Unprecedented boron-functionalized carborane derivatives by facile and selective cobalt-induced B-H activation

Article abstract of DOI:10.1021/ja4047075

The 16-electron complex CpCoS₂C₂B₁₀H ₁₀ (1) is found to react with the alkynes HCi - CC(O)R [R = methyl (Me), phenyl (Ph), styryl (St), ferrocenyl (Fc)] at ambient temperature to give two types of 17-electron cobalt complexes 2a-d and 3a-d containing unique B(3)/B(6)-norbornyl carborane moieties. A formation mechanism via a tandem sequence of metal-induced B-H activation, B-Cp formation, Cp delivery and Diels-Alder addition is proposed on the basis of DFT calculations. The reactivity of these paramagnetic 17-electron complexes has been studied: Exposed to a combination of air, moisture and silica, complexes 2a-d undergo alkyl C-S cleavage to give 16-electron complexes 4a-c containing a boron-norbornadienyl moiety, and simultaneous carboranyl C-S cleavage to afford cobalt-free carborane derivatives 5a-d containing a boron-norbornyl unit. Both 2a-d and 3a-d allow further alkyne insertion into the Co-S bond to generate cobalt-free boron-norbornyl carborane derivatives (Z/E)-7a-d and (Z/E)-8a-d, both containing a vinyl sulfido group. Addition of AlCl₃ not only promotes the conversion of 2a-d, but also leads predominantly to (E)-9a-d as retro-Diels-Alder products. Upon heating, the isomerization from E to Z-configuration of the vinyl group and reorganization of the norbornyl moiety of (Z/E)-7a-d occur to lead to (Z)-9a-d as well as the unexpected [1,2]-H shifted products (Z)-10b,c. Thus, the 17-electron complexes 2a-d and 3a-d serve as intermediates for synthesis of variety of boron-functionalized carborane derivatives. In this study, efficient routes have been developed through cobalt-mediated B-H activation to prepare boron-functionalized carborane derivatives that are unavailable by conventional routes.

Full text of DOI:10.1021/ja4047075

Article
pubs.acs.org/JACS
Unprecedented Boron-Functionalized Carborane Derivatives by
Facile and Selective Cobalt-Induced B−H Activation
Zhaojin Wang, Hongde Ye, Yuguang Li, Yizhi Li, and Hong Yan*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
Jiangsu 210093, China
S
* Supporting Information
ABSTRACT: The 16-electron complex CpCoS2C2B10H10 (1)
is found to react with the alkynes HCCC(O)R [R = methyl
(Me), phenyl (Ph), styryl (St), ferrocenyl (Fc)] at ambient
temperature to give two types of 17-electron cobalt complexes
2a−d and 3a−d containing unique B(3)/B(6)-norbornyl
carborane moieties. A formation mechanism via a tandem
sequence of metal-induced B−H activation, B−Cp formation,
Cp delivery and Diels−Alder addition is proposed on the basis
of DFT calculations. The reactivity of these paramagnetic 17-
electron complexes has been studied: Exposed to a
combination of air, moisture and silica, complexes 2a−d
undergo alkyl C−S cleavage to give 16-electron complexes 4a−
c containing a boron-norbornadienyl moiety, and simultaneous carboranyl C−S cleavage to afford cobalt-free carborane
derivatives 5a−d containing a boron-norbornyl unit. Both 2a−d and 3a−d allow further alkyne insertion into the Co−S bond to
generate cobalt-free boron−norbornyl carborane derivatives (Z/E)-7a−d and (Z/E)-8a−d, both containing a vinyl sulfido group.
Addition of AlCl3 not only promotes the conversion of 2a−d, but also leads predominantly to (E)-9a−d as retro-Diels−Alder
products. Upon heating, the isomerization from E to Z-configuration of the vinyl group and reorganization of the norbornyl
moiety of (Z/E)-7a−d occur to lead to (Z)-9a−d as well as the unexpected [1,2]-H shifted products (Z)-10b,c. Thus, the 17-
electron complexes 2a−d and 3a−d serve as intermediates for synthesis of variety of boron-functionalized carborane derivatives.
