Frustrated Lewis acid/Br?nsted base catalysts for direct enantioselective α-amination of carbonyl compounds

Article abstract of DOI:10.1021/jacs.6b11908

A method for enantioselective direct α-amination reaction catalyzed by a sterically "frustrated" Lewis acid/Br?nsted base complex is disclosed. Cooperative functioning of the Lewis acid and Br?nsted base components gives rise to in situ enolate generation from monocarbonyl compounds. Subsequent reaction with hydrogen-bond activated dialkyl azodicarboxylates delivers α-aminocarbonyl compounds in high enantiomeric purity.

Full text of DOI:10.1021/jacs.6b11908

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Communication
Frustrated Lewis Acid/Brønsted Base Catalysts for Direct
Enantioselective #-Amination of Carbonyl Compounds
Ming Shang, Xiaoxu Wang, Seung Moh Koo, Jennifer Youn, Jessica
Z Chan, Wenzhi Yao, Brian Thomas Hastings, and Masayuki Wasa
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Dec 2016
Downloaded from http://pubs.acs.org on December 17, 2016
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Journal of the American Chemical Society
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Frustrated Lewis Acid/Brønsted Base Catalysts for Direct Enantioselective -Amination of
Carbonyl Compounds
10
11
12
13
14
15
Ming Shang, Xiaoxu Wang, Seung Moh Koo, Jennifer Youn, Jessica Z. Chan, Wenzhi Yao, Brian T. Hastings and
Masayuki Wasa*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
RECEIVED DATE (automatically inserted by publisher); wasa@bc.edu
Scheme 1. “Indirect” and “Direct” -Amination Reactions
A: "Indirect" asymmetric amination reactions of preformed enolates
Abstract: A method for enantioselective direct -amination
reaction catalyzed by sterically “frustrated” Lewis
16
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51
52
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54
55
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58
59
60
a
acid/Brønsted base complex is disclosed. Cooperative
functioning of the Lewis acid and Brønsted base
components gives rise to in-situ enolate generation from
monocarbonyl compounds. Subsequent reaction with
hydrogen-bond activated dialkyl azodicarboxylates delivers
-aminocarbonyl compounds in high enantiomeric purity.
R
O
R
O
OR
N N
RO
B
N
O
CO2R
CO2R
O
R2BOTf
1 eq.
O HN
N
O
H
N
+
R
O
X
X
O
O
R
1 eq. Et3N
R
R
R
cat. chiral
1)
O
OR
N N
RO
CO2R
CO2R
TMSO
O
Lewis acid
O HN
N
LDA
1) 1 eq.
O
H
+
Stereoselective synthesis of C–N bonds, which can be found in
a large number of biologically active molecules, represents a
frontier endeavor in chemistry.1-6 Electrophilic amine sources
such as dialkyl azodicarboxylates, nitrosoarenes, and oxaziridines
have been used extensively in the diastereo- or enantio-controlled
generation of amine-bearing stereogenic centers through reaction
with preformed enolate equivalents (Scheme 1A).3-6 Recently,
enantioselective “direct” amination of carbonyl compounds
involving in-situ nucleophile generation has emerged as an atom-
and step-economical approach for preparation of these important
molecules (Scheme 1B, 1C).2,7,8 One strategy entails the use of a
cooperative Lewis acid/Brønsted base catalyst which promotes
both deprotonation of a carbonyl pronucleophile to generate an
enolate equivalent and its enantioselective reaction with the
amination reagent.2,9-11 However, a key unsolved issue in
enantioselective cooperative catalysis is that mutual quenching
can occur to inhibit the desired transformation. To address this
complication, the majority of bifunctional catalysts (e.g., C2-C4)
have been equipped with mildly to moderately acidic and basic
groups that only allow for deprotonation of preactivated substrates
with acidic C–H bonds (e.g., 1,3-dicarbonyl and -arylcarbonyl
compounds).10 With stronger chiral acid and/or base catalysts,
self-quenching can be problematic.7,8
While contemplating the design of an enantioselective
cooperative acid/base catalyst capable of promoting reactions
between N-based electrophiles and unactivated carbonyl
pronucleophiles, we considered a system that would contain an
unquenched and more strongly Lewis acidic fragment along with
a hindered Brønsted base unit (Scheme 1B). The frustrated Lewis
pairs (FLPs) pioneered by Stephan and Erker consist of acidic and
basic fragments that are not able to associate easily because of
steric factors.12 However, FLP-catalyzed enantioselective
processes remain limited in large extent to hydrogenation or
hydrosilylation processes.12-14 By exploiting strongly acidic
B(C6F5)3 and hindered 1,2,2,6,6-pentamethylpiperidine (PMP),
we recently demonstrated that these catalysts can overcome the
self-quenching problem and promote the direct Mannich-type
reaction.15 Herein, we disclose the development of
enantioselective direct -amination of unactivated carbonyl
compounds catalyzed by a readily accessible class of chiral
frustrated B(C6F5)3/amine complexes.
X
X
2) HF THF
2)1 eq. TMSCl
R
R
R
B: "Direct" amination of in-situ genenated enolate (This work)
O
CO2R
O
BY3
cat.
O HN
N
N
N
OR
H
+
X
RO
X
CO2R
cat. NR3
R
1
2
3
R
O
X = R, OR, NR2, SR
catalysts
BY3
NR3
+
R3N
H
Y3B
BY3
H
NR3
O
Y3B
O
2
O
O
NR3
H
N
OR
X
X
X
N
R
+
RO2C
R
R
Catalyst design:
R
N
Ar
R
B(C6F5)3
NR3
+
F3B NH3
"Classic"
B
BF3 + NH3
Ar
Ar
R
Frustrated
acid/base pair
acid/base adduct
Cy
C: Previously used catalysts in direct -amination reactions
Cy
Et
CF3
Cy
O
O
S
O
O
P
N
OH
O
Cy
N
N
N
CF3
OH
H
HO
H
H
N
Me Me
Cy
N
C1
C2
C3
C4
Cy
We envisioned a set of transformations that would begin by a
boron-based Lewis acid binding to carbonyl pronucleophiles to
enhance the acidity of an -C–H bond. Ensuing deprotonation by
a hindered (“soft”) amine would then result in the formation of a
tightly bound ionic pair consisting of a boron enolate and an
ammonium cation;16 the latter component may then serve as a
Brønsted acid to activate electrophilic amination reagent 2 while
precisely positioning it for reaction with the enolate component to
afford aminocarbonyl products 3 (Scheme 1B). A critical
advantage of the proposed strategy is that tethering of acidic and
basic catalyst components is not necessary, allowing for facile and
independent modification of each component for optimization of
reaction efficiency and/or enantioselectivity.
We began by examining the ability of achiral Lewis
acid/Brønsted base catalysts to promote the desired
transformation. -Tetralone (1a) and dialkyl azodicarboxylates
(2) were reacted with B(C6F5)3/amines serving as potential
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catalysts (Table 1). No product was formed in the absence of an
Table 2. Catalytic Amination of Different Pronucleophiles a,b
1
2
3
4
5
6
7
8
amine or B(C6F5)3 (entries 1, 2). With 5.0 mol% of B(C6F5)3 and
10 mol% of Et3N the reaction between 1a and dimethyl
azodicarboxylate (2a, DMAD) in toluene at 22 °C afforded 3a in
32% yield (Table 1, entry 3). With the less basic N,N-
dimethylaniline there was hardly any product obtained (entry 5,
<5% yield) and with DBU and Barton’s base, yields were low as
well (entries 6, 7). In contrast, the transformation proceeded
efficiently when PMP was employed (entry 9, >95%). These
observations are consistent with the hypothesis that highly acidic
B(C6F5)3 and sterically hindered PMP can serve as an effective
catalyst combination for the direct -amination reaction. Alkyl-
substituents of azodicarboxylate were found to have a strong
influence on efficiency, as use of the more hindered diethyl,
benzyl and tert-butyl-substituted electrophiles led to diminished
yields (entries 10-12). The reaction was higher yielding in a non-
polar solvent (e.g., toluene), which is consistent with the
hypothesis that ionic and H-bonding interactions are likely critical
(Scheme 1B).8,16
O
CO2Me
O
B(C6F5)3
cat.
O HN
N
N
N
OMe
H
+
X
MeO
X
CO2Me
cat. amine
R
O
1
2a
3b-3p
R
O
toluene, 22 °C, 12h
CO2Me
CO2Me
CO2Me
O HN
CO2Me
HN
N
HN
O
N
N
CO2Me
CO2Me
Me
3d, 89% yieldc
3b, 97% yieldc
3c
, 98% yield
c
9
CO2Me
CO2Me
CO2Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
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45
46
47
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59
60
O
HN
N
O HN
N
O HN
N
CO2Me
CO2Me
CO2Me
Me Me
3e, 80% yieldc
3f, 94% yieldd
3g, 70% yieldd
CO2Me
O HN
CO2Me
O HN
CO2Me
O HN
N
N
O
CO2Me
N
O
CO2Me
MeO
CO2Me
Ph
Table 1. Evaluation of Reaction Parameters a,b
3h, 78% yielde
3i, 88% yieldf
3j, 69% yieldf
CO2Me
CO2Me
CO2Me
CO2Me
HN
O
HN
N
O HN
N
O
N
CO2Me
N
N
CO2Me
Me
Ph
N
3l, 60% yieldg
3m
, 60% yield
g
3k, 65% yieldg
CO2Me
CO2Me
CO2Me
HN
N
HN
N
S
O
O HN
N
CO2Me
CO2Me
Me
N
S
EtS
CO2Me
3o, 97% yieldf
3p, 98% yieldc
3n, 35% yieldf
a
Conditions: pronucleophile (0.2 mmol), dimethyl azodicarboxylate (0.3 mmol),
B(C6F5)3, amine, toluene (1.0 mL), under N2, 22°C, 12h. Yield of isolated and
purified products. B(C6F5)3 (5 mol%) and PMP (10 mol%) were used. B(C6F5)3
(10 mol%) and N-methylmorpholine (20 mol%) were used. B(C6F5)3 (10 mol%)
b
c
d
e
f
and PMP (10 mol%) were used. B(C6F5)3 (10 mol%) and PMP (20 mol%) were
used. g B(C6F5)3 (10 mol%) and Barton’s base (20 mol%) were used.
The tertiary amine moiety of C-mono was expected to play the
role of a Brønsted base in deprotonation of B(C6F5)3-activated 1a;
after deprotonation, the amine group would be transformed into a
Brønsted acid which could associate with a basic moiety of 2a by
a single H-bonding interaction (A). Catalyst-activated enolate and
DMAD fixed within the catalyst framework would undergo
enantiodetermining C–N bond formation to give 3a. However,
transformations between -tetralone 1a and DMAD 2a with chiral
amine catalysts such as C5, C6 and (−)-sparteine (C7) afforded
rac-3a (Table 3). Accordingly, we posited that a single H-bonding
interaction between chiral ammonium ion and 2a might not be
sufficient for promoting a highly enantioselective C–N bond
forming process (Scheme 2, A).8,16 For a more directional
catalyst–electrophile binding, we decided to evaluate the dual H-
bond donors derived from the amine groups of C-di. We surmised
a
Conditions: -tetralone (0.2 mmol), dialkyl azodicarboxylate (0.3 mmol), acid (5
b
mol%), base (10 mol%), toluene (1.0 mL), under N2, 22°C, 12h. Yields were
determined by H NMR analysis of unpurified reaction mixtures with mesitylene as
the internal standard. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, Barton’s base =
1
c
2-tert-butyl-1,1,3,3-tetramethylguanidine, TMP
=
2,2,6,6-tetramethylpiperidine,
PMP = 1,2,2,6,6-pentamethylpiperidine.
Cyclic as well as acyclic ketones participate effectively in
direct -amination reactions with 2a catalyzed by 5 mol% of
B(C6F5)3 and 10 mol% of PMP (Table 2, 3b-3e). Cyclopentanone
and cycloheptanone which lack a fused aromatic group gave 3d in
89% yield and 3e in 80% yield. Using 10 mol% of B(C6F5)3 and
20 mol% of less hindered N-methylmorphiline, ’-disubstituted
aminoketones 3f and 3g were obtained in 94% and 70% yield,
respectively. Lactone containing 3i and 3j were isolated in 88%
and 69% yield in the presence of 10 mol% of B(C6F5)3 and 20
mol% of PMP. The more basic Barton’s base was required for
deprotonation of amides. Acyclic and cyclic amides were readily
transformed to the corresponding products in 60–65% yield (3k-
m). Thioesters could be deprotonated by PMP, delivering 3n and
3o in 35% and 97% yield, respectively. -Amination of 2-
pyrrolidinethione proceeded to afford 3p in quantitative yield.
We then focused on the development of an enantioselective
version of the catalytic process with 1a serving as the model
substrate (Scheme 2 and Table 3). Chiral amine catalysts were
easily prepared from commercial chiral amines, and their
derivatization, storage and handling are less complicated
compared to chiral boron-based Lewis acids.12,13 Therefore, we
chose to center our initial studies on the development of catalysts
that consist of B(C6F5)3 and chiral amine catalysts (Scheme 2).
Scheme 2. Designing of Chiral Amine Catalysts
O
CO2Me
O
5 mol% B(C6F5)3
O HN
N
N
N
OMe
H
+
X
X
CO2Me
MeO
10 mol% chiral amine
conditions
R
R
1
2a
3
O
N
R
R
EWG
R
R
R
chiral amine
=
chiral amine
N
=
H
N
R
C-mono
C-di
R
R
Ar3B
R
Ar3B
O
O
N
R
N
H
N EWG
H
X
H
N
R
R
X
O
R
R
O
MeO2C N
O
MeO
N
N
+
+
OMe
OMe
A: single H-bonding
- inferior directionality
B: dual H-bonding
- improved directionality
- Brønsted and Lewis acid activation of 2a
- Brønsted acid activation of 2a
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that the second N–H unit of C-di, attached to an electron-
withdrawing substituent, would function as an additional H-bond
donor (B). Dual H-bonding interactions could as a result offer
substituted -tetralone derivatives 3q-3v were converted to the
1
2
3
4
5
6
7
8
corresponding products in 90:10 to >99:1 er. Nitro-substituted 3w
was generated in 86% yield and 79:21 er, probably due to
competing H-bonding by the nitro group. Chroman-4-one was
converted to 3x in 87% yield and 94:6 er; 3y (85% yield, 68:32
er) and 3z (47% yield, 85:15 er) were prepared through reactions
with -indanone and 1-benzosuberone. Using more hindered
diethyl azodicarboxylate (DEAD), 3a-DEAD was obtained in
78% yield and 93:7 er.17 The catalytic protocol is scalable, as
highlighted by the gram-scale synthesis of 3a (72% yield, 98:2 er)
in the presence of 2.5 mol% B(C6F5)3 and 5 mol% C9 (Scheme
3). However, our studies indicate that the tertiary amine moiety of
C9 is not sufficiently basic for efficient deprotonation of esters,
amides and thioesters. To address this latter reactivity issue and to
expand the scope of the enantioselective method, we are
evaluating the effectiveness of a number of catalysts that contain a
more basic guanidine derivative.
additional
electrophile
activation,
accelerating
the
enantioselective C–N bond formation by increasing
conformational restriction (vs single H-bonding in A).8
Table 3. Evaluation of Chiral Amine Catalysts a,b
O
CO2Me
O
B(C6F5)3
5 mol%
O HN
N
N
N
OMe
H
+
CO2Me
MeO
10 mol% chiral amine
9
1a
O
3a
2a
toluene, 46 °C, 24h
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H
N
t-Bu
N
H
OBn
Me
N
Bn
N
Me Me
C7
C5
C6
3a, 93% yield
50:50 er
3a
3a, 95% yield
50:50 er
, 71% yield
52:48 er
Ph
O
Ph O
Table 4. Direct Enantioselective -Amination Reactions a,b
22°C: 3a, 69% yield, 82:18 er
0°C: 3a, 78% yield, 84:16 er
20°C: 3a, 80% yield, 96:4 er
78°C: 3a, 53% yield, 91:9 er
Ph
Ph
N
Ot-Bu
N
CF3
H
H
N
N
Me Me
Me Me
C9
C8
3a, 88% yield
97:3 er
3a, 15% yield
86:14 er
Ph
O
Ph
O
Ph
O O
S
Ph
Ph
Ph
N
CF3
N
CCl3
N
CF3
C11
H
N
H
H
N
N
C10
C12
Me Me
Me Me
3a, 31% yield
72:28 er
3a
3a, 81% yield
97:3 er
, 61% yield
94:6 er
OMe
Ph
O
Cl
O
Ph
Me
N
CF3
MeO
O
N
H
N
N
CF3
C14
Me
CF3
H
Cl
N
H
N
C13
Me
Me Me
C15
Me Me
3a, 38% yield
80:20 er
3a, 48% yield
92:8 er
3a, 62% yield
98:2 er
Ph
O
O
O
Ph
N
Me
Me
CF3
N
CF3
C17
N
CF3
C16
H
H
N
N
N
Me
Me Me
Me Me
C18
3a, 62% yield
5:95 er
3a, 64% yield
97:3 er
3a, 86% yield
80:20 er
a Conditions: -tetralone (0.2 mmol), dimethyl azodicarboxylate (0.3 mmol), acid (5
b
mol%), base (10 mol%), toluene (1.0 mL), under N2, –46°C, 24 h. Yields were
1
determined by H NMR analysis of unpurified reaction mixtures with mesitylene as
the internal standard. Er was determined byHPLC analysis of the purified product.
(1R,2R)-(+)-1,2-Diphenylethylenediamine was converted
into diamine catalysts (C8-C13). N-Boc-substituted C8 generated
3a inefficiently (15% yield) but in a promising 86:14 er. Yield as
well as enantioselectivity were improved (88% and 97:3 er) when
the more electron-withdrawing and less hindered N-trifluroacetyl-
substituted C9 was utilized. Enantioselectivity proved to be highly
dependent on the reaction temperature: 3a was obtained in 82:18
er at 22 °C, 84:16 er at 0 °C, and 96:4 er at –20 °C (69–80%
yield); at –78 °C, 3a was obtained in 53% yield and 91:9 er.
Diamines containing N-trichloroacetyl (C10) and N-triflyl (C11)
groups gave 3a in 97:3 and 72:28 er, respectively. Installation of
other N,N,-dialkyl groups to diamine catalysts (C12, C13)
resulted in lower yield and er. Chiral 1,2-diarylethylenediamines
were converted to C14, C15 and C16. ortho-Chlorophenyl (C14)
and 1-naphthyl (C16)-substituted catalysts were less
enantioselective. The highest enantioselectivity (98:2 er) was
observed with C15 but at the cost of diminished yield (62%).
Reaction with cyclohexyldiamine C17 generated 3a in 64% yield
and 97:3 er. With C18, prepared by N-methylation of C9, 3a was
formed in 80:20 er (vs 97:3 er with C9). These results clearly
point to the effectiveness of dual H-bonding strategy.
a
Conditions: pronucleophile (0.2 mmol), dialkyl azodicarboxylate (0.3 mmol),
b
B(C6F5)3 (5 mol%), PMP (10 mol%), toluene, under N2, –46°C, 24 h. Yields of
purified products. Er was determined byHPLC analysis. c The absolute configuration
of 3a-DEAD was determined to be R (see reference 17).
Scheme 3. Gram Scale Synthesis of 3a
In summary, we have developed a catalytic method for
B(C6F5)3/amine-catalyzed direct -amination reaction that
provides access to an assortment of -aminocarbonyl compounds
with ketones, esters, amides, thioesters and thioamides serving as
suitable pro-nucleophiles. We have also been able to develop an
efficient enantioselective variant of the process. Based on our
mechanistic hypothesis, it should be possible to broaden the scope
of this enantioselective addition reaction through design of more
potent chiral Lewis acid/Brønsted base catalyst combinations.
Investigations along these lines are currently underway.
A range of cyclic ketones are suitable for enantioselective
direct -amination reactions catalyzed by 5 mol% B(C6F5)3 and
10 mol% C9 (Table 4). Methoxy, fluoro, chloro and bromo-
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(12) For reviews of frustrated Lewis pair chemistry, see: (a) Stephan, D. W.;
Author Information
Erker, G. Angew. Chem., Int. Ed. 2015, 54, 6400. (b) Stephan, D. W. J. Am.
Chem. Soc. 2015, 137, 10018. (c) Oestreich, M.; Hermeke, J.; Mohr, J.
Chem. Soc. Rev. 2015, 44, 2202.
1
2
Corresponding Author
*wasa@bc.edu
(13) For a review of asymmetric FLP chemistry, see: Feng, X.; Du, H.
Tetrahedron Lett. 2014, 55, 6959.
3
4
5
6
7
8
9
Notes
The authors declare no competing financial interest.
(14) For FLP-catalyzed asymmetric hydrogenation and hydrosilylation
reactions, see: (a) Chen, D.; Klankermayer, J. Chem. Commun. 2008,
2130. (b) Chen, D.; Wang, Y.; Klankermayer, J. Angew. Chem., Int. Ed.
2010, 49, 9475. (c) Heiden, Z. M.; Stephan, D. W. Chem. Commun. 2011,
47, 5729. (d) Sumerin, V.; Chernichenko, K.; Nieger, M.; Leskelä, M.;
Rieger, B.; Repo, T. Adv. Synth. Catal. 2011, 353, 2093. (e) Liu, Y.; Du,
H. J. Am. Chem. Soc. 2013, 135, 6810. (f) Wang, X.; Kehr, G.; Daniliuc,
C. G.; Erker, G. J. Am. Chem. Soc. 2014, 136, 3293. (g) Wei, S.; Du, H. J.
Am. Chem. Soc. 2014, 136, 12261. (h) Ren, X.; Du, H. J. Am. Chem. Soc.
2016, 138, 810.
Acknowledgements. We gratefully acknowledge Boston College for
financial support. We are thankful to Professors Amir H. Hoveyda,
James P. Morken (Boston College) and Mr. Richard Y. Liu (MIT) for
helpful discussions.
Supporting Information Available: Experimental procedures and
spectral data for all new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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