximum (π-π*)
was observed at 339 nm in CDCl3. The color of (E)-1 is yellow,
which is in contrast to the previously reported red of tetracoordinate
2-(phenylazo)phenylsilanes, suggesting the perturbation of the
electronic structure of the azo moiety.4a
Scheme 1
-70 °C, a triplet (1JSiF ) 299 Hz) was observed at δSi -20.0, which
significantly shifted downfield compared to (E)-1. These results
suggest that (Z)-1 possesses a tetracoordinate silicon atom in the
absence of Si‚‚‚N interaction. The configuration of the lone pair
of nitrogen atom, which is at a great distance from the silicon atom,
is responsible for these spectral features of (Z)-1.
Reaction of (E)-1 with an excess amount of KF in the presence
of 18-crown-6 in CDCl3 at rt for a period of 10 min afforded 82%
yield of tetrafluorosilicate 2. A similar reaction in benzene-d6 at rt
for 30 min gave 98% yield of 2. The octahedral structure of 2 with
a hexacoordinate silicon atom was determined by NMR spectros-
copy and X-ray crystallographic analysis (Figure 1).7 Conversely,
no change was observed on treatment of (Z)-1 with an excess
amount of KF in the presence of 18-crown-6 in CDCl3 at rt for 30
min in the dark, which is in contrast to the case of (E)-1. Silicate
2 was obtained in 36% yield after a period of 2 days. However,
photoirradiation (λ ) 445 nm) of tetracoordinate (Z)-1 with an
Isomerization of (E)-1 to (Z)-1 in CDCl3 was carried out
quantitatively by irradiation (λ ) 360 nm) for 40 min (Scheme
1).8 A solution of (Z)-1 showed its absorption maximum (n-π*)
at 443 nm. Irradiation (λ ) 445 nm) of (Z)-1 at rt for 2 h caused
regeneration of (E)-1 (72%).8 (Z)-1 was thermally isomerized to
(E)-1 quantitatively after 11 days, during which it was maintained
at rt in the dark. In the 29Si NMR spectrum of (Z)-1 in CDCl3 at
9
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J. AM. CHEM. SOC. 2004, 126, 6250-6251
10.1021/ja037404m CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Chart 1
excess amount of KF in CDCl3 at rt for 30 min gave 91% yield of
2. Isomerization of (Z)-1 to (E)-1 and the successive formation of
2 should have occurred instead of direct formation of 2 from (Z)-
1. These results clearly illustrate the change in reactivities of 1
induced by photoirradiation.
coordination number of silicon induced by photoirradiation. To
promote the reaction, it is important that both the nucleophilic and
electrophilic parts are activated by the Si‚‚‚N interaction im-
mediately. Such a type of reaction control can lead to new ways of
starting or stopping a reaction without changing any other conditions
that are suitable for a certain reaction.
Considering that the allylation of azobenzene with allylmagne-
sium chloride yields 1-allyl-1,2-diphenylhydrazine (89%) and that
the nucleophilicity of an allyl group in allyl-substituted silicon
compounds is raised as the coordination number of silicon
increases,1 the main formation mechanism of 2 is described as
follows: The silicon atom of (E)-1 is fluorinated by fluoride ion
to generate intermediary silicate (E)-3. The nitrogen of (E)-3 is
allylated to obtain 4 (Scheme 2).9 Protonation at the other nitrogen
of 4 with water contained in the reaction solution and additional
fluorination at silicon with fluoride ion resulted in the formation
of 2. Monitoring the reaction solution revealed complete disap-
pearance of the absorbance due to the azo unit in UV/vis
spectroscopy and downfield shift of methylene protons of the allyl
group (δ 3.56 (brs, 1H), 4.32 (brs, 1H)) compared to those of (E)-1
Acknowledgment. We thank Shin-etsu Chemical Co., Ltd., and
Tosoh Finechem Corp. for gifts of chlorosilanes and alkyllithiums,
respectively. This work was partially supported by Grants-in-Aid
for The 21st Century COE Program for Frontiers in Fundamental
Chemistry (T.K.) and for Scientific Research Nos. 15036217 (T.K.),
15105001 (T.K.), and 14740395 (N.K.) from Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Synthetic procedures and
spectral data for (E)-1, (Z)-1, 2, (E)-5, and 6 (PDF) and X-ray
crystallographic files in CIF format for (E)-1 and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
(δ 1.87-1.94 (m, 2H)) in H NMR spectroscopy. These results
References
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The fluorination proceeded easily in both (E)-1 and the inter-
mediate with a pentacoordinate silicon atom2 because the Si‚‚‚N
interaction could increase the electrophilicity of the silicon atom
in analogy with trifluoro[2-(phenylazo)phenyl]silane. Contrary to
the behavior of (E)-1, neither allyldifluorophenylsilane nor (Z)-1
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(E)-tetrafluorosilicate (E)-52 with crotyltrifluorosilane and CsF in
refluxing THF for 24 h and successive hydrolysis, which yielded
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120 K, R1 (I > 2σ(I)) ) 0.038, wR2 (all data) ) 0.092, GOF ) 1.051.
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,
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