In this study, efficient routes have been developed through cobalt-mediated B−H activation to prepare boron-functionalized
carborane derivatives that are unavailable by conventional routes.
from a major restriction that the key starting material 3-iodo-o-
carborane was typically synthesized by deboronation (using a
base) to form nido-carborane, followed by cage reconstruction
using BI3, thus limiting options for further substitution in these
two positions.10 Moreover, metal-promoted selective boron-
substitution has proven difficult although significant advances
have been achieved,11−14 among which boron-functionalization
by organic groups was rare.13,14
Selective B−M (M = Rh, Ir, Ru, Os) and B−C (B-alkyl and
B-vinyl) formation via intramolecular metal-induced o-carbor-
ane B(3)/B(6)−H activation has been reported.15 Recent
advances in cobalt-induced B−H/C−H activation leading to
B−C coupling and cobalt-bound-Cp (Cp = cyclopentadienyl)
involved Diels−Alder reaction leading to boron-norbornyl
formation have brought renewed interest.16,17 Of these
examples, the formation of unprecedented boron-norbornyl
group observed in the reaction of CpCoS2C2B10H10 and HC
CC(O)Ph appeared far from a conventional route,17 therefore
inspiring us to make further exploration. In this study we have
extended the scope of alkynes, investigated the complicated
INTRODUCTION
■
Dicarba-closo-dodecaboranes (o-, m-, and p-carboranes) have
been used for decades as versatile building blocks for
construction of functional materials and pharmaceuticals1−3
because of their useful properties such as thermal stability,
unique structures and electronic effects, enriched boron content
and low biotoxicity. To enable those diverse applications,
structural modifications of carboranes have been the first
concern. Substitution in the carbon positions was readily
achieved via deprotonation of C−H as early as the discovery of
carboranes in 1960s. Substitution at the boron atoms, however,
remains less explored because of the less acidic B−H bonds.3f,4
In particular, selective boron-functionalization of carborane
clusters is still a synthetic challenge because of the chemically
close B−H bonds and limited functional groups.5 This situation
seriously restricts further development of applications of
carborane derivatives. Until now boron-substitution of o-
carborane was mainly accomplished by electrophilic substitu-
tion to give, for example, boron-alkylation,6 boron-halogen7 and
boron-chalcogen,8 but the reactive sites of these examples were
mostly localized on B(9)/B(12), which is attributed to the
inherent charge distribution of the o-carborane cage.9
Substitution in the alternative B(3)/B(6) positions suffered
Received: May 10, 2013
Published: July 8, 2013
© 2013 American Chemical Society
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mechanism by theoretical calculations and examined the
reactivity that has led to synthesis of numerous boron-
functionalized carborane derivatives. The in-depth study
demonstrates the feasibility of cobalt-induced selective B−H
functionalization that can provide a variety of boron-function-
alized carborane derivatives bearing in situ generated norbornyl
or norbornadienyl functional group.
RESULTS AND DISCUSSION
■
1. Synthesis and Characterization of 17-Electron
Complexes 2a−d and 3a−d. The reactions of
CpCoS2C2B10H10 (1) and selected four alkynes HCCC(O)R
(R = Me, Ph, St, Fc) at ambient temperature led to two types of
17e complexes 2a−d and 3a−d with combined isolated yields
40−50% based on complex 1 (Scheme 1).
Scheme 1. Synthesis of 2a−d and 3a−d
The single crystal structure of 2a (Figure 1a) shows a new-
generated norbornyl moiety that combines to the starting 16e
complex 1 through both C−S and B−C bonds. The norbornyl
unit that bears endo configuration18 appears to be constructed
by the original alkyne (blue) and Cp (red) (Scheme 1). The
acetyl group and S(2) are in anti-arrangement with respect to
C(3)−C(4) bond. As one Co−S bond has been changed to a
coordinating Co ← S bond, complex 2a acts as a 17e species,
and the cobalt center is in a formal charge of +2. The
paramagnetic characteristic of species 2a−d are indicated by
NMR data as shown by the broad signals (15 to −10 ppm for
1H NMR and 20 to −30 ppm for 11B NMR) that could not be
accurately identified (Supporting Information (SI), Figures
S3.1/2). Crystal structure of 3b (Figure 1b) shows the cobalt
center is bound to two B(3)/B(6)-norbornyl carborane units
through Co ← S and Co−S bonds as in 2a. The core geometry
of 3b exhibits distorted trigonal bipyramid in which S(2)/S(4)
are located above/below S(1)−O(1)−S(3) plane, and the angle
between Co(1)−S(1)−S(2) and Co(1)−S(3)−S(4) planes is
61.8°, in sharp contrast to the known planar cobaltadithiolene
complexes.19 Unlike 2a, the Co−S distances in 3b are
significantly different with Co−S 2.25 Å and Co ← S 2.50 Å.
Because of the oxygen-coordination to cobalt, the benzoyl
group in the substituted norbornyl moiety takes syn-arrange-
ment with S(2), whereas the other benzoyl adopts anti-
arrangement with S(4), the same as in 2a. In view of the
structure of 3b, it appears to be a combination of two molecules
of 2b with loss of a Cp2Co fragment. The electron count of
cobalt in 3b is also 17 with cobalt in formal charge of +2. In
contrast to 2a−d, the effect of paramagnetism of 3a−d on
NMR is much larger; for example, the signals span from 100 to
−15 ppm for 1H NMR and from 300 to −20 ppm for 11B NMR
(SI, Figures S3.3/4).
