Cobalt-Catalyzed Regio- and Stereoselective Hydroboration of Allenes

Article abstract of DOI:10.1002/anie.201915716

An efficient pincer-ligand-based cobalt-complex-catalyzed allene hydroboration affording Z-allylic boronates is described. The reaction demonstrates an excellent regio- as well as Z-stereoselectivity and a wide substrate scope that tolerates many functional groups. Based on solvent-assisted electrospray ionization mass spectrometry (SAESI-MS) studies, a rationale for the cobalt-catalyzed hydroboration involving the highly selective insertion of an allene into the Co?H bond to form Z-allylic cobalt intermediates is proposed.

Full text of DOI:10.1002/anie.201915716

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Accepted Article
Title: Cobalt-Catalyzed Regio- and Stereoselective Hydroboration of
Allenes
Authors: Can Li, Zheng Yang, Lei Wang, yinlong Guo, Zheng Huang,
and Shengming Ma
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915716
Angew. Chem. 10.1002/ange.201915716
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10.1002/anie.201915716
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Cobalt-Catalyzed Regio- and Stereoselective Hydroboration of
Allenes
Can Li,[a,b]§ Zheng Yang,[a]§ Lei Wang,[a] Yinlong Guo,*[a] Zheng Huang,*[a] and Shengming Ma*[a,c]
Dedication ((optional))
a) Non-catalytic hydroboration of allenes with a borane reagent (Roush)
Et2O•Li
Abstract: An efficient pincer ligand-based cobalt complex-catalyzed
allene hydroboration affording Z-allylic boronates is described. The
reaction enjoys an excellent regio- as well as Z-stereoselectivity and
a wide substrate scope tolerating many functional groups. Based on
SAESI/MS studies, a rationale for the cobalt-catalyzed hydroboration
involving the highly selective insertion of allene into the Co-H bond
to form Z-allylic cobalt intermediates is proposed.
H
ate complex
TMS-Cl
H
R1
•
B
Me3Si
R1
B
SiMe3
ate complex
OH
R2CHO
after workup
R = c-C6H11, 86%, syn/anti = 78:22
R = n-C8H17, 81%, syn/anti = 60:40
R2
R1
Organic boronic acid derivatives are of low toxicity and decent
stability, and thus serve as versatile intermediates for many
synthetic transformations.1,2 Of particular interest is the stereo-
defined allylic boronic acid derivatives, which may provide a
synthetically attractive allylic entity into organic skeletons.
Compared to previous approaches to allylic boronic acid
derivatives,3 transition metal-catalyzed hydroboration of readily
available allenes with pinacolborane (HBpin) would be, in
principle, a very efficient method featuring excellent atom
economy. However, compared with the well-established catalytic
hydroboration of alkenes,4 alkynes,5 and 1,3-dienes,6 the
hydroboration of allenes has remained underdeveloped.7,8 In
2009, a one-pot two-step reaction to form homoallylic alcohols
involving non-catalytic hydroboration of alkyl-substituted terminal
allenes with a chiral borane reagent has been reported, however,
the stereoselectivity is rather poor (Scheme 1a).9 In 2013, Tsuji
et al. reported a copper-catalyzed hydroboration of mono-
substituted terminal allenes to form linear E-allylic boronates as
the major products, although the E/Z selectivity is subject to the
steric effect of the R group (Scheme 1b).10 In 1999, Miyaura and
coworkers reported the preparation of linear Z-allylic boronates
as major products via platinum-catalyzed hydroboration of 2,3-
propadienyl TBS or methyl ether with Z/E ratios ranging from
84:16~91:9 (Scheme 1c).11 Herein, we report a cobalt-catalyzed
hydroboration of allenes with pinacolborane (HBpin) to provide
Z-allylboronates with an excellent regio- and stereoselectivity
(Scheme 1d).
