Catalytic asymmetric boration of acyclic α,β-unsaturated esters and nitriles

Article abstract of DOI:10.1002/anie.200703699

(Chemical Equation Presented) Asymmetrie β-boration of acyclic α,β-unsaturated carbonyl compounds provides ready access to enantioenriched functionalized organoboron compounds. A range of acyclic unsaturated esters and nitriles reacted with high enantioselectivity and good yields at room temperature using a copper catalyst and planar chiral diphosphine ligands (L).

Full text of DOI:10.1002/anie.200703699

Angewandte
Chemie
DOI: 10.1002/anie.200703699
Asymmetric Catalysis
Catalytic Asymmetric Boration of Acyclic a,b-Unsaturated
Esters and Nitriles**
Ji-Eon Lee and Jaesook Yun*
Organoboranes are versatile synthetic intermediates for the
preparation of a wide range of organic molecules. An
increasing effort has been devoted to the efficient synthesis
of organoboron compounds. One of the important tools for
the synthesis of organoboranes is transition-metal-catalyzed
addition of diboron reagents such as bis(pinacolato)diboron
to carbon–carbon multiple bonds, which has been the subject
of extensive research.[1] In comparison with electron-rich
alkene or alkyne substrates, the reaction with a,b-unsaturated
carbonyl compounds has not been studied as extensively.
Since introducing a boronate group at the b-position to a
carbonyl using conventional hydroboration methods is not
possible, the metal-catalyzed b-boration of a,b-unsaturated
carbonyl compounds provides an interesting approach. Such
reactions have been reported using systems based on plati-
num,[2] rhodium,[3] and copper[4] but with limitations such as
high catalyst loading, high temperature, low to moderate
yield, and narrow substrate scope.
be deleterious to enantioselectivity, minimizing the back-
ground reaction would be the key for the successful develop-
ment of an asymmetric variant of the b-boration.
In preliminary experiments, we chose cinnamonitrile as a
model substrate and (R)-(S)-josiphos (L1, Scheme 1) as a
nonracemic ligand on the basis of its successful use in the
Recently, we reported an efficient copper-catalyzed
addition of bis(pinacolato)diboron (B2pin2) to a range of
a,b-unsaturated carbonyl compounds, the substrate scope of
which was extended from enones to more challenging a,b-
unsaturated esters, nitriles, and phosphonates.[5] In view of the
variety of stereospecific transformations available to stereo-
genic carbon–boron bonds,[6] we envisioned that catalytic
enantioselective boration of a,b-unsaturated carbonyl com-
pounds would easily provide functionalized enantioenriched
organoboron compounds. However, an asymmetric boration
of such compounds has not been reported yet. Herein, we
describe the enantioselective b-boration of a,b-unsaturated
esters and nitriles catalyzed by a nonracemic copper phos-
phine complex.
In our previous study on the copper-catalyzed b-boration,
we found that both a suitable ligand and methanol additive
were required for complete conversion. Especially the alcohol
was critical to the enhanced rate of reaction; even a reaction
with no ligand proceeded with great conversion in the
presence of methanol. Because this methanol effect could
Scheme 1. Structures of ligands.
asymmetric reduction of acrylonitriles,[7] and we examined a
range of reaction conditions. Variable enantiomeric excesses
(50–85% ee) were obtained without reproducibility when a
1:1 combination of CuCl and NaOtBu was employed. We
surmised that insufficient reaction of the two inorganic salts in
THF, an inadvertent shortage of the nonracemic ligand
relative to copper, or a combination of these two factors
might increase the concentration of catalytic species that
cause nonselective reactions. Increasing the amount of base to
1.5 equivalents relative to copper and measuring the exact
amount or a slight excess of ligand were effective and
reproducibly afforded the b-borylated product with a higher
enantioselectivity. Using a given set of conditions (3 mol%
CuCl, 4.5 mol% NaOtBu, 3 mol% ligand, 1.1 equiv B2pin2,
2 equiv MeOH, THF, room temperature), a series of other
ligands[8] were screened, including bidentate phosphines and
P,N ligands (Scheme 1); representative results are shown in
Table 1. Josiphos (L1) and mandyphos (L2) were equally
effective in giving the best results. Moreover, L2 was less
sensitive to the ratio of copper to base and consistently gave a
reproducible enantiomeric excess with both 1:1 and 1:1.5
copper/base. The C2-symmetric ligands L3 and L4 displayed
[*] J.-E. Lee, Prof. J. Yun
Department of Chemistry and Institute of Basic Science
Sungkyunkwan University
Suwon 440-746 (Korea)
Fax: (+82)31-290-7075
E-mail: jaesook@skku.edu
[**] This work was supported by the Faculty Research Fund 2005,
Sungkyunkwan University. We thank Solvias for supplying the
ligands used in this study.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
Table 1: Enantioselective b-boration of cinnamonitrile.[a]
Table 2: Asymmetric b-boration of acyclic a,b-unsaturated esters and
nitriles.[a]
Entry
Ligand
Yield [%][b]
ee [%][c]
Entry Substrate
Condition[b] Yield of 2
ee of 3
[%][c]
[%][d]
1
2
L1
L2
L2
L3
L4
L5
97
93
96
92
92
93
94
94
94
80
À3
55
3[d]
4
1
2
3
A
A
A
94
92
97
90 (R)
91 (S)
89
5
6
[a] Complete conversion was achieved within 7 h. [b] Yield of isolated
borylated product. [c] Enantiomeric excess of the corresponding b-
hydroxy nitrile compound obtained by oxidation with NaBO3 in THF/H2O
(1:1). [d] 3 mol% NaOtBu.
4
5
A
B
93
94
90 (S)
87 (S)
inferior enantioselectivities. The P,N ligand L5 also gave
satisfactory conversion within the reaction time but yielded
the product with poor enantioselectivity.
6
7
A
A
90
87
91 (S)
On the basis of these results, we chose condition A
(2 mol% CuCl, 3 mol% NaOtBu, 4 mol% L1)[9] and con-
dition B (3 mol% CuCl, 3 mol% NaOtBu, 3 mol% L2) as the
optimal reaction conditions along with two equivalents of
MeOH in THF at room temperature and applied them to the
enantioselective conjugate addition of diboronate to various
a,b-unsaturated esters and nitriles (Table 2). All reactions
proceeded smoothly in reasonable reaction times (7–24 h) to
provide the addition products in excellent yields and with high
levels of enantioselectivity. All of the products could be
isolated by silica gel chromatography and further transformed
by oxidation to the corresponding hydroxy compounds[10] for
the determination of the enantiomeric excess. Assignments of
stereochemistry were made by comparison of the optical
rotations of the hydroxy compounds 3 with those in the
literature.[11]
88
8
9
A
B
95
89
87
84
10
11
12
A
A
A
93
94
90
82
90 (S)
92
13
B
94
91
Both b-alkyl substituted (entries 1–3, Table 2) and b-aryl
substituted unsaturated esters (entries 4–6, Table 2) provided
the products with almost the same levels of enantioselectivity
(89–91% ee). However, meta substitution and ortho substitu-
tion at the aryl ring slightly lowered the enantioselectivity of
the reaction (entries 7–9, Table 2). The heteroaromatic thio-
phene-substituted substrate 1h afforded 2h with a lower
enantioselectivity (82% ee) as well. The nitrile substrates
were generally more efficient than the ester substrates with
regard to both reactivity[12] and enantioselectivity and
afforded the addition products with high enantioselectivities
regardless of the substitution pattern at the aromatic ring
(entries 11–13, Table 2).
