NHC-copper-thiophene-2-carboxylate complex for the hydroboration of terminal alkynes

Article abstract of DOI:10.1039/c9ob00839j

An air-stable N-heterocyclic carbene-copper thiophene-2-carboxylate (CuTC) complex has been prepared for the stereoselective hydroboration of terminal alkynes using pinacolborane (HBpin) or 1,8-naphthalenediaminatoborane (HBdan). The newly synthesized complex can be directly activated by hydroboranes without a cocatalyst such as a base, and exhibits high reactivity for the hydroboration of alkynes under mild conditions. A gram-scale hydroboration of terminal phenylacetylene demonstrated the applicability of the copper complex for the preparation of alkenyl boronates.

Full text of DOI:10.1039/c9ob00839j

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NHC-copper-thiophene-2-carboxylate complex
for the hydroboration of terminal alkynes†
Cite this: DOI: 10.1039/c9ob00839j
Won Jun Jang, a Byung-Nam Kang,a Ji Hun Lee,a Yoon Mi Choi,b
Received 11th April 2019,
Accepted 8th May 2019
a
Chong-Hyeak Kimb and Jaesook Yun
*
DOI: 10.1039/c9ob00839j
rsc.li/obc
An air-stable N-heterocyclic carbene–copper thiophene-2-car- ligands served as appropriate precursors in the absence of an
boxylate (CuTC) complex has been prepared for the stereoselective alkoxide base.7,8
hydroboration of terminal alkynes using pinacolborane (HBpin) or
Very recently, the Bertrand9 and Bai groups10 also reported
1,8-naphthalenediaminatoborane (HBdan). The newly synthesized the use of NHC–CuOPh and NHC–CuCl/base as catalysts in the
complex can be directly activated by hydroboranes without a respective hydroboration of alkynes with pinacolborane.
cocatalyst such as a base, and exhibits high reactivity for the However, the use of a phenoxide ligand or an alkoxide base
hydroboration of alkynes under mild conditions. A gram-scale was required for catalyst activity in both methods. Based on
hydroboration of terminal phenylacetylene demonstrated the these facts, we believe the synthesis of air-stable copper com-
applicability of the copper complex for the preparation of alkenyl plexes that can be activated without an additional cocatalyst
boronates.
would expand the utility of the copper-catalyzed hydroboration
process. Herein, we report the first synthesis of a new NHC–
CuTC complex 1, its characterization, and successful catalysis
with regard to the stereoselective hydroboration of terminal
alkynes.
We aimed to synthesize the SIPr–CuTC complex 1, since the
SIPr (= 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene)
ligand has previously displayed higher reactivity than other
conventional NHC ligands with regard to alkyne hydroboration
with HBdan.7 The synthesis of 1 was successfully achieved
through the reaction of a free carbene SIPr and CuTC (copper
thiophene-2-carboxylate) in toluene at room temperature; the
Transition-metal catalyzed hydroboration of alkynes with
hydroboranes enables direct access to alkenylboron com-
pounds with high atom-economy, which are important syn-
thons for cross-coupling reactions in organic synthesis.1 While
hydroboration reactions using rare transition metals as cata-
lysts have been investigated in earlier research,2 hydroboration
methods using inexpensive metals including copper,3 cobalt,4
and iron5 have been recently reported and continue to be
explored.
In a continuation of the efforts made since our first report
with regard to the asymmetric hydroboration of alkenes by
bisphosphine-coordinated copper catalyst with pinacolbor-
ane,6 our group recently reported a stereodivergent synthesis
of alkenylboron compounds through the copper-catalyzed
hydroboration of terminal aryl alkynes to prepare (E) or (Z)-
alkenylboron compounds with 1,8-naphthalenediaminatobor-
ane (HBdan) as the hydroborane reagent.7 While copper chlor-
ide-based precatalysts such as NHC–CuCl always require a
base cocatalyst for catalyst activation in the hydroboration, we
found that copper-carboxylates coordinated with phosphine
product was isolated as
a white solid with 71% yield
(Scheme 1). The compound was characterized by NMR spec-
troscopy, elemental analysis and X-ray crystallography. Single
crystals of SIPr–CuTC suitable for X-ray crystallography were
obtained via the slow diffusion of pentane into its concen-
trated CH2Cl2 solution and its structure can be seen in
Fig. 1.11 The thienyl group of 1 exhibited rotational disorder as
modeled via a PART command. For clarity, the major com-
aDepartment of Chemistry, Sungkyunkwan University, Suwon 16419, Korea.
E-mail: jaesook@skku.edu
bChemical Analysis Center, Korea Research Institute of Chemical Technology
(KRICT), Daejeon 34114, Korea
†Electronic supplementary information (ESI) available. CCDC 1908511. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9ob00839j
Scheme 1 Synthesis of (SIPr)CuTC.
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Table 2 Substrate scope
Fig. 1 X-ray structure of 1. Thermal ellipsoids are shown at their 25%
probability level. Selected bond distances (Å) and angles (°): Cu(1)–C(6)
1.870(1), Cu(1)–O(1) 1.875(1), O(1)–C(1) 1.183(3), C(1)–O(2) 1.214(3),
C(6)–N(1) 1.334 (1), C(6)–N(2) 1.337(1), C(6)–Cu(1)–O(1) 179.14(7), Cu(1)–
O(1)–C(1) 122.03(1), O(1)–C(1)–O(2) 129.7(2), Cu(1)–C(6)–N(1) 130.2(1),
Cu(1)–C(6)–N(2) 121.7(1), N(1)–C(6)–N(2) 107.9(1).
ponent of the two disorder parts is shown with hydrogen
atoms omitted. The Cu–Ccarbene bond distance of SIPr–CuTC
was 1.870(1) Å, which was within that of Cu–Ccarbene bonds
