Cobalt-Catalyzed Ligand-Controlled Regioselective Hydroboration/Cyclization of 1,6-Enynes

Article abstract of DOI:10.1021/acscatal.6b02816

A ligand-controlled cobalt-catalyzed regioselective hydroboration/cyclization of 1,6-enynes with HBPin was developed by switching the size of the coordinated side arm to afford alkenylboronates and alkylboronates, respectively. Gram-scale reactions could be easily conducted, which is beneficial for further derivatizations. A primary mechanism was proposed on the basis of substrate-controlled experiments and deuterium experiments.

Full text of DOI:10.1021/acscatal.6b02816

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Article
Cobalt-Catalyzed Ligand-Controlled Regioselective
Hydroboration/Cyclization of 1,6-Enynes
Tuo Xi, and Zhan Lu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02816 • Publication Date (Web): 01 Dec 2016
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Cobalt-Catalyzed Ligand-Controlled Regioselective Hydrobora-
tion/Cyclization of 1,6-Enynes
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Tuo Xi and Zhan Lu*
Department of Chemistry, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China
KEYWORDS: ligand-controlled, cobalt-catalyzed, hydroboration, cyclization, 1,6-enynes
ABSTRACT: A ligand-controlled cobalt-catalyzed regioselective hydroboration/cyclization of 1,6-enynes with
HBPin was developed by switching the size of the coordinated side arm to afford alkenylboronates and alkylboro-
nates, respectively. The gram-scale reactions could be easily conducted which is benefit for further derivatizations.
A primary mechanism was proposed based on substrate-controlled experiments and deuterium experiments.
Selective hydroboration1 of carbon-carbon multiple bonds is
one of direct, atom-economic, and powerful methods to syn-
thesize alkyl or alkenyl organoboron compounds,2 which are
very important intermediates for organic synthesis, material
science and pharmaceutical.3
afford alkenylboronates and alkylboronates, respectively
(Scheme 1, c).
Scheme 1. Hydroboration/cyclization of 1,6-enynes
Cyclization of enynes is a direct and useful method to effi-
ciently construct cyclic compounds.4 However, hydroboration
of 1,6-enynes with hydroboranes are less formulated. In 2006,
Widenhoefer and co-works reported an effective rhodium-
catalyzed protocol for asymmetric hydroboration/cyclization
of 1,6-enynes with catecholborane followed by oxidation or
arylation to afford the corresponding chiral cyclic compounds
(Scheme 1, a). 5 Tian and Lin reported copper-catalyzed
asymmetric borylative cyclization of cyclohexadienone-
containing 1,6-enynes with bis(pinacolato)diboron (Scheme 1,
a).6 Cárdenas and co-workers7 developed a series of facile
palladium-catalyzed regioselectivity-controllable borylative
cyclization of 1,6-enynes with (BPin)2 by switching different
protecting groups on substrates (Scheme 1, a and b). However,
the O-tether and N-tether substrates have shown poor reactivi-
ties.
We began our studies by investigating the model reaction of
1,6-enyne 1a with HBpin in the presence of 5 mol% of
OIP.CoCl2 as a precatalyst and 15 mol% of NaBHEt3 as a re-
ductant in a solution of THF at ambient temperature for 5 h
(table 1, entry 1). The alkylboronate cyclic product 2a was
observed in 66% yield. It should be noted that 2a would de-
compose on silica gel and could be converted to the corres-
Although cobalt-catalyzed hydroboration of alkenes8 or al-
kynes9 have been explored, to the best of our knowledge, co-
balt-catalyzed selective hydroboration/cyclization of 1,6-
enynes with HBpin has not been previously reported. Herein,
we developed
a ligand-controlled regioselective cobalt-
catalyzed hydroboration/cyclization reaction of 1,6-enynes to
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ponding alcohol 5a through oxidation of 2a with H2O2.7c Vari-
yields; 1-En-6,8-diyne could be reacted with HBpin to give 5s,
3
4
5
6
7
8
9
ous solvents, such as Et2O, dioxane, toluene and DCM (entries
2-5), were used instead of THF, toluene was more suitable to
give 5a in 72% yield. The reductant reagents (NaBHsBu3,
LiBHEt3, MeLi, MeMgBr, Et2Zn) were also investigated to
give 3a in promising yields (SI Table 1). Interestingly, using
BIP instead of OIP as a ligand, the reaction afforded the reduc-
tive cyclization product 4a in 25% yield without any hydrobo-
ration products (entry 6). Using BOP or OP as ligands, the
yields of 2a were diminished (entries 7 and 8). Unexpectedly,
albeit in 16% yield. The reaction of terminal or Me3Si-
substituted alkynes (1x-y) did not occur.
