An organocatalytic method for constructing pyrroles: Via the cycloisomerisation of Z -1-iodo-4- N -methylbenzenesulfonyl-1,6-enynes

Article abstract of DOI:10.1039/c9ob01123d

A new cycloisomerisation of Z-1-iodo-4-N-methylbenzenesulfonyl-1,6-enynes to functionalized pyrroles was realized in the presence of an organomolecule (4,4′-bis(1,1-dimethylethyl)-2,2′-bipyridine) and KOtBu. The transformations were performed efficiently to produce different kinds of functionalized pyrroles within 10 min. This is the first example of an organomolecule promoted methodology with vinyl iodides from a non-aromatic system to an aromatic system, which offers an excellent option toward establishing a new horizon for cross-coupling reactions of vinyl halides. Preliminary mechanistic studies were performed and a crude radical pathway was proposed.

Full text of DOI:10.1039/c9ob01123d

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10.1039/C9OB01123D.
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DOI: 10.1039/C9OB01123D
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An Organocatalytic Method for Constructing Pyrroles via
Cycloisomerisation of Z-1-Iodo-4-N-Methylbenzenesulfonyl-1,6-
Enynes
Received 00th January 20xx,
Accepted 00th January 20xx
Long Meng,a Xiaochen Chi,a Xi Sun,a Chengqiang Cao,a Bing Ai,a Qing Liu,a Pingping Zhao,b Zengdian
Zhao,a Yunhui Donga, * and Hui Liua,*
DOI: 10.1039/x0xx00000x
A new cycloisomerisation of Z-1-iodo-4-N-methylbenzenesulfonyl- revealed that the new mode of carbon-halogen bond
1,6-enynes to functionalized pyrroles was realized in the presence activation is realized through organic molecules, which is
of organomolecule (4,4'-bis(1,1-dimethylethyl)-2,2'-bipyridine) transformed into a “ super electron donor ” to initiates the
and KOtBu. The transformations performed efficiently to produce cleavage of the C-X bond by single electron transfer (SET).17,19
kinds of functionalized pyrroles within 10 min. This is the first Obviously, these organic molecules are thermostable and
example that organomolecule promotes methodology with vinyl distinct from conventional radical initiators, while they could
iodides from non-aromatic system to aromatic system, which enable an efficiently unprecedented initiation system for
offers an excellent option toward establishing a new horizon for radical chain reactions with new advances.
cross-coupling reactions of vinyl halides. Preliminary mechanistic
studies were performed and a crude radical pathway was
proposed.
Construction of C-C bond is of great importance in synthetic
chemistry.1 During the past decades, transition-metal-
catalyzed C-C bond construction is emerging as a valuable and
efficient alternative in the construction of cyclic compounds.2
In pharmaceutical chemistry, to remove transition-metal
impurities from products is necessary and difficult, hence to
develop alternative routes is extremely worthwhile.
Organic halides (Csp2-X) is an important category of
intermediates in organic synthesis.3 In addition to
stoichiometric metalation or transition-metal catalysis,4
organic molecules promoted C-X bond activation has attracted
great attention in recent years. Since the pioneering work from
the research groups of Shi,5 Shirakawa/Hayashi,6 and
Kwong/Lei7 on the cross-coupling between aryl halides and
Scheme
1 Organic Molecules Promoted Carbon – Halogen Bond Activation and
Transformation.
arenes with the aid of organo-catalysts, the electron-induced
catalysis has been extensive investigated.8 So far, a variety of
organic molecules, such as 1,10-phenanthrolines,5,6,9,10 1,2-
Notably, the reported examples are mainly focusing on
diamines,7,9f,10
derivatives,13
1,2-diols,11
amino
carbenes,14
acid,12
hydrazine
cross-coupling between aryl halides and arenes to form biaryls
(a, scheme 1), and Heck-type reaction between aryl halides
and olefins to form aryl-substituted alkenes (b, scheme 1).
However, there are rare examples for the coupling of vinyl
iodides via organocatalytic method.
N-heterocyclic
and N-
methylanilines11b,15 had been applied to promote the
activation of haloarene with the assistance of strong base,
which was considered as base-promoted homolytic aromatic
substitution(BHAS).8,16 The previous mechanistic studies
Pyrrole derivates are not only prevalent in a wide variety
of important natural products and pharmaceuticals, but also
used as building blocks in organic synthesis.21 For the synthesis
of pyrroles, Vessally and co-workers have contributed two
elegant review recently, during which N-propargylamines as a
building block are introduced.22 However, the requirement of
aSchool of Chemistry & Chemical Engineering, Shandong University of Technology,
266 West Xincun Road, Zibo 255049, P. R. China
bCollege of chemical and environmental engineering, Shandong university of
science and technology, 579 Qianwangang Road, Huangdao District, Qingdao,
266590, P.R.China
*E-mail: huiliu1030@sdut.edu.cn.
