Substitution reactions at tetracoordinate boron: Synthesis of N-heterocyclic carbene boranes with boron-heteroatom bonds

Article abstract of DOI:10.1021/ja107025y

Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene (dipp-Imd-BH₂X, X = halide or sulfonate) have been prepared from the parent borane dipp-Imd-BH₃ by (1) substitution reactions with R-X (X = halide or sulfonate), (2) reactions with electrophiles (like I₂ or NIS), or (3) acid/base reactions with HX (provided that HX has a pK_a of about 2 or less). Dipp-Imd-BH ₂I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH₂OTf is best prepared by reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and many have been characterized by X-ray crystallography. Several are members of very rare classes of functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.).

Full text of DOI:10.1021/ja107025y

Published on Web 10/01/2010
Substitution Reactions at Tetracoordinate Boron: Synthesis of
N-Heterocyclic Carbene Boranes with Boron-Heteroatom
Bonds
Andrey Solovyev,† Qianli Chu,† Steven J. Geib,† Louis Fensterbank,‡
Max Malacria,‡ Emmanuel Lacoˆte,*,‡ and Dennis P. Curran*,†
Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, and UPMC
UniV Paris 06, Institut Parisien de Chimie Mole´culaire (UMR CNRS 7201), C. 229, 4 place
Jussieu, 75005 Paris, France
Received August 17, 2010; E-mail: emmanuel.lacote@upmc.fr; curran@pitt.edu
Abstract: Boryl halide, carboxylate and sulfonate complexes of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene (dipp-Imd-BH2X, X ) halide or sulfonate) have been prepared from the parent borane dipp-Imd-
BH3 by (1) substitution reactions with R-X (X ) halide or sulfonate), (2) reactions with electrophiles (like
I2 or NIS), or (3) acid/base reactions with HX (provided that HX has a pKa of about 2 or less). Dipp-Imd-
BH2I is most conveniently prepared by reaction with diiodine while dipp-Imd-BH2OTf is best prepared by
reaction with triflic acid. These and other less reactive complexes behave as electrophiles and can be
substituted by a wide range of heteroatom nucleophiles including halides, thiolates and other sulfur-based
nucleophiles, isocyanate, azide, nitrite, and cyanide. The resulting products are remarkably stable, and
many have been characterized by X-ray crystallography. Several are members of very rare classes of
functionalized boron compounds (boron azide, nitro compound, nitrous ester, etc.).
Introduction
Most classes of complexes between neutral Lewis bases and
borane (LB•BH3) are transient.1 For example, complexes with
ethers, sulfides, alkenes, carbonyl groups, etc. readily equilibrate
in solution. Such exchanges often precede the hydroboration
reaction of a functional group by borane. In other words,
complexes of most Lewis bases with borane express free borane
chemistry. Amine-boranes (R3N•BH3) are more stable and have
a considerable chemistry of their own,2 but most still release
borane under suitable thermal conditions. Phosphine-boranes
(R3P•BH3) are even more stable.3
Perhaps the most stable class of borane-Lewis base com-
plexes are those with stable carbenes.4 A number of N-
heterocyclic carbene boranes (NHC-boranes) have been recently
† University of Pittsburgh.
‡ Institut Parisien de Chimie Mole´culaire.
(1) (a) Brown, H. C. In ComprehensiVe Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E., Eds.; Pergamon: Oxford,
1982; Vol. 7, p 111. (b) Pelter, A.; Smith, K.; Brown, H. C. Borane
Reagents; Academic Press: New York, 1988.
Figure 1. Selected examples of stable NHC-borane complexes and derived
reactive intermediates.
described (Figure 1), and most are robust complexes that are
stable to ambient conditions and chromatography. None has yet
shown any tendency to express borane chemistry. Unlike
boranes and borohydrides, most NHC-boranes are resistant to
oxidation as well as to acid and base hydrolysis.
Early work on NHC-boranes focused on structure, and
interesting complexes of BH3,5 BF3,6 and other simple boranes7,8
were characterized by crystallography and other means. More
recently, the chemistry of such complexes has garnered attention.
The NHC ligand has been used to stabilize boron-boron double
bonds8 and to generate unusual reactive intermediates like boryl
radicals,9 borenium cations,10 and, most recently, boryl anions
(2) (a) Carboni, B.; Monnier, L. Tetrahedron 1999, 55, 1197–1248. (b)
Kanth, J. V. B. Aldrichimica Acta 2002, 35, 57–66. (c) Carboni, B.;
Carreaux, F. In Science of Synthesis, Organometallics: Boron Com-
pounds; Kaufmann, D. E., Matteson, D. S., Eds.; Verlag: Stuttgart,
2004; Vol. 6, pp 455-484. (d) Burnham, B. S. Curr. Med. Chem.
2005, 12, 1995–2010. (e) Staubitz, A.; Robertson, A. P. M.; Sloan,
M. E.; Manners, I. Chem. ReV. 2010, 110, 4023–4078.
(3) Gaumont, A. C.; Carboni, B. In Science of Synthesis, Organometallics:
Boron Compounds; Kaufmann, D. E., Matteson, D. S., Eds.; Verlag:
Stuttgart, 2004; Vol. 6, pp 485-512.
(4) Carbene boranes for other classes of stable carbenes: (a) Cattoe¨n, X.;
Gornitzka, H.; Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2004,
126, 1342–1343. (b) Monot, J.; Fensterbank, L.; Malacria, M.; Lacoˆte,
E.; Geib, S. J.; Curran, D. P. Beilstein J. Org. Chem. 2010, 6, 709–
712.
9
15072 J. AM. CHEM. SOC. 2010, 132, 15072–15080
10.1021/ja107025y  2010 American Chemical Society

Substituted NHC-Boranes
A R T I C L E S
(Figure 1).11 NHC-boranes are also starting to be used as
synthetic reagents in radical,12 ionic,13 and organometallic
reactions.14 The structures and therefore reactions of these
species appear to be unique in boron chemistry. For example,
NHC-boryl radicals have a very different structure from amine-
boryl and phosphine-boryl radicals.9b
With few exceptions, NHC-borane complexes have been
prepared by direct complexation of the NHC with a suitable
borane (BH3, BF3, etc.). The BH3 complexes were initially
prepared from BH3•THF or BH3•SMe2, but they can now be
prepared from inexpensive, easy to handle amine-boranes as
well.15 While apparently very general, the scope of this direct
complexation method is limited by the availability of boranes.
Compounds such as acyl boranes that are not readily available
by direct complexation can now be made from boryl anions.11b
We became interested in developing methods to synthesize
new NHC-boranes directly from existing NHC-boranes for two
reasons. First, we wanted to better understand the existing
chemistry of NHC-boranes. For example, recent work with
NHC-boranes as synthetic reagents has focused on the trans-
formation of the organic component of the reaction. But each
Figure 2. Generic structures of new substituted 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene carbene boranes.
transformation must also have a parallel new NHC-borane
product. In radical reactions of xanthates, the NHC-borane
products have been isolated and characterized,9a and we set out
to do likewise for representative ionic transformations. Second,
we wanted to expand the available classes of NHC-boranes in
order to study their structures and reaction chemistry. For
example, common functional groups in carbon chemistry like
azides and nitro groups are rarely found bound to boron atoms.
These and many other functionalized NHC-borane complexes
could not be produced by direct complexation because the
needed borane is not available.
