Cobalt-Catalyzed Asymmetric Sequential Hydroboration/Hydrogenation of Internal Alkynes

Article abstract of DOI:10.1021/jacs.7b09832

A highly regio- and enantioselective cobalt-catalyzed hydroboration/hydrogenation of internal alkynes with HBpin and a hydrogen balloon in one pot was developed. A new type of chiral imidazoline iminopyridine (IIP) ligand was introduced for the first time in this novel and efficient strategy. This protocol used relatively simple and available starting materials with good functional group tolerance to construct more valuable chiral secondary organoboronates. The primary mechanistic studies illustrated that the cobalt-catalyzed regioselective hydroboration of alkynes did initially occur followed by HBpin-promoted and cobalt-catalyzed enantioselective hydrogenation of alkenylboronates.

Full text of DOI:10.1021/jacs.7b09832

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Communication
Cobalt-Catalyzed Asymmetric Sequential
Hydroboration/Hydrogenation of Internal Alkynes
jun guo, Biao Cheng, Xuzhong Shen, and Zhan Lu
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 18 Oct 2017
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Cobalt-Catalyzed
Asymmetric
Sequential
Hydrobora-
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tion/Hydrogenation of Internal Alkynes
Jun Guo, Biao Cheng, Xuzhong Shen, Zhan Lu*
Department of Chemistry, Zhejiang University, Hangzhou 310058, China
Supporting Information Placeholder
Scheme 1 The asymmetric hydrogenation strategies for the construction of
chiral secondary organoboronates.
ABSTRACT: A highly regio- and enantioselective cobalt-
catalyzed hydroboration/hydrogenation of internal alkynes with
HBpin and a hydrogen balloon in one pot was developed. A new
type of chiral imidazoline iminopyridine (IIP) ligand was intro-
duced for the first time in this novel and efficient strategy. This
protocol used relatively simple and available starting materials
with good functional group tolerance to construct more valuable
chiral secondary organoboronates. The primary mechanistic stud-
ies illustrated that the cobalt-catalyzed regioselective hydrobora-
tion of alkynes did initially occur followed by HBpin-promoted
and cobalt-catalyzed enantioselective hydrogenation of alkenyl-
boronates.
controlled carefully for making this transformation synthetically
useful. Very recently, our group reported a cobalt-catalyzed regio-
and enantioselective sequential hydrosilylation/hydrogenation of
terminal alkynes.7 Although the internal alkynes were not suitable
for the previously reported reaction, it still strongly encourages us
to explore new asymmetric transformation of internal alkynes.
Here, we developed a highly regio- and enantioselective cobalt-
catalyzed hydroboration/hydrogenation of internal alkynes in one
pot to afford the chiral secondary organoboronates.
Chiral organoboronates are very useful building blocks for the
synthesis of complicated chiral molecules through efficient C-Y
(Y = C, N, O etc.) bonds formation from carbon-boron bonds.1 To
date, several strategies, such as stoichiometric stereospecific
organoboronate homologation reaction2 and catalytic asymmetric
alkene hydroboration3, have been widely used for the
enantioselective construction of carbon-boron bond. Compared to
the above two methodologies, asymmetric hydrogenation of
alkenylboronic esters provides an attractive alternative strategy
(Scheme 1). Although asymmetric hydrogenation of
alkenylboronic esters4 have been reported to afford chiral
organoboronates by Miyaura, Morken, Andersson, Pfaltz, and
other groups, the catalysts are still limited to noble transition
metals rhodium5 and iridium6 with chiral phosphine containing
ligands. It will be ideal to realize asymmetric hydrogenation of
alkenylboronates using earth-abundant transition metal catalyst
not only for fundmental studies but also for potentially broader
utility. Additionally, the functional group tolerance of this
transformation has not been described. Furthermore, the
preparation and purification of some unstable alkenylboronates
increase difficulties for this type of transformation. Thus, the
development of novel strategies for asymmetric hydrogenation of
alkenylboronates is still highly desirable.
