Hydrogen and amine activation by a frustrated Lewis pair of a bulky N-heterocyclic carbene and B(C6F5)3

Article abstract of DOI:10.1002/anie.200802596

(Chemical Equation Presented) Size matters: The frustrated Lewis pair derived from B(C₆F₅)₃ and the sterically encumbered N-heterocyclic carbene N,N′-tBu₂C₃H ₂N₂ (1) cleaves dihydrogen heterolytically to give a imidazolium borate (see scheme, left), and cleaves amine N-H bonds to form aminoborate salts (right) or aminoboranes.

Full text of DOI:10.1002/anie.200802596

Angewandte
Chemie
DOI: 10.1002/anie.200802596
H2 Activation
Hydrogen and Amine Activation by a Frustrated Lewis Pair of a Bulky
N-Heterocyclic Carbene and B(C6F5)3**
Preston A. Chase and Douglas W. Stephan*
Dedicated to Professor R. J. Puddephatt
N-Heterocyclic carbenes (NHCs),[1] one class of stable and
isolable carbenes,[2] are a well-studied ligand that can often
replace phosphines in organometallic complexes, which have
found extensive use in catalysis.[3,4] NHCs are also utilized as
organocatalysts in their own right,[5–7] and there are growing
number of reports of unique and unexpected reactivity of a
variety of stable carbenes. For example, recent studies from
Bertrand and co-workers showed that certain (amino)-
(alkyl)carbenes react directly with dihydrogen and ammo-
range of compounds known to be reactive in FLP-type
chemistry is relatively small to date.[20–22] Herein, we present
bulky NHCs which are viable basic partners with the Lewis
acid tris(pentafluorophenyl)borane[23–25] in FLP chemistry,
showing bond activation reactivity with dihydrogen and
amines. In the later case, the NHC also catalyzes an
elimination reaction with primary and secondary alkyl
amines adducts of tris(pentafluorophenyl)borane to directly
and cleanly generate a variety of aminoboranes.
nia,[8] mimicking metalloid oxidative addition processes
In our initial efforts to uncover carbene-based FLPs, the
reaction of the NHC 1,3-bis(2,6-diisopropylphenyl)-1,3-imi-
dazol-2-ylidene (IDipp) with B(C6F5)3 was performed. The
11B NMR spectrum of the resulting product (1) shows a single
resonance at À15.6 ppm, which is indicative of a four-
coordinate boron center, suggesting the formation of the
simple Lewis adduct of formulation (IDipp)B(C6F5)3
[9,10]
(Scheme 1a). Small main-group species, such as P4
also
1
(Scheme 2). The H, 19F and 13C{1H} NMR spectra of 1 are
Scheme 1. Selected examples of main-group compounds that activate
dihydrogen and other small molecules: a) an (amino)(alkyl)carbene,
b) a mixture of bulky phosphines and boranes, c) a para-phosphino-
phenylborane.
Scheme 2. Reactivity of B(C6F5)3 with the NHCs IDipp and ItBuwith
H2. N.R.=no reaction.
react with carbenes. Current work from our group involves
the reactivity of “frustrated Lewis pairs” (FLPs),[11,12]
a
combination of Lewis acids and bases in which steric
encumbrance prevents or limits adduct formation. These
unquenched sites can have unique reactivity. For example,
mixtures of bulky phosphines and boranes react directly with
dihydrogen[13,14] and olefins (Scheme 1b).[15,16] Furthermore,
the compound (2,4,6-Me3C6H2)2P(C6F4)B(C6F5)2 is the first
example of a main group system that can reversibly react with
dihydrogen (Scheme 1c).[17] The hydrogen-activating ability
of these species has been exploited in metal-free catalytic
reduction of imines, nitriles, and aziridines.[18,19] However, the
all quite complex, which is attributable to restricted rotation
in this bulky adduct. In addition this formulation was
confirmed by an X-ray crystal structure determination
(Figure 1). Power and Phillips described the only reported
NHC/B(C6F5)3 adduct, which contains 1,3,4,5-tetramethyl-
À
1,3-imidazol-2-ylidene. This latter species had
a B
C(carbene) bond length of 1.6407(18) Š,[26] which compares
À
with the B C distance of 1.663(5) Š found in 1. The greater
steric demands presented by the iPr groups on the arene
[*] Dr. P. A. Chase, Prof. Dr. D. W. Stephan
À
substituents in IDipp presumably account for the longer B C
Department of Chemistry, University of Toronto
80 St George St, Toronto, Ontario, M5S3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
bond. It is noteworthy that Lewis acid–base adduct 1 proved
unreactive towards dihydrogen or olefins Scheme 2), and is
not stable for extended periods in solution, decomposing to
give a mixture of products.