b) Cu-catalyzed hydroboration of allenes forming E-allylboronates (Tsuji)
Cu catalyst
R
CF3Ar-Xantphos
+
HBpin
R
•
Bpin
R = aryl, t-alkyl: exclusive E-selectivity
R = 1o-alkyl, E/Z ~ 90:10
c) Pt-catalyzed hydroboration of allenyl ether with Z-selectivity (Miyaura)
Pt(dba)2
PCy3 or P(t-Bu)3
+
RO
•
HBpin
RO
Bpin
R = Me, L = PCy3, 65%, Z/E = 84/16
R = TBS, L = PCy3, 77%, Z/E = 91/9
R = TBS, L = P(t-Bu)3, 86%, Z/E = 90/10
d) Co-PNN-catalyzed hydroboration of allenes affording Z-allylboronates
(This work)
Rs
Rs
RL
RL
•
Co-PNN catalyst
HBpin
+
Bpin
Rs = H, alkyl,
L = alkyl, alkoxyl, silyl, siloxy
excellent regio- and Z-selectivity
R
Scheme 1. Hydroboration of allenes affording alkenyl or allylic boronates.
Based on the pincer ligand-based iron and cobalt complexes
developed in Huang’s group,12 initially, (tBuPONNiPr)FeCl2 (1) was
chosen as the precatalyst for the hydroboration of 1,2-
undecadiene (3a) with HBpin in toluene. Unfortunately, only a
complicated mixture was formed. Subsequently, a series of
complexes of Fe and Co supported by various PCNN ligands12
were investigated. To our delight, using (tBuPCNNiPr)FeCl2 (2a) as
the precatalyst, the reaction afforded Z-4a in a high yield
together with small amounts of regioisomers 5a and 6a as
[a]
C. Li, Dr. Z. Yang, L. Wang, Prof. Dr. Y.-L. Guo, Prof. Dr. Z. Huang,
Prof. Dr. S.-M. Ma
1
determined by H NMR analysis of the crude reaction mixture
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032, P. R. China
E-mail: masm@sioc.ac.cn; huangzh@sioc.ac.cn; ylguo@sioc.ac.cn.
C. Li
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
Prof. Dr. S.-M. Ma
(Table 1, entry 2). The difference between 1 and 2a is largely
due to the increased steric hindrance around the metal center.
Interestingly, using the cobalt complex with the same pincer
ligand 2b with its structure established by X-ray diffraction
(Table 1),13 the reaction afforded Z-4a exclusively, albeit with
40% of 3a being recovered. Further screening of the
precatalysts revealed that Fe complex 2c with a less hindered
iPrPCNNiPr ligand is more active than the Co variant 2d with the
same ligand, but the former is much less selective (Table 1,
compare entry 5 with entry 6). The Co complex 2e with the least
sterically demanding ligand among the PNN ligands investigated
here is less efficient than 2b, albeit with an excellent selectivity
[b]
[c]
[§]
Department of Chemistry, Fudan University
220 Handan Lu, Shanghai 200433, P. R. China
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201915716
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(Table 1, entry 8 vs. entry 2). Further optimization led to the
observation that increasing the loading of precatalyst 2b from 1
to 3 mol% afforded the hydroboration product Z-4a in 90% yield
without the detection of other isomers (Table 1, entry 4). The
yield dropped when the reaction was conducted in ethyl ether,
hexane, or THF, although the selectivity remained unaffected
(Table 1, entries 9-11). No conversion was observed in the
absence of the metal complex or the catalyst activator NaBHEt3
(Table 1, entries 12 and 13). Therefore, the conditions used in
entry 4 were chosen as the optimized reaction conditions for
further study.
Next, we investigated the scope of cobalt-catalyzed allene
hydroboration. Various functionalized allenes underwent
hydroboration with HBpin in high yields and excellent regio- and
stereoselectivity
at
ambient
temperature
(Table
2).
Hydroboration of alkyl substituted allenes proceeded smoothly
under the standard reaction conditions (4a-4e). The gram-scale
reaction of 3a with HBpin (5 mmol) afforded 4a in 83% yield.