Here we describe a detailed study of substitution reactions
of the prototypical 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene class of carbene borane complexes (hereafter abbreviated
dipp-Imd-BH3, dipp-Imd-BH2OTf, etc.). Borane complexes of
the dipp ligand were introduced by Robinson,8 and we chose
this ligand because it is featured in many other recent applica-
tions of NHC-boranes. We describe the synthesis of over two
dozen new complexes with one (NHC-BH2X) or two (NHC-
BHX2) heteroatoms or heteroatom-based functional groups
(Figure 2). These are made by ionic substitutions, reactions with
electrophiles, acid/base reactions, and, perhaps most generally,
direct displacement of leaving groups on tetracoordinate boron
with nucleophiles. Many of the complexes are new types of
NHC-boranes, and indeed several, including azide, nitro, and
nitrous ester, are rare examples of stable compounds of boron
bonded to such functional groups.
(5) BH3 complexes: (a) Kuhn, N.; Henkel, G.; Kratz, T.; Kreutzberg, J.;
Boese, R.; Maulitz, A. H. Chem. Ber. 1993, 126, 2041–2045. (b)
Ramnial, T.; Jong, H.; McKenzie, I. D.; Jennings, M.; Clyburne,
J. A. C. Chem. Commun. 2003, 1722–1723. (c) Yamaguchi, Y.;
Kashiwabara, T.; Ogata, K.; Miura, Y.; Nakamura, Y.; Kobayashi,
K.; Ito, T. Chem. Commun. 2004, 2160–2161.
(6) BF3 complexes: (a) Arduengo, A. J., III; Davidson, F.; Krafczyk, R.;
Marshall, W. J.; Schmutzler, R. Monatsh. Chem. 2000, 131, 251–
265. (b) Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H.
Inorg. Chim. Acta 2003, 352, 143–150.
(7) Other complexes: (a) Lambert, C.; Lopez-Solera, I.; Raithby, P. R.
Organometallics 1996, 15, 452–455. (b) Tamm, M.; Lu¨gger, T.; Hahn,
E. F. Organometallics 1996, 15, 1251–1256. (c) Phillips, A. D.; Power,
P. P. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2005, 61,
o291-o293. (d) Chase, P. A.; Stephan, D. W. Angew. Chem., Int.
Ed. 2008, 47, 7433–7437.
(8) (a) Wang, Y.; Quillian, B.; Wei, P.; Wannere, C. S.; Xie, Y.; King,
R. B.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. J. Am.
Chem. Soc. 2007, 129, 12412–12413. (b) Wang, Y.; Quillian, B.; Wei,
P.; Xie, Y.; Wannere, C. S.; King, R. B.; Schaefer, H. F.; Schleyer,
P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 3298–3299.
(9) (a) Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.;
Lacoˆte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P.
J. Am. Chem. Soc. 2009, 131, 11256–11262. (b) Walton, J. C.; Maklouf
Brahmi, M.; Fensterbank, L.; Lacoˆte, E.; Malacria, M.; Chu, Q.; Ueng,
S.-H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 2350–
2358. (c) Hioe, J.; Karton, A.; Martin, J. M. L.; Zipse, H. Chem.sEur.
J. 2010, 16, 6861–6865. (d) Solovyev, A.; Ueng, S.-H.; Monot, J.;
Fensterbank, L.; Malacria, M.; Lacoˆte, E.; Curran, D. P. Org. Lett.
2010, 12, 2998–3001.
(10) (a) Weber, L.; Dobbert, E.; Stammler, H.-G.; Neumann, B.; Boese,
R.; Bla¨ser, D. Chem. Ber. 1997, 130, 705–710. (b) Matsumoto, T.;
Gabba¨ı, F. P. Organometallics 2009, 28, 4252–4253.
Results and Discussion
Substitution Reactions with Halides and Sulfonates. We have
recently described thermal reductions of halides and sulfonates
with various NHC-boranes.13a Reaction temperatures vary from
rt for 1°-alkyl triflates up to 150 °C or more for less reactive
substrates or leaving groups. The reactions are thought to occur
by ionic mechanisms for primary halides and sulfonates.
However, compounds like carbon tetrachloride and carbon
tetrabromide are also reduced, possibly by a radical chain
pathway as suggested for the reaction between amine-boranes
and CCl4 or CCl3Br.16
(11) (a) Braunschweig, H.; Chiu, C.-W.; Radacki, K.; Kupfer, T. Angew.
Chem., Int. Ed. 2010, 49, 2041–2044. (b) Monot, J.; Solovyev, A.;
´
Bonin-Dubarle, H.; Derat, E.; Curran, D. P.; Robert, M.; Fensterbank,
L.; Malacria, M.; Lacoˆte, E. Angew. Chem., Int. Ed. 2010, DOI:
10.1002/anie.201004215.
´
(12) (a) Ueng, S.-H.; Makhlouf Brahmi, M.; Derat, E.; Fensterbank, L.;
Lacoˆte, E.; Malacria, M.; Curran, D. P. J. Am. Chem. Soc. 2008, 130,
10082–10083. (b) Ueng, S.-H.; Fensterbank, L.; Lacte, E.; Malacria,
M.; Curran, D. P. Org. Lett. 2010, 12, 3002–3005.
(13) (a) Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S.-H.; Curran,
D. P.; Malacria, M.; Fensterbank, L.; Lacoˆte, E. Chem.sEur. J. 2009,
15, 12937–12940. (b) Lindsay, D. M.; McArthur, D. Chem. Commun.
2010, 46, 2474–2476. (c) McArthur, D., PhD thesis, University of
Bristol, 2009.
The focus of that preliminary work was on optimizing the
reaction conditions and on isolating the products formed from
transformation of the halide or sulfonate. To learn about the
(14) Monot, J.; Makhlouf Brahmi, M.; Ueng, S.-H.; Robert, C.; Desage-El
Murr, M.; Curran, D. P.; Malacria, M.; Fensterbank, L.; Lacoˆte, E.
Org. Lett. 2009, 11, 4914–4917.
(15) Makhlouf Brahmi, M.; Monot, J.; Desage-El Murr, M.; Curran, D. P.;
Fensterbank, L.; Lacoˆte, E.; Malacria, M. J. Org. Chem. 2010, ASAP
(DOI: 10.1021/jo101301d).
(16) Ryschkewitsch, G. E.; Miller, V. R. J. Am. Chem. Soc. 1973, 95, 2836–
2839.
9
J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010 15073

A R T I C L E S
Solovyev et al.
Table 1. Formation of dipp-Imd-BH2X from Reduction of Alkyl
Halides and Sulfonates
temp
(°C) prod, X ) yielda b (CDCl3), δ
11B NMR
Figure 3. ORTEP diagram of the X-ray crystal structure of 4•C6H6. The
benzene molecule is omitted. Selected bond lengths (Å), angles (deg), and
torsion angles (deg): B-I 2.313(2), B-C(1) 1.593(3), I-B-C(1) 108.5(1),
I-B-C(1)-N(1) -98.6(2), I-B-C(1)-N(2) 86.4(2).
,
Entry
R-X
equiv
solvent
1
2
3
4
5
6
7
8
9
C12H25Cl
CCl4
C12H25Br
C12H25Br
CBr4
C12H25I
C4H9I
C12H25OTs
1
64
BTF
CCl4/CDCl3
140 2, Cl
80 2, Cl
140 3, Br
140 3, Br
80 3, Br
180 4, I
180 4, I
N.D.
