Evidence for an intermediate in the methylation of CB11H 12- with methyl triflate: Comparison of electrophilic substitution in cage boranes and in arenes

Article abstract of DOI:10.1002/cplu.201300219

The trideuteriomethylation of BH vertices in CB₁₁H ₁₂^- and its derivatives with CD₃OTf (OTf=triflate, trifluoromethanesulfonate) yields a mixture of B-CD₃ and B-CHD₂ substitution products, thus demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed if CD₃OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σadduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square-pyramidal methane attached axially to a naked boron vertex. The subsequent proton or deuteron loss is presently not understood in detail. A general comparison of electrophilic substitution on closo-boranes and arenes is provided and similarities as well as differences are discussed. A recalculation of the optimized geometry of the CB₁₁Me₁₂^. radical produced a second Jahn-Teller distorted minimum and resulted in a somewhat improved agreement between calculated and measured proton hyperfine coupling constants. Scrambled protons: Methylation of HCB₁₁H ₁₂^- and similar anions with deuterated methyl triflate (CD₃OTf) produces a mixture of CD₃- and CHD ₂-substituted anions (see scheme), thus demonstrating that the reaction mechanism involves an intermediate in which protons can scramble. A comparison with electrophilic substitution on arenes shows similarities and differences. Copyright

Full text of DOI:10.1002/cplu.201300219

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DOI: 10.1002/cplu.201300219
Evidence for an Intermediate in the Methylation of
À
CB11H12 with Methyl Triflate: Comparison of Electrophilic
Substitution in Cage Boranes and in Arenes
[a]
[a, b]
Raꢁl Crespo,[c] Mari-Carmen Piqueras,[c] and Josef Michl*[a, d]
ˇ
Jirꢀ Kaleta, Akin Akdag,
In memory of Detlef Schrçder
À
The trideuteriomethylation of BH vertices in CB11H12 and its
derivatives with CD3OTf (OTf=triflate, trifluoromethanesulfo-
nate) yields a mixture of BÀCD3 and BÀCHD2 substitution prod-
ucts, thus demonstrating the intermediacy of a species with
a long enough lifetime for hydrogen scrambling between the
boron vertex and the methyl substituent. No such scrambling
is observed if CD3OTf is used to methylate toluene. According
to density functional theory calculations, the intermediate in
BH vertex methylation is a three-center bonded s adduct of
a methyl cation to the BH bond and the proton scrambling
occurs via a transition structure containing a distorted square-
pyramidal methane attached axially to a “naked” boron vertex.
The subsequent proton or deuteron loss is presently not un-
derstood in detail. A general comparison of electrophilic sub-
stitution on closo-boranes and arenes is provided and similari-
ties as well as differences are discussed. A recalculation of the
C
optimized geometry of the CB11Me12 radical produced
a second Jahn–Teller distorted minimum and resulted in
a somewhat improved agreement between calculated and
measured proton hyperfine coupling constants.
Introduction
Electrophilic substitution on BH bonds in closo-boranes and
-carboranes plays an important role in their chemistry,[1–3] and
is comparable with electrophilic substitution on benzene and
other p-electron aromatic systems (Scheme 1). The exocyclic
bonds in closo-boranes and arenes are analogous: the boron
cages carry two-electron s BH bonds, built from exo-directed
radial sp hybrids on boron, and at the end of the substitution
process these bonds are replaced with similar two-electron
s bonds BX. In arenes, two-electron s CH bonds, built from
exo-directed sp2 hybrids on carbon, are substituted with similar
two-electron CX bonds.
Scheme 1. Electrophilic substitution on BH and CH bonds.
The electrophile used in the substitution reaction on BH ver-
tices can be a donor of a proton (X=H),[4] a halogen cation
(X=halogen),[5–7] a carbocation (X=alkyl),[8,9] a metal cation
(X=HgR),[10] and many others. We are particularly interested in
electrophilic alkylation.[8,11–14] In spite of the preparative impor-
tance of these substitution reactions, little is known with cer-
tainty about their mechanism. It is usually assumed that they
proceed via an intermediate adduct of X+ to one of the BH
vertices, [XBH]+, followed by deprotonation, as suggested by
calculations.[15,16]
[a] Dr. J. Kaleta, Dr. A. Akdag, Prof. J. Michl
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic
Flemingovo nꢀm. 2, 16610 Prague (Czech Republic)
E-mail: michl@eefus.colorado.edu
[b] Dr. A. Akdag
Department of Chemistry
Middle East Technical University
06800 Ankara (Turkey)
[c] Prof. R. Crespo, Prof. M.-C. Piqueras
Department de Quꢁmica Fꢁsica
Universitat de Valꢂncia
We now present experimental evidence for the intermediacy
À
of such an adduct in the methylation of the CB11H12 anion
Dr. Moliner 50, 46100 Burjassot (Spain)
and its derivatives with methyl triflate (TfOMe), and computa-
tional evidence for its anticipated three-center bonded s-com-
plex nature. Initial results were collected during a recent prep-
aration[17] of all 16 symmetrically CH3/CD3 substituted deriva-
tives of the anion CB11(CH3)12À. The anions were oxidized to
[d] Prof. J. Michl
Department of Chemistry and Biochemistry
University of Colorado
Boulder, CO 80309-0215 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300219.
C
the radicals CB11(CH3)12 and the relative proton hyperfine cou-
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pling constants in their four inequivalent positions were de-
duced from deuterium-substitution-induced line narrowing.
The constants were thought to be in only imperfect agreement
with the values calculated by density functional theory (DFT).
