Sulfinate-Organocatalyzed (3+2) Annulation Reaction of Propargyl or Allenyl Sulfones with Activated Imines

Article abstract of DOI:10.1002/ejoc.201800749

An operationally simple methodology for the synthesis of 4-sulfonyl-3-pyrrolines is described using a propargylic sulfone and N-sulfonyl imines as substrates. This annulation process is initiated by an arenesulfinate organocatalyst, which allows a smooth isomerization of the alkynyl precursor into the corresponding allene, followed by the generation of a highly reactive allyl sulfone anion. An asymmetric version involving an unprecedented enantiopure sulfinate–ammonium cooperative ion pair (PhSO₂^– R₄N⁺*) was investigated. A proof-of-concept, with enantiomeric excesses of up to 41 %, was obtained according to a preliminary screening of commercially available chiral phase-transfer catalysts.

Full text of DOI:10.1002/ejoc.201800749

A Journal of
Accepted Article
Title: Sulfinate-Organocatalyzed (3+2) Annulation Reaction of
Propargyl or Allenyl Sulfones with Activated Imines
Authors: Thomas Martzel, Jean-François Lohier, Annie-Claude
Gaumont, Jean-François Brière, and Stephane Perrio
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800749
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800749
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10.1002/ejoc.201800749
European Journal of Organic Chemistry
COMMUNICATION
Sulfinate-Organocatalyzed (3+2) Annulation Reaction of
Propargyl or Allenyl Sulfones with Activated Imines
Thomas Martzel,[a] Jean-François Lohier,[a] Annie-Claude Gaumont,[a] Jean-François Brière,[b] and
Stéphane Perrio*[a]
Dedication ((optional))
Abstract: An operationally simple methodology for the synthesis of
4-sulfonyl-3-pyrrolines is described using a propargylic sulfone and
N-sulfonyl imines as substrates. This annulation process is initiated
conspicuously absent.
Interestingly, the group of Robina disclosed an original
approach to 4-sulfonyl 3-pyrrolines involving N-sulfonyl imines
as electrophiles (Scheme 1, eq. 2).9 Sodium nitrite (NaNO2) was
introduced as the initial reaction promoter but the authors
speculated the intermediary formation of a sulfinate.10 The
reported protocol was tedious and required the use of a syringe
pump, in order to generate small amounts of the key allyl sulfone
anion intermediate and thus to minimize unwanted
polymerisation events.
by an arenesulfinate organocatalyst, which allows
a smooth
isomerization of the alkynyl precursor into the corresponding allene,
followed by the generation of a highly reactive allyl sulfone anion. An
asymmetric version involving an unprecedented enantiopure
–
sulfinate–ammonium cooperative ion pair (PhSO2 R4N+*) was
investigated. A proof-of-concept, with enantiomeric excesses of up
to 41%, was obtained according to a preliminary screening of
commercially available chiral phase transfer catalysts.
Padwa: annulation with Michael acceptors (eq. 1)
SO2Ph
Na
PhO2S
SO2Ph
Introduction
PhO2S
EWG
PhSO2Na(cat)
EWG = CN, C(O)Me
β
–
Sulfinates RSO2 (sulfinic acid anions) are readily available,
EWG
EWG
bench-stable, non-hygroscopic and easy to handle sulfur
species, mainly associated to sodium cations.1 Sulfinate salts
exhibit a wide range of chemical reactivity, based on nucleophilic
or radical pathways, thus acting as relevant precursors of
Robina: annulation with imines (eq. 2)
SO2Ph
SO2Ph
PhO2S
N
Ar
Na
Ar
R1O2S
NO2Na(cat)
PhSO2Na(cat)
PhO2S
Ar
SO2R1
–
N
sulfones. The pKa value of about 2 for RSO2H/RSO2 explains
N
the good leaving group ability of sufinates.2 This feature and the
intrinsic nucleophilicity3 of sulfinates afford an ideal combination
for applications of these anions as Lewis bases in
organocatalysis. However, to the best of our knowledge, very
few example of sulfinate-organocatalyzed reactions have been
reported so far in the literature.4–6 The main approach in the area
refers to the so-called Padwa reaction, which was introduced in
the late 80’s.6 This strategy consists in a (3+2) annulation
reaction between an allenyl sulfone and Michael acceptors to
furnish 1-sulfonyl cyclopentenes (Scheme 1, eq. 1). The process
is triggered by the nucleophilic attack of the sulfinate catalyst
(probably through its soft sulfur centre) at the central carbon
atom of the cumulative diene to generate an allyl sulfone anion
intermediate. Diastereoselective versions were briefly developed
by the group of García Ruano using chiral furanones as
electrophilic reagents.7 The group of Hale described also an
example involving a chiral cyclohexenone, which resulted in the
elaboration of a key precursor for the formal total synthesis of
the naturally occurring (–)-echinosporin and (+)-brefeldin A.8 To
the best of our knowledge, enantioselective variants remain
R1O2S
Yield = 39–70 %
(5 examples)
Ar = Ph, 4-MeO-C6H4, 4-Cl-C6H4, 4-O2N-C6H4
R1 = Tol, (CH2)2SiMe3
Scheme 1. Sulfinate-mediated (3+2) annulations with allenyl sulfones.
