Cleavage of the C=N bond in carbodiimides via release of high ring strain: A new strategy for the selective synthesis of 2-aminoaryl alkynyl imines

Article abstract of DOI:10.1002/chem.201304701

A novel pattern of the cleavage and reorganization of C=N bond in the multicomponent reaction (MCR) of terminal alkynes or haloalkynes, carbodiimides, and benzynes is achieved for the first time to construct efficiently 2-aminoaryl alkynyl imines. The selective formation and ring-opening of the azetine intermediate with the high ring strain is essential for this reaction. Further transformation of 2-aminoaryl alkynyl imines via the Cu-catalyzed cycloisomerization is explored to provide steroselectively the bi-, tri-, and tetracyclic fused pyrrolines. A novel pattern in the cleavage and reorganization of the C=N bond in the multicomponent reaction of terminal alkynes or haloalkynes, carbodiimides, and benzynes is achieved for the first time to construct efficiently 2-aminoaryl alkynyl imines. The selective formation and ring-opening of the azetine intermediate with high ring strain is essential for this reaction (see scheme; Tf=trifluoromethanesulfonyl, TMS=trimethylsilyl). Copyright

Full text of DOI:10.1002/chem.201304701

DOI: 10.1002/chem.201304701
Communication
&
Multicomponent Reactions
Cleavage of the C=N Bond in Carbodiimides via Release of
High Ring Strain: A New Strategy for the Selective Synthesis of
2-Aminoaryl Alkynyl Imines
Yi Zhou,[a] Yue Chi,[a] Fei Zhao,[a] Wen-Xiong Zhang,*[a, b] and Zhenfeng Xi[a]
Abstract: A novel pattern of the cleavage and reorganiza-
tion of C=N bond in the multicomponent reaction (MCR)
of terminal alkynes or haloalkynes, carbodiimides, and
benzynes is achieved for the first time to construct effi-
ciently 2-aminoaryl alkynyl imines. The selective formation
and ring-opening of the azetine intermediate with the
high ring strain is essential for this reaction. Further trans-
formation of 2-aminoaryl alkynyl imines via the Cu-cata-
lyzed cycloisomerization is explored to provide steroselec-
tively the bi-, tri-, and tetracyclic fused pyrrolines.
The cleavage of C=N double bonds is of significant synthetic
interest, and is a more challenging issue than that of CÀN
single bonds because of the stronger bond energy.[1] In con-
trast to the cleavage of CÀN single bonds,[1] the cleavage of
C=N double bonds is much less explored.[2–5] Imines, which
have the typical C=N double bonds, could undergo through
acid or transition-metal mediated metathesis to cleave C=N
double bonds.[2] In case of carbodiimide having the cumulative
C=N double bonds,[6,7] three types of the cleavage and reor-
ganization of C=N double bonds have been reported. At the
early stage, the cleavage of C=N double bonds of carbodiimide
were reported in the stoichiometric metathesis reaction of car-
bodiimide with transition-metal imido complexes or unsaturat-
ed substrates to reconstruct different carbodiimide derivatives
(Scheme 1a).[3] The dual nucleophilic addition/elimination
could also lead to the cleavage of C=N double bonds
(Scheme 1b).[4] We reported the cleavage and reorganization
of C=N and C(sp3)ÀH bonds in the reaction of carbodiimide
with lithium alkynethiolate to efficiently construct 2,3-dihydro-
pyrimidinthiones (Scheme 1c).[5] In this process, these three
cleaved fragments were reconnected by the formation of two
Scheme 1. Reconnected models of the cleaved fragments.
CÀN single bonds, one C=C double bond, and one CÀH bond.
However, as far as we are aware, the reconnected pattern be-
tween “NR1” and “C=NÀR2” units is not reported (Scheme 1d).
