Cobalt-Catalyzed Diborylation of 1,1-disubstituted Vinylarenes: A Practical Route to Branched gem-Bis(boryl)alkanes

Article abstract of DOI:10.1002/anie.201710389

We report the first catalytic diborylation of 1,1-disubstituted vinylarenes with pinacolborane using a cobalt catalyst generated from bench-stable Co(acac)₂ and xantphos. A wide range of 1,1-disubstituted vinylarenes underwent this transformati

Full text of DOI:10.1002/anie.201710389

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Accepted Article
Title: Cobalt-Catalyzed Diborylation of 1,1-disubstituted Vinylarenes: A
Practical Access to Branched gem-Bis(boryl)alkanes
Authors: Wei Jie Teo and Shaozhong Ge
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710389
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Angewandte Chemie International Edition
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COMMUNICATION
Cobalt-Catalyzed Diborylation of 1,1-disubstituted Vinylarenes: A
Practical Access to Branched gem-Bis(boryl)alkanes
Wei Jie Teo and Shaozhong Ge*
Abstract: We report the first catalytic diborylation of 1,1-
catalytic hydroboration of vinylboronates, which can be prepared
disubstituted vinylarenes with pinacolborane using a cobalt catalyst
by hydroboration of alkynes, also affords gem-bis(boryl)alkanes.
generated from bench-stable Co(acac)
2
and xantphos. A wide range
For example, Hall and Yun developed
a
Cu-catalyzed
of 1,1-disubstituted vinylarenes underwent this transformation to
produce the corresponding gem-bis(boryl)alkanes in modest to high
isolated yields. This cobalt-catalyzed protocol can be readily
conducted on a gram scale without the use of a dry box and
represents a practical and effective approach to access a wide
range of branched gem-bis(boryl)alkanes.
asymmetric hydroboration of vinyl-Bdan with HBpin to prepare
[
17]
chiral gem-bis(boryl)alkanes (Scheme 1B).
and Xiao have reported a Ni-catalyzed diborylation of alkenes
with B pin in the presence of bases at 130 °C, but only
monosubstituted alkenes undergo this Ni-catalyzed diborylation
Very recently, Fu
2
2
[
18]
(Scheme 1C).
However, gem-bis(boryl)alkanes prepared by
these catalytic dihydroboration or diborylation reactions of
alkynes or alkenes are limited to the ones with methylene
groups at the β-position.
Organoboronate compounds are synthetically valuable building
blocks in organic synthesis due to their low toxicity and high
[
1]
functional-group compatibility.
As an important class of
organoboryl compounds, gem-bis(boryl)alkanes can undergo a
wide range of organic transformations. For examples, the
carboanions generated from gem-bis(boryl)alkanes in the
presence of bases can selectively react with a variety of
[
2]
electrophiles, such as alkylhalides,
N-heteroaromatic N-
In addition, gem-
[
3]
[4]
[5]
oxides,
alkenes,
and carbonyls.
bis(boryl)alkanes can undergo various metal-catalyzed cross-
[
6]
[7]
coupling reactions, such as arylation and vinylation, allylic
[
8]
[9]
[10]
substitution,
alkylation,
epoxide ring-opening
and 1,2-
[
11]
[12]
addition of imines
and carbonyl groups.
Furthermore, the
recent studies on sequential reactions using gem-
bis(boryl)alkanes have highlighted their synthetic utilities as one
of boryl groups remains intact after conversions of one boryl
[
7a, 7c, 5c]
group.
Therefore, it is highly desirable to develop
practical and atom-economic protocols, especially the ones that
can address the limitations of current synthetic methodologies,
for syntheses of gem-bis(boryl)alkanes from readily accessible
starting materials.
Scheme 1. Catalytic protocols for the synthesis of gem-bis(boryl)alkanes.
Conventionally, gem-bis(boryl)alkanes can be prepared via
Diborylation of methylene C-H bonds has also been
the lithiation of bis(boryl)methane with LDA or Li(TMP) followed
[
19]
[
13]
developed to prepare gem-bis(boryl)alkanes, but the bis(boryl)
products were limited to 1,1-benzyldiboronates. Very recently,
Chirik reported a Co-catalyzed diborylation of non-benzylic C-H
bonds in branched alkylarenes to access gem-bis(boryl)alkanes
by the reactions with alkylhalides
or the reactions of gem-
[
14]
2 2
dihaloalkanes with B pin .