Considering the high interest of organic chemists for the
development of synthetic approaches to original and
medicinally-relevant
five-membered
nitrogen-containing
heterocycles,11 we believed that this annulation process which
leads to unique functionalized pyrroline platforms deserves
further attention.12 The process could stand further
improvements in terms of operational simplicity, substrate
compatibility and reaction efficiency. Proving the intermediacy of
a
sulfinate catalyst could also open the way to an
enantioselective version of the reaction.
In this respect, we conceived that the readily available
propargylic sulfone isomer 1 could be a convenient source13 of
the allenyl sulfone 2 upon the influence of a sulfinate catalyst, to
provide a user-friendly one-pot annulation procedure and to
prevent, thereby side reactions such as polymerization (Figure
1). The introduced sulfinate catalyst would act both as a
Brønsted base, thus permitting an in situ isomerisation of alkyne
1 into allene 2, and as a Lewis base in the consecutive
annulation pathway. Based on this hypothesis, the highly
reactive allenyl sulfone 2 and allyl sulfone species I would be
generated gradually in the reaction mixture. Furthermore,
according to the anionic character of all intermediates, the use of
[a]
[b]
T. Martzel, J.-F. Lohier, A.-C. Gaumont, and S. Perrio*
Normandie Univ, ENSICAEN, UNICAEN, CNRS, LCMT, 14000
Caen, France
E-mail: stephane.perrio@ensicaen.fr,
https://www.lcmt.ensicaen.fr/cv-stephane-perrio/
J.-F. Brière
Normandie Univ, INSA Rouen, UNIROUEN, CNRS, COBRA, 76000
Rouen, France
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejoc.201800749
European Journal of Organic Chemistry
COMMUNICATION
phase transfer conditions (PTC) could be an entry to an
asymmetric version of the reaction.14 Indeed, the fact that the
incoming sulfinate remains on the products 4, whereas the
liberated one originates from the allenyl sulfone, implies the use
of identical sulfonyl groups for the sulfinate catalyst and the
allenic substrate and thus precludes to take profit of a chiral
sulfinate.7a Alternatively, the use of an enantiopure sulfinate–
32% yield when the reaction was performed in EtOH alone,
while only trace amounts of 4a (< 5% yield)19 were delivered in
THF. We believe that the THF/EtOH mixture has a beneficial
impact on the solubilization of the reagents (THF) and the
stabilization of the highly reactive intermediates (EtOH) through
hydrogen bonding.
PhO2S
–
ammonium salt PhSO2 R4N+* (formed in situ in a catalytic
R1
SO2Ph
R1
PhSO2Na (25 mol-%)
2:1 THF/EtOH , r.t., 24 h
N
EWG
+
amount from PhSO2Nacat and R4N*Xcat), would allow the chiral
ammonium cation R4N+* to influence the stereoselective issues
of the anionic intermediates, meanwhile the sulfinate would still
act as a Lewis base.15–17 This methodology would design the first
N
1
3
4
EWG
4a, R2 = R3 = H, 65%
R2
4b, R2 = H, R3 = Cl, 59%
4c, R2 = H, R3 = F, 53%
4d, R2 = H, R3 = Me, 49%
4e, R2 = H, R3 = OMe, 33%
4f, R2 = MeO, R3 = H, 40%
PhO2S
R3
enantioselective version of
a sulfinate-mediated annulation
N
reaction, but also would highlight the unprecedented use of
SO2Ph
–
PhSO2 R4N+* as a cooperative ion pair.18 We are pleased to
present herein the results of this investigation.
