Mechanistic studies on the metal-free activation of dihydrogen by antiaromatic pentarylboroles

Article abstract of DOI:10.1021/ja311842r

The perfluoro- and perprotiopentaphenylboroles 1 and 2 react with dihydrogen to effect H-H bond cleavage and formation of boracyclopentene products. The mechanism of this reaction has been studied experimentally through evaluation of the kinetic properties of the slower reaction between 2 and H₂. The reaction is first-order in both [borole] and [H₂] with activation parameters of ΔH^? = 34(8) kJ/mol and ΔS^? = -146(25) J mol^-1 K^-1. A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H-H cleavage in the rate-limiting transition state. To explain the stereochemistry of the observed products, a ring-opening/ring- closing mechanism is proposed and supported by the separate synthesis of a proposed intermediate and its observed conversion to product. Furthermore, extensive DFT mapping of the reaction mechanism supports the plausibility of this proposal. The study illustrates a new mechanism for the activation of H₂ by a strong main group Lewis acid in the absence of an external base, a process driven in part by the antiaromaticity of the borole rings in 1 and 2.

Full text of DOI:10.1021/ja311842r

Article
pubs.acs.org/JACS
Mechanistic Studies on the Metal-Free Activation of Dihydrogen by
Antiaromatic Pentarylboroles
Adrian Y. Houghton,† Virve A. Karttunen,‡ Cheng Fan,† Warren E. Piers,*,† and Heikki M. Tuononen*,‡
†Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
‡Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The perfluoro- and perprotiopentaphenylboroles 1
and 2 react with dihydrogen to effect H−H bond cleavage and
formation of boracyclopentene products. The mechanism of this
reaction has been studied experimentally through evaluation of the
kinetic properties of the slower reaction between 2 and H2. The
reaction is first-order in both [borole] and [H2] with activation
parameters of ΔH⧧ = 34(8) kJ/mol and ΔS⧧ = −146(25) J mol−1
K−1. A minimal kinetic isotope effect of 1.10(5) was observed, suggesting an asynchronous geometry for H−H cleavage in the
rate-limiting transition state. To explain the stereochemistry of the observed products, a ring-opening/ring-closing mechanism is
proposed and supported by the separate synthesis of a proposed intermediate and its observed conversion to product.
Furthermore, extensive DFT mapping of the reaction mechanism supports the plausibility of this proposal. The study illustrates a
new mechanism for the activation of H2 by a strong main group Lewis acid in the absence of an external base, a process driven in
part by the antiaromaticity of the borole rings in 1 and 2.
In 2006, Stephan and co-workers found11 that sterically bulky
INTRODUCTION
■
Lewis acids combined with likewise sterically endowed Lewis
bases are capable of activating H2reversiblywith remark-
ably low barriers. The acids are almost invariably perfluoroaryl
boranes of high Lewis acid strength, but wide variation in the
nature of the Lewis base is possible, and there are now many
examples of this phenomenon in the literature.12 The
mechanism by which these systems polarize the H−H bond
has been probed in the seminal B(C6F5)3/PR3 (R = mesityl,
tBu) systems both experimentally13,14 and computation-
ally,15−17 and it is currently believed that a preformed
“encounter complex”, held together by weak van der Waals
interactions, creates a pocket in which the H2 can bind; the
electric field created by the empty orbital on the boron and the
filled orbital on phosphorus splits the H−H bond heterolyti-
cally with a very low barrier.17 No experimental evidence for
this encounter complex has yet been found, but the
computational support for this picture is convincing for this
particular system.