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN/
[**] The support of NSERC of Canada is gratefully acknowledged.
Angew. Chem. Int. Ed. 2008, 47, 7433 –7437
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7433

Communications
=
reduction of imines, such as tBuN CPh(H), over a 13 hour
period at 258C in CD2Cl2. This latter observation is consistent
with the greater basicity of carbene in comparison to
phosphine, precluding protonation of imine and thus initia-
tion of reduction.[18,19]
It is notable that Bertrand et al. observed no reaction of
NHC-type carbenes with dihydrogen.[8] Furthermore, B-
(C6F5)3 alone does not appreciably interact with dihydro-
gen.[13,14] This fact suggests that approach of the Lewis acid
ItBu and base B(C6F5)3 is prevented by the steric demands of
the tert-butyl substituents of the carbene, precluding dative
bond formation. Such pairing could provide a reactive pocket
for the activation of dihydrogen. Indeed, this proposition is
supported by recently published DFT studies for the closely
related situation observed for the approach of sterically
demanding phosphine and borane.[14,16]
The carbene ItBu also reacts with ammonia, aniline, and
diphenylamine adducts of B(C6F5)3 to give new species 3–5,
respectively, as evidenced by the observation signals in the 1H
NMR spectrum for the imidazolium protons at 9.53, 7.75, and
7.62 ppm, respectively. Owing to the FLP nature of ItBu and
B(C6F5)3, no base exchange reactions are observed. The
11B{1H} NMR spectra for 3–5 revealed single resonances at
À10.0, À9.7, and À7.6 ppm, respectively, which is consistent
with generation of borate anions. For 4, the 19F NMR
spectrum exhibits three peaks for the ortho, para, and meta
fluorine atoms at À133.2, À161.9, À165.9 ppm, respectively.
The corresponding 19F NMR spectrum for 5 is more compli-
cated at room temperature, which is consistent with restricted
rotation arising from the increased steric bulk in the anion.
These data support the formulation of the products as
[ItBuH][H2NB(C6F5)3] (3) and [ItBuH][PhRNB(C6F5)3] R =
H (4), Ph (5). In the case of 4, the structure was confirmed by
an X-ray crystallographic study (Figure 3). The disorder of
the [ItBu]+ cation in 4 precludes metric comparison with
compound 2. Nonetheless, the closest approach of the cation
and anion observed was between a C6F5 fluorine atom and a
Figure 1. POV-ray depiction of the X-ray crystallographic structure of 1.
Hydrogen atoms have been omitted for clarity.
In a similar fashion to IDipp, reactions of 1,3-di-tert-butyl-
1,3-imidazol-2-ylidene (ItBu) and B(C6F5)3 gave many prod-
ucts upon mixing at room temperature. Based on NMR
spectroscopic data, some of these species appear to be the
result of nucleophilic attack of aryl carbons in B(C6F5)3.
However, performance of the reaction at À608C in toluene
resulted in the 1H, 19F, and 11B NMR spectra that indicate no
interaction between ItBu and B(C6F5)3. This observation
supports the notion that this combination of carbene and
tris(pentafluorophenyl)borane should behave as a FLP capa-
ble of reaction with small molecules. Indeed, addition of
dihydrogen to a solution of equimolar amounts of ItBu and
B(C6F5)3 at À788C results in the clean formation of the ionic
complex[I tBuH][HB(C6F5)3] (2) in 97% yield (Scheme 2).