It is of note that the nature of the electron withdrawing
group (CO2Et vs. CN) affected the enantioselectivity. For
example, when the mandyphos ligand was used, substrate 1d,
an ester analogue of cinnamonitrile, afforded 2d in 87% ee
(entry 5, Table 2), while cinnamonitrile gave 94% ee (entry 3,
Table 1). The unsaturated ester 1g provided 2g in 84% ee
(entry 9, Table 2), and the corresponding nitrile 1k led to
91% ee (entry 13, Table 2). We also observed that the methyl
ester (89% ee) and the more bulky tert-butyl ester (89% ee)
derivatives of cinnamic acid provided asymmetric inductions
[a] EWG=electron-withdrawing group (CN or C(O)OR’). [b] Condi-
tion A: 2% CuCl, 3% NaOtBu, 4% L1; condition B: 3% CuCl, 3%
NaOtBu, 3% L2 with 1.1 equiv B2pin2 in THF at room temperature.
[c] Yield of borylated product (2) isolated by chromatography. [d] Deter-
mined by either GC or HPLC.
that were very similar to that found for 1d under the
condition A (90% ee, Table 2; Scheme 2). The results suggest
that the structures of different ester moieties do not
significantly influence the enantioselectivity of this reaction.
In summary, we have described the first asymmetric b-
boration of acyclic a,b-unsaturated carbonyl compounds that
provides ready access to functionalized chiral organoboron
compounds under mild reaction conditions. Excellent yields
and high enantiomeric excesses were obtained using planar
chiral ligands L1 and L2 at room temperature.
Experimental Section
General procedure (condition A): THF (0.45 mL) was added under
nitrogen to CuCl (0.010 mmol, 1.0 mg), NaOtBu (0.015 mmol,
1.4 mg), and (R)-(S)-josiphos ligand (0.020 mmol, 12.8 mg) in an
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[2] a) Y. G. Lawson, M. J. G. Lesley, T. B. Marder, N. C. Norman,
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[6] a) Organoboranes for syntheses, ACS symposium series 783
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[8] Other diphosphine ligands, such as various josiphos analogues
and binap, were also surveyed in the boron addition reaction
with 1e but gave poorer enantioselectivities (less than 50% ee).
In this case, the best result was again obtained with josiphos.
Other ligands, such as 2-(diphenylphosphino-2’-methoxy-1,1’-
binaphthyl) (mop) and binapthol-derived phosphoramidite,
gave incomplete conversion and very low asymmetric inductions
(less than 7% ee).
[9] The condition A gave slightly better enantioselectivities (0–
2% ee) than the reaction conditions using the 1:1.5:1 combina-
tion of copper/base/L1.
[10] C. N. Farthing, S. P. Marsden, Tetrahedron Lett. 2000, 41, 4235 –
4238.
[11] The major enantiomer of the other unknown products 3 was
assumed to have the same configuration as the known cases (3a,
3b, 3d, 3e, and 3i).
[12] With the ester substrates, longer reaction times (16–24 h) were
required than with the nitriles substrates (less than 12 h) for
complete conversion.
Scheme 2. Effect of ester structure on the enantioselectivity of the
conjugate addition.
oven-dried Schlenk tube. The reaction mixture was stirred for 30 min
at room temperature, at which time bis(pinacolato)diboron
(0.55 mmol, 139.7 mg) and THF (0.30 mL) were added. The reaction
mixture was stirred for 10 min. Then, the a,b-unsaturated carbonyl
compound (0.5 mmol) and subseqently MeOH (1.0 mmol, 0.04 mL)
were added. The reaction tube was washed with THF (0.2 mL),
sealed, and stirred until no starting material was detected by TLC.
The reaction mixture was filtered through a pad of celite and
concentrated. The product was purified by silica gel chromatography.
Received: August 13, 2007
Published online: October 8, 2007
Keywords: asymmetric catalysis · boron · conjugate addition ·
.
copper · unsaturated carbonyl compounds
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Angew. Chem. Int. Ed. 2008, 47, 145 –147
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