Copper-catalyzed cyclization and azidation of γ,δ-unsaturated ketone O-benzoyl oximes

Article abstract of DOI:10.1002/adsc.201400681

An intramolecular imination/azidation sequence has been realized through the tetrakis(acetonitrile)copper(I) hexafluorophophate [Cu(CH₃CN)₄PF₆]-catalyzed reaction of γ,δ-unsaturated ketone O-benzoyl oximes with trimethylsi

Full text of DOI:10.1002/adsc.201400681

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DOI: 10.1002/adsc.201400681
Copper-Catalyzed Cyclization and Azidation of g,d-Unsaturated
Ketone O-Benzoyl Oximes
Hailin Su,a Weifei Li,a Zhaoli Xuan,a and Wei Yua,*
a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-891-2582; e-mail: yuwei@lzu.edu.cn
Received: July 14, 2014; Revised: August 29, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400681.
À
Abstract: An intramolecular imination/azidation
sequence has been realized through the tetrak-
to promote the reductive N O bond cleavage of O-
acyl oximes and subsequent C N bond formation,
À
A
hexafluorophophate
and this imination strategy has recently found wide
applications in the synthesis of nitrogen heterocy-
cles.[8,9] The use of copper(I) salts toward this goal
was first reported by Narasaka et al.[10] These re-
searchers found that CuBr·SMe2 could effectively cat-
alyze the cyclization of g,d-unsaturated ketone O-al-
koxycarbonyl or O-pentafluorobenzoyl oximes to
afford dihydropyrrole products.[10a] When the reaction
was performed in the presence of an excess amount
of LiBr·H2O, 2-bromomethyldihydropyrrole products
were obtained in high yields. Inspired by this work,
we envisioned that by using nucleophilic azides as the
nitrogen source, the azidyl group would be introduced
onto the alkene moiety after cyclization.[11–13] Thus, an
intramolecular imination/azidation sequence could be
realized (Scheme 1). As the azidyl group is syntheti-
cally versatile,[11] the thus formed dihydropyrrole
products could serve as precursors for other useful
[Cu(CH3CN)4PF6]-catalyzed reaction of g,d-unsatu-
rated ketone O-benzoyl oximes with trimethylsilyl
azide (TMSN3). The reaction proceeds via the
À
À
copper-mediated N O cleavage and subsequent C
N forming 5-exo cyclization. The thus formed inter-
mediate is then azidated to afford the correspond-
ing dihydropyrrole product. Preliminary mechanis-
tic investigations suggest that the cyclization step
does not involve a radical intermediate.
Keywords: azidation; copper catalysis; cyclization;
dihydropyrroles; g,d-unsaturated O-benzoyl ketox-
imes
Olefin diamination reactions have received much at- compounds in organic synthesis.
tention from organic chemists due to the importance
To achieve this goal, we initiated the study by
of 1,2-diamine-bearing compounds.[1] As a result, sig- choosing TMSN3 and NaN3 as the azidation agents,
nificant progress has been made recently in this and several low-valent copper species such as
aspect with the development of new transition metal- CuBr·SMe2, Cu(CH3CN)4PF6 (shorten as CuPF6
catalyzed proc2] When the olefin diamination is realiz- below), Cu powder and Cu2O as catalyst to promote
ed in an intramolecular manner by tethering one (or the reaction of (E)-1-phenylpent-4-en-1-one O-benzo-
two) nitrogen atom(s) with the alkene moiety, func- yl oxime (1a).[14] Preliminary results showed that
tionalized nitrogen heterocycles will be furnished. As when TMSN3 was used as the azide source, the de-
such, several important methodologies employing pal- sired reaction could happen, but NaN3 was invalid
ladium,[3] nickel,[4] gold,[5] and copper[6] as catalyst under the same conditions. CuPF6 was the most effec-
have been developed for this purpose. However, tive catalyst among these copper species. When 1a
these studies mainly deal with unsaturated amide or
sulfonamide derivatives, whereas reactions involving
other types of nitrogen sources have much less been
explored.
À
Oxime derivatives can be cleaved at the N O bond
by the action of transition metals to form active
iminyl intermediates, which can be tailored to form
new C N bonds. Copper salts are effective catalysts g,d-unsaturated O-acyl oximes
Scheme 1. Expected intramolecular imination/azidation of
[7]
À
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Table 1. Screening of conditions for the reaction of 1a.[a]
Entry
Copper
(equiv.)
