Intramolecular halogen stabilization of silylium ions directs gearing dynamics

Article abstract of DOI:10.1021/ja9109665

2,6-Bis(2,6-difluorophenyl)phenyldimethylsilanium ion (1a) adopts a ground-state C₂ trigonal-bipyramidal geometry in which fluorines from opposing 2,6-difluorophenyl groups coordinate to the apical positions of the pentavalent silyl cation. Exchange of fluorine at silicon occurs by a disrotatory gearing mechanism wherein one fluorine remains coordinated to silicon throughout the circuit. The cogwheel transition state is C_s-symmetric, with one ring having a dihedral angle of 0° and the other a dihedral angle of 90°. This correlated dynamic process is a function of the coordinating halogen. The chloro derivative (1b) adopts a similar ground-state geometry, but exchange of chlorine at silicon follows a conrotatory process.

Full text of DOI:10.1021/ja9109665

Published on Web 05/25/2010
Intramolecular Halogen Stabilization of Silylium Ions Directs Gearing
Dynamics
Paola Romanato, Simon Duttwyler, Anthony Linden, Kim K. Baldridge,* and Jay S. Siegel*
Organic Chemistry Institute, UniVersity of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
Received December 30, 2009; E-mail: kimb@oci.uzh.ch; jss@oci.uzh.ch
Silicon cations1 are highly reactive Lewis acids that are useful in
Scheme 1
chemical synthesis.2,3 Even relatively weak Lewis bases, such as the
π-basic solvent toluene, form tetrahedral complexes with silylium
ions.4,5 Intramolecular π coordination in cationic silicon species with
a 2,6-diarylphenyl scaffold distorts the ideal C2V-symmetric geometry
to adopt the C1-symmetric geometry of a Wheland-like complex.6 The
energy difference between the C2V and C1 forms decreases with
decreasing π basicity of the flanking aromatic rings. Systems with
lateral rings having lower basicity than benzene are likely candidates
for cations without internal π coordination. On the basis of this idea,
silyl cations 1 bearing 2,6-dihalophenyl substituents were synthesized.
(∆δ ≈ 3 ppm). Computations further predicted that cation 1a should
However, instead of free tricoordinate ions, NMR and X-ray crystal-
undergo fluorine exchange at silicon via a disrotatory gearing of
lographic studies revealed a trigonal-bipyramidal geometry with
the lateral aryl rings (Figure 3); the circuit (B1-A2)2 was predicted
bridging halogen atoms as the apical ligands.7
to require only 4.5 kcal mol-1. In contrast, cation 1b was predicted
to exchange chloro substituents at silicon via a B1-A1-B1
conrotatory librational process.
Crystals of composition [1a][CB11H6Cl6] were obtained from a
-
C6H5Cl/C6H14 mixture using the carborane CB11H6Cl6 as the
counterion.16,17 X-ray crystallographic analysis revealed the cation
structure to be essentially the C2 trigonal-bipyramidal B1-C2 form
Double Negishi coupling on triazene 58 led to diarylphenyltria-
zene 4, which gave the corresponding iodoterphenyl 3a when treated
with iodine.9 Lithiation of 310 followed by reaction with chlo-
rodimethylsilane afforded the silanes 2. Hydride abstraction by
[Ph3C][B(C6F5)4] gave the silylium ions [1][B(C6F5)4] (Scheme 1).
The 29Si NMR resonances of 1a (89 ppm) and 1b (91 ppm) are
downfield-shifted relative to their neutral precursors (-20 ppm), an
indication that species with partial positive charge on silicon are formed.
While these values are still far from that of a free silylium ion,11 they
indicate deshielding in comparison with classical silanium ions.12
1H and 29Si NMR spectra of 1a revealed magnetic coupling to the
fluorine substituents (Figure 1). The signal of the methyl groups
attached to silicon in 1a appeared as a quintet, and the same multiplicity
was maintained upon cooling to 223 K. Additionally, the silicon signal,
which appeared as a broad resonance at room temperature, was
resolved into a quintet at lower temperature.13 Coordination by fluorine
can explain the multiplicity and the coupling constant (J ) 32 Hz) of
the silicon signal; the effect of this interaction reached the methyl
groups covalently bonded to silicon. 19F NMR spectroscopy revealed
signal isochrony for the fluorine substituents over the temperature range
293-223 K. Computational analysis of a conformational graph showed
that this equivalence originated from a dynamic exchange of lower-
symmetry conformations rather than a static conformer having C2V
symmetry.
Figure 1. Variable-temperature 1H and 29Si NMR analysis of 1a.
