Rhodium-catalyzed dehydrogenative borylation of aliphatic terminal alkenes with pinacolborane

Article abstract of DOI:10.1002/anie.201506328

Aliphatic terminal alkenes react with pinacolborane at ambient temperature to afford dehydrogenative borylation compounds as the major product when iPr-Foxap is used as the ligand with cationic rhodium(I) in the presence of norbornene, which acts as the s

Full text of DOI:10.1002/anie.201506328

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506328
German Edition: DOI: 10.1002/ange.201506328
Alkenes
Rhodium-Catalyzed Dehydrogenative Borylation of Aliphatic
Terminal Alkenes with Pinacolborane
Masao Morimoto, Tomoya Miura,* and Masahiro Murakami*
Abstract: Aliphatic terminal alkenes react with pinacolborane
at ambient temperature to afford dehydrogenative borylation
compounds as the major product when iPr-Foxap is used as the
ligand with cationic rhodium(I) in the presence of norbornene,
which acts as the sacrificial hydrogen acceptor. The reaction is
applied to the one-pot syntheses of aldehydes and homoallylic
alcohols from aliphatic terminal alkenes.
arylethenes and alkoxyethenes. There is no facile method to
obtain the dehydrogenative borylation compounds from
aliphatic terminal alkenes and HBpin.[10,11] Herein, we
report a rhodium(I)-catalyzed reaction of aliphatic terminal
alkenes with HBpin, which preferentially produce dehydro-
genative borylation compounds (Figure 1b). The use of iPr-
Foxap as the ligand of cationic rhodium(I) and norbornene as
the sacrificial hydrogen acceptor is the key for the dehydro-
genative borylation. Hydroboration of terminal alkynes with
HBpin is a straightforward and reliable method for the
stereoselective preparation of E-alkenyl pinacolboronates.[12]
Even Z isomers have become accessible by hydroboration of
terminal alkynes.[13] Sometimes, over-reduction of the alkyne
to give saturated diboronates compounds,[14] along with issues
of regioselectivity, complicates this route. The attractiveness
of the dehydrogenative borylation is the use of readily
available terminal alkenes as starting materials instead of
terminal alkynes.
T
he hydroboration of alkenes with borane reagents to give
the corresponding alkylboranes is a fundamental textbook
reaction. The use of transition-metal catalysts makes it
possible to use dialkoxyborane reagents [HB(OR)2] for the
hydroboration under mild reaction conditions.[1] A variety of
transition-metal complexes such as rhodium(I),[2] iridium(I),[3]
ruthenium(II),[4] iron(0),[5] and cobalt(I)[6] catalyze the hydro-
boration of terminal alkenes with pinacolborane (HBpin),
thus forming alkyl pinacolboronates in a regioselective way
[Figure 1a]. Interestingly, the dehydrogenative borylation
Initially, 4-phenylbut-1-ene (1a, 1.0 equiv) was subjected
to the reaction with HBpin (2, 1.7 equiv) in the presence of
[2c]
[Rh(cod)2]BF4
and norbornene (nbe; 2.3 equiv) as the
sacrificial hydrogen acceptor (Table 1, entry 1). After the
reaction mixture was stirred at 288C for 9 hours, a mixture of
the dehydrogenative borylation products 3a and 4a, and the
hydroboration product 5a was formed in a ratio of 3a/4a/5a =
15:5:80, albeit in 20% total yield. The hydrogenation of 1a
also occurred as a side reaction. Next, various ligands were
examined using [Rh(cod)2]BF4 as the catalyst precursor.
