Pd-catalyzed hydroborylation of alkynes: A ligand controlled regioselectivity switch for the synthesis of α- Or β-vinylboronates

Article abstract of DOI:10.1021/acs.orglett.5b03416

A ligand controlled selective hydroborylation of alkynes to α- or β-vinylboronates has been developed using a Pd catalyst. The high α-selectivity displayed by this reaction can be switched to furnish β-vinylboronates by altering the ligand from a trialkylphosphine to N-heterocyclic carbene. A variety of terminal alkynes are shown to furnish the corresponding α- or β-vinylboronates in good to excellent selectivity and yield. The mechanistic studies suggest that the solvent is the proton source and bromobenzene functions as an important additive in driving this reaction forward.

Full text of DOI:10.1021/acs.orglett.5b03416

Letter
pubs.acs.org/OrgLett
Pd-Catalyzed Hydroborylation of Alkynes: A Ligand Controlled
Regioselectivity Switch for the Synthesis of α- or β‑Vinylboronates
Devi Prasan Ojha and Kandikere Ramaiah Prabhu*
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
S
* Supporting Information
ABSTRACT: A ligand controlled selective hydroborylation of
alkynes to α- or β-vinylboronates has been developed using a Pd
catalyst. The high α-selectivity displayed by this reaction can be
switched to furnish β-vinylboronates by altering the ligand from a
trialkylphosphine to N-heterocyclic carbene. A variety of terminal
alkynes are shown to furnish the corresponding α- or β-vinylboronates in good to excellent selectivity and yield. The mechanistic
studies suggest that the solvent is the proton source and bromobenzene functions as an important additive in driving this reaction
forward.
he hydroborylation of alkynes is one of the fundamental
reactions for the synthesis of vinylboron compounds in
organometallic species; and (iii) quenching of the organometallic
species with electrophiles. The regioselectivity observed in the
addition M-Bpin species is governed by the steric and electronics
factors of both metal complex and alkyne substitutions. In
continuation of our work on Pd-catalyzed reactions,8 and based
on the literature precedence on borylation of alkenes,4i we
envisioned borylation reactions of alkynes using an aryl halide
and Pd catalyst. Thus, herein we reveal an unprecedented Pd-
catalyzed hydroborylation of terminal alkynes to selectively
furnish α- or β-vinylboronates wherein the ligand plays a decisive
role in the regioselectivity of the reaction.
The optimization studies were initiated with nonyne (1a) and
B2Pin2 by employing various ligands. After evaluating several
palladium catalysts, ligands, and additives, we found that a
combination of [Pd(OAc)2]3 (5 mol %), PCy3 (10 mol %),
CF3CH2OH, and bromobenzene in toluene furnished the
product 2a in 85% yield with excellent regioselectivity (90% α-
selectivity, entry 1, Table 1). The aryl bromide was found to
furnish phenylboronic acid pinacol ester (4) as the byproduct in
almost quantitative yields.
T
organic chemistry. Vinylboronate esters are an important class of
intermediates used in a variety of organic transformations,1 such
as Suzuki−Miyaura coupling, Chan−Lam coupling, Petasis
reaction, Hayashi−Miyaura conjugate addition, and stereo-
specific C−C bond forming reactions.2−4 Studies on hydro-
borylation of alkynes have resulted in the discovery and use of
various metal catalysts such as Cu, Rh, Ni, Co, etc., to
accompolish the synthesis of vinylboron species.3,4 A vast
majority of these hydroborylations furnish the corresponding β-
isomers,5 whereas reports of selective formation of α-borylated
products are limited. Synthesis of α-vinylboronates requires a
multistep reaction sequence, and these utilize vinyl halides as
precursors. Therefore, direct one-step synthesis of α-vinyl-
boronates using alkynes as a precursor is necessary, but it is a
challenging task. The only method for selective hydroborylation
of terminal alkynes to α-vinylboronates was reported by
Hoveyda in 2011, by employing NHC−Cu complexes (Scheme
1). However, α-vinylboronates can also be obtained using
Once the optimal conditions were established, an intensive
screening study was performed to validate the reaction
conditions. The reaction in the absence of a palladium catalyst
did not furnish any product, whereas the reaction in the absence
of ligands was ineffective (entries 2−3, Table 1). Use of other
monophosphine ligands, such as XPhos, t-Bu-XPhos, and P(o-
tolyl)3, furnished the expected product 2a in only trace amounts
(entries 4−6). The use of a bis-phosphine ligand, rac-BINAP,
also proved to be completely ineffective (entry 7). Interestingly,
the use of an N-heterocyclic carbene precursor, IPr·HCl, as a
ligand with a catalytic amount of tert-BuOK (12 mol %) was
found to completely reverse the selectivity of the reaction,
furnishing β-vinylboronate 3a in high selectivity and good yield
(83% selectivity and 80% yield, entry 9). Using the Pd(dppf)Cl2
Scheme 1. Regioselective Hydroboration of Alkynes
aliphatic allenes.6 Further, using a directing group strategy,
Carretero’s group has documented the Cu-catalyzed selective
borylation of internal alkynes (Scheme 1).7 Nevertheless, until
now a direct method for selective hydroboration of alkynes to
obtain α-vinylboronates using Pd catalysts has not been reported.
