CAAC boranes. synthesis and characterization of cyclic (alkyl) (amino) carbene borane complexes from BF3 and BH3

Article abstract of DOI:10.3762/bjoc.6.82

In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF₃×Et₂O provided the first two examples of CAAC-BF₃ complexes: 1-(2,6-diisopropylphenyl)-3,5,5- trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and 2-(2,6- diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC-BF₃ complexes are robust compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spectroscopy and X-ray crystallography. In contrast, a CAAC complex with borane (BH₃) was readily formed in situ according to ¹H and ¹¹B NMR analysis, but did not survive the workup conditions. These results set the stage for further studies of the chemistry of CAAC boranes.

Full text of DOI:10.3762/bjoc.6.82

CAAC Boranes. Synthesis and characterization of
cyclic (alkyl) (amino) carbene borane complexes
from BF3 and BH3
Julien Monot1,2, Louis Fensterbank*1, Max Malacria*1, Emmanuel Lacôte*1,
Steven J. Geib2 and Dennis P. Curran*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 709–712.
1UPMC Univ. Paris 06, Institut parisien de chimie moléculaire (UMR
CNRS 7201), C 229, 4 place Jussieu, 75005 Paris, France and
2Department of Chemistry, University of Pittsburgh, Pittsburgh, PA
15260, USA
doi:10.3762/bjoc.6.82
Received: 14 June 2010
Accepted: 26 July 2010
Published: 02 August 2010
Email:
Julien Monot - jam287@pitt.edu; Louis Fensterbank* -
louis.fensterbank@upmc.fr; Max Malacria* - max.malacria@upmc.fr;
Emmanuel Lacôte* - emmanuel.lacote@upmc.fr; Steven J. Geib -
geib@pitt.edu; Dennis P. Curran* - curran@pitt.edu
Editor-in-Chief: J. Clayden
© 2010 Monot et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
borane Lewis acid Lewis base complexes; carbene–borane
complexes; cyclic (alkyl) (amino) carbenes; N-heterocyclic carbenes;
stable carbenes
Abstract
In situ formation of two cyclic (alkyl) (amino) carbenes (CAACs) followed by addition of BF3•Et2O provided the first two
examples of CAAC–BF3 complexes: 1-(2,6-diisopropylphenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoroborane, and
2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-1-ylidene trifluoroborane. These CAAC–BF3 complexes are robust
compounds that are stable to ambient laboratory conditions and silica gel chromatography. They were characterized by spec-
troscopy and X-ray crystallography. In contrast, a CAAC complex with borane (BH3) was readily formed in situ according to 1H
and 11B NMR analysis, but did not survive the workup conditions. These results set the stage for further studies of the chemistry of
CAAC boranes.
Introduction
Lewis acid/Lewis base complexes of N-heterocyclic carbenes classes of Lewis base complexes of boranes with neutral mole-
and boranes (NHC–boranes) are readily prepared from NHC’s cules (ethers, sulfides, etc.), NHC–borane complexes are highly
and boranes by direct complexation [1-4]. Unlike many other stable in diverse environments. Complexes such as those shown
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Beilstein J. Org. Chem. 2010, 6, 709–712.
in Figure 1 are white solids that can often be chromatographed studies that a representative CAAC–BH3 complex can be gener-
if desired. Many such complexes resist decomplexation, oxi- ated in solution.
dation, and both acidic and basic hydrolysis. Such
NHC–boranes are beginning to be used as synthetic reagents, Results and Discussion
with applications in radical [5-9], ionic [10,11] and CAAC trifluoroborane complexes 3a and 3b were readily
organometallic [12] reactions. They are also precursors for prepared as summarized in Scheme 1. Deprotonation of the 3,4-
making higher complexes with unusual bonding patterns such dihydro-2H-pyrrolium hydrogen dichloride salt 1a with
as boron–boron double bonds [3], or for making unusual reac- NaHMDS in THF at −78 °C, followed by warming to room
tive intermediates on boron [6,7,13].
temperature gave a pale yellow solution of the free carbene 2a
[20]. A broad resonance at −312 ppm in the 13C NMR spec-
trum of this solution indicated that 2a had formed. Boron triflu-
oride etherate (BF3•Et2O) was then added at −78 °C and the
resulting solution was stirred overnight. Rapid filtration through
silica gel afforded the pure complex 3a (1-(2,6-diisopropyl-
phenyl)-3,5,5-trimethyl-3-phenylpyrrolidin-2-ylidene trifluoro-
borane) as white crystals in 61% yield. CAAC borane 3b
(2-(2,6-diisopropylphenyl)-3,3-dimethyl-2-azaspiro[4.5]decan-
1-ylidene trifluoroborane) was made by a similar procedure
starting from triflate salt 1b and was isolated in 64% yield.
Figure 1: Representative complexes of N-heterocyclic carbenes and
boranes (NHC–boranes).