Despite the low polarity and high bond strength of the H−H
bond in dihydrogen, its activation is remarkably facile in the
presence of transition metal compounds via oxidative
addition,1,2 σ-bond metathesis,3,4 and electrophilic 1,2 addition
type mechanisms.5,6 Thus, the catalytic addition of H2 to a
variety of unsaturated organic functions and the hydrogenolysis
of many M−X bonds are well known and understood
phenomena that are widely deployed reactions in the chemical
industry.7
While transition metal catalyzed hydrogen utilization is of
tremendous economic and practical importance, transition
metals and their complexes can be costly, and toxicity issues can
also present challenges. Therefore, there has been significant
interest in “metal-free” processes that do not require transition
metals to activate and deliver hydrogen to substrates of
interest.8 Main group element based compounds that cleave
hydrogen are less common, but a growing body of literature,
beginning with the seminal discoveries of Power et al.,9 suggest
that main group elements can also cleave dihydrogen via low-
energy pathways that synergistically depopulate the H−H σ
bonding orbital while populating the H−H σ* antibonding
orbital. For example, Bertrand and co-workers showed10 that
stable singlet carbenes react rapidly with H2 via a mechanism
that is heterolytic in character but involves polarization of the
H−H bond by donation of carbene electrons into its σ* orbital
and transfer of hydride to the carbene carbon. In both the
Power and Bertrand systems, application to catalytic hydro-
genation is limited by the lack of reversibility in the hydrogen
cleavage reaction.
For bases with less symmetry complementarity, like imines18
or carbenes,19 similar encounter complexes have been described
in silico but are somewhat less favored than those found for the
borane/phosphine complexes. Thus, alternate mechanisms
might be operative in these systems. For example, in a
mechanism reminiscent of that proposed for B(C6F5)3-
catalyzed hydrosilation reactions,20−24 the highly Lewis acidic
borane might bind hydrogen via the σ bonding electrons; the
base may then deprotonate the coordinated H2 to generate the
Received: December 4, 2012
Published: December 21, 2012
© 2012 American Chemical Society
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ion pair that is the product of dihydrogen activation. This
process resembles the binding and deprotonation of
dihydrogen to transition metal complexes.25−27
borane products are formed. The nature of the isomers 3 was
determined by a combination of 1H NMR spectroscopy and X-
ray crystallography of the borapentenes themselves and their
pyridine adducts. Although no structures have been deter-
mined, the unfluorinated isomers of 4 exhibited chemical and
spectral behavior similar to that observed for 3, and trans-4, as
well as its pyridine adduct, were isolated analytically pure.
In both reactions, the trans isomer is favored; the numbers in
parentheses in Scheme 1 indicate the relative amounts of each
isomer under these ambient conditions. This ratio is a kinetic
ratio and does not change upon heating the mixture to higher
temperatures (50−80 °C). Interestingly, solid samples of 1 also
react with H2 to give the products, but under these conditions,
the cis isomer of 3 is favored, being produced in a 10:1 ratio
relative to the trans isomer. Irradiation of solutions of 3 thusly
enriched in the cis isomer at 254 nm results in complete
conversion to trans-3 after about 4 days. Therefore, the trans
isomer is also the thermodynamically most favored isomer in
these systems, a notion supported by DFT calculations (see
below).
Proposed Mechanism. The reaction of boroles 1 and 2
has been probed mechanistically via kinetic studies, the separate
generation of a key intermediate and demonstration that it
proceeds to products, and DFT studies. A mechanism that is
consistent with these investigations is shown in Scheme 2. The
first step entails reversible formation of a borole dihydrogen
adduct (I). This step is faster for the more Lewis acidic
perfluorinated borole 1 and accounts for the qualitatively much
faster rates of reaction of 1 with H2 in comparison to that
observed for 2. Attempts to observe these H2 adducts met with
failure; solubility issues thwarted the examination of solutions
of either 1 or 2 under H2 at low temperatures. At ambient
temperatures, addition of the coordinated H2 across an intra-
ring B−C bond (step b) produced the reactive zwitterionic
intermediate cis-II, which features a carbocationic center that is
stabilized by virtue of the fact that it is both benzylic and allylic;
the adjacent borate center is probably also a stabilizing
In this context, then, our observation that the very highly
Lewis acidic perfluoropentaphenylborole28 complex reacts
essentially instantly with dihydrogen29 to form the cis and
trans isomers of the pentaarylboracyclopentenes is significant.