Whereas the use of simple phosphine ligands such as PPh3 and
dppe preferentially yielded the hydroboration product 5a
(entries 2 and 3), a P,N bidentate ligand (L1) gave a better
product selectivity for 3a (3a/4a/5a = 40:2:58; entry 4). A
commercially available P, N bidentate ligand, iPr-Foxap,[15]
exhibited a dramatic effect to favor the formation of 3a.[16]
After chromatographic purification, the product 3a was
obtained along with 4a and 5a (3a/4a/5a = 91:3:6) in 86%
total yield (entry 5).[17] The E/Z ratio of 3a was 85:15. The
counterions of rhodium(I) complexes also affected the
product selectivity. The tetraphenylborate complex [Rh-
(cod)2]BPh4 showed a comparable product selectivity (3a/
4a/5a = 93:3:4), but the hexafluorophosphate complex [Rh-
(cod)2]PF6 resulted in a lower product selectivity (3a/4a/5a =
28:3:69; entries 6 and 7). While the neutral complex [RhCl-
(cod)]2 is known as the effective precursor for the dehydro-
genative borylation of styrene,[8a,c] it gave a result inferior to
that of [Rh(cod)2]BF4 in terms of both yield and product
selectivity (entry 8). Furthermore, the choice of hydrogen
acceptor was important. When norbornadiene or styrene was
used as the hydrogen acceptor, the yield of 3a markedly
decreased (entries 9 and 10).[18]
Figure 1. Two pathways of the borylation reaction of aliphatic terminal
alkenes with pinacolborane (HBpin).
competes with the hydroboration in some cases to afford
alkenyl pinacolboronates as the major product.[7–9] For
example, Masuda et al. reported that the reaction of styrene
with HBpin in the presence of neutral [RhCl(cod)]2 gave the
dehydrogenative borylation product along with a small
amount of the hydroboration products.[8a,c] Concurrently,
ethylbenzene was generated along with styryl pinacolboro-
nate in similar amounts, thus showing that a half of styrene
was used as the hydrogen acceptor. Therefore, an excess
amount of styrene was required. Moreover, the substrates for
the successful dehydrogenative borylation are limited to
[*] Dr. M. Morimoto, Dr. T. Miura, Prof. Dr. M. Murakami
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University
Katsura, Kyoto 615-8510 (Japan)
E-mail: tmiura@sbchem.kyoto-u.ac.jp
murakami@sbchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506328.
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Table 1: Optimization of reaction conditions for the dehydrogenative
borylation of 4-phenylbut-1-ene (1a) with HBpin (2).[a]
of the B H bond of 2 onto rhodium(I) affords the boryl-
(hydride)rhodium species A. Subsequent insertion of the
À
alkene 1 into the Rh B bond of A takes place to give the
alkyl-rhodium intermediate B. The initial conformer under-
À
goes rotation along the C C bond axis to form the other
conformer C. Then, syn b-hydride elimination furnishes the
E isomer of the alkenyl boronate 3. The (dihydride)rhodium
species D reacts with norbornene to generate an active
rhodium(I) species together with norbornane.[17,20] The
strained structure of norbornene enhances the reactivity
toward D.[18] Therefore, hydroboration of norbornene is
preferred over the alkenyl pinacolboronate 3a.
The following experiments were carried out to obtain
mechanistic insights into the stereoselectivity. First, the
rhodium(I)-catalyzed reaction of 1a with 2 was monitored
by GC after 20 minutes [Eq. (1)]. The E/Z ratio of 3a was
74:26 at 18% conversion of 1a. Thus, the E/Z ratio of 3a
changed during the reaction (vs. 9 h; Table 1, entry 5).
Entry [Rh]
Ligand
Conv.
[%]
Yield
[%][b]
3a(E/Z)/4a/
5a[c]
1
[Rh(cod)2]BF4 none
>95
20
15(>95/
5):5:80
27(89/11):3:70
0:0:100
40(87/13):2:58
91(85/15):3:6
[d]
2
3
4
5
[Rh(cod)2]BF4 PPh3
88
50
63
37
49
50
92 (86)
[Rh(cod)2]BF4 dppe
[Rh(cod)2]BF4 L1
[Rh(cod)2]BF4 iPr-
Foxap
>95
6
7
[Rh(cod)2]BPh4 iPr-
Foxap
[Rh(cod)2]PF6 iPr-
Foxap
95
79
93
22
72
90 (78)
58
93(83/17):3:4
28(81/19):3:69
77(83/17):3:20
21(71/29):5:74
73(82/18):4:23
8[e] [RhCl(cod)]2
iPr-
Foxap
82 (76)
19
9[f] [Rh(cod)2]BF4 iPr-
Foxap
10[g] [Rh(cod)2]BF4 iPr-
Foxap
26
[a] On a 0.50 mmol scale. [b] Total yield of 3, 4, and 5 as detemined by
GC. The value within parentheses reflects the total yield of the product
after chromatographic purification. [c] Product ratio determined by GC.