We have now located an essentially isoergic second minimum
in the potential energy surface of the Jahn–Teller distorted
C
ground state of CB11(CH3)12 . If DFT is used to calculate the
proton hyperfine coupling constants averaged over the two
minima, a reasonable agreement with experiment is found.
Results
Our initial intention was to introduce CD3 groups at the BH
À
vertices of selected derivatives of the CB11H12 anion by reac-
tion with TfOCD3 in sulfolane.[17] As usual,[18] CaH2 was present
to remove the triflic acid byproduct. Unexpectedly, the reac-
tions yielded mixtures of CD3- and CHD2-substituted products,
although the isotopic purity of TfOCD3, declared by the suppli-
er and independently verified, was 99 atom% D. The presence
Scheme 2. Stepwise preparation of BÀCHD2-free Cs[1-CH3CB11(CD3)11].
with slight modifications that simplified the isolation and pu-
rification of the product, and proceeded smoothly. The second
step was disappointing. Although the dehalogenation using
Birch reduction in liquid [D3]ammonia at À788C was complete
of BÀCHD2 groups was revealed in IR spectra by characteristic
À1
À
vibrations at 2872 cmÀ1 [strong, n(CÀH)] and 1276 cm [weak,
in less than 5 min at À788C (ESI-MS) and 1-CH3CB11D11 was
~
d(CÀH)] and in 2H NMR spectra (broad doublets with JH,D
=
the major ionic component of the crude reaction mixture
(charged boron-containing impurities formed less than 15% of
the total negative ESI-MS signal), its yield was very small,
barely sufficient for subsequent use without any purification.
The identity of the product was beyond doubt, but no attempt
was made to prepare an analytically pure sample. Upon treat-
ment with TfOCD3 in sulfolane in the presence of CaH2,
2.1 Hz). Formation of BÀCH2D and BÀCH3 groups was not ob-
served.
À
Ten CB11H12 anions variously substituted with CH3À, CD3À,
IÀ, and iPr3SiÀ groups [1-CH3, 1-CD3, 1-iPr3Si, 1-CH3-12-I, 1-CD3-
12-I-, 1-iPr3Si-12-I, 1,(7–12)-Me7, 1-CD3-(7–12)-Me6, 1,(7–11)-Me6,
1-CD3-(7–11)Me)] were treated with TfOCD3 and CaH2 in sulfo-
lane at room temperature (see Scheme 1 for vertex number-
ing). The ratio of CHD2 to CD3 groups in each of the four types
of positions of substitution in the product was determined by
NMR spectroscopy and mass spectrometry.[17] The results were
the same within experimental
À
1-CH3CB11(CD3)11 was formed and a comparison of 1H and
2H NMR spectra (Figure 1) showed clearly that only BÀCD3 and
no BÀCHD2 groups were now present. Infrared and high-reso-
lution mass spectra confirmed this conclusion.
error in all ten cases: in posi-
tions 2–6, the CHD2/CD3 ratio
was 0.6:0.4, and in positions 7–
12, it was 0.5:0.5. Once the final
substitution
product
was
formed, the content of hydrogen
and deuterium atoms in its
methyl groups was immutable. It
did not change on stirring a solu-
tion of the product of 11-fold tri-
deuteriomethylation of Cs[1-
CH3CB11H11] in anhydrous sulfo-
lane for 7 days alone, with CaH2,
or with TfOCD3 and CaH2.
These results indicated that
the source of the hydrogen
atom in the CHD2 group is the
starting carborate anion itself;
therefore, we prepared the
À
1-CH3CB11D11 anion as shown in
Scheme 2 and repeated the tri-
deuteriomethylation experiment.
The first step in the synthesis fol-
1
2
1
2
Figure 1. Comparison of H NMR (A) and H NMR spectra (B) of Cs[1-CH3CB11(CD3)11] with H NMR (C) and H NMR
lowed
a published procedure spectra (D) of the product of its methylation with TfOCD3. The peak at 2.05 ppm is a result of the acetone solvent.
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À
Attempts to convert all BH bonds in CB11H12 into BD bonds
by treatment with D2SO4, either concentrated or diluted with
D2O, or with DCl in D2O, produced mixtures of partially (mostly
six times) deuterated carborane anions with products carrying
an OSO3D or OD substituent on the anionic carborane cage,
and were abandoned as unpromising.
model (CPCM)[20] to take into account solvation in sulfolane
(e=46) and assumed that there is no association with the tri-
flate counterion. The force constant matrix was evaluated and
contributions from zero-point energy (ZPE), heat capacity, and
entropy were included in the free energies DG8 reported.
The species [CD3BH]+ and [CHD2BD]+ were found to corre-
spond to a minimum on the potential energy (DE) surface, and
had the expected[21] triangular structure. The bond lengths and
valence angles are given in Table 2. The transition state con-
A similar trideuteriomethylation reaction was run on toluene,
known[19] to be methylated with methyl triflate. The xylenes
formed contained only CH3 and CD3 groups and there was no
indication of any hydrogen scrambling.
The observations described above demonstrate that the BH
methylation reaction proceeds through an intermediate with
a lifetime sufficient for proton scrambling. To identify a possible
mechanism for the scrambling, we performed DFT (B3LYP/SVP)
calculations on the anticipated [CD3BH]+ intermediate at all
Table 2. Selected bond lengths [ꢃ] and valence angles [8] calculated
(B3LYP/SVP) for intermediates (i) in the methylation of CB11H12 in posi-
tions 2, 7, and 12, and for transition states (t) of their proton scrambling
reactions (see Figure 2).