With no external base present, it seems clear that the H2 must
interact with the boron center in some fashion in order that this
reaction proceeds. In our initial report, we speculated that H2
binding to the boron atom of the borole was followed by ring-
opening to a 1-bora-pentadiene that then gave the observed cis
and trans products via an electrocyclic ring closure and 1,2
hydride migration process.30,31 Herein we present an
experimental and computational study of this mechanism,
using both the fully fluorinated system 1 and its unfluorinated
pentaphenylborole32,33 analogue 2.
RESULTS AND DISCUSSION
■
Activation of H2 by Pentaarylboroles. Reaction of the
pentaarylboroles 1 and 2 with 1 atm of dihydrogen in
methylene chloride occurs rapidly in the case of fully
fluorinated 1, and over the course of 4−5 h with the less
Lewis acidic 2, to yield the 1-bora-3-cyclopentene products 3
and 4, respectively (Scheme 1).29 The reactions are
Scheme 1
accompanied by a striking color change from the deep blue
or purple colors of the boroles34 to light yellow as the cyclic
Scheme 2
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influence. Again, this intermediate is not observed directly, due
to low-energy decomposition pathways that lead either directly
to the cis isomers of the product 1-bora-3-cyclopentenes as
indicated by step c or to an interconverting pair of 1-bora-2,4-
pentadienyl rotamers (cis/trans-III) via a B−C bond-cleaving
ring-opening, step d. Note that this is the only path by which
the trans isomers of 3 and 4 can form, since path c leads only to
the cis isomers; path d must therefore be kinetically competitive
with the seemingly more straightforward migration of hydride
from cis-II to give cis-3/4. The rotomers of III are in rapid
exchange by rotation about the B−C single bond (step e), and
each leads to one of the product isomers by a rapid conrotatory
ring closure back to the cyclic zwitterions cis/trans-II that lead
to the observed products via 1,2-hydride migration.
a pseudo-first-order plot. These results are consistent with the
second-order formation of the H2 adduct I, at least in the case
of unfluorinated 2.
A kinetic isotope effect (KIE) for this reaction was measured
by two methods. First, the ratio of kobs values for the separate
reaction of 2 with H2 and D2 showed a small effect of 1.10(5)
(Figure 2). Second, exposure of 2 to a 1:1 mixture of H2:D2
Compound 1 reacts too rapidly with H2 at temperatures at
which it exhibits reasonable solubility in CD2Cl2 to follow by
NMR spectroscopy. Fortunately, the less Lewis acidic borole 2
has no such limitations, and its reactivity with H2 can be
followed conveniently over the course of 3−6 h to >4 half-lives
1
using H NMR spectroscopy. Methodology similar to that
reported by Parkin and co-workers35 was employed. In a typical
procedure, an NMR tube was charged with a CD2Cl2 solution
containing 2 and mesitylene as an internal standard. After
degassing, dry H2 was admitted to a defined pressure and
temperature and the reaction followed by recording spectra
every 5−10 min; between data collection steps the sample was
agitated to ensure that the solution remained saturated with H2.
The [H2] in solution was measured from the signal at 4.61
ppm, with the assumption that 25% of the H2 existed as para-
hydrogen.36 It should be noted that, while the measurements
indicated that [H2] in solution was roughly equal to [2], the
total H2 in the 2.7 mL NMR tube was in at least 10-fold excess
of the total 2 (0.0080 mmol). This method therefore relied on
the assumption that agitation of the sample would replenish the
dissolved H2 much faster than the rate of reaction. That every
trial obeyed a pseudo-first-order rate law indicates that this is a
valid assumption.
Figure 2. Pseudo-first-order plots of the reaction between 2 and H2
(red) and D2 (blue).
resulted in a 1.1(1):1 ratio of cis/trans-4 and d2-cis/trans-4
products, corroborating the result. The reaction of fully
fluorinated 1 with a 1:1 mixture of H2:D2 gave a slightly larger
KIE of 1.2(1). The small KIEs observed are consistent with
binding of H2 followed by a step (b, Scheme 2) in which the
H−H bond is cleaved via an asynchronous transition state.