Eight conformations of 1 can be arranged into a bipartite graph
(K4,4) with conrotatory or disrotatory (correlated one-ring-flip)
connections (Figure 2).14 Calculations at the density functional
theory level favored a C2 ground state (B1-C2) for 1a and 1b in
which the silicon interacts with one halogen atom from each of
the opposing flanking rings (Table 1);15 the experimental 29Si NMR
values matched well the computed values for the B1-C2 conformers
Figure 2. Bipartite conformational graph of 1 viewed along the Si-Caryl axis.
Blue paths, disrotatory; red paths, conrotatory; bold blue path, gearing circuit.
GS, ground state; TS, transition state; HSP, higher-order stationary point. Bold
black lines, lateral aryl rings; wedges, methyl groups at silicon.
9
7828 J. AM. CHEM. SOC. 2010, 132, 7828–7829
10.1021/ja9109665  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. B98/DZ(2df,pd)-Calculated Relative Gas-Phase Energies
(kcal mol-1) and B98/DZ+(2df,pd) 29Si NMR Shifts (ppm)a for 1a
and 1bb
rings in silanes 2 versus cations 1 suggests a similar coordination
mode in 1a and 1b.19 For the favored C2 conformation of 1b, the
Si-Cl distance (coordinating chlorine atoms) was predicted to be
2.661 Å, ∼0.61 Å longer than in a single Si-Cl bond.20
1a
1b
29Si NMR shift
29Si NMR shift
This new class of silylium ions, coordinated by neutral halogen
+
atoms that are part of C(Ar)-X bonds, displays binding to SiR3
Erel
calcd
exptl
Erel
calcd
exptl
that is energetically comparable to that of a π-basic ligand such as
benzene. Desymmetrization of the meta-terphenyl scaffold via
replacement of one halogenated ring with a methylated ring can
refine the energetic details of lone-pair (halogen) or π (aryl) donor
stabilization of silyl cations.
A1-C2V
A2-Cs
A3-C2V
B1-C2
B2-Cs
13.2
4.5
22.0
0.0
272.2
160.3
33.6
-
-
9.3
16.6
79.8
0.0
208.3
195.5
13.6
-
-
-
-
87.6c
69.2
88.6
-
93.8c
66.3
90.5
-
9.6
21.4
a NMR shift data calibrated relative to TMS. b For additional
computational details, see the Supporting Information. c B98/
DZ+(2df,pd) calibrated 29Si NMR shifts in toluene: 1a, 87.2; 1b, 93.7.
Acknowledgment. This work was supported by the Swiss
National Science Foundation.
Supporting Information Available: Experimental procedures,
computational
details,
and
CIFs
for
[1a][CB11H6Cl6],
[1a·THF][B(C6F5)4],16 and [1a·Et2O][B(C6F5)4].16 This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Kochina, T. A.; Vrazhnov, D. V.; Sinotova, E. N.; Voronkov, M. G.
Russ. Chem. ReV. 2006, 75, 95. (b) Reed, C. A. Acc. Chem. Res. 1998, 31,
325. (c) Lambert, J. B.; Zhao, Y.; Zhang, S. M. J. Phys. Org. Chem. 2001,
14, 370.
Figure 3. Proposed conformational gearing circuit.
(2) For catalytic activation of carbonyl groups, see: (a) Klare, H.; Bergander,
K.; Oestreich, M. Angew. Chem., Int. Ed. 2009, 48, 9077. (b) Hara, K.;
Akiyama, R.; Sawamura, M. Org. Lett. 2005, 7, 5621.
(3) For Lewis base activation of silicon Lewis acids, see: Denmark, S. E.;
Chung, W. J. Org. Chem. 2008, 73, 4582.
(4) This paper uses current IUPAC recommendations for cationic species (see:
Powell, W. H. Pure Appl. Chem. 1993, 65, 1357). The term silylium ion
refers to a tricoordinate silicon cation. A silanium ion is a pentacoordinate
species with a formal positive charge at silicon. The expression silyl cation
is used for species without specification of the coordination number.
(5) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13, 2430.
(6) Duttwyler, S.; Do, Q.; Linden, A.; Baldridge, K. K.; Siegel, J. S. Angew.
Chem., Int. Ed. 2008, 47, 1719.
(7) For halogen f Si coordination in silylium ions, see: (a) Lehmann, M.;
Schulz, A.; Villinger, A. Angew. Chem., Int. Ed. 2009, 48, 7444. (b) Panisch,
R.; Bolte, M.; Mu¨ller, T. J. Am. Chem. Soc. 2006, 128, 9676. (c) Ku¨ppers,
T.; Bernhardt, E.; Eujen, R.; Willner, H.; Lehmann, C. W. Angew. Chem.,
Int. Ed. 2007, 46, 6346. (d) Hoffmann, S. P.; Kato, T.; Tham, F. S.; Reed,
C. A. Chem. Commun. 2006, 767. (e) Xie, Z.; Manning, J.; Reed, R. W.;
Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am. Chem. Soc.
1996, 118, 2922.