Figure 1. Crystal structures of 2a (a) and 3b (b). Selected bond
lengths [Å] and angles [°]: 2a, Co1−S1 2.1563(6), Co1−S2
2.1762(6), C1−S1 1.785(2), C2−S2 1.807(2), C3−S2 1.864(2),
C1−C2 1.663(3), C3−C4 1.528(3), C6−C7 1.314(4), C8−B3
1.570(3); 3b, Co1−O1 2.036(2), Co1−S1 2.2461(1), Co1−S2
2.5499(9), Co1−S3 2.2423(1), Co1−S4 2.4577(9), C10−O1
1.215(4), C26−O2 1.200(4); S1−Co1−S2 89.97(3), S3−Co1−S4
90.78(3).
formation. Even though metal-bound Cp were reactive in many
reactions,20 including in Diels−Alder addition,21 its intra-
molecular combination with olefinic unit and substitution at
B(3)/B(6) site of carborane could be only observed in few
examples contributed by our group.16,17
2. Formation Mechanisms of 2a−d and 3a−d: A DFT
Study. Complexes 2a−d and 3a−d are unique; thus,
mechanistic study is essential to better understand their
formation processes. However, elucidation of the formation
mechanism by routine analytical means such as NMR and MS
is not realistic because of the complicated reaction mixture
along with paramagnetic disturbance (SI, 5.1). Therefore,
theoretical calculations in combination of experimental
evidence were utilized to demonstrate the mechanistic
processes. DFT calculations on the formation of 2a were
conducted using the um0622/6-31g(d, p) basis set for all
noncobalt atoms and lanl2dz23 for cobalt, as is a common
treatment for this class of organometallic complexes.24 The
calculated energy profile is shown in Figure 2.
As implied by the solid-state structures of complexes 2a and
3b, sequential steps such as B−H activation and Cp-
participated Diels−Alder addition must be involved in their
The sequence starts from alkyne insertion into a Co−S bond
via a concerted transition state (TS1, energy barrier 14.5 kcal/
mol) and is favored by −9.3 kcal/mol energy change leading to
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Figure 2. Energy profile for 2a formation. The relative free energy and the electronic energy (in the bracket) for each intermediate or transition state
are given in kcal/mol.
−
intermediate I (see an isolated analogue generated from 1 and
MeO2CCCCO2Me).25 In the following step, the CpCo
group is reoriented (via TS2, energy barrier 25.7 kcal/mol) to
give a B−H σ complex II with an energy demand of 16.1 kcal/
mol. In II, typical stabilization of B−H σ bond to Co (LP*,
anti-bond orbital of Lewis pair) is calculated as 43.2 kcal/mol
from natural bonding orbitals (NBO) analysis.26 This σ-
coordination leads to the B−H bond weakening, as indicated by
a longer B(3)−H bond (1.24 vs 1.18 Å in I, SI, Figure S6.2.2).
The B−H activation that further leads to hydrogen transfer
from B(3) to vinyl is an energy-favored process (ΔG = −12.8
kcal/mol), whereas the transition state (TS3, energy barrier 7.9
kcal/mol) that connects II and III is located as a Co−H
structure in which the B(3)···H distance is 1.54 Å.
The structure of III has two notable features, i.e., Co−B
bond, which is optimized as 2.03 Å (referred to 2.01 Å in one
related example15i), and vinyl coordination to cobalt. Similar
complexes with a M−B bond (M = Rh, Ir, Ru, Os)15b,d,27 were
sufficiently stable to allow isolation and even characterization
by X-ray crystallography. However, III can easily convert to IV
by vinyl stretch with energy consumption of 4.1 kcal/mol. In
the optimized structure of IV, the shortest distance between
C(η5-Cp) and B(3) is 2.81 Å, thus allowing B−Cp28 formation
(V) through reductive elimination, as the energy barrier (TS4,
Co···B···C29 transition structure) for this step is only 11.2 kcal/
mol. Within V, cobalt is coordinated by Cp (neutral) in η4-
mode with +1 valence state. Note that TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) inhibited the reaction, which may
oxidize this Co(I) species (SI, 5.2).
Starting from V, the involvement of C5H5 or Cp• was
empirically excluded because of very high energy activation (SI,
6.3). The direct Cp delivery between IV and V supplies the
required C5H5 unit, and intermediate VI was located (only 8.5
kcal/mol above IV and V) in which IV and V share one Cp
ligand, similar to many triple-decker complexes.30 The Cp
ligand combines to Co(III)/Co(I) in η3/η2 modes simulta-
neously, and each cobalt in VI possesses 16 electrons.
