Two Distinct Ag(I)- And Au(I)-Catalyzed Olefinations between α-Diazo Esters and N-Boc-Derived Imines

Article abstract of DOI:10.1021/acs.orglett.9b02343

Metal-catalyzed reactions between α-diazo esters and imines were well-known to yield aziridine derivatives exclusively. This work reports two new olefination reactions between N-Boc-derived (Boc = tert-Butyloxycarbonyl) imines and α-diazo esters with Ag(I) and Au(I) catalysts, respectively. Our mechanistic studies reveal that these new olefinations involve an initial attack of diazo esters on metal/imine complexes to form Mannich-addition intermediates, which subsequently afford α-aryl-β-aminoacrylates via a Roskamp reaction, or to form β-aryl-β-aminoacrylates via the formation of silver carbenes.

Full text of DOI:10.1021/acs.orglett.9b02343

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Two Distinct Ag(I)- and Au(I)-Catalyzed Olefinations between
α‑Diazo Esters and N‑Boc-Derived Imines
Rahul Dadabhau Kardile and Rai-Shung Liu*
Frontier Research Center on Fundamental and Applied Sciences of Matters, Department of Chemistry, National Tsing-Hua
University, Hsinchu, Taiwan, ROC
S
* Supporting Information
ABSTRACT: Metal-catalyzed reactions between α-diazo
esters and imines were well-known to yield aziridine
derivatives exclusively. This work reports two new olefination
reactions between N-Boc-derived (Boc = tert-Butyloxycarbon-
yl) imines and α-diazo esters with Ag(I) and Au(I) catalysts,
respectively. Our mechanistic studies reveal that these new
olefinations involve an initial attack of diazo esters on metal/
imine complexes to form Mannich-addition intermediates,
which subsequently afford α-aryl-β-aminoacrylates via a Roskamp reaction, or to form β-aryl-β-aminoacrylates via the formation
of silver carbenes.
esters via α-oxo metal carbenes (In-1) that are subsequently
captured by nucleophilic imines to yield aziridine products.3
Such aziridine syntheses have been achieved with less acidic
metal catalysts including Rh(II), Ru(II), Cu(I), Re(I), and
Co(II). Alternatively, in the case of Brønsted acids and BF3,4
diazo esters serve as nucleophiles to attack metal/imine pairs
to form intermediates of Mannich type (In-2), which also
afford aziridines preferably (Scheme 1, eq 2). Equation 3
(Scheme 1) highlights one specific example that employs (R)-
VANOL to catalyze an asymmetric [2 + 1] annulation of N-
Boc-derived (Boc = tert-Butyloxycarbonyl) imines with α-diazo
esters;4h herein, α-aryl-β-aminoacrylate was formed also in
small proportions, indicative of an unknown path. We are
aware of no example to obviate this aziridine chemoselectivity
unless α-diazo esters are replaced with α-diazo nitriles.5 In
seeking new chemoselectivity beyond aziridine formation, this
work reports two new olefinations between α-diazo esters and
N-Boc-derived imines using Au(I) or Ag(I) catalysts without
additive. With a cationic Au(I) catalyst, these imine/α-diazo
ester mixtures yield α-aryl-β-aminoacrylates, involving a
Roskamp reaction6 of intermediates In-2′, whereas a Ag(I)
catalyst enables the same intermediates to afford β-aryl-β-
aminoacrylates via silver carbene intermediates. Our mecha-
nistic analysis supports the roles of intermediates ln-2′ that
have been verified with deuterium labeling and other control
experiments.
α-Diazo carbonyl compounds have found widespread
applications in organic synthesis, because they readily form
reactive metal carbenes.1 These metal carbenes undergo
various and useful transformations such as X−H insertions
(X = C, N, O, S, Si), cyclopropanation, Stevens and Wolff
rearrangements, and ylide-based cycloaddition reactions.2
Equation 1 (Scheme 1) shows one well-known system for
catalytic [2 + 1] annulations of numerous imines with diazo
Scheme 1. Aziridine Routes
Importantly, these two new catalytic reactions are accessible
to two β-amino acrylates with the same imines and α-diazo
esters; these functionalized acrylates are recognized as useful
building blocks to construct numerous heterocyclic rings
Received: July 7, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02343
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Letter
including pyrroles,7 indoles,8 oxazoles,9 pyrazoles,10 quino-
supported by H nuclear Overhauser effect (NOE) spectra
and X-ray diffraction of their related 4k and 5i (Supporting
Information).
1
lines,11 pyridines,12 and dihydropyridines.13
Table 1 shows catalytic optimization between imine 1a and
ethyl α-diazoacetate 2a (1.5 equiv) using various Lewis acid
We assessed the generality of the two reactions with various
N-Boc- and N-phenyl derived imines 1 and ethyl diazoacetate
2a using (C6F5)3PAuCl/AgNTf2 and AgOTf; the results are
summarized in Table 2. In the case of (C6F5)3PAuCl/AgNTf2,
Table 1. Optimization and Catalyst Screening
Table 2. Au(I) and Ag(I)-Catalyzed Reactions on Various
Imines
b
yield (%)
entry
catalyst
solvent
T (h)
3a
4a
1
2
3
4
5
6
7
8
(PhO)3PAuCl/AgNTf2
DCM
DCM
DCM
DCM
DCM
DCM
DCE
toluene
DCM
DCM
toluene
toluene
DCM
DCM
05
05
04
02
03
06
12
12
24
12
06
05
03
15
40
30
41
80
61
30
74
25
25
30
11
10
08
05
c
IPrAuCl/AgNTf2
d
LAuCl/AgNTf2
(C6F5)3PAuCl/AgNTf2
(C6F5)3PAuCl/AgSbF6
(C6F5)3PAuCl/AgOTf
(C6F5)3PAuCl/AgNTf2
(C6F5)3PAuCl/AgNTf2
AgNTf2
05
25
05
25
40
53
71
9
10
11
12
13
14
AgOTf
AgOTf
Rh2(OAc)4
e
BF3
65
10
15
10
e
HOTf
a
b
[1a] = 0.12 M. Product yields are reported after purification from a
c
silica column. IPr = 1,3-bis(diisopropylphenyl) imidazol-2-ylidene.
d
e
L = P(t-Bu)2(o-biphenyl). 10 mol % of catalyst was used.
catalysts. Our goal was to switch the chemoselectivity to
nonaziridine products. We tested the reaction with
(PhO)3PAuCl/AgNTf2 (5 mol %) in dichloromethane/
molecular sieves 4 Å (DCM, 5 h) near 23 °C that afforded
α-phenyl-β-aminoacrylate 3a and its β-phenyl isomer 4a,
respectively, in 40% and 10% yields (entry 1). With other gold
catalysts, L′AuCl/AgNTf2 (L′ = IPr and P(t-Bu)2(o-
biphenyl)), these two reactants gave products 3a and 4a in
30−41% and 5−8% yields, respectively (entries 2 and 3). To
our delight, a switch to (C6F5)3PAuCl/AgNTf2 (5 mol %)
afforded α-phenyl-β-aminoacrylate 3a in 80% yield (entry 4).
Other silver salts (C6F5)3PAuCl/AgX (X = SbF6 and OTf)
became less chemoselective (entries 5 and 6); we noted that
compound 4a was obtained in 25% yield because of the less
acidic nature of OTf-. For (C6F5)3PAuCl/AgNTf2, dichloro-
ethane (DCE) provided a satisfactory yield (74%) of
compound 3a, whereas toluene gave a 1:1 mixture of 3a and
4a (entries 7 and 8). For AgNTf2 alone in DCM, the reaction
chemoselectivity altered to give compounds 3a and 4a in 25%
and 40% yields, respectively (entry 9). Inspired by this
discovery, we tested AgOTf in DCM and toluene, which
increased the yields of desired 4a to 53% and 71%, respectively
(entries 10 and 11). The use of Rh2(OAc)4 led only to diazo
decomposition to give a mixture of diethyl fumarate and
malonate. For a strong Lewis acid like BF3·Et2O, we obtained
1,2-phenyl migration product 3a as major species (65%)
together with its isomer 4a in 15% yield (entry 13). In
contrast, HOTf (10 mol %) gave the two products 3a and 3a′
in low yields (entry 14). Compounds 3a and 4a presumably
bear a Z-configuration to form a hydrogen bond between their
amino groups and esters. This structural assignment is
a
b
[1a] = 0.12 M. Product yields are reported after purification from a
c
silica column. An aziridine was isolated in 5% yield.
only α-aryl-β-aminoacrylates 3 were produced exclusively, with
satisfactory yields. For AgOTf, β-aryl-β-aminoacrylates 4 were
produced dominantly with 62−74% yields, together with α-aryl
substituted isomers 3 in small proportions (3−12%). In all
cases, we isolated no aziridine derivative in a tractable amount.
We prepared N-Boc-derived imines 1b−1f bearing various 4-
phenyl substituents (X = Me, OMe, F, Cl, Br); their resulting
α-aryl-β-aminoacrylates 3b−3f were isolated in 74−82% yields
with (C6F5)3PAuCl/AgNTf2 (entries 1−5). When reactions of
these imines were catalyzed with AgOTf, their β-aryl-β-
aminoacrylates 4b−4f were also produced efficiently (68−
74%) together with byproducts 3b−3f in 3−7% yields (entries
1′−5′). 2-Methylphenyl imine 1g was also compatible with
Au(I) and Ag(I) catalysts, respectively, rendering β-amino-
acrylates 3g and 4g in 71% and 62% yields (entries 6 and 6′).
N-Boc-derived imines 1h−1j bearing heteroaromatic moieties
(Ar = 2-furyl and 2- and 3-thienyl) were suitable for these two
catalytic reactions, affording 3h−3j and 4h−4j in 71−78% and
67−72% yields, respectively (entries 7−9 and 7′−9′). Both
Au(I) and Ag(I) catalysts were amenable to imine 1k bearing a
2-naphthyl group, giving desired products 3k and 4k in
satisfactory yields (entries 10 and 10′). The molecular
B
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structure of compound 4k was determined with X-ray
diffraction (Supporting Information). We performed the
reactions also on a distinct N-phenyl imine 1l that delivered
the desired olefin 3l in 80% yield with gold catalyst, and
afforded a mixture of two olefins 4l/3l = 5:1 in 89% yield with
AgOTf (entries 11 and 11′).
catalyst, the resulting product d1-4a comprised 0.83 D at the
C(α)-carbon, showing a minute loss of deuterium (eq 5). We
prepared also deuterated α-diazo ester d1-2a bearing 0.54 D at
the C(α)-carbon; its catalytic reactions with gold and silver
catalysts yielded compounds 3a and 4a bearing no deuterium
for either sample, indicating a complete loss of deuterium (eq
6). We prepared also an aziridine derivative 714 to examine its
We examined further the two reactions with various α-diazo
carbonyls 2 and N-Boc-derived imine 1a; the results are
summarized in Table 3. We prepared alkyl-derived α-diazo
Table 3. Catalytic Reactions with Various α-Diazo Carbonyl
Species
role in this catalytic system. As shown in eq 7, this sample
remained unreacted in toluene (25 °C, 12 h) with AgOTf, but
it was convertible to heterocycle 8 efficiently with the gold
catalyst (eq 7). Accordingly, an aziridine species such as 7 is
not a precursor for our targets, β-aminoacrylates 3a and 4a. We
prepared also β-amino α-diazo ester 915 to examine its catalytic
role; notably, this species gave our desired compound 4a with
either AgOTf or P(C6F5)3AuCl/AgNTf2 (eq 8). In contrast,
the other product 3a was unattainable from species 9 even with
the gold catalyst.
a
b
[1a] = 0.12 M. Product yields are reported after purification from a
silica column.
