Palladium-catalyzed tunable functionalization of allylic imidates: Regioselective aminodiacetoxylation and aziridination

Article abstract of DOI:10.1002/anie.201103500

In control: The title reaction has been shown to be highly general and efficient, thus exhibiting valuable potential synthetic utility (see scheme; DMF=N,N-dimethylformamide). Isotope labeling and two proposed mechanistic pathways, which invoke a Pd^I

Full text of DOI:10.1002/anie.201103500

DOI: 10.1002/anie.201103500
Palladium Catalysis
Palladium-Catalyzed Tunable Functionalization of Allylic Imidates:
Regioselective Aminodiacetoxylation and Aziridination**
Sunliang Cui, Lukasz Wojtas, and Jon C. Antilla*
Recently, palladium-catalyzed reactions that appear to
involve PdII/PdIV catalytic cycles have proven to be interesting
and attractive to synthetic chemists. These transformations
offer access to novel organic products, complementing those
obtained by conventional Pd0/PdII-catalyzed processes.[1] The
Pd-catalyzed oxidative difunctionalization of alkenes is
captivating because it is utilized to access diamination,[2]
aminooxygenation,[3] dioxygenation,[4] aminofluorination,[5]
and cyclopropanation products.[6] One common feature of
these reactions is the use of PhI(OAc)2 as an oxidant to
À
À
intercept the Pd C bond of the intermediate to facilitate C X
(X = C, N, O, F) bond formation.[7] Despite these advance-
Scheme 1. DMF=N,N-dimethylformamide.
ments, PdII/PdIV-catalyzed and tunable functionalization to
provide divergent C X bond formation remains elusive.
[8]
À
Table 1: Optimization of the Pd-catalyzed aminodiacetoxylation of allylic
imidate 1.[a]
The PdII-catalyzed aza-Claisen rearrangement of allylic
imidates, involving an alkyl palladium intermediate, is a well
known process for the synthesis of allylic amines.[9] During
our current study, we reasoned that the alkyl palladium
intermediate formed in the aza-Claisen rearrangement of
allylic imidates could be further transformed by oxidation to
the PdIV species, using PhI(OAc)2 as an oxidant, to provide
Entry
R
Catalyst
Solvent Product Yield
[%][b]
À
new C X bonds. Herein, we report a novel and general
1
2
3
4
5
6
Ph (1a)
1a
1a
1a
1a
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
CH2Cl2 2a
CH3CN 2a
toluene 2a
62
61
42
palladium-catalyzed tunable functionalization of allylic imi-
dates, thus demonstrating regioselective aminodiacetoxyla-
tion and aziridination under mild reaction conditions
(Scheme 1).
AcOH
CH2Cl2 2a
CH2Cl2 2b
2a
trace
56
77
[PdCl2(CH3CN)2]
Our initial investigations began with allyl N-phenylbenz-
imidate (1a). Treatment of 1a with 10 mol% Pd(OAc)2 and 3
equivalents of PhI(OAc)2 in CH2Cl2 at room temperature
afforded an unexpected 2-amino-1,3-diacetoxylation product
2a in 62% yield (Table 1, entry 1). The reaction was highly
regioselective, with no additional functionalized isomers
detected. Control experiments indicated that in the absence
of Pd(OAc)2 aminodiacetoxylation was not observed. The use
of other PdII sources and different solvents uniformly failed to
improve the yield (Table 1, entries 2–5). Based on an initial
mechanistic hypothesis for this reaction, we believed that
electron-donating substituents could stabilize a postulated
intermediate, thus facilitating this transformation. Indeed,
4-CH3C6H4 Pd(OAc)2
(1b)
PMP (1c)
1c
1c
1c
1c
7[c]
8
9
10
11
Pd(OAc)2
CH2Cl2 2c
CH2Cl2 2c
CH2Cl2 2c
CH3CN 2c
89
78
65
80
[PdCl2(CH3CN)2]
[PdCl2(PhCN)2]
Pd(OAc)2
Pd(OAc)2/1,10-phen- CH2Cl2 2c
anthroline (10%)
trace
[a] Reaction conditions: 1 (0.30 mmol), catalyst (10 mol%), PhI(OAc)2
(0.90 mmol), solvent (1 mL), at RT. [b] The yield of isolated product
calculated with 1 as the limiting reagent. [c] PMP=para-methoxyphenyl.
when 1b and 1c were subjected to these reaction conditions
(Table 1, entries 6 and 7), 2b and 2c were obtained in 77%
and 89% yield respectively. These remarkable improvements
demonstrated that electron-rich substituents could enhance
reactivity. With this result in hand, we attempted to further
optimize the reaction conditions using 1c as a model
substrate, and found other combinations of Pd sources and
solvents did not promote the process (Table 1, entries 8–10).
