Asymmetric diboration of terminal alkenes with a rhodium catalyst and subsequent oxidation: Enantioselective synthesis of optically active 1,2-diols

Article abstract of DOI:10.1002/anie.201305181

Pin it down: A highly enantioselective diboration of terminal alkenes with chiral 1 and bis(pinacolato)diboron (B₂pin₂) was realized. Subsequent oxidation of the diboron adducts with sodium peroxoborate readily gave the corresponding optically active 1,2-diols in high yields and high enantioselectivities. Copyright

Full text of DOI:10.1002/anie.201305181

Angewandte
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DOI: 10.1002/anie.201305181
Asymmetric Catalysis
Asymmetric Diboration of Terminal Alkenes with Rhodium Catalyst
and Subsequent Oxidation: Enantioselective Synthesis of Optically
Active 1,2-Diols**
Kenji Toribatake and Hisao Nishiyama*
Optically active 1,2-diols are an important class of organic
compounds used for synthetic intermediates of bioactive or
pharmaceutical compounds.[1] They have commonly been
synthesized by direct asymmetric dihydroxylation of alkenes
with osmium reagents,[2] asymmetric hydrogenation of a-
hydroxyketones,[3] asymmetric hydrolytic kinetic resolution of
epoxides,[4] and enzymatic reactions,[5] to name a few. How-
ever, efficient, reliable, and practical synthetic methods of
optically active 1,2-diols are still in demand to solve challeng-
ing subjects such as expanding substrate scope, attaining
a high level of enantioselectivity, lowering catalyst loading, or
making the reaction environmentally benign. In that sense,
direct dihydroxylation of alkenes with osmium-free catalysts
has recently attracted attention.[6] Some iron catalysts such as
non-heme iron enzyme mimics have also been explored.[7]
Alternatively, asymmetric 1,2-diboration of alkenes
(ADA) and subsequent oxidation can provide optically
active 1,2-diols (Scheme 1).[8,9] In 2003, Morken et al.
disubstituted alkenes and terminal alkenes only afforded
moderate ee values. In 2009, Morken et al. also developed
platinum-catalyzed asymmetric diboration of terminal
alkenes to attain ee values of up to 94% for the corresponding
1,2-diol products.[11]
In this context, it should also be mentioned that mono-
boration of conjugated electron-deficient alkenes with dibor-
ons can be applied to the preparation of b-hydroxy carbonyl
compounds, and it has been established as a reliable synthetic
method.[12] Very recently, the asymmetric catalytic conjugate
b-boration was carried out by metal-free organocatalysts such
as chiral or nonchiral N-heterocyclic carbenes (NHCs) or
phosphines.[13] Furthermore, catalytic diboration of non-
activated alkenes using Lewis base chiral alkoxides was
realized by Gulyꢀs, Fernꢀndez, and co-workers.[14] Thus,
catalytic di- and monoboration of alkenes are at the cutting
and leading edge of organic synthesis.
As we studied the asymmetric conjugate monoboration of
a,b-unsaturated carbonyl compounds with chiral rhodium-
[bis(oxazolinyl)phenyl] complexes ([Rh(Phebox)]; 1), and
bis(pinacolato)diboron (B2pin2),[15] we became strongly
intrigued by ADA (Figure 1). Herein, we report a highly
Scheme 1. Catalytic diboration of alkenes and subsequent oxidation to
form 1,2-diols.
reported the first enantioselective 1,2-diboration of alkenes
catalyzed by a rhodium catalyst with bis(catecholatodiboron)
(B2cat2) in the presence of chiral phosphine ligands.[10] In their
work, trans 1,2-disubstituted alkenes and terminal alkenes
bearing tertiary alkyl groups were subjected to the catalytic
diboration and subsequent oxidation to give optically active
1,2-diols with high enantioselectivities, whereas some cis-
Figure 1. Chiral rhodium[bis(oxazolinyl)phenyl] complexes.
efficient ADA with a [Rh(Phebox)] catalyst and subsequent
oxidation, thus producing optically active 1,2-diols.
The mixture of p-chlorostyrene (2; 0.50 mmol) as
a selected substrate and B2pin2 (1.2 equiv) in THF (1 mL)
was treated at 608C with 1 mol% of [Rh(Phebox-ip)] (1a),
and subsequent oxidation with NaBO3·4(H2O) to give the 1,2-
diol 3 in 20% yield with 66% ee (entry 1, Table 1). To our
delight, the addition of NaOtBu (5 mol%) promoted the
diboration reaction smoothly to form the 1,2-diboration
product in 1 hour, and subsequent oxidation (in the same
pot) with sodium peroxoborate in THF and water at room
temperature gave 3 in 94% yield with an ee value of more
than 99% (entry 2). At 308C, the addition of NaOtBu was
effective but resulted in a slight decrease in the ee value
(entry 3). The reason of the decrease was not identified, and
[*] K. Toribatake, Prof. Dr. H. Nishiyama
Department of Applied Chemistry
Graduated School of Engineering
Nagoya University, Chikusa, Nagoya, 464-8603 (Japan)
E-mail: hnishi@apchem.nagoya-u.ac.jp
Homepage: http://www.apchem.nagoya-u.ac.jp/06-II-1/nisilab/
index.html
[**] This research was partly supported by the Japan Society for the
Promotion of Science (No. 2562007).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305181.
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Table 1: Asymmetric diboration and subsequent oxidation of p-chloro-
styrene with the [Rh(Phebox)] catalysts 1a–e.[a]
Table 2: Asymmetric diboration and subsequent oxidation of aryl-
substituted terminal alkenes using [Rh(Phebox-ip)] (1a).
Entry
Cat. 1
Additive
Solvent
T
[8C]
Yield
[%]
ee
[%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1a
–
THF
THF
THF
THF
THF
THF
THF
THF
60
60
30
60
60
60
60
60
60
60
80
80
80
60
60
60
60
60
20
94
93
92
93
23
29
20
91
50
79
85
46
95
91
23
88
89
66
>99
88[b]
NaOtBu
NaOtBu
NaOEt
KOtBu
NaOAc
Na2CO3
K2CO3
Cs2CO3
NaOtBu
NaOtBu
KOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
93
82
76
77
55
76
61
96
84
61
95
94
12
99
98
9
THF
10
11
12
13
14
15
16
17
18[c]
C6H5CH3
C6H5CH3
C6H5CH3
Dioxane
THF
THF
THF
THF
THF
[a] Reaction conditions: p-Chlorostyrene (2; 0.50 mmol), [Rh(Phebox)]
cat. (1.0 mol%), B2pin2 (1.2 equiv, 0.60 mmol), additive (5 mol%), THF
(1.0 mL), reaction time (1 h). The diboration product was subsequently
oxidized with NaBO3·4H2O. [b] Average of three reactions; 87–93%
yield, 88–90% ee. [c] p-Chlorostyrene (2; 1.38 g, 10.0 mmol), cat. 1a
(0.2 mol%), B2pin2 (1.2 equiv, 12.0 mmol), NaOtBu (5 mol%), THF
(20 mL), 608C, 1 h, product 3 (1.53 g, 8.9 mmol).
the phenomenon was confirmed by running the reaction three
times. NaOEt and KOtBu worked efficiently to afford good
yields and ee values (entries 4 and 5). Sodium acetate and
carbonate and potassium carbonate decreased the yield to 20–
29% with moderate ee values (entries 6–8). Cesium carbon-
ate enhanced the diboration, but resulted in only 76% ee
(entry 9). Toluene in place of THF required a slightly higher
reaction temperature (808C) to give 96% ee (entries 10–12).
1,4-Dioxane was not the solvent of choice (entry 13). Under
the optimum reaction conditions (entry 2), several substitu-
ents on the oxazoline skeleton of the catalyst were examined,
and the benzyl and phenyl substituents afforded the best
results, thus affording 3 in over 90% yield with 94 and
95% ee, respectively (entries 14 and 15). However, a tert-
butyl group resulted in a diminished reaction yield (entry 16).
The catalyst 1e,[15d] having dimethyl substituents on the
phenyl skeleton, was also effective for producing a high
yield with high enatioselectivity (entry 17). More noteworthy
is that the reaction could also be practically manipulated on
gram scale with respect to 2 by using a 0.2 mol% catalyst
loading (substrate/catalyst = 500; entry 18).
Reaction conditions: Aryl-substituted alkene (5; 0.50 mmol), [Rh(Phe-
box-ip)] (1a; 1.0 mol%), B2pin2 (1.2 equiv, 0.60 mmol), NaOtBu
(5 mol%), THF (1.0 mL), reaction time (1 h). The diboration product
was subsequently oxidized with NaBO3·4H2O. The yields are those of the
isolated products and the ee values were determined by HPLC using
a chiral stationary phase. Small amounts of pinacol contaminated 6h, 6i,
and 6j.
We next examined other aryl-substituted terminal alkenes
as substrates and used the reaction conditions used in entry 2
of Table 1. The results are shown in Table 2. Styrene resulted
in 99% ee for the diol 6a. Substituted styrenes and naphtha-
lene derivatives were borated in 60–96% yields with high
enantioselectivities of up to 99%. The optically active diol 6j
derived from 3,5-bis(trifluoromethyl)styrene served as a pre-
cursor for production of the substance P antagonist as an
antidepressant.[1b] A vinyl furan was nicely transformed into
the corresponding diol 6m in 95% ee.
Next, several functionalized terminal alkenes were sub-
jected to the diboration under the standard reaction con-
ditions (Table 3). 3-Phenylpropene and 4-phenyl-1-butene
2
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Table 3: Asymmetric diboration and subsequent oxidation of substituted
terminal alkenes using [[Rh(Phebox-ip)] (1a).
Figure 2. Proposed catalytic cycle.
Our proposed catalytic cycle is shown in Figure 2. We
attempted to follow the reaction of the [Rh(Phebox)(OAc)2-
(H2O)] complex and B2pin2 by 1H NMR spectroscopy, but we
could not confirm the corresponding boryl-rhodium spe-
cies.[15] However, formation of a boryl acetate was confirmed,
using 11B NMR spectroscopy, by the reaction of [Rh(Phebox-
ip) (OAc)2(H2O)] (1a) and B2pin2 at 608C for 30 minutes (see
the Supporting Information). Therefore, on the basis of the
fact that NaOtBu accelerated the diboration, the s-bond
metathesis or the transmetallation is thought to be included in
the catalytic cycle. The boryl–RhIII species A coordinates to
an alkene and subsequent insertion of the boryl group to form
an alkyl–Rh species (B). Next, s-bond metathesis or the
transmetallation produces the diboration product and regen-
erates the active tBuO–RhIII (C) or A. Recently, Bo,
Fernꢀndez, and co-workers reported that Rh/NHC complexes
mediate diboration of cyclic alkenes by a mechanism that
includes an oxidative addition pathway to form a diboryl–Rh
species without using a basic additive.[16] Therefore, the
catalytic cycle may contain the generation of a RhI species
and oxidative addition of diboron to form a RhIIIB2 species.
Hypothetical transition-state model structures are also
proposed (Figure 3). On the basis of the absolute configu-
ration of the 1,2-diol product (R), the Si face of styrene binds
to the rhodium atom to give the corresponding alkyl–Rh
species, having the boryl group at the apical position, with
subsequent syn diboration giving the product. The final
oxidation treatment generates the 1,2-diol with retention.
The structure D is likely to give the major enantiomer rather
than the Re-face attack on the structure E because of steric
hindrance. In the case of the boryl group in the equatorial
position, the Si-face attack is more sterically encumebred for
the structure F rather than for the structure G. Notably, G
cannot give the desired chirality of the product diol.
