191 7 Electrophilic Formation of Aromatic C–S Bonds 7.1 Sulfonylation 7.1.1 General Aspects The preparation of arenesulfonic acids, arenesulfonyl chlorides, and sulfones from arenes is a widely used and versatile reaction (Scheme 7.1). The yields are generally high because the products are unreactive and only slowly sulfonylate the excess arene. Sulfones are, though, often obtained as byproducts in small quantities in the synthesis of aromatic sulfonic acids or sulfonyl halides [1]. O H 2 SO 4 (20% SO 3 ) 40 °C, 18 h, then Na 2 CO 3 O S NaO O O 81% WO 2008040934 NH 3 HSO 4 Cl 200 °C neat, vacuum NH 2 Cl SO 3 H 48% 04jmc6948 Cl NH 2 12 eq ClSO 3 H 12 eq NaCl 160 °C, 3 h NH 3 , H 2 O 100 °C, 1 h Cl NH 2 S S Cl Cl O O O O Cl NH 2 S S NH 2 H 2 N O O O O 30% (two steps) 60joc965 N O Br Br Br N O Br Br Br S F O O FSO 3 H 160 °C, 3 h 76% 83joc3220 Scheme 7.1 Examples of the preparation of arene sulfonic acids and sulfonyl halides [2–5]. Side Reactions in Organic Synthesis II: Aromatic Substitutions, First Edition. Florencio Zaragoza D¨ orwald. c 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.
Transcript
191
7Electrophilic Formation of Aromatic C–S Bonds
7.1Sulfonylation
7.1.1General Aspects
The preparation of arenesulfonic acids, arenesulfonyl chlorides, and sulfones fromarenes is a widely used and versatile reaction (Scheme 7.1). The yields are generallyhigh because the products are unreactive and only slowly sulfonylate the excessarene. Sulfones are, though, often obtained as byproducts in small quantities inthe synthesis of aromatic sulfonic acids or sulfonyl halides [1].
O
H2SO4 (20% SO3)40 °C, 18 h, then
Na2CO3 OS
NaO
O O
81%WO 2008040934
NH3HSO4Cl200 °C
neat, vacuum NH2Cl
SO3H48%
04jmc6948
Cl NH2
12 eq ClSO3H12 eq NaCl160 °C, 3 h
NH3, H2O100 °C, 1 h
Cl NH2
SSClCl
O O O O
Cl NH2
SSNH2H2N
O O O O30%
(two steps)60joc965
N
O Br
Br
Br
N
O Br
Br
Br
SF
OOFSO3H
160 °C, 3 h
76%83joc3220
Scheme 7.1 Examples of the preparation of arene sulfonic acids and sulfonyl halides [2–5].
192 7 Electrophilic Formation of Aromatic C–S Bonds
The AlCl3-mediated conversion of arenesulfonyl chlorides into diarylsulfonesproceeds smoothly with simple substrates (Scheme 7.2). For alkanesulfonyl halides,though, special catalysts and conditions are required [6]. Sulfonic anhydridessulfonylate arenes more cleanly than sulfonyl halides [7, 8] because the latter oftenact as halogenating reagents or oxidants.
SCl
O O5.6 eq C6H6
1.2 eq AlCl320 °C, 16 h
92%
SO O 10 eq ClSO3H
50 °C, 6 h
100%
SO O
SCl
O O
08jmc7670
SO
S
O OO O
2 eq AlCl3excess C6H6
77%
S
O O
SCl
OO2 eq AlCl3excess C6H6
2%
54ja1222
F
F
Cl
2 eq (MeSO2)2O
0.1 eq F3CSO3H
130 °C, 21 h
43%
2 eq (EtSO2)2O
2 eq F3CSO3H
130 °C, 18 h
13%
F
F
Cl
S O
O
F
F
Cl
S O
O
08oprd1060
N
+
SCl
OO
1 eq 3 eq
10% Pd(MeCN)2Cl2
2 eq K2CO3, 4 Å MS
dioxane, 120 °C, 6 h N
S
OO
79%
(does not work with
alkanesulfonyl chlorides)
09ja3466
N +
1 eq 3 eq
SCl
O O
Br
5% [Ru(p-cymene)Cl2]22 eq K2CO3
MeCN, 115 °C, 15 h S
O O
Br
N74%
11ja19298
Scheme 7.2 Examples of aromatic sulfonylations [7–11]. Further examples: [6, 12, 13].