However, the use of strong bases
or limited availability of dibromo-analogues limits the scope of
gem-bis(boryl)alkane products. Recently, Liu has reported a new
approach to prepare gem-bis(boryl)alkanes through a base-
induced borylation of C-OBpin bonds in α-oxyboronate
[
19b]
containing a branched alkyl group (Scheme 1D).
However,
this transformation requires very high catalyst loading and
copious amounts of boron reagents, and the desired gem-
bis(boryl)alkane products were obtained in low yields (15-30%).
In view of broad synthetic applications of gem-bis(boryl)alkanes,
we are interested in develop a selective and efficient protocol to
synthesize gem-bis(boryl) compounds containing branched alkyl
groups through a Co-catalyzed diborylation of 1,1-disubstituted
vinylarenes with pinacolborane (Scheme 1E).
[
15]
alkylboronates (RC(OBpin)Bpin) with B
2
Pin
2
.
However, this
borylation reaction requires boryl-pre-functionalized starting
materials.
To develop more atom-economic approaches to prepare
gem-bis(boryl)alkanes, catalytic dihydroboration of alkynes was
studied and various transition metal catalysts based on Rh, Cu
[
16]
and Co have been developed (Scheme 1A).
Alternatively,
Recently, cobalt complexes have been emerging as active
[
20]
catalysts for hydroboration
and dehydrogenative borylation
[
21]
reactions
of alkenes or alkynes to prepare organoboron
[*]
W. J. Teo, Prof. Dr. S. Ge
Department of Chemistry, National University of Singapore
Science Drive 3, Singapore 117543 (Singapore)
compounds with one or multiple boryl groups. During our studies
3
on the Co-catalyzed hydrofunctionalization of unsaturated
E-mail: chmgsh@nus.edu.sg
Homepage: www.geresearchgroup.com
[21f,22]
hydrocarbons,
we found that the reaction of α-
and
xantphos afforded the hydroboration product 1a, together with
methylstyrene with HBpin in the presence of Co(acac)
2
Supporting information for this article is available on the WWW
under http://angewandte.org or from the author.
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Angewandte Chemie International Edition
10.1002/anie.201710389
COMMUNICATION
an unexpected gem-bis(boryl) product 2a (eq 1). In this reaction,
substituted vinylarenes containing electronically varied aryl
groups reacted smoothly at 50 °C, yielding the corresponding
gem-bis(boryl)alkenes (2a−2r) in modest to high isolated yields
4
0% of α-methylstyrene was sacrificed and concomitantly
hydrogenated to cumene. In order to develop this reaction into a
[
23]
selective protocol for the synthesis of gem-bis(boryl) compounds, (55−87%).
The GC-MS analysis on the crude mixtures of
we tested various alkenes as hydrogen acceptors and cobalt
catalysts generated from Co(acac) and various bisphosphine
these reactions indicated the formation of trace amounts (< 5%)
of hydroboration products and iso-propylarenes that were
generated by hydrogenation of α-methyl vinylarenes. In addition,
we also tested vinylarene with various aliphatic substituents at
the α-position for this transformation. These vinylarenes
containing sterically varied linear (2s−2aa) or cyclic (2ab−2af)
aliphatic groups also reacted smoothly to produce the desired
bis(boryl) compounds (2s−2af), albeit in modest yields. This
cobalt-catalyzed bisborylation reaction of vinylarenes tolerates
various functionalities, including ether (2b−2d, 2f, 2i, 2ac, and
2
ligands for this transformation. The selected experiments are
summarized in Table 1.
