Mechanistic Study of Highly Efficient Direct 1,2-Carboboration of Alkynes with 9-Borafluorenes

Article abstract of DOI:10.1002/chem.201801818

We recently reported a new one-pot transformation of alkynes into 9,10-diarylphenanthrene derivatives, which proceeds through efficient catalyst-free 1,2-carboboration of alkynes with 9-chloro-9-borafluorene (1_Cl), which yields a chlorodibenzoborepin, followed by oxidative deborylation/C?C coupling of the resultant chlorodibenzoborepin. Herein, based on new experimental observations for the catalyst-free 1,2-carboboration by using diphenylacetylenes and 1_Br or 1_OTf as well as results from theoretical investigations, we show how the substituent on the boron atom of 9-borafluorene affects the reactivity toward alkynes. Kinetic studies indicated that the 1,2-carboboration of diphenylacetylene with the borafluorenes can be described as a second-order reaction. The reaction rates became larger with the increase in the acceptor numbers of the borafluorenes (1_Br>1_OTf>1_Cl), which was evaluated by the Gutmann–Beckett method based on a Lewis acid/base complexation in solution. Interestingly, thermodynamic parameters obtained experimentally indicated that the term of activation entropy, rather than the term of activation enthalpy, largely contributes to the reaction rate. This experimental result was also supported by DFT calculations. Overall, among the borafluorenes examined, 1_Br exhibited the highest reactivity toward a wide variety of substituted diarylacetylenes. Similar to the case of chlorodibenzoborepin, when the dibenzoborepin obtained from 1_Br or 1_OTf was oxidized by using FeCl₃, an efficient deborylation/C?C coupling took place to give the corresponding 9,10-diarylphenanthrene derivatives in high yields.

Full text of DOI:10.1002/chem.201801818

DOI: 10.1002/chem.201801818
Full Paper
&
Direct 1,2-Carboboration
Mechanistic Study of Highly Efficient Direct 1,2-Carboboration of
Alkynes with 9-Borafluorenes
Yoshiaki Shoji,[a] Naoki Shigeno,[a] Kumiko Takenouchi,[a] Manabu Sugimoto,[b] and
Takanori Fukushima*[a]
Abstract: We recently reported a new one-pot transforma-
tion of alkynes into 9,10-diarylphenanthrene derivatives,
which proceeds through efficient catalyst-free 1,2-carbobora-
tion of alkynes with 9-chloro-9-borafluorene (1Cl), which
yields a chlorodibenzoborepin, followed by oxidative debor-
ylation/CÀC coupling of the resultant chlorodibenzoborepin.
Herein, based on new experimental observations for the cat-
alyst-free 1,2-carboboration by using diphenylacetylenes and
1Br or 1OTf as well as results from theoretical investigations,
we show how the substituent on the boron atom of 9-bora-
fluorene affects the reactivity toward alkynes. Kinetic studies
indicated that the 1,2-carboboration of diphenylacetylene
with the borafluorenes can be described as a second-order
reaction. The reaction rates became larger with the increase
in the acceptor numbers of the borafluorenes (1Br >1OTf >
1Cl), which was evaluated by the Gutmann–Beckett method
based on a Lewis acid/base complexation in solution. Inter-
estingly, thermodynamic parameters obtained experimental-
ly indicated that the term of activation entropy, rather than
the term of activation enthalpy, largely contributes to the re-
action rate. This experimental result was also supported by
DFT calculations. Overall, among the borafluorenes exam-
ined, 1Br exhibited the highest reactivity toward a wide varie-
ty of substituted diarylacetylenes. Similar to the case of
chlorodibenzoborepin, when the dibenzoborepin obtained
from 1Br or 1OTf was oxidized by using FeCl3, an efficient de-
borylation/CÀC coupling took place to give the correspond-
ing 9,10-diarylphenanthrene derivatives in high yields.
Introduction
The insertion reactions of unsaturated and/or polarized sub-
strates into a BÀC bond of organoboranes have attracted con-
siderable attention for organic transformation.[1–8] Among
them, the carboboration of alkynes with organoboron com-
pounds, which is accompanied by the simultaneous formation
of CÀC and CÀB bonds, represents a useful boron-mediated or-
ganic transformation, particularly for the regio- and stereose-
lective synthesis of substituted alkenes (Figure 1a).[1,2] Al-
though there are many examples of metal-catalyzed carbobo-
ration,[2,3] catalyst-free direct carboboration has recently re-
ceived growing attention because of their high cost efficiency
and low environmental load.[1a,4,5] However, as long as alkynes
are not activated by, for example, heteroatom or transition-
metal substituents, catalyst-free alkyne carboboration can be
achieved only when strongly electrophilic boranes such as
[a] Prof. Dr. Y. Shoji, N. Shigeno, K. Takenouchi, Prof. Dr. T. Fukushima
Laboratory for Chemistry and Life Science
Institute of Innovative Research, Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
E-mail: fukushima@res.titech.ac.jp
[b] Prof. Dr. M. Sugimoto
Figure 1. (a) Schematic representation of 1,2- or 1,1-carboboration of al-
kynes. (b) 1,2-Carboboration of alkynes with borafluorene (1Cl) that yields
borepin 2 and subsequent oxidative deborylation/CÀC coupling of the bore-
pin product that leads to an extended p-conjugated system with a 9,10-dia-
rylphenanthrene skeleton. (c) Chemical structures of three borafluorene de-
rivatives with a Cl (1Cl), Br (1Br), or OTf (1OTf) group used in this study.