7.1 Sulfonylation 193
7.1.2Typical Side Reactions
One reason for the poor yield of sulfonylations with sulfonyl halides is that thelatter often act as halogenating agents instead of sulfonylating agents (Scheme 7.3).In contrast to acyl anions, sulfinates (RSO2
−) are stable and good leaving groups,and often react as such (e.g., as leaving group in β-eliminations, RSO2X as syntheticequivalent of X+). Even the preparation of sulfonamides from sulfonyl chloridescan give low yields when conducted under anhydrous conditions. High yieldsof sulfonamides are usually obtained only under Schotten–Baumann conditions(in the presence of aqueous base). Hydrolysis of the sulfonyl halide is seldom aproblem, because sulfonyl halides, in particular arenesulfonyl chlorides, do notreadily react with water or aqueous bases.
NH
1 eq MeMgBr, 20 °C, 1 h
then 1 eq PhSO2Cl
THF, 20 °C, 3 h
75% NH
O
PhSO2ClKOH, H2O
C6H6
90%
N
SO2Ph
N
Cl
NH
O
S
O
Ph
87joc3404
Scheme 7.3 Oxidation of an indole with benzenesulfonyl chloride [14].
Acid-catalyzed aromatic sulfonylations with sulfonic acid derivatives arereversible, which can lead to the isomerization to more stable isomers, totranssulfonations, or to desulfonations (Scheme 7.4). Such reactions can besuppressed by reducing the reaction temperature and the amount and strength ofacid used.
Sulfonic acid derivatives can act as oxidants, and convert electron-rich arenesinto radical cations. Biaryls are the typical byproducts formed from intermediateradicals (Scheme 7.5). In the presence of transition metals, arenesulfonyl halidesand other derivatives of arenesulfonic acids can also act as arylating reagents [17].
Sulfuryl chloride (SO2Cl2) is not a convenient reagent for the preparation ofsulfonyl chlorides or sulfones, because this reagent is an even more powerfulchlorinating agent than arenesulfonyl chlorides (Scheme 7.6). Only if metallatedarenes are to be chlorosulfonylated under anhydrous conditions can sulfurylchloride be used as electrophile, but the yields tend to be low. The conversion ofarenes into arenesulfonyl halides works best with halosulfonic acids (HalSO3H). In
194 7 Electrophilic Formation of Aromatic C–S Bonds
S
OO
+
MeO
1.0 eq 3.0 eq
1.5 eq F3CSO3H
150 °C, 4 h
75%MeO
S
O O
85joc3334
F
+ SCl
O OS
O O
F
S
O O
F
1 eqexcess
(solvent)
1 eq GaCl360 °C, 24 h
+
73% 26%
06oprd334
Scheme 7.4 Isomerization of sulfones [15, 16].
N
Tf2O, CH2Cl20−20 °C
N
S CF3
OO
+N N
20% 60%
N
1.1 eq Tf2O
MeNO2
0−20 °C, 16 h+
N
1.0 eq 1.2 eq
N
S CF3
OO
88%
2 eq KOtBu
DMF−50 °C, 16 h
N
S
N
F3C O
O
SCF3
O
O
N
S
N
F3C O
O
81%
11ol4854
11ol4854
N
SCl
O O
+
10% PdCl2(MeCN)2
1 eq Ag2CO3
3 eq CuBr, MS 4 Å
dioxane, 160 °C, 12 h N
67%
1 eq 3 eq09ja3466
Scheme 7.5 Oxidations and arylations with sulfonic acid derivatives [10, 18].
7.2 Sulfinylation 195
MeO
1.05 eq SO2Cl21% AlCl3
PhCl, 20 °C, 22 h
MeO
Cl
95%
12oprd148
Cl
NH
O
2.1 eq BuLi
THF, −20 °C, 2 h
then 5 eq SO2Cl2−78 °C, 2 h, then 20 °C, 16 h
50% N
O
SCl
O
O
N
O
LiN
O
Li
09cej8283
Scheme 7.6 Reaction of arenes with sulfuryl chloride [20, 21].
the presence of iodine, though, even chlorosulfonic acid can act as a chlorinatingreagent [19].
