New fluorinated 9-borafluorene Lewis acids [24]

Full text of DOI:10.1021/ja005607u

J. Am. Chem. Soc. 2000, 122, 12911-12912
12911
Scheme 1
New Fluorinated 9-Borafluorene Lewis Acids
Preston A. Chase,† Warren E. Piers,*,† and Brian O. Patrick‡
Department of Chemistry, UniVersity of Calgary
2500 UniVersity DriVe N.W.
Calgary, Alberta T2N 1N4, Canada
Department of Chemistry, UniVersity of British Columbia
2036 Main Mall, VancouVer
British Columbia V6T 1Z1, Canada
ReceiVed September 13, 2000
Tris(pentafluorophenyl)borane1 and its derivatives are important
Lewis acids for olefin polymerization and organic synthesis.2 For
both applications, Lewis acid strength is a key factor in the
borane’s performance. Attempts to improve the Lewis acidity of
the parent B(C6F5)3 borane have focused on the use of more
heavily fluorinated aryl groups, e.g. perfluorobiphenyl3 or per-
fluoronaphthyl.4 While this has been shown to be effective both
experimentally and computationally,5 the limitation of this ap-
proach lies in the increasing front- and back-strain engendered
by sterically larger groups, which eventually counters the gains
made by the increased electron-withdrawing power of more
fluorine substituents.
A second strategy for ramping up the Lewis acid strength of a
boron center is to incorporate it into a five-membered borole ring,6
which by virtue of the 4π electron count is antiaromatic. The
tendency to remove the boron p-orbital from conjugation in the
borole ring increases the boron Lewis acidity relative to noncyclic
analogues;6,7 furthermore, the relief of strain energy in the borole
ring upon boron pyramidilization also contributes to the enhanced
Lewis acidity of boroles. Here we report the synthesis of the
fluorinated biphenyl-based borole derivatives (C12F8)B-R [R )
CH3 (1a), C6F5 (1b), Br (1c)]8 and demonstrate their enhanced
Lewis acidity relative to the more heavily fluorinated (C6F5)2B-R
analogues.
via X-ray crystallography (Figure 1).12 The boron center is planar
(∑C-B-C ) 360.0°) but the C(5)-B(1)-C(5*) angle is compressed
to 103.6(2)°. The borafluorene ring is essentially planar, and the
C6F5 ring is tilted out of this plane by 53.2(1)°, precluding
significant conjugation between the aryl and 9-borafluorene π
systems. In contrast to the blue pentaphenylborole (λmax ) 570
nm, πfπ*),6 compounds 1 are pale yellow or orange in color
(λmax 1a, 398 nm; 1b, 440 nm), comparable to the nonfluorinated
9-borafluorene analogues8 (λmax ) 385-405 nm).
Several observations show that 9-borafluorenes 1a and 1b are
slightly stronger Lewis acids than their bis-pentafluorophenyl
13
analogues (i.e. H3CB(C6F5)2 and B(C6F5)3). In a competition
reaction, 1b competes more effectively for acetonitrile compared
to B(C6F5)3.14 Marks et al. has used the Childs method15 to situate
a variety of perfluoroaryl boranes on a scale containing a variety
of main group Lewis acids (relative to BBr3 at 1.00).2a By this
method, 1a has a relative value of 0.58 ( 0.02 (cf. the value of
0.56 ( 0.02 for H3CB(C6F5)2) and 1b places at 0.70 ( 0.02 (cf.
0.68 ( 0.02 for B(C6F5)316). Finally, Laszlo and Teston used
semiempirical MNDO calculations to compute the energy of the
π* molecular orbital in LA‚crotonaldehyde adducts, which is
related to the Childs Lewis acidity.17 Again, 1a and 1b prove to
be stronger Lewis acids than RB(C6F5)2.11 Thus, the antiaromatic
nature of the central borole moiety more than compensates for
the loss of two fluorine atoms.5 Using these data and that
published by Marks et al.,2a a relative scale of Lewis acidity for
a variety of perfluorinated boranes and 9-borafluroenes 1 (toward
crotonaldehyde) is PBB > PNB > 1b > B(C6F5)3 . 1a >
H3CB(C6F5)2.
The synthetic route to 9-borafluorenes 1 is a boron-tin ex-
change reaction analogous to that used to prepare pentaphenyl-
borole6 (Scheme 1). We have improved the published synthesis
of the stannole 29 by using more dilute conditions. Reaction of
this material with BBr3 gives 1c, which is methylated using
Cp2Zr(CH3)2 to give 1a (11B NMR ) 67.3 ppm). Direct reaction
of 2 with Cl2BC6F510 leads to the fully fluorinated 9-borafluorene
1b (11B NMR ) 57.0 ppm).11
Both 1a and 1b were characterized by NMR and UV-vis
spectroscopy and the solid-state structure of 1b was determined
† University of Calgary.