Faculty of Advanced Science and Technology, Kumamoto University
2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 (Japan)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201801818.
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(C6F5)3B are employed.[1,4] Furthermore, previously reported ex-
amples of catalyst-free alkyne carboboration are mostly 1,1-
type (Figure 1a),[1,4] and 1,2-type carboboration has been
known to occur in only limited cases, where specific boron
species such as diarylchloroborane,[5a] borenium ion
[R2B+(:L)],[5b,c] and borinium ion [R2B+][5d] were used. Pentaaryl-
borole derivatives have also been reported to show a formal
1,2-carboboration reaction of alkynes without catalysis.[9] This
reaction proceeds through sequential steps involving the
Diels–Alder reaction between alkyne and borole, a sigmatropic
shift of the resulting boryl group, and electrocyclic ring-open-
ing, eventually giving rise to a boron-containing unsaturated
seven-membered ring compound, that is, a borepin.[9] Al-
though catalyst-free, direct 1,2-carboboration has rarely been
explored to date, the development of readily available and effi-
cient 1,2-carboboration reagents would substantially expand
the synthetic utility for substituted alkenes and in turn the
scope of boron-mediated organic transformations.
Figure 2. Selected molecular orbitals and energies of (a) 1Cl, (b) 1Br, and
(c) 1OTf in the optimized geometries at the wB97X-D/[SDD for Cl, Br, and S, 6-
31G(d,p) for C, B, O, F, and H] level.
We recently reported that 9-chloro-9-borafluorene[10] (1Cl)
causes direct 1,2-carboboration of various internal and terminal
alkynes to give dibenzoborepin derivatives such as 2Cl (Fig-
ure 1b).[11] Notably, the resultant dibenzoborepin derivatives
undergo deborylation/CÀC coupling upon one-electron oxida-
tion to transform into aromatic compounds efficiently (Fig-
ure 1b).[11] The sequential 1,2-carboboration and oxidative de-
borylation/CÀC coupling reactions can be performed in a one-
pot fashion, thus enabling facile synthesis of a wide variety of
extended p-conjugated systems with 9,10-diarylphenanthrene
and/or dibenzo[g,p]chrysene skeletons (Figure 1b). The finding
of this new direct 1,2-carboboration that proceeds between al-
kynes and the simple aromatic borane prompted us to further
investigate how the Lewis acidity of the borane compound af-
fects the reaction efficiency. Here, we report mechanistic as-
pects of the direct alkyne 1,2-carboboration through kinetic
and theoretical studies on the reactions between diarylacety-
lenes and three borafluorene derivatives with a Cl, Br, or tri-
solely located on the phenylene rings (Figure 2) and lie in
almost identical energy levels (À8.04, À8.06, and À8.00 eV, re-
spectively). The contributions of the vacant 2p orbitals of
boron are reflected in the lowest unoccupied molecular orbi-
tals (LUMOs; Figure 2). Based on the calculated LUMO levels of
1Cl (À0.62 eV), 1Br (À0.72 eV), and 1OTf (À0.37 eV), 1Br is expect-
ed to exhibit the highest electron-accepting properties. The
calculated natural bond orbital (NBO) charges on the boron
atoms of 1Cl, 1Br, and 1OTf are +0.74, +0.64, and +1.05, indicat-
ing the higher polarized nature of 1OTf compared with 1Cl and
1
Br (Figures S3–S5 in the Supporting Information).
NMR spectroscopy provided insight into the Lewis acidity of
the borafluorenes in solution. The 11B NMR spectra of 1Cl, 1Br,
and 1OTf in CDCl3 at 258C showed a signal arising from the
boron atom at d=63.6, 65.7, and 50.2 ppm, respectively (Fig-
ure S11 in the Supporting Information).[14] This observation was
consistent with the order of the electron-accepting ability
(1Br >1Cl >1OTf) expected from the calculated LUMO energies.
To further evaluate Lewis acidity of the borafluorenes by using
the Gutmann–Beckett method,[15] we measured changes in the
31P NMR chemical shift of triethylphosphine oxide (Et3PO) upon
addition of the borafluorene. Although the 31P NMR signal of
Et3PO alone was observed at d=55.2 ppm in CDCl3 at 258C, it
appeared at d=81.5, 83.2, or 82.3 ppm in the presence of five
equivalents of 1Cl, 1Br, or 1OTf, respectively, owing to the forma-
tion of the corresponding borafluorene-Et3PO adduct (Table 1
and Figure S12 in the Supporting Information). The values of
acceptor number (AN), defined as AN=(d31PÀ41.0)ꢁ[100/
(86.14À41.0)],[15] were determined to be 89.7, 93.5, and 91.5
for 1Cl, 1Br, and 1OTf, respectively. Based on this criterion relying
on Lewis acid–base complexation in solution, 1Br is considered
to be the strongest Lewis acid among the three borafluorenes.
The Gutmann–Beckett analysis also showed that 1OTf is a stron-
ger Lewis acid than 1Cl, whereas the theoretical calculation
suggested that 1OTf (LUMO level: À0.37 eV) has a lower elec-
tron-accepting ability than 1Cl (LUMO level: À0.62 eV).
fluoromethane sulfonyloxy (OTf) group on the boron atom (1Cl,
[13]
1Br,[12] and 1OTf
,
Figure 1c).