(8) Liu, C.-Y.; Knochel, P. Org. Lett. 2005, 7, 2543.
(9) Moore, J. S. Tetrahedron Lett. 1994, 35, 5539.
(10) For the synthesis of 3b, see: Saednya, A.; Hart, H. Synthesis 1996, 1455.
(11) A typical value would be 225 ppm, as for trimesitylsilylium ion. See: Kim,
K.-C.; Reed, C. A.; Elliot, D. W.; Mueller, L. J.; Tham, F.; Lin, L.; Lambert,
J. B. Science 2002, 297, 825.
(12) For silanium ions, see: (a) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009,
42, 303. (b) Chauhan, M.; Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reye´, C.
Organometallics 1996, 15, 4326. (c) Belzner, J.; Scha¨r, D.; Kneisel, B. O.;
Herbst-Irmer, R. Organometallics 1995, 14, 1840. (d) Berlekamp, U.-H.;
Jutzi, P.; Mix, A.; Neumann, B.; Stammler, H.-G.; Schoeller, W. W. Angew.
Chem., Int. Ed. 1999, 38, 2048. (e) Ebata, K.; Inada, T.; Kabuto, C.; Sakurai,
H. J. Am. Chem. Soc. 1994, 116, 3595.
(13) For an earlier example of a rapidly exchanging F-Si system, see: Olah,
G. A.; Mo, Y. K. J. Am. Chem. Soc. 1971, 93, 4942.
(14) Mislow, K. Chemtracts: Org. Chem 1989, 2, 151.
Figure 4. ORTEP plot of [1a][CB11H6Cl6] with 30% probability ellipsoids;
the anion and hydrogen atoms have been omitted. Dashed lines show the Si-F
interactions. The conformation is consistent with the B1-C2 form (cf. Figure
1).
Table 2. Selected Bond Lengths (Å) and Dihedral Angles (deg) for
the Calculated C2 Conformer and the Single-Crystal X-Ray
Structure of 1a
C2 calcd
parameter
gas-phasea
in tolueneb
exptl
F2 f Si
F4 f Si
2.118 [2.126]
2.118 [2.126]
1.397 [1.397]
1.397 [1.397]
1.331 [1.332]
1.331 [1.332]
35.14
2.133
2.133
1.394
1.394
1.336
1.336
36.25
36.25
0.0
2.151(2)
2.065(2)
1.410(3)
1.421(3)
1.352(3)
1.351(3)
38.28(13)d
31.14(13)e
0.0184(7)
C14-F2
C20-F4
C10-F1
C16-F3
dihedral anglec
35.14
0.0
dfp-Sif
a B98/DZ(2df,pd) [B98/DZ+(2df,pd)]. b B98/DZ+(2df,pd) in toluene.
c Angle between the least-squares planes of a flanking ring and the central
ring. d Between the ring containing F2 and the central ring. e Between the
central ring and the ring containing F4. f Distance between the Si atom and
the plane defined by the three C atoms bound to Si.
(15) The C1 starting geometry converged to the C2 structure.
(16) Initial attempts to crystallize 1a showed that traces of THF and Et2O had
coordinated to the silicon center. Cocrystals containing [1a·THF][B(C6F5)4]
and [1a · Et2O][B(C6F5)4] were obtained, and X-ray analysis displayed a
distorted-tetrahedral arrangement around silicon (see the Supporting
Information).
(17) For the carborane CB11H6Cl6-, see: Reed, C. A. Acc. Chem. Res. 1998,
31, 133.
predicted computationally (Figure 2; cf. Figure 2). The F2-Si and
F4-Si distances are longer than a typical F-Si bond by 0.55 and
0.46 Å, respectively (Table 2).18 The sum of C-Si-C angles,
359.9(1)°, and the F2-Si-F4 angle, 174.20(6)°, indicate that the
fluorine atoms occupy the apical positions of a trigonal bipyramid.
Spectroscopic data and computations support similar ground
states for 1a and 1b. The observed 29Si NMR shifts for 1a and 1b
indicate a slightly weaker electron donation to silicon by 1b.
Comparison of the differences in the 13C NMR shifts of the flanking
(18) The Si-F bond length of 1.600(1) Å in Me3SiF (see: Rempfer, B.;
Oberhammer, H.; Auner, N. J. Am. Chem. Soc. 1986, 108, 3893) was used
as a reference.
(19) For comparison of the 13C NMR shifts in cations 1, see the Supporting
Information. Analysis of the 13C NMR shifts of analogues with methylated
flanking rings revealed π coordination via Cortho with a resonance pattern
typical of Wheland intermediates (ref 6). The data for cations 1 are not
consistent with this kind of electron donation.
(20) Reference 7a indicates a Si-Cl bond length of 2.055(2) Å for Me3SiCl.
JA9109665
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