Although recent years have witnessed a significant develop-
ment of aryne chemistry, the reaction chemistry of carbodii-
mides with benzynes is not reported.[8,9] Herein, we wish to
report a room temperature and transition-metal-free multicom-
ponent reaction of readily available terminal alkynes or haloal-
kynes, carbodiimides, and two molecules of benzynes to con-
struct 2-aminoaryl alkynyl imines. The alkynyl imines are useful
synthons in organic synthesis because of their polyfunctionali-
ty,[10,11] however, such 2-aminoaryl alkynyl imines could not ac-
cessed by other methods. In this process, two fragments
formed via the cleavage of C=N bond of the carbodiimide are
both incorporated into 2-aminoaryl alkynyl imines by the for-
mation of one CÀH (Br and I), two CÀN, and two CÀC bonds.
The selective formation and ring opening of the azetine inter-
mediate with the high ring strain is essential for this reaction.
Furthermore, the mechanism and application of 2-aminoaryl
alkynyl imines are explored.
The multicomponent coupling[12] of phenylethyne (1a), N,N’-
diisopropylcarbodiimide (DIC, 2a), 2-(trimethylsilyl) phenyltri-
flate (3a) was chosen as a model to establish the reaction con-
ditions (Table 1). As control experiment, no reaction was ob-
served between 1a and 2a or between 1a and 3a in the pres-
ence of CsF in MeCN at room temperature (entries 1 and 2).
However, the reaction of 2a with 3a gave two hydrolysis prod-
ucts 5a and 6a (entries 3 and 4). Interestingly, the multicom-
ponent coupling of 1a, 2a, and 3a yielded the unexpected al-
[a] Y. Zhou, Y. Chi, Dr. F. Zhao, Prof. Dr. W.-X. Zhang, Prof. Dr. Z. Xi
Beijing National Laboratory for Molecular Sciences (BNLMS),
Key Laboratory of Bioorganic Chemistry
and Molecular Engineering of Ministry of Education, College of Chemistry
Peking University, Beijing 100871 (P.R. China)
Fax: (+86)10-62751708
E-mail: wx_zhang@pku.edu.cn
[b] Prof. Dr. W.-X. Zhang
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304701.
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In addition to aryne precursor 3a, other aryne pre-
cursors have been tested under the optimized reac-
tion conditions to provide the corresponding prod-
ucts 4v–x in high yields (Table 3).
Table 1. Condition screening.[a]
Although the 2-aminoaryl alkynyl imines could, in
1
principle, have two possible isomers E or Z, the H
and 13C NMR spectra of the resulting 2-aminoaryl al-
kynyl imine compounds (4a–x) suggested that only
one isomer is present for each compound in solution.
The X-ray structure analyses of 4h and 4a-Br re-
vealed that it is E in the solid state (Figure 1).
Entry Substrates
n
FÀ source
Solvent T [8C] t [h] Yield [%][c]
4a
5a
6a
1
1a+2a
0
1
1
2
2
2
2
2
3
1
2
2
CsF
CsF
CsF
CsF
CsF
TBAF
MeCN
MeCN
MeCN
MeCN
MeCN
THF
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
12
12
12
12
12
12
9
9
9
9
9
–
–
0
0
82
8
95
64
83
63
0
–
–
–
–
2
1a+3a
In order to gain mechanistic insight into the pres-
ent MCRs, a deuterium labeling experiment was con-
ducted. When phenylacetylene-D was used under the
standard conditions, the deuterated product 4a-D
was isolated in 87% yield with 95% D-incorporation
on the ortho position of the phenyl ring (reaction 1).
Then, we aimed to isolate and characterize the
possible intermediates by exploring the different sub-
strates. When N-phenyl-N’-cyclohexyl carbodiimide
was allowed to react with benzyne and phenyle-
thyne, 4y was formed in only 20% yield. In the
above reaction, 9-imino-9,10-dihydroacridine 7 cou-
pled by two benzynes and one carbodiimide was iso-
lated as the major product in 58% yield (reaction 2).