Thereafter, through η3-Cp dissociation and electron transfer,
the smooth formation of Co(II) intermediates VI′ and VII
occurs with 21.3 kcal/mol energy lowering. Next, [1,5]-H
shift31 from VII to VIII occurs and releases 5.3 kcal/mol
energy, by overcoming a 19.4 kcal/mol barrier (TS5). The
Diels−Alder process that leads to the final boron-norbornyl
moiety in 2a proceeds smoothly from VIII with further energy
lowering of 16.2 kcal/mol. Hence these results deduce an
interesting sequence: the cobalt-induced B−H activation leads
to reactive Co−B species IV, which rationalizes the key
combination of carboranyl and cyclopentadienyl units, thus
supplying an appropriate platform for boron-norbornyl
formation in 2a.
The formation of 3a−d follows from 2a−d as the boron-
norbornyl moiety should be established prior to the
bipyramidal core geometry. However, no direct conversion
from isolated 2a−d to 3a−d was experimentally observed,
implying that an in situ generated mediator is involved. Since
intermediate VI′ exhibits severe coordination unsaturation on
cobalt, Cp might deliver from 2a−d to VI′ to lead to
intermediates IV and IX (Scheme 2). To compensate electron
shortage in IX, the carbonyl group is driven to coordinate to
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Scheme 2. Proposed Formation Mechanism of 3a−d
Figure 3. Crystal structures of 4a (a) and 5b (b). Select bond lengths
[Å]: 4a, Co1−S1 2.1315, Co1−S2 2.1318, C1−C2 1.635(5), C3−C4
1.294(6), C6−C7 1.355(6), C8−B3 1.589(6); 5b, C1−C2 1.658(3),
C2−S1 1.787(2), C3−S1 1.838(2), C3−C4 1.548(3), C6−C7
1.326(3), B3−C8 1.574(3).
cobalt (X). The further combination of X with 2a−d gives rise
to 3a−d, accompanied by loss of [CpCo] fragment which could
be captured by complex 1 to form a dinuclear species
(CpCo)2S2−C2B10H10.25b,c Note that in coordinative solvents
such as THF, 2a−d can ideally convert to 3a−d with better
carborane economy, which may be attributed to THF
stabilization of intermediates such as V and VI′, whereas in
CH2Cl2 or toluene the conversion is less efficient as shown by
the corresponding ratios of 2a−d:3a−d (SI, 1.1 and 4.1).
3. Redox Conversions of 2a−d and 3a−d. Formation of
4a−c and 5a−d. Despite electron unsaturation on cobalt,
complexes 2a−d were found to be stable in air and moisture in
both solution and solid state, with no detectable decomposition
observed for 5 days in solution. However, when exposed to
silica in air, visible transformation with color change from
yellow to brown was observed, yielding oxidized products 4a−c
and cobalt-free 5a−d (Scheme 3). The conversions could reach
5a−d were also isolated and characterized both in solution
and solid state. Surprisingly, crystal structure of 5b (Figure 3b)
shows that [CpCoS] fragment has been dissociated with a
Ccab−H (cab = carborane) bond recovery, but the original endo
1
norbornyl moiety retains. Among the H NMR signals of 5b,
the newly formed Ccab−H signal was observed at 3.77 ppm as a
singlet. H(3) and H(4) were observed at 4.58 and 3.86 ppm,
respectively, with a 5.0 Hz coupling, which is in agreement with
anti-arrangement as revealed in solid state.32 H(6) appears at
6.03 ppm as a double of doublets (J = 5.5, 2.5 Hz). Because of
the binding to norbornyl, the 11B resonance of B(3) is located
at 1.1 ppm, which is the lowest one among all the boron signals.
In the 13C NMR spectrum, C(3) is shifted to the lower field at
69.7 ppm compared to other alkyl signals (e.g., C(4) at 53.4
ppm) as it is attached to the carboranyl thiolate unit. Two
carborane skeletal carbons show signals at 73.6 and 84.0 ppm.
It follows the consistent structures both in solution and solid
state.
Scheme 3. Synthesis of 4a−c and 5a−d by Treatment of 2a−
d
The conversions of 2a−d to 4a−c and 5a−d involve both
alkyl and carboranyl C−S cleavage. Similar metal-induced alkyl
C−S cleavage leading to vinyl derivatives has been extensively
reported,33 whereas carboranyl C−S cleavage is fairly rare; for
example, the carboranyl cyclothioether cleavage was induced
only by electrochemical methods.34 In this study the alkyl C−S
cleavage to 4a−c is more competitive (>67%), and carboranyl
C−S cleavage to 5a−d plays a minor role (<21%).