There is a wealth of potential applications for these compounds,
not only as Lewis acids, but also as electron deficient ligands.18
In terms of metallocene activation, both 1a and 1b are effective
‡ University of British Columbia.
(1) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218.
(2) (a) Chen, Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (b) Piers,
W. E.; Chivers, T. Chem. Soc. ReV. 1997, 345. (c) Ishihara, K.; Yamamoto,
H. Eur. J. Org. Chem. 1999, 527.
(11) Attempts to prepare 1b directly from the 2,2′-dilithioperfluorbiphenyl
reagent and Cl2B(C6F5) led to isolation of the Et2O adduct of 1b; the base
could not be removed under conditions which did not decompose the
compound.
(3) (a) Chen, Y. X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am.
Chem. Soc. 1998, 120, 6287. (b) Li, L.; Stern, C. L.; Marks, T. J.
Organometallics 2000, 19, 3332.
(4) Li, L.; Marks, T. J. Organometallics 1998, 17, 3996.
(5) Vanka, K.; Chan, M. S. W.; Pye, C. C.; Ziegler, T. Organometallics
2000, 19, 1841.
(6) (a) Eisch, J. J.; Galle, J. E.; Kozima, S. J. Am. Chem. Soc. 1986, 108,
379. (b) Sebald, A.; Wrackmeyer, B. J. Organomet. Chem. 1986, 307, 157.
(c) Herberich, G. E.; Buller, G.; Hessner, B.; Oschmann, W. J. Organomet.
Chem. 1980, 195, 249.
(12) For full details see the Supporting Information.
(13) 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
1998, 17, 3557.
(14) At room temperature Keq ) 1.30(3) for B(C6F5)3‚NCCH3 + 1b S
1b‚NCCH3 + B(C6F5)3.
(15) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982, 60,
801.
(7) Antiaromaticity also plays a role in the high Lewis acidity observed in
perfluorinated 9,10-boraanthracenes: (a) Williams, V. C.; Dai, C.; Liz, Z.;
Collins, S.; Piers, W. E.; Clegg, W.; Marder, T. B. Angew. Chem., Int. Ed.
1999, 38, 3695. (b) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P.
N.; Marks, T. J. Angew. Chem., Int. Ed. 2000, 39, 1312.
(8) Nonfluorinated 9-borafluorenes have been reported: Ko¨ster, R.; Bene-
dikt, G. Angew. Chem., Int. Ed. Engl. 1963, 2, 323.
(16) Marks et al. report a value of 0.77 on the Child’s scale for B(C6F5)3.2a
While the value we obtain is somewhat lower, it is consistent with the order
of Lewis acidity obtained for 1b and B(C6F5)3 based on the competition and
computational experiments described.
(17) Laszlo, P.; Teston, M. J. Am. Chem. Soc. 1990, 112, 8750.
(18) Leading reference on borollide anions: Herberich, G. E. In Compre-
hensiVe Organometallic Chemistry II; Abel. E. W., Stone, F. G. A., Wilkinson,
G., Eds.; Pergamon Press: Oxford, 1995; Vol. 1, p 197.
(9) Cohen, S. C.; Massey, A. G. J. Organomet. Chem. 1967, 10, 471.
(10) Chambers, R. D.; Chivers, T. J. Chem. Soc. 1965, 3933.
10.1021/ja005607u CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/06/2000

12912 J. Am. Chem. Soc., Vol. 122, No. 51, 2000
Communications to the Editor
Figure 2. ORTEP diagram of 3a (50% ellipsoids). Selected bond
distances (Å): B(1)-C(1), 1.666(5); B(1)-C(2), 1.625(4); B(1)-C(3),
1.635(4); B(1)-C(14), 1.617(4); Zr(1)-C(1), 2.506(3); Zr(1)-C(15),
2.252(4). Selected bond angles (deg): C(15)-Zr(1)-C(1), 96.0(1); Zr(1)-
C(1)-B(1), 169.5(2); C(1)-B(1)-C(2), 110.7(3); C(1)-B(1)-C(3),
111.5(2); C(1)-B(1)-C(14), 113.0(2); C(2)-B(1)-C(3), 111.8(2); C(2)-
B(1)-C(14), 112.1(2); C(3)-B(1)-C(14), 97.2(2).
