3244
J. Am. Chem. Soc. 1999, 121, 3244-3245
Thus, treatment of the known compound 1,2-(BCl2)2C6H411 with
slightly more than 2 equiv of Zn(C6F5)2 led to smooth conversion
to the desired product 1-H4 as shown in eq 1. While the first
New Bifunctional Perfluoroaryl Boranes. Synthesis
and Reactivity of the ortho-Phenylene-Bridged
Diboranes 1,2-[B(C6F5)2]2C6X4 (X ) H, F)′
12
V. Clifford Williams,† Warren E. Piers,*,† William Clegg,‡
Mark R. J. Elsegood,‡ Scott Collins,§ and Todd B. Marder|
Department of Chemistry, UniVersity of Calgary
2500 UniVersity DriVe N. W.
Calgary, Alberta, Canada T2N 1N4
Department of Chemistry, UniVersity of Newcastle
Newcastle upon Tyne NE1 7RU, England
Department of Chemistry, UniVersity of Waterloo
Waterloo, Ontario, Canada N2L 3G1
Department of Chemistry, South Road
UniVersity of Durham, Durham DH1 3LE, England
two -C6F5 groups are incorporated relatively easily, the third and
fourth substitutions require heating to 120 °C to ensure complete
C6F5 transfer. 1H NMR spectra of the reaction in progress revealed
the sequential nature of the substitution process eventually yielding
1-H4, characterized by a complex multiplet centered around 7.73
ppm for the aromatic protons of the symmetrical AA′BB′
phenylene backbone. In addition to full spectroscopic character-
ization, the molecular structure of 1-H4 has been determined (see
Supporting Information).
Boranes of general formula RB(C6F5)2 are prone to proteolytic
loss of RH13 and are less Lewis acid than B(C6F5)3;14 furthermore,
anions formed via methyl abstraction using such boranes (i.e.,
[R(CH3)B(C6F5)2]-) are unstable toward transfer of C6F5 groups
back to the metal.8 Indeed, 1-H4 suffers from some of these
problems in addition to being poorly soluble in aromatic solvents,
which are usually employed in solution olefin polymerization
processes. Accordingly, a more desirable ortho-phenylene-bridged
diborane would include a fully fluorinated backbone which, like
the parent borane B(C6F5)3, should be more stable to proteolysis15
and -C6F5 transfer and offer better solubility properties. The fully
fluorinated analogue of 1, i.e., 1-F4, however, represents a more
formidable synthetic challenge since the deactivating effect of
four fluorine substituents precludes the use of a route analogous
to that employed for preparing 1-H4. Not only were we unable
to prepare 1,2-(Me3Si)2C6F4 but a model reaction between BCl3
and C6F5SiMe3 showed also that the silyl methyl groups prefer-
entially underwent metathesis to boron instead of the required
C6F5.
We thus turned to the known mercury trimer [(C6F4)Hg]316 as
a synthon for installing the 1,2-BX2 units on the tetrafluoroben-
zene ring. When this reagent is treated with an excess of BCl3,
the desired 1,2-bis-(dichloroboryl)tetrafluorobenzene product is
observed at early stages of the reaction, but is thermally unstable
toward loss of BCl3 and production of octafluoro-9,10-dichloro-
9,10-diboraanthracene, which is the main product of this reac-
tion.17 Eisch et al. have recently reported a similar condensation
reaction involving unfluorinated diborane 1,2-(BCl2)2C6H4 at
elevated temperatures.18 Evidently, this is a more facile process
when the backbone is fluorinated.19 Fortunately, the reaction of
[(C6F4)Hg]3 with BBr3 produces 1,2-bis-(dibromoboryl)tetrafluo-
robenzene, with only ∼5-10% of the diboraanthracene product
ReceiVed January 11, 1999
Highly electrophilic boranes containing perfluorinated aryl
groups are effective activators for olefin polymerization using d0
transition metal catalysts.1 To date, the most effective examples2
have been monofunctional boranes, but it has been suggested that
bifunctional Lewis acids might offer advantages since the
counterions formed from diboranes are potentially less coordinat-
ing than [RB(ArF)3]-.3 To this end, we have been developing
routes to various diboranes of general formula (C6F5)2B-linker-
B(C6F5)2 for testing as metallocene activators. Although such
compounds are most relevant in the olefin polymerization arena,
in a more general sense bifunctional boron-based Lewis acids
are finding application in such diverse areas as organic synthesis,4
new materials,5 selective anion binding,6 and molecular recogni-
tion.7 Thus far, diboranes incorporating B(C6F5)2 units have been
limited to those with one-carbon linkers,3a,8 which have some
limitations and are, at any rate, best suited to binding hydride
anions. Herein we report synthetic routes to diboranes containing
bis-(pentafluorophenyl)boryl groups tethered by the two-carbon
perprotio and perfluoro ortho-phenylene bridges.
