Activation of hydrogen and hydrogenation catalysis by a borenium cation

Article abstract of DOI:10.1021/ja307995f

The readily prepared borenium salt [(IiPr₂)(BC₈H ₁₄)][B(C₆F₅)₄] (2) [IiPr₂ = C₃H₂(NiPr)₂] is shown to activate H ₂ heterolytically in the presence of tBu₃P. Compound 2 also acts as a catalyst for the metal-free hydrogenation of imines and enamines at room temperature.

Full text of DOI:10.1021/ja307995f

Communication
pubs.acs.org/JACS
Activation of Hydrogen and Hydrogenation Catalysis by a Borenium
Cation
Jeffrey M. Farrell, Jillian A. Hatnean, and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S
* Supporting Information
the potential of borenium cation-based FLP hydrogenation
ABSTRACT: The readily prepared borenium salt
[(IiPr2)(BC8H14)][B(C6F5)4] (2) [IiPr2 = C3H2(NiPr)2]
is shown to activate H2 heterolytically in the presence of
tBu3P. Compound 2 also acts as a catalyst for the metal-
free hydrogenation of imines and enamines at room
temperature.
catalysts. In this work, a carbene-stabilized borenium cation
derived from the ubiquitous borane reagent 9-
borabicyclo[3.3.1]nonane (9-BBN) has been prepared and
shown to effect the stoichiometric heterolytic activation of H2
in combination with a bulky base. Moreover, this borenium
cation-based FLP has been demonstrated to be a highly
effective and functional-group-tolerant catalyst for the rapid
hydrogenation of imines and enamines at room temperature.
The NHC adduct of 9-BBN, (IiPr2)(HBC8H14) (1) [IiPr2 =
C3H2(NiPr)2], was readily prepared via the stoichiometric
combination of 1,3-diisopropylimidazol-2-ylidene and 9-BBN
dimer in toluene at room temperature and recrystallized in 76%
yield. While a B−H resonance was not observed at room
he addition of H2 to unsaturated molecules is without
Tquestion the most used transformation in chemical
industry.1 This process must be facilitated by a catalyst. Despite
the fact that both heterogeneous and homogeneous hydro-
genation catalysts are firmly established technologies, research
continues to uncover new ways to effect this important
reaction. For example, recent efforts from several research
groups2 have exploited iron catalysts, while Harder and co-
workers3 have utilized calcium complexes for hydrogenation
catalysis. In addition, we4 and others5 have focused on main-
group catalysts derived from “frustrated Lewis pairs” (FLPs) for
metal-free hydrogenations. While the latter discovery has
provided an unprecedented role for main-group chemistry in
hydrogenation catalysis, all of the FLP hydrogenation catalysts
known to date require neutral boron species with highly
electron-withdrawing fluorinated substituents.4a The presence
of such electron-withdrawing substituents deters hydride
delivery and thus slows the catalysis. In addition, synthetic
challenges associated with such air- and moisture-sensitive
boranes preclude facile access to families of catalysts for process
optimization and selectivity control.
1
temperature in the H NMR spectrum of 1, the 11B NMR
spectrum showed a doublet resonance at −16.64 ppm (1JBH
=
80 Hz). The formulation of 1 as a pseudotetrahedral molecule
was confirmed by X-ray crystallography (Figure 1a).
Figure 1. POV-ray drawings of (a) 1 and (b) the cation of 2 (C, black;
N, blue; B, yellow-green; H, gray). H atoms except for BH have been
omitted for clarity.
A lesser studied, but important, class of boron-based Lewis
acids are borenium cations.6 Considerable electrophilicity is
imparted to these boron species by their cationic charge.