Bpin
Bpin
Bpin
Co(acac)2 (2 mol %)
xantphos (2 mol %)
Me
Me
Me + HBpin
+
+
(1)
THF, 50 °C, 12 h
Me
Me
1
a, 20%
2a, 40%
40%
2
This hydroboration reaction catalyzed by Co(acac) /xantphos
in the presence of 1.2 equivalent of norbornene afforded the
desired bis(boryl)alkene 2a in 88% yield with an excellent
chemoselectivity (96% for 2a, entry 1 in Table 1). Decreasing
2
af), thioether (2j), siloxy (2q and 2z), chloro (2m), tertiary
amine (2k and 2x), chloro (2m), sulfonyl (2o), acetal (2r),
carboxylic ester (2y), pinacolboryl (2aa), and carbamate (2ae
and 2af) moieties. Ketone, and free hydroxyl groups are
incompatible with the reaction conditions. However, vinylarenes
containing a ketone (2r) protected as acetal and an alcohol (2q
and 2z) protected as silyl ether reacted to afford the
corresponding bis(boryl) product in good yields.
the amount of norbornene to
1 equivalent resulted in a
decreased selectivity to 2a (entry 2 in Table 1). Similarly high
yield and selectivity for 2a were obtained for this reaction when
norbornadiene was used as a hydrogen acceptor (entry 3 in
Table 1). When other alkenes, such as cyclohexene, tert-
butylethylene, or 3-ethyl-2-pentene, were used as hydrogen
acceptors, this reaction showed significantly lower selectivity for
[
a]
Table 2. Scope of 1,1-disubstituted vinylarenes
2
a (entries 4-6 in Table 1). However, when catalyzed by the
Co(acac)2 (2 mol %)
xantphos (2 mol %)
Bpin
Bpin
R
combination of Co(acac) and other bisphosphine ligands, such
2
norbornene (1.2 equiv.)
R
+ HBpin
as dppe, dcpe, dppp, dppbz, or dpephos, these reactions
afforded the hydroboration product 1a together with 2-
norbornylboronate as major products and only small amounts of
product 2a were detected (entries 7-11 in Table 1).
R'
THF, 50 °C, 12 h
R'
2
a-2af
Bpin
Bpin
Me
Bpin
Bpin
Me
OMe
Bpin
Bpin
Me
Bpin
Bpin
Me
Table 1. Evaluation of conditions for Co-catalyzed diborylation of α-
methylstyrene with pinacolborane
OEt
MeO
[
a]
2
a, 73%
Bpin
2b, 80%
2c, 87%
2d, 60%
Bpin
Me
Co(acac)2 (2 mol %)
ligand (2 mol %)
hydrogen acceptor (1.2 equiv.)
Bpin
Bpin
Me
Bpin
Bpin
Me
Bpin
Bpin
Me
Bpin
Me
Bpin
Bpin
Me
Me
O
Me
Me + HBpin
ligand
+
THF, 50 °C, 12 h
O
tBu
Me
Me
e, 68%
1
a
2a
hydrogen acceptor Conversion (%)[b] Yield of 2a (%)[b] 1a:2a[b]
2
f, 55%
2g, 66%
2h, 70%
2
entry
Bpin
Bpin
Me
Bpin
Bpin
Me
Bpin
Bpin
Me
Bpin
Bpin
Me
1
2
xantphos
xantphos
norbornene (nbe)
nbe (1 equiv.)
>99
>99
88
74
4:96
12:88
F3C
3
4
5
6
7
8
9
xantphos
xantphos
xantphos
xantphos
dppe
norborndiene
>99
>99
>99
>99
20
83
63
47
39
<5
5:95
24:76
16:84
34:66
94:6
R
Me2N
X
R = OMe, 2i, 71%
R = SMe, 2j, 67%
2k, 59%
X = F, 2l, 66%
X = Cl, 2m, 31%[b]
2n, 77%
cyclohexene
Bpin
Bpin
Me
tert-butylethylene
Bpin
Bpin
Me
Bpin
Bpin
Me
Bpin
Bpin
3-ethyl-2-pentene
Me
O
nbe
nbe
nbe
nbe
nbe
MeO2S
MeO2C
TBSO
Bpin
O
Me
2r, 60%
dcpe
24
<5
10
<5
22
97:3
70:30
99:1
o, 40%[b,c]
2p, 25%[b,c]
2
2q, 65%
dppp
38
Bpin
Bpin
Bpin
Me
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Me
1
0
dppbz
55
Ph
Me
Me
11
dpephos
32
15:85
Me
2u, 65%
Bpin
Me
Me Me
2s, 67%
Bpin Bpin
2t, 62%
Bpin
2v, 40%
2w, 45%
Bpin Bpin
Cy2P
Ph2P
PCy2 dcpe
PPh2
PPh2
PPh2
Bpin
Bpin
dppe
O
O
CO2Me
OTIPS
Bpin
N
PPh2
PPh2
3
3
PPh2
PPh2
N
Ph2P
PPh2 dppp
dppbz
Ph
xantphos
dpephos
2
x, 65%
2y, 63%
2z, 45%
2aa, 63%
Bpin Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
[
a] Conditions: α-methylstyrene (0.250 mmol), pinacoborane (0.525 mmol),
Co(acac) (5.0 µmol), ligand (5.0 µmol), hydrogen acceptor (0.300 mmol), THF
1 mL), 50 °C, 12 h; The conversion of α-methylstyrene, yield of 2a, and the
MeO
2
b
(
N
N
O
ratios of 1a:2a were determined with gas chromatography (GC) analysis with
Boc
2ae, 61%
Boc
2af, 55%
2
ab, 51%
2ac, 62%
2ad, 45%
dodecane as an internal standard.