À
À
three inequivalent boron vertices in CB11H12 (Figure 2 and
Species
Bond or angle
Position
7
2
12
Table 1). We used the conductor-like polarizable continuum
i
BÀC
1.982
1.166
1.104
1.091
1.091
88.46
116.41
114.33
109.29
110.18
114.83
2.030
1.161
1.103
1.091
1.091
90.36
112.95
116.25
110.63
109.52
114.48
2.064
1.158
1.102
1.091
1.091
91.67
112.16
115.91
110.99
109.73
114.16
CÀH1
CÀH2
CÀH3
CÀH4
H1ÀCÀH2
H1ÀCÀH3
H1ÀCÀH4
H2ÀCÀH3
H2ÀCÀH4
H3ÀCÀH4
t
BÀC
1.609
1.614
1.615
BÀCÀH1
BÀCÀH2
BÀCÀH3
BÀCÀH4
H1ÀCÀH2
H1ÀCÀH3
H1ÀCÀH4
H2ÀCÀH3
H2ÀCÀH4
H3ÀCÀH4
112.14
113.11
119.25
117.50
48.53
119.32
81.68
81.88
113.09
112.29
118.60
118.75
48.60
118.62
81.05
81.73
112.53
112.54
118.65
118.64
48.39
118.81
81.46
81.48
119.10
100.29
119.00
100.25
118.80
100.47
Figure 2. DFT optimized structures of the methylation intermediate i and of
the transition state t for hydrogen scrambling. See Table 2.
necting the minima had a structure similar to one of those
+ [22,23]
known from CH5
,
with methane shaped as a distorted
square pyramid that is attached axially to the “naked” boron
vertex. The simplest description of the transition-state geome-
try is to say that it contains an H2 molecule complexed with
a carboranylmethyl cation. The calculated isotope effects on
the equilibrium and the activation energy of the scrambling re-
action were small and very similar at all boron vertices at
Table 1. Energies[a] [kcalmolÀ1] of optimized structures of intermediates i’
and i’’ in the methylation of CB11H12 with TfOMe in positions 2, 7, and
À
12, and of the hydrogen scrambling transition state t.
Compound
DE
0.00
DE+ZPE
DH8
DG8
À
CB11H12 (Table 1). For this anion, the calculated free energies
CB11H12À +TfOMe
2i’+TfOÀ
149.52
166.36
183.20
166.41
160.92
181.28
160.97
158.41
180.12
158.47
161.54
178.93
195.34
178.97
173.52
193.36
173.57
171.16
191.79
171.22
107.44
124.30
141.57
124.37
118.68
139.80
118.76
115.47
139.13
115.55
of the intermediate relative to the starting materials were
23.55, 17.90, and 15.54 kcalmolÀ1 in positions 2, 7, and 12, re-
spectively.
23.55
42.31
23.55
17.90
40.16
17.90
15.54
39.32
15.54
2t+TfOÀ
2i’’+TfOÀ
7i’+TfOÀ
Finally, we complement the previously published[17] calculat-
7t+TfOÀ
7i’’+TfOÀ
12i’+TfOÀ
12t+TfOÀ
12i’’+TfOÀ
C
ed properties of the radical CB11Me12 . In addition to the report-
ed optimized geometry of C1 symmetry with a ground-state
electronic wave function of A’ symmetry, we now describe an
isomeric C1 geometry with an A’’ electronic ground state
(Figure 3). Both were found by DFT [M06-2X/6-311G(d,p)] ge-
ometry optimization and neither has any imaginary vibrational
[a] At 298.15 K, relative to the potential energy DE of the starting materi-
als (CB11H12À +TfOMe, DE=À1319.573650 Hartree).
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Table 3. Proton hyperfine coupling constants (G) in the A’ and A’’ forms
C
of CB11(CH3)12 in positions 1, 2, 7, and 12.
a
aH(a)
calcd.[a]
aH(a)/aH(7)
calcd.[a]
aH(a)/aH(7)
obsd.[b]
A’
A’’
ave.
A’
A’’
ave.
1
2
7
12
0.21
2.61
4.75
2.48
0.32
3.60
4.97
0.49
0.27
3.10
4.86
1.48
0.04
0.55
1
0.06
0.72
1
0.05
0.64
1
0.18Æ0.09
0.71Æ0.02
1.00Æ0.03
0.52Æ0.05
0.52
0.10
0.31
Figure 3. Geometries of the A’ (left) and A’’ (right) forms of Jahn–Teller dis-
[a] Calculated using the B3LYP/EPRIII method[25, 26] at an M06-2X/6-
311G(d,p)[27] optimized geometry, assuming an equal mixture of the two
forms of the radical. [b] Ref. [17].
C
torted CB11(CH3)12
.
frequencies (Figure 3). The A’ ground-state isomer is slightly
more distorted from the Cs symmetry (the methyl groups in A’
are less overlapped than in A’’). In the A’ ground-state isomer
the BÀB neighbor distances in the proximate pentagon (B2–
B6) range from 1.754 to 1.783 ꢃ and in the A’’ ground-state
isomer they lie between 1.753 and 1.820 ꢃ. Neighbor distances
in the B7–B11 pentagon are between 1.776 and 1.811 ꢃ for the
A’ isomer and between 1.761 and 1.836 ꢃ for the A’’ isomer.
Discussion
An intermediate in BH methylation
The results described above provide firm experimental evi-
À
dence that the methylation of BH vertices in CB11H12 and its
C
The variation is smaller than in the parent CB11H12 radical at
derivatives is not a one-step process but proceeds through an
intermediate, the lifetime of which is sufficient for hydrogen
exchange between the incoming methyl group and the react-
ing vertex. It is reasonable to accept the computational result
and take this intermediate to be a triangular s complex in
which a methyl cation has been added to the BH bond. The
computed relative energies of the intermediates formed in the
the B3LYP/6-31G* level of calculation.[24] The differences ob-
served for the A’ isomer are smaller than those obtained for
the A’’ isomer, similarly to the parent CB11H12 . The potential
C
energy DE value for the A’ minimum is 0.45 kcalmolÀ1 below
that of the A’’ minimum, but the order of the free energy DG8
values, which include the ZPE, thermal corrections, and entro-
py, is the opposite: the A’’ minimum lies below the A’ mini-
mum, by 0.38 kcalmolÀ1 at 77 K and by 0.51 kcalmolÀ1 at
298 K. Spin density distributions at both geometries are pre-
sented in Figure 4.
