Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3Does What B(C6F5)3Cannot Do!

Article abstract of DOI:10.1002/chem.201603466

The transition-metal-free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5-bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron-deficient diboranes [H₂BAr^F]₂and [(Ar^F)(H)B(μ-H)₂BAr^F₂]. These in situ-generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin-derived anti-Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.

Full text of DOI:10.1002/chem.201603466

A Journal of
Accepted Article
Title: Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane:
BArF3 Does What B(C6F5)3 Cannot Do!
Authors: Qin Yin; Sebastian Kemper; Hendrik Klare; Martin Oestreich
This manuscript has been accepted after peer review and the authors have elected to
post their Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by using the Di-
gital Object Identifier (DOI) given below. The VoR will be published online in Early
View as soon as possible and may be different to this Accepted Article as a result
of editing. Readers should obtain the VoR from the journal website shown below
when it is published to ensure accuracy of information. The authors are responsible
for the content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201603466
Link to VoR: http://dx.doi.org/10.1002/chem.201603466
Supported by

Chemistry - A European Journal
10.1002/chem.201603466
COMMUNICATION
Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with
F
Pinacolborane: BAr Does What B(C F ) Cannot Do!**
3
6 5 3
Qin Yin, Sebastian Kemper, Hendrik F. T. Klare,* and Martin Oestreich*
Dedicated to Professor Gerhard Erker on the occasion of his 70th birthday
Abstract: The transition-metal-free hydroboration of various alkenes
with pinacolborane (HBpin) initiated by tris[3,5-
bis(trifluoromethyl)phenyl]borane (BAr ) is reported. The choice of
the boron Lewis acid is crucial as the more prominent boron Lewis
acid tris(pentafluorophenyl)borane (B(C ) is reluctant to react.
is found to engage in substituent redistribution
The analogous hydroboration would require the catalytic
generation of the boron electrophile for its reaction with the
alkene nucleophile. The feasibility of this strategy is documented
by several examples using borenium ions or equivalents
F
3
[
9–14]
6
F
5
)
3
thereof.
generated by heterolytic B–H bond cleavage and, in fact,
B(C is known to facilitate hydride abstraction from
commonly used hydroboranes such as catecholborane
These cationic boron intermediates are typically
F
Unlike B(C
with HBpin, resulting in the formation of Ar Bpin and the electron-
6 5 3 3
F ) , BAr
F
6 5 3
F )
F
F
F
2 2 2 2
deficient diboranes [H BAr ] and [(Ar )(H)B(µ-H) BAr ]. These in-
[15–17]
[11,18]
situ-generated hydroboranes undergo regioselective hydroboration
of styrene derivatives as well as aliphatic alkenes with cis
diastereoselectivity. Another ligand metathesis of these adducts with
HBpin subsequently affords the corresponding HBpin-derived anti-
Markovnikov adducts. The reactive hydroboranes are regenerated in
this step, thereby closing the catalytic cycle.
(HBcat),
borabicyclo[3.3.1]nonane (9-BBN).
assistance of Lewis base (LB) such as
pinacolborane
(HBpin),
and
this, the
tertiary
9-
[
13,19,20]
For
a
a
amine/phosphine or N-heterocyclic carbene is essential, mainly
to energetically favour the B–H heterolysis in the neutral
tricoordinate hydroborane by coordination and to stabilize the
resulting boron cation in form of its borenium ion. Due to the
increased hydridic character of the B–H bond, the Lewis base
Hydroboration, i.e., the (formal) addition of B‒H bonds
adduct of the hydroborane rather than the borohydride
across multiple bonds, finds wide application in synthetic
–
[
HB(C
6
F
5
)
3
] functions as the actual reducing agent in borenium
[
1,2]
chemistry.
To overcome reactivity and selectivity issues in
[11]
ion-catalyzed reductions.
these reactions, various transition-metal-catalyzed protocols
[
1]
were developed. However, a practical transition-metal-free
approach to alkene hydroboration with simple hydroboranes that
employs a Lewis acid as a catalyst is missing although Finke
and Moretto had already introduced such a straightforward
technique to the related hydrosilylation nearly four decades
[
3–7]
ago.
3
By replacing AlCl , which had initially been used as the
[
4]
catalyst for this transformation, with the strong boron Lewis
acid B(C
[
8]
6
5
F )
3
,
Gevorgyan and co-workers disclosed a highly
efficient alkene hydrosilylation that proceeds with anti-
[
5]
Markovnikov regioselectivity (Scheme 1A).