Under the identified conditions (entry 1 in Table 1), we
studied the scope of 1,1-disubstituted vinylarenes that undergo
this cobalt-catalyzed bisborylation reaction and the results are
summarized in Table 2. In general, a wide range of α-methyl-
[a] Conditions: 1,1-disubstituted vinylarene (0.250 mmol), pinacolborane
0.525 mmol), norbornene (0.300 mmol), Co(acac) (5.0 µmol), xantphos (5.0
µmol), THF (1 mL), 50 °C, 12 h; isolated yields; [b] 10 mol % catalytic loading;
c] 70 °C.
(
2
[
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Angewandte Chemie International Edition
10.1002/anie.201710389
COMMUNICATION
After establishing the scope of this bisborylation reaction, we
formation of norbornane-d
2
. Subsequently, we conducted this
subsequently demonstrated synthetic utilities of this protocol
bisborylation reaction with α-ethyl-4-tert-butyl-styrene-d
HBpin and found that both deuterium atoms were retained in the
product 2ag-d , but with one deuterium shifted from terminal
2
and
(
Scheme 2). First, we tested a gram scale reaction of 1-
methylene-1,2,3,4-tetrahydronaphthalene and HBpin with
Co(acac) and xantphos weighed on bench top, and this reaction
2
2
vinylic carbon to the benzylic carbon of 2ag (Scheme 3B).
proceeded smoothly under the developed conditions to produce
Furthermore, we conducted a crossover experiment using a 1:1
product 2ab in 54% isolated yield (Scheme 2A). Bis(boryl)alkane
2
mixture of α-ethyl-4-tert-butyl-styrene-d and α-methylstyene,
2
ab was oxidized with NaBO
3
to form an aldehyde 3 in 59%
and found that this reaction did not afford any H/D-scrambled
products (Scheme 3C). The result of this crossover experiment
suggests that the deuterium shift observed in Scheme 3B
occurred intramolecularly.
yield (Scheme 2B), which provides a facile pathway to achieve
aldehydes through functionalization of alkenes. This
bis(boryl)alkane 2ab also underwent a boron-Wittig reaction with
an aldehyde to afford ketone 4 in 65% isolated yield (Scheme
Based on the results of deuterium labelling experiments and
the precedent of cobalt-catalyzed hydroboration and
2
C). Furthermore, Suzuki-Miyaura cross-coupling between 2ab
and 4-iodoanisole occurred in a diastereoselective fashion in the
[
20,21]
t
dehydrogenative borylation reactions of alkenes,
proposed possible catalytic cycle for this Co-catalyzed
bisborylation reaction (Scheme 4A). Activation of Co(acac) with
HBpin in the presence of a bisphosphine ligand generates a
we
3 2
presence of 5 mol% of Pd(P Bu ) , affording a benzyl boronate 5
a
with a high diastereoselectivity (dr > 20:1), albeit in a modest
2
isolated yield (Scheme 2D).
[
21c,24]
H
O
Co(I)-H species,
the reaction with norbornene and HBpin through intermediate I-
which turns into a Co(I)-Bpin species by
NaBO3•4H2O (10 equiv.)
H2O/THF, RT, 1h
[
25]
(B)
A.
Subsequently, 2,1-migratory insertion of a vinylarene with
1
) LiTMP (1 equiv.)