1
À
H NMR signals of 2 for the imidazolium C H and hydrido-
À
borate B H hydrogen atoms are clearly observed at 8.20 and
3.63 ppm, respectively, with 19F and 11B NMR spectra indica-
tive of the [HB(C6F5)3]À anion. The formulation of 2 was also
confirmed by X-ray crystallography (Figure 2). The geometry
of the [HB(C6F5)3]À anion is unexceptional.[13,18] The structure
of the [ItBuH]+ cation has not been previously reported, but
the metric parameters are similar to those seen in an N-
adamanyl variant and the complex[(I tBu)PdI2PPh3].[27]
Unlike [R3PH][HB(C6F5)3], no proton–hydride dihydrogen
bond is observed.[13] The closest ion–ion contact is between
À
tBu hydrogen atom (2.588 Š). In the anion, the B N bond
À
length is 1.532(8) Š, which is significantly shorter than the B
N bonds in other crystallographically characterized amino-
borates of B(C6F5)3 (1.628(3) and 1.636(3) Š in [Na(OEt2)4]-
[H2N{B(C6F5)3}2],[28] 1.576(4) Š in [LiOEt2][C4H4NB-
(C6F5)3],[29] and 1.565(4) Š in [HNEt3][(indolate)B-
(C6F5)3]).[30] The ammonia, aniline, and diphenylamine
À
the B H hydride and a proton of the imidazolium tBu group
(2.592 Š). Compound 2 has a similar thermal stability to
[R3PH][HB(C6F5)3], failing to liberate dihydrogen upon
heating to 1508C for 4 days. In contrast to the phosphonium
salts, 2 does not mediate the stoichiometric or catalytic
[31]
adducts of B(C6F5)3 are deprotonated by ItBu to generate
Figure 3. POV-ray depiction of the X-ray crystallographic structure of 4.
À
Figure 2. POV-ray depiction of 2. Hydrogen atoms of the tBugroups
omitted for clarity.
Non-N H hydrogen atoms have been omitted for clarity. One orienta-
tion of the disordered cation is shown.
7434
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 7433 –7437

Angewandte
Chemie
ionic complexes 3–5, respectively, which is analogous to
reaction with dihydrogen (Scheme 3).
In the corresponding reactions with the primary and
secondary alkyl amines, adducts with ItBu result in the
in carbene. These results provide further evidence that the
concept of FLPs can be broadened resulting in the activation
of a variety of small molecules and the discovery of new
catalytic processes. A variety of new FLP systems are being
investigated, and the their utility in stoichiometric and
catalytic transformation are the subjects of current investiga-
tions.
Experimental Section
General considerations: All manipulations were performed on a
double manifold N2(H2)/vacuum line with Schlenk glassware, or in an
N2-filled M-Braun or Vac Atmospheres glove box. The N2 and H2
gases were dried by passage through a Dririte column. Solvents
(Aldrich) were dried using an Innovative Technologies solvent
system. NMR spectra were obtained on a Bruker Avance 300 MHz
spectrometer, and spectra were referenced to residual solvent (1H,
13C) or externally (11B: BF3OEt2, 19F: CFCl3, 31P: 85% H3PO4). NMR
solvents were purchased from Cambridge Isotopes, dried over CaH2
or Na/benzophenone, vacuum distilled prior to use and stored over
4 Š molecular sieves in the glovebox. All amines were purchase from
Aldrich; liquid amines were dried over 4 Š molecular sieves and solid
reagents were used as received. ItBu and IDipp were purchased from
Strem Chemicals and used as received. B(C6F5)3 was generously
provided by Nova Chemicals. The adducts tBuNH2B(C6F5)3, H3NB-
(C6F5)3 and EtNH2B(C6F5)3 were prepared as previously de-
scribed.[19,36]
À
Scheme 3. N H bond cleavage of amines with ItBu/B(C6F5)3.
formation of the aminoboranes RR’NB(C6F5)2 (R = H, R’ =
Et (6), tBu (7); R = R’ = Et (8)). The generated C6F5H and
unreacted ItBu were observed by 19F and H NMR spectra.
1
The observation of a broad signal centered around 33 ppm in
the 11B NMR spectra for 6–8 suggest three-coordinate mono-
meric aminoboranes. In addition, restricted rotation about the
newly formed covalent B N bond is evident in the 19F NMR
À
spectra. For example, the unsymmetrical aminoborane 7 has
sixseparate 19F signals at 258C. This is consistent with a
significant overlap of the orbital occupied by the nitrogen
lone pair and the empty boron pZ orbital.[32,33]
1: A solution of B(C6F5)3 (0.100 g, 0.20 mmol) in toluene (5 mL)
was cooled to À788C. IDipp (0.076 g, 0.20 mmol) was added to the
cooled solution in toluene (9 mL) with a syringe. The reaction was
stirred for 5 min then allowed to warm to room temperature over 2 h.