Notably, this
methodology follows an ionic mechanism, in which the Si–H
1
bond of the hydrosilane is activated by reversible η -coordination
6 5 3
to B(C F ) , thereby allowing for nucleophilic substitution of the
hydride at the silicon atom by the alkene π-nucleophile. The
resulting ion pair composed of a β-silicon-stabilized carbenium
ion and a borohydride then undergoes sterically controlled
hydride transfer to form the hydrosilylation adduct concomitant
with regeneration of the Lewis-acid catalyst. Unlike typical
transition-metal-mediated cis-selective hydrosilylations, trans-
addition of the Si–H bond across the C–C double bond is
obtained.
Scheme 1. Boron Lewis acid-catalyzed hydrosilylation (known) and
hydroboration (unknown) of alkenes. Ar = 3,5-bis(trifluoromethyl)phenyl.
F
We
show
here
that
rarely
used
tris[3,5-
F
[21]
bis(trifluoromethyl)phenyl]borane (BAr
3
)
enables the catalytic
hydroboration of various alkenes with HBpin without the
assistance of an external base (Scheme 1B). Moreover, we
F
reveal that BAr
3 6 5 3
(active) and equally potent B(C F ) (inactive)
[
+]
[*]
Dr. Q. Yin, Dr. S. Kemper, Dr. H. F. T. Klare, Prof. Dr. M.
Oestreich
behave totally differently in this transformation. This led us to
investigate the mechanism of action to explain this striking
reactivity difference. The verified reaction pathway also
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: hendrik.klare@tu-berlin.de
distinguishes itself from those of Lewis-acid-catalyzed
martin.oestreich@tu-berlin.de
[3–7]
hydrosilylations
or hydroborations involving borenium- or
Homepage: http://www.organometallics.tu-berlin.de
NMR spectroscopic measurements
[9–14]
+
boronium-ion catalysis.
[
[
]
F
**]
Ar = 3,5-Bis(trifluoromethyl)phenyl
While parent borane (B
2
6 3
H as well as BH ·LB with LB =
Supporting information for this article is given via a link at the end of
the document.
Me
2
S or THF) and dialkylboranes such as 9-BBN are known to
readily add across alkenes, boronic esters are usually reluctant

Chemistry - A European Journal
10.1002/chem.201603466
COMMUNICATION
to react in the absence of
a
catalyst even at elevated
F
F
F
8
9
1
1a
1a
1a
1a
1a
2a
2a
2a
2a
BAr
BAr
BAr
3
3
3
50
80
50
50
50
50
80
50
80
4
93:7
>99:1
—
52
[
22]
temperature.
We therefore selected HBpin (3) for it to react
4
49
with styrene (1a) or oct-1-ene (2a) (Table 1). As expected,
treatment of neat 1a with HBpin (3) at 80°C for prolonged
reaction times only showed little conversion (entry 1). When then
tested various Lewis acids as potential catalysts for this
[
d]
0
24
15
15
40
24
40
24
0
11
12
BF
3
·OEt
2
99:1
>99:1
—
20
transformation. As opposed to the related hydrosilylation, AlCl
3
[
4]
BPh
3
10
had no effect (entry 2). We then turned toward boron Lewis
acids but BCl provided no improvement (entry 3). More
electron-deficient B(C , the superior catalyst for Lewis acid-
3
13
B(C
B(C
6
F
5
)
3
traces
traces
81
6 5 3
F )
[
5]
catalyzed hydrosilylation (Scheme 1A), showed no catalytic
activity in the hydroboration (entries 4 and 5). This result did not
come as a surprise as Crudden and co-workers had already
14
6
F
5
)
3
—
F
3
15
BAr
99:1
>99:1
F
reported that B(C
6
F
5
)
3
accelerates the Rh(I)-catalyzed
16
BAr 3
80
hydroboration of alkenes but does not promote this reaction
[
18]
alone.
Inspired by Crudden’s related imine hydroboration
[a] All reactions were performed on a 0.2 mmol scale in a sealed tube. [b] The
regioselectivity was determined by GLC analysis. [c] Isolated yield after
purification by flash chromatography on silica gel. [d] DABCO (2 mol%) was
used as additive. DABCO = 1,4-diazabicyclo[2.2.2]octane.