3, 59%
this Co(I)-Bpin species forms a sterically hindered alkylcobalt
intermediate I-B, which undergoes valence tautomerization to
less hindered alkylcobalt species I-C through a cyclic σ,π-
THF, -78−0 °C, 4 h
Co(acac)2 (2 mol %)
xantphos (2 mol %)
nbe (1.2 equiv.)
2
) PhCH2CHO (1 equiv.)
) NaBO3•4H2O (10 equiv.)
H2O/THF, RT, 2 h
Bpin
Bpin
Ph
O
3
[
26]
THF, 50 °C, 12 h
(A)
transition state. The alkylcobalt I-C then turns over with HBpin
+
(C)
HBpin
to afford the bis(boryl)alkane product and regenerates the Co(I)-
2
ab
4
, 65%
1
.07 g, 54% yield
H
species. To gain further support for this pathway, we
rationalize that the allylboronate can undergo migratory
insertion into Co(I)-H species to form the alkylcobalt
MeO
4
-iodoanisole (1 equiv.)
6
t
Pd(P Bu3)2 (5 mol%)
Bpin
a
TBAH (5 equiv.)
H2O/1,4-Dioxane, RT, 12h
D)
intermediate I-B, which reacts with HBpin and releases the
product 2a (Scheme 4B). Indeed, the hydroboration of the
(
5, 42%; >20:1 dr
alkylboronate with HBpin catalyzed by Co(acac)
occurred smoothly to afforded the gem-bis(boryl)alkane 2a in
1% yield (Scheme 4C).
2
/xantphos
Scheme 2. Synthetic utilities of this cobalt-catalyzed bis(borylation) reaction.
5
7
1
7% D
(A)
Co(acac)2 + L + HBpin
5
0% D
H C
Bpin
Bpin
3
L = xantphos
2
Co(acac) (2 mol %)
Bpin
Bpin
Et
Bpin
D/H
(H/D)2
Me
xantphos (2 mol %)
nbe (1.2 equiv.)
nbe
A)
Et
Ph
(L)Co-H
2a
+
+
nba-d
3%
2
tBu
H
THF, 50 °C, 12 h
5
+
(H/D)
2
(L)Co
HBpin
tBu
tBu
(by 2H NMR)
HBpin
Bpin
minor pathway)
DBpin
2
ag, 51%
1ag, 30%
I-A
HBpin
nba
1
3% D
No deuterium incorporation
(
CH3
Ph
Bpin
9
7% D
9
8% D
9
5% D
Co(acac) (2 mol %)
2
D
H
D
D
D
xantphos (2 mol %)
nbe (1.2 equiv.)
Bpin
Bpin
(L)Co-Bpin
(
L)Co
HBpin
H3C
(minor pathway)
Bpin
Me
B)
+ HBpin
Et
THF, 50 °C, 12 h
Et
I-C
D
tBu
t
Bu
95% D
Ph 1a
Ph
2
ag-d , 57%
2
Ph
Me
H
9
8% D
D
95% D
Me
Ph
(
L)Co
(L)Co
H
Bpin
Et
2
Co(acac) (2 mol %)
Bpin
I-B
xantphos (2 mol %)
nbe (1.2 equiv.)
97% D
Bpin
tBu
D
D
0
0
.5 equiv.
+
Bpin
Bpin
Bpin
Me
(B)
HBpin (2.1 equiv.)
Me
Ph
HBpin
C)
+
Bpin
Bpin
(
L)Co
H3C
Ph
THF, 50 °C, 12 h
Et
+
(L)Co-H
Ph
(C)
Bpin
Bpin
Me
.5 equiv.
- (L)Co-H
tBu
95% D
, 30%
Bpin
I-B
6
6
2a
2
ag-d
2
2a, 34%
No deuterium incorporation
Co(acac)2 (2 mol %)
xantphos (2 mol %)
Bpin
Bpin
Me
+
HBpin
Scheme 3. Deuterium-labelling experiments
THF, 50 °C, 12 h
Ph
2
a, 51% GC yield
To provide insights into the reaction mechanism, we
conducted a series of deuterium-labelling experiments (Scheme
Scheme 4. Proposed catalytic pathway for the cobalt-catalyzed diborylation of
1,1-disubstituted vinylarenes
3
). The bisborylation reaction of α-ethyl-4-tert-butyl-styrene with
DBpin under standard conditions afforded the desired gem-
bis(boryl)alkane 2ag in 51% isolated yield (Scheme 3A).