(1.80–2.20 Å).12 The distance was slightly shorter than that for
similar complexes such as SIPr–CuOPh13a and SIPr–CuOAc.13b
The C(6)–Cu(1)–O(1) linkage was linear with a bond angle of
179.14(7)° and the bond length of Cu(1)–O(1) was 1.875(1) Å.
The complex was bench-stable and could be stored for months
without losing its activity.
in the incomplete conversion of phenylacetylene (2a). SIPr–
copper(I) acetate demonstrated good reactivity with HBpin
(entry 2), but exhibited reduced reactivity with less active
HBdan as the hydroborane source (entry 5). Finally, the
SIPrCuTC complex was highly efficient for the hydroboration
of 2a with both HBpin (entry 3) and HBdan (entries 6 and 7).15
Catalyst loadings as low as 1 mol% of 1 resulted in the desired
product with good yields (entry 7).
Next, we prepared other known NHC–copper carboxylates,
14
SIPr–CuOAc13b and IPr–Cu(OAc)2 by following literature pro-
We applied the optimized reaction conditions using the
copper complex 1 to the hydroboration of various terminal
alkynes (Table 2). With pinacolborane as the hydroborane
reagent, a variety of alkenyl pinacolboronates (3) were efficien-
tly prepared. Aryl alkynes with an electron withdrawing- or
donating substituent and alkyl alkynes were suitable substrates
for the reaction. An ester-group was compatible with the reac-
tion conditions (3e), affording the target product with a good
yield. In addition, with HBdan, a variety of alkenyl boron com-
pounds possessing 1,8-diaminonaphthalene as a boronyl pro-
tective group (4) were efficiently prepared. However, alkyl
alkynes were unreactive for the hydroboration with HBdan.
Such Bdan-protected alkenylboron compounds were useful for
the iterative synthesis of oligoarene derivatives via Suzuki–
Miyaura coupling reactions.16
cedures and screened their efficiency in the hydroboration of
phenylacetylene (2a) together with 1 (Table 1). IPr–Cu(OAc)2
(IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) was
the least efficient among the complexes screened for hydro-
boration with HBpin (entry 1) and HBdan (entry 4), resulting
Table 1 Screening NHC–Cu carboxylates for the hydroboration of
terminal alkynes
The hydroboration protocol using catalyst 1 without a coca-
talyst was also suitable for large-scale synthesis (Scheme 2). A
0.1 mol% loading of 1 was efficient enough to achieve the
hydroboration reaction at a 20 mmol scale of phenylacetylene
in 2 h, resulting in the production of 3a in 96% yield.
NHC–Cu
complex
Hydroborane
(HBY2)
Conv.a
(%)
Entry
Yieldb (%)
1
2
3
IPr–Cu(OAc)2
SIPr–CuOAc
SIPr–CuTC (1)
IPr–Cu(OAc)2
SIPr–CuOAc
SIPr–CuTC (1)
SIPr–CuTC (1)
HBpin
HBpin
HBpin
HBdan
HBdan
HBdan
HBdan
75
64 (3a)
93 (3a)
98 (3a)
29 (4a)
>98
>98
44
53
>98
>98
4c
5c
6c
7c,e
41 (4a)
80 (4a) (87 : 13)d
82 (4a)
a Determined by 1H NMR spectroscopy with N,N-dimethylformamide
as an internal standard. b Isolated yield. c The reaction time was 24 h.
d Combined yield of co-eluted 4a and 5a. e 1 mol% catalyst was used.
Scheme 2 Gram-scale synthesis of 3a.
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Notes and references
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Scheme 3
experiment.
A
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A proposed catalytic cycle for alkyne hydroboration can be
seen in Scheme 3. The active SIPrCu–H is formed in situ by
reaction with HBpin and reacts with the alkyne (2a) to form
the hydrocuprated intermediate.17 The organocopper inter-
mediate subsequently reacts with HBpin to generate the alke-
nylboron product (3) and the active catalyst. NMR study
showed that the alkynyl proton of 2a remained intact when
mixed with a stoichiometric amount of the catalyst 1,18 elimi-
nating the pathway to copper-acetylide complex formation
unlike Bertrand’s catalyst.9 Furthermore, the hydroboration of
deuterium-labeled phenylacetylene (2a-d) afforded
a (E)-
alkenyl product (3a-d) with high deuterium retention (99%).18
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Conclusions
In summary, we prepared a new NHC–Cu-thienyl carboxylate
complex, SIPr–CuTC and successfully applied it for the stereo-
selective hydroboration of terminal alkynes with pinacolborane
or 1,8-naphthalenediaminatoborane as the hydroborane reagent.
The new copper complex was air-stable and storable at ambient
temperatures without any precautions and could be activated by
hydroboranes without additional base. Moreover, the protocol
could be easily scaled to gram-scale quantities with a low catalyst
loading of 0.1 mol% for the formation of alkenylboron com-
pounds with high atom-efficiency. We expect the SIPr–CuTC
complex to expand the utility of the hydroboration process; more
catalytic applications of the complex are underway.
Conflicts of interest
12 J. C. Y. Lin, R. T. W. Huang, C. S. Lee, A. Bhattacharyya,
W. S. Hwang and I. J. B. Lin, Chem. Rev., 2009, 109, 3561–
3598.
There are no conflicts to declare.
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T. R. Cundari, A. W. Pierpont, J. L. Petersen and
P. D. Boyle, Inorg. Chem., 2006, 45, 9032–9045;
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Acknowledgements
This study was supported by National Research Foundation
of Korea (NRF) grants (No. 2016R1A2B4011719 and
2019R1A2C2005706), funded by the Korean government (MEST).
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14 J. Yun, D. Kim and H. Yun, Chem. Commun., 2005, 5181–5183.
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