+
–
using the borenium ion [(DABCO)Bpin] [HB(C
,4-diazabicyclo[2.2.2]octane) as catalyst, we decided to add
catalytic amounts of DABCO to probe whether this method could
be extended to the alkene hydroboration (entry 6). Again, no
6
5
F )
3
] (DABCO =
[
11]
1
adduct formation was detected. In stark contrast, the use of
F
BAr
3
had a dramatic effect on the reaction outcome: full
We then subjected oct-1-ene (2a) as a representative
example for an aliphatic alkene to the reaction with HBpin (3)
conversion of styrene (1a) was now observed at room
temperature, and pinacol boronic ester 4a was obtained in 31%
isolated yield with high anti-Markovnikov regioselectivity (entry
(
(
Table 1, entries 13–16). B(C
entries 13 and 14) while BAr
6
F
F
5
)
3
did not promote this reaction
3
displayed high catalytic activity,
7). Polymerization of 1a accounts for the moderate yield. Raising
furnishing the anti-Markovnikov adduct 6a with excellent
regioselectivity (entries 15 and 16). Isolated yields are higher
with α-olefins than with styrenes as polymerization is not
competing.
the temperature to 50°C significantly shortened the reaction time,
affording 4a in 52% yield after 4 h (entry 8). Attempts to further
optimize the reaction conditions were largely unsuccessful; e.g.,
heating to 80°C gave comparable results (entry 9).The addition
of DABCO completely suppressed the hydroboration process
With the optimized protocol in hand, we applied it to
differently substituted styrene derivatives (Scheme 2). The new
method emerged as widely applicable, and both electron-
donating and -withdrawing groups were tolerated on the
benzene ring, furnishing the anti-Markovnikov hydroboration
adducts exclusively. Styrenes 1b–d decorated with a methyl
substituent at the aryl group were converted with excellent
regioselectivities independent of the substitution pattern.
However, yields were rather moderate (45–53%) as
polymerization was again an issue and became prevalent in the
case of 4-methoxystyrene (1e) where only traces of adduct 4e
were seen. Conversely, styrenes with an electron-withdrawing
group (1f–h) performed significant-ly better, affording pinacol
boronic esters 4f–h in higher yields (68–71%). Similarly, 2-
vinylnaphthalene (1i) underwent smooth reaction in 81% yield.
α- and β-Substituted styrene, e.g., 1j–k, and 1m, derivatives
were also tested successfully. Diminished reactivity was
generally observed for these substrates but using two
equivalents of HBpin (3) under slightly forcing conditions (80°C
for 40 h) led to improved yields (e.g., 86% for 4k). For un-known
reasons, (E)-β-methylstyrene (1l) proved to be unreactive while
(
entry 10). Less Lewis-acidic BF
turnover albeit with considerably lower efficiency (entries 11 and
2).
3 2 3
·OEt and BPh also showed
1
[
a]
Table 1: Development of a Lewis acid-catalyzed hydroboration of alkenes.
[
b]
[c]
Entry
Alkene
Catalyst
T [°C]
t [h]
rs
Yield [%]
1
2
3
4
5
1a
1a
1a
1a
1a
1a
1a
—
80
50
50
RT
50
50
RT
24
15
15
15
15
15
15
—
—
—
—
—
—
traces
AlCl
3
traces
BCl
B(C
B(C
B(C
3
traces
(
E)-stilbene (1m) was cleanly converted into 4m in 87% yield.
6
F
F
F
3
)
5 3
)
5 3
)
5 3
0
6
6
0
[
d]
6
7
0
F
BAr
95:5
31

Chemistry - A European Journal
10.1002/chem.201603466
COMMUNICATION
Scheme 3. Substrate scope II: Hydroboration of aliphatic alkenes. TIPS =
Triisopropylsilyl.
F
3
To study the diastereoselectivity of the BAr
-catalyzed
hydroboration, we subjected 1-methyl- and 1-phenylcyclohexene
k and 2l, respectively to the standard protocol but both did not
react (Scheme 4, top). We therefore switched to deuterium-
labelled phenylacetylene (8-d ) that cleanly converted into (E)-9-
, as verified by NMR spectroscopy (Scheme 4, bottom). This
2
Scheme 2. Substrate scope I: Hydroboration of styrene derivatives.
