Competition between π-arene and lone-pair halogen coordination of silylium ions?

Article abstract of DOI:10.1021/ja2040392

In 2,6-diarylphenylSiR₂ cations, the 2,6-diarylphenyl (m-terphenyl) scaffold blocks incoming nucleophiles and stabilizes the positive charge at silicon by lateral ring interactions. Direct ortho-halogen and π-electron-rich face coordination to

Full text of DOI:10.1021/ja2040392

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Competition between π-Arene and Lone-Pair Halogen Coordination
of Silylium Ions?
Paola Romanato, Simon Duttwyler, Anthony Linden, Kim K. Baldridge,* and Jay S. Siegel*
Organic Chemistry Institute, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
S Supporting Information
b
ABSTRACT: In 2,6-diarylphenylSiR2 cations, the 2,6-
diarylphenyl (m-terphenyl) scaffold blocks incoming
nucleophiles and stabilizes the positive charge at silicon by
lateral ring interactions. Direct ortho-halogen and π-electron-
rich face coordination to silicon has been seen. For a series of
cations bearing 2,6-difluoro-20,60-dimethyl-Xn-substituted
rings, the relative contribution of these two modes of
stabilization has been assessed. Direct coordination from
an aryl fluoride is found to be comparable to that from the
mesityl π-face.
Figure 1. 29Si NMR shifts of different terphenylsilylium ions: (black)
molecules with preferential π-arenefSi interactions; (blue) molecules with
ꢀ
preferential halogenfSi interactions. Solvent, C6D6; anion, B(C6F5)4
.
he quest for stable silylium ions, R3Si+,1 has led to the use of
the 2,6-diarylphenyl (m-terphenyl) scaffold as a substituent,
T
Scheme 1
which can block incoming nucleophiles and stabilize the positive
charge at silicon by lateral ring interactions. When the lateral rings
are π-electron-rich because of methyl groups, single η1 π coordina-
tion to the silicon center dominates.1c In contrast, halogenfSi
interactions dominate when chlorine or fluorine atoms are at the
ortho positions of the flanking rings (I and II in Figure 1).2,3 Are
the energetic details of these two modes of stabilization comparable?
Would π effects compete or cooperate with the halogenfSi
interactions? This study of a series of cations 1 bearing 2,6-
difluoro- and 2,6-dimethyl-Xn-substituted rings indicates a “friendly”
competition between the two modes of stabilization: lower-
basicity xylyl and mesityl rings (1a, 1b) contribute less than the
FfSi interactions, whereas higher-basicity duryl and penta-
methylphenyl rings (1c, 1d) contribute more than the FfSi
interactions.
tautomers in which SiꢀF coupling is detectable (Figure 2). The
unresolved signals of 1c and 1d imply weak interactions with the
ortho fluorine atoms, whereas an analysis of the 13C NMR shifts of
the lateral rings in these cations is consistent with η1 π coordina-
tion by the Cortho atoms of the methylated rings.6 Thus evolves a
picture for 1aꢀd in which, among structures of an equilibrium
that is fast on the NMR time scale, FfSi interactions contribute
demonstrably in 1a and 1b but to a lesser extent in 1c and 1d.
M06-2X/Def2-TZVPP calculations predicted cations 1 to
adopt a C1-symmetric geometry with a low barrier to dynamic
exchange of silicon among the preferential coordination sites.7 In
the specific cases, two minima were predicted for 1a and 1b, with
halogenfSi preferred over π-arenefSi, whereas only a single
π-arenefSi conformation was predicted for 1c and 1d.
Single Negishi coupling of triazene 34 followed by treatment
of 4 with iodine afforded biphenyl 5. Hart-type coupling5 of 5 and
subsequent lithiation/silylation furnished silanes 7. Cations 1
were prepared by hydride abstraction using [Ph3C][B(C6F5)4]
(Scheme 1).
NMR spectroscopy studies suggest analogies between 1a/1b
and IIa and between 1c/1d and Ia (Figure 1). The experimental
and calculated 29Si NMR shift data (Table 1) show 1a and 1b
(δ ≈ 100 ppm) to be more deshielded than 1c and 1d (δ ≈
60 ppm). In 1a and 1b, the signal multiplicity for the SiMe2
fragment (1H, 13C, 29Si) indicates a dynamic equilibrium of
Computational structural predictions fully matched the solid-
state structures of 1a and 1d (Figure 3), which were obtained as
Received: May 11, 2011
Published: July 18, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja2040392 J. Am. Chem. Soc. 2011, 133, 11844–11846
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Table 1. M06-L/Def2-TZVPP//M06-2X/Def2-TZVPP CSGT