-18.7
-18.7
-23.0
-23.0
-23.0
-33.0d
84%a
70%b
19%a
56%b
90%b
c
1.5 toluene
1.5 toluene
1
1
1
The latter procedure was more convenient because 4 could be
isolated by simple evaporation of the solvent. This compound
is hydrolytically sensitive (see below) and cannot be purified
by flash chromatography.
c
C6D6
C6D6
PhH
100%b -33.0d
1.5 toluene
140 5, OTs 70%b
140 6, OMs 60%a
25 7, OTf N.D.
-11.4
-11.0
-8.8
C12H25OMs 1.5 toluene
1.5 1,4-dioxane
The structure of 4 as a benzene solvate was confirmed by
X-ray crystallographic analysis (Figure 3). The B-C(1) bond
(1.593(3) Å) is slightly longer than the B-Ccarbene bond in 1
(1.585(4) Å).8 The boron atom lies in the plane of an
imidazolium ring (torsion angles (deg): C(2)-N(1)-C(1)-B
-175.7(2), C(3)-N(2)-C(1)-B 175.5(2)), and the B-I bond
is almost orthogonal to that plane (torsion angles (deg):
I-B-C(1)-N(1) -98.6(2), I-B-C(1)-N(2) 86.4(2)). The
B-I bond (2.313(2) Å) in 4 is longer than the bonds of the
same type in c-Hex3P-BI3 (2.236(3), 2.237(3), 2.249(3) Å).18
NHC-boryl sulfonate esters dipp-Imd-BH2OSO2R 5-7 were
prepared by reduction of corresponding alkyl sulfonates (entries
8-10). Pure tosylate dipp-Imd-BH2OTs 5 was isolated in 70%
yield by cooling and filtration of the reaction mixture (entry 8).
The crude mesylate dipp-Imd-BH2OMs 6 was contaminated with
a small amount of corresponding imidazolium mesylate salt
(dipp-Imd•HOMs). However, 6 was stable to flash chromatog-
raphy and was obtained in pure form in 60% yield (entry 9).
The reaction with dodecyl triflate is so fast that it can be
performed at room temperature (entry 10). The formation of
dipp-Imd-BH2OTf 7 was confirmed by 11B NMR of the crude
reaction mixture, but the triflate could not be isolated in a pure
form because of its susceptibility toward hydrolysis to the
imidazolium triflate salt (dipp-Imd•HOTf).
10 C12H25OTf
a Isolated yield after flash chromatography. b Isolated yield after
filtration. c 72 h. d C6D6.
products formed from the NHC-borane component, we con-
ducted a series of reactions that were followed by 11B NMR
spectroscopy. In most cases, the boron products of these
reactions were subsequently isolated. Table 1 summarizes
selected results from this series of experiments.
In a typical reaction, dipp-Imd-borane 1 (1 equiv) and dodecyl
chloride (1 equiv) were heated in benzotrifluoride17 (BTF,
C6H5CF3) at 140 °C for a fixed time period of 48 h. At that
time, about 50% of borane 1 was converted to chloroborane 2
as indicated by 11B NMR spectroscopy (entry 1). An easier route
to make 2 involved heating of 1 in carbon tetrachloride at reflux
for 72 h (entry 2). Full conversion to 2 was observed. Chloride
2 was a robust compound that was isolated in 84% yield after
flash chromatography. A second sample of 2 was prepared by
direct complexation of the carbene with BH2Cl•SMe2, though
in much lower yield (27%, see Supporting Information).
The bromoborane 3 was cleanly formed by reduction of
dodecyl bromide (1.5 equiv) with 1 in toluene at 140 °C. It
precipitated directly upon cooling and was isolated in 70% yield
by simple filtration (entry 3). However, the sample isolated in
this way contained about 10% of the corresponding imidazolium
bromide (dipp-Imd•HBr). We repeated the experiment, this time
conducting flash chromatography after filtration. However,
bromoborane 3 proved to be sensitive to silica gel; it was isolated
in good purity but in only 19% yield (entry 4). Reaction of 1
with carbon tetrabromide (1 equiv) at 80 °C in benzene-d6 for
72 h provided a better route to 3. The bromoborane again
precipitated on cooling and was isolated by filtration (hexane
wash) in 56% yield in pure form (entry 5). Experiments
conducted with more than 1 equiv of carbon tetrabromide
showed partial conversion to the dibromoborane 8, which was
also produced by reaction with dibromine as described below.
Again, another sample of 3 was made by direct complexation
with Me2S•BH2Br (see Supporting Information).
The structure of a representative sulfonate, dipp-Imd-BH2OTs
5, was solved by X-ray crystallographic analysis.19 Tosylate 5
exists as two similar but crystallographically independent
molecules, only one of which is shown in Figure 4. In contrast
to 4, the tosyloxy substituent at the boron atom in 5 is not
orthogonal to the plane of the imidazolylidene ring. Instead, it
is closer to coplanar; the N-C-B-OSO2Ar angle is 9.4° in
the molecule shown in Figure 4 and -35.2° in the other
molecule (see Supporting Information).
The 11B NMR spectrum of precursor dipp-Imd-BH3 1 exhibits
1
a clear quartet, with JB-H ) 88 Hz. In contrast, substituted
derivatives dipp-Imd-BH2X 2-7 (Table 1) each showed a broad
signal due to the quadrupole moment of the boron atom.20
Iodoborane 4 was cleanly produced by the reduction of either
dodecyl iodide (entry 6, 90%) or butyl iodide (entry 7, 100%).
(18) Braunschweig, H.; Radacki, K.; Uttinger, K. Inorg. Chem. 2007, 46,
8796–8800.
(19) The structure of phosphine-boryl sulfonate c-Hex3P-BH2OSO2CF3
makes an interesting comparison: Imamoto, T.; Asakura, K.; Tsuruta,
H.; Kishikawa, K.; Yamaguchi, K. Tetrahedron Lett. 1996, 37, 503–
504.
(17) (a) Ogawa, A.; Curran, D. P. J. Org. Chem. 1997, 62, 450–451. (b)
Maul, J. J.; Ostrowski, P. J.; Ublacker, G. A.; Linclau, B.; Curran,
D. P. In Modern SolVents in Organic Synthesis; Knochel, P., Ed.;
Topics in Current Chemistry; Springer-Verlag: Berlin, 1999; Vol. 206,
pp 80-104.
(20) (a) Beall, H.; Bushweller, C. H. Chem. ReV. 1973, 73, 465–486. (b)
Herˇma´nek, S. Chem. ReV. 1992, 92, 325–362. (c) Rapp, B.; Drake,
J. E. Inorg. Chem. 1973, 12, 2868–2873.
9
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Substituted NHC-Boranes
A R T I C L E S
Scheme 1. Reactions of 1 with Halogen-Based Electrophiles
Figure 4. One of the two similar crystallograhic molecules of dipp-Imd-
BH2OTs 5. Selected bond lengths (Å), angles (deg), and torsion angles (deg):
S(1)-O(3) 1.500(3), B(1)-O(3) 1.522(7), B(1)-C(1) 1.606(7), S(2)-O(6)
1.518(3), B(2)-O(6) 1.557(8), B(2)-C(35) 1.597(7), S(1)-O(3)-B(1)
123.9(3), O(3)-B(1)-C(1) 108.8(5), S(2)-O(6)-B(2) 122.8(3), O(6)-
B(2)-C(35) 105.9(4), N(1)-C(1)-B(1)-O(3) 9.4(8), N(2)-C(1)-B(1)-O(3)
-169.1(4), N(3)-C(35)-B(2)-O(6) 147.4(4), N(4)-C(35)-B(2)-O(6)
-35.2(7).
the boron atom can be disassociated from it NHC ligand. Such
transformations could well be useful in some settings.
