Lawrence Berkeley National Laboratory
Recent Work
Title
Cage-Walking: Vertex Differentiation by Palladium-Catalyzed Isomerization of B(9)-Bromo-
meta-Carborane.
Permalink
https://escholarship.org/uc/item/0x57v855
Journal
Journal of the American Chemical Society, 139(23)
ISSN
0002-7863
Authors
Dziedzic, Rafal M
Martin, Joshua L
Axtell, Jonathan C
et al.
Publication Date
2017-06-01
DOI
10.1021/jacs.7b04080
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
Cage-Walking: Vertex Differentiation by Palladium-Catalyzed
Isomerization of B(9)-Bromo-meta-Carborane
Rafal M. Dziedzic,
†
Joshua L. Martin,
†
Jonathan C. Axtell,
†
Liban M. A. Saleh,
†
Ta-Chung Ong,
†
Yun-Fang Yang,
†
Marco S. Messina,
†
Arnold L. Rheingold,
‡
K. N. Houk,
†
and Alexander M. Spokoyny*
,†,§
†
Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles,
California 90095, United States
‡
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093,
United States
§
California NanoSystems Institute (CNSI), University of California, Los Angeles, 570 Westwood Plaza, Los Angeles, California
90095, United States
*
S
Supporting Information
ABSTRACT: We report the first observed Pd-catalyzed
isomerization (“cage-walking”) of B(9)-bromo-meta-car-
borane during Pd-catalyzed cross-coupling, which enables
the formation of B−O and B−N bonds at all boron
vertices (B(2), B(4), B(5), and B(9)) of meta-carborane.
Experimental and theoretical studies suggest this isomer-
ization mechanism is strongly influenced by the steric
crowding at the Pd catalyst by either a biaryl phosphine
ligand and/or substrate. Ultimately, this “cage-walking”
proces s provides a unique pathway to preferentially
introduce functional groups at the B(2) vertex using
B(9)-bromo-meta-carborane as the sole starting material
through substrate control.
I
somerization mechanisms such as chain-walking via β-
hydride elimination/reinsertion and aryne-based rearrange-
ments (Figure 1A) are ubiquitous in metal-catalyzed trans-
formations of organic molecules.
1,2
Through judicious choice of
catalyst design, these mechanistic pathways can be biased to
form specific regioisomers. Thus, metal-catalyzed isomerization
control can provide a means of incorporating functional groups
in molecules at positions remote from where initial bond
activation occurs.
1−3
Boron clusters are unique molecular scaffolds that feature
three-dimensional (3D) electronic delocalization.
4
Specifically,
in the case of icosahedral carboranes (C
2
B
10
H
12
)this
delocalization is nonuniform.
5
This charge distribution makes
carboranes an interesting alternative to classical carbon-based
structural building blocks such as aryl and alkyl groups.
6
Because of their inherent robustness, carboranes can be
promising molecular building blocks for applications ranging
from pharmacophores to photoactive materials.
7
Ultimately,
vertex-specific functionalization routes (vertex differentiation)
are critical for constructing carborane-containing molecules and
materials.
7,8
Recent developments in carborane functionalization have
relied on several metal-catalyzed routes, including B− H
activation (either directed or undirected) and cross-coupling
of halogenated carborane electrophiles at both C and B
vertices.
8,9
Even so, these approaches provide limited access to
rational, vertex-specificB−H functionalization. Surprisingly,
metal-catalyzed isomerization reactivity commonly observed
with classical organic substrates (vide supra) has never been
reported for any boron cluster systems, including carboranes.
Herein we disclose our discovery of a Pd-catalyzed activation of
B(9)-bromo-meta-carborane (Br−B(9)), which can undergo
subsequent “cage-walking”, leading to the formation of B(2)-,
B(4)-, B(5)-, and B(9)-functionalized clusters in the presence
of a suitable nucleophile (Figure 1B).
Recently we reported the Pd-catalyzed cross-coupling of Br−
B(9) to generate B(9) −O and B(9)−N bonds with a wide
range of substrates.