PhO2S
O
PhO2S
N
N
SO2Ph
SO2Ph
4g, 49%
SO2Ph
4h, 53%
SO2Ph
PhO2S
PhO2S
1
S
2
Ph
SO2Ph
R4N*
4j, 53%
N
α
PhO2S
N
O2S
R4N*X
PhSO2
R4N*
SO2Ph 4i, 58%
PhSO2
Na
OMe
I
R1
N
NaX
*
PhO2S
PhO2S
Ph
PhO2S
Ph
SO2tBu
3
R1
R2O2S
N
N
N
SO2Me
4k, 40%
4l, 44%
4
SO2R2
SO2Ph
SO2Ph
Unappropriate imines
EWG = Boc (3m)
PhO2S
R1
R4N*
SO2R2
PhO2S
*
R1
*
Ph
N
P(O)Ph2 (3n)
P(O)(OEt)2 (3o)
S(O)tBu (3p)
O
N
N
N
EWG
R4N*
II
O2S
SO2R2
III
(3q)
Figure 1. Proposed catalytic cycle from propargyl sulfone, mediated by a
sulfinate-ammonium cooperative ion pair.
Scheme 3. Substrate scope with respect to the activated imine 3.
Satisfied with the chemical efficacy of the process in the
THF/EtOH system for the model reaction and its operational
simplicity without the need of a syringe pump, these conditions
were next used for the evaluation of the scope of the reaction
(Scheme 3).20 Imines derived from electron poor (3b and 3c), as
well as electron rich (3d–f) benzaldehydes provided the desired
heterocycles 4b–f with yields in the range of 33–59%. A 2-
naphthyl group (imine 3g) is also tolerated, leading to 4g in a
49% yield. In addition, imines derived from heteroaromatic
aldehydes such as 3h (R1 = 2-furyl) and 3i (R1 = 2-thienyl) were
also good substrates, furnishing 4h and 4i in 53 and 58% yields,
respectively. The influence of the sulfonyl group of the imine was
then examined. A switch to the N-(4-methoxyphenyl)sulfonyl
derivative 3j allowed formation of 4j in a 53% yield. Use of the
N-methylsulfonyl analogue 3k led to 4k in a moderate 40% yield.
A similar level of conversion was obtained with the N-tert-
butylsulfonyl precursor 3l (44% yield), and the structure of the
target 4l was confirmed by a single-crystal X-ray diffraction
analysis.21 The scope of the process was further examined
through the investigation of imines activated by an N-tert-
butoxycarbonyl (3m), an N-phosphinoyl (3n), an N-phosphoryl
(3o), an N-sulfinyl (3p) or an N-sulfamoyl (3q) group.22
Unfortunately, none of them furnished the desired cycloadducts.
An unwanted hydrolytic reaction rapidly consumed imine 3m
(EWG = Boc), whereas a lack of reactivity and full recovery of
3n-q was observed.
Results and Discussion
Investigation of the racemic version
The validation of our working hypothesis began with the
reaction of propargyl sulfone 1 (1.5 equiv.) and N-sulfonyl imine
3a, at room temperature, in the presence of a catalytic amount
of sodium benzenesulfinate (25 mol-%) (Scheme 2).
PhO2S
SO2Ph
Ph
PhSO2Na (25 mol-%)
Solvent, r.t., 24 h
Ph
SO2Ph
+
N
N
PhO2S
1
3a
4a
2:1 THF/EtOH: 65%
EtOH: 32%
THF: < 5%
Scheme 2. Optimization of the reaction conditions with imine 3a.
The reaction was carried out in a 2:1 THF/EtOH mixture.
This solvent combination was previously identified as an
appropriate reaction medium in the inspiring Robina’s report.9
Pleasingly, the anticipated product 4a was obtained in a 65%
isolated yield after 24 h of reaction. Other conditions were tested
but all of them led to inferior results (see Supporting Information).
As the most representative examples, 4a was produced in a
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10.1002/ejoc.201800749
European Journal of Organic Chemistry
COMMUNICATION
Having developed an operationally simple procedure giving
rise to racemic 3-pyrrolines 4 through a (3+2) annulation
reaction between N-sulfonyl imines 3 and allenyl sulfone,
generated in situ from isomeric propargylic sulfone 1, we turned
our attention to the enantioselective version.
At the outset, the evaluation of the asymmetric version was
carried out with imine 3a. The influence of various reaction
parameters, including the solvent, the concentration, the
catalytic PhSO2Na/R4N+* X– loading and the temperature, was
examined.24 Details of this seminal study are reported in
Supporting Information. Et2O, CH3Cl and CH2Cl2 failed to
promote the process25 and the best reaction performance was
obtained in THF26 solution at an ambient temperature. Pleasingly,
the catalyst loading could be reduced from 25 to 10 mol-%,
without significant erosion of the reactivity. Furthermore,
improved chemical yields were obtained when the allene and the
imine were added together, via a syringe pump to the solution of
the catalytic system. The slow addition of both reagents
probably prevents polymerization of the highly reactive allyl
sulfone anion and hydrolysis of the imine reagent.
Investigation of the enantioselective version
A preliminary experiment was carried out with allenyl sulfone
2 and N-sulfonyl imine 3a to embrace the question of the
racemic background rate (Scheme 4). In order to favor tight ion
pair species, the reaction was arbitrarily performed in pure THF.
In the presence of PhSO2Na (25 mol-%) and n-Bu4NBr (25 mol-
%), a smooth annulation reaction took place to furnish, after 24
hours at a room temperature, the anticipated adduct 4a in a 45%
isolated yield.23 By way of comparison, a slower transformation
(< 15% yield with a similar reaction time) was observed without
the ammonium catalyst. Unfortunately, attempts starting with the
propargyl sulfone 1, in the THF solution, failed to produce the
annulation product 4a, as a result of an ineffective alkyne-allene
isomerisation. Pleasingly, the results obtained with the
preformed allenyl sulfone 2 highlighted a marked effect of phase
transfer conditions on the process efficiency and were
encouraging for the development of the asymmetric version with
chiral ammonium salts R4N+* X–. On these grounds, the
investigation was pursued with the preformed cumulene 2.
These parameters being set, the screening of enantiopure
quaternary ammonium salts C1–C8,27 displaying architecturally
distinct types, was then investigated (Figure 2). The reaction
with the chloro-substituted imine 3b, in the presence of 10 mol-
% of the organocatalytic system (PhSO2Na/C1–C8) was chosen
as the model reaction (Table 2).
Table 2. Screening of enantiopure phase transfer catalysts towards an
enantioselective version.[a]
Cl
SO2Ph
PhO2S
PhO2S
SO2Ph
+
PhSO2Na
(25 mol-%)
Ph
PhSO2Na (10 mol-%)
Cl
Ph
SO2Ph
+
N
N
C1–C8 (10 mol-%)
N
N
THF, r.t., 24 h
PhO2S
2
3b
4b
2
3a
4a
SO2Ph
PhO2S
THF, r.t., 24 h
n-Bu4NBr (25 mol-%): 45%
n-Bu4NBr (0 mol-%): 15%
Entry
Catalyst
C1
Yield (%)[b]
ee (%)[c]
1
2
3
4
5
6
7
8
37
59
41
41
48
32
31
20
22
–2
–2
28
0
Scheme 4. Effect of phase transfer conditions.
C2
C3
F
C4
F
F
Br
F
Br
C5
F
F
n-Bu
n-Bu
C6
13
2
N
N
C7
F
F
C8
0
C2
F
C1
F
F
F
[a] Reaction conditions: PhSO2Na (10 mol-%) and catalyst C (10 mol-%) in
THF (0.1 M). A solution (THF, 0.1 M) of allenyl sulfone 2 (1.5 equiv.) and N-
sulfonyl imine 3b (1 equiv.) was slowly added to the previous suspension via a
syringe pump (speed of 0.25 mL/h) at r.t. The mixture was stirred for 24 h. [b]
BF4
R1
N
Ph
Ph
Cl
Ar
Me
Tol
N
N
N
O
O
Tol
Tol
Isolated yield. [c] Determined by HPLC: IA column, 1mL.min–1
,
n-
OR2
heptane/iPrOH/CH2Cl2 mixture (80:15:5 ratio) as eluent.
Tol
Ar = 9-anthracenyl
Me
C3
C4: R1 = R2 = H (CD)
C5: R1 = H, R2 = allyl (CD)
C6: R1 = OMe, R2 = H (QN)
Use of the fully synthetic N-spiro C2-symmetric quaternary
ammonium salt C1 (first generation Maruoka’s catalyst) led to
the desired heterocycle 4b, in 22% enantiomeric excess (ee)
and 37% yield (entry 1). With the second-generation Maruoka’s
scaffold C2, a significant improvement of the yield to 59% (entry
2) was obtained but the product was almost racemic (ee = –2%).
The tartrate-derived diammonium salt C3 (TaDiAS) led to 4b in a
41% yield (entry 3), but without enantioselectivity (ee = –2%).
The N-anthracenylmethyl cinchonidinium chloride C4 furnished a
sample of 4b, in 28% enantiomeric excess and 41% yield (entry
4). The analogous O-allyl cinchonidinium derivative C5 allowed
the formation of 4b in an improved 48% yield, but unfortunately
BF4
R1
HO
Cl
C7: R1 = R2 = H (CN)
C8: R1 = OMe, R2 = H (QD)
N
Ar
N
Ar = 9-anthracenyl
Figure 2. Enantiopure quaternary ammonium salts evaluated. CD:
cinchonidinium, QN: quininium, CN: cinchoninium, QD: quinidinium.
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10.1002/ejoc.201800749
European Journal of Organic Chemistry
COMMUNICATION
in a racemic manner (entry 5). The results obtained with C4 and
C5 highlight that the presence of the free 9-OH in the Cinchona
available enantiopure cinchonidinium salt C4, the chiral 3-
pyrrolines were obtained with ee ranging from 25 to 41%, and
yield up to 42%. We believe that the evaluation of a library of
alternative ammonium catalysts derived from the Cinchona
chiral pool and displaying various substituents on the
quinuclidine nitrogen center could infer improved levels of
stereocontrol of the reaction. These proof-of-principle results
also highlight the unprecedented use of an enantiopure
sulfinate–ammonium (RSO2– R4N+*) cooperative ion pair through
phase transfer conditions. This offers an attractive opportunity in
asymmetric organocatalysis making use of sulfinates as Lewis
bases. Future efforts aiming at broadening this strategy are
currently underway in our laboratories.
alkaloid catalyst is crucial to achieve
a
reasonable
enantioselectivity through ion-pairing interactions. The
analogous 9-hydroxy quininium chloride C6 allowed the isolation
of 4b in a 32% yield, but only in a 13% ee (entry 6). The
pseudoenantiomeric derivatives C7 (cinchoninium salt) and C8
(quinidinium salt) appeared as completely inefficient in terms of
enantioselectivity (entries 7 and 8). Finally, a range of other 9-
hydroxy cinchonidinium salts, through the variation of the Ar
group on the quinuclidine moiety, proved ineffectual, by
comparison with the C4 catalyst previously tested (see
Supporting Information).
The proof of concept for the enantioselective version was
next examined with various N-sulfonyl imines, under the
established conditions in the presence of the currrently optimal
cinchonidinium salt C4 (Scheme 5).
Experimental Section
Typical Procedure for the Racemic Version: Propargylic sulfone 1 (1.5
equiv.), N–sulfonyl imine 3 (1.0 equiv.) and sodium benzenesulfinate (25
mol-%) were suspended in a THF/EtOH mixture (2:1 ratio). The reaction
mixture was stirred at room temperature for 24 h, filtered on a syringe
filter with a PTFE membrane (0.2 µm pore size) and concentrated under
reduced pressure. The resulting crude product was then purified by
column chromatography on silica gel [eluent: n-pentane/EtOAc/CH2Cl2
(7:1.5:1.5)] to afford the analytically pure pyrroline 4.
PhO2S
SO2Ph
R1
PhSO2Na (10 mol-%)
R1
SO2Ph
C4 (10 mol-%)
N
+
N
THF, r.t., 24 h
PhO2S
2
3
4
PhO2S
PhO2S
Cl
PhO2S
Ph
N
N
N
Typical Procedure for the Enantioselective Version: Sodium
benzenesulfinate (10 mol-%) and N-anthracenylmethyl cinchonidinium
chloride C4 (10 mol-%) were suspended in THF. N-sulfonyl imine 3 (1
equiv.) and allenyl sulfone 2 (1.5 equiv.) were dissolved in THF and
slowly added to the previous suspension via a syringe pump (speed of
0.25 mL/h). The reaction mixture was stirred at room temperature until
total disappearance of the allenyl sulfone (TLC) and concentrated under
reduced pressure. The resulting crude product was purified by column
chromatography on silica gel [eluent: n-pentane/EtOAc/CH2Cl2
(7:1.5:1.5)] to afford the analytically pure pyrroline 4.
SO2Ph
(+)-4a
33%, 25% ee
PhO2S
SO2Ph
SO2Ph
(+)-4g
17%, 41% ee
(+)-4b
41%, 28% ee
PhO2S
O
S
N
N
SO2Ph
(+)-4h
42%, 28% ee
SO2Ph
(+)-4i
29%, 30% ee
Scheme 5. Proof of concept towards an enantioselective version.
Supporting Information (see footnote on the first page of the article):
Experimental procedures, optimization of reaction conditions, compound
characterization data, copies of 1H and 13C NMR spectra, HPLC
chromatographs, and crystallographic data for 2 and 4l.
3-Pyrroline 4a was obtained in
a 33% yield and an
enantiomeric excess of 25% starting with parent imine 3a (R1 =
Ph). The best ee value (41%) was obtained for compound 4g
containing a 2-naphthyl group, but at the expense of the yield Acknowledgements
(17%). Hetero-aromatic imines 3h (R1 = 2-furyl) and 3i (R1 = 2-
thienyl) were also suitable substrates, providing the related
enantioenriched products 4h (ee = 28%) and 4i (ee = 30%) in 42
and 29% yield, respectively. The major enantiomers produced
were shown to be uniformly dextrorotatory and display also the
shortest retention time on the HPLC chromatographs (see
Supporting Information). We believe that the same sense of
asymmetric induction with the phase transfer catalyst C4 has
taken place in all the cases.
We acknowledge financial support from Region Normandie
(CRUNCh network), CNRS, EFRD and Labex SynOrg (ANR-11-
LABX-0029). This work has been also partially supported by
INSA Rouen and Rouen Normandy University. We wish to thank
C. Bellanger and D. Leparfait (Université de Caen Normandie)
for contributing in the synthesis of starting materials and the
optimization of the protocols.
Keywords: Sulfinates • Organocatalysis • Allenyl sulfones •
Lewis base • Phase transfer
Conclusions
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400.
In conclusion, we have developed an operationally simple
procedure for the preparation of racemic 3-pyrrolines by a (3+2)
annulation reaction between N-sulfonyl imines and parent allenyl
sulfone, generated in situ from the isomeric propargylic sulfone.
We also report an extension to enantioenriched products, under
asymmetric phase transfer conditions, in which the preformed
allenyl sulfone is the suitable substrate. Using the commercially
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10.1002/ejoc.201800749
European Journal of Organic Chemistry
COMMUNICATION
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This article is protected by copyright. All rights reserved.

10.1002/ejoc.201800749
European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Lewis base organocatalysis
PhO2S
R1
Thomas Martzel, Jean-François Lohier,
Annie-Claude Gaumont, Jean-François
Brière and Stéphane Perrio*
SO2Ph
>
R1
Y = Na, 33-65% yield
Y = R4N*, up to 41% ee
or
*
Y
PhSO2
>
+
N
N
SO2R2
SO2R2
SO2Ph
Page No. – Page No.
The synthesis of 4-sulfonyl-3-pyrrolines, from a propargylic sulfone and activated
imines, is described. The annulation is initiated by sodium benzenesulfinate as
organocatalyst, which allows isomerization of the alkynyl precursor into the
analogous allene. A proof of concept towards an asymmetric version involving an
Sulfinate-Organocatalyzed (3+2)
Annulation Reaction of Propargyl or
Allenyl Sulfones with Activated
Imines
–
unprecedented enantiopure sulfinate–ammonium (PhSO2 R4N+*) cooperative ion
pair was then highlighted (ee up to 41%).
This article is protected by copyright. All rights reserved.