esters 2b−2e (R = t-butyl, isobutyl, benzyl, and cinnamyl);
their reactions with imine 1a under the gold catalyst afforded
the corresponding α-phenyl-β-aminoacrylates 5b−5e in
satisfactory yields (75−82%, entries 1−4); no 1,2-hydrogen
migration byproduct 6 was formed in a tractable amount. With
AgOTf, a 1,2-hydrogen shift occurred with the same α-diazo
esters 2b−2e, yielding β-phenyl-β-aminoacrylates 6b−6e in
61−71% yields, together with 5b−5e in minor proportions
(7−10%, entries 1′−4′). These reaction patterns worked also
for aryl-derived α-diazo esters 2f−2h (X = H, OMe, and Cl);
products of two distinct classes 5f−5h (76−83%, entries 5−7)
and 6f−6h (64−73%, entries 5′−7′) were obtained,
respectively. α-Diazo thioester 2i also followed Au(I) and
Ag(I)-directed chemoselectivities, giving the desired α- and β-
phenyl acrylates 5i and 6i in 75% and 54% yields (entries 8 and
8′). The molecular structure of species 5i was confirmed with
X-ray diffraction (Supporting Information).
Scheme 2 depicts a plausible mechanism to rationalize the
two olefination products 3a and 4a. The control experiment in
eq 6 shows a complete loss of deuterium for initial d1-2a; to
rationalize this observation, we postulate an initial attack of
diazo ester d1-2a at catalyst-bound imine 1a that serves an
electrophile. For AgOTf catalysis, the diazo deuterium of
resulting Mannich-addition intermediate A is very acidic and
Scheme 2. A Plausible Reaction Mechanism
We performed deuterium-labeling experiments to elucidate
the mechanisms of formation of α- and β-aryl-β-aminoacrylates
3 and 4. We prepared imine d1-1a containing 0.85 D at the
imine proton. With the gold catalyst, its resulting product d1-
3a was analyzed to contain 0.84 D at the olefinic C(β)-carbon,
indicative a complete deuterium transfer (eq 5). With a AgOTf
C
DOI: 10.1021/acs.orglett.9b02343
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Letter
easily deprotonated by OTf− to generate a β-amino-α-diazo
ester B. This OTf− effect is also noted in gold catalyst
P(C6F5)3AuOTf that leads to formation of undesired 3a in
25% yield (Table 1, entry 6). A further protodeauration of
species B affords compound 9 that has been confirmed to be
the precursor for β-phenyl-β-amino acrylate 4a. Accordingly,
we postulate a silver carbene C to induce a 1,2-hydrogen shift
to yield species D and ultimately the observed product 4a. For
the gold catalyst, its corresponding intermediate A′ undergoes
a Roskamp reaction6 to induce a 1,2-phenyl migration, yielding
species B and ultimately β-amino-α-acrylate 3a. In gold
catalysis, the C−D bond of species d1-1a remains intact, thus
retaining the initial deuterium content as shown in eq 5. In
silver catalysis, 1,2-hydrogen migration of silver carbenes C
also occurs with a complete transfer of deuterium16,17 when d1-
imine 1a was used. This proposed mechanism is compatible
with our two deuterium-labeling experiments.
In summary, catalytic reactions of N-Boc-derived imines and
α-diazo esters lead to two unprecedented olefinations using
Au(I) and AgOTf catalysts. These catalytic systems represent
the first examples to obviate the well-known aziridine route.
We performed control experiments in a series to reveal that
these new olefinations involve initial formation of Mannich
addition intermediates that yielded novel β-amino-α-phenyl
acrylates via a Roskamp reaction. In contrast, AgOTf enables
the same intermediates to generate silver carbenes to induce a
1,2-hydrogen shift. Further elaborations of these nonaziridine
routes will be explored in future investigations.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors thank the Ministry of Science and Technology
(MOST 107-3017-F-007-002) and the Ministry of Education
(MOE 106N506CE1), Taiwan, for supporting this work.
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