The IP.CoCl2-catalyzed hydroboration/cyclization with
various enynes was summarized in Table 3. It should be noted
that the reactions were conducted in 5.0 mmol scale. 1,6-
Enynes with various linkers such as O-tether, N-tether, C-
tether were tolerated to give 3a-3e in 36-68% yields; The reac-
tion of sterically hindered ortho-methylphenyl enyne 1f af-
forded a mixture of regioisomers 3f and 2f in 30% and 14%
yields, respectively; meta- and para-methylphenyl enynes
were delivered to the corresponding products 3g and 3h in 65%
and 64% yields, respectively; The reactions of enynes contain-
ing electron-donating and withdrawing groups, such as meth-
oxyl (3i, 3t), phenyl (3m), chloro (3u) and cyano (3v) groups
could smoothly undergo hydroboration/cyclization to afford
the cyclic products in 29-63% yields; Alkyl substituted enynes
(1q-r) could be reacted to give cyclic products in 19-56%
yields. The terminal alkynes (1x) would be converted to 3x in
41% yield. Me3Si-substituted alkyne (1y) could not be suitable.
The stereochemistry of hydroboration products was deter-
mined by X-ray diffraction of compound 3m.12
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when remove of the side arm oxazoline on OIP,10 11 IP.CoCl2
could catalyze the cyclization reaction to deliver a regioselec-
tivity-switchable alkenyl boronate 3a in 68% yield without 2a
(entry 9). The control experiments without reductant or cata-
lyst or ligand did not afford any cyclic products (entries 10
and 11). Unfortunately, no enantioselectivities were observed
when chiral OIP ligand was used.
Table 1. Optimizations of hydroboration/cyclization of 1,6-
enynesa
Table 2. OIP.CoCl2-catalyzed hydroboration/cyclization of
1,6-enynes followed by oxidationa
Entry
Enynes Products
Yield
(%)b
Yield
of 2aa
(%)
Yield
of 3aa
(%)
Yield
of 4aa
(%)
Entry
[Co]
Solvent
1
2
3
4
1a
1b
1c
1d
5a : X = O, R = Ph
72
71
49
84
5b : X = NMe, R = Ph
5c : X = NBn, R = Ph
5d : X = NTs, R = Ph
1
2
OIP.CoCl2
OIP.CoCl2
OIP.CoCl2
OIP.CoCl2
OIP.CoCl2
BIP.CoCl2
BOP.CoCl2
OP.CoCl2
IP.CoCl2
OIP.CoCl2
--
THF
66
50
72b
69
0
0
0
0
0
Et2O
3
toluene
Dioxane
DCM
0
0
5
1e
75
5e: X=
, R = Ph
4
0
0
6
7
8
9
10
11
12
13
14
15
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
5f : X = O, R = 2-MeC6H4
5g : X = O, R = 3-MeC6H4
5h : X = O, R = 4-MeC6H4
5i : X = O, R = 4-MeOC6H4
5j : X = NTs, R = 4-MeOC6H4
5k : X = NTs, R = 3-MeOC6H4
5l : X = NTs, R = 4-MeC6H4
5m : X = NTs, R = 4-PhC6H4
5n : X = NTs, R = 1-naphthyl
5o : X = NTs, R = 2-thienyl
46
60
52
44
74
63
67
62
31
64
5
0
0
6
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
0
0
25
0
7
41
7
0
8
0
68c
0
9
0
0
10
11d
12
0
0
0
0
0
0
CoCl2
0
0
0
16
1p
5p : X = NTs, R = 2-(N-methyl)-indolyl
56
a Yields determined by 1H NMR analysis using TMSPh as an internal standard.
bIsolated yield of the corresponding alcohol 5a followed by oxidation of 2a.
cIsolated yield. d Without NaBHEt3.
