Enantioselective addition of boronates to acyl imines catalyzed by chiral biphenols

Full text of DOI:10.1002/anie.200901023

Angewandte
Chemie
DOI: 10.1002/anie.200901023
Asymmetric Catalysis
Enantioselective Addition of Boronates to Acyl Imines Catalyzed by
Chiral Biphenols**
Joshua A. Bishop, Sha Lou, and Scott E. Schaus*
Chiral biphenols are privileged catalyst structures[1] utilized in
a wide range of reactions which continues to expand.[2] The
accessibility of the chiral framework and structural variants is
a key aspect of the utility this class exhibits in asymmetric
catalysis.[3] More recently, chiral biphenol catalysts have
proven to be effective catalysts for asymmetric conjugate
addition reactions, the asymmetric allylboration of ketones[4]
and acyl imines,[5] as well as the asymmetric three component
Petasis condensation reaction of secondary amines, glyox-
ylates, and alkenyl boronates.[6] An important mechanistic
facet in each of the reactions is the exchange of one of the
boronate alkoxy groups with the biphenol to create a more
reactive boronate species.[4,7] We sought to expand the
repertoire of boronate nucleophiles that will react with acyl
imines using chiral biphenol catalysts. Our approach towards
the rapid identification of the optimal catalyst for each
nucleophilic addition reaction was to screen a collection of
chiral biphenols.[8] Screening chiral catalyst collections has
proven to be an effective method for catalyst identification[9]
and reaction discovery.[10] In this approach the efficient
identification of the optimal catalyst is maximized and
unexpected results or patterns in reactivity can be rapidly
elucidated. Herein we describe the identification and appli-
cation of chiral biphenol catalysts for the addition of aryl,
vinyl, and alkynyl boronates to acyl imines by a catalyst
screening approach.
Our investigations began with the identification of
reaction conditions that promote the addition of aryl, vinyl,
and alkynyl boronates to acyl imines [Eq. (1)]. For each
boronate, the nucleophilic addition proceeded only by the
inclusion of a biphenol catalyst; a yield of less than 5% was
obtained in the absence of any biphenol catalyst. In develop-
ing a general protocol for a catalyst screening process, the di-
n-butyl boronate was determined to be optimal because of its
hydrolytic stability. Good yields could be obtained for each of
the nucleophiles using binol (binol = 2,2ꢀ-dihydroxy-1,1ꢀ-
binaphthyl) as the catalyst. The next step was to perform a
screen by using a collection of chiral biphenol catalysts in
each of the boronate addition reactions. Twelve chiral
biphenols were screened as catalysts (Figure 1a) in the
presence of benzoyl imine 7 and the aryl, alkenyl, and alkynyl
boronates 6, 9, and 11 respectively (Figure 1b). Comparing
the enantiomeric ratio of each nucleophile as a function of the
catalyst employed illustrated notable trends (Figure 1c). The
use of catalyst 4b with aryl nucleophile 6 yielded the desired
diaryl amide with excellent selectivity, whereas use of catalyst
4b in the presence of alkenyl boronate 9 or alkynyl
nucleophile 11 afforded the corresponding product in lower
selectivities. In each case, a different binol catalyst structure
proved to be the most effective for each boronate nucleophile.
However, a catalyst was identified that afforded the addition
product in greater than 95:5 enantiomeric ratio for each of the
boronate nucleophiles investigated.
The scope of the reaction was investigated for each of the
boronate nucleophiles. In general, the optimal catalyst
identified in the screening experiments, 4b, proved effective
for all of the aryl boronates evaluated in the reaction. Aryl
boronates (Table 1, entries 1–6), aryl (Table 1, entries 7–8)
and aliphatic (Table 1, entry 9) imines, as well as acyl imine
substituents (Table 1, entries 10–12) were found to be effec-
tive in the biphenol-catalyzed addition reaction, each afford-
ing the corresponding amide in good yields (> 70% yield of
the isolated product) and enantioselectivities (> 95:5 e.r.).
