Ring-opening protonolysis of strained silicon-containing rings: A new approach to ions with silylium character

Full text of DOI:10.1021/ja993602v

2126
J. Am. Chem. Soc. 2000, 122, 2126-2127
Scheme 1
Ring-Opening Protonolysis of Strained
Silicon-Containing Rings: A New Approach to Ions
with Silylium Character
Mark J. MacLachlan, Sara C. Bourke, Alan J. Lough, and
Ian Manners*
Department of Chemistry, UniVersity of Toronto
80 St. George Street, Toronto, ON, Canada M5S 3H6
ReceiVed October 7, 1999
The existence of silylium ions [R3Si]+ has aroused a great deal
of controversy and the earliest reported examples were subse-
quently shown to have their perchlorate “counteranion” covalently
bound.1-5 Silylium ions are much more reactive than carbocations,
[R3C]+, and are generally solvated in solution. However, Lambert
and co-workers have shown that [Mes3Si]+ is an almost com-
pletely isolated cation in the presence of a noncoordinating
counterion such as the tetrakis(pentafluorophenyl)borate anion.6
Silylium ions are interesting from a fundamental perspective, as
reactive intermediates, and as synthetic reagents as a consequence
of their Lewis acidity.1-3,7 For example, tetracoordinate Si cations
[R3SiL]+ have been postulated to be reactive intermediates in the
cationic polymerization of hexamethylcyclotrisiloxane,
[Me2SiO]3.8
There are currently two main routes for the formation of cations
with high silylium character.1-4 The first involves electrophilic
abstraction of X- from four-coordinate R3SiX molecules and is
driven by the insolubility of metal salts (e.g. eq 1a), or the
formation of a strong C-H bond (e.g. eq 1b). The second route
involves electrophilic addition of E+ to an allyl substituent on
Si, leading to elimination of CH2dCH-CH2E (eq 2). In this paper,
we present a novel ring-opening route to silylium ions that is
free of byproducts.
reactive cationic silicon intermediate that could extract F- from
- 2,10
BF4
.
Coordination of an electron-rich iron atom from a
ferrocenyl substituent might be expected to stabilize a silylium
ion,4b,11 in a manner similar to the carbonium ion FcCPh2+ 12 We
.
report here that when an acid with a noncoordinating anion Y-
such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-)13
is used, the ring-opening addition to [1]silaferrocenophanes, fcSiR2
(fc ) (η-C5H4)2Fe) generates novel solvated Si cations.
To investigate if solvated Si cations are formed in the reactions
of 1a and 1b with H(OEt2)(THF)TFPB, we characterized the
products by low-temperature NMR spectroscopy (Figure 1). In
the reaction of H(OEt2)(THF)TFPB with 1a at ca. -60 °C, a new
29Si NMR resonance was observed at 49.7 ppm, remarkably
downfield from that of 1a (δ ) -3.0 ppm in CD2Cl2).14 The 29Si
NMR chemical shift of the product is consistent with similar ether-
coordinated silylium species prepared by Sakurai and co-workers15
(see Table 1) and suggested that the solvated Si cation [3a]+ is
formed at low temperature. The 1H NMR spectrum at this
temperature was consistent with this interpretation and showed
sets of peaks assigned to ferrocenyl and SiMe2 groups together
with resonances characteristic of free diethyl ether and one set
consistent with a coordinated THF molecule. This indicated that
the four-coordinate silylium ion [3a]+ had formed. The THF
multiplets were shifted downfield (δ ) 4.35, 2.15 ppm) from the
shifts expected for THF in CD2Cl2, as would be expected for
ligation to a silicon center with significant cationic character.
Some broadening of the coordinated THF resonances was
observed, possibly indicative of coordinative exchange of the
ligand.15 In the 13C NMR spectrum similar resonances for
We have recently shown that ring-opening addition of HCl to
the strained Si-C bonds of [1]silaferrocenophanes (1) is a
convenient and controlled route to ferrocenylchlorosilanes (2)
(Scheme 1).9 When 1b was reacted with HBF4 a mixture of
ferrocenylfluorosilanes, such as Fc3SiF and Fc2SiF2 (Fc ) (η-
C5H4)Fe(η-C5H5)), resulted, suggesting the presence of a highly
(1) (a) Lambert, J. B.; Kania, L.; Zhang, S. Chem. ReV. 1995, 95, 1191-
1201. (b) Lambert, J. B.; Zhang, S.; Ciro, S. M. Organometallics 1994, 13,
2430-2443.