Results and Discussion
We prepared 1Br from 9,9-dimethyl-9-stannafluorene and BBr3
in 78% yield, according to the procedure reported previous-
ly.[12a,b] Compound 1OTf was obtained as yellow needle crystals
in 74% yield by the treatment of 1Cl with Me3SiOTf (5 equiv) in
CH2Cl2 at 258C.[14] Attempts for elemental analysis of this crys-
talline product have not been successful because of the highly
hygroscopic nature of 1OTf. Instead, we confirmed the purity of
1
the product (>97%) by quantitative H NMR analysis in CDCl3
at 258C by using hexamethylbenzene as an internal standard
(Figures S14–S16 in the Supporting Information).[14]
To evaluate the electronic structures of 1Cl, 1Br, and 1OTf (in
vacuum), density functional theory (DFT) calculations were car-
ried out by using the wB97X-D/[SDD for Cl, Br, and S, 6-
31G(d,p) for C, B, O, F, and H] level (Tables S4–S6 in the Sup-
porting Information).[14] In the optimized geometries of 1Cl, 1Br,
and 1OTf, the highest occupied molecular orbitals (HOMOs) are
Similar to the case of 1Cl,[11] when diphenylacetylene was re-
acted with 1Br or 1OTf at 808C in 1,2-dichloroethane (1,2-DCE),
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Table 1. Experimentally and theoretically evaluated electronic properties
of the borafluorenes (1Cl, 1Br, and 1OTf).
LUMO level NBO charge of boron[a] 11B NMR 31P NMR AN[d]
[eV][a]
[ppm][b] [ppm][c]
1Cl
1Br
À0.62
À0.72
+0.74
+0.64
+1.05
63.6
65.7
50.2
81.5
83.2
82.3
89.7
93.5
91.5
1OTf À0.37
[a] wB97X-D/[SDD for Cl, Br, and S, 6-31G(d,p) for C, B, O, F, and H] level.
[b] In CDCl3 at 258C. [c] The 31P NMR chemical shift of Et3PO (0.1m) in the
presence of 5 equivalents of the borafluorene in CDCl3 at 258C. [d] Ac-
ceptor number (AN) is defined as AN=(d31PÀ41.0)ꢁ[100/(86.14À41.0)].[15]
direct 1,2-carboboration took place to give the corresponding
borepin (2Br or 2OTf) quantitatively. The borepin products were
1
unambiguously characterized by H, 11B, 13C, and 19F NMR spec-
troscopy, IR spectroscopy, and X-ray crystallography (Fig-
ures S1, S2, and S17–S23 in the Supporting Information).[14] To
evaluate the rate constant (k) of the 1,2-carboboration, a CDCl3
solution containing an equimolar amount of diphenylacetylene
and the borafluorene (1Cl, 1Br, or 1OTf), together with an internal
standard (hexamethylbenzene), was prepared in a sealed NMR
tube (initial concentrations: [1Cl]=0.3m, [1Br]=0.1m, and
1
[1OTf]=0.3m). The reaction was monitored by H NMR spectros-
copy at a given temperature (Tables S1–S3 in the Supporting
Information). In every case, the signals resulting from the cor-
responding borepin product (2Cl, 2Br, or 2OTf) monotonically in-
creased with the decrease in those resulting from diphenylace-
tylene and the borafluorene, and no other detectable product
was observed (Figure S13 in the Supporting Information). The
1,2-carboboration was characterized as a second-order reaction
(Figure 3), and the obtained k values at 50–808C for 1Cl and at
30–608C for 1Br and 1OTf are summarized in Table 2. Notably,
the rate constant at 508C for the reaction with 1Br or 1OTf was
approximately 20 or 6 times greater than that for the reaction
with 1Cl (Table 2). The k values increased (1Br >1OTf >1Cl) with
the increase in the AN of the borafluorenes (Table 1).
Figure 3. Plot of 1/[borafluorene] [mÀ1] versus reaction time t [s] for the reac-
tion of an equimolar mixture of diphenylacetylene and (a) 1Cl, (b) 1Br, or
(c) 1OTf in CDCl3. Initial concentration: [1Cl]=[1OTf]=0.3m, [1Br]=0.1m. See
also Tables S1–S3 in the Supporting Information.
Table 2. Second-order rate constants (k) for the 1,2-carboboration be-
tween diphenylacetylene and the borafluorenes (1Cl, 1Br, and 1OTf) in
CDCl3.[a]
As shown in Figure 4a, the plot of lnk versus 1/T [KÀ1] for
each 1,2-carboboration showed a linear relationship, from
which the values of activation energy (Ea) and frequency factor
(A) were obtained (Table 3). In Ea, only small differences
(<1 kcalmolÀ1) were seen among the reactions with 1Cl, 1Br,
and 1OTf. However, there were significant differences in A, and
the reaction with 1Br showed the highest A value (2.19ꢁ
106 mÀ1 À1), which may account for the greatest k value for 1Br.