Addition of 1,10-phenanthroline as a ligand inhibited the
reaction (Table 1, entry 11). Notably the reaction proceeds
smoothly at room temperature under an atmosphere of air
[*] Dr. S. Cui, Prof. Dr. J. C. Antilla
Department of Chemistry, University of South Florida
4202 E. Fowler Avenue, Tampa, FL 33620 (USA)
E-mail: jantilla@usf.edu
Dr. L. Wojtas
Department of Chemistry X-Ray Facility, University of South Florida
4202 E. Fowler Avenue, Tampa, FL 33620 (USA)
[**] We thank the University of South Florida for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103500.
Angew. Chem. Int. Ed. 2011, 50, 8927 –8930
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8927

Communications
with high regioselectivity. Additional control reactions under
an argon atmosphere gave identical results.
utility. The reaction is tolerant of various functional groups,
e.g., -CN, -CO2CH3, -Cl, -Br, -CF3, thus showing a broad
substrate scope. Interestingly, aryl–bromide bonds were
unaffected by the reaction conditions (Table 2, entries 5 and
16). The general structure of the products was confirmed by
X-ray single-crystal analysis of compound 2k (see the
Supporting Information), in addition to standard character-
ization.
Furthermore, we discovered that the addition of water to
this reaction could lead to a rather surprising formation of
aziridine products. Optimization of the reaction conditions
showed that the use of [PdCl2(CH3CN)2] as a catalyst, DMF/
H2O (20:1) as cosolvent, and Na2CO3 as an additive led to
divergent reactivity of the allylic imidate, exclusively promot-
ing the aziridination (for optimization, see the Supporting
Information). The generality of the reaction was then
established, with various functionalized aziridines synthesized
in moderate to good yields (Table 3). The structure of the
products was further confirmed by X-ray single-crystal
2-Amino-1,3-diol subunits constitute an important triad
pattern found in many natural molecules of biological interest
and attract much synthetic attention.[10] Encouraged by our
results, we applied the aminodiacetoxylation protocol to a
variety of allylic imidates, which are readily available from the
respective allyl alcohol and imidoyl chloride.[11] As summar-
ized in Table 2, electron-rich N-aryl allylic imidates furnished
Table 2: Pd-catalyzed aminodiacetoxylation of allylic imidates.[a]
Entry
R1
R2
R3
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Ph
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
Ph
PMP
PMP
PMP
PMP
PMP
PMP
H
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
Et
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
2t[c]
2u[d]
89
81
86
76
75
78
56
63
92
88
92
65
72
88
89
74
92
53
52
4-CH3C6H4
4-CH3OC6H4
4-ClC6H4
4-BrC6H4
4-CH3O2CC6H4
4-NCC6H4
3-CF3C6H4
2,6-(CH3)2C6H3
isopropyl
cyclohexyl
Cl(CH2)3
Ph
Table 3: Pd-catalyzed aziridination of allylic imidates.[a]
9
10
11
12
13
14
15
16
17
18
19
Entry
R1
R2
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3 f
3g
3h
3i
78
79
64
69
71
66
58
55
58
73
46
4-CH3C6H4
4-TsOC6H4
4-ClC6H4
4-BrC6H4
4-CH3O2CC6H4
4-NCC6H4
3-CF3C6H4
3-NO2C6H4
2,6-(CH3)2C6H3
Ph
Ph
4-CH3C6H4
4-BrC6H4
isopropyl
Ph
4-CH3C6H4
Et
[a] Reaction conditions: 1 (0.30 mmol), PhI(OAc)2 (0.90 mmol), CH2Cl2
(1 mL), RT, 1–5 h. [b] The yield of isolated product calculated with 1 as
the limiting reagent. [c] The aza-Claisen rearrangement product was
detected and 1H NMR analysis of the reaction mixture showed a ratio of
1:1.54 versus 2t. [d] The aza-Claisen rearrangement product was
detected and 1H NMR analysis of the reaction mixture showed a ratio of
1:1.62 versus 2u.