Additive
Solvent
Yield of
2a [%][b]
1
2
3
4
5
6
7
8
CuPF6 (0.2)
Cu2O (0.2)
Cu (0.2)
none
none
none
none
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
54
23
13
24
66
47[c]
58[d]
58
42
34
53
50
60
55
53
58
40
37
10
50
Figure 1. Structure of ligands L1–L7.
CuBr (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.1)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
CuPF6 (0.2)
was treated with 3.0 equiv. of TMSN3 and 0.2 equiv. of
CuPF6 in 1,2-dichloroethane (DCE) at 808C, the de-
sired product 2a was obtained in a yield of 54%.
Using other solvents such as toluene, dichlorome-
thane (DCM), 1,4-dioxane, acetonitrile and dime-
thoxyethane (DME) delivered inferior results. To fur-
ther improve the yield of 2a, we evaluated the effect
of several common copper ligands including 1,3-dicar-
bonyl compounds L1, L2, L3 and L4, picolinic acid
(L5), 2,2’-bipyridine (L6) and TMEDA (L7) (Figure 1).
It was found that the yield of 2a could be enhanced
by using 0.2 equiv. of L1 together with 0.2 equiv. of
CuPF6 (Table 1, entry 5). The yield was somewhat
lower when smaller amounts of TMSN3 were used.
Other ligands were less effective in comparison with
L1. The representative screening experiments are
summarized in Table 1.
L1 (0.2)
L1 (0.2)
L1 (0.2)
L1 (0.4)
L1 (0.2)
L1 (0.2)
L1 (0.2)
L1 (0.2)
L2 (0.2)
L3 (0.2)
L4 (0.2)
L5 (0.2)
L6 (0.2)
L7 (0.2)
L1 (0.2)
L1 (0.2)
L1 (0.2)
L1 (0.2)
9
DCE
10
11
12
13
14
15
16
17
18
19
20
21
22
DCE[e]
DCE[f]
DCE[g]
DCE
DCE
DCE
DCE
DCE
DCE
DCM
toluene
acetonitrile
1,4-dioxane
[h]
–
[h]
–
The optimized conditions (Table 1, entry 5) were
then applied to a variety of g,d-unsaturated O-benzo-
yl oxime derivatives 1b–1u, and the results are listed
in Table 2. In general, the cyclization products were
obtained in moderate to good yields. For 1-aryl-substi-
tuted oxime derivatives, the yield was lower when the
aryl ring was an electron-rich furan or thiophene (en-
tries 5 and 6). Better results were obtained when the
substrates were disubstituted with methyl groups at
the a-position (entries 12–19), which reflects the fa-
vorable Thorpe–Ingold effect. On the other hand, if
the a-carbon is tertiary, the azidation will take place
at the a-position as well (entries 8, 9 and 20). The
products 3h, 3i and 3t were isolated from the reaction oxidative addition of Cu(I) to the N O bond. In the
mixture as single isomers. It is also interesting to see present cases, two mechanisms can be proposed ac-
that for substrates 1k, 1n and 1s, the reaction deliv- cording to the indicated literature to account for the
ered olefination product 4 as well as azidation prod- formation of 2. They are illustrated in Scheme 2. In
uct 2 (entries 11, 14, 19). The olefination products like the first mechanism, the reaction is initiated by single
4 are generally formed from the palladium-catalyzed electron transfer (SET) between 1 and Cu(I), fol-
[a]
The reaction was performed in a 0.05M solution of 1a
and 3.0 equiv. of TMSN3 contained in a sealed tube at
808C unless otherwise specified. Compound 1a has the E
configuration.[14]
chloromethane.
Isolated yield.
DCE=dichloroethane;
DCM=di-
[b]
[c]
[d]
[e]
[f]
1.0 equiv. of TMSN3 was used.
2.0 equiv. of TMSN3 were used.
At room temperature.
At 508C.
At 1008C.
Starting material decomposed.
[g]
[h]
[8]
À
À
cyclization of g,d-unsaturated ketone pentafluoroben- lowed by the N O cleavage to deliver iminyl radical
zoyl oximes.[7a,b,15] When compounds 1p and 1q (en- B [path (a)] The latter is then converted to carbon
tries 16 and 17) were subjected to the present condi- radical C in the cyclization step [path (c)]. In the SET
tions, the benzoyloxy transfer products 5p and 5q step, Cu(I) is oxidized to Cu(II), which takes the form
were also generated, respectively, besides 2p and 2q.