Electrophilic Halogenations. Seeking even simpler ways to
make bromide 3 and iodide 4 for downstream applications, we
tried several direct electrophilic halogenation reactions of
carbene borane 1 (Scheme 1). These were patterned after
classical halogenation reactions of amine-boranes.21 Bromination
reactions were not selective. For example, treatment of 1 with
0.5 equiv of Br2 resulted in a rapid consummation of the Br2
with formation of a mixture of monobromide 3, dibromide 8,
and unreacted 1, as assessed by 11B NMR spectroscopy. In
contrast, when 2 equiv of Br2 were used, the known boron
tribromide complex 98 was the only product detected by 11B
NMR. The complete formation of 9 with only 2 equiv of Br2
means that acid/base reactions must occur with the HBr that is
generated. Reactions with N-bromosuccinimide (NBS) were also
not selective.
In contrast, the addition of 1 equiv of N-iodosuccinimide
(NIS) to 1 at rt quickly and cleanly provided boron iodide 4 as
the only boron-containing product according to 11B NMR
analysis. However, the separation of iodide 4 from succinimide
is not convenient. Addition of 0.5 equiv of I2 to 1 also cleanly
and quickly provided a solution of 4. Presumably, I2 consumes
half of 1 to produce 4 and HI. The so formed HI consumes the
other half of 1 in an acid/base reaction to produce 4 and H2.
The reaction of 1 with I2 is the most convenient way to make
4, which can then be used directly for displacement reactions
(see below) or for reductive lithiations to make boryl anions.11b
Brønsted Acid/Base Reactions. Touchstones for NHC-boranes
include neutral amine- and phosphine-boranes and anionic
borohydrides with electron-withdrawing groups like NaBH3CN.
All of these compounds react with Brønsted acids to some
degree.22 The reactions of 1 with Br2 and I2 indicate that it also
reacts with strong acids. During the course of this work, a thesis
of D. McArthur described reactions of Mes-Imd-BH3 with HCl,
TsOH, and TfOH, and his results are consistent with ours.13c
To learn more about the acid/base chemistry of 1, we reacted
it with a series of acids whose pKa’s ranged widely from about
-14 to +14. Selected results are compiled in Table 2 in order
of decreasing acidity of the acid. In a typical experiment (entry
1), 1 equiv of triflic acid was added to a solution of 1 in CDCl3
Heating of CDCl3 solutions of 2 or 6 to 50 °C resulted in sharp
1
triplets with JB-H ) 107 and 104 Hz respectively. The larger
1JB-H of these complexes compared to dipp-Imd-BH3 1 is due
to the increased s-character of B-H bonds in 2-7.20c
Stability of dipp-Imd-BH2X Complexes (2-7) toward
Hydrolysis. The six boron halides (2-4) and boryl sulfonates
(5-7) are white solids that vary greatly in their stability toward
moisture and column chromatography. At one extreme, dipp-
Imd-BH2Cl 2 survives column chromatography and can be
exposed to water or dissolved in MeOH without decomposition.
At the other extreme, the triflate 7 could only be generated in
situ, and the iodide 4 could be isolated by filtration but suffered
fast hydrolysis or methanolysis. In the middle are the mesylate
6, tosylate 5, and bromide 3. The mesylate 6 may be the most
stable of the three, since it survived flash chromatography
reasonably well. Clearly there is a qualitative correlation
between the leaving group ability of the group X and the
hydrolytic sensitivity of the corresponding borane.
Addition of water or methanol to dipp-Imd-BH2I was ac-
companied by vigorous evolution of a gas (presumably H2) with
formation of a corresponding imidazolium iodide (dipp-Imd•HI).
This salt was isolated from one experiment, and its structure
was confirmed by X-ray analysis (see Supporting Information).
A
11B NMR spectrum of the reaction with methanol showed a
broad signal at +18.7 ppm that can be attributed to B(OMe)3.20b
Accordingly, we formulate the hydrolysis (R ) H) or metha-
nolysis (R ) Me) reaction as shown below:
dipp-Imd-BH2X + 3ROH f
dipp-Imd•HX + B(OR)3 + 2H2
This sensitivity to hydrolysis is unique to NHC-boranes
containing good leaving groups. For example, precursor 1 is
very stable to water, and indeed a related water-soluble carbene
borane is known that is stable for weeks.12b Likewise, many of
the usual new functionalized compounds presented below are
not especially sensitive to hydrolysis. Additional information
on the hydrolysis of the iodide 4, including a possible sequence
of steps, is provided in the Supporting Information. This
hydrolysis is one of the few reactions discovered so far in which
(21) (a) No¨th, H.; Beyer, H. Chem. Ber. 1960, 93, 2251–2263. (b) Douglass,
J. E. J. Org. Chem. 1966, 31, 962–963.
(22) (a) Drake, J. E.; Simpson, J. J. Chem. Soc. A 1968, 974–979. (b)
Ryschkewitsch, G. E. Inorg. Nucl. Chem. Lett. 1971, 7, 99–101. (c)
Imamoto, T.; Oshiki, T. Tetrahedron Lett. 1989, 30, 383–384. (d)
Oshiki, T.; Imamoto, T. Bull. Chem. Soc. Jpn. 1990, 63, 2846–2849.
9
J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010 15075

A R T I C L E S
Solovyev et al.
Table 2. Brønsted Acid/Base Reactions of 1
Scheme 2. Reaction of 1 with 2.3 equiv of TfOH Produces
Ditriflate 13
temp
(°C)
11B NMR
(CDCl3), δ
Entry
H-X
pKa
prod, X )
yield
1
2
3
4
5
6
7
8
TfOH
-14
-9
0
0
0
7, OTf
3, Br
2, Cl
NMRa
-8.8
produced only monosubstituted complex 2 or 6. In contrast, the
reaction of 1 with 2.3 equiv of triflic acid in CH2Cl2 quickly
produced a solution of dipp-Imd-BH(OTf)2 13 (Scheme 2). This
ditriflate was not isolated, but its formation was indicated by
11B NMR spectroscopy by a new, broad signal at -2.5 ppm.
Subsequent double nucleophilic substitution reactions (see
below) also confirmed that 13 was formed. Addition of 5 equiv
of triflic acid to 1 produced only ditriflate 13; no tritriflate was
detected.
Nucleophilic Substitution Reactions. One of the characteristic
reactions of boranes ligated to amines or phosphines and bearing
leaving groups is nucleophilic substitution.19,23 In addition,
amine-ligated boryl iodides and triflates have also been used in
hydroboration reactions.24 In these reactions, the alkene is
proposed to displace the iodide or triflate to start the reaction.
We already observed that the NHC-BH2X compounds with the
best leaving groups react more or less readily with water and
alcohols. Accordingly, we posited that NHC-boranes bearing
leaving groups would react as electrophiles in nucleophilic
substitution reactions. Having ready access to the needed boryl
halides and sulfonates, we undertook a detailed study of such
reactions. Many of the most interesting results are summarized
in Table 3.