In a final set of kinetic experiments, the activation parameters
for the reaction were derived from an Eyring plot (Figure 3):
The proposed mechanism predicts that the reaction should
be first-order in both [H2] and [2]. Indeed, by varying the
pressure of H2 from ca. 1 to 4 atm (10−40 times the amount of
2), the observed rate constant (kobs) changed in direct
proportion to [H2] (Figure 1). The slope of this ln-vs-ln plot
is 1.01, indicating the order in [H2] to be 1; plots of kobs vs
[H2]n (n = 1, 2, Figure S1) showed that only the n = 1 plot was
linear, with a slope of 0.12 s−1. First-order behavior in [2] is
evidenced by the fact that every reaction profile can be fitted to
Figure 3. Eyring plot for the reaction of 2 (0.016 M) with H2 (ca. 1
atm) in CD2Cl2 at 283, 298, 309, and 316 K.
ΔH⧧ = 34(8) kJ/mol and ΔS⧧ = −146(25) J mol−1 K−1.
Although the temperature range over which these measure-
ments were conveniently made is quite narrow, the negative
ΔS⧧ value is consistent with a bimolecular process. Using these
activation parameters, the ΔG⧧ at 298 K is 78(16) kJ/mol.
While these kinetic experiments are consistent with the first
steps in the proposed mechanism, the subsequent steps remain
rather speculative. We thus sought to provide more evidence
for the proposal through the separate synthesis of a 1-bora-2,4-
pentadiene complex analogous to cis- or trans-III. Early
attempts by Zweifel and co-workers30,31 to generate such
boranes led directly and rapidly to 1-bora-3-cyclopentenes
analogous to products 3 and 4 herein, and so we have only
succeeded in generating III (Ar = C6H5) in situ.
Figure 1. Plot of ln kobs against ln [H2] with [2] = 0.016 M under ca.
1−3.8 atm H2 in CD2Cl2 at 293 K for the determination of the
reaction order in [H2] using the isolation method.
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This was accomplished by the synthesis of the borinic ester 5
via the protic ring-opening of 2 by treatment with 1 equiv of
phenol (Scheme 3). After exploring various hydroboration-
between C3 and C4 toward the trigonal plane about B1; the
nonbonding distance between C4 and B1 is only 2.79 Å.
As shown in Scheme 3, treatment of 5 with 1 equiv of DIBAl-
H rapidly gave trans-4 (exclusively), as evidenced by the
emergence of the singlet at 4.85 ppm in the 1H NMR spectrum
characteristic of this compound. Addition of a few drops of
pyridine resulted in the quantitative conversion to the pyridine
adduct of trans-4. The spectroscopic signature of the
compounds produced from 5 and DIBAl-H is identical to
that from 2 and H2 (Figure 5), with the exception that no
Scheme 3
based routes (as originally established by Zweifel), this proved
the most convenient way to generate 5. Of significance is the
fact that the π-donating phenoxy group on boron in 5 stabilizes
this compound toward ring-closingso much so that 5 can be
isolated as a pale yellow solid in 52% yield after recrystallization
from hot toluene. The 11B NMR spectrum of 5 contains a
broad signal that appears at 44.8 ppm, which is comparable to
that found for Ph2B(OMe) (45.2 ppm).37 Furthermore, a
crystal suitable for X-ray analysis was obtained by slow
evaporation from dichloromethane; its molecular structure
and selected metrical parameters are given in Figure 4.
Figure 5. Comparison of the 1H NMR of trans-4 and the
corresponding pyridine adduct trans-4-py generated from 2 and H2
(top) and 5 and DIBAl-H (bottom).
evidence for the cis isomer of 4 is observed in the former
reaction. While the reasons for this are not clear, it may reflect
the fact that, in the H2 reaction with 2, the cis isomer arises
exclusively via path c depicted in Scheme 2.
These experimental studies provide convincing support for
the mechanism proposed in Scheme 1, but the facility of these
reactions and the limitations imposed by the physical properties
of these sparingly soluble, highly Lewis acidic and reactive
boroles necessarily dictate that aspects of the proposed
mechanism remain experimentally opaque. Therefore, we
turned to DFT calculations to probe the veracity of the overall
mechanistic proposal and computed a complete energy surface
for the reactions of both 1 and 2 with dihydrogen.