However, no reaction was observed between 7 and
3
2a+3a
29
22
0
67
0
11
0
13
33
48
12
21
trace
23
10
17
4
2a+3a
5
6
7
8
1a+2a+3a
1a+2a+3a
1a+2a+3a
1a+2a+3a
1a+2a+3a
1a+2a+3a
1a+2a+3a
1a+2a+3a
KF, [18]crown-6 THF
KF, [18]crown-6 THF[d]
KF, [18]crown-6 THF
KF, [18]crown-6 THF
KF, [18]crown-6 THF
KF, [18]crown-6 THF
9
10
11
12
trace trace
60
9
trace trace trace
[a] Conditions: terminal alkynes (0.5 mmol), carbodiimide (0.5 mmol), benzyne precur-
sors, fluoride source (1.2 mmol) in solvent (3 mL), unless otherwise noted. [b] H2O
(5 mmol). [c] Isolated yields. [d] THF was not dried.
kynyl imine 4a in 82% yield of the isolated product followed
by a small amount of 6a. After many screening experiments,
the optimal condition was established. Thus, as shown in
entry 7, the mixture of 1a, 2a, and 3a (2 equiv) was treated
with the effective fluoride source KF/[18]crown-6 (2.4 equiv) in
THF at room temperature for 9 h to give 4a in 95% yield.
Summarized in Table 2 are representative results from the
MCRs among terminal alkynes, carbodiimides, and 2-(trimethyl-
silyl) phenyltriflate (3a). Carbodiimides, such as iPrN=C=NiPr,
CyN=C=NCy, tBuN=C=NtBu could be used as suitable sub-
strates to yield the corresponding compounds 4a–c in moder-
ate to high isolated yields. When the unsymmetrical carbodii-
mide, such as tBuN=C=NEt was applied, the sole regioisomer
4d was obtained in 74% isolated yield. The exclusive forma-
tion of 4d is ascribed to the steric hindrance of tert-butyl
group resulting in the prior reactivity of benzyne with the C=N
double bond adjoining the ethyl group of the carbodiimide. A
large range of polyfunctional terminal alkynes could be used
for the MCRs. The reaction was not affected by either electron-
withdrawing or electron-donating substituents or their posi-
tions at the phenyl ring (4e–k). Naphthalenyl or thiophenyl
substituted terminal alkynes were also applicable to yield the
corresponding products 4l and 4m. In addition, a wide variety
of the aliphatic alkynes also smoothly underwent this reaction
to generate the corresponding products 4n–t. The keto group
in 4u survived the present conditions. It was noted that, in ad-
dition to terminal alkynes, bromoalkyne or iodoalkyne could
also act as suitable substrates under the same conditions to
provide the bromo- or iodo-substituted 2-aminoaryl alkynyl
imines 4a-Br, 4n-Br, and 4a-I.
phenylethyne under the same condition. This result showed
that the formation of 4y and 7 was a competitive process, and
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of arynes to give the final product 9 with the incor-
poration of two different arynes. Also, the formation
of 9 could be achieved by one-pot sequential process
from two different arynes.
Table 2. Formation of the 2-aminoaryl alkynyl imines 4a–u.[a–c]
Based on the above evidence, a proposed mecha-
nism for the formation of 4–6 is shown in Scheme 2
(see the Supporting Information for other possible
pathways). The benzyne precursor first reacts with
fluoride source to generate in situ benzyne. Then an
azetine intermediate A is formed via [2+2]-cycloaddi-
tion between C=N double bond of the carbodiimide
with benzyne.[13] Owing to the high ring strain, the
regioselective ring-opening of A leads to the forma-
tion of A’. The addition of a terminal alkyne to A’
could undergo through the six-membered ring transi-
tion state B to yield the intermediate C. The subse-
quent NÀH bond arylation of C by benzyne should
give the final product 4. In the presence of the trace
of water, 5 is easily observed. The subsequent reac-
tion of 5 with benzyne gives 6.