In a typical experiment, boryl triflate 7 was generated in situ
by addition of TfOH to 1 in CDCl3. Tetrabutylammonium
fluoride (Bu4NF, 2 equiv) was then added, and the mixture was
stirred at room temperature for 20 h. Evaporation of the solvent
and flash chromatography provided pure fluoroborane 14 as a
white solid in 40% yield. Complete characterization data of 14,
including analysis of its characteristic 11B and 19F NMR spectra,
are presented in the Supporting Information.
Iodide 4 was also used in several substitution reactions, and
this was generated in situ by the reaction of 1 with 0.5 equiv of
I2. In one case, mesylate 6 was used, and this was generated in
situ by the reaction of 1 with MsOH. A very diverse range of
substitution products were formed, as shown in Table 3.
Remarkably, all of these products are stable white solids that
were isolated by flash chromatography. Because many of the
compounds in Table 3 have functional groups that are rarely
found bound to boron, the following discussion of the results
in the table focuses on structure. However, complete details of
the preparation and isolation are provided in the Supporting
Information. Solvent choices were usually based on the solubility
of the nucleophile. Results with additional nucleophiles beyond
HBrb
NMRa -23.0
HClc d
-8
81%
-18.7
,
TsOHe
-2.6 60 5, OTs
NMRa -11.2
MsOHd
-2.6
0
6, OMs
88%
85%
-11.0
-11.4
-11.1
-11.9
CF3COOH
Cl2CHCOOH
NCCH2COOHe
-0.3 25 10, OCOCF3
1.3 60 11, OCOCHCl2 67%
2.5 60 12, OCOCH2CN 24%
a The product was observed by NMR spectroscopy but was not
isolated in a pure form. b 33% solution of HBr in AcOH. c 4 M solution
in 1,4-dioxane. d 2 equiv of acid. e Solid acids that required heating to
dissolve.
at 0 °C. A rapid reaction ensued with essentially immediate
evolution of hydrogen gas. After 5 min, analysis of the reaction
mixture by 11B NMR spectroscopy showed complete conversion
to the boron triflate 7. As mentioned above, triflate 7 is not
easily isolated, so this is a very convenient way to generate it
for use in situ.
Similar reactions of 1 with strong acids HBr, HCl, MsOH,
CF3COOH (entries 2, 3, 5, 6) at 0 °C were also accompanied
by intense bubbling, and 11B NMR spectroscopy at the 5 min
time point showed complete conversion to the corresponding
products. The chloride 2, mesylate 6, and trifluoroacetate 10
are stable compounds that were isolated by flash chromatog-
raphy in good yields (81-88%). However, in practice this is
not needed since the solutions produced on mixing contain only
the product and can be used directly in onward reactions.
TsOH is poorly soluble in CDCl3 at room temperature;
however, warming the mixture to 60 °C resulted in dissolution
and clean conversion to 5 (entry 4). The reaction with
Cl2CHCOOH was slow at room temperature (30% conversion
after 3 h according to 11B NMR spectroscopy) even though the
acid is soluble in CDCl3. But the complete conversion to 11
was observed after heating at 50 °C for 18 h (entry 7). Heating
of NCCH2CO2H at 60 °C to effect dissolution resulted in a slow
reaction (20 h) to provide 12, which was isolated in 24% yield.
The complex 1 did not react with weak acids such as
Ph2P(dO)OH (pKa ) 2.3), p-CH3C6H4COOH (pKa ) 4.4),
CH3COOH (pKa ) 4.9), or phenol (pKa ) 10) even after a
lengthy heating of the solutions in CDCl3 at 60 °C. It is already
well established that 1 does not react with water or alcohols
either. To summarize, 1 reacts rapidly at room temperature or
below with Brønsted acids of pKa’s of 0 or below (provided
that the acids are dissolved). A transition occurs in the range of
pKa 1-2, where reactions become slower. Less acidic acids do
not react with 1, even on heating at 60 °C. These results suggest
that acid catalyzed reductions or other reactions with NHC-
boranes might be possible.
(23) (a) Ryschkewitsch, G. E. J. Am. Chem. Soc. 1967, 89, 3145–3148.
(b) Bratt, P. J.; Brown, M. P.; Seddon, K. R. J. Chem. Soc., Dalton
Trans. 1974, 2161–2163. (c) Denniston, M. L.; Chiusano, M. A.;
Martin, D. R. J. Inorg. Nucl. Chem. 1976, 38, 979–981.
We also briefly studied the further acid/base reactions of
several of the heterosubstituted NHC-boranes, but these proved
to be considerably less reactive than 1. Dipp-Imd-BH2OC-
(dO)CF3 10 was the only product detected by 11B NMR
spectroscopy when 5 equiv of CF3COOH were added to dipp-
Imd-BH3 1. Similarly, adding 2 equiv of either HCl or MsOH
(24) (a) Scheideman, M.; Shapland, P.; Vedejs, E. J. Am. Chem. Soc. 2003,
125, 10502–10503. (b) Clay, J. M.; Vedejs, E. J. Am. Chem. Soc.
2005, 127, 5766–5767. (c) Karatjas, A. G.; Vedejs, E. J. Org. Chem.
2008, 73, 9508–9510. (d) In contrast to the above hydroboration
reactions with pyridine borane derivatives, no reaction was observed
between dipp-Imd-BH2OTf or dipp-Imd-BH2I and C12H25CHdCH2 in
CH2Cl2/THF at rt.
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Substituted NHC-Boranes
A R T I C L E S
Table 3. Nucleophilic Substitution of dipp-Imd-BH2X 4, 6, or 7
11B NMR
Entry
X
nucleophile
solvent
prod, Nu
yield (CDCl3), δ
1
2
3
4
5
6
7
8
9
OTf Bu4NF
CDCl3/THF 14, F
CDCl3/THF 2, Cl
40% -6.1 (app q)
38% -18.7 (br s)
31% -22.6 (br s)
58% -24.9 (br s)
42% -23.5 (br s)
I
BnEt3NCl
OTf PhMgBr
THF
CDCl3/THF 15, SPh
THF 16, SPT
3, Br
Figure 5. X-ray crystallographic structure of dipp-Imd-BH2SO2PT 26.
Selected bond lengths (Å), angles (deg), and torsion angles: B-S 1.908(1),
S-C(28) 1.821(1), B-C(1) 1.586(2), B-S-C(28) 113.16(6), S-B-C(1)
112.59(9), B-S-C(28)-N(3) -46.9(1), B-S-C(28)-N(6) 146.6(1),
S-B-C(1)-N(1) 88.6(1), S-B-C(1)-N(2) -100.9(1).
a
OTf
I
PhSLi
a b
,
PTSLi
OTf EtOC(dS)SK CDCl3/THF 17, SC(dS)OEt 35% -24.4 (br s)
c
OTf
I
CDCl3/THF 18, SC(dS)SPh 65% -22.9 (br s)
PhSC(dS)SLi
NaSCN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
19, NCS
20, NCO
21, N3
21, N3
21, N3
22, ONO
23, NO2
22, ONO
23, NO2
24% -23.2 (br s)
35% -22.8 (t)
42% -17.2 (t)
74% -17.2 (t)
83% -17.2 (t)
12% -10.1 (t)
7% -13.5 (br s)
27% -10.1 (t)
OTf KOCN
hexylphosphine.25 In that compound, the B-S bond is a part
of a five-membered cycle yet has similar geometry parameters:
B-S 1.899(4), S-C 1.793(3), B-S-C 100.7(1).