DFT Calculations. The mechanism outlined in Scheme 2
was examined in detail at the PBE1PBE/def-TZVP level of
theory, employing the polarizable continuum model for the
treatment of solvent effects (methylene chloride). The acquired
solution-state energies are presented in Figure 6 for both the
fluorinated (1) and unfluorinated (2) pentaphenylboroles.
The initial step on the reaction pathway is the reversible
formation of a stable adduct (I) between the borole and a
molecule of dihydrogen (step a in Scheme 2). This step is
endergonic for both boroles, around 20 kJ/mol less so for the
fully fluorinated system. Furthermore, the energy of the
transition state (TS0) is virtually on par with that of the
adduct. Consequently, these intermediates can release H2 or
access a transition state (TS1), leading to addition of H2 across
Figure 4. Thermal ellipsoid diagram (50%) of 5. Selected bond
lengths (Å) and angles (°): C1−C2 1.361(3), C2−C3 1.486(3), C3−
C4 1.354(3), B1−C1 1.571(3), B1−O1 1.367(3), B1−C11 1.568(3);
C1−B1−C11 122.05(17), C1−B1−O1 122.99(18), C11−B1−O1
114.76(17), and C1−C2−C3−C4 35.47.
Significantly, 5 adopts a cisoid 1-bora-2,4-pentadienyl
geometry, which is required for the electrocylic cyclization to
occur. Bond distances are as one would expect for localized
double and single bonds; the short B1−O1 distance of 1.367(3)
Å is indicative of strong π bonding in this linkage. The C1−
C2−C3−C4 dihedral angle of 35.47(5)° orients the π bond
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Figure 6. Calculated Gibbs free energies (kJ/mol) for the reaction of
boroles 1 (blue) and 2 (red) with H2.
an internal B−C bond to give intermediates cis-II (step b in
Scheme 2). In this transformation, the π-electrons of the
antiaromatic system function as an internal Lewis base,
resulting in an overall heterolytic cleavage of the H−H bond.
The energies for TS1 are only 61 kJ/mol for 1 and 74 kJ/mol
for 2, in agreement with the experimentally observed reaction
rates and the experimentally determined activation energy of
78(16) kJ/mol for borole 2.
Continuing from the intermediates cis-II, the reactions can
follow two alternate mechanisms whose transition states TS2
and TS3 have very similar energies. This indicates that both
pathways are equally probable, in agreement with the observed
formation of both cis and trans isomers. The transition states
TS2 and TS3 are close in energy not only to each other but
also to cis-II, which suggests that the intermediate is short-lived
and that formation of intermediates cis-II is effectively
irreversible.
The TS2 involves a hydride transfer from boron to carbon
(step c in Scheme 2) and leads to the formation of the cis-3/4
products. However, the internal B−C bond lengths are
significantly divergent in the cis-II structure: 1.566 vs 1.731 Å
for the fluorinated system and 1.584 vs 1.711 Å in the phenyl-
substituted borole. Consequently, TS3 leads to B−C bond
cleavage and ring-opening (step d) to yield the cis-1-bora-2,4-
pentadienyl rotamer (cis-III), which is the first intermediate on
the pathway leading to products trans-3/4. The transformation
of the cis-III rotamer to the corresponding trans isomer
proceeds through TS4 (step e), with activation barriers of only
28 and 30 kJ/mol for boroles 1 and 2, respectively. A
subsequent, virtually barrierless (TS5) ring closure yields the
trans-II intermediate (step f), which readily undergoes hydride
migration from boron to carbon (TS6, analogous to step c) to
give the trans-3/4 products, thus completing the reaction
pathway. In agreement with the photochemical experiments,
the trans-3 product is found to be more stable than the
corresponding cis isomer for the fully fluorinated compound 1
(by 21 kJ/mol).38
As is evident from Figure 6, the relative energies of transition
states TS2−TS6 are significantly below that of the initial adduct
I and the transition state of the rate-limiting step, TS1.