7.2Sulfinylation
7.2.1General Aspects
Sulfoxides can be prepared by direct sulfinylation of unsubstituted arenes withsulfinic acid derivatives (Scheme 7.7), but numerous byproducts may result. Analternative to the direct sulfinylation (or sulfonylation) is sulfenylation followed byoxidation [8, 22].
7.2.2Typical Side Reactions
As shown in Schemes 7.7 and 7.8, sulfinylations are problematic because theresulting sulfinic acids and sulfoxides can react as electrophiles under the reactionconditions required for their preparation. Even poor nucleophiles (e.g., halides)can add to the cationic intermediates generated from sulfoxides and electrophiles.Aliphatic alcohols will usually be oxidized to aldehydes or ketones by sulfoxides inthe presence of electrophiles (Swern and Pfitzner–Moffatt oxidations).
Similar to sulfonylations, sulfinylations are reversible. Treatment of aromaticsulfoxides with acids or other catalysts can bring about rearrangement to more
196 7 Electrophilic Formation of Aromatic C–S Bonds
F
1.1 eq AlCl3HCl, excess SO2
CS2, 20 °C, 16 h, then NaOH, H2O
F
SONa
O
75%
35ja2166
MeO
+S
MeO
O2 eq 1 eq
1 eq AlCl3DCE, 25 °C, 15 h
S
O
MeO
53%
11joc4635
N
N
F3C Cl
Cl
CN
NH2
1.1 eq F3CSO2Na
1.5 eq Et3NHCl
6.5 eq PhMe, 1.2 eq SOCl2
5 °C, 0.5 h, then add pyrazole
50 °C, 6 hN
N
F3C Cl
Cl
CN
NH2
S
CF3
O
N
N
F3C Cl
Cl
CN
HN S
O
CF3
70%
US 2012309806
F3CS
ONa
Obenzene
TfOH, 20 °C, 24 h
50%
SF3C
OTf2O, 20 °C, 7 d S
CF3
SF3C
56%
TfO
06ang1279
OH
+ SPhCl
O AlCl3, CH2Cl2
90%
OH
SPh
OSOCl2
25 °C, 0.5 h
OH
SPh
Cl
78%
94joc3248
Scheme 7.7 Examples of aromatic sulfinylations [23–27].
7.2 Sulfinylation 197
1 eq F3CSO2K
2 eq (F3CSO2)2O
20 °C, 7 d
SCF3
SCF3
O
S
CF3
+ +
47% 7% 12%
09ejoc1390
O
SO
NH
0.1 eq NEt3PhMe, 111 °C, 24 h
SN
O
S
HN
O
S
N
O
+ +
69% 15% 10%
S N
O
− OH
94joc8076
Scheme 7.8 Decomposition reactions of sulfoxides [28, 29].
stable isomers (Scheme 7.9). Moreover, acids such as AlCl3 can cause the dispro-portionation of sulfoxides into sulfones and thioethers. Strong bases [30], OsO4
[31], or electrophiles (as in the Pummerer rearrangement) can also induce thedisproportionation of sulfoxides or the β-elimination of ArSO− [32]. Because of thishigh reactivity of sulfoxides, dimethyl sulfoxide (DMSO) is a hazardous solvent,ill-suited for most large-scale applications.
N
CN
SO
N
CN
S
OTFA, CH2Cl2
20 °C, 2 h
97%
89jmc1202
Scheme 7.9 Acid-mediated rearrangement of a sulfoxide [33]. Further examples: [34].
The preparation of sulfinic acid chlorides or sulfoxides from thionyl chloridehas been reported [35], but must be done carefully. In addition to the problems
198 7 Electrophilic Formation of Aromatic C–S Bonds
MeO
10 eq SOCl22% catalyst60 °C, 1 h
MeO
SCl
O
+
MeO
S
OMe14%99%
40%traces
03sl631
catalyst:Sc(OTf)3
BiCl3
N
O
excess SOCl2DME, 80 °C, 2 h
95%
N
Cl
S
O
OH Cl
O
Cl
O
oil, 210 °C, 0.5 hN
ClHCl
90%
76joc3406
HN
O
NO
NMe2 excess SOCl280 °C, 1.5 h
67%
N
Cl
NO
NMe2
O
N
Cl
NO
NMe2
S
Cl
O01joc612
MeO
MeON
O
O
OHO O
excess SOCl220 °C, 4 h
79%
MeO
MeON
O
OO
O
78joc3781
CO2H
F3C
2.3 eq SOCl20.14 eq pyridine
135−145 °C, 12 h
F3C SCOCl
Cl
23%
11ja3764
Scheme 7.10 Byproducts formed during reactions with thionyl chloride [36–40]. Furtherexamples: [41].