9
This cross-coupling relied on biaryl
phosphine ligands to generate monoligated palladium(0)
species ([LPd]) capable of undergoing oxidative addition
into the B−Br bond of Br−B(9). To our surprise, during the
course of subsequent investigations, when the DavePhos (L1)
or SPhos (L2) ligand was replaced with the bulkier XPhos
Received: April 25, 2017
Published: May 25, 2017
Figure 1. (A) Pd-catalyzed olefin isomerization through β-hydride
elimination and arene regioisomer formation through a proposed
ben zyne intermediate. (B) Pd-catalyzed isomerization of meta-
carboranyl through “cage-walking”.
Communication
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congener (L3) in the presence of alcohol or amine substrates,
we consistently observed not one but rather three distinct peaks
with identical m/z by gas chromatography−mass spectrometry
(GC−MS). For example, using 3,5-dimethylphenol (R1)asa
cross-coupling partner with Br−B(9), we observed several
products with identical m/z (see the Supporting Information
(SI)). Upon chromatographic separation of the reaction
mixture on silica gel, we identified four distinct R1-carborane
compounds by
11
B,
1
H, and
13
C NMR spectroscopy (Figure 2).
The isolated carborane-containing molecules show a distinct
downfield singlet in the
11
B NMR spectrum corresponding to
R1 bound at a B(2), B(4), B(5), or B(9) vertex of meta-
carborane (R1−B(2), R1−B(4), R1−B(5), and R1−B(9),
respectively). Although we were unable to chromatographically
separate R1−B(5) and R1−B(4), we identified the isomer ratio
as 15:85 by
11
B and
1
H NMR spectroscopy: R1−B(4) is C
1
-
symmetric, resulting in 10
11
B NMR resonances (one singlet
and nine doublets), whereas R1−B(5) contains a mirror plane,
resulting in six
11
B NMR resonances (one singlet and five
doublets). Thus, the more intense singlet at ∼3 ppm ( Figure
2B) is assigned to the dominant pattern of R1−B(4). Similarly,
two sets of
1
H and
13
C NMR resonances corresponding to R1−
B(5) and R1−B(4) were observed in a 15:85 signal ratio for
the CH aromatic and aliphatic regions, respectively (see the SI).
These structural assignments are further supported by single-
crystal X-ray diffraction studies of the four regioisomers (Figure
2C). Interestingly, R1−B(4) is the only monofunctionalized
meta-carborane regioisomer that exhibits chirality. R1−B(4)
crystallized as two distinct polymorphs, with both polymorphs
containing equal amounts of the two enantiomers in the unit
cell. Chiral HPLC analysis further supports the presence of two
R1−B(4) enantiomers in the isolated mixture (Figure S12).
To further assess the generality of this isomerization process,
we examined three biaryl phosphine ligands and several
substrates to ge nerate B−O- and B− N-bound carborane
regioisomers (Figure 3). Consistent with our previous report,
[L1Pd] and [L2Pd] generate B(9) isomers almost exclusively
with O- and N-based nucleophiles.
9
However, [L3Pd]
generates appreciable amounts of regioisomers under the
same conditions. Noteworthy was the presence of bromo-
meta-carborane regioisomers when the cross-coupling reactions
were stopped early, indicating that isomerization of Br−B(9)
occurs in addition to the cross-coupling reaction. Furthermore,
Br−B(9) forms bromo-meta-carborane isomers in the presence
of [L3Pd] precatalyst and triethylamine, implying that
isomerization can occur prior to transmetalation of a cross-
coupling partner and subsequent reductive elimination of the
B-functionalized meta-carborane. Hence, this metal-catalyzed
isomerization may provide a convenient pathway to B(2)-,
B(4)-, and B(5)-functionalized meta-carborane species that
circumvents laborious and often low-yielding protocols such as
deboronation/capitation o r thermal isomerization strat-
egies.
10,11
Since bromo-meta-carboranyl isomerization can occur before
all of the carborane regioisomers are depleted by cross-coupling
(vide supra), we hypothesized that the isomerization process
might operate separately from the main cross-coupling cycle.
To further explore the isomerization mechanics, we attempted
Figure 2. (A) Reaction conditions that result in the formation of R1-meta-carborane regioisomers. (B)
11
B NMR spectra of the isolated regioisomers.
