Stereodivergent Synthesis of N-Heterocycles by Catalyst-Controlled, Activity-Directed Tandem Annulation of Diazo Compounds with Amino Alkynes

Article abstract of DOI:10.1002/anie.201507122

A stereodivergent synthesis of five-membered N-heterocycles, such as 2,3-dihydropyrroles, and 2-methylene and 3-methylene pyrrolidines, has been developed through a tandem annulation of amino alkynes with diazo compounds and involves the trapping of in situ formed intermediates. Mechanistic investigations indicate that the copper-catalyzed tandem annulations proceed by allenoate formation and subsequent intramolecular hydroamination. In contrast, the rhodium-catalyzed protocol features a carbenoid insertion into the N-H bond and subsequent Conia-ene cyclization. Different paths: A stereodivergent synthesis of five-membered N-heterocycles has been developed and involves trapping of the in situ formed intermediates. The copper-catalyzed annulation proceeds by allenoate formation and subsequent intramolecular hydroamination. The rhodium-catalyzed route features a carbenoid insertion into the N-H bond and subsequent Conia-ene cyclization. PG=protecting group.

Full text of DOI:10.1002/anie.201507122

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201507122
German Edition: DOI: 10.1002/ange.201507122
Heterocycles
Stereodivergent Synthesis of N-Heterocycles by Catalyst-Controlled,
Activity-Directed Tandem Annulation of Diazo Compounds with
Amino Alkynes
Kai Liu, Chenghao Zhu, Junxiang Min, Shiyong Peng, Guangyang Xu, and Jiangtao Sun*
Dedicated to Professor Hongwen Hu on the occasion of his 90th birthday
Abstract: A stereodivergent synthesis of five-membered N-
heterocycles, such as 2,3-dihydropyrroles, and 2-methylene and
3
-methylene pyrrolidines, has been developed through
a tandem annulation of amino alkynes with diazo compounds
and involves the trapping of in situ formed intermediates.
Mechanistic investigations indicate that the copper-catalyzed
tandem annulations proceed by allenoate formation and
subsequent intramolecular hydroamination. In contrast, the
rhodium-catalyzed protocol features a carbenoid insertion into
the NÀH bond and subsequent Conia-ene cyclization.
T
he in situ trapping of an active intermediate, generated
from diazo compounds with diverse functionality, by another
nucleophile/electrophile has promoted the discovery of
synthetically useful transformations, which allow rapid assem-
bly of structurally complex molecules from readily available
[
1]
starting materials. For example, by trapping the protic
onium ylides, generated from metal carbenoid XÀH inser-
tions (X = N, O, S etc.), with different electrophiles, Hu and
co-workers developed an array of novel multicomponent
Scheme 1. Strategies on in situ trapping of reaction intermediates.
PG=protecting group.
reactions
Scheme 1). Ni and Montgomery reported a [4+2+1] cyclo-
addition by nickel-catalyzed tandem vinyl nickel carbene
for
accessing
polyfunctional
molecules
[
2]
(
[3]
formation and cyclization. Saµ and co-workers successively
described novel cyclizations by trapping the vinyl ruthenium
carbene intermediates generated in situ from alkyne and
tion of such N-heterocyles generally requires diverse strat-
[7]
egies from quite different starting materials. Thus, a direct,
controllable, and efficient synthetic method utilizing similar
starting materials to selectively and divergently access such
scaffolds is highly desirable. Herein, we describe a stereo-
divergent strategy by trapping in situ formed intermediates to
construct diverse five-membered N-heterocycles in a single
step.
[
4]
TMSCHN2. Very recently, Moody and co-workers reported
the synthesis of highly substituted tetrahydrofurans by either
a copper- or rhodium-catalyzed carbene OÀH insertion/
[
5]
inramolecular aldol reaction sequence. The high efficiency
of this strategy and the ability to construct multiple bonds in
one step has attracted considerable attention and resulted in
In continuation of our interest in diazo-based carbene
[
6]
[8]
the discovery of many novel transformations.
transfer,
we devised a catalyst-controlled, reactivity-
Five-membered N-heterocycles, such as 2,3-dihydropyr-
roles, 2-methylene pyrrolidines, and 3-methylene pyrroli-
dines, are motifs found in numerous natural products, as well
as in biologically active pharmaceuticals. However, prepara-
directed tandem annulation to access different polysubstitut-
ed five-membered N-herterocylces. We anticipated that there
are two possible pathways for the metal-catalyzed tandem
reaction between diazo compounds and amino alkynes: 1) the
prioritized metal-catalyzed NÀH insertion could generate
active ylide, which would then be trapped by the alkynyl
moiety and undergo a formal 5-exo-dig cyclization to give 3-
methylene pyrrolidines; 2) the metal-catalyzed cross-coupling
of an alkyne with diazo compounds could furnish allenoate
intermediates as the first step, and they would then undergo
either formal 5-exo-dig or 5-endo-dig intramolecular hydro-
amination to give the corresponding cycloaddition products
(Scheme 1). Obviously, the challenges are controlling the
[
+]
[+]
[
*] K. Liu, C. Zhu, J. Min, Dr. S. Peng, G. Xu, Prof. Dr. J. Sun
School of Pharmaceutical Engineering & Life Science
Changzhou University
Changzhou 213164 (China)
E-mail: jtsun08@gmail.com
+
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507122.
1
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Chemie
competitive ring-closing modes and avoiding reaction termi-
ZnCl , only the insertion product 8a was isolated (entry 15).
2
nation at either the first NÀH insertion or cross-coupling
Also, reducing the catalyst loading led to low efficiency
(entry 16). Other rhodium complexes were ineffective in this
stage.
Initially, the reaction of the chiral homopropargyl sulfon-
amide 1a with phenyl diazoacetate (3a) was investigated
reaction. Notably, the [Rh (esp) ]-catalyzed reaction of 2a
and 3a gave the insertion product 8b in 77% yield (entry 19).
2
2
(
Table 1). Screening the reaction parameters indicated that
the tandem annulation occurred in the presence of CuCl
10 mol%) in acetonitrile and provided 2,3-dihydropyrrole
a in 70% yield, with 4.3:1 d.r. and 97:3 e.r. (entry 3). Other
Furthermore, in the presence of 10 mol% of ZnCl , 7m was
isolated in 36% yield together with 22% yield of 8b
(entry 20).
2
(
5
Based on the above optimization (Table 1, entry 3), the
substrate scope for the synthesis of 2,3-dihydropyrroles was
investigated (Table 2). Generally, the chiral homopropargyl
sulfonamides containing either electron-rich or electron-
deficient aryl groups, adjacent to the N-atom, all gave the
corresponding products in moderate to high yields and with
good d.r. values. In particular, 5a and 5b were obtained in
97:3 e.r. and no obvious erosion of enantiopurity was
observed. The thiophene- or furan-substituted chiral sulfon-
amides were also tolerated and gave the desired products (5 f,
5g, and 5n) in moderate yield. The alkyl-substituted chiral
sulfonamides also worked well and afforded the chiral
dihydropyrrolidines in slightly lower yields compared with
copper(I) and copper(II) catalysts gave lower yields. Next, the
reaction of the N-benzyl amino alkyne 2a with 3a was
examined. With CHCl as the solvent, (E)-6a was isolated in
8
also gave (E)-6a in low to moderate yields. The tandem
annulation of 2a with 4a proceeded smoothly to give the 3-
methylene pyrrolidine 7a as a single isomer in 78% yield and
with 99:1 e.r. catalyzed by 3 mol% of [Rh (esp) ] and
3
2% yield with a 98:2 e.r. (entry 9). Other copper catalysts
2
2
[
9]
1
0 mol% of ZnCl in CH Cl (entry 14). The presence of
2 2 2
ZnCl was beneficial to the cyclization. By using 3 mol% of
2
[
Rh (esp) ] alone, 7a was observed in 32% yield together
2 2
with 26% yield of 8a (entry 13). By using In(OTf) instead of
2
[
a,b,c]
Table 1: Optimization of the tandem annulations.
Entry
1
Catalyst
CuI
Alkyne
Diazo
Solvent
Yield [%]
(d.r., e.r.)
5
6 (E/Z, e.r.)
7 (e.r.)
1a
3a
MeCN
5a, 30 (2:1, 97:3)
0
0
(
R=Ts)
(R’=Ph)
3a
3a
3a
3a
2
3
4
5
6
CuBr
CuCl
Cu(OTf)2
Cu(OAc)2
CuBr2
1a
1a
1a
1a
1a
MeCN
MeCN
MeCN
MeCN
MeCN
5a, 47 (3:1, 97:3)
5a, 70 (4.3:1, 97:3)
–
5a, 29 (2.5:1, –)
5a, 26 (3:1, –)
0
0
–
0
0
0
0
–
0
0
3a
7
CuI
2a
3a
CHCl3
0
6a, 45 (1:0, 97:3)
0
(
R=Bn)
8
9
1
1
1
CuBr
CuCl
Cu(OTf)2
Cu(OAc)2
CuBr2
2a
2a
2a
2a
2a
3a
3a
3a
3a
3a
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
0
0
–
0
0
6a, 61 (1:0, 97:3)
6a, 82 (1:0, 98:2)
–
6a, 50 (1:0, 97:3)
6a, 23 (1:0)
0
0
–
0
0
0
1
2
1
3
[Rh (esp) ]
2a
4a
CH Cl
0
0
7a, 32; 8a, 26
2
2
2
2
(
R’=CO Me)
2
1
1
1
1
1
4
5
6
7
8
[Rh (esp) ]/ZnCl
2a
2a
2a
2a
2a
4a
4a
4a
4a
4a
CH Cl
0
0
0
–
–
0
0
0
–
–
7a, 78 (99:1)
7a, <5; 8a, 72
7a, 23
–
–
2
2
2
2
2
2
2
2
2
[Rh (esp) ]/In(OTf)
CH Cl
2
2
3
2
[
d]
[Rh (esp) ]/ZnCl
CH Cl
2
2
2
2
[Rh (OAc) ]/ZnCl
CH Cl
2
4
2
2
[{Cp*RhCl } ]/ZnCl
CH Cl
2
2
2
2
1
2
9
0
[Rh (esp) ]
2a
2a
3a
3a
CH Cl
0
0
0
0
7m, 0; 8b, 77
7m, 36; 8b, 22
2
2
2
2
2
[Rh (esp) ]/ZnCl
CH Cl
2
2
2
2
[
[
a] Reactions were carried out with either 1 or 2 reacting with either 3 or 4 (0.5 mmol), a copper salt (10 mol%) or rhodium catalyst (3 mol%), at 608C.
b] Yield of the isolated single isomer. Reaction time: 12 h for entries 1 to 6; 18 h for entries 7 to 12; 8 h for entries 13 to 20. [c] The d.r. and E/Z values
1
were determined by H NMR analysis of the crude reaction mixture. The e.r. values were determined by HPLC analysis using a chiral stationary phase.
[
d] 1 mol% of [Rh (esp) ]. Cp*=C Me , esp=a, a, a’, a’-tetramethyl-1,3-benzenedipropionic acid, Tf =trifluoromethanesulfonyl, Ts =4-toluene-
2 2 5 5
sulfonyl.
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Table 2: Scope of copper-catalyzed tandem annulation towards dihy-
dropyrroles.
Table 3: Scope of the copper-catalyzed tandem annulation towards
pyrrolidines.
[
a,b]
[a,b]
1
[
a] Yield of purified product. [b] E/Z was determined by H NMR analysis
of the crude reaction mixture. The e.r. values were determined by HPLC
analysis using a chiral stationary phase.
[
a] Yield of the isolated single isomer. [b] The d.r. values were determined
1
by H NMR analysis of the crude reaction products. The e.r. values were
determined by HPLC analysis using a chiral stationary phase.
[a,b]
Table 4: Scope for the rhodium-catalyzed tandem annulation.
the aryl-substituted substrates. The major isomers had an
S,S configuration, which was confirmed by the X-ray structure
[10]
of 5b’ (diastereomer of 5b). Next, nonsubstituted homo-
propargyl sulfonamides were also examined. Comparatively,
electron-rich phenyl diazoacetates gave the cyclization prod-
ucts in higher yields than the electron-poor ones. Notably,
benzyl diazoacetate was also tolerated and delivered 5x in
7
8% yield.
Next, the copper-catalyzed tandem reaction of the diazo
compounds 3 with 2 to provide pyrrolidines (6) was further
performed under the optimal reaction conditions (Table 1,
entry 9) (Table 3). The (S)-N-benzyl-but-3-yn-1-amines 2
containing either electron-rich or electron-poor phenyl
groups adjacent to the N-atom gave the corresponding
chiral pyrrolidines in moderate to high yields. Among them,
6
a and 6b were isolated in 82 and 70% yield, respectively,
[
a] Yield of purified product. [b] The e.r. values were determined by HPLC
with the corresponding e.r. values of 98:2 and 97:3 e.r. The
chiral alkyl- and thiophene-substituted N-benzyl but-3-yn-1-
amines also worked well in this reaction (6g–l). A structurally
hindered N-benzyl amino alkyne was tolerated and gave 6 f in
analysis using a chiral stationary phase.
products (7n and 7o) were not obtained, probably owing to
the inert NÀH insertion step for these two substrates.
7
8% yield. It is noteworthy that the products with an
E configuration were all favored, and was confirmed by X-
To gain insight into the reaction mechanism, further
investigations were performed. In 2004, Suµrez and Fu
reported that, under nonbasic conditions, copper-catalyzed
coupling of terminal alkynes with diazo compounds selec-
[10]
ray structure of 6m.
The substrate scope and the limitations of the rhodium-
catalyzed 3-methylenepyrrolidine formation with a range of
acceptor/acceptor diazo substrates was further investigated
[
11a]
[11b]
tively afforded 3-alkynoates.
and Wang
Later, the groups of Fox
[11c]
(
4
Table 4). The reaction between 2 and the diazocarboxylate
a proceeded smoothly to furnish the corresponding 3-
successively described that the presence of
a base is essential for the highly efficient allene/allenoate
formation. Since the reaction of 1 with 3 gave the 2,3-
dihydropyrroles directly and the reaction intermediates
cannot be isolated, we performed the reaction of alkynes 9
and 10 with 3a and 3k, respectively (Scheme 2). Under the
same reaction conditions as used for the reactions in Table 2,
the allenoates 9a and 10a were isolated in 66 and 72% yield,
methylene pyrrolidines in moderate to high yields and with
well-maintained optical purity (99:1 e.r. for 7a and 7b). Other
dicarbonyl diazo compounds were also examined. The
diazoesters 4b and 4c were well-tolerated and delivered 7k
and 7l in high yields. However, reactions between 2 and either
the diazoketone 4d or 4e were sluggish and the desired
1
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Scheme 2. Copper-catalyzed cross-coupling of alkynes with diazo com-
pounds.
Scheme 4. Gold-catalyzed cyclization of allene/allenoate. EDG=elec-
tron-donation groups, EWG=electron-withdrawing group.
respectively. These experiments disclosed that, different from
former reports, even in the absence of a base the allenoates
were also obtained as the major products for the simple
copper-catalyzed cross-coupling of diazoacetates with alkynyl
Next, additional control experiments were carried out to
help understand the mechanism. Treatment of 8b with CuCl
under standard reaction conditions given in Table 2 did not
afford 6a, and thus indicated that the formation of pyrroli-
dines (6) does not proceed by tandem NÀH insertion/
[12]
sulfonamides.
These experiments also provided strong
evidence that the copper-catalyzed tandem annulation of
cyclization, but by the prioritized coupling of alkyne with
diazo compounds (Scheme 5a). Furthermore, after treatment
1
with 3 probably proceeded via an allenoate intermediate.