Synthetically useful functional groups such as ketal (4f), OTBS
(4g), benzyloxy (4h), halogen (4i and 4j), ester (4k), and N-
heterocycles (4n, 4o, and 4p) could be tolerated. Even the very
bulky silyl-substituted allene 3l could also be accommodated,
affording the corresponding Z-allylboronate 4l containing both C-
B and C-Si bonds with a 97:3 Z/E selectivity. Exclusive Z-
selectivity was observed even for product 4m, which showed a
Z/E selectivity of ~90:10 as reported in the literature.11 The high
selectivity for the formation of allylic boronate Z-4c from
cyclohexylpropadiene is also noteworthy since it afforded vinylic
boronates under the conditions reported by Tsuji et al.10
Table 1. Optimization of the reaction conditions.[a]
Although cobalt complexes have been reported to catalyze the
hydroboration of alkenes,4e,4g,4h under the optimized conditions,
allenes exhibit a much higher reactivity than both terminal and
internal alkenes as demonstrated by the formation of Z-4q, Z-4r,
and Z-4s in high yields with an exclusive chemoselectivity.
Hydroboration of 1,1-disubstituted allenes also worked well
allowing for the construction of trisubstituted (Z)-allylic boronates
4t-4x. The stereoselectivity is determined by the steric effect of
these two substituents as shown by the Z/E ratios of 4u, 4w, and
4x. Even an estrone 3-methyl ether derived product Z-4v was
formed and isolated in a high yield and an excellent Z/E
selectivity. Double hydroboration of Si- or C-tethered bisallenes
3y and 3z afforded bisallylic boronates 4y and 4z in high yields
and an excellent Z-selectivity via the reaction with 2.2 equiv of
HBpin, without formation of the monohydroboration products.
To demonstrate the synthetic utility of this methodology,
synthetic transformations of (Z)-4a were conducted. Linear allylic
alcohol 7 or branched allylic alcohol 8 could be obtained in high
NMR yield (%)[b]
Z-4a:5a:6a
Recovery
of 3a (%)
Entry
Precatalyst
Solvent
1
2
1
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
Et2O
-
85:7:6
57:0:0
90 (87):0:0
41:12:12
40:0:0
66:13:0
38:0:0
88:0:0
87:0:0
60:0:0
-
-
-
2a
2b
2b
2c
2d
2d
2e
2b
2b
2b
-
3
40
-
4[c]
5
yields using H2O2 or PhNO15 as the oxidant, respectively.
14
-
Suzuki-Miyaura coupling reaction of (Z)-4a and iodobenzene
afforded the corresponding branched allylbenzene derivative (9).
Moreover, (Z)-4a could react with benzaldehyde,16 furnishing
homoallylic alcohol 10 with a high diastereoselectivity.
6
45
22
45
-
7
8
9[c]
10[c]
11[c]
12[c]
13[c,d]
hexane
THF
-
-
toluene
toluene
89
93
2b
-
[a] Reaction conditions: 3a (0.2 mmol), HBpin (0.2 mmol), 2 μmol of
precatalyst, and 4 μmol of NaBHEt3 in toluene (0.5 mL) for 4 h. [b]
Determined by 1H NMR analysis of the crude product with CH3NO2 as the
internal standard. The value in parentheses is the isolated yield of product Z-
4a. [c] 3a (0.2 mmol), HBpin (0.2 mmol), 3 mol% of precatalyst, and 6 mol%
of NaBHEt3 in solvent (1 mL) for 24 h. [d] NaBHEt3 was not added.