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18
19
20
21
1q
1r
1s
1x
1y
5q : X = NTs, R = Me
5r : X = NTs, R = nC4H9
5s : X = O, R = PhCC
5x: X = NTs, R = H
50
50
16
0
With standard conditions in hands, the scope of the
OIP.CoCl2-catalyzed hydroboration/cyclization followed by
oxidation was illustrated in Table 2. Oxygen-, nitrogen- or
carbon-linkers would be tolerated; The enynes with a methyl
group at para-, meta-, or ortho-position of aryl rings could be
delivered to the corresponding products 5f-h in 46-60% yields;
The enynes with electron-rich aryl rings underwent hydrobora-
tion/cyclization reaction to give products in good yields, how-
ever, the reactions of electron-deficient enynes were messy;
Naphthalene (1n), thiofuran (1o), and indole (1p) were toler-
ated to afford cyclic products in 31-64% yields; The alkyl
enynes (1q-r) could also be converted to cyclic products in 50%
5y: X = NTs, R = SiMe3
0
aOIP-conditions: 1 (1 mmol), HBPin (2 mmol), OIP.CoCl2 (5 mol%), NaB-
HEt3 (15 mol%) and toluene (2 mL) at room temperature for 5 hours. Then the
residue was treated with NaOH (3 N, 3 mL) and H2O2 (30%, 3 mL) in ether (6
mL), and stirred for overnight at room temperature.b Isolated yield of 5.
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Table 3. IP.CoCl2-catalyzed hydroboration/cyclization of
1,6-enynesa
High deuterated ratios at the vinyl position under OIP-
conditions and methyl position under IP-conditions were ob-
served.
4
5
6
Scheme 2. Gram scale reactions and further applications.
7
8
9
Entry
Enynes
Products
3a : X = O, R = Ph
3b : X = NMe, R = Ph
3c : X = NBn, R = Ph
3d : X = NTs, R = Ph
Yield (%)b
68
68
51
54
1
2
3
4
1a
1b
1c
1d
10
11
12
13
14
15
16
17
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1e
36
3e: X=
, R = Ph
6
7
8
9
10
11
12
13
14
15
16
17
18
19
1f
1g
1h
1i
3f : X = O, R = 2-MeC6H4
3g : X = O, R = 3-MeC6H4
3h : X = O, R = 4-MeC6H4
3i : X = O, R = 4-MeOC6H4
3l : X = NTs, R = 4-MeC6H4
3m : X = NTs, R = 4-PhC6H4
3q : X = NTs, R = Me
3r : X = NTs, R = nC4H9
3t : X = O, R = 3-MeOC6H4
3u : X = O, R = 4-ClC6H4
3v : X = NTs, R = 4-NCC6H4
3w : X = O, R = 2-thienyl
3x: X = NTs, R = H
30/14c
65
64
60
63
63
56
19
48
36
29
85
41
1l
1m
1q
1r
1t
1u
1v
1w
1x
1y
3y: X = NTs, R = SiMe3
0
a IP-conditions: 1 (5 mmol), HBPin (10 mmol), IP.CoCl2 (5 mol%), NaB-
HEt3 (15 mol%) and toluene (10 mL) at room temperature for 5 hours; b Iso-
lated yield of 3. c Yield of the regioisomer 2f.