The Cu(I)-catalyzed and O-acyl oxime-involved C
N bond forming reactions are interesting from the cated by path (b), the first step involves the oxidative
of Cu(II)N3 in the presence of TMSN3. The oxidation
of C by Cu(II)N3 finally gives 2. Alternatively, as indi-
À
À
mechanistic point of view. The reaction might initially addition of Cu(I) to the N O bond in 1, resulting in
generate iminyl radicals,[10,15d,16,17] or proceed via the the formation of intermediate D. After ligand ex-
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Copper-Catalyzed Cyclization and Azidation
Table 2. Copper-catalyzed cyclization of compounds 1.[a]
[a]
[b]
[c]
[d]
The reaction was performed on a 0.5 mmol scale.
Isolated yield.
Relative configuration was undetermined.
The product was isolated as a single isomer, but its relative configuration was undetermined.
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Hailin Su et al.
Scheme 2. Proposed reaction mechanism for the formation of 2.
ment demonstrated, 1v was mainly converted to com-
pounds 2v and 5v, and only tiny amounts of ring-
opening products were detected after reaction
(<5%). This result suggests that the cyclization step
is less likely a free radical process. Thus, we believe
that path (b) represents a more reasonable mecha-
nism for the present reaction. Ligand L1 might help to
stabilize the Cu(III) species as the yields of products
2 were lower in the absence of it (Table 1, entry 1).
Besides this experiment, we also investigated the
reaction of 1l in the presence of 1,4-cyclohexadiene
(CHD) and 2,2,6,6-tetramethylpiperidine 1-oxyl radi-
cal (TEMPO) (Scheme 4 and Scheme 5). It was ex-
pected that if the cyclization involved generation of
carbon radical C, reduction product 6l would be ob-
tained when CHD was present in the reaction sys-
Scheme 3. The radical clock experiment with compound 1v.
À
À
change between PhCO2 and N3 (from D to E) and tem;[10a,16] and C could also be captured by TEMPO.
subsequent cyclization (and/or through an alternative However, our result showed that when the reaction
sequence of D to F to G), G is produced, which then was performed with 10 equiv. of CHD under other-
undergoes reductive elimination to afford 2. A similar wise the same conditions, only compound 2l was gen-
organic Cu(III) species has been proposed for the erated in a yield of 68% [Scheme 4, (a)]; no 6l was
copper-catalyzed intermolecular olefin aminoacetoxy- obtained even in the absence of TMSN3 [Scheme 4,
lation by Blakey et al.[18] It is also possible that D or (b)]. In the latter case, 5l was formed in a yield of
E is generated through the binding of iminyl radical 28%. The experiment with TEMPO also failed to cap-
B with Cu(II) [path (d)]. At the current stage, howev- ture radical intermediate C (Scheme 5).
er, these two pathways [path (b) and path (d)] cannot
As shown in Table 2, for substrates bearing an in-
be distinguished. Compounds 4, as generated in the ternal aryl-substituted alkene unit such as 1p and 1q,
reaction of 1k, 1n and 1s, might as well derived direct- the reaction afforded 5p and 5q apart from the corre-
ly from intermediates G and F.
sponding azidation products. This phenomenon can
To shed light on the reaction mechanism, com- also be accounted for with the reaction mechanism
pound 1v was prepared and treated with CuPF6 under shown in Scheme 2. We think that the carbons bound
the indicated reaction conditions (Scheme 3). We as- exocyclic to copper in intermediate F and G are of
sumed that if the reaction followed the radical process carbocationic character in the subsequent product-
represented by path (a) and (c), ring-opening prod- forming reductive elimination step, and this process
ucts would be obtained.[19,20] However, as the experi- would be facilitated by the benzene ring attached to
4
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Copper-Catalyzed Cyclization and Azidation
Scheme 4. Reaction of 1l in the presence of CHD.
Scheme 5. Reaction of 1l in the presence of TEMPO.
Scheme 6. Control experiments with 1h.
it. Therefore, in the cases of 1p and 1q, the reductive
elimination at the stage oÀf F became competitive with
ligand exchange with N3 , and thus 5p and 5q were absence of oxygen, 1h itself would act as oxidant to
formed. The findings that, in the reaction of 1v, 5v be- recycle the copper. The fact that only 1h, 1i and 1t
cames the major product is also consistent with this can be transformed to compounds 3 indicates that the
hypothesis: the cyclopropyl group can be a stronger presence of a substituent at the a position is a prereq-
carbocation-stabilizing group than the benzene uisite for the a-azidation to take place. Our control
ring,[21] so the ligand coupling of F during the reaction experiment also showed that ligand L1 had a beneficial
of 1v was more favored. Furthermore, this result sug- effect on this reaction; the yield decreased to some
gests that process from D to G shown in Scheme 2 extent in the absence of it [Scheme 6, (b)]. A tenta-
probably would pass through intermediate F.
