An Upstream By-product from Ester Activation via NHC-Catalysis Catalyzes Downstream Sulfonyl Migration Reaction

Article abstract of DOI:10.1002/asia.201500959

A sequential reaction combining N-heterocyclic carbene (NHC) and N-hydroxyphthalimide (NHPI) catalysis allowed for the upstream by-product NHPI, which was generated in the NHC-catalyzed cycloaddition reaction, to act as the catalyst for a downstream nitro

Full text of DOI:10.1002/asia.201500959

DOI: 10.1002/asia.201500959
Communication
N-Heterocyclic Carbenes
An Upstream By-product from Ester Activation via NHC-Catalysis
Catalyzes Downstream Sulfonyl Migration Reaction
Runfeng Han,[a] Liwenze He,[a] Lin Liu,[a] Xingang Xie,[a] and Xuegong She*[a, b]
a catalytic model remains lacking in N-heterocyclic carbene
Abstract: A sequential reaction combining N-heterocyclic
carbene (NHC) and N-hydroxyphthalimide (NHPI) catalysis
(NHC) chemistry.
N-heterocyclic carbenes (NHCs) have emerged as effective
allowed for the upstream by-product NHPI, which was
organocatalysts for Umpolung of aldehydes.[2] They have also
generated in the NHC-catalyzed cycloaddition reaction, to
been employed for the activation of esters followed by trans-
act as the catalyst for a downstream nitrogen-to-carbon
esterificaton,[3] polymerization[4] and domino cyclization reac-
sulfonyl migration reaction. Enantiomeric excess of the
tions.[5] The formation of azolium enolates could be achieved
major product in the cycloaddition reaction remained
by activation of ketenes,[6] a-functionalized aldehydes,[7]
intact in the follow-up sulfonyl migration reaction.
enals,[8] and aliphatic aldehydes.[9] Very recently, activation of
esters has attracted more and more attention, and several
kinds of esters for the domino cyclization have been devel-
In organic chemistry, by-products of upstream reactions acting
as efficient catalysts for downstream reactions is one of the
most challenging subjects in the synthetic community due to
the ongoing pursuit of atom- and step-economy. In compari-
son with the traditional sequential catalytic model, this atom/
step-economy catalytic model enables one reaction not only
to deliver the final product FP but also to realize the reuse of
by-products or side-products SP; consequently, the atom uti-
lization is obviously improved (Scheme 1). In 1982, Tsuji et al.
reported that alkoxides which were generated in the first step
acted as Brønsted base to promote the palladium-catalyzed
decarboxylative allylation of active methylene compounds.[1a]
So far, there have been several reports targeting this concept
for the reuse of upstream by-products (HCl, Ph3PO, CuI, InX3, I2,
etc.) to promote the next transformation.[1b–k] However, such
oped by different groups (Scheme 2). In 2006, Smith and co-
workers reported the enol ester activation by NHCs followed
by an acyl transfer from oxygen to carbon.[10] Then Lupton’s
group enclosed the activation of other enol esters (Scheme 2,
A1) by NHCs to get a series of useful skeletons.[11] Recently,
Chi’s group made significant advancements on the activation
of phenol esters by NHC (Scheme 2, A2) for the asymmetric
domino reactions, and they have successfully accomplished a-
carbon HOMO activation, b-carbon activation (after activation
of the b-carbon of the saturated compounds, this carbon is nu-
cleophilic), b-carbon LUMO activation (after activation, the b-
carbon of the unsaturated compounds is electrophilic), and g-
carbon activation in this manner.[12] Furthermore, Smith and
co-workers reported that chiral isothiourea behaved as a nucle-
ophlic catalyst for the highly enantioselective synthesis of dihy-
dropyridones.[13] In most cases, the by-product such as 4-nitro-
phenol generated from NHC activation was discarded. We
wondered whether it is possible to utilize such a by-product
(SP in Scheme 1) as the catalyst to promote subsequent trans-
formations of the product formed in the first step (MP in
Scheme 1). Application of this concept is of great importance
in N-heterocyclic carbene chemistry.
However, activation of substituted phenol esters by NHC for
the asymmetric domino Michael addition/lactamization of
acetic 4-nitrophenol esters and a,b-unsaturated imines has
two drawbacks: (1) high catalyst loading (30%)[12a–d] is needed
and (2) the by-product cannot catalyze the next transforma-
tion. To overcome these problems, it is highly desirable to ex-
plore appropriate esters that can produce the side-product ca-
pable to catalyze the next transformation. It has been known
that enals could be transformed to esters via NHC-bound eno-
lates and esterification (forward process). Thus, NHC-bound
enolates could be obtained via ester activation mode, and this
backward process would be interesting and novel in NHC cat-
alysis.
Scheme 1. Hypothesis for the catalytic model (S=substrate, Cat=catalyst,
MP=main product, SP=side-product, FP=final product).
[a] R. Han, L. He, L. Liu, X. Xie, Prof. Dr. X. She
State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering
Lanzhou University
Lanzhou, Gansu, 730000 (P.R. China)
E-mail: shexg@lzu.edu.cn
[b] Prof. Dr. X. She
Collaborative Innovation Centre of Chemical Science and Engineering
Tianjin 300071 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201500959.
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Scheme 2. Esters of acyl azolium precursors for cyclization.
Based on the above considerations, we thought that 1,3-di-
oxoisoindolin-2-yl 2-phenylacetate A3 would be the best
choice (Scheme 2). In this process, the new catalyst N-hydroxy-
phthalimide (NHPI) is generated from NHC catalysis and then
enters another catalytic cycle. Due to the effective role of
NHPI, the precursor of phthalimido-N-oxyl (PINO) radical for
the CÀH activation by hydrogen abstraction[14] and aromatic
oxidation of heterocyclic compounds,[15] a series of radical reac-
tions of the first product could be performed by this concept.
For this, it would be different from the traditional sequential
catalytic mode, in which the second catalyst was added se-
quentially.
Table 1. Scope of the substrates.[a]
In order to verify our design, we initially investigated the
enantioselective activation of 1,3-dioxoisoindolin-2-yl 2-phenyl
acetate (A3) via NHC catalysis to react with a,b-unsaturated
imines.[15] After screening different reaction conditions (see the
Supporting Information), we found that a high yield and selec-
tivity were obtained when 10 mol% NHC precursor B was used
and CH2Cl2 was employed as solvent. Next, the scope and limi-
tation of this system were also explored under the optimized
reaction conditions (Table 1). A variety of a,b-unsaturated
imines and aryl or alkyl acetic esters were successfully convert-
ed into the desired products. We found that the results of this
reaction were determined by steric and electronic effects. The
yield, diastereoselectivity, and enantioselectivity were all de-
creased to some extent when the aromatic ring R1 (-Ph) was re-
placed by 4-Cl-Ph or 4-OMe-Ph except for the yield with R2 =4-
Cl-Ph (3a–3e, Table 1); also the reaction was relatively sensitive
to substituents on the aromatic ring R3 (3 f–3j, Table 1). As for
alkyl acetic esters, a stronger base would be necessary due to
the weak acidity of the a-position of the acyl azolium inter-
mediate.
[a] Reaction conditions: 1 (0.10 mmol), 2 (0.15 mmol, 1.5 equiv), solvent
(0.5 mL); isolated yield; the diastereomeric ratio was determined by
1H NMR spectroscopic analysis of unpurified reaction mixtures; ee value
of the major diastereomer as determined by chiral HPLC analysis. [b] DBU
was used as the base; ee value of the major diastereomer. [c] DBU was
used as the base; ee value of the minor diastereomer.
A moderate yield and low diastereomeric ratio were ob-
tained when 10% catalyst was used, while a higher yield was
obtained to some extent when the catalyst loading was in-
creased to 20%; however, the diastereomeric ratios were still
low due to the lower hindrance of the ethyl and benzyl groups
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compared with the phenyl group (3k–3l, Table 1). However,
excellent and comparable enantioselectivities were detected
under 10% and 20% catalyst loadings. It was obvious that
when phenol esters were replaced by phthalimide esters, the
catalyst loading and base loading could be decreased to 10%
and 2.0 equivalents, respectively, and that no further additive
was needed. These results revealed that esters 2 exhibited
a higher reactivity than 4-nitrophenol esters overall from the
point of catalyst loading, amount of base, and additive. As
esters as substrates are more stable than enals and more con-
venient to use, this formal [4+2] annulation provided a series
of products containing a single aryl substituent at the lactam
a-carbon, while NHC-bound enolates via enals[8a] could not be
obtained. On the other hand, this activation mode provided
trans products, while NHC-bound enolates via enals only pro-
vided cis products.
lated that an excess amount of DIPEA and conjugate acid
[DIPEA-H]+ may disturb the reaction to some extent. When the
mixture of the NHC-catalyzed reaction was washed either with
water or 1n HCl, a satisfactory yield of 4a was obtained
(Scheme 3). However, the yields were decreased sharply either
Scheme 3. Sequential transformation of the two steps.[a] ([a] Condition
a (achiral catalyst B’ instead of chiral catalyst B): 1 (0.10 mmol), 2
(0.15 mmol, 1.5 equiv), NHC catalyst B’ (10 mol%), DIPEA (2.0 equiv), CH2Cl2
(0.5 mL), r.t. for 24 h; [b] Isolated yield over two steps; the minor diastereo-
meric isomer was not detected.)
Next, we investigated whether NHPI could be used as a new
catalyst for the following transformation of the first main prod-
uct 3a. Thus, 3a (12:1 d.r.) was prepared as the substrate for
the designed reaction. When 3a was treated with NHPI and O2
in CHCl3, the sulfonyl migration product 4a was obtained in
only 16% yield (entry 1, Table 2). A higher yield was obtained
in the absence of Co(II) or by replacement of O2 (bubbles) with
an air atmosphere. This result revealed that the by-product
NHPI actually acted as a new catalyst for the second transfor-
mation, and this verified our hypothesis (Scheme 1).
Table 2. Exploring the reaction conditions for the N- to C-sulfonyl migra-
tion.[a]
Subsequent studies explored the possibility for the NHC/
NHPI catalytic cascade process (Table 3). We found that many
substrates could give the desired products successfully, and
electronic effects enormously affected the yields of the sulfonyl
migration products. When substituents on the aromatic ring R2
were electron-donating groups (4d–4e), the migration reac-
tion gave a higher yield; indeed, it is reasonable that an elec-
tron-rich system is beneficial for the oxidation process. No ob-
vious interference of the reaction was observed when substitu-
ents on the aromatic ring R1 were different groups (4b–4c).
The inverse trend was observed when different esters were
used under the same conditions (4 f–4h). In this process, the
enantiomeric excess values of products were all maintained
from the upstream products.
Entry
NHPI
20%
20%
–
50%
20%
20%
20%
20%
Co(OAc)2·4H2O
–
Solvent
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
(CH2Cl)2
CH3CN
CH3OH
Yield [%][b]
16
50
12
1
2
3
4
5c
6
7
8
5%
5%
5%
5%
5%
5%
5%
58
trace
trace
n.d.
This sulfonyl migration reaction may proceed via a radical
pathway. In order to support this idea, TEMPO was added to
the reaction, and the reaction was completely suppressed. This
result provides indirect evidence that the sulfonyl migration re-
action proceeds via a radical pathway[17] (Scheme 4).
n.d.
[a] Reaction conditions: 3a (0.1 mmol), solvent (2.5 mL). [b] Isolated yield;
the minor diastereomeric isomer was not detected in all cases. [c] The re-
action was performed at 608C.
In conclusion, we have developed sequential N-heterocyclic
carbene (NHC) catalysis and N-hydroxyphthalimide (NHPI) cat-
alysis in which the upstream by-product NHPI acts as the cata-
lyst for a downstream nitrogen-to-carbon sulfonyl migration
reaction. In the upstream catalytic system, only a low catalyst
loading was needed and no additive was required for the cycli-
zation of enolate azolium with a,b-unsaturated imines, reveal-
ing that this kind of esters exhibited a high reactivity. This reac-
tion proceeded to provide a high yield as well as satisfactory
enantioselectivity and diastereoselectivity. The downstream
catalytic process allowed for sulfonyl migration of the first
product, while retaining the enantiomeric excess. Further in-
vestigations on the mechanistic studies and other transforma-
when Co(OAc)2·4H2O was added as co-catalyst as the combina-
tion of O2 and CoII could accelerate the formation of a PINO
radical (entry 2, Table 2). Control experiments revealed that
NHPI is necessary for the transformation (entry 3, Table 2).
Pleasingly, increasing the amount of NHPI to 50 mol% gave
a higher yield (entry 4, Table 2), and the structure of 4a was
confirmed by X-ray analysis (CCDC 1059049).[16] However, other
solvents could not provide improved results (entries 5–8,
Table 2).
After the NHC catalytic reaction, Co(OAc)2·4H2O was added
and the reaction was performed under oxygen atmosphere.
However, only trace amounts of 4a were detected. We specu-
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Scheme 4. Radical capture experiment.
tions by this kind of strategy are being pursued in our labora-
tory.
Acknowledgements
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We are grateful for the financial support by the
NSFC(21125207, 21372103, 21472079), and SRFDP (201302111
10018).
Keywords: ester activation · N-heterocyclic carbenes · N-
hydroxyphthalimide · radicals · sulfonyl migration
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could be performed under autooxidation condition. Combining with
our radical capture experiment (Scheme 4), we considered that this sul-
fonyl migration reaction may be via radical process. And the possible
mechanism is being pursued in our laboratory.
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Manuscript received: September 9, 2015
Revised: October 13, 2015
Accepted Article published: November 2, 2015
Final Article published: November 10, 2015
Chem. Asian J. 2016, 11, 193 – 197
www.chemasianj.org
197
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