s
Note that the magnitude of the A values correlates with that
of the AN of the borafluorenes (Table 1). From the plot of ln(k/
T) versus TÀ1 [KÀ1] (Figure 4b), we also determined the thermo-
dynamic parameters (DG°, DH°, and DS°) for the 1,2-carbobo-
ration (Table 3).[16] Clearly, the entropy term largely affects the
reaction rates, rather than the enthalpy term. A notable differ-
ence can be seen for the 1,2-carboboration reactions with 1Cl
and 1Br, which provide DS° values of À39.1Æ2.4 and À31.6Æ
3.3 calKÀ1 molÀ1, respectively. As the TDS° value for 1Br (e.g., at
508C, À10.2Æ1.0 kcalmolÀ1) is negatively much smaller than
that for 1Cl (À12.6Æ0.8 kcalmolÀ1), the difference in the en-
[b]
[b]
[b]
T [K]
k(1Cl) [mÀ1 sÀ1
]
k(1Br) [mÀ1 sÀ1
]
k(1OTf) [mÀ1 sÀ1
]
303.15
313.15
323.15
333.15
343.15
353.15
n.d.[c]
n.d.[c]
(1.44Æ0.05)ꢁ10À3
(3.13Æ0.14)ꢁ10À3
(4.75Æ0.14)ꢁ10À3
(1.04Æ0.05)ꢁ10À2
n.d.[c]
(3.95Æ0.12)ꢁ10À4
(6.09Æ0.14)ꢁ10À4
(1.44Æ0.04)ꢁ10À3
(2.62Æ0.11)ꢁ10À3
n.d.[c]
(2.32Æ0.21)ꢁ10À4
(4.80Æ0.27)ꢁ10À4
(7.53Æ0.24)ꢁ10À4
(1.22Æ0.06)ꢁ10À3
n.d.[c]
n.d.[c]
[a] Reactions were conducted under argon in a sealed NMR tube. [b] Ini-
tial concentrations: [1Cl]=[1OTf]=0.3m and [1Br]=0.1 M. [c] Not deter-
mined.
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Figure 5. (a) Schematic illustration and (b) energy diagrams of the 1,2-carbo-
boration reactions of diphenylacetylene with 1Cl (black) and 1Br (red) to form
borepin 2, calculated at the wB97X-D/[SDD for Cl and Br, 6-31G(d,p) for C, B,
and H] level. BSSE correction was applied to the energies of CPCl and CPBr.
Figure 4. (a) Arrhenius plot and (b) Eyring plot[16] for the reaction of an equi-
molar mixture of diphenylacetylene and 1Cl, 1Br, or 1OTf in CDCl3. Initial con-
centration: [1Cl]=[1OTf]=0.3m, and [1Br]=0.1m.
the borafluorene moiety (1.553 ꢂ) scarcely change from those
in the RTs (Figure S3 in the Supporting Information). In TSCl, a
BÀC bond is newly formed between the boron atom and one
of the two alkyne carbons (BÀCalkyne: 1.651 ꢂ), which is accom-
panied by the elongation of the CalkyneÀCalkyne bond by 0.062 ꢂ
and one of the BÀCipso bonds of the borafluorene moiety by
approximately 0.1 ꢂ compared with those in CPCl (Figures S7
and S8 in the Supporting Information). Accordingly, Cipso of bor-
afluorene and Calkyne that is non-bonded to boron become
close to each other (interatomic distance: 2.155 ꢂ, Figure S8 in
the Supporting Information), leading to an alkyne insertion to
the BÀCipso bond to form a borepin structure (2). Molecular ge-
ometries of CPBr and TSBr are virtually identical to CPCl and TSCl,
respectively, with the exception of the boron–halogen bonds
(Figures S9 and S10 in the Supporting Information).
Table 3. Experimentally obtained thermodynamic parameters for the 1,2-
carboboration between diphenylacetylene and the borafluorenes (1Cl, 1Br,
or 1OTf) in CDCl3.
Ea[a]
[kcalmolÀ1
lnA[a]
DH°[b]
[kcalmolÀ1
DS°[b]
[calKÀ1 molÀ1
DG°
323.15 K
[b]
]
]
]
[kcalmolÀ1
]
1Cl
1Br
12.3Æ0.8
12.7Æ1.1
10.9Æ1.2 11.7Æ0.8
14.6Æ1.7 12.1Æ1.1
13.8Æ1.9 12.5Æ1.3
À39.1Æ2.4
À31.6Æ3.3
À33.2Æ4.0
24.9Æ1.8
22.3Æ2.1
23.2Æ2.6
1OTf 13.1Æ1.3
[a] Obtained from the Arrhenius plot by using k values in Table 2. [b] Ob-
tained from the Eyring plot[16] by using k values in Table 2.
thalpy terms between 1Br and 1Cl is compensated, resulting in
a positively smaller DG° value for 1Br than 1Cl.
When looking at the total free energies of each reaction
step, the formations of CPCl and CPBr are slightly endothermic
(DG= +2.9 and +4.6 kcalmolÀ1, respectively; Figure 5b). The
DG° value for the 1,2-carboboration to form 2Cl was calculated
to be +17.2 kcalmolÀ1, which was greater than that for the re-
action to form 2Br (+11.9 kcalmolÀ1). This trend is consistent
with that of DG° obtained experimentally (Table 3). The DS°
values were also calculated based on translational, rotational,
and vibrational modes for the computed geometries (Table S14
in the Supporting Information), where the calculated TDS°
value at 298.15 K for 1Cl was negative (À3.0 kcalmolÀ1), where-
as that for 1Br is positive (+0.8 kcalmolÀ1). Although the differ-
ence in the DS° values (3.8 kcalmolÀ1) between the reactions
with 1Cl and 1Br seems not so large at this calculation level, the
computational results may support the experimental observa-
tion that the 1,2-carboboration with 1Br is more entropically fa-
vored than that with 1Cl.