Although the KIEs observed are quite small, the low
concentration of adduct I and the highly asynchronous
geometry for the H−H cleavage in TS1 (Figure 7) are
consistent with this observation. Indeed, in agreement with the
KIE measurements, the reaction profiles shown in Figure 2
Figure 7. Computed structures for the H2 adduct I for the fully
fluorinated borole 1 (top) and the transition state TS1 (bottom) for
addition of H2 across the internal B(1)−C(1) bond to form cis-II.
Selected bond lengths (Å): (top) H(1)−H(2) 0.814, F(1)−H(1)
2.166, F(2)−H(2) 2.309, C(1)−H(1) 2.056, C(2)−H(2) 2.111,
B(1)−H(1) 1.438, B(1)−H(2) 1.443; (bottom) H(1)−H(2) 1.057,
F(1)−H(1) 2.266, F(2)−H(2) 2.391, C(1)−H(1) 1.450, C(2)−H(2)
2.142, B(1)−H(1) 1.328, B(1)−H(2) 1.304.
were found to be essentially independent of the isotope used in
the calculations: substituting hydrogen in H2 with deuterium
had only a very minor (around 0−2 kJ/mol) effect on the
relative Gibbs free energies calculated for both I and TS1.
A comparison of the two reaction profiles in Figure 6 shows
that the overall energetics of the mechanism are largely
independent of the identity of the borole, as in many steps the
blue and red lines overlap or are otherwise very close to each
other. The biggest differences between the two profiles are
observed for the trans-3/4 product as well as for the initial
adduct I and the transition state TS1. In each of these cases, the
fully fluorinated system is about 20 kJ/mol more stable than its
phenyl analogue, suggesting that the difference in Gibbs energy
could be related to the short F···H and F···B contacts observed
in the optimized structures. This was examined by conducting
calculations on boroles which had their phenyl substituents
only partially fluorinated.
The calculations show that, for 1, the structures I and TS1
gain extra stabilization from two short F···H contacts (2.106−
2.391 Å) that make an approximately 10 kJ/mol total
contribution to their relative energies. Consequently, the first
step on the reaction pathway is faster for the borole 1 not just
because of its greater Lewis acidity but also because of the
possibility to form van der Waals interactions that its phenyl-
substituted analogue 2 inevitably lacks. In a similar fashion, the
lower relative energy of trans-3 compared to both trans-4 and
cis-3 results in part from the two short F···B contacts (2.716 Å)
in its structure. Selective F-to-H replacements showed that the
combined effect of these interactions on the relative energy is 8
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[H2] in solution was measured from the signal at 4.61 ppm, assuming a
3:1 ratio of ortho hydrogen:para hydrogen. The [4] in solution was
measured from the allylic proton signals at 4.85 ppm (trans-4) and
4.50 ppm (cis-4) and from the aryl proton signals at 7.78 ppm (trans-
4) and 7.68 ppm (cis-4). This was then used to infer the disappearance
of product; hence the plots in the SI are shown as ln([2]). Activation
parameters were determined by measuring rate constants at 10, 25, 31,
and 43 °C, and agitation was achieved by inverting the NMR tube five
times between readings. Higher pressures of H2 were achieved by
introducing H2 at different temperatures: ∼1 atm at 20 °C, ∼1.5 atm
at −72 °C (dry ice/acetone bath), ∼2 atm at −130 °C (pentane/liquid
N2), and ∼3.8 atm at −196 °C (liquid N2).
Reactions with 1:1 H2:D2. Three J. Young NMR tubes were
charged with a CD2Cl2 solution (0.5 mL) containing either 1 or 2 and
toluene as an internal standard. The solutions were then subjected to
three freeze−pump−thaw cycles and allowed to equilibrate to room
temperature. To the first tube was added H2. A 1:1 H2:D2 mixture was
created by first placing both the vacuum and atmosphere lines under
full (static) vacuum and then introducing H2 (300 mmHg) followed
by D2 (300 mmHg) and letting the mixture equilibrate for a minimum
of 4 h. The J. Young tube was then opened to allow the gas mixture in.
This was then repeated with the third J. Young tube, adding D2 to the
vacuum line first, followed by H2. All three tubes were agitated with a
modified Kugelrohr apparatus overnight. The 1H NMR spectra of each
mixture were recorded, and the ratio of H2:D2 products was obtained
by integration of appropriate signals.
kJ/mol, which accounts for almost half of the difference
between trans-3 and trans-4 structures.
As a whole, the results from theoretical calculations add
considerable support for the pathway presented in Scheme 2.
Nevertheless, in an effort to test the plausibility of alternate
mechanistic possibilities, we performed a more comprehensive
scan of the potential energy surfaces of 1 and 2 with H2. As
expected, the environment at and around the boron atom was
found to be the single reactive site in both boroles. However,
we were able to characterize an additional transition state in
which H2 adds to the external B−C bond involving the
aromatic substituent bound to the boron center. Significantly,
these transition states were found to be 72 and 46 kJ/mol
higher in energy than the TS1 of boroles 1 and 2,
respectively.39 Furthermore, following the internal reaction
coordinate toward products clearly showed that the charac-
terized transition state leads to breakup of the B−C bond and
subsequent formation of a borole with a B−H functionality and
a molecule of benzene/pentafluorobenzene. Since neither ArH
nor products arising from the highly reactive H-borole34,40 were
observed, this pathway is not competitive in the reaction of
boroles with H2.
CONCLUSIONS
■
Synthesis of 5. A solution of phenol (47 mg, 0.50 mmol) in
dichloromethane (10 mL) was added to a cold (0 °C) solution of
pentaphenylborole 2 (222 mg, 0.50 mmol) in dichloromethane (20
mL) via syringe over 30 min. The purple solution turned pale yellow
almost immediately, and the ice bath was removed. After 30 min the
volatiles were removed in vacuo, and yellow solid was recrystallized
from hot toluene and washed with cold hexanes (2 × 3 mL). A pale
yellow powder was obtained (140 mg, 52% yield). Crystals suitable for
X-ray analysis were grown from slow evaporation from dichloro-
The facile reaction of dihydrogen with antiaromatic boroles 1
and 2 illustrates that strongly Lewis acidic main group Lewis
acids can bind hydrogen and activate it in the absence of an
external Lewis base. Although dihydrogen adducts of Lewis acid
centers incapable of π backbonding are ephemeral, these
reactions show that, when a thermodynamic driving force is
present, such adducts can lead to reactivity other than simple
release of the bound H2. Here, the antiaromaticity associated
with the borole ring is a likely contributor to this driving force.
For other Lewis acidic boranes, such as B(C6F5)3, adducts with
H2 have been shown computationally to lie in energetic minima
but are not thought to lie on the reaction coordinate for
hydrogen splitting in the presence of a bulky trialkylphosphine
Lewis base such as PtBu3. Little, however, is known regarding
the mechanism of H2 activation by B(C6F5)3 in partnership
with other, non-C3v-symmetric Lewis bases. The role of H2
adducts of boranes may be more pronounced in the
mechanisms involving these systems.
1
methane. H NMR (400 MHz, CD2Cl2): δ = 7.90 (m, 2H, ArH), 7.41
(m, 1H, ArH), 7.33 (m, 2H, ArH), 7.19−6.87 (m, 20H, ArH + CC-
H), 6.83 (m, 2H, ArH), 6.76 (m, 2H, ArH), 6.70 (m, 2H, ArH). 13C
NMR (100 MHz, CD2Cl2): δ = 155.66, 152.88, 146.95, 145.64,
142.26, 139.31, 138.14, 136.70, 135.11, 131.98, 131.07, 130.80, 130.73,
130.55, 130.10, 129.08, 127.78, 127.69, 127.48, 127.40, 127.28, 127.08,
126.86, 126.48, 125.94, 123.20, 120.16. (Two peaks for the carbons
bonded to boron were not observed due to the quadrupolar
relaxation.) 11B NMR (128 MHz, CD2Cl2): δ = 45(br). HRMS
(TOF MS EI+): calcd for C40H31BO 538.2468, found 538.2450.
Reaction of 5 with DiBAl-H and Pyridine. A solution of 5 26 mg
(0.050 mmol) in CD2Cl2 (∼0.7 mL) was combined with DiBAl-H (50
uL, 1.0 M in hexanes) and shaken. After 20 min, two drops of pyridine
The work described here suggests that an H2 adduct of a
pentaarylborole might be observable experimentally under the
right conditions. Unfortunately, the solubility properties of 1
and 2 (particularly 1) have precluded us from pursuing low-
temperature spectroscopic studies. With solubilizing groups,
such studies may yield interesting results; efforts along these
lines are currently underway.
1
was added to the NMR tube and the H NMR spectrum recorded.
Reaction of 2 with H2 and then Pyridine. In a J. Young NMR
tube, a solution of 2 (5 mg, 0.01 mmol) in CD2Cl2 (∼0.7 mL) was
degassed at −78 °C and then charged with H2 gas (ca. 1 atm). The
tube was then rotated on a modified Kugelrohr apparatus for 10 h.
Two drops of pyridine was added to the tube and the 1H NMR
spectrum recorded.
Computational Details. All calculations were done with the
program packages Turbomole 6.341 and Gaussian09.42 Geometries of
the studied systems were optimized in the gas phase as well as in
solution (methylene chloride) using the PBE1PBE density func-
tional43−46 in combination with Ahlrichs’s TZVP basis sets.47 The
polarizable continuum model, as implemented in Gaussian 09, was
used for the treatment of solvent effects.48 The nature of stationary
points found was assessed by calculating full Hessian matrices. To
ensure that the calculated energetics are not significantly dependent on
the employed basis set, the reaction profile of borole 1 was
recalculated in the gas phase using Ahlrichs’s def2-TZVPP basis
sets.49,50 Doubling the size of the basis sets resulted in only minor 1−
10 kJ/mol changes to the relative energies. The program gOpenMol
was used for all visualizations of molecular structures.51,52
EXPERIMENTAL SECTION
■
General procedures are detailed in the Supporting Information.
Kinetics Experiments. A J. Young NMR tube was charged with a
CD2Cl2 solution (0.5 mL) containing 2 (0.016 M) and mesitylene
(0.004 M) as an internal standard. The solution was then subjected to
three freeze−pump−thaw cycles and allowed to equilibrate at a given
temperature. At this time H2 gas was introduced and allowed to fill the
tube for 2 min, taking care to not agitate it. The tube was carried to an
NMR spectrometer in a dry ice/acetone bath and allowed to warm to
the appropriate reaction temperature. Agitation of the tube was
counted as t = 0. Spectra were recorded every 5−10 min using a delay
of 45 s (∼5 times T1 for the methyl groups of mesitylene in CD2Cl2).
For experiments at room temperature, a modified Kugelrohr apparatus
was used to consistently agitate the samples between readings. The
946
dx.doi.org/10.1021/ja311842r | J. Am. Chem. Soc. 2013, 135, 941−947

Journal of the American Chemical Society
Article
(23) Rendler, S.; Oestreich, M. Angew. Chem., Int. Ed. 2008, 47,
5997−6000.
ASSOCIATED CONTENT
* Supporting Information
Crystallographic data files for 5 (CCDC 912008) as well as
relative energies and .xyz coordinates of the calculated
structures and complete ref 42. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
wpiers@ucalgary.ca; heikki.m.tuononen@jyu.fi
Notes
■
(29) Fan, C.; Mercier, L. G.; Piers, W. E.; Tuononen, H. M.; Parvez,
M. J. Am. Chem. Soc. 2010, 132, 9604−9606.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Funding for the experimental work described was provided by
the Natural Sciences and Engineering Research Council of
Canada in the form of a Discovery Grant to W.E.P. Funding for
the computational work described was provided by the
Academy of Finland and the Technology Industries of Finland
Centennial Foundation in the form of a research grant to
H.M.T. W.E.P. thanks the Canada Council for the Arts for a
Killam Research Fellowship (2012-2014).
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