10 OTf NaN3
11
12
13 OTf NaNO2
14 AgNO2
15 OTf Bu4NCN
16
17
I
NaN3
d
OMs NaN3
Reactions of triflate 7 with commercially available potassium
ethyl xanthogenate KSC(dS)OEt (entry 6) and lithium carbon-
otrithiolate LiSC(dS)SPh (prepared in situ from PhSH, BuLi,
and CS2, entry 7) gave corresponding products 17 and 18
with a general formula dipp-Imd-BH2SC(dS)R. Dipp-Imd-
BH2SC(dS)OEt 17 resembles one of the products of the radical
reduction of primary xanthate with 1, and it has the same
chemical shift in the 11B NMR spectrum as that reported for
dipp-Imd-BH2SC(dS)O(CH2)6OBn.9a
I
DMSO
10% -13.5 (br s)
e
CDCl3/THF 24, CN
-36.7 (t)
81%
I
I
NaCN
AgCN
DMSO
DMSO
24, CN
25, NC
21% -36.7 (t)
55% -24.0 (br s)
f
a Prepared in situ by the reaction between thiol and n-BuLi. b PT )
1-phenyltetrazol-5-yl. c Prepared in situ from PhSLi and CS2. d At 80
°C. e An inseparable 10:1 mixture of 24 and 25. f 100 °C.
The chemistry of carbon compounds is replete with molecules
containing both C-H bonds and functional groups multiply
bonded to heteroatoms. The list is almost endless: carbonyl
groups, nitro groups, nitriles, azides, isocyanates, and so on. In
contrast, compounds containing B-H bonds and such functional
groups are quite rare, especially so when the functional group
is directly bonded to the boron. This is because most types of
B-H bonds reduce multiple bonds with heteroatoms.
As a class, NHC-boranes present an interesting opportunity
to explore such unusual functionalized compounds because they
do not dissociate easily (no free borane reactions), because they
are only very weak hydride donors, and because substitution
reactions are readily effected. In practice, the scope of displace-
ment reactions of NHC-BH2X compounds with nucleophiles
bearing heteroatomic multiple bonds and the stability of the
resulting products extended well beyond our initial imagination
(Table 3, entries 8-17).
The major product isolated from the substitution reaction
between iodide 4 and NaSCN was isothiocyanate dipp-Imd-
BH2NCS 19 (entry 8). The related ambident nucleophile,
potassium cyanate KOCN, also was borylated on the nitrogen
atom to produce isocyanate dipp-Imd-BH2NCO 20 in 35% yield
(entry 9). The regioselectivities of both reactions were initially
assigned based on the chemical shifts in the 11B NMR spectra
and on analyses of the IR spectra, as discussed in the Supporting
Information.
Scheme 3. Oxidation of Sulfide 16 to Sulfone 26
those in Table 3 are also briefly summarized in the Supporting
Information. For example, reduction of triflate 7 by LiAlD4 is
a convenient method to make dipp-Imd-BH2D (1-d1).
Substitution of iodide 4 by chloride (BnEt3NCl) (entry 2)
provided the chloride 2 (38% yield), while the reaction of
triflate 7 with PhMgBr (entry 3) provided none of the phenyl
substitution product (dipp-Imd-BH2Ph) but instead returned
the now familiar bromide 3 (31% yield after column
chromatography).
The reaction of 7 with PhSLi (generated separately in situ
from PhSH and BuLi) produced dipp-Imd-BH2SPh 15 in 58%
yield as a stable white solid (entry 4). Lithium 1-phenyl-1H-
tetrazole-5-thiolate (LiSPT) reacted with 4 to give dipp-Imd-
BH2SPT 16 in 42% yield (entry 5). This compound was
subsequently oxidized with m-chloroperoxybenzoic acid
(mCPBA) to dipp-Imd-BH2SO2PT 26, a stable, readily isolated
compound (Scheme 3). The transformation is remarkable
because the B-H bonds of the complex are inert to such a strong
oxidant.
Later, both structures were confirmed by X-ray crystallographic
analysis, as shown in Figure 6. The bond lengths in the B-X-C-Y
of moiety are similar to those in H3N-BH2NCS:26a B-N 1.534(8),
The structure of borane sulfone 26 was confirmed by X-ray
analysis (Figure 5). The B-S bond is almost orthogonal to the
plane of the imidazolylidene ring (torsion angle S-B-C(1)-N(1)
is 89°). The only other boron-substituted sulfone characterized
by X-ray analysis is (P-B)-(2-(borylsulfonyl)phenyl)dicyclo-
(25) Imamoto, T.; Hirakawa, E.; Yamanoi, Y.; Inoue, T.; Yamaguchi, K.;
Seki, H. J. Org. Chem. 1995, 60, 7697–7700.
(26) (a) Kendall, D. S.; Lipscomb, W. N. Inorg. Chem. 1973, 12, 2920–
2922. (b) Shelly, K.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem.
1992, 31, 2889–2892.
9
J. AM. CHEM. SOC. VOL. 132, NO. 42, 2010 15077

A R T I C L E S
Solovyev et al.
Figure 7. X-ray crystallographic structure of dipp-Imd-BH2N3 21. Selected
bond lengths (Å), angles (deg), and torsion angles: N(5)-N(4) 1.137(2),
N(4)-N(3) 1.212(2), B-N(3) 1.573(2), B-C(1) 1.614(2), N(5)-N(4)-N(3)
174.5(2), B-N(3)-N(4) 119.2(1), N(3)-B-C(1) 106.7(1), N(3)-B-
C(1)-N(1) 59.0(2), N(3)-B-C(1)-N(2) -119.9(2), B-N(3)-N(4)-N(5)
-178(2).
Figure 6. X-ray crystallographic structures of dipp-Imd-BH2NCS 19 (top)
and dipp-Imd-BH2NCO 20 (bottom). Selected bond lengths (Å), angles
(deg), and torsion angles for 19: S-C(28) 1.648(3), C(28)-N(3) 1.089(4),
B-N(3)1.533(4),B-C(1)1.608(4),N(3)-C(28)-S178.2(3),B-N(3)-C(28)
169.2(3), N(3)-B-C(1) 109.8(2), N(3)-B-C(1)-N(1) -79.1(3), N(3)-
B-C(1)-N(2) 102.2(3), B-N(3)-C(28)-S -31(10). Selected bond lengths
(Å), angles (deg), and torsion angles for 20: O-C(28) 1.207(3), C(28)-N(3)
1.129(3), B-N(3) 1.543(3), B-C(1) 1.613(3), N(3)-C(28)-O 178.7(3),
B-N(3)-C(28) 166.6(2), N(3)-B-C(1) 107.7(2), N(3)-B-C(1)-N(1)
65.6(3), N(3)-B-C(1)-N(2) -112.5(2), B-N(3)-C(28)-O 150(11).
Figure 8. X-ray crystallographic structure of dipp-Imd-BH2ONO 22.
Selected bond lengths (Å), angles (deg), and torsion angles: O(2)-N(3)
1.150(4), N(3)-O(1) 1.388(4), B-O(1) 1.512(3), B-C(1) 1.616(3),
O(2)-N(3)-O(1) 110.3(3), B-O(1)-N(3) 108.2(2), O(1)-B-C(1) 110.5(2),
O(1)-B-C(1)-N(1) -124.5(2), O(1)-B-C(1)-N(2) 57.1(3), B-O(1)-
N(3)-O(2) 175.1(3).