The metal catalyzed hydroborylations of alkynes proceed in a
three-step process: (i) formation of a metal-BPin species; (ii)
addition of a metal-Bpin species across the alkynes, generating an
Received: December 7, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03416
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
a b
,
Table 1. Optimization Studies
Table 2. Synthesis of α-Vinylboronates
c
yield
(%)
entry
1
reagents and conditions
α:β
B2Pin2, [Pd(OAc)2]3 (5 mol %), PCy3 (10 mol %), 85
PhBr (1.1 equiv), TFE (2 equiv), toluene, 80 °C
90:10
Deviations from the standard condition
2
w/o [Pd(OAc)2]3
NR
−
3
in the absence of ligands
XPhos instead of PCy3
tBuXphos instead of PCy3
P(o-tol)3 instead of PCy3
rac-Binap instead of PCy3
1,10-Phen instead of PCy3
IPr·HCl instead of PCy3
Pd(dppf)Cl2 instead of [Pd(OAc)2]3
w/o PhBr
trace
10
−
60:40
4
5
NR
15
−
6
65:35
70:30
−
7
30
8
NR
80
d
9
17:83
10
11
12
13
NR
NR
22
−
−
−
71:29
PhI instead of PhBr
MeOH instead of TFE
38
a
Reaction conditions: 1a (1 mmol), B2Pin2 (1.1 equiv), toluene (1.5
b
c
d
mL), 12 h. TFE: trifluoroethanol. Isolated yields. tert-BuOK (12
mol %). NR = No reaction.
catalyst instead of the standard catalyst in the reaction did not
furnish the expected product (entry 10). Moreover, the reaction
in the absence of phenyl bromide did not furnish any product
(entry 11) indicating that the oxidative oxidative insertion of an
aryl halide is a necessary process for the reaction to initiate. The
utility of phenyl iodide was found to be detrimental, as the
reaction furnished a quantitative amount of Suzuki product 4
along with the borylated product 2a in low yield (22%, entry 12).
It was also found that MeOH as a protonating source was not
effective in improving the yield of the product 2a (38%, entry
13).
With these optimized conditions, we started examining the
scope of the hydroboration reaction with a variety of alkynes with
varying reactivity (Table 2). Terminal aliphatic alkynes having
linear hydrocarbon chains furnished the corresponding borylated
products (2a−2e) in high regioselectivity (>90% α-selectivity)
and in good yields (entries 1−5, Table 2).9 The high α-selectivity
observed for the hydroborylation of n-alkyl substituted
acetylenes is rare, and to the best of our knowledge such a
transformation requires a multistep reaction sequence.10 Further
we explored the potential of the present strategy by subjecting
substrates which possess reactive functional groups. Thus, the
reaction of substrate with a nitrile functional group such as 5-
cyanopentyne (1f), under the optimal catalytic conditions,
furnished exclusively the α-vinylboronate (2f, >98:2) in excellent
yield (89%, entry 6, Table 2). Considering the coordinating
ability of a cyano group with Pd, the selectivity observed in this
hydroborylation reaction is unique.11 An aryl acetylene, 1-
ethynyl-4-methylbenzene (1g), under the optimal reaction
conditions afforded the corresponding α-borylated product 2g
in excellent yield with high regioselectivity (87% yield and 98% α-
selectivity, entry 7).
a
Reaction conditions: 1a (1 mmol), B2Pin2 (1.1 equiv), PhBr (1.1
equiv), [Pd] (5 mol %), ligand (10 mol %), TFE (2 equiv), toluene
(1.5 mL), 80 °C, 12 h. Isolated yields. 3a (E/Z 65:35). 3b (E/Z
50:50). 3c (E/Z 25:75). 3d (E/Z 70:30).
b
c
d
e
f
propargyl substrates such as phenyl propargyl ether (1h) and O-
propargyl ether of cholesterol (1i) underwent a facile hydro-
borylation reaction to furnish the corresponding α-vinyl-
boronates 2h and 2i in good yields (entries 8 and 9) and
moderate to high regioselectivity. Further, the propargyl ester of
phenyl acetic acid also underwent facile reaction to furnish the
corresponding α-vinylboronates in moderate selectivity (60:40)
but high yields (93%, entry 10, Table 2).
While propargyl ethers exhibit very good α-selectivity, the
reactions of ω-hydroxy alkynes under the same reaction
conditions, surprisingly furnished the β-borylated products.
This complete reversal of selectivity may be attributed to the
coordination of the catalytic species to the free-hydroxyl groups
in the molecule or on the basis of hydroxyl directed oxidative
metalation across the alkyne.13 Thus, the reactions of pent-4-yn-
1-ol and but-2-yn-1-ol under the optimal conditions furnished
the β-borylated products 2k and 2l in good yields (63% and 71%,
Table 3) with excellent β-selectivity (95% and 84%, respectively).
Hydroboration of terminal aliphatic alkynes using metal
complexes of Rh, Ti, Ir, Cu, and Zr which lead to the formation
of β-vinylboronates is known in literature.14 However, such
reactions are unknown with Pd complexes. We explored a few
representative n-alkyl acetylenes to check the generality of the
Propargyloxy functional groups are sensitive substrates due to
their intrinsic ability to form π-allyl complexes with palladium.12
Moreover, it is well-known that heteroatoms coordinate to a
nearby metal atom, affecting the regioselectivity of the reactions
and yields. Surprisingly, under the present reaction conditions,
B
DOI: 10.1021/acs.orglett.5b03416
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ab
,
We found borylated product 2a only in the presence of PhBr
(20%, Scheme 2). We believe that, as shown in Scheme 2, that
PhBr is essential for the recycling of the metal catalyst. Based on
these observed facts, we propose a tentative mechanism (Scheme
3). The reaction starts with Pd(0) and undergoes oxidative
Table 3. Synthesis of β-Vinylboronates
Scheme 3. A Plausible Reaction Mechanism
a
Reaction conditions: 1a (1 mmol), B2Pin2 (1.1 equiv), PhBr (1.1
equiv), [Pd] (5 mol %), ligand (10 mol %), tBuOK (12 mol %), TFE
b
(2 equiv), toluene (1.5 mL), 80 °C, 12 h. Isolated yields.
selectivity switch. Accordingly, we used IPr·HCl (N-heterocyclic
carbene precursor) as a ligand in the presence of a base, tert-
BuOK, with non-1-yne (1a), and this reaction resulted in an
efficient β-borylation (84:16) to form 3a in good yield (80%
yield, entry 1, Table 3). A similar outcome was also observed with
octyne (1b) and heptyne (1c) to obtain the corresponding
borylated products 3b and 3c in good yields (76% and 85%) with
high β-selectivity (92% and 91%, respectively, entries 2 and 3).
An arylacetylene, phenyl acetylene, was also tested under similar
reaction conditions, which afforded the expected product 3d in
81% yield with excellent β-selectivity (98%, entry 4).
A competitive experiment was undertaken to determine if the
electronic nature of the alkyne has any role in the reaction.
Accordingly, a reaction of 1-ethynyl-4-methylbenzene (1g) and
5-cyanopentyne (1f) under standard reaction conditions resulted
in the formation of the corresponding products, 2f and 2g, in the
ratio 70:30 (Scheme 2). As expected, this shows that aliphatic
insertion into PhBr, to form a Pd(II) species, Int 1.15 The ligand
in Int 1 exchanges with alkyne to form the π-coordinated
intermediate Int 2. Subsequently, B2Pin2 reacts with Int 2, to
eliminate Ph-BPin, generating the active reactive species Int 3,16
which undergoes migratory insertion into the alkyne. This
migratory insertion is dictated by the ligands around the Pd
center. Although, we have not provided any evidence, we believe
that, because NHCs are better electron donors than phosphine
ligands, Int 4a is formed with the boron insertion at the α-
position and the palladium species is formed at the β-position.
Similarly, with phosphine as the ligand the Int 4a is formed with
boron insertion at the β-position and Pd species at the α-
position. Further, these species undergo a protonative
demetalation to release the corresponding products, regenerat-
ing the Pd(0) catalysts via an alcohol oxidation process.17 The
reason for this ligand directed divergence in regioselectivity is still
unknown, and studies are underway in our laboratory to further
understand their roles.18
Scheme 2. Mechanistic Studies
To conclude, a Pd-catalyzed hydroborylation of a range of
terminal alkynes to synthesize their α-vinylboronates in excellent
selectivity has been uncovered. This selectivity can be reversed by
using N-heterocyclic carbene ligands to obtain β-vinylboronates.
A mechanistic study was undertaken to elucidate the reaction
pathway of the catalytic borylation process. These studies have
suggested that the solvent CF3CH2OH is the proton source for
the reaction. Additionally, it was also found that bromobenzene is
essential for the reaction and is an important driving force for this
reaction. Further studies are underway for the hydroborylation of
other classes of alkynes.
a
b
Reaction conditions: 1g (0.5 mmol). 1f (0.5 mmol). Isolated yield.
c
d
e
1
CF3CD2OD (2 equiv). Yield based on H NMR. 1a (1 mmol), no
f
PhBr, K2PdCl6 (5 mol %) instead of [Pd(OAc)2]3. 1a (1 mmol), PhBr
(1.1 equiv), K2PdCl6 (5 mol %) instead of [Pd(OAc)2]3.
alkynes, which are inherently electron rich, are more favorable as
substrates in comparison to aryl alkynes. Further, to confirm that
trifluoroethanol (TFE) is the source of the proton, a deuterium
labeling experiment was carried out using deuterated TFE
(CF3CD2OD). Indeed, this reaction has shown the deuterium
incorporation in the product (Scheme 2) at the terminal end with
an E/Z ratio of 40:60.
Using a premade commercially available Pd(IV) source,
K2PdCl6, we reacted non-1-yne (1a) under the optimal
conditions in the presence of PhBr and in the absence of PhBr.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03416.
Experimental procedures, characterization data and
spectra for all compounds (PDF)
C
DOI: 10.1021/acs.orglett.5b03416
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(6) (a) Lee, Y.; Jang, H.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131,
18234. (b) Jang, H.; Zhugralin, A. R.; Lee, Y.; Hoveyda, A. H. J. Am.
Chem. Soc. 2011, 133, 7859. For α-selective hydroborylation of allenes,
see: (c) Yamamoto, Y.; Fujikawa, R.; Yamada, A.; Miyaura, N. Chem.
Lett. 1999, 10, 1069. (d) Yuan, W.; Ma, S. Adv. Synth. Catal. 2012, 354,
1867.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: prabhu@orgchem.iisc.ernet.in.
Notes
(7) (a) Moure, A. L.; Arrayas
Carretero, J. C. J. Am. Chem. Soc. 2012, 134, 7219. (b) Moure, A. L.;
Mauleon, P.; Arrayas, R. G.; Carretero, J. C. Org. Lett. 2013, 15, 2054.
(c) García-Rubia, A. G.; Romero-Revilla, J. A.; Mauleon, P.; Arrayas, R.
G.; Carretero, J. C. J. Am. Chem. Soc. 2015, 137, 6857.
(8) (a) Ojha, D. P.; Prabhu, K. R. J. Org. Chem. 2013, 78, 12136.
(b) Ojha, D. P.; Prabhu, K. R. J. Org. Chem. 2012, 77, 11027.
(9) The trace amount of β-borylated products was obtained in both E
and Z mixtures (Table 2). 3a (E/Z 65:35), 3b (E/Z 50:50), 3c (E/Z
25:75), 3d (E/Z 70:30).
(10) (a) Takagi, J.; Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Am.
Chem. Soc. 2002, 124, 8001. (b) Takahashi, K.; Takagi, J.; Ishiyama, T.;
Miyaura, N. Chem. Lett. 2000, 2, 126. (c) Coombs, J. R.; Zhang, L.;
Morken, J. P. Org. Lett. 2015, 17, 1708. (d) Ganic, A.; Pfaltz, A. Chem. -
Eur. J. 2012, 18, 6724.
(11) Leow, D.; Li, G.; Mei, T.-S.; Yu, J.-Q. Nature 2012, 486, 518.
(12) (a) Wang, Y. L.; Zhang, W. L.; Ma, S. M. J. Am. Chem. Soc. 2013,
135, 11517. (b) Ito, H.; Sasaki, Y.; Sawamura, M. J. Am. Chem. Soc. 2008,
́ ́
, R. G.; Cardenas, D. J.; Alonso, I.;
The authors declare no competing financial interest.
́
́
ACKNOWLEDGMENTS
■
́
́
We dedicate this work to Prof. S. Chandrasekaran on the
occasion of his 70th birthday. This work was supported by SERB
(No. SB/S1/OC-56/2013), New Delhi, the Indian Institute of
Science and RL Fine Chem. We are thankful to Dr. S.
Raghothama (NRC, IISc) and Dr. A. R. Ramesha (RL Fine
Chem) for useful discussions. We thank the editor and reviewers
for useful suggestions regarding the mechanism. D.P.O. thanks
UGC, New-Delhi for a senior research fellowship.
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DOI: 10.1021/acs.orglett.5b03416
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