Asymmetric β-boration of α,β-unsaturated carbonyl compounds promoted by chiral rhodium-bisoxazolinylphenyl catalysts

Article abstract of DOI:10.1039/b915759j

Chiral rhodium-bisoxazolinylphenyl acetate complexes exhibited high catalytic activity for the β-boration of α,β-unsaturated carbonyl compounds with bis(pinacolato)diboron in the presence of sodium t-butoxide with enantioselectivity up to 97%.

Full text of DOI:10.1039/b915759j

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Asymmetric b-boration of a,b-unsaturated carbonyl compounds promoted
by chiral rhodium–bisoxazolinylphenyl catalystswz
Takushi Shiomi, Takahiro Adachi, Kenji Toribatake, Li Zhou and Hisao Nishiyama*
Received (in College Park, MD, USA) 31st July 2009, Accepted 9th September 2009
First published as an Advance Article on the web 16th September 2009
DOI: 10.1039/b915759j
Chiral rhodium–bisoxazolinylphenyl acetate complexes exhibited
high catalytic activity for the b-boration of a,b-unsaturated
carbonyl compounds with bis(pinacolato)diboron in the presence
of sodium t-butoxide with enantioselectivity up to 97%.
in the presence of Rh(Phebox-ip) 1a (1.0 mol%) in a toluene
solution. The catalytic boration was found to proceed at 80 1C
for 12 h to give the desired adduct 3 in 64% yield (94% ee).
The reaction at 60 1C for 12 h formed a trace amount of 3. To
our delight, the addition of a catalytic amount of NaOt-Bu
(5.0 mol%) significantly accelerated the boration, which
finished at 80 1C within 30 min to furnish the product 3 in 84%
yield (Scheme 2). The enantioselectivity of 95% was measured
through obtaining the corresponding b-hydroxylester 4 via the
oxidation of 3 with sodium peroxoborate (NaBO3(H2O)4,
5.0 equiv.) in THF/H2O. Use of Rh(Phebox-sb) 1b resulted
in the highest ee of 97% whereas use of benzyl and phenyl
catalysts 1c and 1d decreased the yields and ees.y
Catalytic addition reactions of diborons to alkenes and
alkynes via metal-boryl intermediates have the potential to
be highly versatile organic syntheses and transformations to
provide a variety of useful functionalized compounds.1 In
pioneering work, Suzuki and Miyaura disclosed that efficient
addition of diborons can be promoted by Pt catalysts.2 In the
case of electron-deficient olefins such as a,b-unsaturated
carbonyl compounds, the conjugate boration can be promoted
with Pt,3 Rh,4 Cu,5 and Ni6 catalysts to produce the corres-
ponding b-boryl carbonyl derivatives. In 2008, Yun et al. first
reported the asymmetric b-boration of a,b-unsaturated nitriles
and esters using Cu/chiral ferrocenylphosphine catalyst and
bis(pinacolato)diboron (B2pin2) to attain 94% ee and 91% ee
with cinnamonitriles and cinnamates, respectively, in the presence
of alcohol additives.7a In excellent work, Yun et al. extended
the asymmetric boration to a,b-unsaturated ketones and
The procedures with Yun’s copper catalysts5c,7 and
Yorimitsu–Oshima’s nickel catalyst6 need proton sources such as
methanol or ethanol to accelerate the boration by decomposi-
tion of the intermediate a-carbon metalated species. In this
context, Rh(Phebox) catalyst does not need protic additives
for the boration, the same as the result with Wilkinson’s
catalyst, Rh(PPh3)3Cl, previously reported by Kabalka
et al.4 Addition of ethanol into the reaction mixture with
Rh(Phebox) catalyst rather provided the undesirable
conjugate reduction product of the cinnamate.
amides as substrates.7b,c Recently, Pe
rez and Fernandez
´ ´
demonstrated chiral Cu/N-heterocarbene catalysts showing
Next, we tried a one-pot procedure of the sequence of the
boration and oxidation in order to obtain the optically active
b-hydroxyesters efficiently. After the catalytic boration with
1 mol% of Rh(Phebox) catalysts 1 and B2pin2 (1.2 equiv.), the
toluene solvent was removed under reduced pressure, then
water, THF, and sodium peroxoborate were added. The
mixture was stirred at room temperature to give the b-hydroxy-
esters in good total yields up to 89% (Table 1). Rh(Phebox-sb)
1b kept a high enantioselectivity of 95% (Entry 2). The
activity for the b-boration of a,b-unsaturated esters and
´
aldehydes, and Fernandez and Guiry reported axially chiral
P,N ligand/Cu catalysts for the reaction.8a,b It is of importance
that the corresponding boryl compounds can readily be converted
into optically active secondary alcohols by oxidation. Thus,
this attractive asymmetric b-boration has only just begun
being investigated with chiral copper catalysts, and further
studies on the scope of catalysts, substrates, and conditions
will expand the synthetic availability.
reaction of isopropyl cinnamate resulted in
a similar
We have so far demonstrated highly enantioselective
conjugate reductions and coupling reactions using chiral
rhodium-bisoxazolinylphenyl catalysts, Rh(Phebox) (Scheme 1).9
We now wish to report the efficient asymmetric b-boration of
a,b-unsaturated compounds with our rhodium catalysts.
The boration of ethyl (E)-cinnamate 2 (0.5 mmol) was
carried out with bis(pinacolato)diboron (B2pin2, 1.1 equiv.)
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Chikusa, Nagoya, 464-8603, Japan.
E-mail: hnishi@apchem.nagoya-u.ac.jp; Fax: +81-52-789-3209
w This article is part of a ChemComm ‘Catalysis in Organic Synthesis’
web-theme issue showcasing high quality research in organic
chemistry. Please see our website (http://www.rsc.org/chemcomm/
organicwebtheme2009) to access the other papers in this issue.
z Electronic supplementary information (ESI) available: Typical
experimental procedures, analytical data, and NMR spectra for
products. See DOI: 10.1039/b915759j. See DOI: 10.1039/b915759j
Scheme 1 Rh(Phebox) complexes and asymmetric b-boration of
a,b-unsaturated compounds.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5987–5989 | 5987

to give 11 and 12 in good yields and moderate ee. The reaction
of b-boryl acrylamide 13 with 1a was also achieved in 70%
yield with a high ee of 97%.
Scheme 2 Rh(Phebox) catalysed asymmetric b-boration of ethyl
cinnamate.
enantioselectivity (Entries 5 and 6), but that of the tert-butyl
ester decreased it (Entries 9 and 10). The benzyl catalyst 1c
decreased the enantioselectivity, and the phenyl catalyst 1d
decreased both the yield and the enantioselectivity (Entries 3,
4, 7, and 8).
Under conditions similar to those of Table 1, other
substrates such as esters, enones, and an amide were subjected
to the boration with the catalysts 1a and 1b and B2pin2
(1.2 equiv.). Interestingly, the reaction of ethyl (Z)-cinnamate
(Z)-2 was carried out with the catalyst 1a (1.0 mol%) to give
the adduct 3 with the same absolute configuration S in 77%
yield and 93% ee. The reaction of the electron-donating ethyl
4-MeO-cinnamate resulted in formation of the adduct 7 in
good yields with up to 94% ee whereas the reaction of the
electron-withdrawing 4-CF3 substituted cinnamate did not
give the adduct even at 100 1C. The electronic effect cannot
be explained at the moment. b-Alkyl substituted acrylates were
smoothly borylated to give the adducts 9 and 10 in good yields
and selectivity, respectively. The reaction of enones proceeded
We propose a possible catalytic cycle, which can be initiated
by s-bond metathesis to form boryl–rhodium(III) species A
(Scheme 3) and not via oxidative addition of the B–B bond to
the rhodium atom.1d,e The metathesis of the B–B bond can be
accelerated by the basic additive, as recently described by
Hoveyda et al.10 Then, an a,b-unsaturated carbonyl substrate
can be introduced to give B followed by insertion to give
rhodium-enolate C. Finally, exchange reaction by diboron
forms the boration product D and regenerates the boryl–rhodium
species A.
Table 1 One-pot procedure for the asymmetric b-boration of cinnamates
catalysed by Rh(Phebox) complexes and subsequent oxidationa
Judging from the absolute configuration of the product 3,
we propose the stereochemical sequences illustrated in Fig. 1.
When the transition states i–iv with two configurations of the
boryl group, apical (i and iii) and equatorial (ii, iv), are
hypothesised, the structures i and ii with Si-face attack by
the boryl group are most likely to form the major enantiomer
S of 3. The attack on the Re-face (iii, iv) may cause steric
repulsion between the isopropyl group and the substrate. The
formation of (S)-3 from (Z)-2 cannot be clarified yet.
Entry
R
Cat. 1
Product
Yield (%)
% ee
1
2
3
4
5
6
7
8
9
10
Et
Et
Et
Et
i-Pr
i-Pr
i-Pr
i-Pr
t-Bu
t-Bu
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
4
4
4
4
5
5
5
5
6
6
82
71
87
28
89
89
60
19
80
75
94
95
58
66
93
93
41
54
86
85
a
Cinnamate (0.50 mmol), Rh(Phebox) 1 (0.005 mmol, 1.0 mol%),
B2pin2 (0.60 mmol), NaOt-Bu (0.025–0.029 mmol), toluene (1.0 mL),
at 80 1C, 0.5 h; NaBO3(H2O)4 (2.5 mmol), H2O (1.0 mL), THF
(1.0 mL), at room temperature, ca. 1 h.
Scheme 3 Plausible catalytic cycle.
ꢀc
This journal is The Royal Society of Chemistry 2009
5988 | Chem. Commun., 2009, 5987–5989

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This is the first example of chiral rhodium-catalysed
asymmetric b-boration of a,b-unsaturated carbonyl compounds
with high enantioselectivity. We assume that the borylation
takes place via s-bond metathesis of the diboron on rhodium(III)
species. We are currently studying the scope and limitations of
a variety of substrates and intermediate rhodium species to
clarify the reaction mechanisms.
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This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
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460:18065011) and the Japan Society for the Promotion of
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Notes and references
y Typical procedure: Rh(Phebox-sb) 1b (2.8 mg, 0.0050 mmol),
bis(pinacolato)diboron (140 mg, 0.55 mmol), and NaOt-Bu (2.4 mg,
0.025 mmol) were placed in a flask with a stirring bar. Under an argon
atmosphere, ethyl cinnamate 2 (88.1 mg, 0.50 mmol) and toluene
(1.0 mL) were added. The mixture was stirred at 80 1C for 0.5 h. At
room temperature, the mixture was directly charged to a silica-gel
column with an eluent of hexane/ethyl acetate to give the borylated
product 3 in 86% yield (131 mg, 0.43 mmol). Spectroscopic data were
in accordance with those reported previously.5c,7a,11 A part of the
product was subjected to oxidation with sodium peroxoborate to give
the alcohol 4; chromatography: DAICEL-CHIRALCEL OD-H,
hexane/2-propanol = 90 : 10 (0.5 mL/min), retention time = 15.2 min
(major, S) and 18.3 min (minor, R), 96.6% ee; [a]D25 = ꢁ46.5 (c 1.04,
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28
one-pot reaction, bis(pinacolato)diboron (152 mg, 0.60 mmol) was
used, and after the boration the solvent was removed under reduced
pressure. To the residue, water (1.0 mL), THF (1.0 mL), and
NaBO3(H2O)4 (384 mg, 2.5 mmol) were added. The mixture was
stirred at room temperature for 1 h, and then was extracted with ethyl
acetate. The crude product was purified by silica-gel column chromato-
graphy to give the corresponding alcohol 4 in 71 % yield (69.2 mg,
0.357 mmol) with 95% ee.
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ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5987–5989 | 5989