À
three inequivalent positions in CB11H12 follow the well-estab-
lished order of their reactivity in electrophilic substitution, 12>
7>2.
The nature of the hydrogen scrambling process suggested
by the DFT results also appears plausible, and the transition
state can be viewed as a complex of a methane molecule with
a naked boron vertex or, even better, as a complex of a hydro-
gen molecule (atoms H1 and H2, which are only 0.98 ꢃ apart)
with a carboranylmethyl cation. The calculated isotope effects
on the rate and equilibrium in the hydrogen scrambling reac-
tion are small and nearly the same in all three positions. The
proposed mechanism for the methylation reaction is summar-
ized in Scheme 3.
The present experimental and computational study is limited
to establishing the existence of an intermediate in the methyl-
ation of a BH vertex in a closo-carborane and proposing
a mechanism for proton scrambling between the boron vertex
and the methyl substituent. It is likely that the nature of the in-
termediate in the methylation reaction is prototypical of other
electrophilic substitution reactions on BH vertices of cage bor-
anes and carboranes and that the main features of the results
are generally applicable.
Figure 4. Spin density distribution in A’ (left) and A’’ (right) minimum geom-
C
etry of CB11(CH3)12
.
The relative proton hyperfine coupling constants for both
geometries and their averages are compared with experimen-
tal values in Table 3.
Additional experimental and computational studies will be
needed to establish the nature of the transition states that
lead from the starting materials to the intermediate by transfer
of a methyl cation from methyl triflate, and from the inter-
mediate to the products by transfer of a proton to a base. The
latter should be subject to a large primary isotope effect.[28,29]
As indicated in Scheme 3, it is not clear whether the bridging
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Figure 5. Hybridization of a vertex atom in a closo-borane (A) and in an
arene (B). In red, orbitals used to form the delocalized system; in blue, orbi-
tals used to attach a substituent; and in black, orbitals used to form the
s perimeter.
This difference has far-reaching consequences for electro-
philic substitution. In an arene, an attacking electrophile can
recruit two electrons from the electron-rich delocalized p sy-
stem and use them to make a new two-electron s bond (Whe-
land intermediate, e.g., protonated benzene[30]), thus taking
one vertex out of the delocalized system. In a closo-borane, re-
cruiting electrons from the electron-poor delocalized system is
not an option, and electrons for a new bond need to be taken
from the radial two-electron s BH bonds. For example, proto-
nation of a borane cage may occur on a single vertex, on an
edge, or on a triangular face,[2,31] but in none of these cases
does it suppress the participation of any vertices in the delo-
calized electronic system. On evaluating relative reactivities of
different vertices in an arene, it is reasonable to examine first
the energetic cost of taking a vertex out of p conjugation. In
closo-boranes and -carboranes, such a consideration is largely
irrelevant and one needs to examine first the Lewis basicity of
the BH bonds available.
Scheme 3. Proposed mechanism for the methylation of BH vertices. Both
possibilities for the loss of H+ or D+ to a base are indicated, from the
bridge position or from the methyl group.
proton or one of the other three protons on the methyl sub-
stituent is transferred from the intermediate to a base, which
of the three methyl protons is most easily removed, and
whether their positions can be interchanged by methyl rota-
tion. Until these factors are determined, the significance of the
observed isotope effects will remain ambiguous.
An examination of the kinetics of the methylation reaction,
including isotope effects, would go a long way toward eluci-
dating the situation and establishing the nature of the rate-de-
termining step in the overall process. This would be best done
on a presently unknown highly substituted derivative of
À
CB11H12 that has only three free BH vertices, one in each of
Before concluding that the commonly invoked analogy be-
tween electrophilic substitution on arenes and on closo-bor-
anes is entirely spurious, one needs to recognize that there is
more than one mechanism for electrophilic substitution on an
arene. It has just been noted that a reaction via the classical
Wheland intermediate, in which one vertex is taken out of con-
jugation (Scheme 4, mechanism (i)), is without a counterpart in
closo-borane chemistry. However, there are at least three other
fundamentally distinct mechanisms for electrophilic aromatic
substitution in the ground electronic state, and two of them
are analogous in arenes and in boranes. As in all of these pro-
cesses an initial p complex[32,33] between the reaction partners
the three inequivalent positions, or on a series of derivatives
with a single free BH vertex in one of the positions.
Analogies in electrophilic substitution on closo-boranes and
on arenes
Closo-boranes and -carboranes are frequently referred to as ar-
omatic, one of the reasons being their preference for substitu-
tion over addition reactions. The present acquisition of experi-
mental evidence for the mechanism of electrophilic methyla-
tion of the CB11H12À cage provides an opportunity for a compre-
hensive comparison of the presumably analogous electrophilic
substitution processes on closo-boranes and on arenes.
The basic similarity is shown in Scheme 1, but how much
farther does it really extend? In an arene two sp2 orbitals on
carbon make two two-electron s bonds to their carbon perim-
eter neighbors, its exo-directed sp2 hybrid is used to attach
a substituent, and its p orbital becomes a part of the delocal-
À
ized p system (Figure 5). In a species such as CB11H12 the delo-
calized orbitals are not of p character and instead are formed
from the inward-pointing sp hybrid and the two tangential
p valence electrons of boron that are of local s symmetry.