X-ray crystallographic analysis confirmed the structure of 21
(Figure 7). The geometrical parameters are similar to those of
another four-coordinate boron azide Py-9-BBN-N3: B-NR
1.588(3), NR-Nꢀ 1.196(3), Nꢀ-Nγ 1.140(3), B-NR-Nꢀ 119.5(2),
NR-Nꢀ-Nγ 176.3(3).28 In contrast to dipp-Imd-BH2NCO 20,
the B-N(3)-N(4) angle of 119° was consistent with sp2-
hybridization of NR. Like azides bonded to carbon, the NR-Nꢀ
bond (1.212(2) Å) is longer than the Nꢀ-Nγ bond (1.137(2)
Å).
The reaction of triflate 7 with NaNO2 (entry 13) gave two
products: the less polar boryl nitrite dipp-Imd-BH2ONO 22
(12%) and more polar nitroborane dipp-Imd-BH2NO2 23 (7%).
These were stable toward aqueous workup and were easily
separated by column chromatography. Reaction of iodide 4 with
AgNO2 (entry 14) gave the same products in better yields: 22
(27%) and 23 (10%). Reversing the pairings by reacting 7 with
AgNO2 and 4 with NaNO2 also gave the same products
according to 11B NMR analysis of crude mixtures, but in lower
yields.
N-C 1.137(8), C-S 1.627(6), B-N-C 177.5(6), N-C-S
179.2(5) and in a polyborane derivative [Et3NH]2[closo-2-
B10H9NCO]:26b B-N 1.498(7), N-C 1.128(7), C-O 1.187(7),
B-N-C 172.3(3), N-C-O 177.8(4). The bond lengths suggest
that the C-N bond may have some triple bond character as
illustrated in the resonance forms 20a, 20b. The resonance form
20b is stabilized because the positive nitrogen atom is adjacent to
the negative boron atom. This resonance may also explain the high
value of the B-N(3)-C(28) angle of 167°, which is closer to that
expected for sp-hybridization of the nitrogen atom (180° angle)
than that for sp2-hybridization (120° angle).
Synthesis of borane azide dipp-Imd-BH2N3 21 has been
achieved from three different precursors: triflate 7, iodide 4,
and mesylate 6 (entries 10-12) in 42%, 74%, and 83% yields,
respectively. Heating of the reaction mixture at 80 °C was
required for mesylate 6 because no reaction was observed at
room temperature. Azide 21 is a stable white solid with a distinct
melting point. It is much more stable toward hydrolysis than
Me3N-BH2N3, which has been prepared by pyrolysis of
[H2B(NMe3)2]N3 or from Me3N-BH2I by exchange on Amberlite
IRA-400 resin in the azide form.27 The incorporation of an azide
group was evident from the IR spectrum (strong band at 2096
cm-1) and the high resolution mass spectrum.
Infrared spectroscopy was used for a preliminary assignment
of the structures, and confirmation again came from the X-ray
analyses of both samples (Figures 8, 9). For dipp-Imd-
BH2sOsNdO 22, the N(3)sO(1) bond (1.388(4) Å) is longer
than the N(3)sO(2) bond (1.150(4) Å) and these values are
(28) (a) Fraenk, W.; Habereder, T.; Klapo¨tke, T. M.; No¨th, H.; Polborn,
K. J. Chem. Soc., Dalton Trans. 1999, 4283–4286. (b) See also: Kim,
Y.; Hudnall, T. W.; Bouhadir, G.; Bourissou, D.; Gabba¨ı, F. Chem.
Commun. 2009, 3729–3731.
(27) (a) Miller, N. E.; Chamberland, B. L.; Muetterties, E. L. Inorg. Chem.
1964, 3, 1064–1065. (b) Skillern, K. R.; Kelly, H. C. Inorg. Chem.
1977, 16, 3000–3005.
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Substituted NHC-Boranes
A R T I C L E S
Scheme 4. Methylation and Basic Hydrolysis of Cyanoborane 24
Figure 9. X-ray crystallographic structure of dipp-Imd-BH2NO2 23.
Selected bond lengths (Å), angles (deg), and torsion angles: O(2)-N(3)
1.235(2), N(3)-O(1) 1.230(2), B-N(3) 1.591(2), B-C(1) 1.604(2),
C(1)-N(1) 1.354(2), O(1)-N(3)-O(2) 120.6(1), B-N(3)-O(1) 120.9(1),
B-N(3)-O(2) 118.5(1), N(3)-B-C(1) 107.4(1), N(3)-B-C(1)-N(1)
-66.4(2), N(3)-B-C(1)-N(2) 114.6(1), C(1)-B-N(3)-O(1) 131.7(1),
C(1)-B-N(3)-O(2) -50.4(2).
Scheme 5. One-Pot Double Substitution Reactions
close to single NsO and double NdO bond lengths, respec-
tively. The O(1)sN(3)sO(2) angle of 110° shows that the nitrite
nitrogen atom has an accessible lone electron pair. In contrast,
BsN(3)sO(1), BsN(3)sO(2), and O(1)sN(3)sO(2) angles
in 23 are 121°, 119°, and 121° confirming a planar sp2-
hybridized nitrogen atom of a nitro group. The distances
N(3)sO(1) and N(3)sO(2) are essentially equal (1.230(2) and
1.235(2) Å). A nitroborane salt Cs[(CF3)3BNO2] has similar
structural parameters: BsN 1.613(9), NsO 1.218(7) and
1.230(8), BsNsO 119.3(6) and 121.5(6), OsNsO 119.1(6).29
To the best of our knowledge, 22 is the first boryl nitrite to
be isolated and characterized. Previously only IR observation
of transient CatBONO in a mixture with other compounds was
reported.30 Monosubstituted nitroboranes are also extremely rare.
The substituents at the boron atom in 20-23 are not
orthogonal to the plane of the imidazolylidene ring but form
torsion angles between 50° and 70° (Figures 6-9).
21-22, and 24 were sharp triplets even at room temperature.
This can be explained by the lower polarity of the B-X bonds
in these compounds compared with complexes 2-7 and thus a
more symmetrical distribution of charge around the boron atom.
For example, in the 11B NMR spectrum of dipp-Imd-BH2(thexyl)
a triplet was also observed at room temperature.9b
The reaction between 7 and Bu4NCN (2 equiv) provided
boron nitrile dipp-Imd-BH2CN 24 in 81% yield mixed with an
inseparable impurity (∼10%) (entry 15). To show that this
product was the nitrile 24, it was converted to a boryl amide as
described for ligated cyanoboranes Me3N-BH2CN and Ph3P-
BH2CN.31 First, 24 was methylated with Me3O+BF4-, and the
(supposed) intermediate tetrafluoroborate was treated with 1 M
NaOH (Scheme 4). After workup and column chromatography,
amide 27 was isolated in 61% yield as a white solid. Once again,
the robust nature of the NHC-borane component under rather
vigorous conditions is remarkable.
A pure sample of nitrile 24 was prepared by substitution of
iodide 4 with NaCN in DMSO (entry 16). In contrast, reaction
of 4 with AgCN in DMSO (entry 17) gave a different
compound, tentatively assigned as the isonitrile 25 based on
HRMS and 11B NMR data. These complexes showed charac-
We closed this study by demonstrating that double substitu-
tions at boron can occur by generation of a bis-electrophile and
reaction with nucleophiles. Examples of such substitutions are
shown in Scheme 5.