Vinyl boronates (Table 2, entries 1–5) also afforded the
corresponding allylic amide products in high yield and
selectivity. Catalyst 4d also promoted the vinyl addition to a
series of substituted aryl, heteroaryl, and alkyl imines
(Table 2, entries 6–8) as well as substituted acyl imines
(Table 2, entries 9–11) in good yields and selectivities
(> 70% yield, > 95:5 e.r.).
[*] J. A. Bishop, Dr. S. Lou,[+] Prof. Dr. S. E. Schaus
Department of Chemistry
Center for Chemical Methodology and Library Development at
Boston University (CMLD-BU)
Life Science and Engineering Building, Boston University
24 Cummington Street, Boston, MA 02215 (USA)
Fax: (+1)617-353-6466
Similarly, substituted alkynyl boronates (Table 3,
entries 1–6) proved successful as the desired propargyl
amides were isolated in high yield and selectivity. Aryl,
heteroaryl, and aliphatic (Table 3, entries 7–9), as well as acyl-
substituted imines (Table 3, entries 10 and 11) produced the
desired products in good yields and selectivities by using
catalyst 5d.
E-mail: seschaus@bu.edu
[+] Current address: Department of Chemistry
Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139 (USA)
[**] This research was supported by the NIH (R01 GM078240 and P50
GM067041) and Amgen, Inc.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901023.
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Table 1: Asymmetric arylboration of acyl imines.[a]
Entry Ar
R2
R3
Yield [%][b] e.r.[c]
1
2
3
4
5
6
7
8
9
4-CH3OPh
4-CH3Ph
3,4-OCH2OPh Ph
4-BrPh
2-BrPh
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
80
82
91
95
72
98
80
89
70
98:2
98:2
96:4
99:1
97:3
98:2
98.5:1.5
96:4
95.5:4.5
98:2
Ph
Ph
Ph
4-BrPh
2-C4H3S Ph
Cy
Ph
4-ClPh
4-CH3OPh
4-CH3OPh
4-CH3OPh
4-CH3OPh
Ph
=
10
(E)-PhCH CH 87
11
12
4-CH3OPh
4-CH3OPh
Ph
Ph
83
98:2
Cy
71
97.5:2.5
[a] Reaction conditions: catalyst 4b (0.0375 mmol), arylboronate
(0.25 mmol), and acyl imine (0.25 mmol) in toluene (2.5 mL) for 18 h
at 08C!RT. [b] Yield of isolated product. [c] Determined by chiral HPLC
methods. Cy=cyclohexyl.
Table 2: Asymmetric vinylboration of acyl imines.[a]
Entry R1
R2
R3
Yield [%][b] e.r.[c]
1
2
3
4
5
6
7
8
9
Ph
2-C4H3S
3-FPh
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
85
88
82
83
75
80
91
74
97.5:2.5
99:1
96.5:3.5
96:4
98:2
95.5:4.5
95:5
4-CH3OPh Ph
Cy
Ph
Ph
Ph
Ph
Ph
4-Br-Ph
2-C4H3S Ph
c-C6H11
Ph
95.5:4.5
98.5:1.5
=
Ph
(E)-CH CHPh 87
10
11
Ph
Ph
Ph
Ph
83
97.5:2.5
97.5:2.5
Cy
82
[a] Reaction conditions: catalyst 4d (0.0375 mmol), alkenyl boronate
(0.25 mmol), and acyl imine (0.25 mmol) in toluene (2.5 mL) for 18 h,
08C!RT. [b] Yield of isolated product. [c] Determined by chiral HPLC
methods.
Figure 1. a) Chiral biphenols. b) Nucleophilic boronate reactions pro-
moted by chiral biphenols. c) Catalyst screen for enantioselectivity.