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10.1002/anie.201915716
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Table 2. Z-selective hydroboration of various allenes.[a]
R2
R2
R1
2b (3.0 mol%)
NaBHEt3 (6.0 mol%)
R1
•
+
HBpin
Bpin
toluene, rt, 24 h
1 equiv
3
Z-4
Ph
O
O
C8H17
Bpin
C6H13
Bpin
Cy
Bpin
Z-4c,83%
Ph
Bpin
Bpin
Bpin
Z-4a,87%, 83%[b]
Z-4b,82%
Z-4d,76%
Z-4e,81%
Z-4f,81%
O
CO2Et
F
Cl
Bpin
TBSO
BnO
O
O
PhMe2Si
Bpin
Bpin
Bpin
Bpin
Bpin
Z-4k,66%
Z-4l,86%
Z/E = 97/3
Z-4g,78%
Z-4h,71%
Z-4i,65%[c]
Z-4j,66%[c]
Bpin
allenes with alkenyl
Ph
TIPSO
Bpin
BocN
O
TsN
Bpin
O
Bpin
Bpin
O
Bpin
N
Ts
Z-4n,68%[d]
Z-4m,52%
Z-4q,72%
Z-4r,52%
Z-4s,60%
Z-4o,79%[e]
Bpin
1,1-disubstituted allenes
O
Bpin
Me
H
N
Ph
S
H
Ph3Si
Bpin
Me2PhSi
Bpin
O
Bpin
Bpin
O
Ph
H
Ph
Z-4p,68%[f]
Z-4x, 70%[k]
Z-4w, 69%[j]
4t, 70%[g]
MeO
Z-4u, 70%[h]
Z-4v, 79%[i]
Z/E = 93/7
Z/E = 90/10
Z/E = 78/22
Z/E = 94/6
bisallenes
2b (6.0 mol%)
NaBHEt3 (12.0 mol%)
Bpin
Bpin
•
•
Bpin
Bpin
Bpin
Bpin
Ph2Si
X
+
HBpin
X
toluene, rt, 24 h
2.2 equiv
4
3
4y,70%
4z,72%
[a] The reaction was carried out with 1 equiv of 3, 1 equiv of HBpin, 3 mol% of 2b, and 6 mol% of NaBHEt3 in toluene (0.2 M) at room temperature for 24 h on
1.0 or 0.5 mmol scale. Yields of isolated products were given. [b] The reaction was carried out on 5 mmol scale to afford 1.16 g of Z-4a. [c] 5 mol% of 2b and 10
mol% of NaBHEt3 were used. [d] The reaction was run for 43 h. [e] The reaction was run for 36 h. [f] The reaction was run for 35 h. [g] The reaction was run for
54 h. [h] The reaction was run for 22 h. [i]The reaction was run for 38.5 h. [j] The reaction was carried out at 50 oC for 48 h. [k] 5 mol% of 2b and 10 mol% of
NaBHEt3 were used. The reaction was carried out at 50 oC for 48 h.
Based on previous reports of cobalt-catalyzed hydroborylation
reactions,4e,18 a rationale for the cobalt-catalyzed hydroboration
of allenes is proposed as shown in Scheme 3. The precatalyst
2b would react with NaBHEt3, affording the catalytically active
Co(I) hydride species Int 1, followed by the coordination with the
terminal C=C double bond of substrate 3a from the less
hindered side to form Int 2-A (Scheme 3b shows that Int 2-A is
sterically more favorable than Int 2-B). Following the insertion of
double bond into the Co-H bond, a Z-allylic cobalt intermediate
Int 3-A is formed. Subsequent reaction with HBpin, via either σ-
bond metathesis or oxidative addition and reductive elimination,
would furnish the hydroboration product Z-4a and regenerate Int
1 (Scheme 3a). Alternatively, the Co(I) hydride species Int 1
may react with HBpin to cobalt(I)-Bpin intermediate Int 1’.17 Int
1’ then undergoes coordination and insertion processes to yield
the vinyl cobalt intermediate Int 3’ through Int 2’. Subsequently
the reaction of Int-3’ with HBpin affords the hydroboration
product Z-4a and regenerates Int-1’ (Scheme 3c).
Scheme 2. Synthetic applications.
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10.1002/anie.201915716
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(m/z 776) were not observed. The structures of Int 1 and Int 3-A
were further confirmed by SAESI-MS/MS experiment (Figure 1c
and 1d). Such SAESI-MS data provide evidence in support of
the catalytic cycle involving the intermediacies of Co(I)-H Int 1
and Z-allyl-cobalt Int 3-A, as shown in Scheme 3a.