For further understanding of the direct 1,2-carboborations,
we performed DFT calculations on the 1,2-carboboration of di-
phenylacetylene with 1Cl or 1Br in vacuum at the wb97X-D/
[SDD for Cl and Br, 6-31G(d,p) for C, B, and H] level (Figure 5
and Tables S4–S13 in the Supporting Information).[14] These
two reactions not only showed the maximum and minimum
rate constants but also allowed us to unequivocally determine
the computational geometry in each reaction step. Gibbs free
energy was calculated by taking into account the zero-point
energy and intramolecular entropy term at 298.15 K. As illus-
trated in Figure 5a, the borafluorene and diphenylacetylene
(reactants; RTs) molecules approach each other to form a com-
plex (CP), which is transformed into a borepin product (2)
through the formation of a transition state (TS) with a B-Cipso
-
Calkyne-Calkyne quasi-four-membered ring structure. In CPCl, distan-
ces between the boron and the two alkyne carbon atoms are
nearly the same (3.308 and 3.338 ꢂ, Figures 5 and S7 in the
Supporting Information), and the two BÀCipso bond lengths in
We compared the reaction efficiency of 1,2-carboborations
of several diarylacetylenes using borafluorenes 1Cl, 1Br, and 1OTf
(Table 4) under identical conditions. When 1Br or 1OTf (0.25m)
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Table 4. 1,2-Carboboration between diarylacetylenes and the borafluor-
enes (1Cl, 1Br, and 1OTf) in 1,2-DCE.[a]
Entry Alkyne
T [8C], t Yield of borepin 2
[h]
[%][b]
23 (1Cl)
99 (1Br)
89 (1OTf
36 (1Cl)
99 (1Br)
91 (1OTf
46 (1Cl)
99 (1Br)
97 (1OTf
12 (1Cl)
92 (1Br)
66 (1OTf
25 (1Cl)
98 (1Br)
1
2
3
4
5
6
25, 6
)
)
)
)
25, 4
25, 1
Figure 6. (a) Oxidative deborylation/CÀC coupling of 2Br and 2OTf. (b) Se-
quential 1,2-carboboration and oxidative deborylation/CÀC coupling reac-
[11a]
tions of p-bis(phenylethynyl)benzene with 1Br or 1Cl
bridged phenanthrene dimer 3.
to give phenylene-
25, 24
25, 68
80, 16
n.d.[c] (1OTf
57 (1Cl)
)
)
sequent treatment of the reaction mixture with FeCl3 (2 equiv)
led to the formation of a phenylene-bridged phenanthrene
dimer in 81% yield (over two steps in a one-pot manner, Fig-
ure 6b). In contrast, as previously reported, a higher reaction
temperature (e.g., 808C) is required for completing the 1,2-car-
boboration between p-bis(phenylethynyl)benzene and 1Cl
within 24 h under otherwise identical conditions (Fig-
ure 6b).[11a]
91 (1Br)
n.d.[c] (1OTf
[a] Initial concentrations: [1Cl]=[1OTf]=[1Br]=0.25m. [b] Yields determined
by 1H NMR spectroscopic analysis of the reaction mixture by using hex-
amethylbenzene or 1,2-dimethoxybenzene as an internal standard.
[c] Not determined owing to the formation of a complex mixture.
was reacted with diphenylacetylene (1.0 equiv) in 1,2-DCE at
258C for 6 h, 1,2-carboboration proceeded quite efficiently to
give the corresponding borepin, whereas the reaction with 1Cl
was sluggish at 258C (entry 1). Similarly, 1Br and 1OTf also
served as a very efficient reagent for a diarylacetylene with
electron-donating p-methyl (entry 2) or p-methoxy groups
(entry 3), as well as a diarylacetylene with electron-withdrawing
p-Br (entry 4), p-CF3 (entry 5), or p-methoxycarbonyl groups
(entry 6). All the reactions with 1Br smoothly proceeded at
258C, except for entry 6, which required a higher temperature
(808C). Although 1OTf afforded excellent-to-moderate results, it
was inferior to 1Br in functional-group tolerance, given the fact
that the reaction between 1OTf and a p-Br- or p-CF3-substituted
diarylacetylene resulted in the formation of a complex mixture
(entries 5 and 6).
Conclusion
We have described the catalyst-free 1,2-carboboration of diary-
lacetylenes by using three 9-borafluorene derivatives 1Cl, 1Br,
and 1OTf. In the present system, alkyne insertion into the
boron–halogen bonds has not been observed, despite the fact
that 1,2-haloboration is more common than 1,2-carboboration
in related systems.[5a] The experimental and computational re-
sults revealed the kinetic and thermodynamic aspects of this
rare class of direct 1,2-carboboration. The order of reaction effi-
ciency (i.e., reaction rate) of the 1,2-carboboration (1Br >1OTf
>
1Cl) was not consistent with those expected from the energy
level of LUMOs and the NBO charges of boron but correlated
with the acceptor number (AN) of the borafluorenes (1Br >
1
OTf >1Cl). AN, a reliable indicator to evaluate Lewis acidity in
Finally, we confirmed that borepin derivatives 2Br and 2OTf
successfully undergo deborylation/CÀC coupling upon one-
electron oxidation to form 9,10-diarylphenanthrene derivatives.
For example, when 2Br or 2OTf was oxidized with FeCl3 (1 equiv)
in a mixture of 1,2-DCE and MeNO2 at 258C, 9,10-diphenylphe-
nanthrene was obtained quantitatively (Figure 6a). Oxidation
of substituted borepin derivatives obtained from 1Br (Table 4)
by using FeCl3 (1 equiv) also afforded the corresponding 9,10-
diarylphenanthrene in good yields, comparable to the case of
the oxidation of borepins obtained from 1Cl.[11] The 1,2-carbo-
boration of p-bis(phenylethynyl)benzene with two equivalents
of 1Br in 1,2-DCE at 258C completed within 24 h, and the sub-
solution, should reflect the balance between the steric factor
and electronic structure of the borafluorenes and may also be
related to the frequency factor for the coordination of the
boron functionality to the alkyne triple bond. Importantly, the
kinetic analysis and DFT calculations strongly suggested that
the entropy term is more important than the enthalpy term in
determining the reaction rate. Overall, 1Br causes the most effi-
cient 1,2-carboboration to give borepin derivatives in excellent
yields even under very mild conditions. The fact that borepins
2Br and 2OTf, similar to borepin 2Cl, underwent deborylation/
CÀC coupling upon one-electron oxidation to form 9,10-diary-
lphenanthrene derivatives, demonstrates the synthetic utility
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of the borafluorene-mediated benzannulation reaction and
should eventually accelerate the development of extended p-
conjugated systems featuring 9,10-diarylphenanthrene and/or
dibenzo[g,p]chrysene skeletons.
Synthesis of borepin 2Br
Under argon at 258C, diphenylacetylene (36.7 mg, 206 mmol) was
added to a dry 1,2-DCE solution (1.0 mL) of 1Br (50 mg, 206 mmol).
The reaction mixture was stirred at 808C for 12 h and then evapo-
rated to dryness under reduced pressure. The residue was recrys-
tallized from dry hexane at À308C to give 2Br as pale-yellow crys-
tals (65.9 mg, 156 mmol) in 76% yield. d.p.: 135.28C; 1H NMR
(400 MHz, CDCl3, 258C): d=7.93 (d, J=7.6 Hz, 1H), 7.88 (d, J=
7.6 Hz, 1H), 7.73–7.69 (m, 2H), 7.52 (t, J=7.5 Hz, 1H), 7.39 (m, 1H),
7.25–7.20 (m, 2H), 7.13–7.05 (m, 8H), 6.99–6.92 ppm (m, 2H);
11B NMR (128 MHz, CDCl3, 258C): d=65.0 ppm; 13C NMR (100 MHz,
CDCl3, 258C): d=150.6, 143.1, 142.9, 140.0, 139.2, 138.2, 132.3,
132.1, 132.0 (two peaks), 130.6, 130.4, 129.2, 127.6, 127.5 (two
peaks), 127.2, 126.8, 126.6, 126.0 ppm; FTIR (ATR): n˜ =3087, 2921,
2852, 1577, 1466, 1437, 1366, 1296, 1275, 1250, 1209, 1168, 1127,
1075, 1028, 929, 908, 872, 764, 751, 735, 703, 691, 660, 629, 611,
Experimental Section
Materials
Handling of air- and/or moisture-sensitive compounds was per-
formed in a glovebox under argon. n-Hexane was dried by passage
through an activated alumina column and a Q-5 column (Nikko
Hansen & Co., Ltd.). 1,2-Dichloroethane (1,2-DCE), CDCl3, and
MeNO2 were dried over CaH2 and freshly distilled prior to use. 9-
Chloro-9-borafluorene (1Cl),[10] 9-bromo-9-borafluorene (1Br),[12]
bis(4-methylphenyl)acetylene,[17] bis(4-methoxyphenyl)acetylene,[17]
bis(4-trifluoromethylphenyl)acetylene,[17] and bis(4-methyoxycarbo-
nylphenyl)acetylene[17] were prepared according to previously re-
ported procedures.
590, 569, 555, 526, 510 cmÀ1. H, 11B, and 13C NMR spectra of 2Br are
shown in Figures S17, S18, and S19 in the Supporting Information,
respectively.
1
Synthesis of Borepin 2OTf
Under argon at 258C, diphenylacetylene (57.1 mg, 320 mmol) was
added to
a dry 1,2-DCE solution (1.3 mL) of 1OTf (100 mg,
Methods
320 mmol). The reaction mixture was stirred at 808C for 12 h and
then evaporated to dryness under reduced pressure. The residue
was recrystallized from dry hexane at À308C to give 2OTf as pale-
yellow crystals (115.3 mg, 235 mmol) in 73% yield. d.p.: 135.28C;
1H NMR (400 MHz, CDCl3, 258C): d=7.96 (d, J=7.8 Hz, 1H), 7.89 (d,
J=7.6 Hz, 1H), 7.83–7.74 (m, 2H), 7.55 (t, J=7.6 Hz, 1H), 7.44 (m,
1H), 7.31–7.20 (m, 2H), 7.18–7.06 (m, 6H), 6.99–6.85 ppm (m, 4H);
11B NMR (128 MHz, CDCl3, 258C): d=48.0 ppm; 13C NMR (100 MHz,
CDCl3, 258C): d=155.6, 145.2, 142.4, 139.0, 137.8, 132.9, 132.3,
132.2, 130.4, 130.2 (two peaks), 129.7, 128.0 (two peaks), 127.6,
127.3, 127.1, 127.0, 126.5, 118.4 ppm (CF3, q, JCF =316 Hz); 19F NMR
(376 MHz, CDCl3, 258C): d=À76.3 ppm; FTIR (ATR): n˜ =2952, 2923,
2855, 1592, 1486, 1433, 1409, 1323, 1208, 1186, 1141, 1103, 1073,
1053, 1032, 1015, 930, 911, 878, 796, 768, 741, 728, 698, 672, 650,
Melting points (m.p.) and decomposition points (d.p.) were record-
ed with a Yanaco model MP-500D melting-point apparatus. Fourier
transform infrared (FTIR) spectra were recorded at 258C with a
JASCO model FT/IR-4100 Fourier transform infrared spectrometer
with an attenuated total reflection (ATR) equipment (ATR PRO450-
S). Nuclear magnetic resonance (NMR) spectroscopy measurements
were carried out with a Bruker model AVANCE-400 spectrometer
1
(400.0 MHz for H, 128.3 MHz for 11B, 100.6 MHz for 13C, 376.4 MHz
for 19F, 162 MHz for 31P), where chemical shifts (d) were determined
1
with respect to residual solvent peaks for H (residual non-deuter-
1
ated solvent in CDCl3: H(d)=7.26 ppm), external BF3OEt2 in CDCl3
for 11B (11B(d)=0.0 ppm), residual solvent peaks for 13C (CDCl3:
13C(d)=78.0 ppm), external CF3CO2H in CDCl3 for 19F (19F(d)=
À76.6 ppm), and external H3PO4 in D2O for 31P (31P(d)=0.0 ppm).
The absolute values of the coupling constants are given in Hertz
(Hz), regardless of their signs. Multiplicities are abbreviated as sin-
glet (s), doublet (d), multiplet (m), and broad (br). Mass spectrome-
try measurements were carried out with a Bruker micrOTOF II mass
spectrometer equipped with an atmospheric pressure chemical
ionization (APCI) probe.
1
615 cmÀ1. H, 11B, 13C, and 19F NMR spectra of 2OTf are shown in Fig-
ures S20, S21, S22, and S23 in the Supporting Information, respec-
tively.
Synthesis of 9,10-diphenylphenanthrene by the oxidation of
2Br with FeCl3
Under argon at 258C, a dry MeNO2 solution (2.0 mL) of FeCl3
(81 mg, 0.50 mmol) was added to a dry 1,2-DCE solution (2.0 mL)
of 2Br (211 mg, 0.50 mmol). After stirring for 1 h at 258C, the result-
ing mixture was poured into MeOH (100 mL), diluted with water,
and extracted with CH2Cl2. The organic layer was washed with
water, dried over anhydrous Na2SO4, and evaporated to dryness
under reduced pressure. The obtained residue was dissolved in
CH2Cl2, and the solution was passed through a plug of Florisil, to
allow isolation of 9,10-diphenylphenanthrene as colorless crystals
(162 mg, 0.49 mmol) in 98% yield. m.p. (in a sealed tube under
argon): 2428C; 1H NMR (400 MHz, CDCl3): d=8.81 (d, J=8.2 Hz,
2H), 7.67 (ddd, J=8.3, 7.7, 1.3 Hz, 2H), 7.56 (dd, J=8.3, 1.2 Hz, 2H),
7.48 (ddd, J=8.3, 7.7, 1.1 Hz, 2H), 7.28–7.12 ppm (m, 10H);
13C NMR (100 MHz, CDCl3): d=139.7, 137.3, 132.0, 131.1, 130.1,
128.0, 127.7, 126.8, 126.6, 126.5, 122.6 ppm; FTIR (ATR): n˜ =3100,
3056, 3048, 3027, 1606, 1584, 1575, 1527, 1487, 1441, 1419, 1321,
Synthesis of borafluorene 1OTf
Under argon at 258C, trimethylsilyl trifluoromethanesulfonate
(16.7 g, 75.1 mmol) was added to a dry 1,2-DCE solution (20 mL) of
1Cl (2.98 g, 15.0 mmol). The resulting mixture was stirred at 258C
for 12 h and then evaporated to dryness under reduced pressure.
The residue was recrystallized from dry hexane at À308C to give
1OTf as yellow needles (3.47 g, 11.1 mmol) in 74% yield. m.p. (in a
1
sealed tube under argon): 91.28C; H NMR (400 MHz, CDCl3, 258C):
d=7.62 (d, J=7.3 Hz, 2H), 7.43–7.37 (m, 4H), 7.15 ppm (ddd, J=
7.3, 7.9, 1.9 Hz, 2H); 11B NMR (128 MHz, CDCl3, 258C): d=50.2 ppm;
13C NMR (125 MHz, CDCl3, 258C): d=154.0, 136.1, 134.1, 132.3 (B-
Cipso), 128.9, 120.4, 118.6 ppm (CF3, q,
J
CF =317 Hz); 19F NMR
(376 MHz, CDCl3, 258C): d=À75.6 ppm. 1H, 11B, 13C, and 19F NMR
spectra of 1OTf are shown in Figures S14, S11, S15, and S16 in the
Supporting Information, respectively.
1139, 1073, 1047, 1028, 999, 885, 760, 750, 727, 701, 630 cmÀ1
.
APCI-TOF MS: calcd for C26H18 [M]+: m/z=330.14; found: 330.14.
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Synthesis of 9,10-diphenylphenanthrene by the oxidation of
2OTf with FeCl3
Electronic structure calculations
Density functional theory (DFT) calculations were performed by
using the Gaussian 09 program package.[22] All geometry optimiza-
tions including the transitions states were performed by using the
wB97X-D functional. The SDD effective core potential and its basis
set were used for Cl, Br, and S, whereas the 6-31G(d,p) basis set
was used for H, C, B, O, and F. The normal coordinate analysis was
carried out for all stationary points to characterize the optimized
structures on the energy hypersurface and to evaluate the zero-
point energy at each stationary point. The free energy changes
along the reaction path were evaluated by taking into account the
intramolecular entropy terms at 298.15 K. For CPCl and CPBr, the
basis-set superposition error (BSSE) was evaluated by using the
counterpoise method, where BSSE in CPCl and CPBr were 4.0 and
4.1 kcalmolÀ1, respectively. The charge distributions were calculat-
ed with the natural bond orbital (NBO) 5.9 program package.[23–25]
The Cartesian coordinates and energies of the computed structures
are listed in Tables S4–S13 in the Supporting Information.