Recently, we reported the molecular structure of the base-free
zinc compound Zn(C6F5)2,9 which we tested as a -C6F5 transfer
agent.10 While it was not selective in reactions with BCl3, it is an
effective reagent for converting -BCl2 units to -B(C6F5)2 groups.
* Correspondence author. Telephone: 403-220-5746. Fax: 403-289-9488.
E-mail: wpiers@ucalgary.ca.
† University of Calgary.
‡ University of Newcastle.
§ University of Waterloo.
| University of Durham.
(1) Piers, W. E.; Chivers, T. Chem. Soc. ReV. 1997, 345.
(2) (a) Li, L.; Marks, T. J. Organometallics 1998, 17, 3996. (b) Chen, Y.-
X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998,
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(3) (a) Jia, L.; Yang, X.; Stern, C.; Marks, T. J. Organometallics, 1994,
13, 3755. (b) Marks, T. J.; Jia, L.; Yang, X. U.S. Patent 5,447,895, 1995,
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(5) Fontani, M.; Peters, F.; Scherer, W.; Wachter, W.; Wagner, M.; Zanello,
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(6) (a) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420. (b) Katz, H. E. J.
Org. Chem. 1985, 50, 2575. (c) Katz, H. E. Organometallics 1987, 6, 1134.
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1998, 17, 4769.
(11) Kaufmann, D. Chem. Ber. 1987, 120, 901.
(12) Experimental details and spectrocopic data for the new compounds
prepared herein are given in the Supporting Information.
(13) Parks, D. J.; Piers, W. E., Yap, G. P. A. Organometallics 1998, 17,
5492.
(14) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998,
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(15) (a) Siedle, A. R.; Newmark, R. A., Lamanna, W. M.; Huffman, J. C.
Organometallics 1993, 12, 1491. (b) Danopoulos, A. D.; Galsworthy, J. R.;
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(16) Sartori, P.; Golloch, A. Chem. Ber. 1968, 101, 2004.
(17) This compound and some of its derivatives have been fully character-
ized and will be the subject of a future publication.
(18) Eisch, J. J.; Kotowicz, B. W. Eur. J. Inorg. Chem. 1998, 761.
(19) For another example, see: Tschinkl, M. Schier, A.; Riede, J.; Gabbai,
F. P. Inorg. Chem. 1998, 37, 5097.
(7) (a) Nozaki, K.; Yoshida, M.; Takaya, H. Bull. Chem. Soc. Jpn. 1996,
69, 2043. (b) Nozaki, K.; Tsutsumi, T.; Takaya, H. J. Org. Chem. 1995, 60,
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1994, 33, 2452. (d) Katz, H. E. J. Org. Chem. 1989, 54, 2179.
(8) Ko¨hler, K.; Piers, W. E.; Xin, S.; Feng, Y.; Bravakis, A. M.; Jarvis, A.
P.; Collins, S.; Clegg, W.; Yap G. P. A.; Marder, T. B. Organometallics
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10.1021/ja990082v CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/19/1999