Initially, we chose the internal alkyne 1a as a simple model
substrate and oxazoline iminopyridine (OIP) cobalt complex as a
precatalyst. The reaction of 1a with HBpin in the presence of 2.5
mol% of cobalt precatalyst L1a·CoCl2 and 7.5 mol% of NaBHEt3
in a solution of Et2O (0.5 M) at room temperature with a hydrogen
balloon for 10 h was carried out to afford a mixture of alkyne
hydroboration products 4a and 5a in 64% and 16% yield,
respectively, however with only few desired sequential products
2a and 3a (< 5% yield) (entry 1, table 1). Using L-phenylalanine-
derivated ligand L1b, the reaction did afford 2a in 13% yield with
49% ee (entry 2). Encouragingly, using L-valine-derivated ligand
L1c, alkyl boronic ester 2a was obtained with 93% ee, however,
in a rather poor yield (entry 3). To our delight, using more
electron-rich N-phenyl protected chiral imidazoline iminopyridine
(IIP) ligand L1d, the yield of 2a was promisingly increased to 58%
yield with 91% ee (entry 4). A significant raise in both yield (81%)
and the enantioselectivity (97% ee) was observed by using less
sterically bulky 2,6-dimethyl imine ligand (L1e) (entry 5).8
Although asymmetric sequential hydroboration/hydrogenation
of alkynes in one pot is an ideal and step-economic strategy, it has
not been previously reported. There are several challenges: 1)
hydroboration and hydrogenation are competitors in a same
catalytic system; 2) The regio- and enantioselectivities have to be
Additionally, the change of the substituent on the shoulder of
imine (L1f and L1g) showed similar reactivities with a slightly
lower ee (entries 6 and 7). When dioxane and toluene were used,
enantioselectivity of 2a was decreased slightly (entries 8 and 9).
Unexpectedly, only hydrogenation product 6a was afforded in 80%
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Table 1. Optimizationsa
Table 2. Scope of chiral organoboronic esters.a
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entry
[Co]
solvent
yield of
2a/3a/4a/5a (%)
<5/<1/64/16
13/<1/50/19
8/<1/50/17
58/<1/18/<1
81/<1/<1<1
82/<1/<1<1
78/<1/<1<1
68/<1/<1<1
74/<1/<1<1
<1/(80)b
ee of 2a
(%)
~
49
93
91
97
93
79
93
93
~
90
1
2
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8
L1a·CoCl2
L1b·CoCl2
L1c·CoCl2
L1d·CoCl2
L1e·CoBr2
L1f·CoBr2
L1g·CoBr2
L1e·CoBr2
L1e·CoBr2
L1e·CoBr2
L1e·CoBr2
L1e·CoBr2
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
dioxane
toluene
THF
9
10
11c
12d
Et2O
Et2O
82/<1/<1<1
82/<1/<1/<1
96
Yields were determined by 1HNMR using TMSPh as an internal standard.
a
b
c
Yield of 6a. Commercially available Et2O was directly used without any
drying process. d 1 mol% of catalyst was used in a solution of Et2O (0.5 M).
yield when THF was used as a solvent (entry 10). The reaction
underwent smoothly using non-predried diethyl ether as a solvent
to afford 2a in a similar yield with a slightly lower ee (entry 11),
which demonstrated that this protocol was not moisture-sensitive.
When decreasing the catalyst loading to 1 mol%, the reaction
could undergo smoothly to afford 2a in 82% yield with 96% ee
(entry 12). The standard conditions are identified as 1 mmol of
alkyne, 2 mmol of HBpin, 1 mol% of L1e·CoBr2 and 3 mol% of
NaBHEt3 in 2 mL of Et2O with a hydrogen balloon.