Nevertheless, a number of research groups6c,7 have shown that
borenium cations can be readily stabilized by a variety of bulky
or electron-donating substituents. A number of these
compounds have been exploited as electrophiles in aromatic
and aliphatic borylations and borylations of arylsilane-
Compound 1 reacted with [Ph3C][B(C6F5)4] in toluene to
give the borenium salt [(IiPr2)(BC8H14)][B(C6F5)4] (2)
(Scheme 1). Recrystallization from chlorobenzene of 2 afforded
Scheme 1. Synthesis and Activation of H2 by 2
s,7a,d,e,n−p,8 and a recent report from Curran, Laco
̂
te, Vedejs,
and co-workers describes the borenium ion-catalyzed hydro-
boration of alkenes.9 Chiral oxazaborolidinium ions have also
been extensively studied in enantioselective catalysis.10
A
number of borenium cations have been prepared from N-
heterocyclic carbene (NHC)−borane adducts.11 Also, NHC−
borane adducts have been demonstrated to be potent
reductants.12 Thus, while NHC−borenium cations are electro-
philic, the corresponding NHC−borane adducts are effective
hydride donors. These complementary features augur well for
Received: August 10, 2012
Published: August 29, 2012
© 2012 American Chemical Society
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Communication
an 81% yield of 2·0.66 C6H5Cl, which showed the expected 11
B
Table 1. Hydrogenation Catalyzed by a Borenium Ion
and 19F NMR resonances for the [B(C6F5)4]− anion. In
addition, the 11B NMR spectrum also showed a broad
resonance at 83.8 ppm, consistent with the generation of a
borenium center.7m The crystallographic data for 2 (Figure 1b)
confirmed a trigonal-planar geometry with a B−CNHC bond
length of 1.580(3) Å, analogous to the value of 1.579(7) Å for
[(C6H2Me3)2B(IMe2)][OTf] as reported by Matsumoto and
Gabbai.7f
An alternative strategy for obtaining the same cation was
derived from the reaction of 1 with B(C6F5)3.13 This afforded
the salt [(IiPr2)(BC8H14)][HB(C6F5)3] (3) (Scheme 1), which
1
is directly analogous to 2, as evidenced by H, 11B, and 19F
NMR spectroscopy. The formation of 3 illustrates the
considerably higher hydridicity of 1 relative to [HB(C6F5)3]−;
alternatively, viewed in terms of Lewis acidity, B(C6F5)3 is a
stronger Lewis acid than the borenium cations in 2 and 3.
The combination of Lewis acid 2 with the Lewis base tBu3P
or H2 independently showed no evidence of reaction as
determined by multinuclear NMR spectroscopy. Nonetheless,
exposure of 2/tBu3P to H2 (4 atm) at room temperature for 48
h revealed the formation of 1 and the generation of the known
salt [tBu3PH][B(C6F5)4] in 68% yield, as evidenced by 1H, 11B,
19F, and 31P NMR spectroscopy (Scheme 1). The analogous
experiment with D2 afforded the corresponding deuterium
splitting products. A 1:1:1 triplet resonance was observed in the
31P NMR spectrum at 59.8 ppm with P−D coupling of 66 Hz,
while a broad resonance at −16.90 ppm was seen in the 11B
NMR spectrum. These observations confirmed that the
borenium cation and phosphine acted in concert to effect the
FLP heterolytic activation of H2 and D2, respectively. On the
other hand, treatment of PhCHNtBu with [tBu3PH][B-
(C6F5)4] and 1 resulted in stoichiometric conversion to the
corresponding amine and the generation of tBu3P and 2, as
evidenced by NMR spectroscopy.
The above stoichiometric reactions in which H2 is activated
and subsequently delivered to a substrate represent the
complementary reactivity required for catalytic hydrogenation.
Indeed, treatment of PhCHNtBu with 2 (1 mol%) under H2
(102 atm) in CH2Cl2 at room temperature resulted in
quantitative reduction to the corresponding amine in 2 h. No
phosphine was employed in this case, as the imine functioned
as the base in the FLP activation.4f Performing the reaction in
C6H5Cl or toluene resulted in slightly diminished yields. This
was attributed to the decreased solubility of 2 in these solvents.
