Highly enantioselective electrophilic α-bromination of enecarbamates: Chiral phosphoric acid and calcium phosphate salt catalysts

Article abstract of DOI:10.1021/ja304095z

Metal-free chiral phosphoric acids and chiral calcium phosphates both catalyze highly enantio- and diastereoselective electrophilic α-bromination of enecarbamates to provide an atom-economical synthesis of enantioenriched vicinal haloamines. Either enantiomer can be formed in good yield with excellent diastereo- and enantioselectivity simply by switching the catalyst from a phosphoric acid to its calcium salt.

Full text of DOI:10.1021/ja304095z

Communication
pubs.acs.org/JACS
Highly Enantioselective Electrophilic α-Bromination of
Enecarbamates: Chiral Phosphoric Acid and Calcium Phosphate Salt
Catalysts
Aurelien Alix,† Claudia Lalli,† Pascal Retailleau, and Geraldine Masson*
́
́
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
S
* Supporting Information
(Scheme 1), leading to a valuable aminal 418 for retro-inverso
synthesis of peptide mimics.19 Herein we report the first
ABSTRACT: Metal-free chiral phosphoric acids and
chiral calcium phosphates both catalyze highly enantio-
and diastereoselective electrophilic α-bromination of
enecarbamates to provide an atom-economical synthesis
of enantioenriched vicinal haloamines. Either enantiomer
can be formed in good yield with excellent diastereo- and
enantioselectivity simply by switching the catalyst from a
phosphoric acid to its calcium salt.
enantioselective α-bromofunctionalization of enecarbamates
through a domino bromination/nucleophilic addition. This
approach also led to the discovery that the chiral phosphoric
acid (3, M = H) and its calcium salt (3, M = Ca) give access to
both enantiomers of the vicinal haloamine product.
To validate our hypothesis, we began our studies by
examinating the reaction of O-benzyl N-(E)-prop-1-en-1-yl
carbamate (1a) with 2 in the presence of a 10 mol % loading of
chiral phosphoric acids 3 at room temperature. As shown in
Table 1, both the reactivity and enantioselectivity were
dramatically affected by the structure of the chiral phosphoric
acid. The highest diastereo- and enantioselectivity were
obtained when (R)-3,3′-bis(2,4,6-triisopropylphenyl)-BINOL
(TRIP) phosphoric acid (3e) was used (9:1 dr, 88% ee;
entry 5).20 Additionally, it was found that a significant
reduction of the catalyst loading to only 1 mol % under
more concentrated conditions (c = 0.1 M) improved the
reaction diastereo- and enantioselectivity (>95:5 dr, 98% ee,
99% yield; entry 9 vs 5). Comparable enantioselectivity was
obtained when the catalyst loading was reduced to 0.1 mol %,
although this gave a reduced yield (entry 11). Reducing the
amount of 1a to 1.2 equiv relative to 2 gave 4a in lower yield
(entry 13). The absolute configuration of the major isomer cis-
4a was unambiguously determined to be 1R,2S by X-ray
analysis [see the Supporting Information (SI)]. As the
stereochemical outcome of electrophilic bromination can
depend on the enecarbamate geometry, the reaction with the
Z isomer of enecarbamate 1a was investigated, and it gave the
same enantiomer of 4a as the major product, albeit with lower
yield (entry 12). However, to our surprise, reversal of the
enantioselectivity of product 4a to supply (1S,2R)-cis-4a was
observed when the catalyst was changed from 3e to its
corresponding metal salt 3f (M = Li) or 3g (M = Ca), still with
excellent ee and dr values (entries 14 and 15).21 Thus, both
enantiomers of 4a could be prepared efficiently.14a,b,21
icinal haloamines are a versatile structural motif for the
V
synthesis of various biologically active compounds and can
serve as key intermediates for further transformations.1−3 To
date, however, only a few examples of their asymmetric
synthesis have been described.4 Among the reported syntheses,
electrophilic halofunctionalization of enamides represents one
of the most convenient and straightforward approaches to
obtain chiral vicinal haloamine compounds in a regio- and
stereoselective manner.5 Despite recent advances in the
catalytic asymmetric halogenation of carbonyl,4c,d,6 acyl
chloride,7 1,3-dicarbonyl,8 oxindole,9 and enyne compounds,10
no example of an enantioselective α-halogenation of enamides
is extant.11
Chiral phosphoric acids12,13 and chiral phosphate metal
salts9d,14 were found to be effective in catalyzing enantiose-
lective additions of enamide compounds15 to various electro-
philes, such as carbonyl,16c imine,16a,b,d−g,j azodicarboxylate,16i
and nitroso compounds.16h A recent report described the
activation of N-halogenosuccinimide toward highly enantiose-
lective reactions catalyzed by chiral phosphoric acid or chiral
sodium phosphate.9d,17 Inspired by the successful employment
of these catalysts and our interest in enantioselective trans-
formations,16e−g,i,j we envisioned that chiral phosphoric acids or
their metal salts (3) could catalyze enantioselective electrophilic
α-halofunctionalization of enamide compounds (Scheme 1).
Moreover, the electrophile N-bromosuccinimide (NBS, 2),
which includes a masked nucleophilic component, could be
incorporated into the electrophilic iminium intermediate 5
We next examined the scope of the enecarbamate
component in this enantioselective domino process catalyzed
by chiral phosphoric acid 3e or its calcium salt 3g.
Representative results are summarized in Table 2. In general,
the 3e-catalyzed reaction (entries 1−13) proceeded smoothly
with high diastereo- and enantioselectivity in excellent yield
Scheme 1. Working Hypothesis
Received: May 3, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja304095z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Optimization of Enantioselective α-Bromination of
Enecarbamates
Table 2. Enantioselective Direct α-Bromination of
a
a
Enecarbamates: Substrate Scope
e
f
g
entry
3 (R/M)
3a (C6H5/H)
mol % 3 yield (%)
ee (%) /dr
a
h
h
h
h
h
h
h
1
2
10
10
10
10
10
5
33
39
44
39
87
87
87
89
99
75
56
67
64
74
78
51/(75:25)
52/(75:25)
62/(85:25)
64/(85:25)
88/(90:10)
88/(90:10)
a
a
a
a
a
a
b
b
3b (4-ClC6H4 [H8]/H)
3
3c (2-naphthyl/H)
4
3d (1-naphthyl/H)
5
3e (2,4,6-iPr3C6H2/H)
6
3e
7
3e
2
90/(>95:5)
94/(>95:5)
98/(>95:5)
94/(>95:5)
92/(>95:5)
86/(>95:5)
98/(>95:5)
−96/(>95:5)
−98/(>95:5)
h
h
h
h
h
8
3e
1
9
3e
1
b
b
10
11
12
13
14
15
3e
0.5
0.1
3e
bc
,
3e
1
1
1
1
b
hi
,
3e
bd
,
j
j
3f (2,4,6-iPr3C6H2/Li)
3g (2,4,6-iPr3C6H2/Ca)
bd
,
a
Reaction conditions: 1a (0.1 mmol), 2 (0.05 mmol), and 3 in 1 mL
b
of toluene for 14 h. Same conditions as in footnote a except with 0.5
c
d
mL of toluene. The Z isomer of 1a was used. The reaction was
e
performed for 48 h. Yields of the chromatographically pure major
f
g
diastereoisomer. Determined by chiral HPLC analysis. Determined
h
i
by 1H NMR analysis of the crude material. (1R,2S)-enriched 4a. 1.2
j
equiv of 1a (0.6 mmol) was used. (1S,2R)-enriched 4a.
(>90%). This reaction worked well for a broad range of
substituted enecarbamates bearing a linear or branched alkyl
group or a heteroalkyl substituent. Introduction of steric
hindrance at the α-position of the ene moiety resulted in a
slight decrease in the yield without affecting the enantiose-
lectivity of the process (entry 8). Even the β,β-disubstituted
enecarbamate 1n afforded the corresponding bromoaminal
product 4n containing a quaternary center with 96% ee (entry
13). In contrast, vinyl carbamates substituted with aryl groups
at the β-position failed to participate in this electrophilic α-
bromination, probably because of their lower nucleophilicity.22
Modification of the carbamate moiety from benzyl to ethyl,
propargyl, allyl, or tert-butyl carbamate afforded the bromoa-
minal products (4b−f, entries 1−5) with similar diastereo- and
enantioselectivities. It is noteworthy that no bromination of
unsaturated bonds present in the enecarbamate was observed,
whether in the carbamate moiety (entries 2 and 3) or the lateral
chain (entry 7), thus demonstrating the high chemo- and
regioselectivity of this reaction.17 We also investigated
enantioselective electrophilic α-bromination using the opti-
mized reaction conditions with calcium salt 3g (entries 14−16).
A variety of selected enecarbamates 1 reacted with good
enantioselectivity in all cases affording the opposite enantiomer
of the products formed using the corresponding phosphoric
acid 3e. We also found that, contrary to our previous studies,
the N-protecting group of 1 had an influence on the
enantiomeric excess, as diminished enantioselectivity was
observed when Boc or Alloc was used (entries 14 and 15).
We then became interested in pursuing experiments
designed to test the mechanistic hypothesis outlined in Scheme
a
Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), and 3e (entries
1−13) or 3g (entries 14−16) (1 mol %) in toluene (1.0 mL) at room
b
temperature for 14 h. Yields of chromatographically pure product.
c
d
The reaction was performed for 48 h. Determined by chiral HPLC
e
f
analysis. (1R,2S)-enriched 4. The absolute configuration was not
determined. (1S,2R)-enriched 4.
g
1. Different modes of activation of NBS by 3 could be
proposed, such as hydrogen bonding, metallic bonding, or
proton transfer with formation of a chiral bromonium ion
intermediate.9d,17 Inspired by the elegant work of Denmark and
Burk,17c we investigated the addition of the achiral Lewis base
Ph3PS, which is known to favor the ion-pairing mechanism.
Under these conditions, the two catalysts 3e and 3g gave
B
dx.doi.org/10.1021/ja304095z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
similar results in terms of reaction rate, yield and ee (see the
SI). The phenomenon of enantioselectivity reversal by
phosphate salt 3g was preserved, whereas the same enantiomer
should have been observed with a mechanism involving a chiral
bromonium ion intermediate. We also carried out a control
experiment with a two-component catalyst composed of TRIP
phosphoric acid 3e and Ca(iPrO)2, which furnished the desired
product in 95% ee with the same selectivity as 3e. It can
therefore be suggested that 3e would activate NBS through
either hydrogen bonding or ion pairing. At this stage, it is not
possible to distinguish these two activation modes. On the
other hand, an alkaline-earth-metal bonding interaction
between 3g and NBS would be evoked. Next, we carried out
a nonlinear effect (NLE) experiment to gain further insight into
the structure of the active calcium phosphate salt catalyst. We
observed a positive NLE in the 3g-catalyzed α-bromination of
enecarbamates (see the SI), indicating that the active catalyst is
a dimeric species rather than a CaII-free phosphoric acid or
monophosphate salt.14,23 In addition, it is worth noting that
secondary enecarbamates gave no reaction in the presence of
3e or 3g, suggesting that activation of NBS alone is not
sufficient to achieve effective α-bromination.
Scheme 3. Ring Opening of the Succinimide Moiety
building blocks, such as aziridines,5e,f,i 1,2-ether/thioether-
aminals,5e,g,l imidazolidin-2-ones,5e and pyrrolizidines.24
In conclusion, we have reported the first enantioselective
chiral phosphoric acid-catalyzed electrophilic α-bromination of
enecarbamates.11 The efficiency of the system allows reactions
to be run under mild conditions at room temperature with a
modest amount of catalyst (1 mol %). We have also shown an
unprecedented reversal of enantioselectivity for a catalytic
bromination reaction when the corresponding chiral calcium
phosphate salt is used. The high efficiency and operational
simplicity of this procedure make it a practical protocol for
synthesis of chiral vicinal haloamines.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, characterization data, HPLC enantiomer
analysis, and NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
On the basis of these observations, we assumed that 3e and
3g act as bifunctional catalysts activating both NBS and the
enecarbamate via hydrogen bonding and metal chelation,
respectively. While a precise understanding of the origin of the
enantioselectivity reversal awaits further study, two different
transition state models, 6 and 7, were postulated (Scheme 2).
AUTHOR INFORMATION
Corresponding Author
masson@icsn.cnrs-gif.fr.
■
Author Contributions
Scheme 2. Proposed Transition State Model
†A.A. and C.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the ICSN and CNRS for financial support and ANR
for postdoctoral fellowships to A.A. and C.L. We also
acknowledge Prof. Jieping Zhu for his generous support.
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