(10) MacLachlan, M. J.; Manners, I. Unpublished results.
(2) Reed, C. A. Acc. Chem. Res. 1998, 31, 325-332.
(11) For examples of cationic Si species stabilized by transition metals,
see: (a) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J.
Am. Chem. Soc. 1994, 116, 5495-5496. (b) Straus, D. A.; Grumbine, S. D.;
Tilley, T. D. J. Am. Chem. Soc. 1990, 112, 7801-7802.
(3) Borman, S. Chem. Eng. News 1993, 71 (45), 41-42.
(4) (a) Corey, J. Y. J. Am. Chem. Soc. 1975, 97, 3237-3238. (b) Corey,
J. Y.; Gust, D.; Mislow, K. J. Organomet. Chem. 1975, 101, C7-8.
(5) (a) Barton, T. J.; Hovland, A. K.; Tully, C. R. J. Am. Chem. Soc. 1976,
98, 5695-5696. (b) Lambert, J. B.; Sun, H.-N. J. Am. Chem. Soc. 1976, 98,
5611-5615.
(12) (a) In crystalline FcCPh2+ the CPh2 group is bent toward the Fe atom
with an angle of 20.7° between the Cp-CPh2 bond and the Cp plane. For
further information see: Behrens, U. J. Organomet. Chem. 1979, 182, 89-
98. (b) Fe stabilization of R-carbocation centers has been reviewed. See:
Koridze, A. A. Russ. Chem. ReV. 1986, 55, 113-126.
(6) (a) Lambert, J. B.; Zhao, Y. Angew. Chem., Int. Ed. Engl. 1997, 36,
400-401. (b) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J.
Am. Chem. Soc. 1999, 121, 5001-5008.
(13) (a) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922. (b) The synthesis was carried out as described in ref 13a
except that the NaTFPB used was recrystallized from THF and was shown
(7) For examples of investigation of the catalytic activity of silylium ions
see: (a) Johannsen, M.; Jørgensen, K. A.; Helmchen, G. J. Am. Chem. Soc.
1998, 120, 7637-7638. (b) Oishi, M.; Aratake, S.; Yamamoto, H. J. Am.
Chem. Soc. 1998, 120, 8271-8272.
1
by H NMR to possess three THF molecules of crystallization. The product
of the subsequent step was shown by 1H NMR to be H(OEt2)(THF)TFPB
rather than the expected H(OEt2)2TFPB.
(8) Olah, G. A.; Li, X.-Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am.
Chem. Soc. 1995, 117, 8962-8966.
(14) (a) Foucher, D. A.; Tang, B.-Z.; Manners, I. J. Am. Chem. Soc. 1992,
114, 6246-6248. (b) Fischer, A. B.; Kinney, J. B.; Staley, R. H.; Wrighton,
M. S. J. Am. Chem. Soc. 1979, 101, 6501-6506.
(15) Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114, 6697-
6700.
(9) (a) MacLachlan, M. J.; Ginzburg, M.; Zheng, J.; Kno¨ll, O.; Lough, A.
J.; Manners, I. New J. Chem. 1998, 22, 1409-1415. (b) MacLachlan, M. J.;
Zheng, J.; Lough, A. J.; Manners, I.; Mordas, C.; LeSuer, R.; Geiger, W. E.;
Liable-Sands, L. M.; Rheingold, A. L. Polyhedron, 2000, in press.
10.1021/ja993602v CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/17/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 9, 2000 2127
Figure 2. Molecular structure of [Fc3Si(py)][TFPB] ([4b][TFPB]) with
thermal ellipsoids shown at the 30% probability level. Selected bond
lengths [Å] and angles [deg]: Si(1)-N(31) 1.858(5), Si(1)-C(1) 1.832(6),
Si(1)-C(11) 1.833(6), Si(1)-C(21) 1.843(6), C(1)-Si(1)-C(11) 114.9(3),
C(1)-Si(1)-C(21) 115.0(3), C(11)-Si(1)-C(21) 107.8(3), C(1)-Si(1)-
N(31) 106.8(2), C(11)-Si(1)-N(31) 104.3(2), C(21)-Si(1)-N(31)
107.2(2).
Figure 1. (a) 1H and (b) 13C NMR spectra at -60 °C in CD2Cl2 before
(top) and after (bottom) addition of pyridine to the solvated silylium cation
[3a]+. Peaks marked with an asterisk indicate impurities and unreacted
monomer 1a.