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expensive metal catalysts or by the production of harmful 4-methoxy-2-(4-methoxypyridin-2-yl)pyridine CViewaAnrdticle O4n,li4ne'-
DOI: 10.1039/C9OB01123D
dimethyl-2,2'-bipyridyl D. If 4,4'-bis(1,1-dimethylethyl)-2,2'-
waste streams limits their applications.
On the basis of our research interesting on halides,18 we bipyridine
E
was used 30 mol%, the desired product was
suppose vinyl iodides could also proceed via single electron produced in 77% yield (Entries 11). We also screened different
transfer mechanism in the presence of organic molecules (c, bases and solvents, however, no better yields were obtained
scheme 1). With this conception in mind, we design the (Entries 13-19). Without the addition of organic molecules, the
substrate
synthesize pyrroles. We also proposed
transformation to multi-substituted pyrrole via cascade SET mol%
C-I bond cleavage/allene formation, radical addition, 1,2-H chosen as the optimized conditions. In all cases, no starting
shift and deprotonation (d, scheme 1). material wasrecovered.
1
bearing vinyl iodide and N-propargylamine to starting materials will decomposed rapidly, indicating that the
possible additive is essential to this transformation (entry 20). So 30
with 2.0 equiv. KOtBu in THF at -10 oC for 10 min were
a
2
E
Table 1 Optimization of the reaction conditions for Cycloisomerisation of Z-1- Table 2 Substrates scopea,b
iodo-4-N-methylbenzenesulfonyl-1,6-enynesa
aReaction Conditions: 1a (0.2 mmol), Organic Molecules, base, solvent (2.0 mL),
N2. bIsolated yields.
aReaction Conditions: 1 (0.2 mmol), 30mol% E, KOtBu 2 equiv, THF, N2, -10oC,
30min. bIsolated yields.
To investigate our deduction on the radical chain
transformation of vinyl iodide derivates, (Z)-N-(3-iodo-2-
phenylallyl)-4-methyl-N-(prop-2-yn-1-yl)benzenesulfonamide
1a was synthesized and subjected to conditions as 2,2'-
bipyridine (20 mol%), KOtBu (1.2 equiv.) in THF at room
temperature for 30 min. The desired product was isolated in
10% yield, while no starting material was recovered (Entry 1,
table 1). When the reaction temperature was lowered down to
-10 oC, the yield was slightly increased to 22% (Entries 2 and 3).
To increase the amount of KOtBu to 2.0 equivalents could
improve the yield to 35% (Entry 4). However, there was no
improvement to the reaction was obtained by lowering the
temperature to -20 oC (Entry 5). When the reaction time was
shortened to 10 min, the yield was increased to 54%
dramatically. Subsequently, some organic molecules were
Having established the optimized conditions for the
organocatalytic cycloisomerisation of Z-1-iodo-4-N-
methylbenzenesulfonyl-1,6-enynes (Table 1, entry 11), we set
out to investigate the substrate scope of this reaction (Table 2).
First, we explored different protecting group of the nitrogen,
however, only substrate with tosylate protected amine could
give the desired products, and substrates bearing nosylate,
mesylate and boc groups, decomposed under the standard
conditions. The phenyl group at C2 position were screeninged,
and we found that kinds of alkyls could afford the
corresponding products in moderate to good yields (2b-2f).
The substrate bearing electron donating group methoxy could
deliver 4-(4-methoxyphenyl)-1-tosyl-2-vinyl-1H-pyrrole 2g in
69% yield. The substrate with electron withdrawing group
fluoro performed very well, providing 2h in 79% yield. We also
investigated alkyl substitutions at C2 position, and moderate
yields were obtained (2i and 2j). Subsequently, we tested
investigated. The results showed that 1,10-phenanthroline
B
could not promote this transformation (Entry 7), and 4,4'-
bis(1,1-dimethylethyl)-2,2'-bipyridine
E gave better yield than
2 | J. Name., 2012, 00, 1-3
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several examples bearing substitutions at C4, however, we no adducts was detected even with TEMPO (2E).ViAews AprtricolepOonsliened
could only isolate a complex including two compounds which in scheme 1, allene
might be a keyDiOnIt:e10rm.10e3d9i/aCt9eOBfo01r12t3hDe
could not be determined by NMR and GCMS (2k). The transformation, however, when allene was subjected to the
3
3
substrate with two substitutions at C1 and C2 produced allene standard conditions, no desired product was obtained but only
derivate, but no product 2l detected. The oxygen tethered 1m decomposition was detected, indicating this transformation
was synthesized and examined under the standard conditions, did not go through allene intermediate (Scheme 2G).
while neither starting material nor product 2m was observed.