Unlike the electron-rich delocalized p system of neutral arenes,
which contain one electron per atomic orbital, the delocalized
system of closo-boranes contains only about two-thirds of an
electron per atomic orbital and is electron-poor.
Scheme 4. Mechanisms of electrophilic substitution on an arene.
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may be an intermediate or a side diversion,[34,35] certain steps
may be reversible, and one or another can be rate-determin-
ing, the actual number of possible detailed mechanisms is
even higher.
The four most important possibilities for attack by an elec-
trophile on an arene are summarized in Scheme 4. (i) The most
common[36,37] attack at one of the vertices in which its CH
bond is a bystander and two p electrons are recruited to form
a new bond in a Wheland intermediate[38] or, in some cases,
a Wheland transition state,[39] followed by deprotonation.
(ii) Electron transfer from the arene to the electrophile (E) form-
ing an arene radical cation, which then reacts with a nucleo-
phile (Nu), usually with the radical produced from the original
electrophile by electron transfer,[40–42] but possibly with another
one. There is little doubt that the electronic structure of the
classical transition state of the path that leads to the Wheland
intermediate will often have significant charge-transfer charac-
ter,[43–46] but here one considers cases in which a radical-cation/
radical-anion pair is a true intermediate.[47–50] The CH bond is
again a bystander until deprotonation takes place. (iii) Simulta-
neous attack at two vertices (usually ortho or para to each
other) by a species that not only accepts two p electrons but
also donates two electrons, forming two new bonds. The CH
bonds at both vertices again initially act as bystanders. This ini-
tial step can be followed by a variety of events, only some of
which (deprotonation with a loss of one of the newly formed
bonds[51]) lead directly to a monosubstituted product, whereas
others first lead to monocyclic or bicyclic addition products.
(iv) Insertion into a CH bond through a three-center interac-
tion, in which the p-electron system is a bystander, as in the
activation of aromatic CÀH bonds by metal complexes[52–56]
and activation of aromatic CÀH bonds for addition to CC multi-
ple bonds.[57]
Scheme 5. Mechanisms of electrophilic substitution on a closo-borane.
phenyl cation in the arene series but much more accessible en-
ergetically, and readily adds a nucleophile to yield a substitu-
tion product.
The s-bond insertion mechanism (iv)
Unlike the electron-transfer mechanism of substitution on
boron cages, which bears only a limited resemblance to its
counterpart in arene chemistry, the s-bond insertion mecha-
nism is nearly identical in the two realms. In deltahedral bor-
anes and carboranes, it is believed to proceed by the addition
of an incoming electrophile (X+) to a BH bond under the for-
mation of an intermediate with a positively charged three-
membered ring [XBH]+, and above we have provided support
for this notion (Scheme 5).
The three-center bonded intermediate [XBH]+ is believed to
be capable of losing any one of its three vertices to a Lewis
base and forming an ordinary two-electron bond between the
remaining two vertices. If X+ is lost, no net reaction results. If
H+ is lost, the substitution product with a BX bond is formed.
These two possibilities are exactly analogous to the options
available in arene chemistry. A third possibility, loss of the
boron vertex with the formation of HX, does not normally
occur in an arene, since aryl cations have very high energy. In
polyhedral borane chemistry it is, however, fairly common and
produces a cage with a naked vertex of the type discussed
above. Addition of a nucleophile NuÀ then completes a substi-
tution reaction in which BH has been converted into BNu. In
cage borane chemistry, this mechanism is traditionally referred
to as EINS (electrophile-induced nucleophilic substitution).[2,4,62]
As noted above, there are other possibilities, in that a proton
loss could occur from a substituent X, such as a methyl, ac-
companied by a shift of the bridging proton to X.
Little is known with certainty about the substitution mecha-
nisms on deltahedral boranes and carboranes, since no mecha-
nistic studies seem to have been performed. As noted above,
p electrons are absent and cannot be recruited for the forma-
tion of a new bond, excluding the above mechanisms (i) and
(iii). However, a distant analogy to mechanism (ii) and an essen-
tially exact analogy to mechanism (iv) could exist (see
Scheme 5).
The electron-transfer mechanism (ii)
Like many arenes, boranes and carboranes can be converted
by one-electron oxidation to delocalized radicals,[17,58,59] some
of which are actually isolable (Scheme 5). In arenes, the lost
electron is taken from the p system, and in cage boranes, it is
taken in part from the delocalized system of cluster orbitals
and in part from the radial BH bonds. Unlike arene radical cat-
ions, the CH and other peripheral CX bonds of which are only
weakly perturbed relative to the original arene, peripheral BX
bonds of cage borane radicals therefore have considerable
one-electron character and are capable of transferring the
group X as a radical to a suitable substrate (such as a disilane[60]
or an alkene[61]). This leaves behind a naked vertex with an
empty radial orbital on a B atom, which is analogous to the
Although this picture of electrophilic substitution on bor-
anes and carboranes seems fairly appealing, it is mostly based
on speculation and some calculations, and not on hard experi-
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mental data, which still remain to be collected. The presently
obtained evidence for the intervention of an intermediate rep-
resents a step in this direction.
Experimental Section
Calculations
Density fuÀnctional calculations for the intermediates of the reaction
of CB11H12 with TfOCH3 were performed by using the B3LYP func-
tional with the SVP basis set as defined in the Gaussian 09 program
package.[64] Solvent effects were approximated using the CPCM.[20]
Minima in DE were characterized by all positive second derivatives
and transition states by a single negative second derivative. DFT
À
Comment on the Birch reduction of 1-CH3CB11I11
The disappointing yield in the attempted reduction of this
À
anion to 1-CH3CB11D11 demonstrates the limits of the Birch re-
computations for the radical CB11Me12 used the B3LYP/EPRIII[25,26]
C
duction procedure for the conversion of BI to BH substituents.