Using a procedure similar to that for the oxidation of 2Br, 9,10-di-
phenylphenanthrene was obtained in 96% yield from 2OTf (245 mg,
0.50 mmol) and FeCl3 (81 mg, 0.50 mmol).
Synthesis of 3 from 1Br
A dry 1,2-DCE solution (1.5 mL) of a mixture of 1Br (100 mg,
0.41 mmol) and 1,4-bis(phenylethynyl)benzene (57 mg, 0.21 mmol)
was stirred for 24 h at 258C under argon. A dry MeNO2 solution
(1.0 mL) of FeCl3 (67 mg, 0.41 mmol) was added to the reaction
mixture. The resulting mixture was stirred for 1 h at 258C and
poured into MeOH (80 mL), and the precipitate thus formed was
collected by filtration. The residue was dissolved in CH2Cl2, and the
solution was passed through a plug of Florisil, to allow isolation of
3 (95 mg, 163 mmol) as a colorless powder in 81% yield. d.p.:
1
3718C; H NMR (500 MHz, CDCl3): d=8.83–8.80 (m, 4H), 7.70–7.57
(m, 7H), 7.53–7.46 (m, 4H), 7.35–7.29 (m, 4H), 7.24–7.20 (m, 5H),
7.09–7.07 (m, 3H), 7.02–7.00 ppm (m, 3H); 13C NMR (125 MHz,
CDCl3): d=139.7, 137.7, 137.2, 131.3, 130.8, 130.4, 130.3, 130.0 (two
peaks), 127.9, 127.8, 126.8, 127.6, 126.7, 126.6 (two peaks), 126.3
(two peaks), 122.5, 122.4 ppm; FTIR (KBr): n˜ =3062, 3024, 2880,
1609, 1588, 1528, 1511, 1487, 1446, 1418, 1401, 1352, 1321, 1308,
1286, 1237, 1167, 1141, 1103, 1000, 972, 948, 916, 884, 858, 807,
794, 760, 741, 726, 700, 672, 639, 605 cmÀ1; APCI-TOF MS: calcd for
C46H30 [M]+: m/z=582.23; found: 582.23.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “p-Figuration: Control of Electron
and Structural Dynamism for Innovative Functions” (26102008
for T.F. and 26102015 for M.S.), KAKENHI (17H03020 for Y.S.),
and “Dynamic Alliance for Open Innovation Bridging Human,
Environment and Materials” from MEXT, Japan. Y.S. also ac-
knowledges the supports from the Asahi Glass Foundation.
The authors would like to thank Suzukakedai Materials Analysis
Division, Technical Department, Tokyo Institute of Technology,
for their support with the NMR measurements.
Single-crystal X-ray analysis
Single crystals of 2Br (pale-yellow blocks) and 2OTf (pale-yellow
blocks) were obtained from CH2Cl2/hexane and toluene, respective-
ly. A single-crystal sample of each compound was coated with im-
mersion oil (type B: Code 1248, Cargille Laboratories, Inc.) and
mounted on a micromount. Diffraction data were collected at 90 K
under a cold nitrogen gas stream with a Bruker APEX2 platform-
CCD X-ray diffractometer system, using graphite-monochromated
MoKa radiation (l=0.71073 ꢂ). Intensity data were collected by an
w-scan with 0.58 oscillations for each frame. Bragg spots were inte-
grated by using the ApexII program package,[18] and the empirical
absorption correction (multi-scan) was applied by using the
SADABS program.[19] Structures were solved by a direct method
(SHELXT Version 2014/4)[20] and refined by full-matrix least-squares
(SHELXL Version 2014/7).[21] Hydrogen atoms were placed at calcu-
lated positions and refined by applying riding models.
CCDC 1836231 (2Br) and 1836232 (2OTf) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
Conflict of interest
The authors declare no conflict of interest.
Keywords: borafluorene · boranes · borepin · carboboration ·
Lewis acid
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3
¯
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4721.7(14) ꢂ3, Z=8, densitycalcd =1.379 gcmÀ3
,
T=90 K, 2qmax
57.78, MoKa radiation, l=0.71073 ꢂ, m=0.188 mmÀ1, 39876 reflec-
tions measured, 15392 unique reflections, 1261 parameters, Rint
=
=
0.0202, GOF=1.046, R1=0.0481 (I>2s(I)), wR2=0.1380 (all data),
D1min, max =À0.407, 0.764 eꢂÀ3
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Manuscript received: April 12, 2018
Revised manuscript received: May 21, 2018
Accepted manuscript online: June 19, 2018
Version of record online: && &&, 0000
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Chem. Eur. J. 2018, 24, 1 – 9
www.chemeurj.org
8
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Catalyst-free 1,2-carboborations be-
tween diarylacetylenes and 9-borafluor-
enes (1Cl, 1Br, or 1OTf) are described.
Among the borafluorenes, 1Br exhibited
the highest reactivity, which agrees with
the trend of the acceptor number of
the borafluorenes evaluated by the Gut-
mann–Beckett method. Experimental
and computational results indicated
that the activation entropy term, rather
than activation enthalpy term, largely
contributes to the reaction rate.
&
Direct 1,2-Carboboration
Y. Shoji, N. Shigeno, K. Takenouchi,
M. Sugimoto, T. Fukushima*
&& – &&
Mechanistic Study of Highly Efficient
Direct 1,2-Carboboration of Alkynes
with 9-Borafluorenes
Chem. Eur. J. 2018, 24, 1 – 9
www.chemeurj.org
9
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