[d] Using PPh3 (6 mol%). [e] Using [RhCl(cod)]2 (1 mol%). [f] Using
norbornadiene (2.3 equiv) instead of nbe. [g] Using styrene (2.3 equiv)
instead of nbe.
Secondly, the purified Z isomer of 3a was subjected to the
standard reaction conditions using oct-1-ene (1c) as a sub-
strate [Eq. (2)]. The E/Z isomerization of 3a took place to
give an E/Z = 57:43 mixture. Based on these results, the
stereochemistry seems to be subject to thermodynamics
rather than kinetics.
A variety of terminal alkenes 1 were subjected to the
dehydrogenative borylation with 2 by using a combination of
[Rh(cod)2]BF4/iPr-Foxap and norbornene (Table 2). Mono-
substituted alkenes 1b–f readily reacted with 2 to afford the
corresponding alkenyl pinacolboronates 3b–f with good
yields, product selectivities, and E/Z ratios (entries 1–5),
whereas the reactions of 3-tert-butylprop-1-ene (1g) and
cyclohexylethene (1h) were rather sluggish, probably owing
to the steric hindrance (entries 6 and 7). Functional groups
such as siloxy, chloro, methoxycarbonyl, and epoxy groups
were tolerated in the alkyl chain under the reaction conditions
(entries 8–13). The reaction of 1,1- and 1,2-disubstituted
alkenes such as 1,1-diethylethene and cyclohexene failed to
give the desired alkenyl pinacolboronates.[21] Therefore, in the
case of 2-methylhexa-1,5-diene (1o), including 1,1-disubsti-
tuted alkene moiety, only the terminal monosubstituted
alkene moiety underwent the dehydrogenative borylation to
afford the monoborylated product 3o (entries 14). Similarly,
4,8-dimethylnona-1,7-diene (1p) selectively produces the
monoborylated product 3p (entry 15). 1,1-Dimethylbuta-
Scheme 1. Plausible catalytic cycle for the dehydrogenative borylation.
Although it is difficult to explain the reaction pathway
leading to the alkenyl boronate 3 from the aliphatic terminal
alkene 1 and HBpin (2), a possible mechanism is depicted in
Scheme 1. It is similar to the one proposed by Hartwig et al.
for the iridium(I)-catalyzed dehydrogenative silylation using
norbornene as the hydrogen acceptor.[19] Oxidative addition
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Table 2: Rhodium(I)-catalyzed dehydrogenative borylation of various
terminal alkenes 1 with HBpin (2).[a]
purification procedure. After volatile materials in the reac-
tion mixture of 1a with 2 were removed under reduced
pressure, an aqueous THF solution of sodium perborate was
directly added to the residue including the alkenyl pinacol-
boronate 3a. Oxidation of 3a occurred to form the corre-
sponding aldehyde 6 in 60% yield upon isolation (based on
1a; Scheme 2). Formally, this one-pot reaction achieved anti-
Entry
1
3
Yield [%][b] 3a(E/Z)/4a/5a[c]
1
2
3
1b n=2 3b
1c n=5 3c
1d n=9 3d
79[d]
75
82
90(91/9):4:6
91(91/9):3:6
91(90/10):3:6
4
1e
78[e,f]
71
89(88/12):0:11
Scheme 2. One-pot synthesis of aldehydes from terminal alkenes.
5
1 f
92(91/9):2:6
6
7
1g
1h
56
93(89/11):1:6
Markovnikov oxidation of terminal alkenes at ambient
temperature,[22] thus complementing the Wacker–Tsuji oxi-
dation by palladium catalyst. Furthermore, it avoids the need
for two oxidation steps to convert a hydroboration product
(alkyl boronate) of an alkene into an aldehyde.