In a recent report, Kramer and Fu found that treatment of
of 8a with 10 mol% of ZnCl , 7a was obtained in 84% yield,
2
an allene with a phosphine ligand and PhONa furnished 2,5-
thus suggesting that ZnCl2 could promote the Conia-ene
reaction (Scheme 5b). When 5o was subjected to 3 equiv-
[13]
[16]
dihydropyrrole as a single product (Scheme 3). In contrast,
we found that, after treatment of 9a with one equivalent of
PhONa, the azetidine 11 was obtained in high yield with or
without the addition of a phosphine ligand.
alents of Cs CO , the double bond was isomerized to give
2
3
a product with an exocyclic double bond (13) in 91% yield
(Scheme 5c).
Scheme 3. Cylization of sulfonamide allene/allenoate.
Scheme 5. Control experiments.
[14]
[15]
Previously, the groups of Widenhoefer
and Toste
reported that allenes bearing alkyl substituent(s) at the g-
position can undergo gold(I)-catalyzed intramolecular hydro-
amination at the a-position to produce 2-vinylpyrrolidines
Plausible mechanisms for different annulations have been
proposed (Scheme 6). The reaction of the the homopropargyl
amine with donor/acceptor diazoacetate 3a in the presence of
CuCl generates the intermediate A, which might provide the
alkyne intermediate C by path a1. A formal 5-endo-dig
cyclization then proceeds through TS1 (TS = transition
state) to afford 5. Alternatively, the generation of the
allenoate intermediate D is most likely preferred, and
would undergo either formal 5-exo-dig or 5-endo-dig cyclo-
addition via TS2 to give either 5 or 6, respectively. Exper-
imental results show that N-tosyl amino alkynes are more
likely to form 5, while for N-benzyl amino alkynes, 6 would be
(
Scheme 4). We found that, in the presence of 5 mol% of
PCy AuNTf , the intramolecular hydroamination of the
3
2
allenoate 10a occurred smoothly to deliver the N-tosyl
[10]
tetrahydropyridine 12 in 87% yield. The former reports
and our observations indicate that the substituents on allenes
play a vital role in the ring-closing mode. If an electron-
withdrawing group is present in the g-position, the intra-
molecular hydroamination would occur at the b-position and
proceed by formal 6-exo-dig or 6-endo-dig cyclization/isomer-
ization to give tetrahydropyridine as final product.
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[
[
1
Scheme 6. Proposed reaction mechanisms.
more stable. This reactivity was indirectly proven by a control
experiment (Scheme 5c). For rhodium-catalyzed annulation,
the reaction of 2a with 4a generates the ylide B, and
a subsequent 1,2-H shift affords the insertion product E
Chem. 2012, 124, 747; b) F. Cambeiro, S. López, J. A. Varela, C.
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015, 127, 2762.
(
Scheme 6). Next, ZnCl -initiated Conia-ene reaction via TS3
2
[
[
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In summary, a regiodivergent synthesis of five-membered
N-heterocycles has been developed by trapping the in situ
formed intermediates through a metal-catalyzed tandem
annulation of amino alkynes and diazo compounds. Mecha-
nistic investigations indicate that the copper-catalyzed
tandem annulations proceeded by allenoate formation and
subsequent intramolecular hydroamination. On the contrary,
the rhodium-catalyzed protocol features a carbenoid inser-
tion into the NÀH bond followed by a Conia-ene reaction.
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Acknowledgements
This project is supported by NSFC (No. 21172023), the
Priority Academic Program Development of Jiangsu Higher
Education Institution (PAPD), and Jiangsu Key Laboratory
of Advanced Catalytic Materials and Technology
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Keywords: copper · cyclizations · diazo compounds ·
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29.
[
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Received: July 31, 2015
Published online: September 9, 2015
Angew. Chem. Int. Ed. 2015, 54, 12962 –12967
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www.angewandte.org 12967