Figure 1. ORTEP diagram of 1b (50% ellipsoids). Selected bond
distances (Å): B(1)-C(1), 1.556(4); B(1)-C(5), 1.549(3); C(5)-C(6),
1.424(3); C(6)-C(6*), 1.481(4). Selected bond angles (deg): C(1)-B(1)-
C(5), 128.2(1); C(5)-B(1)-C(5*), 103.6(2); B(1)-C(5)-C(6), 108.7(2);
B(1)-C(5)-C(10), 131.7(2); C(5)-C(6)-C(6*), 109.5(1); C(7)-C(6)-
C(6*), 131.7(1).
behave comparably in terms of ethylene polymerization activity
at 25 °C,19 the latter is rapidly deactivated when polymerizations
are conducted after a 5 min incubation period at 70 °C. Due to
the chelate effect, aryl transfer is precluded and the ion pair 3a
is much more robust under these conditions,20 retaining its activity
(2.3(3) × 104 g mol-1 h-1). 9-Borafluorene Lewis acid 1a is the
prototypical member of a new class of activators for olefin
polymerization processes which contain only two fluorinated aryl
groups and produce anions that do not undergo deactivation via
aryl transfer.21 Ion pair 3b is also highly stable in solution in the
absence of monomer; this catalyst system is approximately twice
as active (4.6(3) × 105 g mol-1 h-1) as the Cp2Zr(CH3)2/B(C6F5)3
catalyst system (2.7(3) × 105 g mol-1 h-1) under identical
conditions.
In conclusion, we have developed a route to a new family of
fluorinated 9-borafluorene Lewis acids and demonstrated their
application in metallocene activation. We are currently continuing
in this vein, in addition to examining their electrochemistry, their
efficacy as Lewis acids for organic synthesis, and the coordination
chemistry of the borollides.
and reaction of both 1a and 1b with Cp2Zr(CH3)2 leads to rapid
ion pair formation (eq 1). The structure of contact ion pair 3a
(an ORTEP diagram is shown in Figure 2) has similar features
to those found for ion pairs involving the [CH3B(C6F5)3]- ion.2a
The [(H3C)2B(C6F5)2]- anion contacts the metallocenium ion
through only one of its methyl groups and the bridging methyl
group is more closely associated with boron (B(1)-C(1) )
1.666(5) Å, cf. B(1)-C(2) ) 1.625(4) Å) than zirconium (Zr(1)-
C(1) ) 2.506(3) Å, cf. Zr(1)-C(15) ) 2.252(4) Å), suggesting
that 1a has effected substantial abstraction of the methide anion
despite its comparatively low Lewis acidity. The boron center is
now pyramidalized to a distorted tetrahedron; the C(3)-B(1)-
C(14) angle is 97.2(2)°.
According to the solid state structure, three chemically distinct
methyl groups should be apparent in solution; however, at 25˚C
the methyl groups rapidly exchange on the NMR time scale. At
-60 °C, the broad signal ascribed to the CH3 groups resolves
into three at 0.32 (Zr-CH3), 0.74 (B-CH3 term), and 0.02 ppm
(B-CH3 bridge). Although fluxional, 3a is remarkably stable in
toluene solution, persisting for several days. This contrasts with
the behavior of the ion pair formed from Cp2Zr(CH3)2 and
H3CB(C6F5)2, for which facile back transfer of a -C6F5 group is
observed under similar conditions to give (H3C)2B(C6F5) and
Cp2Zr(CH3)C6F5.13 This observation reveals another potential
advantage of these compounds: anion stability. Thus, while the
Cp2Zr(CH3)2/1a and Cp2Zr(CH3)2/H3CB(C6F5)2 catalyst systems
Acknowledgment. Financial support for this work was provided by
the NSERC of Canada in the form of a Research Grant to W.E.P. and a
PGS B fellowship to P.A.C..
Supporting Information Available: Experimental and spectroscopic
details for new compounds and tables of crystal data, atomic coordinates,
bond lengths and angles, and anisotropic displacement parameters for
1b and 3a (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA005607U
(19) Activities for these two catalyst systems fall in the range [1.9-2.3(3)]
× 104 g mol-1 h-1
.
(20) With prolonged heating, ion pair 3a undergoes a different decomposi-
tion process involving methane loss.
(21) This is also a common deactivation pathway for the [H3CB(C6F5)3]-
and to a lesser extent the [B(C6F5)4]- anions.2a A borate anion with two C12F8
groups attached to boron is mentioned in ref 2a and also capitalizes on the
chelate effect. See also: Bohnen, H.; Fritze, C.; Kuber, F. PCT International
Patent Application WO 9943685, 1999.