The solvent was removed in vacuo to give the product as a light brown
solid. Note: prolonged stirring led to formation of several other
products. Yield 0.161 g (91%). X-ray quality crystals were obtained
The presence and necessity of the carbene in the
formation of 6–8 suggests that these reactions are catalytic
in carbene, and indeed formation is readily accomplished with
the use of 2–3 mol% of ItBu. In these cases, the quantitative
formation of the aminoboranes[33,34] is complete within 2 h at
258C. Notably, 6–8 are stable in solution under these
conditions. The mechanism of these reactions are under
study, but it may be that deprotonation of the initially formed
amine adduct of B(C6F5)3 by ItBu affords an electron-rich
aminoborate that then reacts with the transient imidazolium
ion to eliminate C6F5H and regenerates the carbene. It is
by cooling
a
toluene/hexanes solution to À358C. 1H NMR
3
([D8]toluene, À608C): d = 6.97 (d, 1H, JHH = 7.6 Hz, ArH), 6.92 (t,
1H, 3JHH = 7.0 Hz, ArH), 6.82 (t, 1H, 3JHH = 7.6 Hz, ArH), 6.58 (d,
3
3
2H, JHH = 7.6 Hz, 2ArH), 6.37 (d, 1H, JHH = 7.7 Hz, ArH), 6.32 (s,
=
=
1H, CH), 6.20 (s, 1H, CH), 3.26–3.02 (overlapping m, 3H,
CHMe2), 1.72 (br m, 1H, CHMe2), 1.37 (d, 3H, 3JHH = 6.6 Hz,
CHMe2), 1.31 (d, 3H, 3JHH = 6.4 Hz, CHMe2), 0.95 (d, 6H, JHH
=
3
3
6.7 Hz, CHMe2), 0.90 (br, 2H, JHH = 6.6 Hz, 2CHMe2), 0.83 (d, 3H,
3JHH = 6.3 Hz, CHMe2), 0.62 ppm (d, 3H, 3JHH = 6.6 Hz, CHMe2).
11B{1H} NMR ([D8]toluene, À608C): d = À15.6 ppm (s). 19F NMR
([D8]toluene, À608C): d = À110.6 (d, 1F, 3JFF = 24 Hz, o-ArCF),
À119.4 (dd, 1F, 3JFF = 52, 29 Hz, o-ArCF), À126.0 (d, 1F, 3JFF = 24 Hz,
important to recognize that Bertrand et al. found that NHCs
alone were unreactive towards N H bonds, although
Clyburne and co-workers have observed a carbene C···H N
[8]
À
À
o-ArCF), À127.8 (d, 1F, 3JFF = 25 Hz, o-ArCF), À132.8 (dd, 1F, 3JFF
=
interaction between IMes and HNPh2.[35] Additionally, phos-
phine–borane FLPs do not directly react with amines, as the
transfer of a phosphonium proton to the nitrogen of an
aminoborate anion is a key step in the mechanism of imine
hydrogenation catalyzed by phosphonium borates. In marked
contrast, the ItBu/B(C6F5)3 FLP mixture gives either an
aminoborate salt or an aminoborane, depending on the nature
of the amine substituents.
51, 25 Hz, o-ArCF), À133.5 (d, 1F, 3JFF = 24 Hz, o-ArCF), À157.1 (m,
3
2F, 2p-ArCF), À158.2 (t, 1F, JFF = 21 Hz, p-ArCF), À162.5 (m, 1F,
m-ArCF), À163.4 (m, 2F, 2 x m-ArCF), À164.3 (m, 2F, 2 x m-ArCF),
À166.3 ppm (t, 1F, 3JFF = 21 Hz, m-ArCF). Elemental analysis (%)
calcd for C45H37BF15N2: C 59.95, H 4.14, N 3.11; found: C 59.51,
¯
H 4.14, N 2.95. X-ray analysis: triclinic, P1, a = 9.1610(11), b =
11.9932(14), c = 19.725(2) Š, a = 78.645(2)8, b = 85.995(2)o, g =
75.689(2)o, V= 2058.3(4) Š3, Data/variables: 7218:563, R = 0.0559,
Rw = 0.1226, GOF = 0.921. CCDC-689780.