The radical initiation mechanism of this kind of reactions
still remains elusive, and only limited experimental efforts
have been made to investigated this issue.17a,b,d,19 Generally,
the organic additive either generates an electron donor or
directly functions as an electron donor, which then undergoes
electron transfer to halides to produce the initiator radical
(Scheme 3). On the basis of our experimental results and the
previous reports, we proposed a radical-type mechanism for
this
cycloisomerisation
of
Z-1-iodo-4-N-
the initiation
methylbenzenesulfonyl-1,6-enynes,
but
mechanism was not revealed in sufficient detail.8a,17c,20,23 As
shown in scheme 3, firstly, the starting materials 1a generates
vinyl radical 1a-I, iodide anion and complex
cleavage of C-I in the presence of complex
complex abstracts a hydrogen from 1a-I at propargylic
9
via homolytic
8
. Then, the
9
position to form a double radical 1a-II. Subsequently, the
intramolecular coupling of the 1a-II provides 1a-III, which
might isomerize to the allenamide 1a-IV in the presence of
KOtBu and produce the corresponding product via
aromatization. On the other hand, complex 10 accepts iodide
anion and release catalyst 8. Finally, double bond migration to
the pyrrole form 2a, with aromatization as the driving force.
Although the mechanism is proposed as below, there are still a
lot of issues which could not be explained for this stage. Such
Scheme 2 Control Experiments.
While the substrate scope is not so satisfactory, we are as the deprotonation of propargylic CH2 group followed by
deeply attracted by the reaction mechanism. To shed a light on endo-cyclization and iodide elimination, and tautomerization
the mechanism of this organic molecules promoted might also lead to the product, while the affection of
cycloisomerization, we conducted
a series of control organocatalyst could not be explained. Further investigations
experiments (Scheme 2). First of all, radical scavenger TEMPO will be kept on in the future.
(3.0 equiv.) was added to test if the reaction was a radical
process or not (Scheme 2A). This transformation was partially
inhibited, delivering cyclization product 2a in 17% yield and
allene derivate
3 in 6% yield. The result indicated this reaction
might undergo a radical pathway. A deuterated substrate 1a-D
(D = 83%) was synthesized and examined under the standard
conditions (Scheme 2B). The reaction worked smoothly to
provide 2a-D in 56% yield, however, only 44% deuterium was
transformed into the products 2a-D. Compared to 83%
deuterium, the loss of deuterium might due to de-protonation
of Csp-D in the presence of strong base KOtBu. The reaction
was also carried out in D8-THF, and the desired product 2a was
isolated in 67% without deuterium erosion (Scheme 2C).
Subsequently, we designed 1,6-enyne compound
4 to
investigate if the iodine atom was necessary or not to this
reaction (Scheme 2D). The results showed no cyclization
Scheme 3 Proposed Mechanism.
product was detected, but only the allenamide
5 was obtained,
indicating iodine atom was essential to this transformation.
According to our design in Scheme 1, the reaction might be
initiated from the iodine atom, which inspired us to capture
In summary, we have developed a new cycloisomerisation
of Z-1-iodo-1,6-enynes in the absence of any transition-metal
catalyst. The using of organomolecule (4,4'-bis(1,1-
dimethylethyl)-2,2'-bipyridine) and KOtBu are adequate to
the proposed vinyl radical intermediate with alkene
6.
However, it decomposed under the standard conditions, and
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promote the cycloisomerisation to an array of privileged
pyrroles from non-aromatic system to aromatic system. The
transformations worked efficiently to produce kinds of
functionalized pyrroles within 10 min. To the best of our
knowledge, such transformations has not been demonstrated
9
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previously. It represents
a conceptual breakthrough in
performing cycloisomerisation of non-aromatic system halides
using an organomolecule. It offers an excellent option toward
establishing a new horizon for cross-coupling reactions of vinyl
halides. We are continuously investigating further cases,
detailed kinetic and mechanism, and will be reported in due
course.
Conflicts of interest
10 S. De, S. Mishra, B. N. Kakde, D. Dey and A. Bisai, J. Org. Chem.,
2013, 78, 7823.
There are no conflicts to declare.
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We thank the Shandong Provincial Natural Science Founda-
tion (ZR2018MB008, ZR2018MB012), the National Natural
Science Foundation of China (21671121), the Shandong
Provincal Key Research and Development program of China
(2017GGX20131).
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