In situ generation of the bis-triflate 13 followed by addition
of an excess of Bu4NF gave difluoride dipp-Imd-BHF2 28 in
27% isolated yield. The substitution pattern at the boron atom
was supported by J-couplings in the 11B NMR and 19F NMR
spectra as well as by the high resolution mass spectrum. The
BF3 analog of 1 is already known33 and the BH2F complex is
reported in Table 3, so this completes the series of fluoroborane
analogs of 1. All three compounds are stable under ambient
lab conditions. An analogous series of trimethylamine-fluo-
roboranes has been made by reaction of Me3N-BH3 with HF.34
However, reaction of Me3N-BH2I does not produce Me3N-BH2F
but instead gives a mixture of Me3N-BF3 and Me3N-BH3.23b
Likewise, dicyanide 29 was prepared from ditriflate 13 and
an excess of Bu4NCN in 54% yield. As in the case of
monocyanide 24 (Table 3, entry 15), this product also contained
about 15% of an inseparable and unidentified impurity that
exhibited a broad multiplet at -26.5 ppm in the 11B NMR
spectrum. Dipp-Imd-BH(CN)2 exhibited a sharp doublet at
-35.7 ppm. Just as monocyanide 24 can be considered as a
boron nuclear analog of acetonitrile, dicyanide 29 is an analog
of malononitrile. Finally, diazide 30 was obtained in 37% yield
when ditriflate 13 was treated with Bu4NN3. This is a white
teristic bands in the IR spectra corresponding to nitrile and
32
isonitrile groups: 24 at 2187; 25 at 2192 cm-1
.
Complexes 2-7 exhibit broad peaks in rt 11B NMR spectra
that resolved to triplets on heating. In contrast, the signals of
(29) (a) Brauer, D. J.; Bu¨rger, H.; Chebude, Y.; Pawelke, G. Eur. J. Inorg.
Chem. 1999, 247–253. (b) Brauer, D. J.; Chebude, Y.; Pawelke, G. J.
Fluorine Chem. 2001, 112, 265–270.
(30) Bettinger, H. F.; Bornemann, H. J. Am. Chem. Soc. 2006, 128, 11128–
11134.
(31) (a) Spielvogel, B. F.; Wojnowich, L.; Das, M. K.; McPhail, A. T.;
Hargrave, K. D. J. Am. Chem. Soc. 1976, 98, 5702–5703. (b) Wisian-
Neilson, P.; Wilkins, M. A.; Weigel, F. C.; Foret, C. J.; Martin, D. R.
J. Inorg. Nucl. Chem. 1981, 43, 457–458.
(33) Makhlouf Brahmi, M., PhD Thesis, UPMC, Paris, 2010.
(34) VanPaasschen, J. M.; Geanangel, R. A. J. Am. Chem. Soc. 1972, 94,
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(32) Vidal, J. L.; Ryschkewitsch, G. E. J. Chem. Soc., Chem. Commun.
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A R T I C L E S
Solovyev et al.
crystalline solid stable below the melting point (220-222 °C).
Slow evolution of gas (N2) was observed when the melted
compound was heated above 240 °C.
acid/base reactions with HX (provided that HX has a pKa of
about 2 or less).
The stabilities of the resulting products to protic media
and chromatography vary widely depending on the leaving
group ability of the boron substituent. Like dipp-Imd-
BH3, dipp-Imd-BH2Cl is a robust compound. At the other
extreme, dipp-Imd-BH2I is most conveniently prepared by
reaction of dipp-Imd-BH3 with diiodine and used directly in
situ, while dipp-Imd-BH2OTf is best prepared for in situ use
by reaction with triflic acid.
Iodide 4, triflate 7, and other less reactive complexes behave
as electrophiles and can be substituted by a wide range of
heteroatom nucleophiles including halides, thiolates and other
sulfur-based nucleophiles, isocyanate, azide, nitrite, and cyanide.
The resulting products are remarkably stable and have been
characterized by X-ray crystallography. Several are members
of very rare classes of boron complexes (boron azides, nitro
compounds, nitrous ester, etc.).
These results pave the way for future studies in several
directions. For example, many other types of functionalized
NHC-borane complexes should be accessible by the types of
electrophilic, acid/base, and substitution reactions described
herein. And because many of the compounds are robust and
readily isolated, they could have interesting applications in
organic synthesis, inorganic synthesis, or other areas.
Elucidating the mechanism(s) of these substitution reactions
will require further study. A nucleophilic substitution of chiral
c-Hex3P-BH(CO2Me)Br with LiCN in DMF-THF and of chiral
Me3N-B(CH2Ph)(CN)H with pyridine proceeded with inversion
of the configuration at the boron atom, indicating that SN2-B
mechanisms should be considered.35 Both SN1-B and SN2-B
mechanisms were suggested for phosphine substitution in chiral
Ph3P-BH(Ipc)CN with PMe2Ph (Ipc ) isopinocampheyl) de-
pending on the solvent.36
The intermediacy of borenium cation [dipp-Imd-BH2]+ or a
solvent complex thereof is also possible. To investigate the latter
possibility, we dissolved iodide dipp-Imd-BH2I 4 in several
solvents and recorded the 11B NMR spectra of the samples. The
following chemical shifts (δ) were observed: C6D6, -33.0; THF,
-32.8; CDCl3, -32.4; (CH3)2CO, -31.6; CH3CN, -23.0; DMF,
-5.2; DMSO-d6, -8.8. The large changes in chemical shift
observed upon dissolution in DMF and DMSO suggest that the
iodide is ionized in these coordinating solvents to give the
complex [dipp-Imd-BH2•solvent]+ I-. Likewise, triflate 7 is
presumably ionized in these solvents as well.
Conclusions
N-Heterocyclic carbene boranes bearing leaving groups
(halogen, sulfonate) have been prepared by several means, and
their substitution reactions have been studied. The NHC-borane
halide and sulfonate complexes of 1,3-bis(2,6-diisopropylphe-
nyl)imidazol-2-ylidene (dipp-Imd-BH2X, X ) halide or sul-
fonate) can be prepared from the parent borane dipp-Imd-BH3
by three complementary routes: (1) substitution reactions with
R-X, (2) reactions with electrophiles (like I2 or NIS), or (3)
Acknowledgment. This work was supported by grants from
the U.S. National Science Foundation (CHE-0645998), UPMC,
CNRS, and the French Agence Nationale de la Recherche (Chaire
d’excellence ANR 08-CEXC-011-01 Borane). We thank Prof. D.
Lindsay for providing a copy of the thesis in ref 13. A.S. thanks
the University of Pittsburgh for a Mellon Predoctoral Fellowship.
Supporting Information Available: Procedures and charac-
terization data for all of the new complexes and cif files of the
crystal structures. This material is available free of charge via
the Internet at http://pubs.acs.org.
(35) (a) Imamoto, T.; Morishita, H. J. Am. Chem. Soc. 2000, 122, 6329–
6330. (b) Charoy, L.; Valleix, A.; Toupet, L.; Le Gall, T.; van Chuong,
P. P.; Mioskowski, C. Chem. Commun. 2000, 2275–2276.
(36) Vedrenne, P.; Le Guen, V.; Toupet, L.; Le Gall, T.; Mioskowski, C.
J. Am. Chem. Soc. 1999, 121, 1090–1091.
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