52[g]
88(>95/5):1:11
We recently reported an enantioselective synthesis of anti-
homoallylic alcohols from terminal alkynes, HBpin, and
aldehydes via the formation of alkenyl pinacolboronates,
which act as g-substituted allylboron species.[23] Thus, the
reaction mixture containing the alkenyl pinacolboronate 3b
was treated with benzaldehyde (7) in the presence of [Ir-
(cod)2]BF4/PCy3 and (R)-TRIP in 1,2-dichloroethane (DCE).
The anti-homoallylic alcohol 8 was obtained with high
diastereo- and enantioselectivities (Scheme 3). The above-
mentioned reactions provide efficient methods to directly
functionalize aliphatic terminal alkenes in one pot.
8
9
1i n=2 3i
1j n=4 3j
83
80
89(90/10):3:8
89(91/9):3:8
10
1k
78[g]
92(83/17):2:6
11
12
13
1l
74[h]
83
90(86/14):3:7
91(90/10):3:6
88(89/11):3:9
1m
1n
77
14
15
1o
1p
71[e]
79[g]
92(91/9):3:5
92(88/12):3:5
16
17
18
1q
1r
80[d]
82[g]
78[i]
91(95/5):0:9
93(85/15):0:7
96(>95/5):0:4
1s
[a] On a 0.50 mmol scale. [b] Total yield of 3, 4, and 5 after chromato-
graphic purification. [c] Product ratio determined by 1H NMR analysis.
[d] Yield determined by NMR spectroscopy. [e] Using nbe (2.3 equiv).
[f] Containing 2-cinnamyl-Bpin (4%). [g] Using nbe (1.7 equiv). [h] Using
nbe (2.5 equiv). [i] Containing 1-(4-methoxyphenyl)ethyl-Bpin (2%).
Scheme 3. One-pot synthesis of homoallylic alcohols from terminal
alkenes. (R)-TRIP=(R)-3,3’-Bis(2,4,6-triisopropylphenyl)-1,1’-
binaphthyl-2,2’-diyl hydrogenphosphate.
In summary, we have disclosed that the combined use of
a cationic rhodium(I) complex, iPr-Foxap, and norbornene
enables the facile preparation of alkenyl pinacolboronates
from aliphatic terminal alkenes and HBpin at ambient
temperature. Since terminal alkenes are more easily acces-
sible and often more desirable starting materials than
terminal alkynes, the reaction represents an interesting
alternative to alkyne hydroboration. Based upon the dehy-
drogenative borylation reaction, the one-pot syntheses of
aldehydes and homoallylic alcohols starting from terminal
alkenes have also been realized. Further studies to elucidate
1,3-diene (1q) was also a suitable substrate to give the
corresponding dienylboronate 3q with high product selectiv-
ity and E/Z ratio (entry 16). Allyltriphenylsilane (1r) and
4-methoxystyrene (1s) successfully participated in this reac-
tion (entries 17 and 18). The E isomer was exclusively formed
with 3s.
The resulting alkenyl pinacolboronates were useful inter-
mediates in organic synthesis.[1b] Thus, we examined one-pot,
two-step transformations via the formation of alkenyl boro-
nates, thus saving time and solvents required for a workup/
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the mechanism of this reaction and to expand its utility are in
progress.
Acknowledgements
This work was supported in part by a Grant-in-Aid for
Scientific Research (S) from MEXT and the ACT-C program
of the JST. We thank Prof. Keiji Morokuma, Dr. Travis V.
Harris, and Dr. W. M. C. Sameera for enriching discussion. M.
Morimoto thanks the JSPS for Young Scientists for a Research
Fellowship.
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Keywords: alkenes · boron · reaction mechanisms · rhodium ·
synthetic methods
How to cite: Angew. Chem. Int. Ed. 2015, 54, 12659–12663
Angew. Chem. 2015, 127, 12850–12854
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Received: July 9, 2015
Revised: August 5, 2015
Published online: September 2, 2015
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