This process is a mainstay of preparative manipulations on the
CB11 cage.[2,14] Anions with up to six iodine substituents were
reduced efficiently in reaction sequences that used the iodine
substituent as a protection of a boron vertex or for acidifica-
tion of the CH vertex, but 11 iodine atoms apparently make
the cage excessively susceptible to over-reduction. Most of the
undecaiodinated carborane anion clearly decomposed during
the reaction to form uncharged volatiles (probably B2D6 and
other low-molecular-weight boranes). This result is somewhat
reminiscent of the rapid and nearly complete decomposition
method at an M06-2X/6-311G(d,p)[27] optimized geometry.
General information and materials
All reactions were performed under an argon atmosphere with dry
solvents, freshly distilled under anhydrous conditions, unless other-
wise noted. Standard Schlenk and vacuum line techniques were
employed for all manipulations of air- or moisture-sensitive com-
pounds. Yields refer to isolated, chromatographically and spectro-
scopically homogeneous materials, unless otherwise stated.
À
À
of HCB11I11 into BF3 and BF4 upon attempted fluorination
Me3NH[CB11H12] was purchased from Katchem Ltd. (Prague, Czech
Republic). [D3]Ammonia (99 atom% D) and [D3]methyl trifluorome-
thanesulfonate (99 atom% D) were purchased from Sigma–Aldrich.
n-Butyllithium (solution in hexane), calcium hydride, cesium chlo-
ride, iodine monochloride, anhydrous sulfolane, and 1,1,2,2-tetra-
chloroethane were purchased and used without further purifica-
tion. Cs[CB11H12] and Cs[1-CH3CB11H11] were prepared from Me3NH-
[CB11H12] according to published procedures,[17,65] and all reactions
were performed on cesium salts.
with 20% F2/N2 in anhydrous HF at À788C, in which only
À
traces of the desired HCB11F11 were detected.[63] The undecaio-
dinated cage is not as robust as we initially assumed.
It is unfortunate that the simple process of D for H exchange
in CB11H12 by electrophilic substitution with a donor of D+
À
cations yields only mixtures of partly deuterated with monohy-
droxylated anions, since the EINS mechanism apparently inter-
À
feres. An efficient preparation of 1-CH3CB11D11 and similar
anions remains to be developed.
Equipment and measurements
Conclusion
The NMR spectra were measured in [D6]acetone and [H6]acetone,
and the following referencing was used: 1H, residual signal of
[D6]acetone (d=2.05 ppm); 2H, signal of [D6]acetone (d=2.05 ppm)
as an internal standard (a few drops of [D6]acetone added to
[H6]acetone); 13C, signal of a [D6]acetone deuteriomethyl group
(d=29.80 ppm); 11B, signal of BF3·Et2O as an external standard in
Experimental evidence has been found for the intervention of
an intermediate in the mechanism of BH vertex methylation
with methyl triflate, and DFT calculations strongly suggest that
its structure is characterized by a protonated CB bond. In this
species, hydrogen atoms scramble between the bridging posi-
tion and a position on the methyl group, and DFT results indi-
cate that the transition state for this process has the structure
of a distorted square-pyramidal methane attached to a naked
boron vertex, perhaps best viewed as a complex of H2 with
a carboranylmethyl cation. We have provided a general over-
view of the similarities and differences between electrophilic
substitution at a BH vertex of a closo-borane and a CH vertex
of an arene.
1
a coaxial capillary (d=0.00 ppm). H and 13C NMR spectra were re-
corded with Bruker Avance 400, 500, and 600 spectrometers work-
ing at 400.1, 499.8, and 600.1 MHz for 1H NMR spectroscopy and
100.6, 125.7, and 150.9 MHz for 13C NMR spectroscopy, respectively.
2H NMR spectra were recorded with a Bruker Avance 600 spec-
1
trometer working at 92.1 MHz. H{11B}, 11B, and 11B{1H} NMR spectra
were accumulated with Bruker Avance 400 and 500 spectrometers
working at 400.1 and 499.8 MHz for 1H NMR spectroscopy and
128.3 and 160.4 MHz for 11B NMR spectroscopy, respectively.
Carbon signals of methyl groups on carborane and the C(1) carbor-
ane vertex are usually not easily detectable directly because of
13C–11B coupling and therefore HSQC and HMBC were used for the
assignment of 13C NMR resonances. Carbon signals of CD3 and
CHD2 groups on carborane were not detected in some cases be-
cause of strong 13C–2H and 13C–11B coupling. Assignments of boron
signals were done by 11B,11B COSY NMR spectroscopy.
À
We have also noted the instability of the HCB11I11 anion to
Birch reduction conditions and have improved the description
C
of the CB11Me12 radical by finding a second isomeric structure
for its Jahn–Teller distorted ground state. Averaged over both
nearly isoenergetic structures, the computed relative proton
hyperfine coupling constants agree fairly closely with those
observed.
The content of 2H was determined by elemental analysis as 1H,
since this method is not able to recognize the difference between
1
2H and H (both of these species were converted to water during
the analysis and its amount was finally determined by thermal con-
ductometry; the thermal conductivity of 1H2O and 2H2O is the
same).