Singlet resonances (no
11
B−
1
H coupling) corresponding to the B −O-bonded vertex are labeled; all other resonances correspond to doublet
resonances arising from
11
B−
1
H couplings. (C) Single-crystal X-ray structures of R1-B(n), n = 2, 4, 5, 9 (ellipsoids drawn at 50% probability and H
atoms omitted for clarity).
Figure 3. Reaction conditions for forming B-functionalized meta-
carborane isomers using different substrates (R1−R3) and biaryl
phosphine ligands (L1−L3). Yields were obtained by GC−MS. See
the SI for full experimental conditions.
Journal of the American Chemical Society Communication
DOI: 10.1021/jacs.7b04080
J. Am. Chem. Soc. 2017, 139, 7729− 7732
7730
to inhibit transmetalation by increasing the steric bulk of the
cross-coupling partner, thereby allowing the active catalyst
species to operate in the isomerization pathway for a longer
time (Figure 4, step II). Indeed, cross-coupling reactions using
bulky L3 and sterically congested 2,6-dimethylphenol (R3)
yielded R3−B(2) as the major product (Figure 3). As a control
experiment, equimolar amounts of 3,5-dimethylphenol (R1)
and 2,4,6-trimethylphenol (R3′, a variant of R3 to permit
separation of the products by GC−MS) were reacted with Br−
B(9) in the presence of [L3Pd] and K
3
PO
4
in 1,4-dioxane at 80
°C(Figures S5 and S6). GC−MS analysis of the reaction
mixture showed complete consumption of Br−B(9) with R1-
meta-carborane isomers as the major products, suggesting that
the size of the nucleophile is linked to the rate of product
formation. Since oxidative addition is likely rapid in this
process,
12
it appears that by decreasing the rate of trans-
metalation and/or reductive elimination one can increase the
yield of the B(2) regioisomer (Figure 4B). This type of Pd-
catalyzed remote vertex functionalization is unprecedented and
demonstrates the utility of a metal-catalyzed route to meta-
carborane vertex differentiation. Importantly, it contrasts with
known thermal rearrangements that are limited to thermally
resistant functional groups (above 300 °C) and produce isomer
mixtures with B(2) substituted meta -carboranes as the minor
product.
11
We attribute this difference in reactivity between “cage-
walking” (when using L3) and cross-coupling exclusively at the
B(9) vertex (when using L1/L2) to steric crowding at the Pd
center. The combination of a sterically demanding ligand and
nucleophile appears to inhibit transmetalation,
13
allowing the
catalyst to operate through several “cage-walking” steps before
re-entering the traditional cross-coupling cycle (vide supra). On
the basis of these observations, we propose a Pd-catalyzed
“cage-walking” mechanism for isomerization of Br−B(9)
(Figure 4B). Beginning with the oxidative addition complex
[LPdBr−B(9)], an open Pd(II) coordination site is generated
by bromide dissociation
2d
(Figure 4, step II-a) to form [LPd−
B(9)]
+
. Consistent with this hypothesis, cross-coupling experi-
ments between Br− B(9) and R3 inthepresenceof
tetrabutylammonium bromide sh ow decreased Br−B(9)
consumption and decreased formation of R3-meta-carborane
(Figure S7). These experiments suggest that bromide
dissociation is an important step in the overall cross-coupling
process.
14
After bromide dissociation, two possible “cage-
walking” pathways were envisioned for the formally cationic
[LPd−B(9)]
+
: (1) deprotonation of an adjacent B−H vertex to
form a B(4),B(9)-bound carborane species that isomerizes
upon reprotonation to form [LPd−B(4)]
+
(Figure S9) and (2)
a Pd-mediated B−H activation that leads to an intramolecular
β-hydride shift (Figure S10). Deuterium labeling experiments
in which 2,6-Me
2
C
6
H
4
OD was used as the nucleophile did not
result in deuterium incorporation at any B−H vertex, as judged
by GC−MS and
2
H and
11
B NMR spectroscopy, likely ruling
out isomerization pathway 1. However, with the deuterated
congener of Br−B(9), 9-Br-10-D-meta-C
2
B
10
H
10
, and 2,6-
Me
2
C
6
H
4
OH as the nucleophile, we observed five B−
2
H
resonances in the
2
H{
11
B} NMR spectrum of R3−B(2),
indicating deuterium scrambling across the carborane B−H
framework (Figure 4A). We postulate that this β-hydride shift
exchanges the B(10) deuterium with an adjacent B(5) proton
and enables “cage-walking” to form [LPd−B(4)]
+
(Figure 4 B,
step II-b). The “cage-walking” process can occur again to
generate [LPd−B(2)]
+
(Figure 4B, step II-c). Similar reports of
metal-catalyzed carborane B−H activation processes have been
reported;
8,15,16
however, they are limited to B−H vertices
adjacent to carborane-bound directing groups, whereas the
presently reported “cage-walking” accesses all of the meta-
carborane B−H vertices from one starting point in a diversity-
oriented fashion.