tive mechanism is proposed in Scheme 7 to account
When 1h, 1i and 1t were used as the substrate, the for the formation of 4.
reactions yielded 3h, 3i and 3t, respectively. This
In summary, we have demonstrated that by using
result is surprising because the introduction of the TMSN3 as the azidating agent, the Cu(I)-catalyzed
second azido group at the a-position requires an oxi- cyclization of g,d-unsaturated ketone O-benzoyl
dative process. Although the mechanism of this oxime derivatives can be achieved in an intramolecu-
second azidation is still unclear, we believe that the
active oxidant in this system is the Cu(II) or Cu(III)
species, which, as suggested in Scheme 2, are generat-
ed in situ during the reaction. However, as CuPF6 was
used in only 0.2 equiv., apparently another stoichio-
metric oxidant is required. Our reactions were per-
formed in a sealed tube, which was not pre-deaerated
before heating.[22] It is possible the air present in the
reaction tube played the role of terminal oxidant to
recycle the Cu(II) or Cu(III) species. To test this hy-
pothesis, we did control experiment with 1h by bub-
bling the DCE solvent with argon for 15 min. before
reaction. Still, compound 3h was obtained after reac-
tion, but the yield was much lower [Scheme 6, (a)].
Apart from 3h, no other product could be isolated in
Scheme 7. Proposed mechanism for the formation of 3h, 3i
pure form and structurally identified. Probably, in the and 3t.
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796–799; h) B. Zhang, A. Studer, Org. Lett. 2014, 16,
1790–1793.
lar imination/azidation pattern, affording the azide-
functionalized dihydropyrrole products in moderate
to good yields. There is evidence to suggest that the
cyclization step does not involve a radical intermedi-
ate. Further investigation is being done in this labora-
tory with an aim to develop an enantioselective ver-
sion of this reaction.
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General Experimental Procedure for the Copper-
Catalyzed Reaction of 1
A
suspension of g,d-unsaturated O-benzoyl ketoximes
1 (0.5 mmol, 1.0 equiv.), CuPF6(CH3CN)4 (37 mg, 0.1 mmol,
0.2 equiv.), TMSN3(173 mg, 1.50 mmol, 3.0 equiv.), and L1
(ethyl 2-oxocyclohexanecarboxylate) (17 mg, 0.1 mmol) in
1,2-dichloroethane (10 mL) was stirred at 808C (oil bath
temperature). After the reaction was complete (in 7 h) as
monitored by TLC, the mixture was cooled to 208C, and
then quenched with a saturated aqueous solution of Na2CO3
(15 mL). The organic and the aqueous layers were separat-
ed, and the latter was extracted with CH2Cl2 (15 mLꢂ3).
The combined organic layers were washed sequentially with
a saturated aqueous solution of Na2CO3 (15 mLꢂ3) and
water (15 mLꢂ3). The organic phase was dried over
Na2SO4, filtered and the filtrate was evaporated under re-
duced pressure with a rotatory evaporator. The residue was
purified by column chromatography to give 2 and (others).
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Acknowledgements
The authors thank the National Natural Science Foundation
of China (No. 21372108) for financial support.
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[20] W. Li, P. Jia, B. Han, D. Li, W. Yu, Tetrahedron 2013,
69, 3274–3280.
[21] F. A. Carey, R. J. Sundberg, Advanced Organic Chemis-
try Part A, 5th edn., Springer, Heidelberg, Berlin, 2007,
p 427.
[14] As previous studies indicated, both E and Z isomers
can react under copper catalysis in the same way.[8,10]
[15] For recent examples, see: a) A. Faulkner, J. F. Bower,
Angew. Chem. 2012, 124, 1707–1711; Angew. Chem. Int.
Ed. 2012, 51, 1675–1679; b) A. Faulkner, J. S. Scott, J. F.
Bower, Chem. Commun. 2013, 49, 1521–1523; c) N. J.
Race, J. F. Bower, Org. Lett. 2013, 15, 4616–4619.
d) After the revised manuscript was submitted we
found that a copper-catalyzed version of this transfor-
mation had been reported very recently by the same
authors, see: A. Faulkner, N. J. Race, J. S. Scott, J. F.
Bower, Chem. Sci. 2014, 5. 2416–2421.
[22] A control experiment indicates that performing the re-
action under an argon atmosphere has little effect for
other substrates.
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