Possible Mechanisms for the Formation of 4a−c and 5a−
d. The simple but specific combination of air, silica and
moisture has led to efficient conversions of 2a−d to 4a−c and
5a−d bearing a boron-norbornadienyl or a boron-norbornyl
group at carborane. Obviously, the valence alteration from
Co(II) to Co(III) indicates an oxidation process involved.
Possible formation on 4a−c is shown in Scheme 4a. In the
presence of air, Co(II) is oxidized to Co(III) to form covalent
Co−S bond and sulfonium intermediate, which subsequently
leads to 4a−c via C(3)−S bond cleavage and loss of proton.
Note that no conversion was observed in the absence of any of
silica, air or moisture. Here silica might act as both a catalyst
and a carrier of O2 and moisture. In contrast, the formation of
5a−d proceeds via a reductive-type pathway (Scheme 4b),
where Co(II) acts as the reductant. Initiated by electron
transfer,35 the carboranyl C−S cleavage gives rise to a [5d −
good yields except 2d after overnight exposure. The crystal
structure of 4a reveals that C(3)−S(2) bond has been cleaved
and a novel norbornadienyl functional group is generated as
shown in Figure 3a. The Co(1)−S(2) bond formation leads 4a
as a diamagnetic 16e Co(III) species, thus allowing NMR
analysis. In the 1H NMR spectrum, a singlet at 8.12 ppm, which
is assigned to H(3), is indicative of the new generated C(3)
C(4) bond. H(6) and H(7) appear at 6.76 and 7.16 ppm as
typical vinyl proton signals. Two H(9) signals show character-
istic doublets at 2.20 and 2.24 ppm with J = 7.0 Hz. By
comparison of the H coupled and H decoupled 11B spectra,
B(3) that is attached to norbornadienyl was assigned at the
lowest field (−0.8 ppm), and the others appear as overlapping
signals at higher fields from −2.8 to −8.6 ppm. The 13C signal
of C(3) was observed at 161.0 ppm, and the two equal
carborane carbons (C(1) and C(2)) show at 94.4 ppm.
Notably, the generated 16e complexes 4a−c are stable to air
and moisture.
1
1
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Scheme 4. Possible Pathways of 2a−d Conversions through
Alkyl C−S (a) and Carboranyl C−S (b) Cleavage
H]−, anion and the resulting [CpCoS] fragment is likely
absorbed on silica. Comparative experiment by treatment of
2a−d with NaBH4 (in THF) led to similar yields of 5a−d but
no formation of 4a−d, further supporting the proposed
reductive C−S cleavage (SI, 5.3).
Figure 4. Crystal structures of 5d (a) and 6d (b). Select bond lengths
[Å]: 5d, C1−C2 1.632(5), C3−C4 1.557(4), C6−C7 1.380(4), B3−
C8 1.588(5); 6d, C1−C2 1.651(5), C3−C4 1.554(5), C6−C7
1.325(6), B3−C8 1.572(5).
The 17e complexes 3a−d are more stable to air and moisture
than 2a−d; however, under the same conditions they can also
finally convert to 5a−d with similar yields. In monitoring the
conversion of 3d to 5d, an intermediate with syn-C(O)Fc group
was observed by 1H NMR (SI, 5.4), which gradually isomerizes
to 5d, indicating that reorientation of −C(O)Fc from syn back
to anti is not a quick process.
Retro-Diels−Alder Reaction of 5d. In Diels−Alder studies,
one can often encounter isomers with endo/exo configurations
according to different cycloaddition type. It is commonly
accepted that endo is kinetically controlled while exo is
thermodynamically controlled.36 Therefore it is expected that
in this study the endo boron-norbornyl products are possible to
transform under thermal conditions. Refluxing in toluene (110
°C) for 10 h, 5d led to a new isomer 6d in 45% yield with 5d
remaining in 49%. In reverse, treatment of the isolated 6d led
to a 40% conversion to 5d, indicating an equilibrium between
the two isomers (Scheme 5). EI-MS analysis of each gave
moved to low field by 0.5−2.5 ppm relative to those in 5d. A
significant 13C signal shift was observed on C(1) from 5d (73.6
ppm) to 6d (63.2 ppm).