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of the reduction, excess sodium was carefully quenched by slow
addition of [D4]MeOH (ꢀ5 mL) until the dark blue color disap-
peared. Cooling was interrupted and the ammonia was carefully al-
lowed to evaporate through a bubbler filled with mineral oil. Vola-
tiles were then removed under reduced pressure. The yellow oily
residue was washed with ether (4ꢄ25 mL) and the yellowish ethe-
real phase was washed with 20% aqueous CsCl (2ꢄ8 mL). The
combined CsCl wash was extracted with ether (2ꢄ30 mL). All ethe-
real phases were combined and solvent was removed under re-
duced pressure. The residue was dried thoroughly using a Kugel-
rohr distillation apparatus (2 h, 1608C, 500 mTorr). The crude prod-
uct (190 mg) was used in the following reaction without further
purification. MS (ESI-): m/z (%): 167.3 (center of isotope cluster);
General procedure for iodination of all BH vertices (GP 1)
A procedure previously published by Wilbur et al.[7] was adapted as
follows. A flame-dried and argon-filled high-pressure Schlenk flask
was charged with dry Cs[1-RCB11H11] (R=H or CH3; 4.000 mmol)
and 1,1,2,2-tetrachloroethane (25 mL). Subsequently, ICl (7.02 mL,
140.000 mmol) was added slowly by syringe. An exothermic iodina-
tion caused massive evolution of gases (according to ESI-MS, the
[1-RCB11H5I6] anion (R=H or CH3) was formed quantitatively during
that time). The Schlenk flask was sealed tightly and the dark reac-
tion mixture was stirred vigorously for 3 days at 1508C. A brown
solid precipitated during that time. The reaction mixture was dilut-
ed with 1,1,2,2-tetrachloroethane (15 mL) and transferred to
a 50 mL round-bottom flask. The temperature of the oil bath was
increased to 1908C and all volatiles were thoroughly removed by
distillation at atmospheric pressure. The brown solid residue was
dissolved in methanol (40 mL), all volatiles were evaporated using
rotary evaporator, and the brownish solid was then triturated with
ice-cold ether (2ꢄ15 mL). The resulting gray solid was suspended
in boiling deionized water (30 mL) and dissolved by careful addi-
tion of methanol. The volume of the solution was reduced to ap-
proximately 25%. The precipitated solid was isolated by filtration,
washed with deionized water (3ꢄ15 mL), ether (1ꢄ5 mL), and
hexane (3ꢄ15 mL), and finally dried thoroughly by using Kugelrohr
distillation apparatus (4 h, 1608C, 500 mTorr) to afford Cs[1-
RCB11I11] as snow-white solids.
1
2
11
HRMS (ESI-) for (C2 H3 H11
170.28160.
B
11
À): m/z calcd: 170.28150; found:
Cesium 1-[H3]methyl-2,3,4,5,6,7,8,9,10,11,12-[D33]undecamethyl-
carba-closo-dodecaborate: An argon-filled flask was charged with
CaH2 (2.105 g, 50.000 mmol) and dry sulfolane (8 mL) was added
by syringe. Subsequently, dry crude Cs[1-CH3-CB11D11] from previ-
ous reaction (ꢀ0.470 mmol) was added. Finally, TfOCD3 (1.13 mL,
10.000 mmol) was added slowly by syringe to the vigorously
stirred white suspension over a period of 20 min. The grayish reac-
tion mixture was stirred vigorously at room temperature for 5 days.
The solution turned brown during that time. Completion of the
methylation was checked by ESI-MS. If some less methylated clus-
ters were still present, another portion of TfOCD3 (2.000 mmol) was
added and stirring was continued for an additional 2 days. The
dense reaction mixture was then diluted with CH2Cl2 (50 mL), and
the solids were removed by vacuum filtration and washed with
CH2Cl2 (4ꢄ50 mL). The filtrate was neutralized with saturated aque-
ous NH3 (30 mL) and volatiles (CH2Cl2 and water) were removed
under reduced pressure. The brown oily residue was washed with
ether (5ꢄ50 mL) and the combined ethereal phases were washed
with 20% aqueous CsCl (2ꢄ20 mL). The brown oil that was left
after the first extraction was dissolved in the combined CsCl wash
and the solution was extracted again with ether (3ꢄ50 mL). All
ethereal phases were combined and the solvent was removed
under reduced pressure, to leave a brownish oily residue. Most of
the sulfolane was distilled from this residue by using a Kugelrohr
distillation apparatus (4 h, 1658C, 500 mTorr). The brown solid resi-
due containing a small amount of sulfolane was then recrystallized
from boiling deionized water. The gray crystals were isolated by fil-
tration, subsequently washed with ice-cold deionized water (3ꢄ
10 mL) and hexane (3ꢄ10 mL), and dried thoroughly by using a Ku-
gelrohr distillation apparatus (6 h, 1658C, 500 mTorr). Final purifica-
tion was performed by column chromatography on silica gel
(CH2Cl2). Cs[1-CH3CB11(CD3)11] was obtained as a snow-white crystal-
line solid (28 mg, 0.059 mmol, 2% after two steps). 1H NMR
(499.8 MHz, [D6]acetone): d=0.81 ppm [s, 3H; C-CH3]; 2H NMR
(92.1 MHz, [H6]acetone): d=À0.54 [br s, 3D; B-CD3(12)], À0.44 [br
s, 15D; B-CD3(7–11)], À0.36 ppm [br s, 15D; B-CD3(2–6)]; 11B NMR
(160.4 MHz, [D6]acetone): d=À10.06 [br s, 5B; B-CD3(2–6)], À7.99
[br s, 5B; B-CD3(7–11)], 0.09 ppm [br s, 1B; B-CD3(12)]; 13C NMR
(125.7 MHz, [D6]acetone): d=13.3 [C-CH3], 56.6 ppm [C-CH3]; IR
Cesium 2,3,4,5,6,7,8,9,10,11,12-undecaiodocarba-closo-dodeca-
borate:[7] Prepared from Cs[CB11H12] (1.100 g, 3.987 mmol) and ICl
(7.02 mL, 140.000 mmol) in 1,1,2,2-tetrachloroethane (25 mL) ac-
cording to GP 1 as a snow-white solid (6.490 g, 3.908 mmol, 98%).