In conclusion, the combination of Lewis bases capable of
FLP chemistry with tris(pentafluorophenyl)borane, which
was previously limited to sterically demanding phosphines
and amines, has been expanded to include sterically encum-
bered NHCs. These systems are shown to effect heterolytic
2: A solution of B(C6F5)3 (0.282 g, 0.55 mmol) in toluene (4 mL)
was placed in a 25 mL round-bottomed flask equipped with a sealable
teflon tap. The solution was freeze–pump–thaw cycled three times,
placed under an atmosphere of N2, and frozen in liquid N2. Under a
strong flow of N2, the teflon tap was quickly replaced with a septa and
a solution of ItBu (0.100 g, 0.55 mmol) in toluene (4 mL) was added
quickly with a syringe. The teflon tap was replaced, the flask
evacuated, and placed under an atmosphere of H2. The flask was
sealed, the reaction stirred at À788C for 2 h and then slowly warmed
overnight. The white, turbid reaction mixture was pumped dry to give
À
cleavage of the H H bond to give imidazolium borates. In
À
addition, this FLP combination cleaves amine N H bonds,
affording aminoborate salts, or mediates loss of C6F5H to give
aminoboranes. The latter case has been shown to be catalytic
Angew. Chem. Int. Ed. 2008, 47, 7433 –7437
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7435

Communications
a white powder. X-ray quality crystals were grown from layering
21 Hz, p-ArF), À160.9 (m, 2F, m-ArCF), À161.2 ppm (m, 2F, m-
ArCF).
pentane onto a CH2Cl2 solution of the product and cooling to À358C.
4
Yield: 0.372 g (97%). 1H NMR (CD2Cl2): d = 8.20 (t, 1H, JHH
=
7: A solution of ItBu (1 mg, 6 mmol, 2 mol%) in dry hexanes
(2 mL) was added to a slurry of tBuNH2B(C6F5)3 (0.160 g, 0.27 mmol)
in dry hexanes (4 mL). The reaction was stirred at room temperature
for 2 h. The solvent was removed in vacuo to give a white powder.
Yield 0.111 g, 98%. 1H NMR (C6D5Br): d = 5.63 (br s, 1H, NH),
1.8 Hz, N2CH), 7.50 (d, 2H, 4JHH = 1.8 Hz, CH), 3.63 (1:1:1:1 q,
1H, 1JHB = 89 Hz, BH), 1.70 ppm (s, 18H, tBu). 13C{1H} NMR
=
(CD2Cl2): 148.7 (d, JCF = 244 Hz, o-ArCF), 138.5 (d, 1JCF = 244 Hz,
1
p-ArCF), 137.2 (d, 1JCF = 245 Hz, m-ArCF), 129.8 (s, N2CH), 126.2 (b,
BC), 121.3 (s, NCH), 61.8 (s, tBu), 30.2 ppm (s, tBu). 11B NMR
1.10 ppm (s, 9H, tBuH). 13C{1H} NMR (C6D5Br): d = 147.3 (d, 1JCF
=
=
=
1
1
(CD2Cl2): d = À25.3 ppm (d, JBH = 89 Hz, BH). 19F NMR (CD2Cl2):
1
244 Hz, o-ArCF), 145.3 (d, JCF = 242 Hz, o-ArCF), 141.5 (d, JCF
1
1
256 Hz, p-ArCF), 141.1 (d, JCF = 253 Hz, p-ArCF), 137.1 (d, JCF
d = À133.9 (d, 6F, JFF = 21 Hz, o-ArCF), À164.9 (t, 3F, JFF = 21 Hz, p-
ArCF), À167.5 ppm (m, 6F, m-ArCF). Elemental analysis (%) calcd
for C29H22BF15N2: C 50.17, H 3.19, N 4.03; found: C 50.39, H 3.03,
N 4.10. X-ray analysis: monoclinic, P21/c, a = 15.4897(14), b =
9.7960(9), c = 20.1789(19) Š, b = 107.0520(10)8, V= 2927.3(5) Š3,
Data/variables: 5167:428, R = 0.0444, Rw = 0.1259, GOF = 0.928.
CCDC-689783.
3–5: These species were prepared in a similar fashion. For 3: ItBu
(0.017 g, 0.09 mmol) in hexanes (2 mL) was added to a solution of
H3NB(C6F5)3 (0.050 g, 0.09 mmol) in toluene (3 mL). After addition,
a light yellow solid precipitated from solution. The vial was rinsed
with hexanes (3 ” 2 mL) and the reaction was stirred for 5 min. The
stirring was stopped and the solid was allowed to settle for 30 min
after which the solvent was decanted. The remaining solid was dried
251 Hz, 2 ” overlapping m-ArCF), 111.2 (br s, ArCB), 52.8 (s, tBu),
31.3 ppm (s, tBu). 11B NMR (C6D5Br): d = 32.7 ppm (br s). 19F NMR
(C6D5Br): d = À130.8 (m, 2F, o-ArCF), À133.2 (m, 2F, o-ArCF),
À151.6 (t, 1F, 3JFF = 21 Hz, p-ArF), À153.0 (t, 1F, 3JFF = 21 Hz, p-
ArF), À161.3 ppm (overlapping m, 4F, m-ArCF). Satisfactory
analytical data could not be obtained owing to air sensitivity.
8: ItBu (1 mg, 0.006 mmol, 3 mol%) was added to a solution of
Et2NHB(C6F5)3 (0.096 g, 0.19 mmol) in hexanes (4 mL) and benzene
(2 mL). The reaction was stirred for 12 h, after which the solvent was
removed in vacuo. The light oil was dissolved in hexanes, and the
solvent was removed in vacuo to give the product as a light orange oil.
3
Yield 0.075 g, 96%. 1H NMR (C6D5Br): d = 3.05 (q, 4H, JHH
=
7.0 Hz, NCH2), 0.98 ppm (t, 6H, 3JHH = 7.0 Hz, NCH2CH3).
13C{1H} NMR (C6D5Br): d = 145.0 (d, JCF = 246 Hz, o-ArCF), 140.5
1
to give the product as pale yellow solid. 3: Yield 0.064 g, 95%.
1
1
(d, JCF = 257 Hz, p-ArCF), 136.4 (d, JCF = 257 Hz, m-ArCF), 111.0
(br s, BArC), 43.4 (s, NCH2), 14.1 ppm (s, NCH2CH3). 11B NMR
(C6D5Br): d = 33.5 ppm (br s). 19F NMR (C6D5Br): d = À132.1 (m, 4F,
1
=
H NMR (CD2Cl2): d = 9.53 (s, 1H, N2CH), 7.42 (s, 2H, CH), 4.00
(br s, 2H, NH2), 1.72 ppm (s, 18H, tBu). 13C{1H} NMR (CD2Cl2): d =
148.6 (d, 1JCF = 244 Hz, o-ArCF), 140.1 (d, 1JCF = 244 Hz, p-ArCF),
137.5 (d, 1JCF = 245 Hz, m-ArCF), 134.4 (s, N2CH), 128.7 (b, BC),
119.9 (s, NCH), 61.4 (s, tBu), 30.4 ppm (s, tBu). 11B NMR (CD2Cl2):
d = À10.0 ppm (br s, BNH2). 19F NMR (CD2Cl2): d = À135.5 (br, 6F,
o-C6F5), À161.2 (br, 3F, p-C6F5), À166.0 ppm (br, 6F, m-C6F5).
Elemental analysis (%) calcd for C29H23BF15N3: C 49.11, H 3.27,
N 5.92; found: C 49.33, H 3.50, N 6.12. 4: Yield 0.063 g (94%).
3
o-ArCF), À152.9 (t, 2F, JFF = 21 Hz, p-ArCF), À161.1 ppm (m, 4F,
m-ArCF). Elemental analysis (%) calcd for C16H10BF10N: C 46.08,
H 2.42, N 3.36; found: C 46.23, H 2.39, N 3.24.
Received: June 3, 2008
Published online: July 25, 2008
3
1H NMR (C6D5Br): d = 7.75 (s, 1H, N2CH), 6.96 (t, 2H, JHH = 8 Hz,
Keywords: boranes · carbenes · donor–acceptor systems ·
3
=
m-ArCH), 6.75 (s, 2H, CHN), 6.60 (d, 2H, JHH = 8 Hz, o-ArCH),
.
hydrogen · steric hindrance
6.35 (t, 1H, 3JHH = 8 Hz, p-ArCH), 4.26 (s, 1H, NH), 1.18 ppm (s,
18H, tBu). 13C{1H} NMR (C6D5Br): d = 152.4 (s, N2CH), 148.2 (d,
1JCF = 235 Hz, o-ArCF), 138.2 (d, 1JCF = 248 Hz, p-ArCF), 136.3 (d,
1JCF = 239 Hz, m-ArCF), 128.4 (s, m-ArCH), 128.2 (s, CH), 119.9 (s,
[1] A. J. Arduengo III, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
=
1991, 113, 361.
=
CHN), 114.4 (s, o-ArCH), 114.1 (br s, BArC), 112.6 (s, p-ArCH),
60.1 (s, tBu), 28.7 ppm (s, tBu). 11B NMR (C6D5Br): d = À9.7 ppm (br
[2] D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev.
2000, 100, 39.
[3] R. H. Crabtree, Coord. Chem. Rev. 2007, 251, 595.
[4] S. P. Nolan, N-Heterocyclic Carbenes in Synthesis Wiley-VCH,
Weinheim, 2006.
[5] D. Enders, T. Balensiefer, Acc. Chem. Res. 2004, 37, 534.
[6] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107,
5606.
[7] N. Marion, S. Diez-Gonzalez, S. P. Nolan, Angew. Chem. 2007,
119, 3046; Angew. Chem. Int. Ed. 2007, 46, 2988.
[8] G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G.
Bertrand, Science 2007, 316, 439.
[9] J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand, J.
Am. Chem. Soc. 2007, 129, 14180.
[10] J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand,
Angew. Chem. 2007, 119, 7182; Angew. Chem. Int. Ed. 2007, 46,
7052.
s). 19F NMR (C6D5Br): d = À133.2 (d, 6F, 3JFF = 22 Hz, o-ArCF),
3
À161.9 (t, 3F, JFF = 21 Hz, p-ArCF), À165.9 ppm (m, 6F, m-ArCF).
Elemental analysis (%) calcd for C35H27BF15N3: C 53.52, H 3.47,
N 5.35; found: C 53.25, H 3.42, N 5.23. X-ray analysis: monoclinic,
C2/c, a = 18.552(2), b = 18.677(2), c = 22.029(2) Š, b = 109.055(2)8,
V= 7215.1(13) Š3, Data/variables: 6363:483, R = 0.0804, Rw = 0.2193,
GOF = 0.955. CCDC-689782. 5: Yield 0.170 g, 98%. 1H NMR
(C6D5Br): d = 7.62 (br s, 1H, N2CH), 7.21 (d, 4H, 3JHH = 8 Hz, o-
ArCH), 6.98 (t, 4H, 3JHH = 8 Hz, m-ArCH), 6.77 (s, 2H, CHN), 6.68
=
(t, 2H, 3JHH = 8 Hz, p-ArCH), 1.17 ppm (s, 18H, tBu). 11B NMR
(C6D5Br): d = À7.6 ppm (s). 19F NMR (C6D5Br): d = À131.4 (d, 2F,
3JFF = 23 Hz, m-ArCF), À131.8 (d, 2F, 3JFF = 25 Hz, o-ArCF), À132.1
3
3
(d, 2F, JFF = 26 Hz, o-ArCF), À162.0 (t, 2F, JFF = 21 Hz, p-ArCF),
À162.3 (t, 1F, 3JFF = 21 Hz, p-ArCF), À166.4 (m, 2F, m-ArCF),
À166.6 (m, 2F, m-ArCF), À167.8 ppm (m, 2F, m-ArCF). Elemental
analysis (%) calcd for C41H31BF15N3: C 57.16, H 3.63, N 4.88; found:
57 03, H 3.50, N 4.85.
[11] A. L. Kenward, W. E. Piers, Angew. Chem. 2008, 120, 38; Angew.
Chem. Int. Ed. 2008, 47, 38.
6: A solution of EtNH2B(C6F5)3 (0.012 g, 22 mmol) in C6D5Br
(0.3 mL) was added to a solution of ItBu (0.004 g, 22 mmol) in C6D5Br
(0.3 mL). The reaction solution was placed in an NMR tube and NMR
spectra were obtained within 5 min of mixing. 1H NMR (C6D5Br): d =
[12] D. W. Stephan, Org. Biomol. Chem. 2008, 6, 1535.
[13] G. C. Welch, D. W. Stephan, J. Am. Chem. Soc. 2007, 129, 1880.
[14] T. A. Rokob, A. Hamza, A. Stirling, T. Soos, I. Pꢀpai, Angew.
Chem. 2008, 120, 2469; Angew. Chem. Int. Ed. 2008, 47, 2435.
[15] J. S. J. McCahill, G. C. Welch, D. W. Stephan, Angew. Chem.
2007, 119, 5056; Angew. Chem. Int. Ed. 2007, 46, 4968.
