Cobalt-Catalyzed Cyclization/Hydroboration of 1,6-Diynes with Pinacolborane

Article abstract of DOI:10.1021/acs.orglett.9b02873

Herein, we report a protocol for cyclization/hydroboration of 1,6-diynes with pinacolborane using a cobalt catalyst generated in situ from a Co(II)-phenanthroline complex, tetrabutylammonium fluoride, and pinacolborane. This protocol, which features good

Full text of DOI:10.1021/acs.orglett.9b02873

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cobalt-Catalyzed Cyclization/Hydroboration of 1,6-Diynes with
Pinacolborane
Qiang Huang, Meng-Yang Hu, and Shou-Fei Zhu*
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071,
China
S
* Supporting Information
ABSTRACT: Herein, we report a protocol for cyclization/
hydroboration of 1,6-diynes with pinacolborane using a cobalt
catalyst generated in situ from a Co(II)−phenanthroline complex,
tetrabutylammonium fluoride, and pinacolborane. This protocol,
which features good functional group tolerance, a broad substrate
scope, and excellent stereoselectivity, enables the synthesis of
useful cyclic 1,3-dienylboron compounds from readily accessible
feedstocks. The proposed mechanism involves low-valent cobalt-
promoted cyclometalation and subsequent hydroboration, as
indicated by control experiments. Our findings demonstrate that
combining hydroboration with C−C bond formation is an
efficient way to synthesize structurally diverse organoboranes.
rganoboron compounds are widely used in pharmaceut-
temperature (Table 1). The use of a cobalt catalyst prepared in
Oical chemistry, agricultural chemistry, materials science, situ from L1·CoCl2 and TMSCH2Li9 afforded the desired
and other fields,1 and the development of C−B bond-forming
cyclic 1,3-dienylboron 2a in moderate yield (46%) with
reactions has long been of interest in organic synthesis.2 Since
excellent stereoselectivity (Z/E > 95:5, entry 1); the main
byproduct was [2 + 2 + 2] cycloaddition product 3a. A control
experiment showed that the ligand was indispensable (entry 2).
Next, we systematically evaluated a variety of ligands,
beginning with a study of the effect of the substituent on the
1,10-phenanthroline ligand. Compared with the unsubstituted
ligand (L1), 2,9-substituted ligands L2−L4 exhibited much
lower stereoselectivities (entries 3−5), whereas 3,8- and 4,7-
substituted ligands L5 and L6 gave much lower yields (entries
6 and 7). Despite having coordination models similar to that of
the 1,10-phenanthroline ligands, bipyridine ligand L7,
pyridine−imine ligand L8, and diimine ligand L9 showed
low activity, as indicated by the extremely low to negligible
yields of 2a (entries 8−10). The flat, rigid structure of 1,10-
phenanthroline may stabilize the active catalyst, as we reported
previously for iron-catalyzed alkene hydrosilylation.10 Triden-
tate ligands L10 and L11 also showed lower activities and
stereoselectivities (entries 11 and 12). Electron-rich mono-
dentate phosphine ligand PBu3 gave an acceptable outcome
(entry 13), but bidentate phosphine ligands L12 and L13 gave
much poorer results (entries 14 and 15). Evaluation of a
number of additional ligands did not result in any improve-
ment relative to 1,10-phenanthroline L1 (see Table S1 for
details).
the seminal work of Brown and colleagues, the hydroboration
of alkenes and alkynes has been developed as an effective
method for constructing C−B bonds from readily accessible
feedstocks. In recent years, a number of studies have shown
that combining hydroboration with C−C bond formation is a
useful way to develop new C−B formation reactions and can
facilitate the synthesis of structurally diverse organoboron
compounds from simple raw materials.3 In 2001, catalytic
cyclization/hydroboration of 1,5- and 1,6-dienes with an
organosamarium complex as the catalyst was realized,4 and
more recently, transition-metal-catalyzed cyclization/hydro-
boration of 1,6-enynes to efficiently afford cyclic alkenyl boron
compounds and allylic boron compounds was reported.5
Surprisingly, there have been no reports on the cyclization/
hydroboration of 1,n-diynes, despite the fact that the expected
products, cyclic 1,3-dienylboron compounds, are versatile
synthons and building blocks.6 A ruthenium-catalyzed reaction
between hepta-1,6-diyne and pinacolborane (HBpin) was