1
d
1
We next focused on the substrate scope of aliphatic alkenes
outcome is good evidence in favor of a 1,2-cis-hydroboration
and against 1,2-trans- or 1,1-hydroborations.
(
Scheme 3). Hydroboration of α-olefins 2b–g proceeded with
excellent anti-Markovnikov regioselectivity (≥ 94:6). The high
isolated yields in the range of 78–92% confirmed that
polymerization is not an issue. Notably, allylsilane 2d, allyl silyl
ether 2e, and allyl aryl ether 2f underwent clean hydroboration
with HBpin (3). An aliphatic bromide substituent as in 2g was
tolerated as well. Even 1,1-disubstituted alkene 2h reacted with
high efficiency. Our survey also included an internal alkene, and
hydroboration of cis-cyclooctene (2i) worked albeit at diminished
reaction rate. However, this situation allows for the
chemoselective hydroboration of a terminal double bond in the
presence of an internal double bond, as exemplified with 2j.
F
3
Scheme 4. Probing the diastereoselectivity of the BAr
-catalyzed
hydroboration.
F
To understand the reactivity difference between BAr
B(C we performed stoichiometric control experiments
3
Scheme 5). Monitoring the reaction of BAr with HBpin (3) at
3
and
6 5 3
F ) ,
F
(
elevated temperature by multinuclear NMR spectroscopy in the
absence of an alkene revealed that ligand redistribution
[
23,24]
F
occurs.
4 equiv) in toluene-d
consumption of the borane and clean formation of Ar Bpin (10)
For example, treatment of BAr
3
with excess HBpin
(
8
at 80°C for 20 hours resulted in complete
F

Chemistry - A European Journal
10.1002/chem.201603466
COMMUNICATION
and diborane (B
Supporting Information for NMR spectra). The detection of minor
amounts of the higher borane B may be ascribed to the
decomposition of B at temperatures higher than 50°C. At
room temperature, the redistribution process proceeds slowly,
2 6
H , 11) (Scheme 5, top and Figure 1; see the
5
H
9
2
H
6
F
presumably as a consequence of the poor solubility of BAr
3
in
toluene. Since B is known to readily undergo addition to C–C
2
H
6
multiple bonds, we added styrene (1a) to the reaction mixture
and observed that 1a is indeed consumed when heated at 80°C
for 24 hours. However, just traces of the hydroboration adduct
4
B
a were formed in the presence of HBpin (3), hence excluding
[
25,26]
2
H
6
as the actual catalyst.
To identify the true nature of the
F
catalyst, we studied the reaction of BAr
3
with HBpin (3) with
varied stoichiometries. In the ideal case (see the box in Scheme
8.5
8.4
8.3
8.2
8.1
8.0
7.9
7.8
7.7
7.6
7.5
7.4
7.3
ppm
1.1
1.0
0.9 ppm
F
5
), BAr
3
reacts with three equivalents of HBpin (3) to generate
F
F
1
three molecules of Ar Bpin and one molecule of BH
3
(dimerizing
was treated with only one or two
equivalents of HBpin (3), we observed two new boron
Figure 1. Monitoring the reaction of BAr
spectroscopy (500 MHz, toluene-d
of the key intermediates (scale of the downfield y-axis is amplified for clarity).
3
with HBpin by
H
NMR
F
8
) using different stoichiometries: detection
2 6 3
to B H ). However, when BAr
F
compounds, eventually identified as diboranes [H
and [(Ar )(H)B(µ-H) BAr ] (17), respectively (Scheme 5, bottom
2
BAr ]
2
(16)
F
F
2 2
[27,28]
Based on literature precedence
stoichiometric control experiments, we delineate the following
mechanism for the BAr
). Initial substituent redistribution between BAr
presumably through σ-bond metathesis (see 20 ) leads to the
formation of electron-deficient hydroboranes 16, 17, and 21 that
instantly undergo concerted 1,2-syn-addition of the B–H bond
across the alkene C–C double bond of
regioselective manner. While only hydroborane 16 and the
mixed dimer 17 were detected in the stoichiometric experiments
cf. Scheme 5), it cannot be ruled out that [H
as the initial intermediate is catalytically competent. Another
ligand exchange between the resulting hydroboration adducts 18
and 19 with HBpin is likely to be the rate-determining step,
and consistent with our
and Figure 1; see the Supporting Information for NMR spectra).