With the optimized reaction conditions, the substrate scope was
illustrated in Table 2. Unless otherwise noted, the rr values of all
the substrates were greater than 20/1 (2/3). The methyl propargyl
ether 1b could be converted to 2b in 75% yield and 95% ee. The
phenyl (1c) or benzyl (1d) homopropargyl ether could also be
delivered to 2c and 2d in 68 - 80% yield and 91 - 95% ee. The
phenyl alkyl (Me, Et, n-Pr, n-Bu, n-Am, i-Bu) alkynes could be
smoothly transformed to the corresponding products in 76 - 94%
yield with greater than 93% ee (2e ~ 2j). The use of NaBHEt3
could efficiently promote these transformations. Here, a more
mild reagent LiOtBu was also found to be suitable as an activator
which has been reported by Thomas group.9 Due to the steric
effect, the hydroboration reaction of more sterically bulky 1k did
occur slowly. It was worthy of noting that the asymmetric
hydrogenation of alkenylboronic ester 4k did undergo smoothly to
afford 2k in 97% yield and 96% ee. The diaryl alkyne could
participate to give 2l in 90% yield with 98% ee. The dibenzyl-
protected propargyl amine 1m could also be transformed
smoothly into chiral amino alcohol derivate 2m in 62% yield with
94% ee after direct oxidation. Due to the low activity in the
hydrogenation step, the 1n containing a free alcohol could be
delivered to chiral 1,4-diol 2n in 64% yield with a slightly lower
ee when 4.0 equivalent of HBpin was used as a hydrogen source
instead of H2 to accelerate the hydrogenation step. The electron-
donating and withdrawing substituents on phenyl ring, such as
alkyl, ether, halide, acetyl, and ester, could be tolerated to afford
2o – 2u and 2w - 2ab in 36 - 86% yields and 84 - 97% ee. Due to
the steric effect, the more sterically bulky ortho- substituted
product 2v could be obtained in 72% yield with 96% ee through
asymmetric hydrogenation of the corresponding alkenylboronic
ester. The polycycles and heterocycles, such as 2-naphthyl (1ac),
a
1 mmol of alkyne, 2 mmol of HBpin, 1 mol% of L1e·CoBr2 and 3 mol% of
b
NaBHEt3 in 2 mL of Et2O with a hydrogen balloon. 0.5 mmol scale with 5
mol% of cobalt complex. LiOtBu instead of NaBHEt3. d 0.5 mmol scale with
c
2.5 mol% of cobalt complex. e from vinyl boronic ester. f 4.0 equivalent HBpin
g
was used instead of H2 and run for 24 h. Isolated yield for corresponding
alcohol in parenthesis and NMR yield for boronic ester outside the parenthesis.
9-H-fluoren-2-yl (1ad), 3-pyridyl (1ae), 5-indyl (1af) and 3-
carbazolyl (1ag), could be tolerated to deliver chiral boronic
esters or the corresponding alcohols after oxidation in 52 - 85%
yields and 60 - 98% ee. Asymmetric diaryl acetylene 1ah could
participate to give 2ah in 44% yield with 94% ee and the other
hydroboration isomer 5ah in 56% yield. When symmetric dec-5-
yne 1ai and ortho-methyl substituted aromatic substrate 1aj were
used, only alkyne hydroboration products 4ai and 4aj were
obtained in 80% and 70% yield, respectively. When
phenylacetylene was used, the terminal hydroboration product and
terminal hydroboration/hydrogenation product were obtained in
33% yield and in 43% yield, respectively.
configuration was verified by comparison of optical rotation of 2a
with previously reported data and the other products were then
assigned by analogy to 2a.11
10
The absolute
The gram scale reaction could be carried out smoothly for 22.5
h to afford 2a in 80% yield and 96% ee. The chiral carbon-boron
bond could be easily converted to C-O bond through oxidation
and C-C bond through cross-coupling in a stereospecific manner12
(Scheme 2).
Several control experiments were conducted to elucidate the
possible reaction pathway. The hydroboration of (Z)-but-1-en-1-
ylbenzene gave (S)-2f in 99% yield and >20/1 rr, however, with
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Scheme 2 Gram scale reaction and transformations of chiral organoboronic
ester 2a
Scheme
3
The proposed mechanisms of sequential hydrobora-
tion/hydrogenation of internal alkynes.
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17% ee (eq. 1), which ruled out the hydrogenation of alkynes
followed by alkenes hydroboration pathway to achieve high
enantioselectivity. It also demonstrated that alkyne hydrogenation
followed by alkene hydroboration could be a side reaction to
decrease the enantioselectivity in some cases. The alkenylboronic
ester 4l could be obtained through alkyne hydroboration. It should
be noted that 4l could not be hydrogenated under the standard
conditions without HBpin (eq. 2). When 0.3 equivalent of HBpin
or B2(pin)2 was added, the hydrogenation reaction underwent
smoothly to afford 2l with similar results (55% yield and 93% ee)
(eq. 3). The use of 0.5 equivalent of HBpin could perfectly
promote the hydrogenation reaction (eq. 4). These results
indicated that the HBpin or boronic group might promote the
trisubstituted alkenylboronates hydrogenation process13. These
control reactions suggested that the reaction should undergo
alkyne hydroboration followed by HBpin-promoted alkene
hydrogenation pathway.