shown to produce the dihydroboration product but not the
cyclization/hydroboration product.7 As part of our ongoing
work on the development of C−B bond-forming reactions,8 we
herein report a protocol for cobalt-catalyzed cyclization/
hydroboration reactions of 1,6-diynes with HBpin. This
protocol enables the synthesis of useful cyclic 1,3-dienylboron
compounds in good yields with excellent stereoselectivities
from readily available starting materials.
Using L1·CoCl2 as the catalyst, we next evaluated other
reaction parameters. We found that in addition to TMSCH2Li,
We began our study with the reaction of dimethyl 2,2-
di(prop-2-yn-1-yl)malonate 1a and HBpin in THF at room
Received: August 13, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02873
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Cobalt-Catalyzed Cyclization/Hydroboration of 1,6-Diyne 1a: Optimization of the Reaction Conditions
b
b
b
b
d
entry
ligand
additive
conv (%)
2a (yield, %)
3a (yield, %)
Z/E
c
1
L1
none
L2
L3
L4
L5
L6
L7
L8
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
TMSCH2Li
LDA
>95
72
>95
93
89
>95
61
77
>95
60
85
86
46
0
24
13
11
12
5
16
trace
trace
10
17
36
trace
12
46
50
59
11
11
25
24
32
8
>95:5
NA
2
3
4
5
6
7
8
9
58:42
23:77
55:45
92:8
>95:5
94:6
NA
6
13
24
13
10
19
11
12
30
12
3
10
11
12
L9
NA
L10
L11
PBu3
L12
L13
L1
40:60
82:18
95:5
NA
58:42
>95:5
>95:5
>95:5
>95:5
>95:5
e
13
>95
66
14
15
>95
>95
>95
>95
>95
>95
c
16
c
17
L1
L1
KOAc
tBuOK
c
18
c
8
19
L1
L1
TBAF
TBAF
80
87 (83)
7
8
cf
,
20
a
Reaction conditions: 1a (0.5 mmol), HBpin (0.5 mmol), ligand (0.015 mmol), CoCl2 (0.015 mmol), additive (0.03 mmol), THF (1.0 mL), rt, 12
b
c
d
1
h. Determined by H NMR using 1,3,5-trimethoxybenzene as an internal standard. Preprepared L1·CoCl2 complex was used as catalyst. Not
analyzed. Used 0.03 mmol of PBu3. Used 5.0 mL of THF. The data in parentheses is isolated yield.
e
f
several other previously reported activators, including organo-
metallic reagents11a,b and bases,11c,e and fluorine reagents11f
could also be used to generate active catalysts with tertbutyl
ammonium fluoride (TBAF) as the best choice, which
improved the yield to 80% (entries 16−19, see also Table
S3). The yield could be further improved to 87% (83%
isolated) by carrying out the reaction in dilute solution (entry
20). Complexes between other metals and 1,10-phenanthroline
ligands gave poor results (see Table S2 for details). The
reaction could be scaled up to gram scale with well retained
yield (80%, 1.34 g of product 2a was obtained), which
demonstrated its potential use in practical synthesis.
With the optimal conditions in hand, we next evaluated
various diyne substrates (Scheme 1), starting with C-tethered
1,6-diynes. We found that one or both of the ester groups in 1a
could be changed to other functional groups, such as a ketone
(2b−2d), a phenyl group (2e), an amide (2f), a nitrile (2g), a
sulfone (2h), and a hydroxy methyl group (2i and 2j), with
little effect on the reaction outcome. These results indicate that
the cobalt catalyst tolerated a variety of functional groups.
Even when one or both of the ester groups in 1a were removed
completely, the reactions nevertheless afforded the desired
cyclization/hydroboration products (2k−2m), although the
yields were lower. In addition to C-tethered 1,6-diynes, N-
tethered and O-tethered 1,6-diynes 1n−1s were also suitable
substrates, smoothly affording the corresponding N- and O-
heterocycles in moderate to good yields. Moreover, even
unsymmetric 1,6-diynes (1r and 1s) could be used. In these
cases, the boryl group added mainly to the less sterically
hindered alkyne. Substrate 1r, which has bulky geminal phenyl
substituents, gave 2r as the sole product, whereas 1s, which has
a less bulky n-pentyl substituent, gave a mixture of
regioisomers 2s and 2s′. Interestingly, although reactions
with common 1,7-diynes failed to afford the desired products,
B
DOI: 10.1021/acs.orglett.9b02873
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Cobalt-Catalyzed Cyclization/Hydroboration of
1,6-Diynes with HBpin: Substrate Scope
Scheme 2. Transformations of the Product 2a
a
a
Reaction conditions: (a) N-phenylmaleimide, THF, rt, 15 h; (b)
H2O2/NaOH, THF, rt, 3 h; (c) CuCl2, THF/H2O, 70 °C, 10 h; (d)
p-nitroiodobenzene, Pd(PPh3)4/K2CO3, toluene/H2O/MeOH, 80
°C,12 h; (e) alkenyl bromides or iodides, Pd(PPh3)4/MeONa,