These in-situ-formed electron-deficient hydroboranes are
F
3
-initiated alkene hydroboration (Scheme
[
27,28]
expected to be highly reactive hydroboration reagents
and
F
6
3
and HBpin
were indeed found to instantly add across 1a with high
‡
[
29]
regioselectivity to afford the boranes 18 and 19.
Substituent
exchange reaction with HBpin (3) at elevated temperature then
yielded the final adduct 4a. Analogous control experiments with
1 in a highly
B(C
unidentified compounds by Lewis acid-promoted ring opening.
Ligand metathesis that would generate the corresponding
6 5 3 2 3
F ) showed decomposition of HBpin into B pin and several
[
31]
F
(
2
BAr ]
2
(21) formed
[
27,32]
2 6 5 2 6 5 2 2
hydroborane [H B(C F )] or Piers’ borane, [HB(C F ) ] ,
was not seen (see the Supporting Information for NMR spectra).
eventually furnishing HBpin-derived adduct 4 concomitant with
F
regeneration of the hydroboration reagents. Hence, BAr
3
acts
only as a precatalyst and does not participate in the overall
catalytic cycle.
Scheme 5. Probing the mechanism: stoichiometric control experiments.
F
Scheme 6. Catalytic cycle for the BAr
shown for styrene as substrate).
3
-initiated hydroboration with HBpin
(

Chemistry - A European Journal
10.1002/chem.201603466
COMMUNICATION
F
In conclusion, the boron Lewis acid BAr
3
was uncovered as
[16] E. R. Clark, A. Del Grosso, M. J. Ingleson, Chem. Eur. J. 2013, 19,
2462–2466.
the precatalyst for the 1,2-cis-hydroboration of alkenes with
HBpin. B(C cleaves the 1,3,2-dioxaborolane ring of HBpin
engages in substituent redistribution with HBpin,
thereby forming electron-deficient hydroboranes as the actual
hydroboration reagents. This unexpected reaction mode
constitutes an alternative to the trans-selective hydroboration
involving borenium-ion catalysis and avoids the need of added
base. Further efforts to expand this methodology are subject of
ongoing investigations in our laboratory.
[17] J. Zheng, X. Fan, B. Zhou, Z. H. Li, H. Wang, Chem. Commun. 2016,
6 5 3
F )
F
3
52, 4655–4658.
whereas BAr
[
18] C. J. Lata. C. M. Crudden, J. Am. Chem. Soc. 2010, 132, 131–137.
19] J. M. Farrell, J. A. Hatnean, D. W. Stephan, J. Am. Chem. Soc. 2012,
[
134, 15728–15731.
[20] P. Eisenberger, B. P. Bestvater, E. C. Keske, C. M. Crudden, Angew.
Chem. Int. Ed. 2015, 54, 2467–2471; Angew. Chem. 2015, 127, 2497–
[13]
2501.
F
[
21] For synthetic applications of BAr
3
, see: a) T. J. Herrington, A. J. W.
Thom, A. J. P. White, A. E. Ashley, Dalton Trans. 2012, 41, 9019–9022;
b) E. L. Kolychev, T. Bannenberg, M. Freytag, C. G. Daniliuc, P. G.
Jones, M. Tamm, Chem. Eur. J. 2012, 18, 16938–16946; c) G.
Hamasaka, H. Tsuji, Y. Uozumi, Synlett 2015, 26, 2037–2041; d) R. J.
Blagg, T. R. Simmons, G. R. Hatton, J. M. Courtney, E. L. Bennett, E. J.
Lawrence, G. G. Wildgoose, Dalton Trans. 2016, 45, 6032–6043; e) Q.
Yin, H. F. T. Klare, M. Oestreich, Angew. Chem. Int. Ed. 2016, 55,
Acknowledgements
Q.Y. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2015‒2016). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship.
3204–3207; Angew. Chem. 2016, 128, 3256–3260.
[
22] For uncatalyzed hydroboration with HBpin, see: a) C. E. Tucker, J.
Davidson, P. Knochel, J. Org. Chem. 1992, 57, 3482–3485; b) I.
Pergament, M. Srebnik, Org. Lett. 2001, 3, 217–219.
Keywords: alkenes • boranes • homogeneous catalysis • hydro-
boration • Lewis acids
[
23] For reviews of redistribution reactions of organoboranes, see: a) R.
Köster, Ann. N. Y. Acad. Sci. 1969, 159, 73–88; b) E.-I. Negishi,
Formation of Carbon–Boron Bonds from Organoboranes by
Redistribution, in Inorganic Reactions and Methods, Vol. 10 (Eds.: J. J.
Zuckerman, A. P. Hagen), Wiley-VCH, Weinheim, Germany, 1989,
Chapter 5.3.2.6.1, pp. 92–97.
[
1]
Recent review of hydroboration of C–C multiple bonds: a) M. Zaidlewicz,
A. Wolan, M. Budny in Comprehensive Organic Synthesis, 2nd ed., Vol.
8
(Eds.: P. Knochel, G. A. Molander), Elsevier, Amsterdam, 2014, pp.
877–963; recent review of hydroboration of C=X bonds: b) C. C. Chong,
R. Kinjo, ACS Catal. 2015, 5, 3238–3259.
[
2]
Boronic Acids: Preparation and Applications in Organic Synthesis,
Medicine and Materials, 2nd ed., Vol. 1 and 2 (Ed.: D. G. Hall), Wiley-
VCH, Weinheim, 2011.
[24] Reported examples of substituent redistributions of boranes: a) D. J.
Parks, W. E. Piers, G. P. A. Yap, Organometallics 1998, 17, 5492–
5503; b) M. Erdmann, C. Rösener, T. Holtrichter-Rößmann, C. G.
Daniliuc, R. Fröhlich, W. Uhl, E.-U. Würthwein, G. Kehr, G. Erker,
Dalton Trans. 2013, 42, 709–718; c) Y. Wang, W. Chen, Z. Lu, Z. H. Li,
H. Wang, Angew. Chem. Int. Ed. 2013, 52, 7496–7499; Angew. Chem.
2013, 125, 7644–7647; d) J. Zheng, Y.-J. Lin, H. Wang, Dalton Trans.
2016, 45, 6088–6093; e) K. Chernichenꢄoꢁ M. ꢅindꢆvistꢁ B. ꢇꢈtaiꢁ M.
ꢉiegerꢁ ꢇ. ꢊorochꢄinaꢁ ꢋ. ꢃꢌpaiꢁ ꢍ. ꢎepoꢁ J. Am. Chem. Soc. 2016, 138,
4860–4868.
[
3]
4]
Recent review of Lewis acid-catalyzed alkene hydrosilylation: Y.
Nakajima, S. Shimada, RSC Adv. 2015, 5, 20603–20616.
For aluminum Lewis acid-catalyzed hydrosilylation, see: a) U. Finke, H.
Moretto, DE2804204, 1978; b) K. Oertle, H. Wetter, Tetrahedron Lett.
[
1985, 26, 5511–5514; c) K. Yamamoto, M. Takemae, Synlett 1990,
259–260; d) Y.-S. Song, B. R. Yoo, G.-H. Lee, I. N. Jung,
Organometallics 1999, 18, 3109–3115.
[
5]
6]
For boron Lewis acid-catalyzed hydrosilylation, see: M. Rubin, T.
Schwier, V. Gevorgyan, J. Org. Chem. 2002, 67, 1936–1940.
For electrophilic phosphonium cation (EPC)-catalyzed hydrosilylation of
alkenes, see: M. Pérez, L. J. Hounjet, C. B. Caputo, R. Dobrovetsky, D.
W. Stephan, J. Am. Chem. Soc. 2013, 135, 18308–18310.
2 6
[25] The potential role of B H to act as the true catalyst is documented in
several hydroboration reactions and is also referred to as a “Trojan
horse”. For examples, see: a) K. Burgess, M. Jaspars, Tetrahedron Lett.
1993, 34, 6813–6816; b) C. Villiers, M. Ephritikhine, Tetrahedron Lett.
2003, 44, 8077–8079; c) S. Harder, J. Spielmann, J. Organomet. Chem.
2012, 698, 7–14.
[
[7]
For the related alkene hydrosilylation employing silicon cations directly,
see: a) J. B. Lambert, Y. Zhao, H. Wu, J. Org. Chem. 1999, 64, 2729–
3 2
[26] We independently tested BH ·SMe as a catalyst (2 mol%) for the
2
736 (intermolecular); b) H.-U. Steinberger, C. Bauch, T. Müller, N.
Auner, Can. J. Chem. 2003, 81, 1223–1227 (intramolecular).
For a recent review of the chemistry of B(C , see: M. Oestreich, J.
hydroboration of styrene with HBpin under neat conditions at 50°C.
However, only 15% conversion of styrene was observed after 14 hours.
[
8]
9]
F
6 5
)
3
6 5 2 2
[27] ꢃiers’ boraneꢁ [HB(C F ) ] , is known to be a highly active hydroboration
Hermeke, J. Mohr, Chem. Soc. Rev. 2015, 44, 2202–2220.
For reviews of borenium ion chemistry, see: a) M. J. Ingleson, Top.
Organomet. Chem. 2015, 49, 39–71; b) T. S. De Vries, A. Prokofjevs, E.
Vedejs, Chem. Rev. 2012, 112, 4246–4282.
reagent: a) D. J. Parks, R. E. v. H. Spence, W. E. Piers, Angew. Chem.
Int. Ed. Engl. 1995, 34, 809–811; Angew. Chem. 1995, 107, 895–897;
b) Ref. [24a].
[
[28] Hoshi and co-workers reported the catalytic hydroboration of terminal
[
10] For a seminal report on borenium ion-catalyzed alkene hydroboration,
see: A. Prokofjevs, A. Boussonnière, L. Li, H. Bonin, E. Lacꢀteꢁ ꢂ. ꢃ.
Curran, E. Vedejs, J. Am. Chem. Soc. 2012, 134, 12281–12288.
alkynes with HBpin using HB(C
treatment of B(C with BH ·SMe
Okimoto, Tetrahedron Lett. 2007, 48, 8475–8478; for the analogous
HBCy -catalyzed hydroboration, see: b) K. Shirakawa, A. Arase, M.
6
F
5
)
2
·SMe
2
generated in-situ by
6
F )
5 3
3
2
: a) M. Hoshi, K. Shirakawa, M.
[11] For borenium ion-catalyzed hydroboration of C=N double bonds, see: P.
Eisenberger, A. M. Bailey, C. M. Crudden, J. Am. Chem. Soc. 2012,
2
Hoshi, Synthesis 2004, 1814–1820.
F
2
[30]
2
134, 17384–17387.
[29] It must be noted here that independently prepared HBAr
·SMe
as
[
[
12] For 1,4-hydroboration of pyridines involving boronium-ions, see: X. Fan,
J. Zheng, Z. H. Li, H. Wang, J. Am. Chem. Soc. 2015, 137, 4916–4919.
13] For borenium ion-catalyzed trans-hydroboration of alkynes, see: J. S.
McGough, S. M. Butler, I. A. Cade, M. J. Ingleson, Chem. Sci. 2016, 7,
6 5 2 2
well as ꢃiers’ boraneꢁ [HB(C F ) ] , indeed proved to be active catalysts
for the hydroboration of styrene with HBpin under neat conditions at
50°C.
[30] K. Samigullin, M. Bolte, H.-W. Lerner, M. Wagner, Organometallics
2014, 33, 3564–3569.
3384–3389.
[
[
14] L. Fan, D. W. Stephan, Dalton Trans. 2016, 45, 9229–9234.
[31] A. Del Grosso, R. G. Pritchard, C. A. Muryn, M. J. Ingleson,
Organometallics 2010, 29, 241–249.
15] M. A. Dureen, A. Lough, T. M. Gilbert, D. W. Stephan, Chem. Commun.
2008, 4303–4305.

Chemistry - A European Journal
10.1002/chem.201603466
COMMUNICATION
Suggestion for the Entry for the Table of Contents
COMMUNICATION
Q. Yin, S. Kemper, H. F. T. Klare,* M.
Oestreich*
Page No. – Page No.
Boron Lewis Acid-Catalyzed
Hydroboration of Alkenes with
F
Pinacolborane: BAr
B(C
3
Does What
6
F
5
)
3
Cannot Do!
F
F
Scrambled Puzzle. Unlike B(C
6 5 3 3
F ) , the rarely used boron Lewis acid BAr (Ar =
3,5-bis(trifluoromethyl)phenyl) initiates the catalytic hydroboration of various
alkenes with pinacolborane. No assistance of an external base is required. An
unexpected mechanism accounts for this striking reactivity difference leading to a
cis-selective hydroboration as opposed to typical Lewis acid-catalyzed
hydrometalation reactions.