Most of the first generation carbene-boranes have been made
from N-heterocyclic carbenes in which the carbene carbon is
stabilized by two donating nitrogen atoms (imidazolylidene,
triazolylidene, etc) or other heteroatoms (phosphorous, silicon,
oxygen) [14-18]. An exception is Bertrand’s BF3 complex of an
acyclic (amino) (aryl) carbene in which the aryl group is
9-anthracenyl [19]. This complex, whose structure is shown in
Figure 2, has also two cation stabilizing groups on the carbene
carbon.
Scheme 1: Synthesis of stable CAAC–BF3 complexes 3a and 3b and
in situ generation of CAAC–BH3 complex 4a.
Figure 2: Bertrand’s amino anthracenyl carbene trifluoroborane com-
plex.
CAAC boranes 3a and 3b were obtained as white crystals, and
the structure of a single crystal of each was solved by X-ray
Accordingly, it is interesting to study borane complexes of other diffraction. Two views of each structure are shown in Figure 3.
types of carbenes. Recently, Bertrand introduced, cyclic (alkyl) Of special interest are the lengths of the Ccarbene–N bonds. At
(amino) carbenes, or CAACs, as a new class of carbene com- 1.294(3) Å for 3a and 1.293(3) Å for 3b, these bonds are
plex in which the central carbene carbon is stabilized by only significantly shorter than analogous bonds on other NHC–BF3
one nitrogen atom [20-23]. Herein we report the synthesis and complexes where the carbene carbon has two nitrogen
characterization (including X-ray structures) of two stable substituents (1.338(5)–1.355(5) Å) [24,25]. Instead, these bond
CAAC–BF3 complexes. We also show through spectroscopic lengths are closer to that of the BF3 complex of the amino-
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Beilstein J. Org. Chem. 2010, 6, 709–712.
Figure 3: X-Ray crystal structures of CAAC–BF3 complexes 3a (top) and 3b (bottom).
anthryl-carbene (1.301(2) Å) shown in Figure 2 [19]. Presum- The reaction mixture was allowed to stand for 15 h, during
ably, this is because there is more demand on the lone nitrogen which time the 11B NMR spectrum was unchanged. This
of CAAC complex to stabilize the positive charge than when suggests that complex 4a is thermally stable. However, stan-
two nitrogen atoms are present. In contrast, the Ccarbene–B bond dard workup and evaporation as used for NHC–boranes and
lengths of 3a (1.674(3) Å) and 3b (1.681(3) Å) are somewhat CAAC–BF3 complexes 3a and 3b did not provide 4a.
longer than those in other NHC–BF3 complexes Evidently, 4a is not as stable towards isolation as the BH3
(1.635(5)–1.668(6) Å) [24,25].
complexes shown in Figure 1. Nonetheless, it should still be
possible to generate this complex in situ and use it directly for
To conclude this preliminary study, we briefly attempted forma- onward reactions.
tion of the borane complex 4a by generation of carbene 2a as
above, followed by the addition of BH3•THF. Indeed, complex Conclusion
4a was directly formed in situ as evidenced by 11B NMR spec- In summary, we have synthesized the first CAAC borane
troscopy. The resonance for BH3•THF was absent and in its complexes. The complexes of trifluoroborane (BF3) are stable
place was a new quartet at −30.0 ppm. Both the chemical shift and were isolated as pure solids and fully characterized by spec-
and multiplicity of this resonance are consistent with the troscopic analysis and X-ray crystallography. The borane (BH3)
CAAC–borane structure 4a. The 1H NMR spectrum of the reac- complex was characterized in situ by NMR spectroscopy, but
tion mixture also showed the formation of 4a (see Supporting did not survive workup and isolation. These results set the stage
Information File 1).
for further studies of the chemistry of CAAC boranes.
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10.Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S.-H.;
Supporting Information
Curran, D. P.; Malacria, M.; Fensterbank, L.; Lacôte, E. Chem.–Eur. J.
2009, 15, 12937–12940. doi:10.1002/chem.200902450
11.Lindsay, D. M.; McArthur, D. Chem. Commun. 2010, 46, 2474–2476.
doi:10.1039/c001466d
Supporting Information File 1
Procedures and characterization of the new complexes.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-82-S1.pdf]
12.Monot, J.; Makhlouf Brahmi, M.; Ueng, S.-H.; Robert, C.;
Desage-El Murr, M.; Curran, D. P.; Malacria, M.; Fensterbank, L.;
Lacôte, E. Org. Lett. 2009, 11, 4914–4917. doi:10.1021/ol902012c
13.Braunschweig, H.; Chiu, C.-W.; Radacki, K.; Kupfer, T.
Angew. Chem., Int. Ed. 2010, 49, 2041–2044.