[16] A. Stirling, A. Hanza, T. A. Rokob, I. Pꢀpai, Chem. Commun.
2008, DOI: 10.1039/b804662j.
3
5.41 (br s, 1H, NH), 2.92 (quin, 2H, JHH = 7.0 Hz, NCH2), 1.00 ppm
3
(t, 3H, JHH = 7.0 Hz, NCH2CH3). 11B NMR (C6D5Br): d = 33.3 ppm
(br s). 19F NMR (C6D5Br): d = À132.0 (m, 2F, o-ArCF), À132.6 (m,
2F, o-ArCF), À150.8 (t, 1F, 3JFF = 21 Hz, p-ArF),À152.2 (t, 1F, 3JFF
=
7436
www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 7433 –7437

Angewandte
Chemie
[17] G. C. Welch, R. R. San Juan, J. D. Masuda, D. W. Stephan,
Science 2006, 314, 1124.
[18] P. A. Chase, T. Jurca, D. W. Stephan, Chem. Commun. 2008,
1701.
[19] P. A. Chase, G. C. Welch, T. Jurca, D. W. Stephan, Angew. Chem.
2007, 119, 8196; Angew. Chem. Int. Ed. 2007, 46, 8050; P. A.
Chase, G. C. Welch, T. Jurca, D. W. Stephan, Angew. Chem. 2007,
119, 9296; Angew. Chem. Int. Ed. 2007, 46, 9136.
[20] L. Cabrera, G. C. Welch, J. D. Masuda, P. Wei, D. W. Stephan,
Inorg. Chim. Acta 2006, 359, 3066.
[28] S. J. Lancaster, A. Rodriguez, A. Lara-Sanchez, M. D. Hannant,
D. A. Walker, D. H. Hughes, M. Bochmann, Organometallics
2002, 21, 451.
[29] G. Kehr, R. Roesmann, R. Froehlich, C. Holst, G. Erker, Eur. J.
Inorg. Chem. 2001, 535.
[30] S. Guidotti, I. Camurati, F. Focante, L. Angellini, G. Moscardi, L.
Resconi, R. Leardini, D. Nanni, P. Mercandelli, A. Sironi, T.
Beringhelli, D. Maggioni, J. Org. Chem. 2003, 68, 5445.
[31] F. Focante, P. Mercandelli, A. Sironi, L. Resconi, Coord. Chem.
Rev. 2006, 250, 170.
[21] P. Spies, G. Erker, G. Kehr, K. Bergander, R. Froehlich, S.
Grimme, D. W. Stephan, Chem. Commun. 2007, 5072.
[22] G. C. Welch, T. Holtrichter-Roessmann, D. W. Stephan, Inorg.
Chem. 2008, 47, 1904.
[32] K. M. Bissett, T. M. Gilbert, Organometallics 2004, 23, 850.
[33] P. P. Power, Chem. Rev. 1999, 99, 3463.
[34] H. F. Bettinger, M. Filthaus, H. Borenmann, I. M. Oppel, Angew.
Chem. 2008, 120, 4822; Angew. Chem. Int. Ed. 2008, 47, 4744.
[35] J. A. Cowan, J. A. C. Clyburne, M. G. Davidson, R. L. W. Harris,
J. A. K. Howard, P. Kꢁpper, M. A. Leech, S. P. Richards, Angew.
Chem. 2002, 114, 1490; Angew. Chem. Int. Ed. 2002, 41, 1432.
[36] A. J. Mountford, S. J. Lancaster, S. J. Coles, P. N. Horton, D. L.
Hughes, M. B. Hursthouse, M. E. Light, Inorg. Chem. 2005, 44,
5921.
[23] G. Erker, Dalton Trans. 2005, 1883.
[24] W. E. Piers, Adv. Organomet. Chem. 2005, 52, 1.
[25] W. E. Piers, T. Chivers, Chem. Soc. Rev. 1997, 26, 345.
[26] A. D. Phillips, P. P. Power, Acta Crystallogr. Sect. C 2005, 61,
o291.
[27] W. A. Herrmann, V. P. W. Boehm, C. W. K. Gstoettmayr, M.
Grosche, C.-P. Reisinger, T. Weskamp, J. Organomet. Chem.
2001, 617–618, 616.
Angew. Chem. Int. Ed. 2008, 47, 7433 –7437
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7437