(a)
(
tBuPCNNiPr)CoCl2 2b
2 NaBHEt3
H
H
H
[CoI]
H
L
(a)
•
Int 1
C8H17
Bpin
C8H17
Z-4a
3a
HBpin
H
[CoI]
•
L
H
H
H
C8H17
Int 3-A
[CoI]
C8H17
Int 2-A
(b)
Small steric
repulsion
H
(b)
L
Co
H
H
H
C8H17
Int 3-A
[CoI]
[CoI]
H
H
C8H17
Int 2-A
H
H
L
Co
H
C8H17
H
H
H
C8H17
Int 3-B
Large steric
repulsion
Int 2-B
(c)
(
tBuPCNNiPr)CoCl2 2b
2 NaBHEt3
(c)
[CoI]
H
Int 1
L
HBpin
H
H
H
L
[CoI] Bpin
•
C8H17
Bpin
Int 1'
C8H17
3a
Z-4a
HBpin
L
[CoI] Bpin
H
[CoI]
H
•
(d)
C8H17
Bpin
C8H17
Int 2'
Int 3'
Scheme 3. Proposed mechanisms.
In order to distinguish between these two pathways, we
sought to identify the catalytic intermediates of the catalytic
reaction by solvent-assisted electrospray ionization−mass
spectrometry (SAESI-MS) and SAESI-MS/MS analysis. Two
species with molecular weight matching those of Co(I)-H Int-1
(m/z 497, [M-H]+) and Z-allylic cobalt Int 3-A (m/z 650, [M]•+)
were detected (Figure 1 and see Supporting Information for
details),18 while the species corresponding to Co(I)-Bpin
intermediate Int-1’ (m/z 624) and vinyl cobalt intermediate Int 3’
Figure 1. SAESI-MS studies. (a) SAESI-MS spectrum of the solution. (b)
Expanded SAESI-MS spectrum, showing the major signal of Int
1
[C28H43CoN2P]+ at m/z 497 and Int 3-A [C39H64CoN2P]•+ at m/z 650. (c) SAESI-
MS/MS spectrum of complex ion Int 1 [C28H43CoN2P]•+ at m/z 497. (d) SAESI-
MS/MS spectrum of complex ion Int 3-A [C39H64CoN2P]•+ at m/z 650.
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1, 1306-1309. i) A. J. MacNair, C. R. P. Millet, G. S. Nichol, A.
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In summary, we have developed
a
highly efficient
hydroboration of allenes catalyzed by PCNN-cobalt complex 2b
and identified two of the catalytic intermediates using SAESI-MS
and SAESI-MS/MS analyses. This reaction features excellent
regio- and Z-stereoselectivity, 100% atom economy, and a
broad substrate scope with functionalized mono- and bisallenes.
In addition, for the reaction of enallenes, the exclusive
hydroboration of the allene moiety was observed, leaving alkene
moiety untouched. (Z)-Allylboronate products could be
transformed into different allylic alcohols, or underwent Suzuki-
Miyaura coupling reaction to form C-C bonds.
[5]
For selected examples of hydroboration of alkynes, see: a) H. Jang, A.
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Ohmiya, M. Sawamura, Org. Lett. 2018, 20, 1861-1865. (h) L. Carreras,
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Acknowledgements
Financial support from National Natural Science Foundation of
China (21690063 for S. Ma; 21825109 for Z. Huang) is greatly
appreciated. We thank Mr. Penglin Wu in this group for
reproducing the results for (Z)-4e, (Z)-4i, and (Z)-4q as
presented in this study.
[6]
[7]
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Keywords: Allene • Hydroboration • Cobalt-catalyzed • (Z)-
allylboronate
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V = 2900.8(6) Å3, T = 193(2) K, Z = 4, reflections collected/unique
24238 / 5094 (Rint = 0.0888),
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Can Li, Zheng Yang, Lei Wang, Yinlong
Guo,* Zheng Huang,* and Shengming
Ma*
Page No. – Page No.
An efficient (PNN)Co-catalyzed allene hydroboration affording Z-allylic boronates is
described. The reaction enjoys an excellent regio- as well as Z-selectivity and a
wide substrate scope.
Cobalt-Catalyzed Regio- and
Stereoselective Hydroboration of
Allenes
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