Through the “cage-walking” process, the carboranyl fragment
eventually binds the Pd center through the most electron-
deficient boron vertex, B(2), resulting in a more electrophilic
Pd center that can overcome the steric repulsion between the
cationic [LPd−B(2)]
+
and the anionic cross-coupling partner.
Density functional theory (DFT) calculations (B3LYP/
LANL2DZ 6-31G* and M06/SDD/6-311++G**, SMD(1,4-
dioxane)) on [LPd−B(9)]
+
,[LPd−B(4)]
+
, and [LPd−B(2)]
+
indicate that [LPd−B(2)]
+
has the most cationic Pd center,
which likely results in a lower transmetalation barrier due to a
stronger electrostatic attraction between the Pd center and the
Figure 4. (A) Deuterium labeling experiments. The resonance at 2.7 ppm is present from polydeuterated Br−B(9). See the SI for full experimental
details. (B) Proposed metal-catalyzed isomerization of bromo-meta-carborane through a “cage-walking” mechanism: (I) oxidative addition; (II-a)
bromide dissociation; (II-b, II-c) “cage-walking”; (II-d) bromide association; (III) transmetalation; (IV) reductive elimination.
Journal of the American Chemical Society Communication
DOI: 10.1021/jacs.7b04080
J. Am. Chem. Soc. 2017, 139, 7729− 7732
7731
phenoxide nucleophile (Figures S13−S16). Furthermore, the
ΔG of B−O and B−N bond formation decreases accordingly,
B(9) > B(5) ∼ B(4) > B(2), for the cross-coupling between
Br−B(9) and R1−R3. Similar electronic effects of substrate
and ligand were observed in Pd-catalyzed aryl halide cross-
coupling.
17
In summary, we have discovered the first example of metal-
catalyzed isomerization (“cage-walking”)ofmeta-carboranyl
fragment. The isomerization process appears to operate in
conjunction with a classical cross-coupling mechanism, leading
to a distribution of carborane regioisomers. The rate of cross-
coupling relative to “cage-walking” can be adjusted to achieve
selective B-vertex functionalization. We have demonstrated this
selectivity by controlling the steric crowding at the Pd center by
appropriate choice of catalyst ligand and cross-coupling
substrate. Preliminary studies have shown that this “cage-
walking” strategy can be applied to carborane B(2)−C
aryl
bond
formation using an arylboronic acid (Figure S17). Overall, this
approach provides a unique pathway to vertex differentiation of
boron clusters.
■
ASSOCIATED CONTENT
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b04080.
Full procedures and additional data (PDF)
Crystallographic data (CIF)
■
AUTHOR INFORMATION
Corresponding Author
*spokoyny@chem.ucla.edu
ORCID
Yun-Fang Yang: 0000-0002-6287-1640
K. N. Houk: 0000-0002-8387-5261
Alexander M. Spokoyny: 0000-0002-5683-6240
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank the donors of the American Chemical Society
Petroleum Research Fund (56562-DNI3 to A.M.S.), UCLA
(startup funds to A.M.S.), NSF (CHE-1048804 and
CHE1361104), 3M (Non-Tenured Faculty Award to A.M.S.),
and the National Defense Science and Engineering Graduate
Fellowship Program (to R.M.D.) for support.
■
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Journal of the American Chemical Society Communication
DOI: 10.1021/jacs.7b04080
J. Am. Chem. Soc. 2017, 139, 7729− 7732
7732