1H NMR (400 MHz, [D6]acetone): d=4.21 ppm [s, 1H; C-H]; 11B NMR
(128 MHz, [D6]acetone): d=À19.74 [br s, 5B; B-I(2–6)], À12.86 [br
s, 5B; B-I(7–11)], À8.45 ppm [br s, 1B; B-I(12)]; 13C NMR (151 MHz,
~
[D6]acetone): d=62.1 ppm [C-H]; IR (KBr): n=3012 (m) and 3002
(w, n(C-H)), 1098 (m, d(C-H)), 1037 (s), 919 (vs), 542 cmÀ1 (vw, n(B-
I)); MS (ESI-): m/z (%): 1529.4 (center of isotope cluster); HRMS (ESI-
) for (HCB11I11À): m/z calcd: 1530.05988; found: 1530.05608; ele-
mental analysis calcd (%) for CsCHB11I11: C 0.72, H 0.06, I 84.05;
found: C 0.81, H 0.19, I 84.07.
Cesium 1-[H3]methyl-2,3,4,5,6,7,8,9,10,11,12-undecaiodo-carba-
closo-dodecaborate: Prepared from Cs[1-CH3CB11H11] (700 mg,
2.414 mmol) and ICl (4.26 mL, 85.000 mmol) in 1,1,2,2-tetrachloro-
ethane (15 mL) according to GP 1 as a snow-white solid (3.013 g,
1
1.799 mmol, 75%). H NMR (400 MHz, [D6]acetone): d=2.17 ppm [s,
3H; C-CH3]; 11B NMR (128 MHz, [D6]acetone): d=À15.49 [br s, 5B;
B-I(2–6)], À12.17 [br s, 5B; B-I(7–11)], À8.73 ppm [br s, 1B; B-I(12)];
13C NMR (125.7 MHz, [D6]acetone): d=35.4 [C-CH3], 62.4 ppm [C-
~
CH3]; IR (KBr): n=2997 (vvw, nas(CH3), C-CH3), 1430 (vw, das(CH3), C-
CH3), 1382 (w, ds(CH3), C-CH3), 1125 (w), 1094 (vw), 976 (w), 948 (s),
933 (vs), 532 cmÀ1 (w, n(B-I)); MS (ESI-): m/z (%): 1542.1 (center of
isotope cluster); HRMS (ESI-) for (C2H3B11I11À): m/z calcd:
1544.07553; found: 1544.07424; elemental analysis calcd (%) for
CsC2H3B11I11: C 1.43, H 0.18, I 83.35; found: C 1.42, H 0.21, I 83.61.
~
(KBr): n=2938 (s, nas(CH3), C-CH3), 2182 (vs, nas(CD3), B-CD3), 2120 (s,
Cesium
1-[H3]methylcarba-closo-[D11]dodecaborate:
Sodium
ns(CD3), B-CD3), 2054 (m), 1376 (w, ds(CH3), C-CH3), 1223 (vs), 1166
(vs), 1061 (m, das(CD3), B-CD3), 1044 (m, 1(CH3), C-CH3), 1011 (w),
980 (w, ds(CD3), B-CD3), 786 cmÀ1 (w, n(B-CD3), B-CD3); MS (ESI-): m/z
(%): 344.6 (center of isotope cluster); HRMS (ESI-) for
(1.241 g, 54.000 mmol) freshly cut into small pieces was carefully
dissolved in liquid [D3]ammonia (ꢀ15 mL) in a high-pressure
Schlenk flask at À788C. The deep blue solution was stirred vigo-
rously at this temperature for 10 min with a glass stirrer. Subse-
quently, a solution of Cs[1-CH3CB11I11] (4.523 g, 2.701 mmol) in dry
[D8]THF (6 mL) was added dropwise to the reaction mixture under
vigorous stirring, which was continued for an additional 10 min.
Progress of the reduction was checked by ESI-MS. On completion
2
11
(C131H3 H33
B
À): m/z calcd: 346.59174; found: 346.59163; elemen-
11
tal analysis calcd (%) for CsC131H3 H33B11: C 32.70, H 8.17, Cs 27.84;
2
found: C 32.94, H 8.28, Cs 27.96.
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À
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TfOCD3 (500 mL, 4.4 mmol) was added to dry toluene (10 mL,
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1
2
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Keywords: alkylation · boranes · carboranes · electrophilic
substitution · hydrogen scrambling
[1] R. N. Grimes, Carboranes, 2nd ed., Academic Press, Oxford, 2011.
[2] C. Douvris, J. Michl, Chem. Rev. 2013, DOI: 10.1021/cr400059k.
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Received: June 5, 2013
Revised: July 8, 2013
Published online on && &&, 0000
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J. Kaleta, A. Akdag, R. Crespo,
M.-C. Piqueras, J. Michl*
&& – &&
Evidence for an Intermediate in the
À
Methylation of CB11H12 with Methyl
Scrambled protons: Methylation of
HCB11H12 and similar anions with deu-
strating that the reaction mechanism in-
volves an intermediate in which protons
can scramble. A comparison with elec-
trophilic substitution on arenes shows
similarities and differences.
Triflate: Comparison of Electrophilic
Substitution in Cage Boranes and in
Arenes
À
terated methyl triflate (CD3OTf) produ-
ces a mixture of CD3- and CHD2-substi-
tuted anions (see scheme), thus demon-
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