7.3 Sulfenylation 199
mentioned above, various functional groups are chlorinated, sulfinylated, dehy-drogenated, or otherwise oxidized by thionyl chloride, which can cause theformation of unexpected products (Scheme 7.10). Because of the many poten-tial side reactions of electrophilic sulfinylations, sulfinic acids are usually preparedby reduction of sulfonyl chlorides, and not by electrophilic sulfinylation withSOCl2 or SO2.
7.3Sulfenylation
7.3.1General Aspects
Arylthioethers can be prepared from electron-rich arenes and disulfides [33],thioethers, sulfenyl halides or sulfenyl amides (RS–NR2) [43], or esters (RS–OR)(Scheme 7.11). These reactions can be catalyzed by acids, oxidants [44], andcopper or palladium complexes. In the presence of strong acids, arenes react withsulfoxides to sulfonium salts, which can be dealkylated with various nucleophilesto yield thioethers [29, 45].
NH2
1 eq
7% AlCl3150 °C, 0.5 h
then add 1 eq MeSSMe150–170 °C, 10 h
NH2 NH2
SMe MeS
+
21% 48%89joc2985
OH O
NH
O
+
N N
NN
SCl
Ph
1 eq 1 eq
OH O
NH
O
S
N N
NN
Ph
CCl4, 77 °C, 16 h
77%
EP 0763526
Scheme 7.11 Electrophilic sulfenylation of arenes [42, 46–51]. Further examples: [52].
200 7 Electrophilic Formation of Aromatic C–S Bonds
N
MeSSMe, air1 eq Cu(OAc)2
DMSO, 130 °C, 24 h N
SMe+
N
SMeMeS
10–20%51%
06ja6790
OMe
OMeMeO
+S
SCO2Me
CO2Me0.2 eq CuI, O2
DMF, 120 °C, 24 h
1.3 eq 1.0 eq
OMe
OMeMeO
SCO2Me
71%
10joc6732
(only works with
trialkoxybenzenes)
N
EtO2C
2 eq K2S2O8
4 eq CuF2
DMSO, 125 °C, 72 h N
EtO2C
+N
EtO2C
SMe SMe
SMe66% 9%
89% conversion
10ol1644
NH
CO2Et +
HS OMe
1.0 eq 1.2 eq
1.2 eq NCSCH2Cl2
−78 °C to 0 °C, 1 h
NH
CO2Et
S
OMe
76%
04ol819
NH
NH OMe
O 1.2 eq F3CSNHPh
2.5 eq TsOH
CH2Cl2, 12 h
NH
N
NH
NH OMe
O
S
H
CF3
OMe
O
SCF3
+
temperature:50 °C20 °C
95%
96% (> 20 : 1)
12joc7538
(1 : 1)
Scheme 7.11 (Continued)
7.3.2Typical Side Reactions
During sulfenylations with disulfides, thiols are formed as byproducts. These arestrongly nucleophilic and can cause unwanted substitution reactions, includingthe dealkylation of thioethers. Further side reactions include multiple sulfenylationand halogenation of the arene (Scheme 7.12).
References 201
F
F
F
F
F
+S
S
0.3 eq CuBr
2 eq tBuOLi
DMSO, 60 °C, 4 h
5 eq 1 eq
85%
F
F
S
F
F
S
12ejoc1953
N
N NH
Ph5 eq MeSSMe
10% CuI, air
DMF, 110 °C, 60 h N
N NH
Ph SMe
N
N NH
Ph SMe
SMe+
71% 15%
13obmc5189
Scheme 7.12 Side reactions during electrophilic aromatic sulfenylations [53, 54].
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