The interconversion between 5d and 6d can be considered as
retro-Diels−Alder/Diels−Alder processes.38 To get a better
understanding, the calculated energy profile (under m06/6-
31g(d,p)/lanl2dz level) is provided in Figure 5. Recognizing
Scheme 5. Thermal Interconversion between 5d and 6d
Figure 5. Relative free energy profile (kcal/mol, at 298 K, values given
in the bracket, at 383 K) calculated for conversion of 5d to 6d.
from the relative free energy of each intermediate or transition
state, the conversion only occurs after overcoming the high
energy barrier for retro-Diels−Alder process (TS5d, 31.5 kcal/
mol vs TS6d, 31.0 kcal/mol, 298 K), in agreement with
observed temperature dependence (no reaction would occur at
room temperature). In this case, conversion from IN1 to IN3
proceeds without high energetic obstacles (ΔG = 3.1 kcal/
mol). Because of the approximately equal energy change to
either side, a similar distribution between 5d and 6d is
expected, as was experimentally observed. Optimized structures
also supply clues for the origin of stereoselectivity. Following
the steric requirement, the [4 + 2] cycloaddition will not
respond if the orientation of dienophile and diene disagrees;
thus, B−Cp in IN1 would not match vinyl group in IN3 unless a
147° rotation (see IN1 and IN3 coordinates, SI, 6.5) occurs
where specific endo configuration is produced. Since limited by
inner strain, the exo isomer is excluded. For this reason, the
interconversion of boron-norbornyl moiety acts as a special
example in the Diels−Alder/retro-Diels−Alder chemistry.
molecular ion peak at m/z 478.2. The crystal structures of 5d
and 6d (Figure 4) reveal the endo norbornyl moiety in both
cases, rather than the common endo/exo modes.37 The
difference between 5d and 6d is the skeleton configuration of
norbornyl group (i.e., R3, S4, S5, R8-5d and S3, R4, R5, S8-6d).
This is also reflected in individual NMR data. Despite the same
5.0 Hz coupling of JH(3)−H(4) in both 5d and 6d, the H(3) signal
in 5d (4.57 ppm) is shifted to 4.06 ppm in 6d. H(1) signal of
5d appears at 3.76 ppm but is shifted to 3.92 ppm in 6d. The
chemical shifts of H(9) protons are changed from 1.98/1.70
ppm in 5d to 2.27/1.88 ppm in 6d. All these might indicate the
steric neighborhood effect between H(1)/H(9) in 5d and
H(1)/H(3) in 6d. The alternation of norbornyl group also led
to obvious 11B NMR signal shifts that boron signals of 6d have
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4. Alkyne Insertion into 2a−d and 3a−d. Reactions of
2a−d and 3a−d toward Alkynes. Not only is the reactivity of
17e complexes 2a−d and 3a−d reflected in the redox
conversions owing to the unsaturated metal center as discussed
above, but also the reactive Co−S bond as in 1 predicts
reactivity toward alkynes. The ferrocenyl-conjugated alkyne
HCCC(O)Fc was chosen for this study on the basis of ease
of isolation (color indicator) and the useful redox properties of
ferrocenyl derivatives in biological systems.39 The reactions of
2a−d and HCCC(O)Fc at ambient temperature led to new
cobalt-free compounds (Z/E)-7a−d, and 3a−d led to addi-
tional compounds (Z/E)-8a−d, as illustrated in Scheme 6. The
Scheme 6. Reactions of 2a−d and 3a−d with HCCC(O)Fc
in Either THF or CH2Cl2
molecular structures of (Z)-7d, (E)-7d and (Z)-8d were
characterized by X-ray diffraction, and two are shown in Figure
6 (see SI, 2.2, for (E)-7d). Apparently, (Z/E)-7d retain the
endo norbornyl moiety from 2d accordingly, and the other
carbon at carborane is consequently substituted by Z/E vinyl
sulfido unit. The 1H NMR data also confirm the endo
configuration with JH(3)−H(4) = 5.0 Hz in both cases, but
JH(10)−H(11) is characteristic for (Z/E)-7d with values of 10.0
and 15.0 Hz, respectively. Other significantly different 1H
signals of the two isomers were observed at 7.37, 6.76 ppm for
(Z)-CHCHC(O)Fc and 2.85, 1.70 ppm for H(9) in (Z)-7d
in contrast to 7.71, 6.82 ppm for (E)-CHCHC(O)Fc and
2.68, 1.65 ppm for H(9) in (E)-7d. A major characteristic of
(Z/E)-8d that distinguishes it from (Z/E)-7d is the syn-
arrangement of −C(O)Fc group that inherits from 3d (Figure
6), as supported by a larger coupling constant JH(3)−H(4) = 9.0
Hz. The different orientation of −C(O)Fc group has led to
H(6) signal in (Z)-8d (6.38 ppm) high-filed shift by 0.35 ppm
from that in (Z)-7d (6.03 ppm) and H(4) signal of (Z)-8d
(3.10 ppm) low-field shift by 0.47 ppm relative to (Z)-7d (3.57
ppm).
Figure 6. Crystal structures of (Z)-7d (a) and (Z)-8d (b). Select bond
lengths [Å]: 7d, C1−C2 1.767(5), C3−C4 1.543(5), C6−C7
1.328(5), C10−C11 1.333(6), B3−C8 1.564(5); 8d, C1−C2
1.801(5), C3−C4 1.553(5), C6−C7 1.311(6), C10−C11 1.336(5),
B3−C8 1.572(5).
norbornyl unit; thus, (Z/E)-8a−d can be isolated. This differs
from the reduction of 3a−d to form 5a−d where −C(O)R
group could turn from syn to anti conformations (SI, 5.4). In
this study differing orientations of −C(O)R group contribute
variety of structural isomers containing norbornyl unit at
carborane.
Possible formation of (Z/E)-7a−d was investigated by
reaction of 2d with excess DCCC(O)Fc in CD2Cl21(Scheme
7). Deuterated (Z)-7d was isolated by TLC and its H NMR
Scheme 7. Proposed Formation of (Z)-7d by Deuterium
Labeling Experiment
The insertion of HCCC(O)Fc toward 2a occurs in 70%
yield at ambient temperature, whereas 2b−d proceed in less
than 15% with both reactants remaining under the identical
condition. The distribution of isomers Z:E is ∼2:1 for all R
groups investigated. Similar reactions toward 3a−d (Scheme 6)
are more efficient with yields of around 80%. The distribution
of the four isomers show (Z)-7b ≈ (Z)-8b and (E)-7b ≈ (E)-
8b, as expected. Methyl derivatives from 3a are against this
trend with (Z/E)-7a being predominant, which might be
caused by easier reorientation of the small acetyl group.
Generally, the presence of additional vinyl sulfido group on
carborane efficiently blocks reorientation of −C(O)R within
spectrum shows a similar deuterium degree for H(10) as in
starting DCCC(O)Fc, but H(11) signal remains unaffected
(SI, Figure S5.5.2), indicating that this hydrogen does not
originate from excess deuterated alkyne. Thus alkyne insertion
into Co−S bond, followed by demetalation through a
hydrolytic process on silica was proposed (Scheme 7). As
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such, the steric factor of Cp ligand from 2a−d should be
responsible for the lower conversions relative to 3a−d.
considerably increases E-selectivity attributed to restricted
isomerization from original E- to Z-configuration. Here AlCl3
also plays an additional role in facilitating endo/endo conversion
of the norbornyl moiety as the alkyl derivatives of AlCl3 often
act as catalysts for Diels−Alder/retro-Diels−Alder reactions.41
One-Pot Synthesis of B−H Functionalized Carborane
Derivatives. On the basis of the favored formation of 3a−d
from complex 1 in THF and efficient alkyne insertion toward
3a−d, the strategy of using complex 1 with excess alkyne was
anticipated to straightforwardly prepare boron-norbornyl
products. Indeed, the reaction of complex 1 and HC
CC(O)Fc in refluxed THF led to (Z/E)-7d and (Z/E)-8d
(Scheme 8) in a yield of over 50% with no observed 2d and 3d,
1
A comparison of the H NMR spectra of the products from
reaction of 2a and HCCC(O)Fc (Figure 7) shows that upon
addition of AlCl3, the ratio of E/Z is increased from 33:67 to
65:35, and (E)-9a accounts for 51%.
Scheme 8. One-Pot Synthesis of (Z/E)-7d and (Z/E)-8d
and the distribution of four isomers is close to that obtained
from 3d. It is interesting that from the neat result of the one-
pot synthesis, the ancillary ligand Cp, generally less reactive, is
efficiently involved in in situ construction of organic boron-
norbornyl carborane derivatives without need of tedious
isolation of 17e intermediates. No doubt that the electron-
deficient terminal alkynes are suitable for this strategy as such
alkynes fit both regio- and stereoselectivity in alkyne insertion
into Co−S bond as well as the requirement for Diels−Alder
addition.
1
Figure 7. Selected H NMR spectra for comparison of the products
from 2a and HCCC(O)Fc: (a) AlCl3-free reaction led to (Z/E)-7a;
(b) AlCl3-mediated reaction led to (Z/E)-7a and (Z/E)-9a.
Retro-Diels−Alder Study on (Z/E)-7a−d. The retro-Diels−
Alder reactions of (Z)-7a−d were studied in refluxed toluene.