We continued our studies by characterizing the boronate
species under the catalytic reaction conditions. Electron-spray
ionization mass spectrometry (ESI-MS) experiments of
reaction mixtures at room temperature containing binol-
derived diols and boronates in the presence and absence of
benzoyl imine 7, were conducted. ESI-MS is an effective
process for the characterization of intermediates that are
otherwise difficult to characterize by purification.[11,5] The
mixture of binol 4b and boronate 9 was analyzed by using a
MicroMass ZQ 2000 mass spectrometer in positive electro-
spray ionization mode. Under these conditions the mass of a
boronate resulting from exchange of one of the alkoxy groups
with binol 4b was observed, without any detectable formation
of the corresponding cyclic boronate, consistent with previous
results obtained for the allylboration reaction. Furthermore,
the use of the chiral cyclic boronate derived from binol 4b and
the boronate 9 resulted in low yield (< 20%) and low
enantioselectivity (55:45 e.r.) when reacted with imine 7
under the same reaction conditions. Although computational
studies have implicated the formation of cyclic boronates
under catalytic conditions,[12] the experimental results
obtained to date demonstrate an acyclic boronate complex
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Table 3: Asymmetric alkynylboration of acyl imines.[a]
Entry
R1
R2
R3
Yield [%][b]
e.r.[c]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-CH3OPh
4-BrPh
2-C3H4S
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
99
76
72
90
71
80
99
62
80
70
69
96:4
93:7
94:6
92:8
97:3
96:4
96:4
93:7
97:3
97:3
95:5
4-FPh
3-C4H3S
n-hexyl
Ph(CH2)2
i-propenyl
Ph
Ph
Ph
Ph
Ph
9[d]
10
11
Ph
4-BrPh
4-OCH3Ph
Scheme 1. Synthesis of the key intermediate in the formal synthesis of
Xyzal.
Ph
[a] Reaction conditions: catalyst 4d (0.023 mmol), alkynyl boronate
(0.115 mmol), acyl imine (0.115 mmol) in toluene (1 mL) for 36 h,
08C!RT. [b] Yield of isolated product. [c] Determined by chiral HPLC
methods. [d] Reaction run with 3 equivalents of alkynyl boronate.
retention of stereochemistry constituting a formal synthesis
of Xyzal (20).[19]
In summary, we have applied a chiral biphenol catalyst
screening protocol for the rapid identification of enantiose-
lective catalytic reactions. The approach successfully identi-
fied a unique catalyst that promoted the reaction enantiose-
lectively for each of the boronates investigated. Furthermore,
the optimal catalyst identified proved general for each class of
boronate nucleophiles. Mechanistic studies demonstrate that
there is an exchange between the boronate and catalyst,
giving rise to the active nucleophilic boronate reagent. The
method was utilized in the enantioselective synthesis of the
antihistamine Xyzal. Continued investigations include use of
the screening approach toward expansion of the scope and
utility of the reaction, as well as detailed mechanistic studies.
under catalytic conditions. The stereochemical model devel-
oped for the reaction of the boronate complex with acyl
imines is consistent with the observed stereoinduction
(Figure 2). The observed enantiofacial selectivity is the
Received: February 22, 2009
Published online: May 8, 2009
Keywords: enantioselectivity · boronates ·
homogeneous catalysis · synthetic methods · natural products
.
Figure 2. Proposed transition state for asymmetric boronate addition
to acyl imines.
result of catalyst coordination to the Z conformer of the
acyl imine. The more reactive Z conformer has been proposed
by Corey et al.[13] and others[14] in reactions involving imines
because of the steric interactions that arise from the metal
reagent and the substituents of the imine. The hydrogen-
bonding character of the biphenol activates the acyl imine
towards nucleophilic attack and orients the boronate complex
towards Re enantiofacial selectivity in the addition reaction.[5]
Lastly, the methodology was utilized in the synthesis of
the known antihistamine levocetirizine (Xyzal). The con-
struction of amine (R)-19 is a key step in the synthesis.[15–17]
Our approach to this desired intermediate used (R)-4b as the
catalyst and aryl boronate 13 f to afford diaryl amide 14 f in
98% yield upon isolation and 98:2 e.r. (Scheme 1). Removal
of the acyl protecting group was accomplished using a method
recently described by Prati et al.,[18] resulting in the produc-
tion of free amine (R)-19 in 80% yield with complete
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