Supporting Information File 2
Cif file of crystal structure of compound 3a.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-82-S2.cif]
doi:10.1002/anie.200906884
14.Alcaraz, G.; Reed, R.; Baceiredo, A.; Bertrand, G. Chem. Commun.
1993, 1354–1355.
15.Lambert, C.; Lopez-Solera, I.; Raithby, P. R. Organometallics 1996, 15,
452–455. doi:10.1021/om950427s
Supporting Information File 3
16.Tamm, M.; Lügger, T.; Hahn, F. E. Organometallics 1996, 15,
1251–1256. doi:10.1021/om950773+
Cif file of crystal structure of compound 3b.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-82-S3.cif]
17.Merceron, N.; Miqueu, K.; Baceiredo, A.; Bertrand, G.
J. Am. Chem. Soc. 2002, 124, 6806–6807. doi:10.1021/ja026556z
18.Snead, D. R.; Ghiviriga, I.; Abboud, K. A.; Hong, S. Org. Lett. 2009, 11,
3274–3277. doi:10.1021/ol9013156
19.Cattoën, X.; Gornitzka, H.; Bourissou, D.; Bertrand, G.
J. Am. Chem. Soc. 2004, 126, 1342–1343. doi:10.1021/ja0396854
20.Lavallo, V.; Canac, Y.; Präsang, C.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2005, 44, 5705–5709.
Acknowledgements
This work was supported by grants from the US National
Science Foundation (CHE-0645998), UPMC, CNRS, and the
French Agence Nationale de la Recherche (ANR, BLAN0309
Radicaux Verts, and 08-CEXC-011-01 Borane). We thank H.
Rousselière and P. Herson (IPCM, Paris) for solving one of the
crystal structures. We thank Dr. G. Mignani of Rhodia, Inc. for
generously providing salt 1a.
doi:10.1002/anie.200501841
21.Lavallo, V.; Canac, Y.; DeHope, A.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2005, 44, 7236–7239.
doi:10.1002/anie.200502566
22.Jazzar, R.; Dewhurst, R. D.; Bourg, J.-B.; Donnadieu, B.; Canac, Y.;
Bertrand, G. Angew. Chem., Int. Ed. 2007, 46, 2899–2902.
doi:10.1002/anie.200605083
23.Zeng, X.; Frey, G. D.; Kinjo, R.; Donnadieu, B.; Bertrand, G.
J. Am. Chem. Soc. 2009, 131, 8690–8696. doi:10.1021/ja902051m
24.Arduengo, A. J., III; Davidson, F.; Krafczyk, R.; Marshall, W. J.;
Schmutzler, R. Monatsh. Chem. 2000, 131, 251–265.
doi:10.1007/s007060070101
References
1. Kuhn, N.; Henkel, G.; Kratz, T.; Kreutzberg, J.; Boese, R.;
Maulitz, A. H. Chem. Ber. 1993, 126, 2041–2045.
doi:10.1002/cber.19931260913
2. Ramnial, T.; Jong, H.; McKenzie, I. D.; Jennings, M.; Clyburne, J. A. C.
Chem. Commun. 2003, 1722–1723. doi:10.1039/b301416a
3. Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King, R. B.;
Schaefer, H. F., III; v. R. Schleyer, P.; Robinson, G. H.
J. Am. Chem. Soc. 2007, 129, 12412–12413. doi:10.1021/ja075932i
4. Yamaguchi, Y.; Kashiwabara, T.; Ogata, K.; Miura, Y.; Nakamura, Y.;
Kobayashi, K.; Ito, T. Chem. Commun. 2004, 2160–2161.
doi:10.1039/b405459h
25.Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H.
Inorg. Chim. Acta 2003, 352, 143–150.
doi:10.1016/S0020-1693(03)00143-9
License and Terms
5. Ueng, S.-H.; Makhlouf Brahmi, M.; Derat, É.; Fensterbank, L.;
Lacôte, E.; Malacria, M.; Curran, D. P. J. Am. Chem. Soc. 2008, 130,
10082–10083. doi:10.1021/ja804150k
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(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
6. Matsumoto, T.; Gabbaï, F. P. Organometallics 2009, 28, 4252–4253.
doi:10.1021/om900476g
7. Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.;
Lacôte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P.
J. Am. Chem. Soc. 2009, 131, 11256–11262. doi:10.1021/ja904103x
8. Walton, J. C.; Makhlouf Brahmi, M.; Fensterbank, L.; Lacôte, E.;
Malacria, M.; Chu, Q.; Ueng, S.-H.; Solovyev, A.; Curran, D. P.
J. Am. Chem. Soc. 2010, 132, 2350–2358. doi:10.1021/ja909502q
9. Tehfe, M.-A.; Makhlouf Brahmi, M.; Fouassier, J.-P.; Curran, D. P.;
Malacria, M.; Fensterbank, L.; Lacôte, E.; Lalevée, J. Macromolecules
2010, 43, 2261–2267. doi:10.1021/ma902492q
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