Reaction conditions: Substituted alkene (7; 0.50 mmol), [Rh(Phebox-ip)]
(1a; 1.0 mol%), B2pin2 (1.2 equiv, 0.60 mmol), NaOtBu (5 mol%), THF
(1.0 mL), reaction time (1 h). The diboration product was subsequently
oxidized with NaBO3·4H2O. The yields are those of the isolated products
and the ee values were determined by HPLC using a chiral stationary
phase. For 8k, the catalyst 1b was used. Small amounts of pinacol
contaminated 8i and 8k. The ee value of 8l was determined from the
corresponding dibenzoate.
produced the diols 8a (95% ee) and 8b (94% ee), respec-
tively. The ee values and yields of the diols 8c and 8d derived
from allylethers were slightly reduced. Interestingly, allylani-
lines were smoothly diborated to give the diols 8e and 8 f in 94
and 92% ee, respectively. 1-Phenylbutadiene was converted
into the 1,2-diol 8g in 90% ee, and it is noteworthy that 2-
methyl and 2-pentyl butadiene resulted in increased ee values
of up to 99% for 8h and 8i. 1,1-Diphenylbutadiene unfortu-
nately resulted in both a low yield and ee value. In terms of
asymmetric diboration of dienes, Morken et al. used a chiral
platinum catalyst to efficiently demonstrate selective 1,2-diol
formation and its application to successive allylation.[11e,f] 1-
Methylstyrene gave the diol 8k with moderate ee value. A
simple aliphatic alkene 1-octene was examined and resulted
in a moderate yield and 93% ee of 8l.
In summary, we have found that chiral [Rh(Phebox)]
acetate complexes can act as efficient catalysts for asymmetric
catalytic diboration of terminal alkenes to afford high
enantioselectivities. The diboration can provide a practical
synthetic method for optically active 1,2-diols from a variety
of substituted alkenes in high yields with high optical purities.
As a limitation of our method, 1,2-disubstituted alkenes
such as b-methylstyrene, 1,2-dihydronaphthalene, or trans-
stilbene could not be diborated under the standard reaction
conditions.
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Keywords: asymmetric catalysis · boron · diols ·
.
enantioselectivity · rhodium
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Figure 3. Proposed transition state. Boryl group at apical position and
syn diboration to Si face of the a-carbon atom (D and E). The
equatorial boryl structures F and G.
Application of this reaction for transformation of a variety of
alkene substrates is currently underway in our laboratory.
Experimental Section
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bis(pinacolato)diboron (152 mg, 0.60 mmol), and NaOtBu (2.5 mg,
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(1.0 mL) were added. The mixture was stirred at 608C for 1 h. At
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n-hexane (1:7 to 3:1) as eluant. The corresponding diol 3, (R)-1-(4-
chlorophenyl)ethane-1,2-diol, was obtained in 94% yield (80.9 mg,
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~
0.469 mmol) as a white solid; m.p. 84–868C. IR (KBr): n ¼3388
1
(broad) cmꢀ1; H NMR (300 MHz, CDCl3): d = 2.11 (br s, 1H), 2.64
(br s, 1H), 3.57 (m, 1H), 3.69 (m, 1H), 4.75 (dd, J = 8.4, 3.3 Hz, 1H),
7.20–7.30 ppm (m, 4H); 13C NMR (75 MHz, CDCl3): d = 67.9, 74.0,
127.2, 128.5, 133.5, 138.6 ppm; Elemental Anal: calcd. (%) for
C8H9ClO2: C 55.67; H 5.26. Found: C 55.94; H 5.34. Chromatography:
Daicel Chiralcel OD-H, n-hexane/2-propanol (98:2, 1.5 mLminꢀ1),
retention time: 46.0 min (major), 52.8 (minor), 99.5% ee (R);
28
25
½aꢁD ¼ꢀ57.4 (c = 1.0, CHCl3); Lit.[17,3a], ½aꢁD ¼ꢀ55.9 (c = 1.0, CHCl3),
96% ee (R).
Received: June 17, 2013
Revised: July 23, 2013
Published online: && &&, &&&&
4
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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.
Angewandte
Communications
Communications
Asymmetric Catalysis
K. Toribatake,
H. Nishiyama*
&&&& — &&&&
Asymmetric Diboration of Terminal
Alkenes with Rhodium Catalyst and
Subsequent Oxidation: Enantioselective
Synthesis of Optically Active 1,2-Diols
Pin it down: A highly enantioselective
diboration of terminal alkenes with chiral
1 and bis(pinacolato)diboron (B2pin2)
was realized. Subsequent oxidation of the
diboron adducts with sodium peroxo-
borate readily gave the corresponding
optically active 1,2-diols in high yields and
high enantioselectivities.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!