Z=H
4a: R1 =R2 =iPr, 95%
4b: R1 =R2 =Cy, 78%
4c: R1 =R2 =tBu, 90%
4d: 74%
4e: R’ =4-Me, 80%
4 f: R’ =4-MeO, 82%
4g: R’ =4-F, 82%
4h: R’ =3-Cl, 82%
4i: R’ =2-Br, 93%
4j: R’ =4-CF3, 92%
4k: R’ =4-CN, 79%
Further transformation of 2-aminoaryl alkynyl
imines was explored (Scheme 3). When N-isopropyl
alkynyl imine 4n was subjected to the Cu-catalyzed
cycloisomerization conditions,[10e,i] the bicyclic fused
pyrroline 10a was formed exclusively in good yield.
Similarly, the Cu-catalyzed cycloisomerization of
N-cyclohexyl alkynyl imine 4p or N-cyclohexyl 2-cy-
clopentylethynyl imine 4r provided the correspond-
ing tricyclic fused pyrroline 10b and the tetracyclic
fused pyrroline 10c in high isolated yields, respec-
tively. The X-ray structure of 10c revealed unambigu-
ously the configuration of the resulting four-mem-
bered ring (Figure 2). The mechanism for the forma-
tion of 10a–c is similar to the process proposed by
Gevorgyan,[10e,i] which possibly undergoes the base-
promoted propargyl-allenyl isomerization/two se-
quential [1,5]-H shift/CuI-catalyzed [2+2]-cycloaddi-
tion (see the Supporting Information for the detailed
mechanism). These results are in striking contrast
with what was observed previously for N-isopropyl al-
kynyl imines possessing an alkyl group at the propar-
gylic position providing the mixtures of the bicyclic
fused pyrrolines in low yields and pyrroles. As far as
we are aware, it is the first time that tri- or tetracyclic
fused pyrrolines are prepared in high yields from al-
kynyl imines. This is probably owing to the strong
electron-donating character of the ortho-PhRN sub-
stituent promoting two sequential 1,5-H shifts for the
formation of fused pyrrolines.
4l: 88%
4o: 73%
4r: 72%
4u: 84%
4m: 75%
4p: 77%
4s: 80%
4n: 70%
4q: 76%
4t: 73%
Z=Br, I
4a-Br: 71%
4n-Br: 63%
4a-I: 58%
[a] Conditions: terminal alkynes (0.5 mmol), carbodiimide (0.5 mmol), benzyne precur-
sor (1.0 mmol, KF (1.2 mmol), and [18]crown-6 (1.2 mmol) in THF (3.0 mL) at room
temperature for 9 h, unless otherwise noted. [b] Yields of isolated product. [c] THF
was dried.
In summary, a one-pot multicomponent reaction of
readily available terminal alkynes, carbodiimides, and
two molecules of benzynes is achieved for the first
also it excluded the possibility between two benzynes and one
C=N bond of carbodiimide for the present reaction process. In-
terestingly, when 4,5-dimethoxy aryne precursor was utilized,
we successfully isolated the product 8 with the NÀH moiety.
Next the isolated products 8 was treated with one equivalent
time to afford 2-aminoaryl alkynyl imines. The reaction has the
following features: 1) is transition-metal free, 2) has mild reac-
tion conditions, 3) is highly efficient, and 4) a new reconnected
pattern between “NR1” and the “C=NÀR2” units is formed via
C=N bond cleavage of the carbodiimide. The selective forma-
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Table 3. Formation of the 2-aminoaryl alkynyl imines 4v–x.[a–c]
4v 87%
4w: 79%
4x: 65%
[a] Conditions: terminal alkynes (0.5 mmol), carbodiimide (0.5 mmol),
aryne precursors (1.0 mmol), KF (1.2 mmol), and [18]crown-6 (1.2 mmol)
in THF (3.0 mL) at room temperature for 9 h, unless otherwise noted.
[b] Yields of isolated product. [c] THF was dried.