As a result, (Z)-9a−d (30−50%) and unexpected products (Z)-
10b,c (25 and 33%) were isolated together with unreacted (Z)-
7a−d (30−45%) (Scheme 10). The 1H NMR monitoring
Lewis Acid (e.g., AlCl3) Improves Reactivity and Selectivity
to Form (E)-9a−d. Electron-deficient alkynes, such as
MeO2CCCCO2Me, exhibited excellent reactivity toward
Co−S bond to alkyne insertion adducts.25 On considering
that Lewis acids such as AlCl3 can further increase electron
deficiency of carbonyl-substituted alkynes through oxygen-
coordination,40 AlCl3 was anticipated to improve the low
reactivity of HCCC(O)Fc and 2a−d shown in Scheme 6. As
predicted, addition of AlCl3 at room temperature resulted in
conversions of 70−95% in contrast to less than 15% in the
absence of AlCl3. More surprisingly, additional products (E)-
9a−d were generated as major products accounting for 40−
50% yields (Scheme 9). The molecular structure of this series is
Scheme 10. Synthesis of (Z)-9a−d and (Z)-10b,c
confirmed that no E-isomers were in this reaction. The crystal
structure of (Z)-10b (Figure 8b) reveals a different boron-
norbornyl unit where B(3) is bound to C(7), rather than C(8)
as in the other products, indicative of H-shift occurrence during
the retro-Diels−Alder process. Correspondingly, boron-nor-
bornyl moiety in (Z)-10b has significantly different NMR data.
For example, H(6) appears at 7.17 ppm, low-field shifted in
comparison to 5.96 ppm in (Z)-7b or 6.08 ppm in (Z)-9b
because of the electron-withdrawing effect of carborane cage.
The new H(8) proton is located at 4.12 ppm. Another distinct
feature was observed in the separation of the two H(9) signals
for 0.17 ppm in (Z)-10b, much smaller than those in (Z)-9b
(0.46 ppm) and (Z)-7b (1.22 ppm). The 13C NMR spectrum
shows a weak and broad peak at 133.8 ppm assigned to C(7)−
B(3).
The formation of (Z)-10b,c is a selection of cycloaddition
between diene and dienophile in the retro-Diels−Alder process.
This might be explained that synchronous vinyl reorientation
and B−Cp rotation give (Z)-9a−d, but if [1,2]-H migration
within Cp occurs during B−Cp rotation, (Z)-10b,c would be
formed. Such a phenomenon has not been described in any
retro-Diels−Alder reaction, but here the [1,2]-H migration has
led to a new type of B−Cp precursors for the following Diels−
Alder process, thus contributing diversity of the boron-
Scheme 9. AlCl3-Mediated Reactions of 2a−d and HC
CC(O)Fc Leading to (E)-9a−d as Major Products
proven by a Z-isomer, for example, (Z)-9b in Figure 8a with a
ferrocenyl-conjugated vinyl sulfido unit attached to carborane,
which is generated from alkyne insertion. In comparison with
2b, the norbornyl moiety in (Z)-9b exhibits an alternative endo
configuration, which is derived from a retro-Diels−Alder
process as occurred from 5d to 6d. This change has resulted
1
in significant differences of H data for H(9) protons, for
example, 2.43, 1.97 ppm in (Z)-9b vs 2.93, 1.71 ppm in (Z)-7b.
The carbonyl oxygen-coordination to AlCl3 not only leads to
further alkyne activation thus improving yields, but also
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mild conditions allow for straightforward norbornyl function-
alization at B(3)/B(6) sites of carborane in good yields. Two
strategies using thermal promotion or Lewis acid mediation
have provided specific retro-Diels−Alder products with differ-
ing boron-norbornyl groups and good selectivity. In this study
cobalt-mediated selective B−H activation and further function-
alization has provided a facile and efficient route to a new
family of functionalized carborane derivatives containing either
norbornadienyl or norbornyl group.
ASSOCIATED CONTENT
* Supporting Information
Experimental section and crystallographic data (CIF) are
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
hyan1965@nju.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Zhenyang Lin and Prof. You Song for their
helpful suggestions. This work was supported by the National
Basic Research Program of China (2010CB923303 and
2013CB922101), the National Science Foundation of China
(20925104, 21271102 and 21021062) and high performance
computational center of Nanjing University. Dedicated to
Professor Dr. Vladimir Bregadze on the occasion of his 75th
birthday.
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CONCLUSIONS
■
Two types of 17e cobalt(II) complexes containing B(3)/B(6)-
norbornyl carborane have been synthesized from
CpCoS2C2B10H10 (1) and electron-deficient alkynes HC
CC(O)R at ambient temperature. Their formation includes
metal-induced selective B−H activation, B−Cp formation and
Cp-involved Diels−Alder addition, as supported by DFT
calculations. These isolatable 17e intermediates exhibit
interesting redox reactions under simple conditions that has
led to both alkyl C−S cleavage and carboranyl C−S cleavage to
yield stable 16e Co(III) complexes bearing a boron-
norbornadienyl unit and organic carborane derivatives bearing
a boron-norbornyl group, respectively. The reactive Co−S
bonds provide further reactivity toward alkynes that has
delivered two series of cobalt-free carborane derivatives
containing a B(3)-norbornyl moiety. In particular, the one-
pot reactions of complex 1 and alkynes HCCC(O)R under
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