However, no deuterium incorporation into 2ag was detected by
Alternatively, the alkylcobalt I-B can undergo β–H elimination
to form a vinylboronate and a Co(I)-H species and subsequent
insertion of the vinylboronate into this Co(I)-H species generates
2
the H NMR analysis. Instead, GC-MS analysis indicated the
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Angewandte Chemie International Edition
10.1002/anie.201710389
COMMUNICATION
the alkylcobalt species I-C [eq (2)]. To test this possibility, we
[8]
J. Kim, S. Park, J. Park, S. H. Cho, Angew. Chem. Int. Ed. 2016, 55,
1
498.
a) K. Endo, T. Ohkubo, T. Ishioka, T. Shibata, J. Org. Chem. 2012, 77,
826; b) Z.-Q. Zhang, C.-T. Yang, L.-J. Liang, B. Xiao, X. Lu, J.-H. Liu,
conducted reactions of a vinylboronate 7 with HBpin catalyzed
[
9]
by Co(acac)
norbornene, and found that the desired bis(boryl)alkane product
a was not formed under both sets of conditions [eq (3)]. The
2
/xantphos in the presence or in the absence of
4
Y.-Y. Sun, T. B. Marder, Y. Fu, Org. Lett. 2014, 16, 6342; c) F. Li, Z.-Q.
Zhang, X. Lu, B. Xiao, Y. Fu, Chem. Commun. 2017, 53, 3551.
2
results of these experiments suggest that vinylboronates are not
intermediates for this diborylation reaction, which is in contrast to
[
10] a) A. Ebrahim-Alkhalil, Z.-Q. Zhang, T.-J. Gong, W. Su, X.-Y. Lu, B.
Xiao, Y. Fu, Chem. Commun. 2016, 52, 4891; b) S. A. Murray, M. Z.
Liang, S. J. Meek, J. Am. Chem. Soc. 2017, 139, 14061.
[
21a]
the triborylation reaction of styrenes to form alkyltriboronates.
[
[
11] a) J. Park, Y. Lee, J. Kim, S. H. Cho, Org. Lett. 2016, 18, 1210; b) J.
Kim, K. Ko, S. H. Cho, Angew. Chem. Int. Ed. 2017, 56, 11584.
H3C
Ph
Bpin
H C
Bpin
3
Me
Ph
CH3
Ph
Ph
12] a) M. V. Joannou, B. S. Moyer, M. J. Goldfogel, S. J. Meek, Angew.
Chem. Int. Ed. 2015, 54, 14141; b) M. V. Joannou, B. S. Moyer, S. J.
Meek, J. Am. Chem. Soc. 2015, 137, 6176; c) S. A. Murray, J. C. Green,
S. B. Tailor, S. J. Meek, Angew. Chem. Int. Ed. 2016, 55, 9065.
(
L)Co
H
(2)
(
L)Co-H
β-H elimination
migratory insertion
Bpin
Bpin
I-B)
(L)Co
(
(I-C)
Co(acac)2 (2 mol %)
xantphos (2 mol %)
w/o norbornene
Bpin
Me + HBpin
, Z/E = 84:16
Bpin
Bpin
Me
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Acknowledgements
This work was supported by the National University of Singapore
(
No. R-143-000-614-133 and R-143-000-630-133). WJT thanks
Dr. Ji’En Wu for the assistance with NMR analysis.
Keywords: cobalt • diborylation • vinylarene • homogeneous
catalysis • gem-bis(boryl)alkane
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Angewandte Chemie International Edition
10.1002/anie.201710389
COMMUNICATION
COMMUNICATION
Wei Jie Teo and Shaozhong Ge*
Page No. – Page No.
Cobalt-Catalyzed Diborylation of 1,1-
disubstituted Vinylarenes: A Practical
Access to Branched gem-
Bis(boryl)alkanes
Alkene Diborylation: A highly chemoselective cobalt-catalyzed diboration of 1,1-
disubstituted vinylarenes with pinacolborane has been developed using a catalyst
2
generated from bench-stable Co(acac) and xantphos (see the Picture). A wide
range of α-vinylarenes reacted to afford the corresponding branched gem-
bis(boryl)alkanes in high isolated yields.
This article is protected by copyright. All rights reserved.