29Si NMR (ppm) Predictions for 1a, 1b, 1c, and 1d in Toluene
Table 2. Selected Bond Lengths (Å) and Angles (deg) for the
Single-Crystal X-ray Structures and the Calculated C1 Con-
formers of 1a and 1d
1a
1b
1c
1d
1a
exptl
1d
exptl
exptl
calcd
101.3
98.9
95.5
97.6
60.1
61.6
57.3
56.2
parameter
F1fSi1
calcd
parameter
calcd
1.8658(8) 1.8880 C16fSi1
1.448(2) 1.4246 C16ꢀC21
1.346(2) 1.3327 C20ꢀC25
356.8(1) 356.58 ∑CꢀSiꢀC
0.190(1) 0.198 dfp-Sia
2.089(2) 2.1703
1.546(2) 1.5355
1.508(3) 1.5048
345.5(2) 349.94
0.413(1) 0.344
C10ꢀF1
C14ꢀF2
∑CꢀSiꢀC
dfp-Sia
dihedral angleb 29.4(1)c
29.90 dihedral angleb 45.3(1)d 50.54
a Distance between the Si atom and the plane defined by the three C
atoms bound to Si. b Angle between the least-squares planes of a flanking
ring and the central ring. c Between the ring containing F1 and the
central ring. d Between the ring containing C16 and the central ring.
Scheme 2
Figure 2. NMR analysis of 1a, 1b, 1c, and 1d: signals for methyl groups
at silicon are shown in blue, signals for silicon in red, and signals for
fluorine in green, each in the fast-exchange limit. Solvent, C6D6; anion,
ꢀ
B(C6F5)4
.
Figure 3. X-ray structures of (left) [1a][CB11H6Cl6] and (right)
[1d][CB11H6Cl6] with 35% probability ellipsoids; anions and hydrogen
atoms have been omitted. Dashed lines show the FfSi and π-arenefSi
interactions.
Figure 4. X-ray structure of 2bꢀCB11H6Cl6 with 35% probability
ellipsoids; hydrogen atoms have been omitted.
CorthoꢀSi π coordination comparable in strength to that of Ib,
although their δ(29Si) signals differed by ∼15 ppm.
solvent-free salts with the carborane anion CB11H6Cl6ꢀ.8 1a
exhibits fluorine coordination with a Si1ꢀF1 distance of
1.8658(8) Å, which is longer than the SiꢀF bond length of
1.600(1) Å in Me3SiF (Table 2).9 X-ray analysis of 1d
revealed π coordination via Cortho with a Si1ꢀC16 distance
of 2.089(2) Å, which is longer than the SiꢀC bond length of
1.875(2) Å in Me4Si.9 In both cations, the dihedral angle
between the coordinating ring and the central ring deviates
significantly from 90°, whereas the noninteracting ring adopts
an almost perpendicular conformation relative to the central
ring.
Crystals of 2b were obtained with the carborane anion
CB11H6Cl6ꢀ. The X-ray analysis revealed an interaction between
a lower-belt chlorine atom of the carborane and silicon
(Figure 4). The Si1ꢀCl1 distance is 2.3130(5) Å, which is
almost identical to that in iPr3ꢀCB11H6Cl6;12 coordination of
2b by the anion causes a pyramidalization of the silicon center, as
shown by the sum of angles around silicon [∑CꢀSiꢀC =
351.40(12)°] and by the corresponding out-of-plane distance
[dfp-Si = 0.3154(4) Å].13
Electron-rich arenes and aryl halides are donors for silylium
ions. A delicate balance between these two coordination modes
in silylium ions and a clear break point between mesityl- and
duryl-substituted cations have been found. Arenes with reduced
π basicity and no possible halogenfSi interactions (2) poorly
accommodate the avidity of Si+ for electron density, allowiꢀng
Aspirations to obtain a truly tricoordinate silylium ion led to
the synthesis of cations 2 (Scheme 2). Hydride abstraction from
ꢀ
8a with different trityl salts [Y = B(C6F5)4ꢀ, CB11H6Cl6
]
showed the formation of Ph3CH; however, cation 2a was not
observed. Instead, several decomposition products, including
fluorosilane 2aꢀF, were formed.10,11 In contrast, 2b was gener-
ated cleanly. 13C NMR analysis6 of [2b][B(C6F5)4] revealed a
coordination by anions as weakly basic as carborane (CB11H6Cl6
to be observed in the crystal.
)
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dx.doi.org/10.1021/ja2040392 |J. Am. Chem. Soc. 2011, 133, 11844–11846

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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, compu-
b
tational details, and CIFs for [1a][CB11H6Cl6], [1d][CB11H6Cl6],
and 2bꢀCB11H6Cl6. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
kimb@oci.uzh.ch; jss@oci.uzh.ch
’ ACKNOWLEDGMENT
This work was supported by the Swiss National Science
Foundation. K.K.B. gratefully acknowledges D. Truhlar for
access to G03-mngfm41 at MSI.
’ REFERENCES
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(13) Bond lengths and angles are listed in the Supporting Information.
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dx.doi.org/10.1021/ja2040392 |J. Am. Chem. Soc. 2011, 133, 11844–11846