It was worth to note that the gram-scale reaction of 1d with
HBPin under OIP-conditions afforded 2d in 95% yield without
decomposition (Scheme 2). The gram-scale reaction of 1a
catalyzed by IP.CoCl2 was readily conducted to give 3a in 68%
yield. Oxidation of alkenyl-boronate 3a with H2O2 can be per-
formed to give the corresponding ketone 10a in 72% yield
with 6/1 dr.5 The pinacol boronates could be easily converted
to the corresponding organotrifluoroborates 6d and 8a, in 80%
yield respectively.8i Additionally, the cyclization followed
Suzuki-coupling reactions of 1a afforded the corresponding
tetra-substituted olefins 9aa-9ae in 45-52% yields, and 6d
could be delivered to tri-substituted olefin 7 in 80% yield.
To demonstrate the details of transformations, control reac-
tions of alkene 11 or alkyne 12 under IP-conditions were car-
ried out to give isomerization products 13 in 29% yield and
hydrogenation products 14 and 15 in 12% and 29% yields,
respectively (eq. 1 and 2), which suggested that cobalt hydride
might be the inital activated species.13 Under OIP-conditions,
the hydroboration products 16 and 17 were afforded in 29%
and 43% yields, respectively (eq. 3 and 4). The observation of
reductive cyclization product 4ab suggested that the initial active
species might also be Co-H species (eq 5). The poor diastereose-
lectivity observed decreased the possibility of cyclometallation
pathway.14 The reaction of α-substituted alkene 1aa afforded
2aa in 29% NMR yield and dihydroboration product 18 in 21%
NMR yield (Scheme 3), which suggested that the alkyne inser-
tion and alkene insertion underwent step by step. We proposed
that the alkene-coordination to cobalt species could control the
regioselectivity in the step of hydroboration of alkyne (see in
SI). The reaction of 1aa with HBpin did not give desired product
under IP-conditions. The reaction of 1,2-disubstituted alkene 1ad
with HBpin did not give desired products either under OIP-
conditions or IP-conditions (Scheme 3). Additionally, the deute-
rium experiments using DBpin15 were conducted (Scheme 4).
Based on above experimental and previously reported re-
sults,8b,8h-i cobalt complex could be reduced by NaBHEt3 to
generate cobalt hydride8h-i species (scheme 4). Using IP as a
ligand, the alkene insertion to cobalt hydride bond might be
the initial step to produce cobalt alkyl species which could be
delivered to cobalt alkenyl species via alkyne insertion to co-
balt carbon bond. The cobalt alkenyl species then underwent
σ-bond metathesis with HBPin followed by reductive elimina-
tion to afford alkenylboronic ester and regenerate cobalt hy-
dride species. Using the OIP ligand, the alkyne insertion to
cobalt hydride bond might occur to afford cobalt carbon spe-
cies which could undergo alkene insertion to give cobalt alkyl
species. The cobalt alkyl species could react with HBPin via
σ-bond metathesis followed by reductive elimination to afford
alkylboronic ester and regenerate cobalt hydride species.
However, the cyclometallation of enyne16 could not be exclu-
sively ruled out.
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Notes
The authors declare no competing financial interest.
Scheme 3. Disubstituted alkene
5
6
7
8
ACKNOWLEDGMENT
Financial support was provided by the National 973 Pro-
gram (2015CB856600) and NSFC (21472162).
9
REFERENCES
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Scheme 4. Deuterium experiments and plausible mechan-
ism.
1
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In summary, we have developed a ligand-controlled cobalt-
catalyzed regioselective hydroboration/cyclization of 1,6-
enynes to afford alkenylboronates and alkylboronates, respec-
tively. The reactions could be easily scaled up and the boro-
nate products could be derivatized to various useful com-
pounds. A primary mechanism was proposed based on sub-
strate-controlled experiments and deuterium experiments. This
interesting side-arm-effect phenomenon will encourage us to
explore more examples and mechanistic details on selectivity-
controllable reactions by using multi-dentate ligands with non-
precious transition metals.
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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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Procedures and analytical data (PDF)
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X-ray diffraction of compound 3m (cif)
AUTHOR INFORMATION
Corresponding Author
*E-mail: luzhan@zju.edu.cn
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We appreciated the suggestion about the possible cyclometallation
mechanism from one of the reviewers.
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