Table 1. 29Si NMR Shifts of Solvated Silylium Ionsa
between 1a and H(OEt2)(THF)TFPB was used. Again, the reaction
was monitored in situ by 1H and 13C NMR at -60 °C.16,17 Once
again pyridine was added to the cooled sample of [3b]+ and a
similar change in the shifts of the THF molecules was detected
by 1H NMR spectroscopy that indicated the formation of [4b]+.16
To confirm the structure of [4b][TFPB], a single-crystal X-ray
diffraction study was undertaken on crystals obtained from CDCl3
at -50 °C.16 Figure 2 shows the structure of [4b][TFPB], where
three ferrocenyl moieties and pyridine are coordinated to the
silicon atom.16 The nearest approach of the anion to the Si atom
is 3.93 Å. The Si-N bond length of 1.86 Å is in accord with
other silylium species coordinated by pyridine such as [Me3Si-
(py)][X] (X ) Br, I).18 From the bond length, we calculated that
the Pauling bond order is 0.61,19 characteristic of a mainly
covalent interaction. The sum of the three C-Si-C bond angles
in the cation is 337.7°, intermediate between that expected for a
trigonal planar 3-coordinate Si cation and a tetrahedral species.
In summary, we report a new, convenient route to solvated Si
cations with ferrocenyl substituents. Low-temperature NMR
studies indicate that addition of H(OEt2)(THF)TFPB to [1]ferro-
cenophanes generates cationic species with silylium character.
The methodology described should be transferable to other
strained silicon-containing heterocycles such as silacyclobutanes
and also to rings containing other Group 14 elements.
cation
T (°C)
29Si (δ)b
ref
[Me3Si(OEt2)]+
-70
-40
-40
-60
-60
66.9
38.0
21.4
49.7
33.3
c
15
15
15
[Ph2MeSi(OEt2)]+
[(2-thienyl)2MeSi(OEt2)]+
[3a]+
[4a]+
[3b]+
[4b]+
this work
this work
this work
this work
25
25.2d
a [TFPB]- salts. b In dichloromethane-d2. c See ref 17. d In chloroform-
d.
ferrocenyl and SiMe2 groups, coordinated THF, and free ether
molecules were observed. No other species, aside from the
expected TFPB anion, were detected in any of the spectra,16
indicating clean ring-opening protonolysis of the [1]silaferro-
cenophane. After the sample had reached room temperature, it
became green, indicative of oxidation to Fe(III) species.
In a further experiment with 1a, an aliquot of pyridine (py)
was added at -60 °C after the formation of [3a]+ had been
confirmed by NMR. Upon addition of the pyridine, the solution
changed from yellow to orange and the 29Si NMR resonance
moved upfield from 49.7 to 33.3 ppm. This was accompanied by
changes to the 1H and 13C NMR spectra that were consistent with
the replacement of coordinated THF with pyridine to afford [4a]+
(Figure 1).16 In particular, resonances consistent with free THF
were then observed. In this case, when the sample was warmed
to 25 °C, [4a]+ proved to be stable.
Acknowledgment. M.J.M. thanks NSERC for a Postgraduate Fel-
lowship (1995-1999). S.C.B. thanks the Ontario Government and the
University of Toronto for Postgraduate Scholarships (1999-2001). I.M.
thanks NSERC for an E.W.R. Steacie Fellowship (1997-1999), the
University of Toronto for a McLean Fellowship (1997-2003), and the
Ontario Government for a PREA Award (1999-2003).
We also investigated the addition of H(OEt2)(THF)TFPB to
the [1]ferrocenophane 1b with two additional bulky ferrocenyl
substituents. A methodology identical to that for the reaction
(16) See Supporting Information for details.
Supporting Information Available: Experimental details and NMR
data for new compounds; Crystallographic details, tables of bond lengths,
bond angles, and atomic coordinates for [4b][TFPB] (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(17) In this case, we were unable to obtain a 29Si NMR signal at low
temperature due to the long relaxation time of the Si center and difficulty
using a DEPT sequence (lack of R-protons).
(18) Hensen, K.; Zengerly, T.; Pickel, P.; Klebe, G. Angew. Chem., Int.
Ed. Engl. 1983, 22, 725-726.
(19) Pauling, L. Science 1994, 263, 983.
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