toluene/MeOH, 80−100 °C, 15 h; (f) 2,3-dicyano-5,6-dichloroben-
zoquinone, toluene, 100 °C, 3 h; (g) (1) glyoxylic, morpholine,
hexafluoropopan-2-ol, rt, 8 h, (2) TMSCH2N2, THF, 4 h.
Scheme 3. Control Experiments
a
Unless otherwise noted, the reaction conditions were the same with
those described in Table 1, entry 20. All of the reactions exhibited full
b
conversion of diynes. Z/E of all the products are >95:5. Used 2.0
c
d
equiv of HBpin. Used 1.5 equiv of HBpin. Reaction time: 10 h.
e
f
Used 3.0 equiv of 1m. Reaction time: 8 h. Used 1.5 equiv of 1p.
Reaction time: 24 h.
g
1,7-diyne 1t, which has a 4,4,5,5-tetraester moiety, smoothly
underwent the cyclization/hydroboration to afford six-
membered-ring product 2t in 65% yield. The highly
substituted linkage between the two alkyne moieties may
have brought them into close proximity, thus promoting the
reaction. 1,6-Diynes with an internal alkyne moiety failed to
afford the desired cyclization/hydroboration products under
the standard reaction conditions.
To demonstrate the potential synthetic utility of this
reaction, we performed several transformations of product 2a
(Scheme 2). A Diels−Alder reaction between 2a and N-
phenylmaleimide afforded structurally complex tricyclic allylic
borate ester 3 in 67% yield with excellent stereoselectivity
(Scheme 2a). Compound 2a could also be oxidized to α,β-
unsaturated aldehyde 4 in excellent yield (Scheme 2b) and
halogenated to afford chloro-1,3-diene 5 with retained
configuration by treatment with CuCl2 (Scheme 2c). The
boryl group of 2a could undergo Suzuki−Miyaura coupling
reactions with p-nitroiodobenzene (Scheme 2d) and various
alkenyl bromides or iodides (Scheme 2e), and the products of
the latter reaction were easily transformed to cyclohexadienes
by means of a thermal 6-π electrocyclization and then oxidized
to multisubstituted benzenes 8 (Scheme 2f). It is easy to
imagine that various multisubstituted benzenes could be
prepared by combining the present method with the Suzuki
coupling and oxidation and might advance related total
synthesis.12 Finally, the boryl group of 2a could be converted
to an amino ester group by means of a Borono−Mannich
reaction (Scheme 2g).
Bpin (Scheme 3a). This result clearly indicates cis addition of
HBpin to diyne 1a. Stoichiometric reactions were then
performed with catalyst precursor L1·CoCl2. First, we carried
out the reaction of L1·CoCl2 with DBpin in the presence of
TBAF to generate the active cobalt catalyst; then we added
diyne 1a and HBpin and subsequently isolated 2a without
deuterium (Scheme 3b). In contrast, when we used HBpin to
generate the active catalyst but used DBpin for the cyclization/
hydroboration, 2a-d was isolated instead (Scheme 3c). From
these results, we deduced that Co−H was not involved in the
active cobalt catalyst reduced from L1·CoCl2. Next, similar
experiments were performed but using a partially ethyl-
substituted borate in the second step (Scheme 3d) or the
first step (Scheme 3e), respectively. The results clearly indicate
To probe the reaction mechanism, we conducted a series of
control experiments (Scheme 3). When DBpin was used
instead of HBpin, the 2a-d that was obtained had cis D and
C
DOI: 10.1021/acs.orglett.9b02873
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
that the boryl of second step transforms to the products, and
thus, Co−B species can be ruled out from active catalysts
reduced from L1·CoCl2.
The authors declare no competing financial interest.
On the basis of the results of control experiments and by
analogy to the mechanisms of other reactions catalyzed by low-
valent cobalt,11,13 we propose the 2e redox catalytic cycle
shown in Scheme 4. First, low-valent cobalt species I, the active
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21625204), the “111” project (B06005) of the Ministry of
Education of China, and the National Program for Special
Support of Eminent Professionals for financial support.
Scheme 4. Proposed Mechanism
DEDICATION
■
This paper is dedicated to the 100th anniversary of Nankai
University.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sfzhu@nankai.edu.cn.
ORCID
Shou-Fei Zhu: 0000-0002-6055-3139
D
DOI: 10.1021/acs.orglett.9b02873
Org. Lett. XXXX, XXX, XXX−XXX

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