9
10
11
H
H
CH3
3j
3k
[a] Reaction conditions: 1 (0.30 mmol), PhI(OAc)2 (0.60 mmol), DMF
(3 mL), Na2CO3 (4m aqueous solution, 150 mL), RT, 2–8 h. [b] The yield
of the isolated product calculated with 1 as the limiting reagent.
the regioselective N-arylaminodiacetoxylation products in
good to excellent yields (Table 2, entries 1–3 and entry 9),
while electron-deficient N-aryl substrates afforded the prod-
ucts in moderate to good yields (Table 2, entries 4–8). The
reaction could also be performed with various N-alkyl allylic
imidates, giving N-alkylaminodiacetoxylation products in
moderate to excellent yields (Table 2, entries 10–12). A
number of disubstituted allylic imidates were also employed
to investigate the scope of the aminodiacetoxylation. The
transformation afforded the corresponding tetrasubstituted
carbon-tethered aminodiacetoxylation products in moderate
to excellent yields (Table 2, entries 13–19), with generality in
both N-aryl and N-alkyl substitution. Notably, tetrasubsti-
tuted carbon-tethered amines are difficult to synthesize by
conventional methods,[12] and this Pd-catalyzed functionaliza-
tion gives facile access to these compounds in a general and
efficient manner, therefore displaying valuable synthetic
analysis of compound 3g (see the Supporting Information),
in addition to standard characterization. Aziridines are
versatile substances for the synthesis of useful nitrogen-
containing compounds. Therefore, we believe this method to
be of potential synthetic utility.[13,14]
Control experiments were conducted to gain insight into
the reaction mechanism. Interestingly, no aminodiacetoxyla-
tion or aziridination was observed when various Pd0/PdII
oxidants, such as Cu(OAc)2, CuCl2, and benzoquinone were
used in place of PhI(OAc)2. On this basis, we believe these
transformations are less likely to proceed through a tradi-
tional Pd0/PdII catalytic cycle. Indeed, when stoichiometric
[PdCl2(CH3CN)2] was used in the absence of PhI(OAc)2,
aziridination was not observed. Furthermore, an isotope
labeling experiment was conducted to probe the aziridination
mechanism, using H218O (97 atom% 18O; Scheme 2). HRMS
showed an 18O atom was incorporated in the product 3a’
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À
N and C O bond formation. This method opens up a new
avenue for the functionalization of allylic imidates under mild
reaction conditions and with broad substrate scope and high
efficiency. Isotope labeling experiments provide evidence for
two possible mechanistic pathways involving a proposed PdII/
PdIV catalytic cycle. Further study of the scope of the reaction
and extension of the method are in progress.
Scheme 2. Isotopic labeling experiment.
(90% 18O enriched, [M + H]+ 286.1317) and IR spectroscopy
=
showed that the C O bond frequency shifted from 1708 to Experimental Section
1682 cmÀ1.[15] These results confirm the C O group of 3a’ is
=
Typical procedure for aminodiacetoxylation: Pd(OAc)2 (6.8 mg,
18O labeled, and the 18O atom was delivered from H218O.
On the basis of these results, a possible catalytic cycle for
this tunable transformation is proposed (Scheme 3). First, the
0.03 mmol) and PhI(OAc)2 (290 mg, 0.9 mmol) were added to a
vial. CH2Cl2 (0.9 mL) was added, followed by the addition of allylic
imidate 1c (80.1 mg, 0.3 mmol) by using a microsyringe. The micro-
syringe was washed with CH2Cl2 (0.1 mL) and this
solution was added to the reaction solution. The
mixture was kept at room temperature under air.
After completion of the reaction, the solution was
purified by flash column chromatography on silica gel
using ethyl acetate/hexanes (v/v, 1:2) as eluent to give
product 2c as a colorless oil (102.8 mg, 89% yield).
1H NMR (CDCl3, 250 mhz): d = 7.17 (m, 5H), 7.02
(m, 2H), 6.57 (d, J = 7.5 Hz, 2H), 4.87 (t, J = 7.5 Hz,
1H), 4.35 (d, J = 7.5 Hz, 4H), 3.64 (s, 3H), 1.95 ppm
(s, 6H); 13C NMR (CDCl3, 62.5 mhz): d = 170.9,
170.5, 160.6, 142.1, 130.6, 129.2, 129.1, 128.2, 127.4,
113.0, 62.2, 57.3, 55.1, 20.8 ppm; HRMS (ESI) (m/z):
calcd for C21H23NO6 [M+H+] 386.1598; found
386.1599.
Typical procedure for aziridination: [PdCl2-
(CH3CN)2]
(7.8 mg,
0.03 mmol),
PhI(OAc)2
(193.2 mg, 0.6 mmol) were added to a vial. DMF
(2.5 mL) and Na2CO3 (4m aqueous solution, 150 mL)
were added, followed by the addition of allylic
imidate 1c (80.1 mg, 0.3 mmol) by using a micro-
syringe. The microsyringe was washed with DMF
(0.5 mL) and this solution was added to the reaction
mixture. The mixture was kept at room temperature
under air. After completion, the solution was decan-
ted into a separatory funnel, diluted with 15 mL Et2O
and 5 mL water. The two layers were separated and
the aqueous layer was extracted with Et2O (3 ꢀ
Scheme 3. Proposed PdII/IV-catalyzed tunable functionalization of allylic imidates.
PdII-mediated reversible 5-exo-trig aminopalladation of the
allylic imidate forms the oxazolinium-substituted alkyl palla-
dium species A,[16] which is oxidized by PhI(OAc)2 to produce
PdIV intermediate B.[17] Intermediate B shows divergent
reactivity: 1) Under reaction conditions for aminodiacetoxy-
lation, B is nucleophilically attacked by acetate, leading to
oxazolinium fragmentation to give C, a subsequent reductive
elimination furnishes the final product 2 and regenerates the
PdII catalyst (path a). 2) Under conditions for aziridination, B
is hydrolyzed to D and a subsequent ring opening gives amine
intermediate E,[18,19] followed by an intramolecular cycliza-
tion/reductive elimination to furnish the aziridination product
3 and the PdII catalyst (path b).[14a] Therefore, the function-
alization is tunable, depending on the reaction conditions.
Moreover, when R2 is an electron-rich group, such as PMP,
the initial aminopalladation is enhanced to facilitate this
transformation.
15 mL). The combined ether layers were washed with brine
(20 mL), dried (Na2SO4), concentrated in vacuo, and purified by
flash column chromatography on Al2O3 using ethyl acetate/hexanes
(v/v, 1:4) as eluent to give aziridine 3a as a colorless oil (66 mg, 78%
yield). 1H NMR (CDCl3, 250 mhz): d = 8.01 (d, J = 10.0 Hz, 2H), 7.16
(m, 2H), 6.91 (m, 5H), 4.58 (dd, J1 = 12.5 Hz, J2 = 5.0 Hz, 1H), 4.11
(dd, J1 = 10.0 Hz, J2 = 7.5 Hz, 1H), 3.80 (s, 3H), 2.48 (m, 1H), 2.24 (d,
J = 2.5 Hz, 1H), 2.13 ppm (d, J = 7.5 Hz, 1H); 13C NMR (CDCl3,
62.5 mhz): d = 166.3, 163.7, 153.9, 131.9, 129.2, 122.8, 122.5, 120.8,
113.9, 66.8, 55.6, 37.8, 31,7 ppm; HRMS (ESI) (m/z): calcd for
C17H17NO3 [M+Na+] 306.1101; found 306.1110.
Received: May 22, 2011
Published online: August 10, 2011
Keywords: allylic imidates · aziridination · oxidation ·
.
palladium · synthetic methods
In summary, we have successfully developed a novel
palladium-catalyzed tunable functionalization of allylic imi-
dates, including regioselective aminodiacetoxylation and
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À
aziridination with switchable reactivity toward divergent C
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Communications
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