The ketimine N-(1-phenylethylidene)aniline, the aldimines N-
benzylidene-tert-butylamine and N-(m-methoxybenzylidene)-
tert-butylamine, and the enamines N-(1-cyclohexenyl)-
piperidine, N-(1-cyclopentenyl)piperidine, and 1,3,3-trimethyl-
2-methyleneindoline were also reduced quantitatively in 2−4 h
using 1−5 mol% catalyst (Table 1). The product amines were
isolated in 79−94% yield. These reductions were generally
faster and higher-yielding than those previously reported
employing electrophilic borane catalysts.4a−g,5 In contrast, the
N-heterocycle 8-methylquinoline was reduced by 2 in only 27%
yield in C6H5Cl and not at all in CH2Cl2. This latter result
stands in contrast to previously reported reductions4e of this
and related heterocycles using B(C6F5)3 and H2.
mol%) at room temperature in CH2Cl2 under H2 (102 atm), N-
benzylidene-tert-butylamine was selectively reduced in the
presence of fenchone, 4,4′-dimethylbenzophenone, 8-methyl-
quinoline, 2-phenylpyridine, and ethyl-4-bromobenzoate. How-
ever, the catalytic reductions were inhibited in the presence of
acetophenone, 2′,4′,6′-triisopropylacetophenone, and 2,2,2-
trifluoroacetophenone. In no case was hydrogenation of the
functional-group-containing surrogate observed. These results
demonstrate that the borenium cation-based FLP catalyst is
significantly more functional group tolerant than metal-free
reductions based on B(C6F5)3.4h
On the basis of the above observations, the mechanism of
action operative in these hydrogenations is thought to involve
heterolytic cleavage of H2 by the 2/imine (or product amine)4f-
based FLP, generating 1 and an iminium [B(C6F5)4]− salt
(Scheme 2). This is consistent with the observation that the
independent combination of 1 and imine gave no reaction.
Subsequent hydride transfer from 1 affords the amine and
regenerates 2 for further catalysis. In support of this
mechanism, we note that the use of 1 in place of 2 under
catalytic conditions gave no reduction. This is consistent with
Scheme 2. Proposed Catalytic Cycle for the Hydrogenation
of Imines by 2
This difference in reactivity led us to further explore the
functional group tolerance and selectivity of catalyst 2. To this
end, a stoichiometric quantity of a surrogate containing a
functional group was included in catalytic mixtures of the
hydrogenation of N-benzylidene-tert-butylamine.4h Using 2 (5
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Communication
(3) Spielmann, J.; Buch, F.; Harder, S. Angew. Chem., Int. Ed. 2008,
47, 9434.
the inability of 1 to transfer hydride to the imine and
demonstrates that initial iminium generation is essential.
Moreover, this observation implies that the mechanism of H2
activation does not involve hydride migration from B to the
CNHC of 1 to generate a neutral boron−nitrogen FLP, but it is
consistent with a mechanism that necessitates the generation of
a Lewis acidic borenium ion. The activation of H2 proceeds
slowly, while the reaction of 1 and the iminium salt [PhCH
NHtBu][B(C6F5)4] results in immediate hydride transfer.
These preliminary data imply that H2 activation is rate-
determining, although a detailed mechanistic study is ongoing.
In summary, we have demonstrated that a borenium cation
can be employed as a Lewis acid in an FLP to activate H2. This
borenium cation, which is readily obtained from an air-stable
precursor, is a new, highly active metal-free catalyst for the
hydrogenation of imines and enamines at room temperature.
Moreover, the borenium cation hydrogenation catalyst exhibits
improved selectivity and functional group tolerance relative to
existing FLP catalysts. The required Lewis acidity for hydrogen
activation is derived from the cationic charge at boron rather
than the incorporation of fluorinated substituents. This finding
provides a new family of readily accessible Lewis acids for FLP
chemistry and catalysis. Accordingly, systematic modifications
of the borenium cation catalysts for chemo-, regio-, and
stereoselectivity are currently under intense study.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
dstephan@chem.utoronto.ca
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.W.S. gratefully acknowledges the financial support of NSERC
of Canada and the award of a Canada Research Chair. J.M.F. is
grateful for the support of an OGS Scholarship.
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