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yield with >20/1 rr and 93% ee (eq. 7). Unexpectedly, the D-
incoporation was not 100% and the amount of added hydrogen
was more than the reductant used. We proposed that the hydrogen
might come from adventitious water. When 2.0 equivalent D2O
was added to the system using 4.0 equivalent HBpin as the
hydrogen source, the significant D-incoporation was appeared at
both C1 and C2 positions in d-2l-c (eq. 8). This demonstrated that
the trace water in the reaction system did not affect the yield and
ee, however, could paly a role of hydrogen source14 in the
presence of HBpin to participate the hydrogenation proccess.
The reaction of 1l with DBpin without hydrogen gas afforded
d-4l in 79% yield with >95% D-incoporation (eq. 5). The d-4l
could also be hydrogenated to afford d-2l-a in 99% yield
with >20/1 rr and 94% ee with around 20% of D-atom loss at C2
position (eq. 6). This demonstrated that alkene insertion to cobalt
hydride bond was reversible. The reaction of 1l with 4.0
equivalent of DBpin without hydrogen gas afforded d-2l-b in 90%
According to the control experiments and deuterium
experiments, the primary mechanisms were proposed in Scheme 3.
The cobalt precatalyst enters the catalytic cycle by reaction with
NaBHEt3 to form cobalt hydride species A. The alkyne
coordination with species A followed by alkyne insertion to the
cobalt hydrogen bond delivers cobalt vinyl species C. The
intermediate C goes through σ-bond metathesis with HBpin to
afford alkenyl boronic esters 4 and regenerate cobalt hydrides A.15
The 4 is employed to the next hydrogenation cycle by alkene
insertion into the cobalt hydrogen bond to form chiral
interconvertible alkyl cobalt species E or F. Species E or F may
slowly react with H2 to afford 2 and regenerate species A (path a).
The mostly possible pathway may be that species E or F
undergoes σ-bond metathesis with HBpin to form cobalt (I) boryl
species G and 2. The active cobalt hydride species A could be
regenerated from species G through two possible pathways. For
path b: cobalt hydride species A can be regenerated by sigma-
bond metathesis of species G with water which was analogous to
the previous reports on palladium catalyzed diboron-mediated
transfer hydrogenation using water14. For path c: the species G
undergoes through oxdative addition of H2 to form cobalt (III)
boryl dihydrides species H which undergoes reductive elimination
to regenerate species A. More mechanistic studies should be
further executed to demonstrate the details for regio- and
enatioselectivities.
In summary, we have developed
a highly regio- and
enantioselective cobalt-catalyzed hydroboration/hydrogenation of
alkynes with HBpin and a hydrogen balloon. A new type of chiral
imidazoline iminopyridine (IIP) cobalt complex has been proven
to be an efficient catalyst precursor. This novel protocol used
relatively simple and available starting materials to construct
more valuable chiral organoboronates. Compared to traditional
asymmetric hydrogenation of alkenylboronates, this one-pot
strategy does avoid making alkenylboronates and emerge the
advantage of atom and step-economy. The primary mechanistic
studies illustrated that regioselectivity was originated from
hydroboration of alkynes and enantioselectivity from
hydrogenation of alkenylboronates. HBpin or boronic group
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process. The further mechanistic studies are underway in our
laboratory.
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(6) For some selected examples, see: a) Paptchikhine, A.; Cheruku, P.;
Engman, M.; Andersson, P. G. Chem. Commun. 2009, 5996-5998. b)
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(8) CCDC number of L1d·CoCl2: 1525168; CCDC number of
L1e·CoBr2: 1559196.
(9) Docherty, J. H.; Peng, J.; Dominey, A. P.; Thomas, S. P. Nature
chem. 2017, 9, 595-600.
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K. Nature Chem. 2014, 6, 584-589.
ASSOCIATED CONTENT
Supporting Information
9
The Supporting Information is available free of charge on the
ACS Publications website. Experimental procedures, characteriza-
tion data for all compounds (PDF). X-ray diffraction of
L1d·CoCl2 (CIF) and L1e·CoBr2 (CIF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail: luzhan@zju.edu.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
Financial support was provided by the National 973 Program
(2015CB856600), NSFC (21472162, 21772171) and Zhejiang
University.
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