Scheme 2. A possible mechanism for the formation of 4–6.
Scheme 3. Synthesis of bi-, tri-, tetracyclic fused pyrrolines 10a–c via
Cu-catalyzed cycloisomerization.
tion and ring opening of the azetine intermediate with the
high ring strain is essential for this reaction. Further transfor-
mation of 2-aminoaryl alkynyl imines via copper-catalyzed
cycloisomerization is explored to provide steroselectively the
bi-, tri-, and tetracyclic fused pyrrolines.
Experimental Section
General procedure for the multicomponent coupling of
terminal alkynes or haloalkynes, carbodiimides, and arynes
Figure 1. ORTEP drawing of 4h (top) and 4a-Br (bottom) with 30% thermal
ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond length (ꢁ):
4h: C1ÀC8 1.439(3), C1ÀC16 1.492(3), C8ÀC9 1.193(3), C1ÀN1 1.286(3),
C2ÀN1 1.463(3), C5ÀN2 1.472(3), C21ÀN2 1.433(3), C22ÀN2 1.403(3), C14ÀCl1
1.743(3); 4a-Br: C1ÀC8 1.456(7), C1ÀC16 1.508(6), C8ÀC9 1.189(6), C1ÀN1
1.250(6), C2ÀN1 1.471(6), C5ÀN2 1.487(6), C21ÀN2 1.425(6), C22ÀN2 1.416(6),
C23ÀBr1 1.901(5).
KF (1.2 mmol, 69.7 mg) was added to a stirred solution of terminal
alkynes or haloalkynes 1 (0.5 mmol), carbodiimides 2 (0.5 mmol), 2-
(trimethylsilyl)aryl triflates 3 (1.0 mmol), and [18]crown-6 (1.2 mmol,
317.2 mg) in THF (3 mL) under a nitrogen atmosphere. The reac-
tion mixture was stirred at room temperature for 9 h, and the re-
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Figure 2. ORTEP drawing of 10c with 30% thermal ellipsoids. Hydrogen
atoms, except those on the C6 and C12 atoms, are omitted for clarity. Select-
ed bond length (ꢁ): C1ÀC6 1.563(5), C1ÀC12 1.580(5), C6ÀC7 1.555(5),
C12ÀC13 1.536(5), C13ÀC14 1.520(5), C7ÀN1 1.476(5), C14ÀN1 1.289(5),
C16ÀN2 1.450(5), C21ÀN2 1.467(4), C27ÀN2 1.399(5).
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sulting mixture was evaporated under vacuum. The residue was
purified by column chromatography on silica gel (petroleum ether/
ethyl acetate=20:1) to give products 4.
Synthesis of 10a–c
In
a 25 mL flask, CuI (0.09 mmol, 17 mg), anhydrous DMA
(1.50 mL), and 2-aminoaryl alkynyl imines 4n, 4p, or 4r
(0.30 mmol) were successively added. After CuI was dissolved, an-
hydrous Et3N (0.22 mL) was added. The mixture was stirred at
110 8C for 12 h with protection from light. When the reaction was
complete, the mixture was cooled to room temperature and H2O
(10 mL) was added. After the above mixture was extracted with
hexane (3ꢂ5 mL), the combined organic phase was dried over
MgSO4. After the solvent was evaporated under reduced pressure,
the residue was purified over a column of silica gel (petroleum
ether/ethyl acetate=10:1) to afford the pure fused pyrrolines
10a–c.
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Acknowledgements
This work was supported by the Natural Science Foundation of
China, and the 973 Program (2011CB808600). We also thank
Prof. Zhang-Jie Shi of Peking University and Prof. Feng Shi of
Henan University for useful discussion.
Keywords: alkynes
·
benzynes
·
C=N activation
·
carbodiimides · multicomponent reactions
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Received: December 1, 2013
Published online on February 2, 2014
Chem. Eur. J. 2014, 20, 2463 – 2468
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2468
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim