Chiral Anion Directed Asymmetric Carbocation-Catalyzed Diels-Alder Reactions

Article abstract of DOI:10.1002/ejoc.201501621

In recent years the carbocation has re-emerged as a highly efficient Lewis acid catalyst for a variety of organic transformations. However, the goal of asymmetric carbocation catalysis has so far been out of reach mainly as a result of difficulties associated with the preparation of stable chiral carbocations. Here, we describe developments towards asymmetric carbocation catalysis based on the concept of chiral-anion-directed catalysis. Chiral tritylium salts can be conveniently prepared in situ by mixing trityl chloride derivatives with chiral phosphonate, phosphoramide, bis(sulfonyl)amide, and bis(sulfuryl)amide silver or sodium salts. It is shown that the bis(sulfuryl)amide/tritylium ion salt catalyzes the Diels-Alder reaction with an up to 53 % enantiomeric excess. The enantioenriched triphenylmethylium/bis(sulfuryl)amide contact ion pair was applied in asymmetric counter-anion-directed catalysis to give Diels-Alder reaction adducts with up to 53 % ee.

Full text of DOI:10.1002/ejoc.201501621

DOI: 10.1002/ejoc.201501621
Full Paper
Asymmetric Carbocation Catalysis
Chiral Anion Directed Asymmetric Carbocation-Catalyzed Diels–
Alder Reactions
Shengjun Ni,[a] Veluru Ramesh Naidu,[a] and Johan Franzén*[a]
Abstract: In recent years the carbocation has re-emerged as rected catalysis. Chiral tritylium salts can be conveniently pre-
a highly efficient Lewis acid catalyst for a variety of organic pared in situ by mixing trityl chloride derivatives with chiral
transformations. However, the goal of asymmetric carbocation phosphonate, phosphoramide, bis(sulfonyl)amide, and bis-
catalysis has so far been out of reach mainly as a result of diffi- (sulfuryl)amide silver or sodium salts. It is shown that the
culties associated with the preparation of stable chiral carbo- bis(sulfuryl)amide/tritylium ion salt catalyzes the Diels–Alder re-
cations. Here, we describe developments towards asymmetric action with an up to 53 % enantiomeric excess.
carbocation catalysis based on the concept of chiral-anion-di-
Introduction
has previously been applied successfully in organocatalysis and
transition-metal catalysis.[9]
Ever since its discovery[1] more then a century ago the triaryl-
methylium (trityl, Tr) ion has been well studied in terms of phys-
ical properties and reactivity.[2] Although trityl ions are highly
versatile Lewis acids owing to their low-lying empty PC-orbital,
they have been almost completely neglected as Lewis acid cata-
lysts,[3] except for a few elegant studies, mainly by Mukaiyama
et al., 30 years ago.[4,5] In light of more recent work, the trityl
ion has reemerged as a highly efficient Lewis acid catalyst for
several different organic transformations.[6,7]
An intriguing quest in the field of carbocation Lewis acid
catalysis is the development of asymmetric reactions. This is
highly challenging as a result of the intrinsic problems associ-
ated with carbocation stability and reactivity. The rather unsta-
ble cationic carbon needs stabilization by at least two or three
aryl groups. Furthermore, α-hydrogen atoms directly attached
to the carbocation or in ortho- or para-positions on the pending
phenyl ring are highly unstable as a result of E1 elimination,
and α-tertiary alkyl groups can cause problems as a result of
1,2-alkyl shifts/elimination processes. These issues create diffi-
culties about how to introduce a chiral environment in close
proximity to the reaction center. So far, the few reported at-
tempts have met with poor success.[8]
Recently, Luo et al. reported a most elegant study on chiral
counterion/trityl ion directed catalysis.[7] They showed that en-
antioenriched trityl phosphates 1a–1c could be used as a
carbocation precursor for asymmetric Lewis acid catalysis. The
labile C–O bond in trityl-phosphates 1a–1c undergoes ionic dis-
sociation upon interaction with a carbonyl electrophile to form
catalytically a substrate–trityl oxonium ion/chiral anion pair,
which can induce stereoselectivity through chiral counterion
control. This strategy was demonstrated for the Michael addi-
tion reaction of indole and 3-methoxyphenol to activated α,ꢀ-
unsaturated ketones, the hetero-Diels–Alder reaction, and the
hetero-ene reaction with moderate to good enantioselectivity
(Scheme 1).
In light of this excellent study we felt obliged to report our
own results in this challenging area.
We have previously shown that the trityl cation (e.g. TrBF4)
is an outstanding catalyst for the Diels–Alder reaction of α,ꢀ-
unsaturated aldehydes with un-activated dienes with catalyst
loadings as low as 500 ppm.[6a,6c] During the past year we have
addressed the challenge of developing an asymmetric catalytic
version of this reaction. Here we present our attempts toward
this goal based on asymmetric counter-anion directed catalysis
(Scheme 2). We based our strategy on the in situ formation of
a contact trityl cation/chiral anion pair from corresponding trityl
chloride TrCl 2 and the silver or sodium salt of chiral phos-
phates 3,[10] phosphoric amides 4,[11] bis(sulfonyl)amide 5,[12] or
bis(sulfuryl)amide 6[13] (Figure 1 and Figure 2). We anticipated
that the chiral ion pair formed would be in equilibrium with
the inactive covalent bound Tr-anion species (Scheme 2). Luo
et al. showed that these types of equilibrium could be shifted
to the left upon addition of a carbonyl substrate.[7] In the pro-
posed catalytic cycle the dienophile, e.g. the free electron pair
on α,ꢀ-unsaturated aldehyde 7, attacks the trityl ion to form an
intermediate oxonium ion/chiral anion pair. This lowers the
One alternative approach is to introduce enantioselectivity
through asymmetric counter-anion-directed catalysis (ACDC) by
means of a chiral anion that can potentially induce enantiose-
lectivity in catalysis through a contact ion pair. This strategy
[a] Department of Chemistry, Organic Chemistry, Royal Institute of Technology
(KTH),
Teknikringen 30, 10044 Stockholm, Sweden
E-mail: jfranze@kth.se
www.kth.se/en/che/divisions/orgkem/research/JohanFranzen
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201501621.
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Figure 1. Trityl ion precursors.
Scheme 1. Asymmetric catalysis using chiral trityl phosphate recently re-
ported by Luo et al.[7]
LUMO of the substrate and enables the enantioselective
cycloaddition reaction with diene 8 to give adducts 9. By using
binol-based bis(sulfuryl)amide 6a we were able to catalyze the
Diels–Alder reaction with up to 53 % enantiomeric excess.
Figure 2. Chiral ions.
Results and Discussion
The silver and sodium salts of chiral phosphate 3, phosphoric
amides 4, bis(sulfonyl)amide 5, and bis(sulfuryl)amide 6 based
on the binol scaffold are easily available chiral anions that have
previously been successfully used in ACDC reactions (Fig-
ure 1).[9] Upon mixing of these salts with TrCl 2a in CH2Cl2, an
immediate precipitation of AgCl or NaCl occurred. These solu-
tions were then directly used as catalysts for the Diels–Alder
reaction of methacrylaldehyde 7 and 2,3-dimethylbutadiene 8
at 0 °C.
It should be noted that by mixing TrCl with bis(sulfonyl)-
amide 5 and bis(sulfuryl)amide 6 heavily colored solutions re-
sulted, which indicates the presences of trityl ion species. How-
ever, chiral phosphate 3 and phosphoric amide 4 gave colorless
or very weakly colored solutions when mixed with TrCl, which
indicates no or very low concentrations of free trityl ions.
From the screening of different counterion scaffolds we
found that phosphoric amide 4a, bis(sulfuryl)amide 6a, and
binol-derived borate ion 10[14] resulted in the formation of po-
tential catalysts for the Diels–Alder reaction of methacrylalde-
hyde (7) and 2,3-dimethylbutadiene (8) in the presence of TrCl
Scheme 2. Strategy for a chiral anion directed asymmetric carbocation cata-
lyzed Diels–Alder reaction.
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2a (Table 1, Entries 2, 4, 5, and 6). However, both phosphate 3a served for non-substituted phosphoric acid 3a (Table 2, Entry 1
and bis(sulfonyl)amide 5 resulted in very poor catalytic activity versus Table 1, Entry 1). For even bulkier VAPOL-phosphonate
and low conversion into adduct 9 under the same reaction con- 3c the reaction was almost completely inhibited and no enan-
ditions (Table 1, Entries 1 and 3).
tioselectivity was observed (Table 2, Entry 2). Interestingly, for
phosphoramides 4a–4d we could observe a trend of increasing
enantioselectivity with increasing bulk from 4 % ee for Ph- (4a)
to 18 % ee for Ph3Si- (4c) and tri-iPr-Ph- groups (4d; Table 2,
Entries 3–5 versus Table 1, Entry 2). What was most striking
was the dramatic increase in reactivity that was observed for
phosphoramide 4d that resulted in complete conversion in 36 h
(Table 2, Entry 5).
Table 1. Screening of the counterion motif.[a]
Entry[a]
Anion
t
Conv.[b]
ee[c]
Table 2. Screening of binol motifs.[a]
[h]
70
100
26
[%]
2
40
5
[%]
13
4
5
28
1
2
3
4
3a
4a
5
6a
16
28
5
6a
90
73
28
Entry[a]
Anion
t [h]
Conv.[b] [%]
ee[c] [%]
6
10
6a
–
91
68
68
23
23
0
4
0
–
–
7[d]
8[e]
9[d]
1
2
3
4
5
6
7
8
9
3b
3c
4b
4c
4d
6b
6c
6d
6e
6f
70
70
64
35
36
20
21
26
22
19
10
3
13
0
8
18
18
2
14
5
11
40
11
18
24
<95
45
80
5
[a] The anion (5 mol-%) and TrCl 2a (5 mol-%) were dissolved in CH2Cl2. After
10 min the solution was cooled to the indicated temperature, methacrylalde-
hyde (7; 1 equiv.) and 2,3-dimethylbutadiene (8; 1 equiv.) were added (0.3 M)
and the mixture was stirred for the indicated time. [b] Determined by 1H
NMR spectroscopy of the crude reaction mixture. [c] Determined by chiral
GC. [d] Reaction without TrCl 2a. [e] Reaction without anion.
20
24
7
6
10
In terms of stereoselectivity bis(sulfuryl)amide 6a was by far
the best motif and gave Diels–Alder adduct 9 in 28 % ee
(Table 1, Entry 5). Unfortunately, phosphoric amide 4a and chi-
ral borate ion 10 gave the Diels–Alder adduct with very poor
and no stereoselectivity, respectively (Table 1, Entries 2 and 6).
In the absence of TrCl 2a no reaction occurred (Table 1, En-
try 7). Furthermore, in the abscess of anion reactivity slowed
significantly and only 4 % conversion was observed after an
extended reaction time, which indicates that TrCl 2a is not re-
sponsible for any background catalysis (Table 1, Entry 8).
[a] The anion (5 mol-%) and TrCl 2a (5 mol-%) were dissolved in CH2Cl2. After
10 min the solution was cooled to the indicated temperature, methacrylalde-
hyde (7; 1 equiv.) and 2,3-dimethylbutadiene (8; 1 equiv.) were added (0.3 M)
and the mixture was stirred for the indicated time. [b] Determined by 1H
NMR spectroscopy of the crude reaction mixture. [c] Determined by chiral
GC.
For our best lead so far, bis(sulfuryl)amide 6a, we were most
disappointed to find that all attempts to increase the stereo-
selectivity of the Diels–Alder reaction by tuning the bulk of the
binol scaffold failed and all substitutions made in the 3,3′-posi-
The trityl cation readily reacts with a number of rather weak tions resulted in significant decreases in enantioselectivity
nucleophiles and bases, such as water and olefins, and is also a (Table 2, Entries 6–10 versus Table 1, Entries 4 and 5). These
reasonable hydride abstractor. Such processes would result in observations are rather puzzling because binol-based organic
the formation of the corresponding acid of the anion, which acids that are non-substituted in the 3,3′-position generally
would be a potent catalyst for the Diels–Alder reaction. How- shows poorer selectivity relative to their bulkier 3,3′-substitude
ever, there are strong preferences that trityl ion catalyzed Diels– counterparts in asymmetric catalysis.
Alder reactions do not operate through carbocation degrada-
tion and Brønsted acid catalysis.[5d,6a,6c] In addition, we found
that the corresponding acid of 6a, bis(sulfuryl)imide 11, is a
more efficient catalyst in terms of reactivity relative to the TrCl
2a/Anion 6a ion pair. However, for bis(sulfuryl)imide 11 enan-
tioselectivity decreased significantly, which lead us to the con-
clusion that these reactions are not under Brønsted acid cataly-
sis (Table 1, Entry 9).
Although discouraged, we continued our optimization based
on the best lead so far, bis(sulfuryl)amide 6a, and TrCl 2a.
Halogenated solvents are superior for this reaction with 1,2-
dichloroethane (DCE) being the fastest (Table 3, Entries 1–3).
The use of more apolar solvent toluene, which should favor a
close contact ion pair, resulted in a significant decrease in rate
and only 15 % ee under these conditions (Table 3, Entry 4).
The use of a more polar solvent had a negative effect on the
Next, we turned our attention towards 3,3′-substitution of enantioselectivity (Table 3, Entries 5–7). As expected, an in-
the binol motif with the expectation that increased steric bulk crease in catalyst loading led to an increase in extent of conver-
would have a positive influence on the enantioselectivity. 3,5- sion, however, there was no effect on enantioselectivity
Bis(trifluoromethyl)phenyl-substituted phosphonate 3b and (Table 3, Entries 8–11). A decrease the reaction temperature had
TrCl 2a gave only 13 % ee under the standard reaction condi- some effect on enantioselectivity; at –20 °C, 35 and 34 % ee
tions (CH2Cl2, 0 °C), which is the same selectivity that was ob- was obtained in CH2Cl2 and DCE, respectively, with DCE giving
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the highest conversion (Table 3, Entries 12 and 13). At –70 °C
in CH2Cl2, the ee increased to 40 %, but unfortunately with very
low conversion (Table 3, Entry 15). It should be noted that the
TrCl 2/Anion 6 catalyzed reactions are very clean and only start-
Table 4. Screening of TrCl derivatives.[a]
1
ing material or product are identified by H NMR spectroscopy
of the crude reaction mixture. The Diels–Alder reaction of meth-
acrylaldehyde (7) and 2,3-dimethylbutadiene (8) catalyzed by
bis(sulfuryl)amide 6a and TrCl 2a gave the corresponding Diels–
Alder adduct in 75 % isolated yield and 29 % ee after 96 h reac-
tion time (Table 3, Entry 3).
Entry
TrCl
T [°C]
Conv.[b] [%]
ee[c] [%]
1
2
3
2a
2b
2c
2d
2e
–20
–20
–20
–20
0
41
23
4
0
33
34
39
36
0
4[d]
5[e]
Table 3. Screening of reaction conditions.[a]
28
6[f, g]
2b
–70
9
53
[a] Anion 6a (5 mol-%) and TrCl 2 (5 mol-%) were dissolved in CH2Cl2. After
10 min the solution was cooled to the indicated temperature, methacrylalde-
hyde (7; 1 equiv.) and 2,3-dimethylbutadiene (8; 1 equiv.) were added (0.3 M)
and the mixture was stirred for the indicated time. [b] Determined by 1H
NMR spectroscopy of the crude reaction mixture. [c] Determined by chiral
GC. [d] Reaction performed at 0 °C. [e] In CH2Cl2. [f] In DCE/CH2Cl2, 2:1. [g] Re-
action time is 45 h.
Entry[a]
Cat.
[mol-%]
Solvent
T
[°C]
t
[h]
Conv.[b]
[%]
ee[c]
[%]
1
2
5
5
CH2Cl2
CHCl3
0
0
68
68
62
29
28
21
3[d]
5
DCE
0
68
78
29
Finally, we attempted the reaction with a few more dienes
and dienophiles with limited success, although with substantial
enantioselectivity demonstrates the concept of chiral-anion-di-
rected carbocation catalysis (Scheme 3). 2,3-Dibenzylbutadiene
(11) gave corresponding Diels–Alder adduct 12 in 20 % ee with
anion 6a and TrCl 2b. Cyclopentadiene (13) gave the bicyclic
adduct 15a in 23 % ee, whereas 1,3-cyclohexadiene (14) did
not react under these conditions. ꢀ-Substituted dienophile 16
gave trans adduct 17 in 22 % ee.
4
5
6
7
8
9
10
11
5
5
5
5
1
10
20
100
PhMe
THF
CH3CN
CH3NO2
DCE
DCE
DCE
DCE
0
0
0
0
0
0
0
0
68
68
68
68
24
24
24
24
10
0
15
–
4
17
60
33
71
83
80
2
26
30
30
22
12
13
5
5
CH2Cl2
DCE
–20
–20
22
22
23
41
35
34
14
15
5
5
CH2Cl2
CH2Cl2
–20
–70
70
70
37
5
35
40
[a] Anion 6a and TrCl 2a were dissolved in CH2Cl2. After 10 min the solution
was cooled to the indicated temperature, methacrylaldehyde (7; 1 equiv.) and
2,3-dimethylbutadiene (8; 1 equiv.) were added (0.3 M) and the mixture was
stirred for the indicated time. [b] Determined by 1H NMR spectroscopy of the
crude reaction mixture. [c] Determined by chiral GC. [d] Adduct 9 was iso-
lated in 75 % yield and 29 % ee after 96 h reaction time.
The Lewis acidity of the trityl ion is easily tuned by variation
of the electronic properties of the aromatic groups.[6a,6c,15] Thus,
we wanted to see how this would affect the stereoselectivity of
the Diels–Alder reaction. As expected from previous studies, the
reactivity decreased as electron density on the aromatic groups
increased e.g. as Lewis acidity of the carbocation decreased
(Table 4, Entries 1–3). Tri-methoxy TrCl 2d was too weak a Lewis
acid to catalyze the Diels–Alder reaction under these conditions
(Table 4, Entry 4). Unfortunately, the Lewis acidity had only a
minor influence on enantioselectivity, although mono-methoxy
TrCl 2b gave a somewhat higher ee relative to TrCl 2a and di-
methoxy TrCl 2c (Table 4, Entries 1–3). Bulkier carbocation 1-
naphthyldiphenylmethylium ion 2e did not influence the select-
ivity or the reactivity of the reaction (Table 4, Entry 5 versus
Table 1, Entry 4).
Scheme 3. Screening of different substrates for the chiral anion directed
Diels–Alder reaction.
Conclusions
Here we have demonstrated that trityl cation/chiral anion con-
Finally, the best selectivity for the Diels–Alder reaction was tact pairs are catalytically active and capable of chiral induction
obtained with anion 6a and TrCl 2b at –70 in a 2:1 mixture of in the Diels–Alder reaction with up to 53 % ee in accordance
DCE/CH2Cl2, which gave the Diels–Alder adduct in 53 % ee but with the concept of chiral-anion-directed carbocation asymmet-
in only 9 % conversion after 45 h.
ric catalysis. Although the results presented here are not of di-
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(400 MHz, CD3OD): δ = 8.08 (s, 2 H, Ar-H), 8.01 (d, J = 8.0 Hz, 2 H,
Ar-H), 7.68 (d, J = 7.2 Hz, 4 H, Ar-H), 7.48 (t, J = 7.2 Hz, 2 H, Ar-H),
7.39 (t, J = 7.6 Hz, 4 H, Ar-H), 7.33–7.28 (m, 4 H, Ar-H), 7.02 (d, J =
8.4 Hz, 2 H, Ar-H) ppm. 13C NMR (125 MHz, CD3OD): δ = 147.2,
140.3, 137.3, 134.2, 133.6, 132.3, 130.7, 129.4, 129.15, 129.05, 128.1,
rect synthetic value, they do provide an intriguing proof of con-
cept to complement recent and more successful work by Luo
et al.[7] in this area.
As a result of the exceptional Lewis acid properties of the
carbocation and their recent application in catalysis for a variety
of transformations, chiral anion directed asymmetric trityl ion
catalysis has the potential to become a highly efficient strategy
in organic synthesis.
We are continuing to explore the scope and limitations of
chiral-anion-directed asymmetric trityl ion catalysis to gain fur-
ther understanding of reactivity, to increase the enantioselectiv-
ity, and to extend the reaction scope. These results will be re-
ported in due course.
127.9, 127.4, 126.3 ppm. HRMS (ESI): m/z
= calcd. for
C32H20NNa2O6S2 [M + Na]+ 624.0527; found 624.0522.
General Procedure for Diels–Alder Reaction (screening condi-
tions): Sodium bis(sulfuryl)amide 6 (0.0075 mmol) and TrCl 2
(0.0075 mmol) were dissolved in CH2Cl2 (0.3 mL). After 10 min, the
solution was cooled to the indicated temperature and methacrylal-
dehyde (0.15 mmol) and 2,3-dimethylbutadiene (0.15 mmol) were
added. The resulting mixture was stirred for the indicated time.
Conversion into product was followed by 1H NMR spectroscopy.
The enantiomers were separated by GC on a CYCLOSIL-B column:
temperature program: 60 °C (10 min)/2 °C min–1/ 130 °C (30 min)/
10 °C min–1/ 180 °C (5 min). Rt (min): 40.9 (minor enantiomer); 41.8
(major enantiomer).
Experimental Section
General: All reactions were performed in pre-dried solvents under
a nitrogen atmosphere. Imidobis(sulfuryl chloride) was prepared in
accordance with literature procedures.[13,16] The 1H and 13C NMR
spectra were recorded at 500 or 400 MHz and 125 MHz, respec-
tively. Chemical shifts are reported relative to CHCl3 (δ = 7.26 ppm),
[D6]DMSO (δ = 2.50 ppm), and CD3OD (δ = 3.32 ppm) resonances
for 1H NMR spectroscopy, and relative to the central CDCl3 reso-
nance (δ = 77.0 ppm) for 13C NMR spectroscopy. Flash chromatogra-
phy and column chromatography were carried out with Merck silica
gel 60 (230–400 mesh).
Preparation of 1,3,4-Trimethylcyclohex-3-ene-1-carbaldehyde
(9): (Table 3, Entry 3). Sodium bis(sulfuryl)amide 6a (17 mg,
0.0375 mmol) and TrCl 2a (10 mg, 0.0375 mmol) were dissolved in
DCE (2.5 mL). After 10 min, the solution was cooled to 0 °C and
methacrylaldehyde (7; 62 μL, 0.75 mmol) and 2,3-dimethylbutadi-
ene (8; 85 μL, 0.75 mmol) were added and the reaction mixture was
stirred at 0 °C for 96 h. The reaction was quenched with a drop of
water and directly purified by flash chromatography (pentane/
Et2O = 20:1) to give compound 9 as a colorless oil (85.0 mg, 75 %,
29 % ee). All characterization data was in accordance with those
previously reported.[17] The enantiomers were separated by GC on
a CYCLOSIL-B column: Temperature program: 60 °C (10 min)/
2 °C min–1/ 130 °C (30 min)/10 °C min–1/ 180 °C (5 min). Rt (min):
40.9 (minor enantiomer); 41.8 (major enantiomer).
General Procedures for the Preparation of Sodium 1,1′-Bi-
naphthyl-2,2′-bis(sulfuryl)amides 6[13]
Method A. Exemplified by the Preparation of Sodium (R)-3,3′-
Diiodo-1,1′-binaphthyl-2,2′-bis(sulfuryl)amide (6c): To a stirred
solution of (R)-3,3′-diiodo-2,2′-dihydroxy-1,1′-binaphthyl (1.13 g,
2.10 mmol) in dry toluene (20 mL) was added sodium hydride (60 %
in mineral oil, 265 mg, 6.62 mmol). The suspension was heated to
130 °C, and imidobis(sulfuryl chloride) (517 mg, 2.41 mmol) in dry
toluene (10 mL) was added over a period of 30 min. The reaction
mixture was stirred at 130 °C for 24 h. After cooling, the solution
was poured into water (10 mL), and all volatile components were
removed under reduced pressure to give a greenish, semisolid resi-
due. Purification by flash chromatography (CH2Cl2/MeOH = 5:1)
Acknowledgments
This work was made possible by grants from the Swedish Re-
search Council (VR). S. N. thanks the Chinese Scholarship Coun-
cil (CSC) for a grant. V. R. N. thanks the Wenner-Gren Founda-
tions for a grant. Carin Larsson (Stockholm University) is ac-
knowledged for HRMS analyses.
1
gave compound 6c as a light yellow solid (736 mg, 50 %). H NMR
(400 MHz, CD3OD): δ = 8.68 (s, 2 H, Ar-H), 7.91 (d, J = 8.4 Hz, 2 H,
Ar-H), 7.49 (t, J = 7.2 Hz, 2 H, Ar-H), 7.32 (t, J = 7.2 Hz, 2 H, Ar-H),
6.81 (d, J = 8.4 Hz, 2 H, Ar-H) ppm. 13C NMR (125 MHz, CD3OD): δ =
149.2, 142.2, 134.5, 134.2, 128.6, 128.27, 128.22, 127.9, 126.2,
90.3 ppm. HRMS (ESI): m/z = calcd. for C20H10I2NNa2O6S2 [M + Na]+
723.7834; found 723.7829.
Keywords: Synthetic methods · Asymmetric catalysis ·
Cycloaddition · Carbocations · Trityl ion
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Method B. Exemplified by the Preparation of Sodium (R)-3,3′-
Diphenyl-1,1′-binaphthyl-2,2′-bis(sulfuryl)amide (6b): To a solu-
tion of sodium (R)-3,3′-diiodo-1,1′-binaphthyl-2,2′-bis(sulfuryl)amide
6c (20.0 mg, 0.03 mmol) and Pd(PPh3)4 (3 mol-%, 1.0 mg,
0.009 mmol) in EtOH (0.1
M) were added phenylboronic acid
(13.4 mg, 0.11 mmol) and Na3PO4 (24.6 mg, 0.15 mmol). The result-
ing mixture was heated to reflux temperatures until all of the start-
ing material had reacted, then cooled to room temperature, and
passed through a pad of Celite. The solvent was evaporated and
the residue was dissolved in CH2Cl2, washed with saturated aque-
ous NH4Cl, water, and brine. The organic phase was dried with
Na2SO4 and concentrated in vacuo to give the crude product. Sub-
sequent purification by flash chromatography (CH2Cl2/MeOH =
15:1) gave compound 6b as a white solid (12 mg, 66 %). 1H NMR
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Received: December 29, 2015
Published Online: February 26, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim